| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Department of Physiology, Asahikawa Medical College, Midorigaoka-higashi 2-1, Asahikawa 078-8510, Japan
2
Department of Physiology, Fukushima Medical University School of Medicine, 1 Hikari-ga-oka, Fukushima 960-1295, Japan
3
Department of General Medicine, Asahikawa Medical College, Midorigaoka-higashi 2-1, Asahikawa 078-8510, Japan
4
Department of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
(Received 25 February 2005;
accepted after revision 8 August 2005;
first published online 25 August 2005)
Corresponding author Y. Koyama: Department of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan. Email: koyamay{at}sss.fukushima-u.ac.jp
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
One of the major orexinergic projections is present in the structures of the midbrain, including the substantia nigra and the mesopontine tegmentum (Peyron et al. 1998; Nambu et al. 1999). The latter contains the mesencephalic locomotor region (MLR) (Grillner et al. 1997; Takakusaki et al. 2003a), and the pedunculopontine and the laterodorsal tegmental nuclei (PPN/LDT) which regulate rapid eye movement (REM) sleep (McCarley et al. 1995; Datta & Siwek, 1997; Koyama & Sakai, 2000). Under normal conditions emotional stimuli induce alert responses which produce an increase in muscle tone and/or locomotor behaviour (Garcia-Rill et al. 2004; Skinner et al. 2004). However, humans and animals with narcolepsy may experience cataplexy, a sudden loss of muscle tone induced by emotional stimuli (Nishino & Mignot, 1997; Nishino, 2003). It is possible therefore that orexinergic projections to these areas may be involved in both locomotion and a loss of muscle tone.
Other investigations have reported that the activation of neurones in the MLR of acute decerebrate cats induced locomotion, and activation of neurones in the ventrolateral PPN-induced muscular atonia that was associated with REM (Takakusaki et al. 2003a, 2004c). The PPN-induced REM and atonia were associated with activation of cholinergic neurones. Moreover, an activation of neurones in the substantia nigra pars reticulata (SNr) prevented PPN-induced REM with atonia via GABAergic projections to the PPN (Takakusaki et al. 2004c). Because the orexinergic system is abnormal in the context of narcolepsy, we hypothesized that orexinergic projections to these midbrain structures may regulate the switching of emotional motor behaviour. Emotional signals elicit locomotor behaviour in the presence of orexins and induce cataplexy in the absence of orexins. Consequently, the goal of the present study was to characterize the role of orexinergic projections to the midbrain in the control of locomotion and postural muscle tone. For this purpose we employed acute decerebrate cats in which the cerebral hemispheres, including the hypothalamus, were removed. We then examined how MLR-induced locomotion, and PPN-induced REM and atonia, were altered after injections of orexin-A into each of the MLR, the PPN, and the SNr. The preliminary results have been published as abstracts (Takakusaki et al. 2004d).
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Animal preparation
The experiments were performed with 24 cats, with a weight from 2.1 to 3.4 kg, from the animal facility at Asahikawa Medical College. Each cat was surgically decerebrated at the precollicular–postmammillary level while under halothane (Halothane, Otsuka, Osaka, Japan; 0.5–3.0%) and nitrous oxide gas (0.5–1.0 l min–1) anaesthesia with oxygen (3.0–5.0 l min–1). The anaesthesia was then discontinued. The trachea was intubated, and a catheter was placed in the femoral artery to monitor blood pressure. Another catheter was placed in the cephalic vein to administer adrenaline (Bosmin, Daiichi Co., Osaka, Japan). The head was fixed in a stereotaxic apparatus, and a rigid spinal frame secured the cat by clamping the dorsal processes of the first three thoracic vertebrae. The limbs rested on a static surface, or on the surface of a treadmill, and a rubber hammock supported the body. The animal's rectal temperature was maintained at 36–37°C by using radiant heat lamps. The mean blood pressure of each cat was maintained greater than 100 mmHg by an intravenous infusion of adrenaline (0.1–0.3 mg kg–1, infusion rate of 0.01 mg min–1), and the end tidal CO2 was maintained between 4% and 6%.
Brainstem stimulation and EMG recording
Each stimulating electrode consisted of a glass micropipette filled with Wood's metal. The tip of the micropipette was replaced with a carbon fibre (diameter, 7 µm; resistance, 0.2–0.5 M
; Takakusaki et al. 2003a, 2004c). The experimental design is schematically illustrated in Fig. 2. A stimulating electrode was inserted into the mesopontine tegmentum (A 1.0–P 3.0, LR 2.0–5.0, H +1.0–5.0). To evoke locomotion, repetitive stimuli with a constant pulse (10–50 µA, and 0.2 ms duration at 50 Hz) were delivered for 5–30 s while the treadmill belt was advanced at a speed of 0.3 m s–1. The same electrode was used for mesopontine stimulation (10–50 µA, and 0.2 ms duration at 50 Hz, lasting for 5–10 s) to evoke REM with atonia while the animal's limbs rested on a stationary surface (Takakusaki et al. 2003a, 2004c). The stimulation was applied by moving the stimulating electrode with an interval of 0.5–1.0 mm in the dorsoventral, mediolateral and rostrocaudal directions so that an optimal site for evoking locomotion, or REM with atonia could be identified in each animal. The optimal stimulus sites for evoking locomotion, so-called midbrain locomotor region (MLR), were mainly located in the cuneiform nucleus (CNF). The sites for evoking muscle tone suppression were located in the ventral and ventrolateral parts of the PPN (Fig. 1).
