A 14C-2-deoxyglucose analysis of the neural pathways of the limbic forebrain in the rat: II. The hypothalamus.

An attempt was made to characterize the nature of the functional organization of the hypothalamus by observing the patterns of uptake of 14C-2-deoxyglucose (2DG) following electrical stimulation of different regions within the preoptico-hypothalamus in the rat. The experimental paradigm consisted of electrical brain stimulation delivered continuously for periods of 30 sec on and 30 sec off for 45 minutes following injection of 2DG. Brains were removed and processed for autoradiography. Activation of the medial forebrain bundle was noted following stimulation of the nucleus accumbens and lateral preoptico-hypothalamus. Activated fibers could be followed only in a caudal direction through the medial forebrain bundle and into the ventral tegmental area as a result of nucleus accumbens stimulation. Stimulation of the lateral preoptic region or of the anterior half of lateral hypothalamus produced activation of the lateral septal nucleus, lateral habenular nucleus, perifornical region, midline thalamus and ventral tegmental area. Since stimulation of the perifornical hypothalamus significantly activated the rostro-caudal extent of the midbrain cental gray, it is suggested that impulses from the lateral hypothalamus reach the lower brainstem via its connections with the perifornical hypothalamus. Ventromedial hypothalamic stimulation activated only the lateral septal nucleus, cortico-medial amygdala and medial preoptico-hypothalamus, while medial preoptico-hypothalamic stimulation resulted in increased 2DG uptake in the midbrain central gray, thus suggesting that medial hypothalamic impulses reach the brainstem by first ascending to the level of the preoptico-hypothalamus. Mammillary body stimulation orthodromically activated fibers in the mammillothalamic and mammillotegmental tracts and antidromically fibers in the fornix for a short distance.

Effect of stimulation of prefrontal cortex and amygdala on diencephalic neurons.

Extracellular unit recordings were obtained from the nucleus medialis dorsalis (MD) and adjacent thalamic nuclei and from the hypothalamus of anesthetized cats. Electrical stimulation of the prefrontal cortex produces suppression of unit activity and rebound excitation in MD. In addition antidromic and short latency excitation was found which was followed by prolonged suppression of firing. This suppression was also followed by a strong postinihibitory activation of unit firing which was, in some cases, again followed by a second phase of inhibition. Amygdala stimulation produced similar results except that antidromic responses were found only in the intralaminar nuclei, and short-latency transynaptic excitation occurred somewhat later. The results are in consonance with the hypothesis that the same interneuronal mechanism is engaged by both afferent sources to MD. In the hypothalamus, fewer units responded to cortical and amygdalar stimulation and alternating sequences of excitation-inhibition-excitation were not found.