Electrically silent neurons in the substantia gelatinosa of the rat spinal cord.

Substantia gelatinosa (SG) neurons are usually categorized on three main types: tonic, adapting and delayed firing (DFNs), based on characteristic firing response evoked by sustained stimulation. Here, the existence of electrically silent neurons (ESNs, 9.3%) is reported by using patch-clamp recording and confocal microscopy in spinal cord slices from 3-5 weeks-old rats. Those neurons does not generate spikes at their resting membrane potential (approximately -69 mV) but only at preliminary depolarization to > -60 mV In the latter case, spikes appeared starting from the end of stimulation, which is characteristic feature of DFNs. With the exception of APs block, all other passive and active electrophysiological properties of ESNs and DFNs were similar. Their main morphological properties (vertical orientation of dendritic tree and axonal trajectory) were close too. A distinctive feature of ESNs was larger amplitude of outward A-type K+ current (K(A)). The results suggest that the latter might cause a block of APs in ESNs, while these cells likely are a functional (i.e., non-firing) subtype of DFNs. The role of DFNs in descending control of pain transmission via modulation of its K(A) is hypothesized.