Dorsal raphe stimulation reduces responses of parafascicular neurons to noxious stimulation

Spontaneous activity, responses to noxious stimulation and response to electrical stimulation of the dorsal raphe were recorded extracellularly from single units in the parafascicularis (PF) nucleus in the rat. Three types of spontaneous activity were found: 'slow' firing units (1-10 spikes/sec), 'bursting' units (bursts of 2-5 spikes/10-20 msec, bursts repeat every 1-2 sec), and 'fast' firing units (15-40 spikes/sec). Noxious stimulation increased the firing rate of 63% of the slow cells and 87% of the bursting cells, while the fast firing units did not respond. Dorsal raphe (DR) stimulation decreased the firing rates of both the slow and bursting PF cells only. The degree of suppression of PF units was directly related to the frequency and intensity of the DR stimulation. When the noxious stimulus was combined with DR stimulation, DR stimulation inhibited the increase in firing rate caused by the noxious stimulus. The firing rate during combined DR stimulation and noxious stimulation averaged 51% of that during the noxious stimulation alone. In several units, DR stimulation had no effect on spontaneous activity when applied alone but did decrease the effects of noxious stimulation when applied simultaneously. The results indicate that in addition to other possible mechanisms, the DR may affect responses to noxious stimuli via an ascending modulation pathway to the parafascicularis nucleus in the thalamus.