Chronic Posterior Pituitary Lobectomy: Prolonged Elevation of Plasma Prolactin and Interruption of Cyclicity

Abstract

We previously reported that the posterior pituitary dopaminergic system participates in the inhibition of prolactin (PRL) secretion in both male and lactating female rats. However, posterior pituitary lobectomy (Lobex) of urethane-anesthetized cycling rats resulted in an elevation in plasma PRL for a short time only. This raises a question regarding the importance of input from the posterior pituitary to the control of PRL secretion during the estrous cycle. The objectives of this study were (1) to examine the chronic effects of Lobex on plasma PRL levels in conscious rats and (2) to determine whether the absence of input from the posterior pituitary interferes with estrous cyclicity. Lobex or sham lobectomy were performed under Brevital anesthesia in estrous rats. Blood was collected from a jugular cannula at hourly intervals on the day of surgery and at 09.00, 13.00, and 17.00 h during the following 4 days. Daily water consumption and vaginal cyclicity were monitored for 14 and 20 days, respectively. Within 2 h after Lobex, the plasma PRL levels rose 3- to 4-fold and remained elevated for 3 days before declining to near control levels on the 4th day. None of the Lobex rats resumed cyclicity within 3–4 days, 50% had an interruption of cyclicity for 4–10 days, and the remainder were noncyclic for more than 11 days. Upon resumption of cyclicity, Lobex rats had 11.3 ± 0.4 oviductal ova which is within the normal range for intact ovulating rats. In contrast, 60% of the sham-operated rats resumed cyclicity within 4 days with an afternoon proestrous surge of PRL, whereas 30% became pseudopregnant, as judged by diurnal peaks of plasma PRL levels and a sustained leukocyte smear for 11–13 days. The daily water consumption of Lobex rats was 3-fold higher than that of sham-lobectomized rats for at least 2 weeks. It is concluded that: (1) Lobex results in a prolonged elevation of plasma PRL levels in conscious rats; (2) the eventual decline in plasma PRL is more likely due to a compensatory activation of the hypothalamic dopaminergic system by the Lobex-induced rise in plasma PRL than to a regeneration of the posterior lobe or restoration of its function; (3) the Lobex-induced PRL rise appears to be the cause of interruption of cyclicity, and (4) the data are consistent with the hypothesis that the dopaminergic inhibition of PRL secretion involves two interdependent systems: one consists of the hypothalamus – long portal vessels – and the second comprises of the posterior pituitary -short portal vessels. Depending upon the endocrine conditions and the nature of hormonal or neuronal stimuli, either one or both systems may be activated.

Immunoneutralization of Endogenous Opioid Peptides Prevents the Suckling-Induced Prolactin Increase and the Inhibition of Tuberoinfundibular Dopaminergic Neurons

“…However, our results do not rule out the possibility that these peptides may also affect the activity of the THDA neurons [12, 13, 14, 15, 16, 17]and/or the activity of prolactin-releasing factors [7, 8, 9, 10], for review see 11]both of which play an important role in prolactin release during lactation. We have previously shown that while β-endorphin antiserum abolished the prolactin secretory response to suckling, it did not prevent the suckling-induced inhibition of TIDA neurons [37], indicating that this peptide has a different mechanism of action.…”

“…While the TIDA system is considered to be a major regulator of prolactin secretion, the tuberohypophyseal dopaminergic (THDA) system, also plays an important role in the regulation of prolactin release during lactation [12, 13, 14, 15, 16, 17]. These neurons arise from the rostral portion of the arcuate nucleus and terminate in the intermediate and neural lobes of the pituitary gland [for review see 6].…”

Characterization of the Dopaminergic Input to the Pituitary Gland throughout the Estrous Cycle of the Rat

“…It has been suggested, however, that IL and NL may contribute greater than 50% of the DA in the AL, presumably through the SPV [6]. Given the importance ascribed to the neurointermediate lobe in the control of the surge of PRL secreted into serum on PRO [13, 14, 15], we compared the changing concentrations of DA and DOPAC in AL, IL, and NL throughout the 4-day estrous cycle of the rat in order to gain insight into the relative contribution of each lobe to the regulation of the secretion of PRL.…”

Prolactin Secretion in Patients with Idiopathic Diabetes Insipidus.

“…It In rats, acute or chronic posterior pituitary lobectomy produces an increase in plasma prolactin [3,4,5]. On the other hand, it is well known that prolactin physiologically regulates the water and electrolyte balance in teleost fish [6].…”

“…However, recent studies strongly suggest that the posterior pituitary plays an important role in prolactin secretion. Ben Jonathan et al [3,4,5] reported that posterior pituitary lobectomy signi- …”