A GABAergic Influence on the Light‐Induced Increase in Dopamine Turnover in the Dark‐Adapted Rat Retina <i>In Vivo</i>

Abstract

Abstract: Light stimulates tyrosine hydroxylase activity and dopamine (DA) turnover in the dark‐adapted rat retina in vivo . The DA neurons are located in the amacrine cell layer and form numerous connections with other cells in this layer. Conceivably, alterations in neurotransmission in these other cells could influence the light‐responding parameters of the DA neurons. Evidence presented in this paper shows that in vivo pharmacologic manipulation of the GABA system modifies the light‐induced change in DA turnover. The decline in DA content following inhibition of tyrosine hydroxylase by α‐methyl‐p‐tyrosine (αMPT, 250 mg/kg, i.p.) was used to estimate DA turnover. The decline in DA content in retinas of the μMPT‐treated rats was significantly enhanced by light exposure for 30 or 60 min. Two doses of the potent GABA agonist muscimol (13.2 or 26.4 μmol/kg, i.v., cumulative) significantly inhibited the light‐induced increase in DA turnover (p &lt;.001). This action was selective for GABA because the GABA antagonist picrotoxinin (1.88 mg/kg, i.v., cumulative) reversed the muscimol‐mediated blockade of the light‐induced stimulation. In fact, DA turnover in the presence of light, muscimol, and picrotoxinin was not different from DA turnover in light alone. These data suggest that there is either a direct or indirect GABAergic input to the DA system of the rat retina. Current studies are aimed at clarifying the physiological role, if any, that this input plays in the normal light response of the retinal DA system.