|
|
|
|
|
Histological control
At the end of an experiment, the stimulus sites were marked by passing a direct current of 30 µA through an electrode for 30 s. The injection sites were also marked with 10% fast green, using the same amount as the substances that had been previously injected. Each cat was then killed with an overdose of sodium pentobarbital (60 mg kg–1, I.P.) anaesthesia. The brainstem was removed and fixed in 10% formalin. Frozen coronal or parasagittal sections (50 µm) were cut and stained with neutral red. The location of the microlesions and diffusion areas of the fast green were identified with the assistance of the stereotaxic atlases of Berman (1968) and Snider & Niemer (1961).
Choline acetyltransferase (ChAT) immunohistochemistry was performed to identify the boundaries of the PPN so that we could elucidate whether the effective stimulus sites for evoking REM and atonia were located within the PPN. Six animals were deeply anaesthetized with Nembutal and transcardially perfused with 0.9% saline followed by a solution of 3.0% paraformaldehyde and 0.01% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4). The brain of each cat was removed and saturated with a cold solution of 30% sucrose, and 50 µm frozen sections were prepared. Following this, ChAT immunohistochemistry was performed by using the peroxidase–antiperoxidase method combined with diaminobenzidine (Mitani et al. 1988; Lai et al. 1993; Takakusaki et al. 2003a, 2004a). Monoclonal anti-ChAT antibody (Boehringer Mannheim, Germany) was used for these preparations.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Before examination of the effects of the orexin injections into the mesopontine tegmentum, we confirmed the stimulus effects of the locomotor region and muscle tone inhibitory region in the mesopontine tegmentum, as described in a previous report (Takakusaki et al. 2003a). The findings shown in Fig. 1A illustrate that repetitive electrical stimulation applied to the CNF induced locomotion on the moving treadmill (Fig. 1Ab). On the other hand, stimulation of the ventral part of the PPN resulted in suppression of postural muscle tone and generation of REM (REM and atonia). The distribution of the optimal stimulus sites for evoking locomotion, and the muscle tone inhibitory region, are shown on parasagittal (Fig. 1Ba) and coronal (Fig. 1Bb) planes of the brainstem. It was confirmed that the locomotor region was mainly located in the CNF and partly included the dorsal region of the PPN. The muscle tone inhibitory region was located in the ventrolateral region of the PPN. The distribution of the cholinergic neurones, which were labelled by ChAT immunohistochemistry on a coronal section of the brainstem, is shown in Fig. 1C. ChAT- positive, cholinergic neurones were located in the LDT and the PPN (Fig. 1Ca). The PPN was defined by loosely arranged cholinergic neurones that surrounded the superior cerebellar peduncle (SCP; Fig. 1Ca). The cholinergic neurones were preferentially distributed in an area corresponding to the inhibitory region, rather than the locomotor region.
A framework for this study
Figure 2 shows a framework for this study. The MLR and muscle tone inhibitory region in the PPN are in close proximity to each other in the lateral part of the midbrain (Takakusaki et al. 2003a, 2004a,c). Activation of the MLR induces locomotor movements via activation of central pattern generators in the spinal cord through the medullary reticulospinal tract (Rossignol, 1996). Activation of the MLR may also activate muscle tone excitatory systems, including the coerulospinal and raphespinal tracts (Mori, 1987; White & Fung, 1989). In contrast, activation of the PPN neurones induces REM and atonia. The PPN-induced muscular atonia is mediated through the pontomedullary reticulospinal tract (inhibitory system; Habaguchi et al. 2002; Takakusaki et al. 2003a, 2004a,c). It is suggested that an interconnection between the mesopontine cholinergic nuclei and the caudoventral PRF could operate as a common generator of REM and ponto-geniculo-occipital waves (Sakai & Jouvet, 1980; Datta & Hobson, 1994; Vanni-Mercier & Debilly, 1998). The PPN-induced REM can be thus attributed to an activation of the REM generator in the PPN and the caudal PRF (Takakusaki et al. 2004c). Electrophysiological (Saitoh et al. 2003) and neuroanatomical (Grofova & Zhou, 1998) studies have suggested that GABAergic neurones in the SNr monosynaptically inhibit the activity of cholinergic PPN neurones. We have demonstrated that the PPN effects were under the control of GABAergic inhibitory projections from the SNr (Takakusaki et al. 2003a, 2004c).
