Action of gamma-aminobutyric acid (GABA) in the isolated photosensory pineal organ.

The effect of GABA (gamma-aminobutyric acid), its agonists (muscimol, baclofen) and antagonist (bicuculline) on pineal ganglion cells of the luminosity type were studied in the isolated, superfused pineal organ of the rainbow trout, Oncorhynchus mykiss. Extracellular recordings revealed that GABA added through the superfusion medium caused a clear alteration of the activity of projecting neurons, which transmit luminosity responses to the brain. Spontaneous discharges of ganglion cells were predominantly suppressed by GABA (33 neurons out of 48), but 10 neurons were clearly excited. Similar effects were observed after addition of muscimol, but not of baclofen. Bicuculline reversed the GABA and muscimol induced inhibition or excitation. In 4 neurons of the luminosity type, GABA caused bidirectional, inhibitory and excitatory responses depending on the state of light- or dark-adaptation. These observations suggest a role of a GABAergic mechanism in the generation and transmission of luminosity responses in the trout pineal organ. It appears that GABA participates in the modulation of light sensitivity during light- and dark-adaptation processes and that this action is mediated by GABAA receptors.