RETICULAR THALAMIC INHIBITORY INPUT TO LATERAL HYPOTHALAMIC NEURONS -A FUNCTIONAL AND HISTOCHEMICAL DETERMINATION

Both the lateral hypothalamus (LH) and reticular thalamus (RT) have be en implicated in the integration of a variety of vital functions. To d etermine relevant connections more specifically between cells of these two regions in the rat, functional neurophysiological and horseradish peroxidase (HRP) histochemical techniques were utilized. Single pulse electrical stimulation of the RT modified the discharge frequency of the majority of LH neurons tested. LH neuronal responses to RT stimula tion included; inhibition (66.32%), excitation followed by inhibition (6.12%), and excitation (7.14%). Because RT stimulation resulted prima rily in the inhibition of LH neuronal activity with a relative short m ean latency of 1.87 +/- 0.23 ms, the existence of a possible direct in hibitory pathway from the RT to LH also was evaluated using HRP histoc hemistry. Retrogradely HRP labeled soma were identified in the RT afte r HRP was ejected extracellularly onto LH neurons, which exhibited a d ecrease in spontaneous activity in response to RT stimulation. These d ata demonstrate direct projections from the RT to the LH. In addition, HRP labeling of neurons and axons in the zona incerta-LH area followi ng LH HRP ejections suggest this route as a synaptic pathway from RT t o LH.