Changes in the local metabolism of dopamine in the anterior and neural lobes but not in the intermediate lobe of the pituitary gland during nursing

Dopamine (DA), produced by tubero-infundibular dopaminergic (TIDA) neurons of the arcuate nucleus (ARN) is the established inhibitor of the secretion of prolactin (PRL). Changes in dopaminergic (DAergic) neuronal activity in the median eminence–long portal vessels (ME–LPV) and/or the concentration of DA in the anterior lobe (AL) are inversely related to the secretion of PRL. However, conflicting reports concerning DAergic neuronal activity during the suckling-induced release of PRL persist. In addition to TIDA neurons, PeVN-hypophysial DAergic (PHDA) and tubero-hypophysial DAergic (THDA) neurons which, respectively, innervate the intermediate lobe (IL) and the IL/neural lobe (NL) also have a significant role. We measured the concentrations of DA and its main metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the median eminence and the three pituitary lobes of lactating mothers. Concentrations of DA and DOPAC from tissues and the concentration of PRL in plasma were measured by HPLC-EC and RIA, respectively. There were no changes in the concentration of DA and DOPAC of the IL due to the suckling stimulus. In the NL, a decrease in the concentration of DOPAC was detected due to the suckling stimulus. In addition, there were no changes of DA or DOPAC in the outer zone of the AL (AL-OZ) due to suckling. However, a decrease in the concentrations of DA and DOPAC was detected in the inner zone of the AL (AL-IZ). These data suggest lactotrophs from the AL-IZ are responsible for the changes in the concentration of plasma PRL in response to the suckling stimulus. In addition TIDA and THDA neurons, but not PHDA neurons, regulate the control of the secretion of PRL in response to suckling.     

Ovarian steroids influence the activity of neuroendocrine dopaminergic neurons

The secretion of prolactin (PRL) from the anterior lobe (AL) of the pituitary gland is tonically inhibited by dopamine (DA) of hypothalamic origin. While ovarian steroids play a role in the regulation of the secretion of PRL, their effect on all three populations of hypothalamic neuroendocrine dopaminergic neurons is not fully understood. In this study we describe the effects of ovarian steroids on regulation of the release of DA from tuberoinfundibular dopaminergic (TIDA), tuberohypophyseal dopaminergic (THDA) and periventricular-hypophyseal dopaminergic (PHDA) neurons. Adult female rats were bilaterally ovariectomized (OVX) and, 10 days following ovariectomy (day 0), injected with corn oil (vehicle), estrogen, or estrogen plus progesterone (day 1). Animals were sacrificed every 2 h from 09.00 to 21.00 h by rapid decapitation. Trunk blood was collected and the concentration of PRL in serum was determined by radioimmunoassay. The median eminence (ME) and the AL, intermediate (IL) and neural (NL) lobes of the pituitary gland were dissected and the concentration of DA and DOPAC in each was measured by HPLC-EC. OVX rats presented small but significant increases in the secretion of PRL at 15.00 and 17.00 h. Replacement of estrogen or estrogen plus progesterone increased the basal concentration of PRL. Moreover, injection of estrogen only, or estrogen plus progesterone increased the concentration of PRL in serum at 15.00 h through 19.00 h, respectively, followed by a decrease to baseline thereafter. The turnover of DA in the ME and NL of OVX rats increased at 13.00 and returned to low levels. Turnover of DA in the IL of OVX rats increased in the morning by 11.00 h and remained elevated before decreasing by 17.00 h. The turnover of DA in the ME, IL and NL of OVX rats increased by 19.00 h. Injection of estrogen advanced the increase of TIDA activity by 2 h in the ME compared to OVX rats. Moreover, administration of estrogen suppressed the activity of THDA and PHDA neurons in the afternoon compared to OVX rats. In estrogen plus progesterone-treated rats, the activity of hypothalamic neuroendocrine dopaminergic neurons terminating in the ME, IL, and NL was inhibited prior to the increase in the secretion of PRL. The concentration of DA in the AL diminished prior to the estrogen-induced increase of PRL. Administration of progesterone, in concert with estrogen, delayed the increase of PRL in serum and the decrease of DA in the AL, compared to estrogen-treated rats, by 4 h. These data suggest a major role for ovarian steroids in controlling increases in the secretion of PRL by not only stimulating PRL release from lactotrophs, but also by inhibiting the activity of all three populations of hypothalamic neuroendocrine DAergic neurons.     

