A direct catecholaminergic projection from the brainstem to the neurohypophysis of the rat

The presence of the catecholamines dopamine and noradrenaline in the posterior pituitary is well documented. Dopaminergic terminals are thought to derive from cells in the periventricular hypothalamus including the rostral arcuate nucleus, but the origin of the noradrenergic terminals is uncertain. The majority of central noradrenaline-containing neurons are in the brainstem and lesions of the ventral noradrenergic tract significantly reduce the noradrenaline content of the neural lobe. We have explored the possibility of a direct noradrenergic projection from the brainstem to the neural lobe using as a retrograde tract tracer horseradish peroxidase alone or in combination with immunohistochemical detection of tyrosine hydroxylase. Horseradish peroxidase was injected into the neural lobes of 20 rats using a ventral approach, and the animals were perfusion fixed 12 h later. In all 20 cases, cells retrogradely labelled with horseradish peroxidase were found not only in the periventricular zone and magnocellular nuclei of the hypothalamus but also in the A2 region of the brainstem. In all six cases simultaneously processed for horseradish peroxidase and tyrosine hydroxylase immunohistochemistry, retrogradely labelled cells in both the arcuate nucleus and A2 region were found to be tyrosine hydroxylase-positive. These findings demonstrate the presence of a direct projection from catecholaminergic neurons in the A2 region of the brainstem to the neurohypophysis. Since catecholaminergic neurons in this region are known to be noradrenergic and lesions of the ventral noradrenergic tract deplete the neurohypophysis of noradrenaline, these neurons may represent an important source of noradrenaline in the neurohypophysis.