Differential responses of identified rat hypothalamic paraventricular neurons to suprachiasmatic nucleus stimulation.

The suprachiasmatic nucleus in the anterior hypothalamus contains a circadian oscillator that is responsible for 24-h rhythms in several behavioral, endocrine and autonomic processes. Efferent suprachiasmatic projections are likely to transmit rhythmic information to brain nuclei controlling these functions. The hypothalamic paraventricular nucleus is considered to be a target of the suprachiasmatic nucleus due to its important role in autonomic and endocrine regulation. The present study applied extracellular electrophysiological techniques to intact animals to look for a possible interaction between suprachiasmatic nucleus efferents and identified neurons in the hypothalamic paraventricular nucleus. Results showed that electrical stimulation of the suprachiasmatic nucleus induced an increase in the excitability of 87% of paraventricular neurons that project to the median eminence and are situated in the medial and dorsal parvocellular subnucleus; neurons with similar projections but located in the periventricular subnucleus displayed a reduction in firing rate following suprachiasmatic stimulation. Electrical activation of the suprachiasmatic nucleus provoked a decrease in excitability in 75% of paraventricular neurons in the posterior magnocellular subnucleus that send axons to the posterior pituitary and in 85% of paraventricular neurons, located in the medial parvocellular subnucleus, that project to the dorsal vagus complex in the brainstem. The data imply that functional and selective neural connections exist between suprachiasmatic nucleus efferents and specific cell groups within the hypothalamic paraventricular nucleus. These projections would be able to convey rhythmic information to certain endocrine and autonomic functions. The anatomical and neurochemical characteristics of the underlying pathways remain to be determined.