Glutamate is considered to be a major excitatory neurotransmitter in the central nervous system. The presence of glutamate-like immunoreactive neurons in the rodent locus coeruleus has been reported previously. In this study we used both immunohistochemical and electrophysiological techniques to answer two major questions: (1) Is there any glutamate-like immunoreactivity in the catecholaminergic coeruleospinal system of the cat? (2) What is the physiological role, if any, of glutamate in descending locus coeruleus control of spinal motoneurons? Following injections of rhodamine-labeled latex microspheres or Fast Blue into the seventh lumbar segment of the spinal cord of the cat, retrogradely labeled cells were found throughout the rostrocaudal extent of the dorsolateral pontine tegmentum. They were primarily observed in the nucleus locus coeruleus and the Kolliker-Fuse nucleus. Some labeled cells were also present in the nucleus subcoeruleus and, to a lesser extent, in the parabrachial nuclei. Data from immunohistochemical studies indicate that 86% of all dorsolateral pontine tegmentum neurons that project to the spinal cord contain glutamate-like immunoreactivity, and 77% co-contain both glutamate- and tyrosine hydroxylase-like immunoreactivity. Electrical stimulation (four pulses of 500 μs duration at 500 Hz; intensity = 50–200 μA) of the locus coeruleus, in decerebrate cats, consistently induced lumbar motoneuron discharges recordable ipsilaterally as ventral root responses. These motoneuronal responses were reversibly antagonized following chemical inactivation of noradrenergic locus coeruleus neurons by local infusion of the α 2 -adrenergic agonist clonidine, suggesting the locus coeruleus neurons to be the main source of evoked ventral root responses. Additionally, the evoked ventral root responses were reversibly reduced by 34.20 ± 4.45% (mean ± S.E.M.) upon intraspinal injections of the non- N-methyl-d-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the ventral horn of seventh lumbar spinal cord segment (three to four injections, 20 nmol in 0.2 μl of 0.1 M Tris-buffered saline for each injection). Similar volumes of vehicle injections had no significant effect on the locus coeruleus-evoked ventral root responses. These ventral root responses were also partially blocked (62.30 ± 11.76%) by intravenous administration of the α 1 -adrenergic receptor antagonist prazosin (20 μg/kg). In the light of several anatomical reports of noradrenergic and glutamatergic terminals in close contact with spinal motoneurons, our present findings suggest that the locus coeruleus-evoked ventral root response probably involves the synaptic release of both norepinephrine and glutamate onto lumbar motoneurons. It is also possible that such motoneuronal responses may arise from the locus coeruleus's effect on promoting the interneuronal release of glutamate.The present data indicate that glutamate is co-localized with tyrosine hydroxylase in the majority of the dorsolateral pontine tegmentum neurons of the cat, including those neurons that project to the ventral horn of the seventh lumbar segment of the spinal cord. Physiologically, glutamate in the pontospinal system is probably involved in the mediation of descending control of lumbar motoneurons, at least in part, via non- N-methyl-d-aspartate receptors. The present results also confirm a previous finding from this laboratory that α 1 -adrenergic receptors play an important role in locus coeruleus descending control of spinal motoneurons. In addition, the possible interactions between co-localized glutamate and norepinephrine and their effects on spinal motoneurons are discussed.
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