Dehydration Increases the Electrically Evoked Dopamine Release from the Neural and Intermediate Lobes of the Rat Hypophysis

Abstract

The effect of dehydration on dopamine (DA) release from the neural and intermediate lobes of the rat pituitary gland after electrical stimulation of the pituitary stalk was studied in vitro. The amount of DA released from pituitary lobes of dehydrated rats (no water for 72 h), in which the hormone secretion from the hypophysis, especially the neural lobe, is known to be greatly enhanced, was significantly higher when compared with that from control tissues. In control and dehydrated rats the evoked DA release from the intermediate lobe was higher than that from the neural lobe. After dehydration, the stimulus-evoked DA release from the neural lobe increased by more than 200%, whereas that from the intermediate lobe increased by only 60%, indicating a preferential activation of the DA system of the neural lobe. There was a decrease in DA released per pulse from the combined neuro-intermediate lobe of control rats with increasing number of stimulation pulses. In contrast, DA release per pulse from the neuro-intermediate lobe of dehydrated rats showed a tendency to increase with increasing number of pulses. As the availability of newly synthesized DA is believed to be the limiting factor for transmitter release, the present observations suggested a higher in vitro DA synthesis rate in the tissues from dehydrated rats. This agrees with in vivo results of Alperand his colleagues who found an increased accumulation of L-3,4-dihydroxyphenylalanine (DOPA) in neuro-intermediate lobes of dehydrated rats after inhibition of DOPA-decarboxylase.

Neurochemical Regulation of Oxytocin Secretion in Lactation*

“…Conflicting views of the role of tuberohypophysial DA have also emerged from studies in which the level of activity of this system has been monitored in correlation with physiologically induced changes in OT secretion. For example, dehydration increased both the synthesis (373) and release (374) of DA in neurohypophysis, but suckling was associated with a transient decrease of DA turnover in this structure (203). One potentially complicating factor is that the dopaminergic innervation of the neurohypophysis has been implicated in the regulation of PRL secretion from the anterior lobe of the pituitary gland (375), and as reviewed in the next section, this hormone can exert modulatory effects on the release of OT.…”

Vasopressin RNA in the neural lobe of the pituitary: dramatic accumulation in response to salt loading.

“…Over a 10-day period of salt loading, the NIL levels of VP RNA increased 25-fold and the OT RNA levels increased 8-fold. As a control, POMC mRNA levels were also measured and these fell by 50% in the NIL during the saline load-a change probably related to the accumulation of dopamine in the NIL during osmotic stimulation (19 POMC gene expression (15). POMC and growth hormone mRNA levels did not change in the anterior pituitary.…”