The parafasciculus thalami as a site for mediating the antinociceptive response to GABAergic drugs

Electrophysiological (single cell) experiments were undertaken to examine whether neurons in the rat parafasciculus thalami (PF) are involved in mediating the antinociceptive response to GABAergic drugs. The results indicated that: noxious stimuli excite most PF neurons; microiontophoretic application of morphine, GABA or the GABA agonist, THIP, attenuated the spontaneous firing rate of PF neurons; morphine, THIP and GABA reduced the neuronal excitation induced by noxious stimuli; application of the GABA receptor antagonist, bicuculline, prevented the effects of THIP and GABA on PF activity; while naloxone blocked the response to morphine on PF neurons, it failed to influence the actions of GABA and THIP; and the injection of THIP or GABA into the PF produced an antinociceptive response as assessed by the rat tail-immersion assay, whereas pentobarbital was inactive. The findings suggest that GABA receptors located in the PF may mediate the antinociceptive response to GABAergic drugs, and that the action of these agents is unrelated to opiate receptors.