Tuberoinfundibular Dopaminergic Neuronal Activity Research Articles

RFamide-related Peptide 3 Signaling via Neuropeptide FF Receptor Stimulates Prolactin Secretion in Female Rats.

The RF-amide peptides comprise a family of neuropeptides that includes the kisspeptin (Kp), the natural ligand of kisspeptin receptor (Kiss1r), and the RFamide-related peptide 3 (RFRP-3) that binds preferentially to the neuropeptide FF receptor 1 (Npffr1). Kp stimulates prolactin (PRL) secretion through the inhibition of tuberoinfundibular dopaminergic (TIDA) neurons. Because Kp also has affinity to Npffr1, we investigated the role of Npffr1 in the control of PRL secretion by Kp and RFRP-3. Intracerebroventricular (ICV) injection of Kp increased PRL and LH secretion in ovariectomized, estradiol-treated rats. The unselective Npffr1 antagonist RF9 prevented these responses, whereas the selective antagonist GJ14 altered PRL but not LH levels. The ICV injection of RFRP-3 in ovariectomized, estradiol-treated rats increased PRL secretion, which was associated with a rise in the dopaminergic activity in the median eminence, but had no effect on LH levels. The RFRP-3-induced increase in PRL secretion was prevented by GJ14. Moreover, the estradiol-induced PRL surge in female rats was blunted by GJ14, along with an amplification of the LH surge. Nevertheless, whole-cell patch clamp recordings showed no effect of RFRP-3 on the electrical activity of TIDA neurons in dopamine transporter-Cre recombinase transgenic female mice. We provide evidence that RFRP-3 binds to Npffr1 to stimulate PRL release, which plays a role in the estradiol-induced PRL surge. This effect of RFRP-3 is apparently not mediated by a reduction in the inhibitory tone of TIDA neurons but possibly involves the activation of a hypothalamic PRL-releasing factor.

Partial loss of arcuate kisspeptin neurons in female rats stimulates luteinizing hormone and decreases prolactin secretion induced by estradiol.

Kisspeptin, neurokinin, and dynorphin (KNDy) neurons in the arcuate nucleus (ARC) control luteinizing hormone (LH) and prolactin (PRL) release, although their role in conveying the effects of estradiol (E2 ) to these hormones is not well understood. We performed a longitudinal evaluation of female rats in which KNDy neurons were ablated using a neurokinin-3 receptor agonist conjugated with saporin (NK3-SAP) to investigate the impact of the reduction of KNDy neurons on the E2 regulation of gonadal and PRL axes. NK3-SAP rats, bearing a moderate loss of ARC kisspeptin-immunoreactive (-IR) neurons (50%-90%), displayed irregular estrous cycles but essentially unaltered follicular development and a normal number of corpora lutea. Rats were then ovariectomized (OVX) and treated with a positive-feedback dose of E2 (OVX + E2 ). LH and PRL were measured in the tail blood by an enzyme-linked immunosorbent assay. The E2 -induced LH surge was amplified, whereas the PRL rise was decreased in NK3-SAP rats compared to Blank-SAP control. After 10 days of no hormonal treatment, basal LH levels were equally elevated in NK3-SAP and controls. Tyrosine hydroxylase (TH) phosphorylation in the median eminence, in turn, was increased in NK3-SAP rats, with no change in the number of ARC TH-IR neurons. Thus, KNDy neurons exert concurrent and opposite roles in the E2 -induced surges of LH and PRL. The partial loss of KNDy neurons disrupts ovarian cyclicity but does not preclude ovulation, consistent with the disinhibition of the LH preovulatory surge. Conversely, KNDy neurons tonically inhibit the enzymatic activity of tuberoinfundibular dopaminergic neurons, which appears to facilitate PRL release in response to E2 .

Kisspeptin Stimulation of Prolactin Secretion Requires Kiss1 Receptor but Not in Tuberoinfundibular Dopaminergic Neurons.