Ovarian steroid hormones modulate circadian rhythms of neuroendocrine dopaminergic neuronal activity

Dopamine (DA) is the primary inhibitor of prolactin (PRL) secretion. Three populations of neuroendocrine dopaminergic neurons (NDNs) designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal DAergic (PHDA) neurons regulate PRL secretion. Given that ovarian steroids modulate both DA release and PRL secretion independently, we characterized the role of steroid hormones in coupling rhythmic NDN activity and PRL secretion. OVX rats under a standard 12:12 L:D cycle (L:D), constant dark (DD), or a 6-h phase-delayed L:D cycle (pdL:D) were treated with Estradiol-17beta (E) or E and Progesterone (E+P). NDN activity, defined by DA:DOPAC ratio in nerve terminals, was determined by HPLC-EC. E or E+P stimulated PRL surges in L:D that persisted under DD. In TIDA neurons, E or E+P treatment reduced the amount of DA released under L:D and DD and advanced the rhythm of DA turnover. E and E+P treatment reduced THDA and PHDA neuron activity under L:D, but did not affect these rhythms under DD. Circadian rhythms of PRL, corticosterone and DA turnover in NDN terminals from steroid treated rats entrained to a pdL:D cycle within 7 days. Therefore, ovarian steroids differentially adjust the timing and magnitude of NDN activity to facilitate coupling of DA release and PRL secretion.     

Hypothalamic neurotransmitter function in experimentally induced hyperprolactinemia

It is known that animals or patients bearing a prolactin (PRL)-secreting tumor (PST) do not suppress PRL levels after administration of indirectly acting dopamine agonists, namely nomifensine (Nom), and are not responsive to the PRL releasing effect of antidopaminergic drugs and opioid peptides. Since the action of these drugs is mediated through the tuberoinfundibular dopaminergic (TIDA) system, these findings have been taken to indicate that animals and humans bearing prolactinomas have a defective TIDA function. Alternatively, PRL unresponsiveness to these drugs could be due to hyperfunction of TIDA system for the feedback action of high PRL levels. To clarify whether hypo- or hyperfunction of the TIDA system was responsible for such behaviour, we tested the effect of a synthetic opioid peptide (FK 33–824), a DA receptor antagonist, domperidone (Dom), and of Nom on PRL secretion in two experimental models of non-tumoral hyperprolactinemia, i.e. rats bearing ectopic pituitaries since 3 days (TP rats), or treated with ovine PRL (oPRL 250 μg, twice daily for 3 days), in which existence of an increased TIDA function has been demonstrated. FK 33–824 (0.5 mg/kg i.p.) increased significantly plasma PRL levels in control rats but failed to do so in TP rats and it elicited a significantly lower PRL response than in controls in rats treated with oPRL. In both experimental models, a PRL secretagogue, e.g. 5-hydroxytryptophan (50 mg/kg i.p.), elicited the same response as in controls, indicating that the pituitayr PRL pool was preserved. Kinetic characteristics of opioid binding sites in the hypothalamus of hyperprolactinemic rats did not differ from those of controls, and also hypothalamic Met-enkephalin concentrations were similar to those of controls. These findings would exclude a defect of endogenous opioid neurotransmission as responsible for the impaired PRL stimulation by the enkephalin analog. Supporting this were the findings that oPRL-treated rats exhibited a growth hormone response to FK 33–824 and a rise in plasma luteinizing hormone after naloxone (5 mg/kg s.c.) similar to those present in controls. These data indicate that the defective PRL responsiveness to opioids has to be attributed to the impairment of a neurotransmitter e.g. DA, functionally located at a site ‘downstream’ from opioid receptors. As already shown for oPRL-treated rats, TP rats exhibited a faster turnover rate of DA in the median eminence of the hypothalamus. In keeping with the idea that the hypothalamic dopaminergic system was involoved in the defective PRL responsiveness to opioids, Dom (50 μg/kg i.p.) induced aignificantly lower increase of PRL levels in TP and oPRL-injected rats than in controls. All these data demonstrate that opioid peptides and DA receptor antagonists are unable to discriminate the functional state of TIDA system. In fact, they fail to elicit a rise in plasma PRL in hyperprolactinemic states related to either TIDA hypo- (PST rats and humans) or hyperfunction (TP and oPRL-treated rats). In contrast to FK 33–824 and Dom, Nom (10 mg/kg i.p.), a drug which fails to lower PRL secretion in animals and patients with prolactinoma, did suppress plasma PRL levels of oPRL-treated rats as those of controls. The latter data indicate that indirectly acting DA agonists can discriminate the functional state of TIDA system; they fail to lower PRL secretion only when TIDA function is defective.     

