GABA is among a broad array of neuroactive substances, which include several neurotransmitters and neuropeptides that have been identified in mammalian taste buds. The present study is the first to characterize the morphological features, distribution patterns and functional consequences of GABAergic taste receptor cells (TRCs) in adult rat posterior lingual epithelium. Subsets of TRCs were identified as the endogenous source of GABA in taste buds of foliate or circumvallate papillae, as demonstrated by localization of immunoreactivity to both GABA and its synthetic enzyme, glutamate decarboxylase (GAD). Double labeling immunofluorescent studies revealed complex co-expression patterns of GAD with TRC population-specific protein markers (i.e. taste-specific G-protein gustducin α subunit, neural cell adhesion molecule and protein gene product 9.5), the presynaptic membrane-specific protein marker synaptosomal-associated protein of 25 kDa (SNAP-25) and neuropeptides (i.e. cholecystokinin and vasoactive intestinal polypeptide), implying that GABAergic TRCs may have diverse functions in taste buds.

GABA receptor subtypes, GABA_A and GABA_B, were also localized to subsets of TRCs in rat foliate and circumvallate papillae. GABA _B receptor R1 subunit-immunoreactivity was observed in a group of TRCs separate from the GAD-containing cells in the same bud, arguing a paracrine role for GABA. Interestingly, peptidergic TRCs appeared to receive differentially distributed modulatory input mediated by the GABA_B receptor. Possible physiological effects of GABA in the taste buds were examined by patch clamp recordings of TRCs acutely dissociated from rat foliate and circumvallate papillae. GABA was demonstrated to enhance chloride currents and inwardly rectifying potassium currents by acting on the GABA_A and GABA _B receptor subtype, respectively. Therefore electrical properties of TRCs may be modulated by GABA.

In summary, GABA produced endogenously in the taste buds may modulate functions of various populations of TRCs by acting on specific receptor subtypes. The results of the present study add GABA to a growing list of recently characterized neuroactive substances in the taste buds. These neuroactive substances mediating cell-to-cell communication within the taste buds are capable of shaping the final output of chemosensory TRCs and therefore may play important roles in peripheral gustatory signal processing.