Kisspeptin has been shown to stimulate prolactin secretion. We investigated whether kisspeptin acts through the Kiss1 receptor (Kiss1r) to regulate dopamine and prolactin. Initially, we evaluated prolactin response in a Kiss1r-deficient mouse line, in which Kiss1r had been knocked into GnRH neurons (Kiss1r-/-R). Intracerebroventricular kisspeptin-10 (Kp-10) increased prolactin release in wild-type but not in Kiss1r-/-R female mice. In ovariectomized, estradiol-treated rats, the Kiss1r antagonist kisspeptin-234 abolished the Kp-10-induced increase in prolactin release but failed to prevent the concomitant reduction in the activity of tuberoinfundibular dopaminergic (TIDA) neurons, as determined by the 3,4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence. Using whole-cell patch clamp recordings in juvenile male rats, we found no direct effect of Kp-10 on the electrical activity of TIDA neurons. In addition, dual-label in situ hybridization in the hypothalamus of female rats showed that Kiss1r is expressed in the periventricular nucleus of the hypothalamus (Pe) and arcuate nucleus of the hypothalamus (ARC) but not in tyrosine hydroxylase (Th)-expressing neurons. Kisspeptin also has affinity for the neuropeptide FF receptor 1 (Npffr1), which was expressed in the majority of Pe dopaminergic neurons but only in a low proportion of TIDA neurons in the ARC. Our findings demonstrate that Kiss1r is necessary to the effect of kisspeptin on prolactin secretion, although TIDA neurons lack Kiss1r and are electrically unresponsive to kisspeptin. Thus, kisspeptin is likely to stimulate prolactin secretion via Kiss1r in nondopaminergic neurons, whereas the colocalization of Npffr1 and Th suggests that Pe dopaminergic neurons may play a role in the kisspeptin-induced inhibition of dopamine release.

Short-term fasting decreases excitatory synaptic inputs to ventromedial tuberoinfundibular dopaminergic neurons and attenuates their activity in male mice

Tuberoinfundibular dopaminergic (TIDA) neurons in the arcuate nucleus (ARC) of the hypothalamus play a role in inhibiting prolactin (PRL) secretion from the anterior pituitary. PRL is involved in a variety of behaviors, including feeding. Consequently, we hypothesized that fasting might reduce the activity of TIDA neurons, which might alter PRL secretion. However, direct examinations of TIDA neuron activity are difficult. Recently, transgenic mice were generated that expressed green fluorescent protein (GFP) under the control of the rat tyrosine hydroxylase gene. We first determined that GFP in the dorsomedial ARC was a reliable marker of TIDA neurons. Then, we performed electrophysiology and immunocytochemistry in GFP-labeled TIDA neurons to examine whether different feeding conditions could change their activity. Eight-week-old male mice were fed or fasted for 24 h. After sacrifice, we prepared acutely isolated brain slices for conducting whole-cell voltage-clamp recordings. TIDA neurons were identified with fluorescence microscopy. The mean amplitude of miniature excitatory postsynaptic currents (mEPSCs) was significantly reduced in fasting mice compared to fed mice, but different feeding conditions did not affect the mean mEPSC intervals. This result suggested that fasting reduced the number of excitatory synaptic inputs to TIDA neurons. To determine whether a reduction in excitatory synaptic inputs would cause a reduction in TIDA neuron activity, we examined the effect of 24-h fasting on c-Fos expression in the ARC. We found that fasting significantly reduced the number of Fos-positive TIDA neurons. In addition, serum PRL levels were significantly increased. Taken together, the present findings suggested that short-term fasting attenuated TIDA neuron activity.

Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats.

BackgroundAn endogenous dopaminergic (DA) tone acting on D3 receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E2) rats. Whether D2 receptor (D2R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.ResultsIntracerebroventricular (icv) injection of PHNO, a D2R agonist, in the morning inhibited TIDA and midbrain DA neurons’ activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D2R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D2R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D2R rhythm was abolished in OVX+E2 rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D2R mRNA for 2 days significantly reduced D2R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D2R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.ConclusionsWe conclude that a diurnal change of D2R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E2 rats.

