Characterization of the parabrachial nucleus input to the hypothalamic paraventricular nucleus in the rat.

The brainstem parabrachial nucleus (PBN) is viewed as an increasingly important site for the transfer of autonomic-related information to more rostral structures in the forebrain including the hypothalamus. In this study, we examined electrophysiologically in vivo and anatomically the nature of PBN input to the hypothalamic paraventricular nucleus (PVN) and particularly to the vasopressin-and oxytocin-secreting magnocellular neurosecretory cells within this nucleus. In urethane-anaesthetized rats, extracellular recordings from 108 antidromically identified neurosecretory PVN cells revealed an excitatory (37/43 cells) and less frequently an inhibitory (6/43 cells) response consequent to electrical stimulation in the PBN. Both vasopressin (12/37 cells)-and oxytocin (9/37 cells)-secreting neurons appear to respond to the PBN stimulus. Four cells projecting to the neurohypophysis could also be antidromically activated from PBN, and this observation may be indicative of collateral branching in some PVN neurosecretory neurons. In addition, recordings from 60 non-magnocellular (i.e. non-neurohypophysially-projecting) PVN cells revealed a facilitatory response (43/60 cells) following PBN stimulation, Iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were made within the rat lateral PBN and brains prepared for immunocytochemical examination of projections to the PVN region. PHA-L-labelled fibres and terminals were visualized within both the parvocellular and magnocellular divisions of the PVN. In addition, labelled fibres were also seen in a region immediately dorsal to the PVN. PHA-L-labelled fibres with axonal varicosities and boutons were visualized over immunocyto-chemically-identified vasopressin and oxytocin neurons within the magnocellular PVN. These convergent electrophysiological and anatomical data provide evidence for a PBN projection to the PVN that is predominantly excitatory to both magnocellular neurosecretory and non-magnocellular cells. Moreover, with respect to vasopressin-and oxytocin-secreting cells, the PBN input appears to be directed at both populations of peptidergic neurons.