Effects of injections of orexin A into the midbrain areas
Orexinergic neurones project to the mesopontine tegmentum, including the MLR, the PPN and the SNr. Consequently, orexin A (0.1–0.25 µl, 60 µM to 1 mM) was injected into each of these areas to characterize how MLR/PPN-induced locomotion and REM and atonia were altered by these orexinergic projections. First, we examined the effects of an orexin injection into the MLR (Fig. 3). Electrical stimulation (30 µA) which was applied to the lateral part of the CNF (indicated by a filled arrow in Fig. 3A) elicited locomotion on the moving treadmill. However stimuli with a strength of 20 µA did not evoke locomotion (Fig. 3Ba). Next, orexin A with a concentration of 200 µM and a volume of 0.25 µl was injected into the region adjacent to the locomotor region (indicated by an open arrow in Fig. 3A). Ten minutes after this injection stimulation with a strength of 20 µA evoked locomotion. Thirty minutes after the injection locomotion was elicited on the treadmill belt (indicated by an open line under the EMG records) without electrical stimulation (Fig. 3Bc). In another cat (Fig. 3C) stimulation of the CNF with a strength of 40 µA elicited locomotion (Fig. 3Ca). Twenty minutes after an injection of orexin (60 µM, 0.25 µl) into the CNF a stimulus strength of 20 µA was enough to evoke locomotion (Fig. 3Cb). Even 60 min after the injection locomotion was still evoked by stimuli with a strength of 30 µA (Fig. 3Cc). The complete effects of the orexin upon locomotion were examined in 10 trials in six cats. In each trial the threshold current to elicit locomotion was reduced (Fig. 3D). Moreover, injections of orexin with higher concentrations (200 and 500 µl) spontaneously induced locomotion without electrical stimulation in two animals.
Next, the effect of orexin injections into the PPN was examined. In the cat illustrated in Fig. 4A, stimulation of the caudal part of the PPN (indicated by a filled arrow in Fig. 4Aa) induced REM and atonia (Fig. 4Ab). Orexin A was then injected into the PPN adjacent to the stimulus site (indicated by an open arrow in Fig. 4Aa). It was generally observed that an orexin injection into the PPN alone did not change the level of muscle tone. However, 30 min after the injection of orexin, REM and atonia were abolished when the PPN was stimulated with the same intensity (Fig. 4Ac). In eight cats, orexin injections into the PPN either abolished the PPN effects, even when stimuli with an intensity of 50 µA were delivered (3 trials in 3 cats), or attenuated the PPN effects (8 trials in 5 cats). Figure 4B illustrates that the threshold stimulus strength which was required to elicit the PPN effect was increased in each trial after the orexin injections. These findings suggest that the orexinergic projection to the PPN suppresses the excitability of PPN neurones that are involved in the generation of REM and atonia.
|
Further attempts were made to test whether orexinergic projections to the SNr could affect the PPN-induced REM and atonia via the GABAergic nigrotegmental projection (see Fig. 2). The results are shown in Fig. 5. After confirming REM and atonia (Fig. 5Ab), which was induced by the PPN stimulation (indicated by a filled arrow in Fig. 5Aa), orexin A was injected into the dorsolateral part of the SNr (indicated by an open arrow in Fig. 5Aa). Although the orexin injection into the SNr did not alter the level of the muscle tone it did result in complete inhibition of the PPN-induced REM and atonia (Fig. 5Ac). In eight trials of four animals, orexin injections increased the stimulus strength that was required to produce the PPN-induced REM and atonia (Fig. 5B). In another cat PPN-induced muscular atonia (Fig. 5Ca) was blocked by an orexin injection into the SNr (Fig. 5Cb). To further determine whether the effect of a nigral orexin injection was mediated through GABAergic projections to the PPN, bicuculline was injected into the PPN. It was observed that the PPN-induced muscular atonia was re-established 5 min after the injection of bicuculline (Fig. 5Cc). These results indicate that PPN-induced REM and atonia is inhibited by the GABAergic nigrotegmental projection.
Orexin injection sites and time course of the orexin effects
Fast green was used to identify the injection sites and to measure the spread of the infusions, which for each injection was limited to an area of approximately 1.0–1.5 mm in diameter. Figure 6 illustrates the locations of the injection sites on coronal (Fig. 6A–C) and parasagittal planes (Fig. 6D) of the brainstem. Ten injection sites which either facilitated MLR-induced locomotion or spontaneously elicited locomotion were located in an area corresponding to the CNF and adjacent region, including the dorsal part of the PPN (Fig. 6A and D). Injection sites which inhibited the PPN effects were located in a region corresponding to the PPN (n = 11, Fig. 6B and D) and the lateral part of the SNr (n = 8, Fig. 6C and D). Two injections, which are indicated by an asterisk in Fig. 6D, not only facilitated MLR-induced locomotion but also inhibited PPN-induced REM and atonia.
|
|
Finally, we elucidated how orexin injections into the PPN modulate the descending excitatory and inhibitory effects on muscle tone (Fig. 8). We first stimulated each of the CNF, the LC, the PRF, the dorsal PPN and the ventral PPN to examine the stimulus effects on soleus muscle activity (Fig. 8B). We then compared these effects with the results obtained by the same stimulus procedures, but after an injection of orexin (Figs 8C and D). Short trains of stimuli which were applied to the CNF and the LC (indicated by open circles in Fig. 8A) induced a mixture of excitatory and inhibitory effects on the muscle tone (1st and 2nd recordings in Fig. 8B). In contrast, stimuli applied to the medial PRF and the dorsal and ventral PPN areas (indicated by filled circles in Fig. 8A) induced prominent inhibitory effects (3rd and 5th recordings in Fig. 8B). The stimuli which were applied to each site approximately 30–60 min after an orexin injection into the left PPN (Fig. 8A) resulted in an increased excitatory effect on the muscle tone from the CNF and the LC. This excitatory effect was accompanied by a prominent decrease in the inhibitory effects from the PRF and the dorsal and ventral PPN (Fig. 8C). Specifically, the duration and amplitude of the inhibitory effects were reduced, while those of the excitatory effects were increased. However, the effects of stimulating each site were not observed after 150–180 min (Fig. 8D). These findings suggest that the orexinergic projection to the PPN facilitates the activities of descending excitatory systems from the CNF and the LC, and suppresses the descending inhibitory system arising from the PPN.