An endogenous dopaminergic tone acting on dopamine D3 receptors may be involved in diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in estrogen-primed ovariectomized rats

The diurnal rhythm of tuberoinfundibular dopaminergic (TIDA) neuron activity, i.e., high in the morning and low in the afternoon, is prerequisite for the afternoon prolactin (PRL) surge in proestrous and estrogen-primed ovariectomized (OVX) female rats. Whether dopamine acts via D(3) receptors in regulating the rhythmic TIDA neuron activity and PRL secretion in estrogen-primed OVX (OVX+E(2)) rats is the focus of this study. Intracerebroventricular (icv) injection of a D(3) receptor agonist, PD128907 (0.1-10 μg/3 μl), in the morning significantly reduced the basal activity of TIDA neurons and increased plasma PRL level. The effects of PD128907 were reversed by co-administration of U99194A, a D(3) receptor antagonist, but not by raclopride, a D(2) receptor antagonist. To determine whether endogenous dopamine acts on D(3) receptors involved in the diurnal changes of the activities, we used both U99194A, a D(3) receptor antagonist, and an antisense oligodeoxynucleotide (ODN) against D(3) receptor mRNA in the study. U99194A (0.1 μg/3 μl, icv) given at 1200 h significantly reversed the lowered TIDA neuron activity and the afternoon PRL surge at 1500 h. Moreover, OVX+E(2) rats pretreated with the antisense ODN (10 μg/3 μl, icv) for 2 days had the same effects as the D(3) receptor antagonist on TIDA neuron activity and the PRL surge. The same treatment with sense ODN had no effect. In conclusion, an endogenous DA tone may act on D(3) receptors to inhibit TIDA neuron activity and in turn stimulate the PRL surge in the afternoon of OVX+E(2) rats.

The spontaneous firing rates of dopamine-inhibited dorsomedial arcuate neurons exhibit a diurnal rhythm in brain slices obtained from ovariectomized plus estrogen-treated rats

The activity of tuberoinfundibular dopaminergic (TIDA) neurons exhibits a diurnal rhythm in female rats, as determined by neurochemical investigation. Whether the spontaneous firing rates of presumed TIDA neurons in the dorsomedial arcuate nucleus (dmARN) also exhibit a diurnal pattern has yet to be ascertained. Single-unit activities of 131 dmARN neurons were recorded in brain slices prepared from 83 ovariectomized plus estrogen-primed rats, and grouped according to their responses to dopamine and the time at which they were observed. In dopamine-inhibited dmARN neurons, significantly lower firing rates were observed in the afternoon compared to those recorded in the morning (2.51 ± 0.27 Hz, n = 15, from 1130 to 1330 h vs. 1.08 ± 0.07 Hz, n = 47, from 1430 to 1630 h). No such change was observed in dopamine-excited or nonresponsive dmARN neurons (1.83 ± 0.32 Hz, n = 9 vs. 1.46 ± 0.17 Hz, n = 21). Four dmARN neurons were continuously recorded from 1130 to 1600 h or even longer until 2000 h. The averaged firing rates decreased significantly between 1300 and 1600 h, two neurons were also inhibited by dopamine and a selective D 2 receptor agonist, PHNO, in both normal and low Ca 2+, high Mg 2+ perfusion mediums. This study revealed the existence of diurnal changes in the firing rates of dopamine-inhibited dmARN neurons. These results are strongly correlated with the rhythmic changes observed in TIDA neuronal activity determined through neurochemical methods.

Mu and kappa opioid receptor expression in the mediobasal hypothalamus and effectiveness of selective antagonists on prolactin release during lactation

Endogenous opioid peptides are involved in prolactin release during lactation, in part by decreasing tuberoinfundibular dopaminergic (TIDA) neuronal activity. Both mu (μ) and kappa (κ) opioid receptors have a role in the suckling-induced prolactin rise after 4–5 h up deprivation. The aim of this study was to investigate effects of μ opioid receptor antagonist, β-funaltrexamine (β-FNA), and κ opioid receptor antagonist, nor-binaltorphimine (nor-BNI), on prolactin secretion and TIDA neuronal activity in lactating rats after 18 h pup deprivation. After 4 h separation from pups, the suckling-induced prolactin rise was abolished by 16 μg nor-BNI and 5 μg β-FNA, coincident with increased dihydroxyphenylacetic acid (DOPAC):dopamine ratio in the stalk-median eminence (SME). However, after 18 h pups separation, these same doses of nor-BNI and β-FNA did not alter the prolactin surge or DOPAC:dopamine ratios in the SME. Higher doses of nor-BNI (32 μg) and β-FNA (10 μg) were required to inhibit suckling-induced prolactin secretion. β-FNA (10 μg) increased the DOPAC:dopamine ratio in the SME, whereas nor-BNI (32 μg) treatment had no effect. The μ and κ opioid receptor mRNA levels in the mediobasal hypothalamus were similar to suckled control rats after 4 h pup deprivation, but increased 1.4-fold after 18 h pup deprivation. These data support involvement of endogenous opioidergic systems in the suckling-induced prolactin rise after a prolonged (18 h) period of pup deprivation, as well as the shorter (4 h) pup deprivation period previously reported. Suppression of TIDA neuronal activity likely played a part in μ opioid receptor input to the suckling-induced prolactin rise after both 4 h and 18 h separation, whereas non-dopaminergic input was implicated with κ opioid receptors after 18 h pup deprivation. Increased μ and κ opioid receptors gene expression in the mediobasal hypothalamus may contribute to reduced effectiveness of opioid receptor antagonists to block suckling-induced prolactin release after 18 h pup deprivation.

