Differing effects of electrical and chemical parabrachial nucleus stimulation on supraoptic vasopressin cells

Electrical stimulation of the parabrachial nucleus stimulates vasopressin secretion. To determine whether this is due to activation of intrinsic projection neurons or fibers of passage we compared, in anaesthetized rats, the effects of electrical and chemical parabrachial nucleus stimulation on the activity of neurosecretory vasopressin cells of the supraoptic nucleus. Electrical stimulation excited 35% and inhibited 14% of vasopressin cells tested (257 tests). Parabrachial nucleus injections of the neuroexcitant glutamate (0.2, 10 or 200 mM) altered the activity of only 6% of vasopressin cells tested (118 tests). At the highest glutamate concentration, which produced clear changes in blood pressure, all responsive units were inhibited (4/22 cells). These data suggest that previous reports indicating a facilitatory role for parabrachial nucleus neurons in the regulation of vasopressin secretion are likely to be attributable to effects on fibers of passage.