Reciprocal connections between the lateral hypothalamus and the frontal complex in the rat: electrophysiological and anatomical observations.

Inputs to rat lateral hypothalamus (LHA) from prefrontal cortex (FC), and vice versa, were studied by intracellular recording, and by retrograde horseradish peroxidase (HRP) method. Stimulation of the FC evoked 3 types of responses: a polysynaptic EPSP-IPSP sequence, IPSPs alone, or antidromic response in LHA neurons. Forty-five per cent of IPSPs were considered to be monosynaptic since the latencies were constant when stimulus intensity was changed. The neurons labeled in the FC following electrophoretic injections of HRP into LHA were located in the medial and sulcal FC. In these cortical areas, not only pyramidal neurons in layer V, but also non-pyramidal neurons in layer VI were labeled. Stimulation of the LHA evoked an EPSP-IPSP sequence, or antidromic response in FC neurons. Some of the fast EPSPs were considered to be monosynaptic. The neurons labeled in the LHA following HRP injection into the FC were either relatively large spherical neurons or small ovoid-shaped neurons. These were distributed diffusely throughout the LHA.