Norepinephrine and serotonin: Opposite effects on the activity of lateral geniculate neurons evoked by optic pathway stimulation

This study compares the responses of single units in the rat dorsal lateral geniculate nucleus (LGNd) to microiontophoretically applied norepinephrine (NE) and serotonin (5-HT). Most of the cells were identified physiologically as P-type (geniculocortical relay) neurons. At low iontophoretic currents (1 to 20 nA), NE caused a delayed increase in the spontaneous firing rate of these units, whereas 5-HT invariably slowed the discharge frequency. To compare the effects of the two monoamines on evoked activity, P-cells were driven by electrical stimulation of the afferent visual pathway at the level of the optic chiasm. NE caused a marked facilitation of both the short-latency (2 to 4 ms) and the delayed (70 to 230 ms) responses to such stimulation. The α-adrenoceptor antagonist phentolamine (10 nA), which by itself had no consistent effect on evoked activity, strongly diminished the response to NE. In contrast to NE, 5-HT was a powerful depressant of electrically evoked activity; neither phentolamine nor the 5-HT antagonist methysergide antagonized this response. Firing of LGNd units evoked by flashes of light was also facilitated by NE and depressed by 5-HT. We conclude that LGNd relay neurons exhibit the following unique features in their responsiveness to monoamines: (i) microiontophoretically applied NE facilitates, but 5-HT depresses, the spontaneous or synaptically evoked activity of virtually every cell; (ii) there is no dissociation between the actions of NE on spontaneous and evoked activity, as is the case in other brain regions.