Dorsal raphe and external electrical stimulation modulate noxious input to single neurons in nucleus parafascicularis...

Spontaneous discharges and nociceptive responses of 47 parafascicularis thalami (PF) neurons were recorded extracellularly and comparisons were made between the effects of these discharges following focal dorsal raphe stimulation (DRS) and bilateral pinnal electrical stimulation (PES). Eighty-three percent of PF neurons (N = 39) responded to noxious stimulus, about 69% of the PF responsive cells (N = 27) were excited during noxious stimuli and thus categorized as "nociceptive-on" cells. The remaining 31% (N = 12) were suppressed by the noxious stimuli, and were categorized as "nociceptive-off" cells. DRS and PES attenuated the spontaneous activity of the "nociceptive-on" neurons as well as the noxious input to these cells, while the spontaneous activity of the "nociceptive-off" cells was suppressed only following DRS and not following PES. Moreover, PES displayed disinhibiting properties, namely, it reduced the suppression effects elicited by noxious input. In conclusion, it was demonstrated that both focal DRS and noninvasive PES were effective in modulating pain input to single neurons in the PF.