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1 Spinal Cord Research Centre, Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada R3E 3J7
2 INSERM E358, Institut F. Magendie, University of Bordeaux II, 33077 Bordeaux cedex, France
3 CNRS UMR 5543, University of Bordeaux II, 33077 Bordeaux cedex, France
Whole-cell patch recordings were made from parasympathetic preganglionic neurones (P-PGNs) and unidentified intermediolateral (IML) neurones in thick slices of the lower lumbar and sacral spinal cord of 14- to 21-day-old rats. The P-PGNs and IML neurones examined were similar in terms of soma sizes, input resistance and capacitance, and displayed a sag conductance as well as rebound firing. In the absence of drugs, the neurones responded with either tonic or adapting firing to depolarizing current steps. However, in the presence of the group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), almost half of the neurones displayed accelerating firing rates during the constant current injection, followed by a sustained after-discharge. In the presence of TTX, plateau potentials were observed. The firing changes and plateaux were blocked by nifedipine, an L-type Ca2+ channel blocker, and (S)-(–)-Bay K8644 was able to produce these firing changes and plateaux in the absence of DHPG, demonstrating the involvement of an L-type Ca2+ conductance. Ca2+-activated nonspecific cationic conductances also appear to contribute to the firing changes. A few neurones displayed membrane oscillations and burst firing in the presence of DHPG. The results suggest that the firing characteristics of both P-PGNs and other neurones likely to be involved in caudal spinal reflex control are not static but, rather, quite dynamic and under metabotropic glutamate receptor modulatory control. Such changes in firing patterns may be involved in normal pelvic parasympathetic reflex function during micturition, defaecation and sexual reflexes, and may contribute to the abnormal output patterns seen with loss of descending brainstem input and visceral or perineal sensory disturbances.
(Received 5 October 2004;
accepted after revision 22 December 2004;
first published online 23 December 2004)
Corresponding author S. Shefchyk: Department of Physiology, University of Manitoba, 730 William Avenue, Winnipeg, MB, Canada R3E 3J7. Email: sjs{at}scrc.umanitoba.ca
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