The effect of neurointermediate lobe denervation on hypothalamic neuroendocrine dopaminergic neurons

The contribution of tuberohypophyseal and periventricular-hypophyseal dopaminergic neurons to the regulation of the secretion of prolactin (PRL) has yet to be clarified. In this study, we used pituitary stalk compression to disrupt hypothalamic neural input to the neurointermediate lobe (NIL). Neurointermediate lobe denervation (NIL-D) selectively disrupts the axons of tuberohypophyseal and periventricular-hypophyseal dopaminergic neurons, while leaving tuberoinfundibular dopaminergic neurons and the vascular supply of the pituitary gland intact. NIL-D was performed in ovariectomized (OVX) rats. The concentration of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME) and various regions of the pituitary gland of OVX and OVX+NIL-D rats were measured by HPLC-EC. The concentration of PRL, α-melanocyte stimulating hormone (α-MSH), and luteinizing hormone (LH) in serum were determined by radioimmunoassay. Successful NIL-D was confirmed by increased water intake. One week after NIL-D, serum PRL and α-MSH were elevated, but there was no change in the concentration of LH in serum. The concentration of DA was increased in the median eminence (ME), decreased in the outer zone of the anterior lobe (AL-OZ), as well as the intermediate (IL) and neural lobes (NL), and remained unchanged in the inner zone of the anterior lobe (AL-IZ). The concentration of DOPAC was increased in the ME and NL, decreased in the IL, and remained unchanged in both the AL-IZ and AL-OZ. These data confirm that pituitary stalk compression denervates the NIL. Moreover, decreases in the concentration of DA in the IL and AL-OZ, coupled with elevation of serum PRL and α-MSH indicate that DA from the NIL contributes to the increased inhibition of the secretion of PRL and α-MSH in OVX rats.     

Distribution of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylglycol (DOPEG) in microdissected brain structures and the pituitary gland: Metabolite changes in the median eminence in response to hyperprolactinemia and suckling

