ADRENERGIC-INNERVATION OF THE MONKEY THALAMUS - AN IMMUNOHISTOCHEMICAL STUDY

The distribution and function of the neurotransmitter adrenaline in th e primate brain are poorly understood. Biochemical studies have shown the presence of adrenaline or its biosynthetic enzyme, phenylethanolam ine-N-methyltransferase, in the rat and human thalamus. However, the d istribution of the adrenergic fibres in the thalamus has only been dem onstrated in rats. We study the adrenergic innervation of the macaque monkey thalamus using immunohistochemistry against phenyletanolamine-N -methyltransferase. The distribution of enyletanolamine-N-methyltransf erase-immunoreactive fibres is markedly heterogeneous and principally restricted to those nuclei, or their portions, that are located in or close to the midline, with the highest density being found in the para ventricular, parafascicular and mediodorsal nuclei. The paraventricula r nucleus is densely innervated by adrenergic axons throughout, while the densest innervation of the parafascicular nucleus is located in it s medial part and the strongest mediodorsal nuclear immunolabelling is found in its most posterior and medial region. Moderate or low concen trations of enyletanolamine-N-methyltransferase-immunopositive fibres are present in the paratenial nucleus, and all parts of the central nu cleus, nucleus reuniens, central medial nucleus, centromedian nucleus, medial geniculate body and medial pulvinar nucleus, while only scatte red immunoreactive axons are found in other thalamic nuclei. The morph ology of the enyletanolamine-N-methyltransferase-immunoreactive axons is quite diverse, as they have different diameters and most are endowe d with diversely-shaped varicosities. These findings are the first mor phological evidence for the presence of adrenergic innervation in the primate thalamus and reveal that this innervation is highly selective, heterogeneous and more widely distributed in primates than in rats. T he thalamic nuclei innervated by adrenaline are connected to widesprea d limbic and associative cortical areas as well as to subcortical stru ctures, in particular the neostriatum and amygdala. We hypothesize tha t thalamic adrenaline may be implicated in emotional, social and atten tional mechanisms through its facilitation of co-ordinated action by t hese brain regions. (C) 1998 IBRO. Published by Elsevier Science Ltd.