Somatostatin 28-immunoreactive inputs to the paraventricular and supraoptic nuclei: principal origin from non-aminergic neurons in the nucleus of the solitary tract.

Immunohistochemical and axonal transport methods were used to chart the distribution of somatostatin-immunoreactive (SS-IR) fibres in the paraventricular (PVH) and supraoptic (SO) nuclei of the rat hypothalamus and to identify the cell group(s) from which they originate. Fibres and varicosities immunoreactive for SS-28 and/or SS-281-12 are found primarily in the parvocellular division of the PVH, though aspects of the magnocellular division, and of the SO, in which oxytocinergic neurons are clustered also receive moderate inputs. Combined retrograde transport-immunohistochemical studies indicated that these arise principally from non-catecholaminergic neurons in the lateral aspect of the commissural part of the nucleus of the solitary tract (NTS). SS-28 has been shown to act within the central nervous system to elicit both oxytocin and vasopressin secretory responses, and may be involved in mediating vasopressin secretory responses to haemorrhage. Direct SS-28-IR inputs to the magnocellular cell groups from the NTS, which receives primary visceral sensory inputs, are in a position to play a role in mediating oxytocin secretory responses to interoceptive stimuli; the pathway(s) and mechanism(s) which allow SS-28 to interact with vasopressinergic neurons are not clear.