Summary Dopamine (DA), norepinephrine (NE), epinephrine (E), 3,4-dihydroxyphenylglycol (DOPEG) and dihydroxyphenylacetic acid (DOPAC) were determined simultaneously by a radioenzymatic, thin-layer chromatographic assay able to detect 1–10 pg of the parent compounds and 80–120 pg of their metabolites. A localization study of these compounds in 20 micro-dissected hypothalamic and limbic structures and the anterior and posterior pituitary glands of male rats was completed. DOPAC was detectable in 14 of 22 structures with the lowest DOPAC/DA ratio being found in the caudate nucleus (7.1%) and the highest in the medial aspect of the ventromedial nucleus of the hypothalamus (422.0%). There was a higher DOPAC/DA ratio in the lateral (21.5%) than in the medial (11.3%) portion of the median eminence suggesting that a greater portion of released DA in the medial median eminence enters the portal circulation. DOPEG was detectable in 6 of 22 structures with DOPEG/NE ratios ranging from 8% (interstitial nucleus of the stria terminalis, ventral aspect) to 32% (medial median eminence). A poor correlation exists between DOPAC and DA concentrations in the various brain regions while there was a stronger relationship between DOPEG and NE concentrations. Male rats were rendered hyperprolactinemic for 48 hours with injections of ovine prolactin (oPRL) every 8 hours (4 mg/kg body weight sc). In such rats there was a suppression of endogenous rat PRL (rPRL) secretion, the DOPAC/DA ratio increased 2.2-fold in the medial (MEm) and 1.9-fold in the lateral median eminence (ME1), and the DA concentration in the anterior pituitary also increased 2.6-fold. In 10 day postpartum lactating rats, suckling produced marked increases in serum rPRL but no change in DOPAC/DA ratios in the ME or in the DA concentration in the anterior pituitary. The data reveal a wide range of DOPAC/ DA ratios (7–422%) in brain regions containing cell bodies, axons and terminals of the different dopaminergic neuronal tracts in brain and pituitary.Considering the DOPAC/DA ratios in the MEm and ME1, it is suggested that a large perturbation of dopaminergic transmission produces a significant ratio change while a smaller perturbation is not detected by this index of neuronal metabolism.Supported by NIH grants NS-14611 (MS) and HD-15955 awarded to MS and Dr. Phyllis M. Wise.Recipient of NIH National Research Service Award AG-05351.Recipient of NIH National Research Service Award HD-06481.Recipient of NIH Research Career Development Award NS-00731.     

Kisspeptin Regulates Tuberoinfundibular Dopaminergic Neurones and Prolactin Secretion in an Oestradiol‐Dependent Manner in Male and Female Rats

Prolactin (PRL) secretion is inhibited by hypothalamic dopamine. Kisspeptin controls luteinising hormone (LH) secretion and is also involved in PRL regulation. We further investigated the effect of kisspeptin‐10 (Kp‐10) on the activity of tuberoinfundibular dopaminergic (TIDA) neurones and the role of oestradiol (E₂) in this mechanism. Female and male rats were injected with i.c.v. Kp‐10 and evaluated for PRL release and the activity of dopamine terminals in the median eminence (ME) and neurointermediate lobe of the pituitary (NIL). Kp‐10 at the doses of 0.6 and 3 nmol increased plasma PRL and decreased 4‐dihydroxyphenylacetic acid (DOPAC) levels in the ME and NIL of ovariectomised (OVX), E₂‐treated rats but had no effect in OVX. In gonad‐intact males, 3 nmol Kp‐10 increased PRL secretion and decreased DOPAC levels in the ME but not in the NIL. Castrated males treated with either testosterone or E₂ also displayed increased PRL secretion and reduced ME DOPAC in response to Kp‐10, whereas castrated rats receiving oil or dihydrotestosterone were unresponsive. By contrast, the LH response to Kp‐10 was not E₂‐dependent in either females or males. Additionally, immunohistochemical double‐labelling demonstrated that TIDA neurones of male rats contain oestrogen receptor (ER)‐α, with a higher proportion of neurones expressing ERα than in dioestrous females. The dopaminergic neurones of periventricular hypothalamic nucleus displayed much lower ERα expression. Thus, TIDA neurones express ERα in male and female rats, and kisspeptin increases PRL secretion through inhibition of TIDA neurones in an E₂‐dependent manner in both sexes. These findings provide new evidence about the role of kisspeptin in the regulation of dopamine and PRL.     

