DOPAMINERGIC TRANSMISSION IN THE RAT RETINA - EVIDENCE FOR VOLUME TRANSMISSION

The study was designed to determine whether dopaminergic neurotransmis sion in the retina car, operate via volume transmission. In double imm unolabelling experiments, a mismatch as well as a match was demonstrat ed in the rat retina between tyrosine hydroxylase (TH) and dopamine (D A) immunoreactive (ir) terminals and cell bodies and dopamine D2 recep tor-like ir cell bodies and processes. The match regions were located in the inner nuclear and plexiform layers (D2 ir cell bodies plus proc esses). The mismatch regions were located in the ganglion cell layer, the outer plexiform layer, and the outer segment of the photoreceptor layer, where very few TH ir terminals can be found in relation to the D2 like ir processes. In similar experiments analyzing D1 receptor lik e ir processes versus TH ir nerve terminals, mainly a mismatch in thei r distribution could be demonstrated, with the D1 like ir processes pr esent in the outer plexiform layer and the outer segment where a misma tch in D2 like receptors also exists. The demonstration of a mismatch between the localization of the TH terminal plexus and the dopamine D2 and D1 receptor subtypes in the outer plexiform layer, the outer segm ent and the ganglion cell layer (only D2 immunoreactivity (IR)) sugges ts that dopamine, mainly from the inner plexiform layer, may reach the D2 and D1 mismatch receptors via diffusion in the extracellular space . After injecting dopamine into the corpus vitreum, dopamine diffuses through the retina, and strong catecholamine (CA) fluorescence appears in the entire inner plexiform layer and the entire outer plexiform la yer, representing the match and mismatch DA receptor areas, respective ly. The DA is probably bound to D1 and D2 receptors in both plexiform layers, since the DA receptor antagonist chlorpromazine fully blocks t he appearance of the DA fluorescence, while only a partial blockade is found after haloperidol treatment which mainly blocks D2 receptors. T hese results indicate that the amacrine and/or interplexiform DA cells , with sparse branches in the outer plexiform layer, can operate via v olume transmission in the rat retina to influence the outer plexiform layer and the outer segment, as well as other layers of the rat retina such as the ganglion cell layer. Copyright (C) 1996 Elsevier Science B.V.