Noradrenergic innervation of vasopressin‐and oxytocin‐containing neurons in the hypothalamic paraventricular nucleus of the macaque monkey: Quantitative analysis using double‐label immunohistochemistry and confocal laser microscopy

Previous reports on the rat and monkey hypothalamus have revealed a dense noradrenergic innervation within the hypothalamic paraventricular nucleus as assessed by dopamine‐β‐hydroxylase immunohistochemistry. These single‐label analyses were unable to delineate the cellular structures which receive this catecholaminergic innervation. Double‐label preparations in the rat hypothalamic paraventricular nucleus have demonstrated synaptic interactions between noradrenergic varicosities and magnocellular neurons. However, the density and distribution of varicosities contacting chemically identified magnocellular neurons have not been assessed at the light or electron microscopic level. In this report, single‐label immunohistochemistry was used to assess the morphology and distribution of vasopressin ‐ and oxytocin‐immunoreactive neurons within the macaque hypothalamic paraventricular nucleus. In addition, double‐label immunohistochemistry was combined with confocal laser scanning microscopy to quantify the number of dopamine‐β‐hydroxylase‐immunoreactive varicosities in apposition to magnocellular neurons expressing vasopressin or oxytocin immunoreactivity. The morphology of chemically identified neurons was also compared to magnocellular neurons in the monkey hypothalamic paraventricular nucleus which were filled with Lucifer Yellow in order to assess the somatodendritic labeling of the immunohistochemical preparation. Qualitative assessment of immunohistochemically identified magnocellular cells indicated that vasopressin‐ and oxytocin‐containing neurons are observed throughout the rostrocaudal extent of the monkey hypothalamic paraventricular nucleus, demarcating this structure from the surrounding anterior hypothalamus. The distribution of the two nonapeptides is complementary, with vasopressin‐immunoreactive neurons having a greater somal volume and located in a more medial aspect of the mid and caudal hypothalamic paraventricular nucleus relative to oxytocin‐immunoreactive perikarya. For the double‐label preparations, a series of confocal optical sections was assessed through the total somal volume of vasopressin‐ and oxytocin‐immunoreactive neurons along with the corresponding dopamine‐β‐hydroxylase‐immunoreactive varicosities in the same volume of tissue, generating a varicosity‐to‐neuron ratio which was further characterized morphologically to assess afferent input to the soma and proximal dendrites. Quantitative analysis revealed that vasopressin‐immunoreactive neurons received approximately two thirds of their dopamine‐β‐hydroxylase‐immunoreactive varicosities in apposition to the proximal dendrites and one third in apposition to the somata. Furthermore, vasopressin‐immunoreactive neurons received a greater innervation density than oxytocin‐immunoreactive neurons, which did not have a differential distribution of varicosities on the proximal dendrites and somata. The distribution of dopamine‐β‐hydroxylase‐immunoreactive afferents on magnocellular neurons in the hypothalamic paraventricular nucleus may reflect a physiological role of this circuit in terms of preferential release of vasopressin from magnocellular neurons upon noradrenergic stimulation.