Is octopamine a ‘false transmitter’? Regional distribution and serial changes in octopamine and noradrenaline following locus coeruleus lesions

Abstract The effects of electrolytic lesions of the locus coeruleus (LC) on endogenous octopamine and noradrenaline levels were observed at various temporal intervals. Noradrenaline was assayed from halved tissue samples by high-performance liquid chromatography (HPLC) while octopamine was determined from the other halfs of the brain samples by radioenzymatic assay methods accompanied by chromatographic separation. Brains were dissected into 8 regions: cerebellum, anterior and posterior cortex, hippocampus, hypothalamus, medulla/pons, midbrain and striatum. Assays were performed at postlesion intervals of 4, 8, 13 and 18 days. The rates of depletion of noradrenaline at the tested intervals roughly matched those of octopamine in the anterior cortex, posterior cortex and striatum. In the cerebellum, hypothalamus, hippocampus and pons/medulla, levels of octopamine diminished later than those of noradrenaline, at times showing an initial increase within the first week after LC lesion. In the midbrain however, this pattern was reversed, octopamine levels initially decreasing in parallel with noradrenaline but subsequently recovering to significantly higher values. The results do not unequivocally support the hypothesis that octopamine is present in the brain solely as a ‘false transmitter’ in noradrenergic neurones. Neither, however, do they provide clear support for earlier contentions that octopamine is likely to play a role in central neurones as a synaptic transmitter, independent of the noradrenergic system.