Reticular thalamic inhibitory input to lateral hypothalamic neurons: a functional and histochemical determination

Both the lateral hypothalamus (LH) and reticular thalamus (RT) have been implicated in the integration of a variety of vital functions. To determine 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 stimulation included; inhibition (66.32%), excitation followed by inhibition (6.12%), and excitation (7.14%). Because RT stimulation resulted primarily in the inhibition of LH neuronal activity with a relative short mean latency of 1.87 ± 0.23 ms, the existence of a possible direct inhibitory pathway from the RT to LH also was evaluated using HRP histochemistry. Retrogradely HRP labeled soma were identified in the RT after HRP was ejected extracellularly onto LH neurons, which exhibited a decrease in spontaneous activity in response to RT stimulation. These data demonstrate direct projections from the RT to the LH. In addition, HRP labeling of neurons and axons in the zona incerta-LH area following LH HRP ejections suggest this route as a synaptic pathway from RT to LH.