Characteristics of rat lateral hypothalamic neuron responses to smell and taste in emotional behavior

Single unit activity in the lateral hypothalamus (LHA) of the rat was recorded while the animal learned to discriminate cue signals. Normally preferred potables (glucose, orange, or grape solution) or intracranial self-stimulation (ICSS) were used as rewards. Electric shock or tail pinch were used as aversive stimuli. The same behavior, licking, was the response required to either obtain the rewarding stimuli or avoid the aversive ones. For positive reinforcement a rat was rewarded with fluid or ICSS upon licking a spout presented in front of its mouth. In negative reinforcement experiments, an aversive stimulus, electric shock or tail pinch, was applied if the rat did not lick the spout. Solutions having smell only, taste only, or smell-plus-taste, were prepared from oranges or grape extract. Of 392 neurons analyzed, 256 responded differentially to rewarding and aversive stimuli, and 138 of these were tested with the 3 different solutions. Similar LHA neural responses occurred during actual drinking of the 3 kinds of solutions, as well as on recognition of the cue signal. Responses to smell only had shorter latency than responses to taste only. Neural activity in response to solutions that could be both smelled and tasted was the sum of activity in response to taste-only solutions plus that in response to smell-only solutions. Cue signal responses were rapidly acquired, usually within 2-5 trials, for both taste-only and smell-only solutions. The results indicate the integration of both taste and olfactory information by the same LHA neurons, and these neurons are involved in cue signal learning. Present results of LHA neuronal responses to taste and smell suggest that the intensity of gustation and olfaction may add together to enhance instinctive hedonic sensations. These neurons are involved in the formation of stimulus-reinforcement association in learning, and in elicitation of conditioned emotional responses.