Effects of Somatostatin-14 on the in Vitro Release of [3H]GABA from Slices of Rat Caudatoputamen

Slices (300 microns) of rat caudatoputamen were incubated in Krebs-Henseleit medium and loaded with [3H]glutamine, part of which was converted to [3H]GABA. This conversion takes place only in GABA-neurons most of which probably contribute to the striatonigral pathway. After a 24 min equilibration period, release of radioactivity was stimulated with veratridine (3.1-4 mumol/l) or K+ (15-25 mmol/l) in the absence or presence of somatostatin-14. From the radioactivity released [3H]GABA was separated by cationic exchange chromatography and measured. Somatostatin-14 affected the release of [3H]GABA in a manner which depended on its concentration as well as on the extent of stimulus-evoked release. Somatostatin-14 (1 nmol/l) enhanced the moderate release (2-4% of tissue content) elicited by veratridine (3.1 mumol/l) or K+ (20 mmol/l), but had no effect on the more pronounced release (5-8% of tissue content) elicited by veratridine (4 mumol/l) or K+ (25 mmol/l). Somatostatin-14 (10 nmol/l) had no effect on the moderate release of [3H]GABA, but diminished the pronounced one. Further experiments provided evidence that the somatostatin-14-induced enhancement was not brought about by a direct action on GABA-neurons but was probably indirect, i.e. mediated by other striatal neurons. In contrast, the diminution of the release of [3H]GABA caused by somatostatin-14 may be due to its direct action on releasing neurons. Two antisera against somatostatin lowered the pronounced release indicating that endogenous somatostatin may also enhance the release of [3H]GABA. In addition, endogenous somatostatin seems also to be able to diminish the release under certain experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)