Dopaminergic Mechanisms Involved in Prolactin Release after Mifepristone and Naloxone Treatment during Late Pregnancy in the Rat

Background/Aims: During late pregnancy, the antiprogesterone mifepristone facilitates prolactin release. This effect is enhanced by administration of the opioid antagonist naloxone, suggesting an inhibitory-neuromodulatory role of the opioid system. Since hypothalamic dopamine (DA) is the main regulator of prolactin release, in this study we explored the role of DA on prolactin release induced by mifepristone and naloxone treatment. Methods/Results: Rats on day 19 of pregnancy were used. Naloxone treatment did not modify the 3,4-dihydroxyphenylacetic acid/DA (DOPAC/DA) ratio or serum prolactin concentration in control rats. After mifepristone treatment, DA activity diminished significantly without modifying serum prolactin levels. Naloxone administration to antiprogesterone-treated rats did not change the DOPAC/DA ratio but increased serum prolactin. Tyrosine hydroxylase (TH) expression in medial basal hypothalamus (MBH) protein extracts was lowered by pretreatment with mifepristone, with no additional effect of naloxone. While mifepristone decreased the intensity of TH immunoreactivity in the arcuate and periventricular nuclei and in fibers of the median eminence, naloxone treatment had no further effect. Conclusions: (1) A reduction of tuberoinfundibular dopaminergic (TIDA) neuron activity is suggested by the fall of the DOPAC/DA ratio and the low expression of MBH TH; (2) this reduction facilitates prolactin secretion by naloxone, indicating that progesterone stimulates DA neurons to maintain low serum prolactin; (3) naloxone action seems to depend on a previous decrease of DA tone induced by mifepristone, without involve a direct effect on neuronal DA activity, and (4) endogenous opioids may inhibit prolactin secretion through a non-dopaminergic neuronal system that regulates prolactin secretion in which as yet undetermined prolactin-releasing factors may participate.

Orphanin FQ/Nociceptin Is a Physiological Regulator of Prolactin Secretion in Female Rats

Orphanin FQ/nociceptin (OFQ/N), the most recently identified endogenous opioid peptide, stimulates prolactin secretion in both male and female rats. OFQ/N, however, did not elicit this stimulatory effect through the mu-, delta-, or kappa-opiate receptor subtype. The role OFQ/N plays in prolactin regulation under physiological conditions and its mechanism of action are not known. The purpose of these studies was to determine the physiological significance and pharmacological specificity of the prolactin secretory response to OFQ/N. In addition, the role of the tuberoinfundibular dopaminergic (TIDA) neurons in mediating this response was examined. Opioid receptor-like-1 (ORL-1) receptors were blocked by pretreatment with compound B (Comp B), a purported OFQ/N antagonist, or receptor synthesis was disrupted by pretreatment with ORL-1 receptor antisense oligonucleotides. The prolactin secretory response to OFQ/N administration in diestrous females was measured. Furthermore, the suckling-induced prolactin response was also determined after Comp B pretreatment. TIDA neuronal activity was quantified in diestrous female rats to determine whether OFQ/N stimulates prolactin release by inhibiting TIDA neurons. OFQ/N significantly inhibited the TIDA neurons by 1 min, preceding the prolactin secretory response. Both Comp B and antisense pretreatment blocked the stimulatory effects of OFQ/N on prolactin release, and Comp B abolished the suckling-induced prolactin response. These studies indicate that OFQ/N is a potent stimulus for prolactin secretion in female rats and that it mediates this effect by rapid and transient inhibition of TIDA neuronal activity. Furthermore, OFQ/N plays a physiologically significant role in the regulation of prolactin secretion during lactation, and it mediates its effects via actions at the ORL-1 receptor subtype.

