The role of GABA in the regulation of the dopamine/tyrosine hydroxylase-containing neurons of the rat retina

Dopamine (DA) and gamma-aminobutyric acid (GABA) are putative neurotransmitters in two separate populations of amacrine neurons in the mammalian retina. Pharmacological studies have been conducted to determine if GABA neurons regulate the neuronal activity of the neurons that secrete DA. Tyrosine hydroxylase (TH) activity, a biochemical indicator of changes in activity of DA/TH-containing neurons, was low in dark-adapted retinas and high in light-exposed retinas. Muscimol (a GABA receptor agonist) produced a dose-related, biphasic effect on the light-evoked activation of TH, when the drug was injected into the vitreous (intravitreal injection) of dark-adapted rats. At low doses, (35 and 60 pmol) muscimol enhanced the light-evoked activation of TH, but at higher doses (greater than or equal to 120 pmol) it inhibited the light-evoked increase in enzyme activity. Muscimol had no significant effect on the TH activity of dark-adapted retinas. GABA antagonists, bicuculline and picrotoxin, produced effects on TH activity that were dependent on both dose and light-exposure. At low doses (0.4-0.5 nmol), bicuculline and picrotoxin both inhibited the light-evoked activation of TH, but had no effect on TH activity of the dark-adapted retinas. At a higher dose (2.0 nmol), both antagonists increased TH activity in the dark-adapted retina and attenuated the further activation of the enzyme by light. Rat retinas were dissociated into suspensions of viable cells in order to investigate the direct effects of muscimol and picrotoxin on the DA/TH-containing cells. The process of dissociating dark-adapted retinas resulted in an apparent activation of TH. Incubation of the cells with muscimol resulted in a decrease of TH activity in a concentration-dependent manner. Picrotoxin antagonized the inhibitory effect of muscimol, but had no effect when incubated alone. The biphasic effects of GABA agonists and antagonists in vivo suggest that a certain subpopulation of GABA neurons are involved in the activation of the DA/TH-containing neurons by photic stimulation, while another subpopulation of GABA neurons produce a tonic inhibition of the DA/TH-containing neurons in darkness. The experiments with retinal cell suspensions indicate that the tonic inhibition is probably mediated by synapses of GABA neurons directly onto the DA/TH-containing cells.