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Disturbances of neurotransmitter systems in narcolepsy and their regulation by orexin
Disturbances of the noradrenergic system have been repeatedly reported in human narcolepsy patients with respect to the induction of cataplexy. The reports have indicated therefore, that an enhancement of the noradrenergic system powerfully reduces cataplexy (Aldrich et al. 1994; Schwartz, 2005). Studies which have used a canine narcolepsy model have also reported that various neurotransmitter systems are affected, including the noradrenergic (Fruhstorfer et al. 1989), adrenergic (Mignot et al. 1993), dopaminergic (Nishino et al. 1991; Reid et al. 1996; Kanbayashi et al. 2000), serotonergic (Nishino et al. 1993) and cholinergic systems (Nishino et al. 1988, 1995; Reid et al. 1994a,b). In particular, an increase in the activity of the noradrenergic system ameliorated cataplexy (Fruhstorfer et al. 1989). But an activation of the cholinergic system caused the symptoms to worsen (Nishino et al. 1988, 1995; Reid et al. 1994a,b). The deficiencies in these neurotransmitter systems were observed in both the brainstem (Reid et al. 1994a,b, 1996) and forebrain structures such as the amygdala (Guilleminault et al. 1998), the basal forebrain (Nishino et al. 1988, 1995) and the basal ganglia. In human narcolepsy patients, for example, an alteration of the dopaminergic system was observed in the basal ganglia (Eisensehr et al. 2003) and the amygdala (Aldrich et al. 1993).
Orexin neurones in the perifornical hypothalamus project to various regions in the nervous system (Peyron et al. 1998). Although most of the anatomical studies were performed in rats similar orexinergic projections have been reported in cats (Zhang et al. 2002, 2004). Monoaminergic neurones are major targets of the orexinergic system. In particular, a direct orexinergic projection to the LC may be in a position to enhance arousal and modulate plasticity in higher brain centres. These effects could occur through the developing noradrenergic neurones, which play an important role in modulating arousal, a vigilance state, selective attention, and memory (Horvath et al. 1999b; Soffin et al. 2002; van den Pol et al. 2002). The orexinergic system also excites dopaminergic (Korotkova et al. 2002), serotonergic (Liu et al. 2002; Soffin et al. 2004; Takahashi et al. 2005) and cholinergic neurones (Burlet et al. 2002; Takahashi et al. 2002; Wu et al. 2004; Fadel et al. 2005). Moreover the orexinergic system exerts excitatory actions on glutamatergic (Li et al. 2002), peptidergic (Horvath et al. 1999a) and GABAergic neurones (Korotkova et al. 2002; Wu et al. 2002) in various brain regions. Orexin neurones, in turn, receive either excitatory or inhibitory effects from these neurotransmitter systems (Fu et al. 2004; Li & van den Pol, 2005; Yamanaka et al. 2003). A loss of orexin may thus lead to a massive imbalance in these systems, resulting in the dysregulation of vigilance states.
It has been shown that canine narcolepsy is caused by exon skipping mutations of the Orexin-receptor-2 gene (Lin et al. 1999; Hungs et al. 2001; Willie et al. 2003). Orexin-2 receptor mRNA has been observed in the cerebral cortex, hippocampus, medial thalamic groups, hypothalamic nuclei, and brainstem regions including the raphe nuclei, the SNr and the PPN (Marcus et al. 2001). Orexin-2 receptors could therefore act to maintain a normal level of muscle tone. Because orexin A activates both orexin-1 and orexin-2 receptors (Willie et al. 2001), in the present study the effects of an injection of orexin A could be due to activation of orexin-2 receptors in the midbrain regions.
Orexinergic modulation of REM sleep and postural muscle tone
Orexin neurones project to the LDT and the PPN (Nambu et al. 1999; Peyron et al. 1998) where both cholinergic neurones (Armstrong et al. 1983; Rye et al. 1987; Span & Grofova, 1992; Takakusaki et al. 1996) and non-cholinergic neurones, including glutamatergic, GABAergic (Ottersen & Storm-Mathisen, 1984; Kosaka et al. 1988) and peptidergic neurones (Vincent et al. 1983) are located. In the present study orexin injections into the PPN or the SNr suppressed PPN-induced REM and atonia. The effects were eliminated, however, by subsequent injections of bicuculline into each area (Figs 4 and 5). One interpretation of the above findings is that the orexin effects are mediated by local GABAergic neurones in the PPN and GABAergic projection neurones arising from the SNr. This possibility is supported by the following evidence. First, orexin injections into the rat PPN increase the release of GABA in the PPN (Koyama et al. 2004). Second, GABAergic neurones in some brain areas are excited by orexin (Korotkova et al. 2002, 2003; Wu et al. 2002). Therefore the orexin effects can be mediated by the activation of GABAergic neurones, which in turn inhibit cholinergic PPN neurones (Torterolo et al. 2002; Pal & Mallick, 2004), resulting in the suppression of REM and atonia. Alternatively, orexin could stimulate presynaptic inhibitory inputs to the cholinergic neurones in the PPN, as has been shown in the LDT (Burlet et al. 2002). It was also demonstrated that orexins increase the frequency of GABAergic mIPSCs in the neurones of the hypothalamus (van den Pol et al. 1998) and hippocampus (Wu et al. 2002). Accordingly, orexin may act on presynaptic terminals of either local GABAergic interneurones in the PPN, or GABAergic neurones arising from the SNr, to facilitate the release of GABA. Immunohistochemical studies would be necessary to identify the orexinergic projections to the GABAergic neurones.