Effects of Opioid Antagonism on Prolactin Secretion and c-Fos/TH Expression During Lactation in Rats

Many studies have established that dopamine (DA) secreted by tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus is the major inhibitory factor controlling prolactin (PRL) secretion from the anterior pituitary. Endogenous opioid peptides (EOPs), mainly the neuropeptide beta-endorphin, facilitate PRL secretion by decreasing TIDA neuronal inhibitory tone in a number of physiological conditions, including pregnancy and lactation. We have previously demonstrated that there are many more c-Fos-expressing neurons than TIDA neurons in the arcuate nucleus, and treatment with naloxone (NAL), an opioid antagonist, activated these neurons in pregnant rats. Our previous data also suggest that the rostral region of the arcuate nucleus is more important than the caudal region in regulating the nocturnal PRL surge in pregnant rats. The aim of this study was to investigate the effects of NAL in regulating TIDA neuronal activity and therefore facilitating PRL secretion during lactation in rats. NAL was continuously infused (0.2 mg/10 microL/min iv) for 1 h before the separated pups returned, and then for 2 or 5 h after the separated pups were returned. Radioimmunoassay (RIA) was used to measure plasma PRL levels, and the immunocytochemical (ICC) staining of c-Fos was performed to detect changes in transcriptional activity of neurons in the hypothalamus. ICC of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis, was performed to visualize TIDA neurons in the arcuate nucleus. The results showed that the peak of the PRL response to suckling was markedly delayed and dampened in NAL-treated rats (p<0.05). The percentage of c-Fos positive TH neurons in the arcute nucleus increased in rats treated with NAL for 5 h after return of pups, but not in rats treated with NAL for 2 h.

Fasting stimulates tuberoinfundibular dopaminergic neuronal activity and inhibits prolactin secretion in oestrogen-primed ovariectomized rats: involvement of orexin A and neuropeptide Y.

Fasting up-regulates central orexigenic systems including orexin A and neuropeptide Y (NPY) and it also inhibits the secretion of prolactin. We hypothesized that fasting may act through orexin A and NPY to influence tuberoinfundibular dopaminergic (TIDA) neurones, the major regulator of prolactin secretion. The effects of orexin A and NPY on TIDA neuronal activity and prolactin secretion were determined in oestrogen-primed ovariectomized rats, and the effects of fasting and the involvement of orexin A and NPY were tested. Orexin A, NPY and its analogs were administered through preimplanted intracerebroventricular (i.c.v.) cannulae. TIDA neuronal activity was determined by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) or 3,4-dihydroxyphenylalanine in the median eminence. i.c.v. injection of NPY (10 microg) or orexin A (1 microg) concomitantly increased median eminence DOPAC and decreased serum prolactin concentrations. The effect of NPY was mimicked by a Y1 receptor agonist at lower doses (0.1 and 1 microg) and no additive effect was observed when orexin A and the Y1 agaonist were coadministered. Moreover, a Y1 receptor antagonist, BIBP, not only blocked the effect of Y1 agaonist, but also that of orexin A. Treatment with BIBP alone decreased median eminence DOPAC and increased serum prolactin concentrations, indicating that endogenous NPY may play a role. Moreover, fasting for 48 h significantly increased TIDA neuronal activity, both in the morning and afternoon, and the effect was reversed by treatment with either BIBP or an antibody against orexin A. The findings support our hypothesis that fasting stimulates TIDA neuronal activity and inhibits prolactin secretion through up-regulated central orexin A and NPY systems.

Involvement of angiotensin II, TRH and prolactin-releasing peptide in the estrogen-induced afternoon prolactin surge in female rats: studies using antisense technology

