Mechanisms mediating the sympathetic silent period: studies in the isolated spinal cord of the neonatal rat.

The properties of the sympathetic silent period were examined using the isolated spinal cord of the neonatal rat. In all spontaneously active sympathetic preganglionic neurons (PGN) examined, repetitive stimulation of the ventral root gave rise to a prolonged inhibition (up to 30 s) of spontaneous activity. The silent period could also be demonstrated in most 'silent' PGN using a condition-test paradigm. The characteristics of the silent period in this preparation resembled those previously reported in the adult cat. Using several paradigms, we were able to show a dissociation between the duration of the silent period and the recent firing history of the PGN. This indicates that the silent period is not due solely to recovery processes intrinsic to the PGN. By stimulating adjacent ventral roots or rostral and caudal bundles of the same ventral root, we found that the system producing the silent period was confined to a restricted anatomical locus. Activation of the silent period was capable of inhibiting glutamate-evoked activity, implying that the inhibition impinges directly on the PGN. We found no evidence of a cholinergic mediation of the silent period. However, experiments with naloxone indicated that opiate mechanisms play an unsuspected role in modulating the silent period. We conclude that at least some component of the silent period is mediated by a synaptic mechanism extrinsic to the PGN.