Regulation of nigrostriatal and tuberoinfundibular-hypophyseal dopaminergic neurons

Studies were undertaken to investigate the observed sexual differences in tuberoinfundibular dopamine (DA) neuronal activity. The ability of prolactin to differentially alter the activity of tuberoinfundibular DA neurons in gonadectomized male and female rats was determined by measuring the rate of accumulation of dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase with NSD 1015 in the median eminence, the terminal region of these neurons. The rate of DOPA accumulation in control female rats was 2–3 times greater than that in male rats, confirming previous observations. Estradiol benzoate (25 µg/kg s.c. × 3 days), which increases serum prolactin, increased the rate of DOPA accumulation in the median eminence of both male and female rats. This effect was not observed in hypophysectomized rats, confirming that the estradiol-induced increase in DOPA accumulation is not due to direct action of estrogen, but to an indirect effect via increases in prolactin secretion. It was unexpectedly found, however, that hypophysectomy per se dramatically reduced the rate of DOPA accumulation in the median eminence of female but not male rats. Tuberoinfundibular neurons in the hypophysectomized female rats did, however, retain the capacity to respond to intracerebroventricular (ICV) administered prolactin (1 µg). Multiple injections of the DA agonist, bromocriptine, which like hypophysectomy decreases circulating concentrations of prolactin also selectively decreased the rate of DOPA accumulation in the median eminence of female rats with little effect in the male. These data suggest that the tuberoinfundibular neurons in the female are more sensitive to circulating levels of prolactin than those in the male. To test this possibility, the capacity of prolactin to stimulate DOPA accumulation in the median eminence was compared in male and female rats. Rat prolactin (0.1–10µg) was administered ICV via previously implanted cannulas. When measured 12 h after prolactin administration, DOPA accumulation in the median eminence of female rats was significantly increased at a lower dose than in the male (0.1 vs. 1.0 µg). This difference in sensitivity to prolactin in male and female rats was clearly demonstrated in rats which were first pretreated with bromocriptine and then the effect of graded doses of ICV prolactin determined. In these animals, doses of prolactin as low as 0.03 µg stimulated DOPA accumulation in the median eminence of female rats, while the minimal effective dose in the male rat was unchanged (1.0 µg). These results indicate that the sensitivity of tuberoinfundibular DA neurons to prolactin is markedly different in male and female rats.

mentioning

confidence: 84%

Sexual Differences in the Sensitivity of Tuberoinfundibular Dopamine Neurons to the Actions of Prolactin

Demarest¹,

Moore²

1981

“…In contrast, when DA is the main inhibi tory factor as is the case of PRL secretion, perinatal administration of the haloperidol increased PRL serum levels during the 1st postnatal month [2,3], The mechanism of these alterations at receptor level cannot be directly deduced from this RIA study but it must be related to the capacity of haloperidol to block the D2 subtype of dopaminergic receptors which are less abundant in the hypothalamus than in the pituitary gland. Hypothalamic D2 receptors are located in the dorsomedial mucleus but are scarce in areas containing TIDA neu rons [28], whose somatodendritic fields are supposedly devoid of D2 autoreceptors [29], Nevertheless, the coexis tence of GHRF and DA in some hypothalamic neurons [30] must be borne in mind as a factor that may be involved in our results albeit by a mechanism not fully understood yet. In the pituitary gland D2 receptors are located on lactotropes.…”

Section: Discussionmentioning

confidence: 87%

Effects of Perinatal Administration of Haloperidol on GH Pituitary Contents and Serum Levels during the First Postnatal Month

Íñiguez

Tamayo²,

Gómez³

et al. 1995

Perinatal dopaminergic blockade with haloperidol caused PRL increases in rat pituitary gland and serum which persisted during the first postnatal month. However the effects of dopamine on the synthesis and secretion of GH at these early ages are unknown. With the aim of investigating the effects of this blockade on postnatal GH secretion, haloperidol (1 mg/kg i.p.) was injected daily to pregnant rats from gestational day 16 until delivery and to pups from untreated mothers between postnatal days 2-6. GH pituitary contents and serum levels were measured weekly by RIA during the first postnatal month. The results showed that haloperidol induced a long-term increase in GH pituitary contents as well as a transient increase in serum levels. The results in serum are similar to those from human neonates indicating that dopamine plays a more important role as controller of the GH secretion in newborns than in adults.

“…On the other hand, the conflicting results obtained with the «MT and dopa accumulation methods may suggest that they are measuring different phenomena. Under most circumstances changes in the rates of «MT-induced decline of catecholamines are reflected in similar changes in the rates of dopa accumulation [9,18]. One must recognize, however, that there are instances where these two biochemi cal techniques do not produce comparable results.…”

Section: Discussionmentioning

confidence: 95%

Biochemical Indices of Catecholaminergic Neuronal Activity in the Median Eminence during the Estrous Cycle of the Rat

Demarest¹,

Johnston²,

Moore³

1981

Self Cite

An attempt was made to correlate the changes in blood concentrations of prolactin and luteinizing hormone (LH) which occur during the estrous cycle of rats with changes in biochemical estimates of the activities of dopamine (DA)- and norepinephrine (NE)-containing neurons in the median eminence. The serum concentrations of prolactin and LH markedly increased on the afternoon and evening (16.00–24.00 h) of proestrus. The steady state concentrations of NE and DA in the median eminence did not change at any time during the estrous cycle, but the α-methyltyrosine-induced decline of NE was increased during the morning of proestrus (8.00–12.00 h) while the decline of DA was decreased (12.00 h). The rate of total catecholamine synthesis (dopa accumulation after a decarboxylase inhibitor) in the median eminence was not markedly altered at any time during the estrous cycle; the lack of change of dopa accumulation on proestrus may be due, in part, to opposing changes in the rates of amine synthesis in NE and DA neurons.