Several types of cholinergic neurones which are related to the sleep–awake cycle are located in the PPN/LDT. These neurones include those that are active during waking and REM sleep (W/REM-on neurones), and those that are specifically active during REM sleep (REM-on neurones). Desynchronization of the EEG and the regulation of wakefulness via ascending projections to the thalamus or cortex could be properties of W/REM-on neurones. A direct activation by orexin of W/REM-on neurones may therefore induce and maintain wakefulness. In contrast, REM-on neurones are thought to induce EEG desynchronization via ascending projections to forebrain structures, and muscular atonia during REM sleep via a descending projection to the PRF. We have demonstrated that non-cholinergic REM-on neurones in the PRF, which are excited by a cholinergic agonist, project to the medulla (Sakai & Koyama, 1996). An activation of cholinergic PPN neurones may thus excite the REM-on neurones in the PRF to suppress muscle tone via the pontomedullary reticulospinal tract (Takakusaki et al. 1994, 2001, 2003b). Because presumably the cholinergic REM-on neurones in the mesopontine tegmentum were excited by bicuculline, REM-on neurones in the PPN/LDT could be inhibited through GABAA receptors during waking (Sakai & Koyama, 1996). Ulloor et al. (2004) demonstrated that GABAB receptors on PPN cholinergic neurones were also involved in the regulation of REM sleep. It is therefore highly probable that, when orexin excites GABAergic neurones in the PPN/LDT, REM-on neurones are more selectively inhibited by GABA than W/REM-on neurones. This would result in suppression of REM sleep and muscular atonia. It has been reported by Xi et al. (2001) that an injection of orexin into the LDT facilitated wakefulness and suppressed REM sleep. The former effect may be attributed to a direct excitatory effect of orexin on the cholinergic neurones (Burlet et al. 2002; Takahashi et al. 2002), while the latter may be mediated through orexin-induced activity of GABAergic neurones.
Orexin injections into the PPN not only suppressed inhibitory effects from the PPN and the PRF but also enhanced excitatory effects from the MLR and the LC (Fig. 8). Descending monoaminergic systems, such as the coerulospinal and the raphespinal tracts, are muscle tone excitatory systems (Fung & Barnes, 1981; Sakai et al. 2000). There are also direct noradrenergic (Semba & Fibiger, 1992) and serotonergic projections (Honda & Semba, 1994) to the PPN/LDT and to the medial PRF (Semba, 1993). The noradrenergic projection inhibits the mesopontine cholinergic neurones (Koyama & Kayama, 1993; Leonald & Llinás, 1994). The serotonergic projection reduces the activity of the inhibitory system arising from the medial PRF (Takakusaki et al. 1993, 1994). Lai et al. (2001) have reported that there was a reduced release of noradrenaline and serotonin in the spinal cord during muscular atonia which was induced by electrical or chemical stimulation applied to the medial PRF. They indicated that the activity of the coerulospinal and raphespinal tracts was inhibited by projections from the medial PRF to the LC and the raphe nuclei. Consequently, there are interconnections between the excitatory and inhibitory systems. The orexinergic projections to the midbrain therefore may control the level of muscle tone by counterbalancing these systems (see Takakusaki et al. 2004b).
