Dorsal raphe stimulation produces inhibitory effect on trigeminal nucleus neurons

Inhibitory effects of conditioning stimulation of the dorsal raphe nucleus (DR) on the neuron activity in the rostral part of spinal trigeminal nucleus (STN) were studied in cats for the purpose of comparison with the inhibition induced by locus coeruleus (LC) stimulation. DR conditioning stimulation reduced the orthodromic field potential in STN elicited by inferior alveolar nerve stimulation, and enhanced the antidromic field potential in the trigeminal nerve evoked by STN stimulation; but the inhibitory effects of DR stimulation were considerably weaker than those of LC stimulation. In tracking experiments near the raphe nucleus, conditioning stimulation of DR itself produced the most pronounced decrease in the STN field potential. Orthodromic spike number of STN relay neurons was significantly reduced by DR conditioning stimulation; however, the threshold for the conditioning stimulus to the DR was much higher than that to the LC. Antidromic spike generation of the STN neurons was unaltered by conditioning stimulation of both DR and LC. DR stimulation elicited a field potential in STN, which followed high frequency stimuli up to 200 HZ. A single fiber action potential was also obtained in STN by DR stimulation. STN stimulation produced a field potential in DR, which followed high frequency stimuli. It is suggested from these findings that conditioning stimulation of DR produces a direct inhibition of transmission in STN neurons; however, this stimulation has less effect on these neurons than does stimulation of the LC.