D1 and D2 receptor-mediated dopaminergic modulation of visual responses in cat dorsal lateral geniculate nucleus

The modulatory effects of dopamine (DA) on the visual responses of relay cells of the dorsal aspect of cat lateral geniculate nucleus (dLGN) were tested using local micro-iontophoretic application of DA and application of the receptor-specific agonists SKF38393 (SKF, D1/D5) and quinpirole (QUIN, D2/D3/D4) in the anaesthetized alcuronium-treated cat. The effects of DA and QUIN were clearly dose-dependent: small amounts caused a weak and transient facilitation of visual activity (10-30% increase) preferentially in Y-type relay cells, which changed to a moderate reduction of visual responses when the dose was increased (50%, maximal 70%). The effect of SKF was mainly suppressive and increased with the amount of drug applied (up to 90% reduction). The selective antagonists SCH23390 (SCH, D1) and sulpiride (SULP, D2) reduced the effects of co-applied DA agonists. We found little evidence for a specific dopaminergic modulation of the surround inhibition (stimulus-driven lateral inhibition) although DA slightly facilitated the transmission of weak signals (small stimuli). Nevertheless, some dopaminergic effects seem to be mediated via inhibitory interneurons regulating the strength of sustained or recurrent inhibition. Application of DA agonists during blockade of GABA(A) receptors indicates a direct suppression of relay cells via D1 receptors, an excitation of relay cells via D2 receptors and--with increasing amounts of D2 agonist--probably also an excitation of inhibitory interneurons, which results in an indirect inhibition of dLGN relay cells (predominantly of the X-type). The results are discussed in relation to the impairment of visual functions in Parkinson's disease.