| The dorsal lateral geniculate nucleus of the thalamus (LGN) is the gateway to the primary visual cortex, providing the first station where the higher visual system may regulate its own input. The LGN is more than a simple relay of the visually-evoked activity of retinal ganglion cells to cortex. Only 10-20 percent of the synapses made onto relay cells originate from retinal afferents, and the remaining synapses comprise a diverse group of excitatory, inhibitory and neuromodulatory influences that are positioned to regulate the flow of visual information through the nucleus. It has long been known that visual information flow through the LGN can be contingent upon the arousal state of the animal, but the precise connection between the known pharmacology and the observed state-dependent phenomena is poorly understood. A potentially powerful influence on the relay properties of LGN neurons is the inhibitory, GABAergic network of the LGN. The activity of this network can be determined by three primary influences: (1) the intrinsic activity of relay neurons within the LGN, (2) global activity of the ascending projection from the brainstem, and (3) focal activity of the descending projection from layer VI of striate cortex. The contribution of GABAergic inhibition to the regulation of retinogeniculate transmission was examined here by the application of {dollar}gamma{dollar}-aminobutyric acid (GABA) and the GABA{dollar}sb{lcub}rm A{rcub}{dollar} antagonist, bicuculline, via a combined recording/iontophoretic electrode that allowed the assessment of drug effects on the input-output transformations of LGN neurons. Iontophoretic application of GABA decreased the fraction of LGN output action potentials to input action potentials from the retina (the transfer ratio). This effect was reversed by bicuculline. GABA reduced, and bicuculline increased visual signal detectability and the individual phasic and tonic components of the visual response at the LGN. The data suggest that the GABAergic, inhibitory circuitry may participate in significant, selective filtering of synaptic efficiency, response strength and pattern of afferent visual signals by extrathalamic influences. |
