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Received October 5, 2004
Revised November 1, 2004
Accepted after revision December 22, 2004
1 University of Manitoba
2 CNRS UMR 5543 University of Bordeaux
3 INSERM E358 Institut G Magendie, University of Bordeaux
* To whom correspondence should be addressed. E-mail: sjs{at}scrc.umanitoba.ca.
Whole cell patch recordings were made from parasympathetic preganglionic neurons (P-PGNs) and unidentified intermediolateral neurons in thick slices of the lower lumbar and sacral spinal cord of 14-21 day old rats. The P-PGNs and intermediolateral neurons examined were similar in terms of soma sizes, input resistance and capacitance and displayed a sag rectification as well as rebound firing. In the absence of drugs, the neurons responded with either tonic or adapting firing to depolarizing current steps. However, in the presence of the group I metabotroptic glutamate receptor agonist, DHPG, almost half of the neurons 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 calcium channel blocker, and s-BAYK 8644 could produce these firing changes and plateaux in the absence of DHPG demonstrating the involvement of an L-type calcium condutance. Calcium activated non-specific cationic conductances also appear to contribute to the firing changes. A few neurons displayed membrane oscillations and burst firing in the presence of DHPG. The results suggest that the firing characteristics of both P-PGNs and other neurons likely involved in caudal spinal reflex control are not static but rather, quite dynamic and under metabotropic glutamate receptors modulatory control. Such changes in firing patterns may be involved in normal pelvic parasympathetic reflex function during micturition, defecation and sexual reflexes and may contribute to the abnormal output patterns seen with loss of descending brainstem input and visceral or perineal sensory disturbances.
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