Neurohypophyseal dopamine biosynthesis in the spontaneously hypertensive rat

A study was performed to investigate the neurohypophyseal dopaminergic axis in terms of its biosynthetic activity and possible changes associated with spontaneous hypertension (SHR). An in vitro system was used in which isolated neuro-intermediate lobes were incubated with the catecholamine precursor, 3H-tyrosine. Dopamine (DA) content and 3H-DA were monitored using electrochemical detection coupled with high pressure liquid chromatographic separation. A time course study showed that there was significant incorporation of 3H-tyrosine into 3H-DA. In the SHR, both neurohypophyseal DA content and biosynthetic activity were reduced. Tissue levels of 3H-DA decreased from 651 to 297 dpm/posterior pituitary. A test of the effect of dehydration on neurohypophyseal dopaminergic activity revealed that water deprivation (48 hrs) caused an increase in DA biosynthesis in the hypertensive, but not the normotensive animal. This may have been due to a greater stimulation of the neurohypophyseal axis in the SHR since these animals showed significantly higher plasma vasopressin levels and hematocrits in response to dehydration. These results demonstrate that the neurohypophysis contains an active dopaminergic system which is altered in genetic hypertension.