Ultrastructural evidence for gamma aminobutyric acid-immunoreactive synapses on somatostatin-immunoreactive perikarya in the periventricular anterior hypothalamus

Somatostatin-immunoreactive perikarya in the periventricular anterior hypothalamus were demonstrated to be surrounded by gamma aminobutyric acid GABA-immunoreactive nerve terminals, by combining pre-embedding im-munocytochemistry for somatostatin and gold labelling post-embedding immunocytochemistry for GAB A. Ultrastructural studies revealed that in each 100-nm section, cells immunoreactive for somatostatin (n = 62) were contacted by a mean of 7.6 ±0.4 terminal profiles of which 3.0 ±0.3 (40%) were GABA-immunoreactive. Most GABA-immunoreactive terminals contained clear rounded vesicles and, where synaptic specializations were well demonstrated, appeared to be symmetric. The finding provides evidence that there is a significant GABA input to somatostatin neurons, an observation strengthening the hypothesis that GABA may inhibit somatostatin neurons, thereby causing increased secretion of growth hormone.