COTRANSMITTER-MEDIATED LOCUS-COERULEUS ACTION ON MOTONEURONS

This article reviews evidence for a direct noradrenergic projection from the dorsolateral pontine tegmentum (DLPT) to spinal motoneurons. The existence of this direct pathway was first inferred by the observation that antidromically evoked responses occur in single cells in the locus coeruleus (LC), a region within the DLPT, following electrical stimulation of the ventral horn of the lumbar spinal cord of the cat. We subsequently confirmed that there is a direct noradrenergic pathway from the LC and adjacent regions of the DLPT to the lumbar ventral horn using anatomical studies that combined retrograde tracing with immunohistochemical identification of neurotransmitters. These anatomical studies further revealed that many of the noradrenergic neurons in the LC and adjacent regions of the DLPT of the cat that send projections to the spinal cord ventral horn also contain colocalized glutamate (Glu) or enkephalin (ENK). Recent studies from our laboratory suggest that Glu and ENK may function as cotransmitters with norepinephrine (NE) in the descending pathway from the DLPT. Electrical stimulation of the LC evokes a depolarizing response in spinal motoneurons that is only partially blocked by alpha(1), adrenergic antagonists. In addition, NE mimicks only the slowly developing and not the fast component of LC-evoked depolarization. Furthermore, the depolarization evoked by LC stimulation is accompanied by a decrease in membrane resistance, whereas that evoked by NE is accompanied by an increased resistance. That Glu may be a second neurotransmitter involved in LC excitation of motoneurons is supported by our observation that the excitatory response evoked in spinal cord ventral roots by electrical stimulation of the LC is attenuated by a non-N-methyl-D-aspartate glutamatergic antagonist. ENK may participate as a cotransmitter with NE to mediate LC effects on lumbar monosynaptic reflex (MSR) amplitude. Electrical stimulation of the LC has a biphasic effect on MSR amplitude, facilitation followed by inhibition. Adrenergic antagonists block only the facilitatory effect of LC stimulation on MSR amplitude, whereas the ENK antagonist naloxone reverses the inhibition. The chemical heterogeneity of the cat DLPT system and the differential responses of motoneurons to the individual cotransmitters help to explain the diversity of postsynaptic potentials that occur following LC stimuli.