Inhibitory pathway from the frontal cortex to the hypothalamic ventromedial nucleus in the rat

Stimulation of the dorsal frontal cortex (area 10, FCtx) evoked two negative waves with short latencies (N1 and N2) followed by a large, longlasting positive wave (P) in the ventromedial nucleus (VMH) as well as in the more dorsal structures in the hypothalamus and the ventral thalamus. Since the N1 followed high frequency stimulation, it was concluded to be due to antidromic activity. Double or triple pulse stimulation summated the P, and reduced the amplitude of the negative waves of the second and the third responses. VMH neuronal discharges were also decreased during the time course of the P. Therefore, the P was concluded to be composed of the IPSPs. VMH neuronal discharges were frequently superimposed on the N2, indicating the origin of this wave to be EPSPs. Strychnine reduced or blocked the inhibition of the VMH neurons caused by not only FCtx stimulation but also glycine application. The results indicate that the VMH receives both excitatory and inhibitory projections from the FCtx, and the inhibition may be transmitted by glycine.