Electrophysiological analysis of inhibitory synaptic mechanisms in the preoptic area of the rat.

1. Extracellular recordings were made from single neurones in the medial preoptic-anterior hypothalamus of rats anaesthetized with urethane or pentobarbitone 2. Stimulation of the arcuate--ventromedial area evoked inhibition in 76% of neurones; inhibition occurred as the initial response in 26% of neurones, followed orthodromic excitation in 26% and antidromic excitation in 24%. Stimulation of the periaqueductal grey evoked inhibition as the initial response in 63% of neurones. 3. Stimulation of both the arcuate--ventromedial area and the periaqueductal grey generated synaptically evoked high-frequency discharges in a few neurones. 4. Short ionophoretic pulses of beta-alanine, gamma-aminobutyric acid (GABA), glycine and taurine inhibited the discharge of most preoptic-anterior hypothalamic neurones. Bicuculline methochloride and picrotoxin antagonized both GABA responses and synaptic inhibition. Strychnine antagonized beta-alanine, glycine and taurine responses without altering synaptic inhibition. 5. Reduction of hypothalamic serotonin (5-HT) levels to about 40% of control values by micro-injection of 5,7-dihydroxytryptamine into the ventral tegmental area, or parenteral injection of parachlorophenylalanine, did not alter the profile of responses evoked by either arcuate-ventromedial or periaqueductal grey stimulation. Micro-injection of tetanus toxin (100-200 MLD) into the preoptic-anterior hypothalamus significantly reduced the frequency of occurrence of inhibition evoked from both sites. 6. Pentobarbitone administered acutely or as the sole anaesthetic increased both the duration and frequency of occurrence of inhibitory responses evoked from the arcuate-ventromedial nuclei. 7. It is suggested that a GABA-linked inhibitory synaptic mechanism, activated by arcuate-ventromedial and periaqueductal grey stimulation, operates in the preoptic-anterior hypothalamus. Such convergent inhibition may be a result of activity in local interneurone circuitry.