Orexinergic control of locomotor behaviour
The MLR excites a spinal stepping generator to evoke locomotion via the medullary reticulospinal tract (Rossignol, 1996; Grillner, 2003). Signals from the MLR may also activate monoaminergic excitatory systems (see Mori, 1987). The lateral hypothalamus is known to be involved in the control of locomotion, especially of appetitive locomotion, while the medial hypothalamus probably controls defensive behaviour associated with darting locomotion (Sinnamon, 1993; Grillner et al. 1997). This emotional locomotor behaviour could be evoked through the projections from the hypothalamus to the midbrain, including the MLR and the medullary reticular formation (MRF) (Grillner et al. 1997). Torterolo et al. (2003) reported that orexinergic neurones expressed FOS only when somatomotor activity was present. The release of orexin in the lateral hypothalamus was higher during wakefulness than during non-REM sleep (Kiyashchenko et al. 2002). It was shown by Matsuzaki et al. (2002) that a central administration of orexins in rats significantly increased locomotor activity and induced changes in behaviour. Because an orexin injection into the MLR induced or facilitated locomotion (Fig. 3), an orexinergic projection to the MLR may be crucial for maintenance of the background excitability of the locomotor system. It follows that orexinergic projections to the midbrain cholinergic system, in addition to those to the dopaminergic and serotonergic systems, play a crucial role in the expression of emotional locomotor behaviour (Matsuzaki et al. 2002). It has been reported that during the initial 10–20 min in a novel environment orexin knockout mice displayed a smaller increase in locomotor activity than wild-type mice even though their wakefulness was normal (Mochizuki et al. 2004). The mesopontine tegmentum integrates the limbic and motor output systems, and concomitant sympathetic adjustments are likely to occur during complex behavioural changes (Smith & DeVito, 1984; Inglis & Winn, 1995; Winn et al. 1997). Krout et al. (2003) have shown that a considerable number of single orexinergic neurones in the lateral hypothalamus, and single cholinergic neurones in the PPN, directly or indirectly project to both the primary motor cortex and the stellate ganglion. This suggests that orexinergic and cholinergic neurones may integrate somatomotor and autonomic functions, and affect different types of behaviour, such as arousal and sleep, and/or locomotion. All of these results suggest that an orexinergic system may separately control arousal systems and locomotor systems, and may link emotional stimuli to eliciting motivated locomotor behaviour.
Role of orexinergic projections to the midbrain in the pathogenesis of narcolepsy
Our interest was how the orexinergic projections to the midbrain contribute to the pathogenesis of narcolepsy. Muscular atonia has been induced by injections of orexins into the medial PRF (Kiyashchenko et al. 2001; Xi et al. 2002, 2003) and the MRF (Mileykovskiy et al. 2002). Under some circumstances orexinergic projections to the above regions may induce muscular atonia. In contrast, the present study revealed that orexinergic projections to the midbrain inhibited REM and atonia. The inhibition could be due to postsynaptic and/or presynaptic effects upon the soma and terminals of GABAergic neurones in the PPN and the SNr which facilitate the release of GABA, as discussed above. A sustained release of GABA cannot be maintained in the absence of orexin effects. These effects may increase the background excitability of systems generating REM and muscular atonia, and predispose affected individuals to attacks of cataplexy in narcolepsy. Therefore a higher sensitivity to orexin of the GABAergic neurones in the PPN and the SNr than of the REM sleep-related cholinergic neurones may underlie the pathogenesis of cataplexy. In this manner, cataplexy could occur by a disinhibition of the REM sleep generating system. In addition, the orexinergic projection may increase the level of muscle tone and facilitate locomotor behaviour (Fig. 9A).
|
However, the above model may not be consistent with previous results. For example, according to this model, in which there is an absence of forebrain structures, a lack of orexin input may result in an increase in the background excitability of the REM sleep generating system. However, an increase in REM sleep has not been consistently observed in human narcolepsy (Aldrich, 1992), or in canine (Mitler & Dement, 1977) and mice (Mochizuki et al. 2004) narcoleptic models. Furthermore, a canine study has demonstrated that the cyclicity of the normal REM sleep interval is not disturbed in the affected animals (Nishino & Mignot, 1997). Such an inconsistency could be derived from the lack of a contribution by forebrain structures in this model, because orexins modulate various neurotransmitter systems in forebrain structures and the brainstem (Selbach et al. 2004). These neurotransmitter systems are altered during a narcoleptic state, as described above.
It is therefore critical to consider how forebrain structures contribute to the pathogenesis in narcolepsy. In fact, activity of cerebral cortex was altered in human narcolepsy patients (Oliviero et al. 2005). Mesopontine tegmentum receives volitional signals from the cerebral cortex and emotional signals from limbic structures such as the hippocampus and the amygdala (see Takakusaki et al. 2004b). Because the basal ganglia receive afferents from these two structures, the mesopontine tegmentum may play key roles in initiation, integration, selection, or switching of volitionally guided and emotionally triggered motor behaviour (Grillner et al. 1997; Jordan, 1998; Takakusaki et al. 2004b). We propose that descending and ascending systems from the brainstem may mediate changes of activity in the cerebral cortex, basal ganglia, limbic structures, and the brainstem, which would result in the generation of narcoleptic symptoms.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Aldrich MS (1992). Narcolepsy. Neurol 42, 34–43.[Medline]
Aldrich MS, Hollingsworth Z & Penney JB (1993). Autoradiographic studies of post-mortem human narcoleptic brain. Neurophysiol Clin 23, 35–45.[CrossRef][Medline]
Aldrich MS, Prokopowicz G, Ockert K, Hollingsworth Z, Penney JB & Albin RL (1994). Neurochemical studies of human narcolepsy: alpha-adrenergic receptor autoradiography of human narcoleptic brain and brainstem. Sleep 17, 598–608.[Medline]
Armstrong DA, Saper CB, Levey AI, Winer BH & Terry RD (1983). Distribution of cholinergic neurons in the rat brain demonstrated by the immunohistochemical localization of choline acethyltransferase. J Comp Neurol 216, 53–68.[CrossRef][Medline]
Berman A (1968). The Brain Stem of the Cat: a Cytoarchitectonic Atlas with Stereotaxic Coordinates. University of Wisconsin Press, Madison.