The roles of endogenous angiotensin II (AII), thyrotropin-releasing hormone (TRH) and prolactin-releasing peptide (PrRP) on the estrogen-induced prolactin (PRL) surge and the diurnal change of tuberoinfundibular dopaminergic (TIDA) neuronal activity were assessed in this study. Ovariectomized, estrogen-primed rats implanted with intracerebroventricular cannula received daily injection of antisense oligodeoxynucleotide (ODN, 10 μg/3 μl) against the mRNA of AII, TRH or PrRP for two days. Artificial cerebrospinal fluid or the sense ODN were used as the control. In the first experiment, serial blood samples (0.3 ml each) were obtained hourly from each rat through a pre-implanted intraatrial catheter from 1100 to 1700h. Half of the rats pretreated with respective antisense ODN received single injections of AII, TRH or PrRP (1 μg each, iv) at 1400h. In the second experiment, groups of rats were decapitated either at 1000 or 1500h. The hypothalamic median eminence tissue of each rat was dissected out and its DOPAC content was used as the index for TIDA neuronal activity. Plasma and serum PRL levels were determined by radioimmunoassay. Pretreatment of antisense ODN against the mRNA of either AII or TRH significantly attenuated the PRL surge; replacement injection of AII or TRH restored the surge. The effect of antisense ODN against PrRP was less significant. None of the treatments significantly affected the diurnal changes of TIDA neuronal activity. In summary, both AII and TRH may play an important role as the PRL-releasing hormone involved in the estrogen-induced afternoon PRL surge.

Effects of Prolactin-Releasing Peptide on Tuberoinfundibular Dopaminergic Neuronal Activity and Prolactin Secretion in Estrogen-Treated Female Rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 µg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D₂ receptor antagonist, sulpiride (1 µg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1–1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats.

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.

Prostaglandins may participate in opioidergic and cholinergic control of the diurnal changes of tuberoinfundibular dopaminergic neuronal activity and serum prolactin level in ovariectomized, estrogen-treated rats

The roles of prostaglandins (PGs) involved in opioidergic, cholinergic, and serotonergic regulation of the diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activities were investigated in ovariectomized, estrogen-treated rats. Treatment with naloxone [10 mg/kg, intraperitoneal (i.p.)], atropine (5 mg/kg, i.p.), or ketanserin (10 mg/kg, i.p.) at either 1200 or 1400 h prevented the afternoon decrease of TIDA neuronal activity and the prolactin (PRL) surge. Intracerebroventricular (i.c.v.) injection of PGE 1 (5 μg/3 μl/rat) at 1500 h significantly reversed the effects of naloxone and atropine, but not that of ketanserin. In ovariectomized, estrogen-primed rats pretreated with indomethacin, i.c.v. injection of either nicotine (10 ng/rat) or fentanyl (10 ng/rat) failed to suppress the TIDA neuronal activity and to stimulate the PRL secretion. These data suggest that PG may be involved in opioidergic and cholinergic control of the diurnal changes of TIDA neuronal activity and the PRL secretion in ovariectomized (OVX) + E 2 rats.

Prostaglandins play an important role in diurnal changes of tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in ovariectomized, estrogen-treated rats

Intracerebroventricular injection of prostaglandins (PGs) has long been shown to stimulate the secretion of prolactin (PRL). The action of specific PG and the underlying mechanism, however, remain inconclusive. In this study, we evaluated the involvement of PGs in the diurnal changes of the tuberoinfundibular dopaminergic (TIDA) neurons and PRL secretion in ovariectomized (OVX) rats treated with estrogen (E 2). Prior treatment of indomethacin (50 mg/kg, subcutaneous) for 24 h significantly prevented the afternoon decrease of TIDA neuronal activity and blocked the PRL surge. Intracerebroventricular injection of PGE 1 (5 μg/3 μl/rat) at 1000 h significantly lowered the activity of TIDA neurons, while similar treatment of PGE 2 was without effect. In OVX + E 2 rats pretreated with indomethacin, PGE 1 given at 15, but not 30 or 180 min before decapitation at 1500 h significantly reversed the effects of indomethacin on TIDA neuronal activity, while the blocked PRL surge was not reversed. The PGs also exhibited a stimulatory effect on nigrostriatal dopamine (DA) neurons with PGE 2 being the most effective agent. These results indicate that endogenous PGs play a significant role in the control of central DA neurons, especially the diurnal changes of TIDA neuronal activity and PRL secretion in OVX + E 2 rats.

