A1 Cell Group Mediates Solitary Nucleus Excitation of Supraoptic Vasopressin Cells

Stimulation of the nucleus tractus solitarius (NTS) excites putative vasopressin-secreting cells of the supraoptic nucleus (SON) via a catecholaminergic projection to hypothalamus. Despite recent evidence of a direct catecholaminergic projection from NTS to SON, we have performed single-unit recording experiments in pentobarbital sodium-anesthetized rats to investigate the possibility that NTS stimulation effects on SON vasopressin cells are indirect, being relayed via the A1 noradrenergic cell group of the caudal ventrolateral medulla. The effects of single-pulse NTS and A1 region stimulation on the activity of antidromically identified SON neurosecretory cells that had been functionally characterized as vasopressin secreting were compared. NTS stimulation excited 81% of all putative vasopressin-secreting cells tested (n = 83), with a mean onset latency of 51 +/- 1 ms. A1 region stimulation excited 76% of all cells tested and 90% of units responsive to NTS stimulation, with a mean latency of 39 +/- 1 ms. Consistent with previous work NTS stimulation excited only a minority of oxytocin cells tested (3/13), and of these two-thirds also responded to A1 stimulation. Bilateral electrolytic lesions of the A1 region abolished the effects of NTS stimulation on putative vasopressin cells. Ipsilateral A1 region injections of the inhibitory neurotransmitter gamma-amino-butyric acid reversibly blocked NTS stimulation effects on putative vasopressin cells in animals where the contralateral A1 region had already been lesioned. These results support the proposal that excitation of SON vasopressin-secreting cells after NTS stimulation is due to activation of a relay projection through the A1 noradrenergic cell group of the caudal ventrolateral medulla.