Burlet S, Tyler CJ & Leonard CS (2002). Direct and indirect excitation of laterodorsal tegmental neurons by hypocretin/orexin peptides: Implication for wakefulness and narcolepsy. J Neurosci 22, 2862–2872.
Chemelli RM, Willie JT, Sinton CM, Elmquist JK, Scammell T, Lee C, Richardson JA, Williams SC, Xiong Y, Kisanuki Y, Fitch TE, Nakazato M, Hammer RE, Saper CB & Yanagisawa M (1999). Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation. Cell 98, 437–451.[CrossRef][Medline]
Datta S & Hobson JA (1994). Neuronal activity in the caudolateral peribrachial pons: relationship to PGO waves and rapid eye movements. J Neurophysiol 71, 95–109.
Datta S & Siwek DF (1997). Excitation of the brain stem pedunculopontine tegmentum cholinergic cells induces wakefulness and REM sleep. J Neurophysiol 77, 2975–2988.
Derryberry D & Tucker DM (1992). Neural mechanisms of emotion. J Consult Clin Psychol 60, 329–338.[CrossRef][Medline]
Eisensehr I, Linke R, Tatsch K, von Lindeiner H, Kharraz B, Gildehaus FJ, Eberle R, Pollmacher T, Schuld A & Noachtar S (2003). Alteration of the striatal dopaminergic system in human narcolepsy. Neurology 60, 1817–1819.
Estabrooke IV, McCarthy MT, Ko E, Chou TC, Chemelli RM, Yanagisawa M, Saper CB & Scammell TE (2001). Fos expression in orexin neurons varies with behavioral state. J Neurosci 21, 1656–1662.
Fadel J, Pasumarthi R & Reznikov LR (2005). Stimulation of cortical acetylcholine release by orexin A. Neuroscience 130, 541–547.[CrossRef][Medline]
Ford B, Holmes CJ, Mainville L & Jones BE (1995). GABAergic neurons in the rat pontomesencephalic tegmentum: codistribution with cholinergic and other tegmental neurons projecting to the posterior lateral hypothalamus. J Comp Neurol 363, 177–196.[CrossRef][Medline]
Fruhstorfer B, Mignot E, Bowersox S, Nishino S, Dement WC & Guilleminault C (1989). Canine narcolepsy is associated with an elevated number of alpha 2-receptors in the locus coeruleus. Brain Res 500, 209–214.[CrossRef][Medline]
Fu LY, Acuna-Goycolea C & van den Pol AN (2004). Neuropeptide Y inhibits hypocretin/orexin neurons by multiple presynaptic and postsynaptic mechanisms: tonic depression of the hypothalamic arousal system. J Neurosci 24, 8741–8751.
Fung SJ & Barnes CD (1981). Evidence of facilitatory coerulospinal action in lumbar motoneurons of cats. Brain Res 216, 299–311.[CrossRef][Medline]
Garcia-Rill E, Homma Y & Skinner RD (2004). Arousal mechanisms related to posture and locomotion: 1. Descending modulation. Prog Brain Res 143, 283–290.[Medline]
Grillner S (2003). The motor infrastructure: from ion channels to neuronal networks. Nat Rev Neurosci 4, 573–586.[Medline]
Grillner S, Georgopoulos AP & Jordan LM (1997). Selection and initiation of motor behavior. In Neurons, Networks, and Motor Behavior, ed. Stein PSG, Grillner S, Selverston AI & Stuart DG, pp. 3–19. MIT Press, Cambridge, MA, USA.
Grofova I & Zhou M (1998). Nigral innervation of cholinergic and glutamatergic cells in the rat mesopontine tegmentum. Light and electron microscopic anterograde tracing and immunohistochemical studies. J Comp Neurol 395, 359–379.[CrossRef][Medline]
Guilleminault C, Heinzer R, Mignot E & Black J (1998). Investigations into the neurologic basis of narcolepsy. Neurology 50, S8–S15.
Habaguchi T, Takakusaki K, Saitoh K, Sugimoto J & Sakamoto T (2002). Medullary reticulospinal tract mediating the generalized motor inhibition in cats. II. Functional organization within the medullary reticular formation with respect to postsynaptic inhibition of forelimb and hindlimb motoneurons. Neurosci 113, 65–77.[CrossRef][Medline]
Honda T & Semba K (1994). Serotoergic synaptic input to cholinergic neurons in the rat mesopontine tegmentum. Brain Res 647, 299–306.[CrossRef][Medline]
Horvath TL, Diano S & van den Pol AN (1999a). Synaptic interaction between hypocretin (orexin) and neuropeptide Y cells in the rodent and primate hypothalamus: a novel circuit implicated in metabolic and endocrine regulations. J Neurosci 19, 1072–1087.
Horvath TL, Peyron C, Diano S, Ivanov A, Aston-Jones G, Kilduff TS & van Den Pol AN (1999b). Hypocretin (orexin) activation and synaptic innervation of the locus coeruleus noradrenergic system. J Comp Neurol 415, 145–159.[CrossRef][Medline]
Hungs M, Fan J, Lin L, Lin X, Maki RA & Mignot E (2001). Identification and functional analysis of mutations in the hypocretin (orexin) genes of narcoleptic canines. Genome Res 11, 531–539.
