Ultrastructural Evidence for Gamma Aminobutyric Acid-Immunoreactive Synapses on Somatostatin-Immunoreactive Perikarya in the Periventricular Anterior Hypothalamus

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Abstract
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 immunocytochemistry for somatostatin and gold labelling post-embedding immunocytochemistry for GABA. 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.

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