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Neuroeffector transmission in arterioles of the guinea-pig choroid

H. Hashitani, A. Windle * and H. Suzuki

1. Using conventional microelectrode techniques, membrane potentials were recorded from smooth muscle cells of guinea-pig choroidal arterioles.

2. Transmural stimulation initiated excitatory junction potentials (EJPs) which were abolished by either guanethidine or ,-methylene-ATP but not by phentolamine, indicating that they resulted from activation of purinoceptors.

3. Trains of stimuli evoked EJPs which were followed by a slow depolarization, an inhibitory junction potential (IJP) or a biphasic membrane potential change which consisted of an IJP and a subsequent slow depolarization.

4. Slow depolarizations were abolished by either phentolamine or guanethidine, indicating that they resulted from activation of -adrenoceptors.

5. IJPs were abolished by atropine but not by guanethidine, and were reduced by 50 % by apamin with the residual response being abolished by charybdotoxin, indicating that they resulted from the activation of muscarinic receptors which open two sets of Ca²⁺-activated K⁺ channels.

6. Most responses were followed by slow hyperpolarizations. These were almost abolished by L-nitroarginine, an effect which was partly overcome by L-arginine, and were abolished by glibenclamide, indicating that they resulted from the release of NO and activation of ATP-sensitive K⁺ channels.

7. Immunohistochemical analysis showed that arterioles were densely innervated by adrenergic nerve fibres. A population of fibres, likely to be cholinergic, was also identified. Furthermore, populations of nerve fibres immunoreactive to antibodies against either nitric oxide synthase (NOS) or substance P (SP) were also identified.

8. These findings indicate that choroidal arterioles of the guinea-pig are innervated by at least three different populations of nerves, adrenergic nerves which evoke excitatory responses, cholinergic nerves which evoke inhibitory responses and a population of nerves which cause the release of NO.

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