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 ≥30%
augmentation of responses in only 5%. The decrement in
response strength was mimicked by application of the α2
adrenoceptor agonist, p-aminoclonidine, the nonspecific
β agonist, isoproterenol, and the β1
agonist, dobutamine. These agents had similar effects on
responses evoked by electrical stimulation of the optic
chiasm and visual cortex. The α1 agonist,
methoxamine, augmented the light-evoked responses of 53%
of 49 SC cells by ≥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 α2 and β1
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.