Involvement of central GABAergic neurons in basal and diurnal changes of tuberoinfundibular dopaminergic neuronal activity and prolactin secretion

Central administration of gamma-aminobutyric acid (GABA) has been shown to stimulate the secretion of prolactin (PRL). Whether GABA acts via dopamine, the major PRL-inhibiting hormone, and which GABA receptor type(s) is involved have not been ascertained. Both GABA A and GABA B receptor agonists and/or antagonists were administered centrally in this study and their effects on both basal and diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activity were determined by measuring the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME). Serum PRL level was determined by RIA. Ovariectomized, estrogen-primed Sprague-Dawley rats implanted with intracerebroventricular (icv) cannulae were used. Muscimol (1 ng/3 μl/rat, icv), a GABA A receptor agonist, but not baclofen (1–100 ng/3 μl/rat, icv), a GABA B receptor agonist, injected in the morning significantly lowered and elevated ME DOPAC and serum PRL levels, respectively at 15 and 30 min. Lower and higher doses of muscimol were not effective. The effects of muscimol could also be prevented by co-administration of bicuculline (0.1–10 ng/3 μl, icv), a GABA A receptor antagonist. When bicuculline (10–500 ng/3 μl, icv) was given in the afternoon (at 1500 h), it significantly reversed the lowered ME DOPAC level in the afternoon and prevented the concurrent PRL surge. We conclude that endogenous GABA acting through GABA A receptors may play a significant role in the control of basal and diurnal changes of TIDA neuronal activity, and in turn, PRL secretion.

Anti-tumor effect of l-deprenyl is associated with enhanced central and peripheral neurotransmission and immune reactivity in rats with carcinogen-induced mammary tumors

l-Deprenyl, a monoamine oxidase-B (MAO-B) inhibitor, has previously been shown to improve immune responses and restore noradrenergic (NA) nerve fibers in the spleen of old rats. In tumor-bearing rats, l-deprenyl inhibited tumor incidence and enhanced tuberoinfundibular dopaminergic (TIDA) neurotransmission in the hypothalamus. The aim of the present study was to investigate whether alterations in sympathetic NA activity and cellular immune responses in the spleen, and TIDA activity in the hypothalamus, accompany deprenyl-induced regression of 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced mammary tumors. Rats with DMBA-induced mammary tumors were treated with 0, 2.5 mg, or 5.0 mg/kg body weight of deprenyl daily for 13 weeks. Saline-treated tumor-bearing rats exhibited reduced splenic IL-2 and IFN-γ levels, and lowered splenic norepinephrine (NE) concentration and hypothalamic dopaminergic activity, compared to rats without tumors. In contrast, treatment with 2.5 mg/kg and 5.0 mg/kg of deprenyl reduced the number and size of mammary tumors. Deprenyl-induced tumor regression was accompanied by increased immune measures in the spleen, including enhanced IL-2 and IFN-γ production, and NK cell activity. Neural measures enhanced by deprenyl included NE concentration in the spleen and TIDA neuronal activity in the hypothalamus. These results suggest that (1) mammary tumorigenesis is associated with the inhibition of sympathetic NA activity in the spleen, TIDA activity in the hypothalamus, and cell-mediated immunity, and (2) reversal of the inhibition of catecholaminergic neuronal activities of the central nervous system and peripheral nervous system by deprenyl may enhance anti-tumor immunity.

Dopamine and 7-OH-DPAT may act on D 3 receptors to inhibit tuberoinfundibular dopaminergic neurons

Whether the tuberoinfundibular dopaminergic (TIDA) neurons resided in the dorsomedial arcuate nucleus (dmARN) can respond to dopamine and a dopamine D 3 receptor agonist, 7-hydroxydipropylaminotetralin (7-OH-DPAT), was the focus of this study. In studies using extracellular single-unit recording of dmARN neurons in brain slices obtained from ovariectomized rats, dopamine and 7-OH-DPAT inhibited 60.1% ( n = 141) and 80.9% ( n = 47) of recorded dmARN neurons, respectively. Other dopamine D 1 or D 2 receptor agonists were not as effective. Intracerebroventricular injection of 7-OH-DPAT (10 −9 mol/3 μl) in ovariectomized, estrogen-primed rats significantly lowered the TIDA neuronal activity as determined by 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence. Co-administration of a putative D 3 receptor antagonist, U-99194A, could prevent the effect of 7-OH-DPAT. Unilateral microinjection of 7-OH-DPAT or dopamine itself (10 −11–10 −9 mol/0.2 μl) into the right dmARN exhibited the same inhibitory effect on TIDA neurons. In all, dopamine may act on D 3 receptors to exhibit an inhibitory effect on its own release from the TIDA neurons.