Effect of electrical stimulation of the arcuate nucleus on neurochemical estimates of tuberoinfundibular and tuberohypophysial dopaminergic neuronal activities

The activity of nigrostriatal dopaminergic neurons has been estimated biochemically by measuring the rates of dopamine (DA) synthesis (accumulation of dihydroxyphenylalanine (DOPA) after NSD 1015) and turnover (decline of DA concentrations after alpha-methyltyrosine) in the striatum. It has been assumed that the activities of tuberoinfundibular dopaminergic (TIDA) and tuberohypophysial dopaminergic (THDA) neurons can also be estimated by making the same measurements in the terminals of these neurons in the median eminence and the posterior pituitary, respectively. In the present study, this assumption was tested directly by measuring the rates of DA synthesis and turnover in the median eminence and posterior pituitary following electrical stimulation of TIDS and THDA cell bodies in the arcuate nucleus. Electrical stimulation of the arcuate nucleus increased the rate of DOPA accumulation and the alpha-methyltyrosine-induced decline of DA concentrations in the median eminence and in the neural and intermediate lobes of the posterior pituitary. gamma-Butyrolactone (GBL), an anesthetic that selectively inhibits DA impulse flow, reduced the rates of DA synthesis and turnover in the median eminence. GBL also increased prolactin secretion which is tonically inhibited by DA released from TIDA neurons. Serum prolactin levels were significantly decreased by arcuate nucleus stimulation in GBL-anesthetized rats. These results indicate that the rates of DA synthesis and turnover within the median eminence and posterior pituitary reflect the activities of TIDA and THDA neurons, respectively.     

Food deprivation alters dopamine utilization in the rat prefrontal cortex and asymmetrically alters amphetamine-induced rotational behavior

The effects of 24 and 48 h of food deprivation on changes in the activity of dopaminergic (DAergic) neurons and d-amphetamine-induced rotational behavior were studied in male and female Long-Evans rats. Food deprivation selectively altered 3,4-dihydroxyphenylacetic acid (DOPAC) in the medial prefrontal cortex (PFC) but not in the nucleus accumbens or striatum: PFC DOPAC was significantly increased and decreased bilaterally after 24 and 48 h of food deprivation, respectively. Left > right hemispheric asymmetries were seen for DOPAC and DOPAC/DA in the control animals. In a separate experiment, 24 h of food deprivation enhanced right rotational behavior, while 48 h significantly increased left rotational behavior. The results are discussed in terms of food deprivation's effects on mesocortical DAergic neurons, previous work on cortical modulation of striatal function and how these effects on rotational behavior may be determined by brain asymmetry.     

Chronic Hyperprolactinemia and Changes in Dopamine Neurons

The tuberoinfundibular dopaminergic (TIDA) system is known to inhibit prolactin (PRL) secretion. In young animals this system responds to acute elevations in serum PRL by increasing its activity. However, this responsiveness is lost in aging rats with chronically high serum PRL levels. The purpose of this study was to induce hyperprolactinemia in rats for extended periods of time and examine its effects on dopaminergic systems in the brain. Hyperprolactinemia was induced by treatment with haloperidol, a dopamine receptor antagonist, and Palkovits' microdissection technique in combination with high-performance liquid chromatography was used to measure neurotransmitter concentrations in several areas of the brain. After 6 months of hyperprolactinemia, dopamine (DA) concentrations in the median eminence (ME) increased by 84% over the control group. Nine months of hyperprolactinemia produced a 50% increase in DA concentrations in the ME over the control group. However, DA response was lost if a 9-month long haloperidol-induced hyperprolactinemia was followed by a 11

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month-long extremely high increase in serum PRL levels produced by implantation of MMQ cells under the kidney capsule. There was no change in the levels of DA, norepinephrine (NE), serotonin (5-HT), or their metabolites in the arcuate nucleus (AN), medial preoptic area (MPA), caudate putamen (CP), substantia nigra (SN), and zona incerta (ZI), except for a decrease in 5-hydroxyindoleacetic acid (5-HIAA) in the AN after 6-months of hyperprolactinemia and an increase in DA concentrations in the AN after 9-months of hyperprolactinemia. These results demonstrate that hyperprolactinemia specifically affects TIDA neurons and these effects vary, depending on the duration and intensity of hyperprolactinemia. The age-related decrease in hypothalamic dopamine function may be associated with increases in PRL secretion.