Contributions of inhibitory mechanisms to the shift responses of X and Y cells in the cat lateral geniculate nucleus.

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Abstract
1. Adult cats were anaesthetized with a mixture of halothane, nitrous oxide and oxygen to record from single neurones of the dorsal lateral geniculate nucleus (d.l.g.n.) with five-barrel glass micro-electrodes. Periphery effects (shift effects) were elicited by large-field phase-reversing gratings presented in the visual field outside the conventional receptive field area. 2. A range of transient excitatory responses was found in X and Y cells. Y cells had phasic shift effects with significantly higher amplitudes and shorter durations (mean 52 impulses/s, 135 ms) than those observed in the tonic shift effects of X cells (mean 24 impulses/s, 169 ms). All Y cells and most X cells responded to stimulation of remote retinal regions. About 7% of the X cells displayed no shift effect. 3. Micro-ionophoresis of the .gamma.-aminobutyric acid (GABA) antagonist bicuculline, acetylcholine (ACh) and L-glutamate specifically influenced the shift effects of X and Y cells. 4. During continuous application of the GABA antagonist bicuculline the differences in maximal response rates and amplitudes of X and Y cells were eliminated. The maintained activity raised predominantly in X cells and the early peak rates increased more in X- than in Y-cell shift effects, leading to equal average peak rates of 100 and response amplitudes of about 85 impulses/s in both cell classes. The characteristic time courses of X- and Y-cell responses were not affected. 5. Micro-ionophoretic application of ACh caused a combination of excitatory and disinhibitory effects. Maintained activity as well as early parts of stimulus-evoked responses were similarly raised in X and Y cells. In addition, the Y-cell shift effects became less phasic by elevation of the late response part. Sodium pentobarbitone, used to block ACh excitation, suppressed the ACh-induced effects in the early phase of the X- and Y-cell shift effects and the increase of maintained activity in Y-cells, while the effect on the late part of Y-cell responses persisted. Elevation of background activity partially remained in X cells, and the X-cell responses became tonically prolonged at the same time. 6. L-Glutamate increased the activity of X and Y cells without changing the characteristic shift-effect properties of both cell classes. 7. It is concluded that different short- and long-lasting inhibitory mechanisms shape the responses of d.l.g.n. neurones to stimulation outside the conventional receptive field. A strong, short-lasting inhibition superimposed on a tonic inhibitory component is predominant in X cells and leads to the low-amplitude shift effects of this cell type. A weaker, longer-lasting inhibitory process prevails in Y cells, contributing to the phasic shift effects with initially high impulse rates typical of Y cells. These findings are interpreted as reflecting differential inhibitory inputs to the two cell classes.