Inglis WL & Winn P (1995). The pedunculopontine tegmental nucleus: where the striatum meets the reticular formation. Prog Neurobiol 47, 1–29.[CrossRef][Medline]
Jordan LM (1998). Initiation of locomotion in mammals. Ann N Y Acad Sci 860, 83–93.
Kanbayashi T, Honda K, Kodama T, Mignot E & Nishino S (2000). Implication of dopaminergic mechanisms in the wake-promoting effects of amphetamine: a study of D- and L-derivatives in canine narcolepsy. Neuroscience 99, 651–659.[CrossRef][Medline]
Kiyashchenko LI, Mileykovskiy BY, Lai YY & Siegel JM (2001). Increased and decreased muscle tone with orexin (hypocretin) microinjections in the locus coeruleus and pontine inhibitory area. J Neurophysiol 85, 2008–2016.
Kiyashchenko LI, Mileykovskiy BY, Maidment N, Lam HA, Wu MF, John J, Peever J & Siegel JM (2002). Release of hypocretin (orexin) during waking and sleep states. J Neurosci 22, 5282–5286.
Korotkova TM, Eriksson KS, Haas HL & Brown RE (2002). Selective excitation of GABAergic neurons in the substantia nigra of the rat by orexin/hypocretin in vitro. Regul Pept 104, 83–89.[CrossRef][Medline]
Korotkova TM, Sergeeva OA, Eriksson KS, Haas HL & Brown RE (2003). Excitation of ventral tegmental area dopaminergic and nondopaminergic neurons by orexins/hypocretins. J Neurosci 23, 7–11.
Kosaka T, Tauchi M & Dahl J (1988). Cholinergic neurons containing GABA-like and/or glutamic acid decarboxylase-like immunoreactivities in various brain regions of the rat. Expl Brain Res 70, 605–617.[Medline]
Koyama Y & Kayama Y (1993). Mutual interactions among cholinergic, noradrenergic and serotonergic neurons studied by iontophoresis of these transmitters in rat brainstem nuclei. Neurosci 55, 1117–1126.[CrossRef][Medline]
Koyama Y & Sakai S (2000). Modulation of presumed cholinergic mesopontine tegmental neurons by acetylcholine and monoamines applied iontophoretically in unanesthetized cats. Neurosci 96, 723–733.[CrossRef][Medline]
Koyama Y, Takahashi K, Kodama T, Honda Y, Takakusaki K & Kayama Y (2004). Orexinergic inhibition on the mesopontine cholinergic neurons mediated through GABAergic neurons. Soc Neurosci Abs 318.3.
Koyama Y, Takahashi K, Kodama T & Kayama Y (2003). State-dependent activity of neurons in the perifornical hypothalamic area during sleep and waking. Neurosci 119, 1209–1219.[CrossRef][Medline]
Krout KE, Mettenleiter TC & Loewy AD (2003). Single CNS neurons link both central motor and cardiosympathetic systems: a double-virus tracing study. Neurosci 118, 853–866.[CrossRef][Medline]
Lai YY, Clements JR & Siegel JM (1993). Glutamatergic and cholinergic projections to the pontine inhibitory area identified with horseradish peroxidase retrograde transport and immunohistochemistry. J Comp Neurol 336, 321–330.[CrossRef][Medline]
Lai YY, Kodama T & Siegel JM (2001). Changes in monoamine release in the ventral horn and hypoglossal nucleus linked to pontine inhibition of muscle tone: an in vivo microdialysis study. J Neurosci 21, 7384–7391.
Lee MG, Hassani OK & Jones BE (2005). Discharge of identified orexin/hypocretin neurons across the sleep-waking cycle. J Neurosci 25, 6716–6720.
), and REM and atonia (•), are indicated on a parasagittal plane (L 4.0). b, locomotion on the moving treadmill elicited by stimulation (40 µA) of the CNF. c, REM and atonia induced by stimulation (30 µA) of the PPN. In b and c the upper recording is an EOG, the middle and lower recordings are EMGs from the left (L) and right (R) soleus (Sol) muscles. The stimulus period is indicated by lines under each recording. B, optimal stimulus sites for evoking locomotion (
, 100 µM; 
, 200 µM; and •, 500 µM. The threshold currents for evoking locomotion before (mean ± standard deviation = 31.2 ± 7.5 µA, median = 30 µA) and after (mean ± standard deviation = 12.5 ± 7.5 µA, median = 15 µA) the orexin injections were significantly different (P = 0.005). Abbreviation: PRF, pontine reticular formation.
, 1000 µM. The stimulus strength which was required to elicit muscular atonia was compared in 7 trials with 4 cats. The threshold currents for eliciting muscular atonia before (23.8 ± 4.9 µA, 25 µA) and after (39.3 ± 4.5 µA, 40 µA) the orexin injections were significantly different (P = 0.011). C, the effects of bicuculline injection into the PPN. a, muscular atonia induced by PPN stimulation (30 µA and 50 Hz). b, 30 min after an orexin injection (1000 µM, 0.25 µl) into the SNr, the PPN-induced muscular atonia was not observed. c, a subsequent bicuculline injection (1 mM, 0.25 µl) into the PPN, 30 min after the orexin injection, reversed the PPN effect. A line below each recording indicates the period of PPN stimulation.
) and the SNr (