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Rat hypothalamic neurons and endocrine cells from the intermediate lobe of the pituitary were grown in dissociated coculture. Neurons positively stained with an antibody against glutamate decarboxylase established apparent contacts with the ␣-melanocyte-stimulating hormone-positive endocrine cells. These sites of contact were intensely labeled with an antibody against the synaptic protein synapsin I and displayed ultrastructural features characteristic of synapses. Using patchclamp recordings, we have demonstrated that these contacts correspond to functional GABAergic synapses. The synaptic currents were blocked reversibly by bicuculline (5 M) and SR95531 (5 M), two competitive antagonists of the GABA A receptor. At a holding potential of Ϫ60 mV, spontaneously occurring IPSCs (s-IPSCs) had small amplitudes (10 -100 pA), whereas electrically evoked IPSCs (ee-IPSCs) had amplitudes up to 1 nA. The rise times of both types of IPSCs were fast (Յ1 msec), and their decaying phases were fitted in most cases with a single exponential function (time constant, 50 msec). The amplitude distribution of s-IPSCs did not reveal clear, equally spaced peaks and was little affected by tetrodotoxin, suggesting that most s-IPSCs were miniature IPSCs. Reduction of extracellular calcium concentration to 0.3 mM induced a marked decrease in s-IPSC frequency and revealed a single amplitude peak at 10 pA, suggesting that a single quantum of GABA activates 8 -10 GABA A channels. Thus, our preparation might be an interesting model to study different aspects of synapse formation between a central neuron and its target as well as the fundamental mechanisms of synaptic transmission at central synapses.

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