Effects of norepinephrine on the activity of visual neurons in the superior colliculus of the hamster.
Single-unit recording and micropressure ejection techniques were used to test the effects of norepinephrine (NE) on the responses of neurons in the superficial layers (the stratum griseum superficiale and stratum opticum) of the hamster's superior colliculus (SC). Application of NE suppressed visually evoked responses by -30% in 75% of 40 neurons tested and produced > or = 30% augmentation of responses in only 5%. The decrement in response strength was mimicked by application of the alpha2 adrenoceptor agonist, p-aminoclonidine, the nonspecific beta agonist, isoproterenol, and the beta1 agonist, dobutamine. These agents had similar effects on responses evoked by electrical stimulation of the optic chiasm and visual cortex. The alpha1 agonist, methoxamine, augmented the light-evoked responses of 53% of 49 SC cells by > or = 30%, but had little effect on responses evoked by electrical stimulation of optic chiasm or visual cortex. The effects of adrenergic agonists upon the glutamate-evoked responses of SC cells that were synaptically "isolated" by concurrent application of Mg2+ were similar to those obtained during visual stimulation. Analysis of effects of NE on visually evoked and background activity indicated that application of this amine did not significantly enhance signal-to-noise ratios for most superficial layer SC neurons, and signal-to-noise ratios were in some cases reduced. These results indicate that NE acts primarily through alpha2 and beta1 receptors to suppress the visual responses of SC neurons. Activation of either of these receptors reduces the responses of SC neurons to either of their two major visual inputs as well as to direct stimulation by glutamate, and it would thus appear that these effects are primarily postsynaptic.