Modulation of lateral geniculate neurone excitability by noradrenaline microiontophoresis or locus coeruleus stimulation.

The dorsal lateral geniculate nucleus (LGNd) receives afferents from the brainstem which regulate its capacity to transmit visual information from the retina to the striate cortex. One such pathway consists of noradrenaline (NA)-containing fibres originating in the locus coeruleus (LC). These provide a dense, uniform, noradrenergic innervation of the LGNd. Electrical stimulation in the LC region has been reported to enhance the responsiveness of LGNd neurones to afferent excitation. Although this effect was abolished when brain NA stores were pharmacologically depleted, it was not established as a direct action of NA on LGNd neurones because of the widespread distribution of LC fibres to many parts of the brain and the long latency of the response. Recently, we observed that NA, applied locally by microiontophoresis with low ejection currents, produced a delayed increase in the firing rate of most spontaneously active LGNd neurones, an effect selectively blocked by alpha-adrenoceptor antagonists. We show here that microiontophoretic NA can mimic the ability of LC stimulation to enhance the synaptic excitation of LGNd neurones. As neither NA nor LC stimulation activated LGNd neurones in the absence of synaptic or glutamate-induced excitation, both appear to act through a neuromodulatory mechanism. The postsynaptic alpha-adrenoceptor antagonist WB-4101 blocks the facilitation produced by locally applied NA and by coeruleo-geniculate pathway stimulation, providing evidence for pharmacological identity of the two effects.