Adrenergic innervation of the monkey thalamus: an immunohistochemical study

The distribution and function of the neurotransmitter adrenaline in the primate brain are poorly understood. Biochemical studies have shown the presence of adrenaline or its biosynthetic enzyme, phenylethanolamine-N-methyltransferase, in the rat and human thalamus. However, the distribution of the adrenergic fibres in the thalamus has only been demonstrated in rats. We study the adrenergic innervation of the macaque monkey thalamus using immunohistochemistry against phenyletanolamine-N-methyltransferase. The distribution of phenyletanolamine-N-methyltransferase-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 paraventricular, parafascicular and mediodorsal nuclei. The paraventricular nucleus is densely innervated by adrenergic axons throughout, while the densest innervation of the parafascicular nucleus is located in its medial part and the strongest mediodorsal nuclear immunolabelling is found in its most posterior and medial region. Moderate or low concentrations of phenyletanolamine-N-methyltransferase-immunopositive fibres are present in the paratenial nucleus, and all parts of the central nucleus, nucleus reuniens, central medial nucleus, centromedian nucleus, medial geniculate body and medial pulvinar nucleus, while only scattered immunoreactive axons are found in other thalamic nuclei. The morphology of the phenyletanolamine-N-methyltransferase-immunoreactive axons is quite diverse, as they have different diameters and most are endowed with diversely-shaped varicosities. These findings are the first morphological 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. The thalamic nuclei innervated by adrenaline are connected to widespread limbic and associative cortical areas as well as to subcortical structures, in particular the neostriatum and amygdala. We hypothesize that thalamic adrenaline may be implicated in emotional, social and attentional mechanisms through its facilitation of co-ordinated action by these brain regions.