Ontogeny of somatostatin-containing neuron system of the rat: immunohistochemical observations. I. Lower brainstem.

The ontogeny of the somatostatin (SRIF) neuron system in the lower brainstem of the rat was analyzed using the indirect immunofluorescence technique of Coons. SRIF-positive structures first appeared in the primordium of the ventral nucleus of the lemniscus lateralis of the rat fetus corresponding to gestational day 15 (12-14-mm embryos). On and after gestational day 15, these structures appeared in progressively more diverse areas of the lower brainstem and continued to show an increase in number and intensity up till birth. The maximum SRIF-positive structures were histochemically identified in the lower brainstem at the perinatal stage. However, after birth, the numbers of SRIF-positive structures in the lower brainstem began to decrease as the rat grew and none or only a few SRIF-positive structures were detected in the adult rats. This ontogenetical study of the SRIF neuron system on the lower brainstem of the rat strongly suggests that SRIF might play an important role in the development of the lower brainstem other than in its neurotransmitter or neuromodulator function.