Light and electron microscopic demonstration of hypothalamic projections to the parabrachial nuclei in the cat.

Hypothalamic connections with the parabrachial nuclei in the cat were studied at light and electron microscopic levels following wheat germ agglutinin-horseradish peroxidase injections into the parabrachial nuclei and electrolytic lesions in the hypothalamus. The greatest concentration of retrogradely labeled neurons occurred in the paraventricular nucleus. Labeled neurons were also seen within the preoptic, anterior, lateral, dorsomedial and ventromedial hypothalamic nuclei. Hypothalamic lesions resulted in the degeneration of terminals forming axosomatic and axodendritic synapses in the parabrachial nuclei, particularly its lateral division. These findings support the idea that hypothalamic connections to specific regions of the parabrachial nuclei may underlie the topographical functional organization demonstrated for these brainstem nuclei.