Excitation and inhibition of neurons in the trigeminal nucleus caudalis following periaqueductal gray stimulation.

Electrical stimulation (3-4 shocks, 300 Hz, 30-150 microamperemeter) of the periaqueductal gray matter (CG) or dorsal raphé nucleus (DR) of decerebrate cats reduced or abolished the jaw-opening reflex response evoked by stimulation of either the tooth pulp or infraorbital nerve. In addition, CG or DR stimulation inhibited the response of 12 out of 16 trigeminal nucleus caudalis neurons to activation of their sensory afferent inputs. Ten other neurons recorded in the same sites, and often at the same time, but which did not respond to the sensory inputs utilized, were excited by identical stimuli to CG or DR. This excitatory response was blocked by intravenously administered naloxone (0.1-0.2 mg/kg). It is suggested that those neurons which are excited by CG and DR stimulation may be interneurons involved in pre- and post-synaptic inhibition of sensory transmission during stimulus-produced or narcotic analgesia.