THE TUBEROMAMMILLARY NUCLEUS REGION AS A REINFORCEMENT INHIBITING SUBSTRATE - FACILITATION OF IPSIHYPOTHALAMIC SELF-STIMULATION BY UNILATERAL IBOTENIC ACID LESIONS

The tuberomammillary nucleus (TM), located in the posterior hypothalam ic 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 ipsih emispheric 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 reinf orcement mechanism. In the present study we examined whether this faci litating effect on reinforcement was due to the destruction of fibers passing through the lesion area or of intrinsic cells, by lesioning su bgroups of the TM with ibotenic acid, an excitatory amino acid, that s electively destroys neural cell bodies, leaving fibers largely intact. Following such lesions in the rostroventral part of the TM the operan t response rates increased over the six days of testing when the anima ls stimulated themselves in the lateral hypothalamus in the hemisphere located ipsilateral but not contralateral to the lesion. No significa nt changes in response rate occurred following the lesion in the cauda l part of the ventral TM. The results indicate that the region influen ced by the lesion exerts inhibitory control over lateral hypothalamic self-stimulation, and that it is possible that histamine-containing ne urons are involved in this effect.