Uptake and autoreceptor-controlled release of [3H]-GABA by the hypothalamic median eminence and pituitary...

The uptake and release of gamma-[3H]-aminobutyric acid ([3H]-GABA) by the median eminence and the neurointermediate lobe of the pituitary was investigated using sucrose homogenates as crude synaptosomal preparations. Uptake in both areas showed predominantly neuronal specificity and similar Km values, but the median eminence had a considerably greater Vmax value. Release of [3H]-GABA could be stimulated by elevated K+ concentrations, in a Ca2+-dependent manner. Stimulated release was reduced by muscimol, implying the existence of presynaptic autoreceptors in both areas. A number of other transmitters and peptide hormones were demonstrated to have no effect on stimulated release of [3H]-GABA in either area.