High Specific Binding of [3H]GABA and [3H]Muscimol to Membranes from Dendrodendritic Synaptosomes of the Rat Olfactory Bulb

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Abstract
Olfactory bulbs contain dendrodendritic synapses, which occur between granule cells and mitral cells, and γ‐aminobutyric acid (GABA) is thought to act as an inhibitory neurotransmitter at these synapses. Synaptosomes derived from the dendrodendritic synapses of the olfactory bulb were shown previously to contain considerable L‐glutamate decarboxylase activity. The subcellular distribution and binding parameters of [3H]GABA and [3H]muscimol binding sites have now been determined in the rat olfactory bulb. Of all fractions examined, crude synaptic membranes (CSM) prepared from the dendrodendritic synaptosomes were shown to have the highest specific binding activity and accounted for nearly all of the total binding activity for both ligands. The specific binding activities for [3H]GABA and for [3H]muscimol were greatly increased after treating the CSM with 0.05% Triton X‐100. Binding was shown to be Na+‐independent, reversible, pharmacologically specific, and saturable. High‐ and low‐affinity sites were detected for both ligands, and both classes of sites had appreciably lower KD values for muscimol (KD1= 3.1 nM, KD2= 25.1 nM) than for GABA (KD1= 8.6 nM; KD2= 63.7 nM). The amounts of the high‐affinity binding sites for muscimol and GABA were similar (Bmax= 1.7 and 1.5 pmol/mg protein, respectively). The results of the present experiments indicate that the GABA and muscimol binding sites represent the GABA postsynaptic receptor, presumably on mitral cell dendrites, and provide further support for the hypothesis that GABA functions as a neurotransmitter at the dendrodendritic synapses in the olfactory bulb.