Mesencephalic reticular formation stimulation effects on hypothalamic neuronal activity.

The effects of mesencephalic reticular formation (RF) single pulse, 0.5 msec and 0-500 microA, stimulation on lateral preoptic-lateral hypothalamic (LPA-LH) neuronal activity were determined in anesthetized rats. In addition, the effects of LH stimulation on neural activity in the RF and periaqueductal gray (PAG) were evaluated. Recordings from 117 neurons indicate reciprocal connections between the LPA-LH and the mesencephalon. Stimulation of the RF affected 70% of the LPA-LH neurons tested. Short latency decreases in activity predominated indicating an inhibitory synaptic input from the RF to the LPA-LH. Short latency increases in discharge frequency were observed infrequently. Stimulation of the LH affected only 32% of the mesencephalic neurons tested. Short latency decreases in activity were usually observed indicating reciprocal inhibitory synaptic connections between the LPA-LH and the RF and periaqueductal gray. Antidromic responses verified these interconnections and revealed relatively slow conduction velocities of approximately 1.0 m/sec. Results are discussed in terms of the involvement of the LPA-LH and RF in sensorimotor functions, spinal motor excitability, and ingestive behavior.