The tuberomammillary nucleus region as a reinforcement inhibiting substrate: facilitation of ipsihypothalamic self-stimulation by unilateral ibotenic acid lesions

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The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, consists of five subgroups and is the only known source of brain histamine. Knowledge about the function of this nucleus is still scarce. In a previous study we found an increase in the rate of ipsihemispheric hypothalamic self-stimulation following a dc lesion in the rostroventral part of this nucleus, suggesting that this region has an inhibitory action on a neuronal reward system or on the brain's reinforcement mechanism. In the present study we examined whether this facilitating effect on reinforcement was due to the destruction of fibers passing through the lesion area or of intrinsic cells, by lesioning subgroups of the TM with ibotenic acid, an excitatory amino acid, that selectively destroys neural cell bodies, leaving fibers largely intact. Following such lesions in the rostroventral part of the TM the operant response rates increased over the six days of testing when the animals stimulated themselves in the lateral hypothalamus in the hemisphere located ipsilateral but not contralateral to the lesion. No significant changes in response rate occurred following the lesion in the caudal part of the ventral TM. The results indicate that the region influenced by the lesion exerts inhibitory control over lateral hypothalamic self-stimulation, and that it is possible that histamine-containing neurons are involved in this effect.