Electrophysiological identification of neurons in the parabrachial nucleus projecting directly to the hypothalamus in the rat.

Experiments were done in urethane anesthetized rats to identify single units in the region of the parabrachial nucleus (PBN) projecting directly to 'cardiovascular' responsive sites in either the paraventricular nucleus of the hypothalamus (PVH) or the supraoptic commissure and nucleus (SOC-SON) region. Fifty-five single units were antidromically activated in the ipsilateral PBN by electrical stimulation of either the PVH (n = 27) or SOC-SON region (n = 28) with latencies corresponding to conduction velocities of 0.3-5.1 m/s. The axons of PBN units projecting to the PVH conducted at significantly slower velocities (0.5 +/- 0.04 m/s) than those projecting to the SOC-SON region (1.6 +/- 0.25 m/s). These data suggest that ascending fibers from the PBN to the PVH are unmyelinated, whereas those to the SOC-SON region are primarily a little myelinated. In addition, since the PBN is known to receive cardiovascular and visceral afferent inputs, it is suggested that these neurons likely function in relaying this afferent information to hypothalamic areas involved in autonomic regulation.