Distribution of dopamine-containing neurons and fibres in the feline medulla oblongata: A comparative study using catecholaminesynthesizing enzyme and dopamine immunohistochemistry

The distribution of dopamine-immunoreactive neurons and fibres in the feline medulla oblongata was examined by immunocytochemistry with antisera to the catecholamine-synthesizing enzymes tyrosine hydroxylase, dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase, and with antisera to the catecholamines dopamine and l-dihydroxyphenylalanine. Neurons immunoreactive for the catecholamine-synthesizing enzymes were found in two regions of the medulla, the ventrolateral A1 region and the dorsomedial A2 region. Double-staining studies with antisera to the enzymes indicated that a population of neurons within both regions were immunoreactive for tyrosine hydroxylase but not dopamine-β -hydroxylase or phenylethanolamine-N-methyltransferase, implying that they synthesize dopamine. Studies using the dopamine antisera demonstrated the presence of dopamine-immunoreactive neurons in both the ventrolateral and dorsomedial regions of the medulla; in the dorsomedial region, they were found in the area postrema, nucleus tractus solitarius and dorsal motor vagal nucleus, mainly at levels caudal to the obex. Dopamine-immunoreactive fibres were found in several areas of the medulla including the nucleus tractus solitarius, inferior olive, dorsal motor vagal, spinal trigeminal, hypoglossal, cuneate, gracile, and raphe nuclei. Double-staining studies with antisera to dopamine and dopamine-β-hydroxylase revealed a population of cells immunoreactive for dopamine alone. The presence of some double-stained neurons, however, implies some cross-reactivity of the dopamine antiserum with noradrenaline or adrenaline and/or recognition of dopamine present as a metabolic intermediary in some noradrenergic neurons. No l-dihydroxyphenylalanine-immunoreactive neurons were found in the medulla, although fibres were seen. These data provide evidence for the existence of catecholamine neurons which utilize dopamine as a final synthetic product within the medulla oblongata.