Response characteristics of lateral hypothalamic neurons to odors in unanesthetized rabbits.

Extracellular responses of 103 neurons to eight odors were studied in the lateral hypothalamic area (LHA) of unanesthetized rabbits. In response to electrical stimulation of the olfactory bulb (OB), 97 units were excited with latencies of 3.7-68.0 ms (mean, 19.4 ms) and the activity of 6 units was suppressed. In the LHA, 90 units of 103 were found that responded to from one to eight odors. The number of units that responded to three odors was the most numerous (22%) among these odor-sensitive units. Lateral hypothalamic (LH) neurons showed high response probabilities to cineole (0.72), cyclotene (0.68), and isovaleric acid (0.54). Of the recorded units, 94% showed an excitatory-type response to both electrical stimulation of the OB and odor stimuli of the olfactory mucosa, while only 6% showed an inhibitory-type response to both stimuli. These results suggest that the olfactory effect on the LHA is predominantly excitatory. Responses to a concentration series of odors were examined in 24 LH neurons. In most cases, response intensity did not increase monotonically with increasing odor concentration. Instead, the concentration-response curves of most units showed a peak or inflections during the concentration series. Compared with results obtained from other olfactory structures in different species, it appears that LH neurons do not contribute much to odor discrimination in rabbits.