Activation of locus coeruleus neurons by peripheral stimuli: Modulation by a collateral inhibitory mechanism

Noradrenergic neurons of the rat locus coeruleus (LC) respond to noxious stimuli or peripheral nerve stimulation with a burst of spikes followed by a period of suppressed activity. During this period of post-activation suppression, responses to additional stimuli were attenuated. After antidromic activation of the LC there was also a period of reduced responsivity, presumably mediated by inhibitory recurrent LC collaterals. The suppression of LC unit firing which follows nerve stimulation was reduced by piperoxane, an α-adrenergic antagonist which is known to block the norepinephrine-mediated autoinhibitory action of recurrent LC axon collaterals. The specificity of piperoxane in blocking norepinephrine was shown by the fact that it did not antagonize several other putative transmitters in the LC (i.e., GABA, glycine, and met-enkephalin). It is concluded that the post-activation reduction of LC neuronal responsivity may be mediated in part through noradrenergic autoinhibitory mechanisms within the LC.