Jenn-Tser Pan

Circadian Changes of Serum Prolactin Levels and Tuberoinfundibular Dopaminergic Neuron Activities in Ovariectomized Rats Treated with or without Estrogen: The Role of the Suprachiasmatic Nuclei

Variations of serum prolactin (PRL) levels and activities of tuberoinfundibular dopaminergic (TIDA) neurons during the afternoon of ovariectomized (OVX) rats treated with or without estrogen were determined in this study. Long-term OVX rats treated with or without polyestradiol phosphate (0.1 mg/rat, s.c.) were decapitated every hour from 10.00 to 19.00 h (except 11.00 and 13.00 h). Serum PRL and median eminence (ME) dihydroxyphenylacetic acid (DOPAC) or dihydroxyphenylalanine (DOPA) levels were determined by radioimmunoassay and high performance liquid chromatography plus electrochemical detection, respectively. A prominent PRL surge started and peaked around 14.00-15.00 h, and remained significantly higher than levels of 10.00 and 12.00 h throughout the afternoon. Significant decreases of ME DOPAC and DOPA concentrations were also observed between 14.00 and 19.00 h. In OVX rats with no estrogen replacement, no PRL surge was observed and the changes of ME DOPAC concentrations during the afternoon were not significant except for that at 17.00 h. The ME DOPA accumulation, however, exhibited significantly lower levels from 14.00 to 19.00 h than that at 12.00 h, indicating that an endogenous rhythm for DA synthesis existed in OVX rats. In estrogen-treated OVX rats bearing bilateral lesions of the suprachiasmatic nuclei, both changes in serum PRL level and TIDA neuron activity were abolished. We conclude that an endogenous rhythm of the activities of TIDA neurons may exist in both OVX and OVX plus estrogen-treated rats. The rhythm is regulated by the suprachiasmatic nuclei and may be amplified by estrogen for the induction of PRL surge.

Circadian change of dopaminergic neuron activity: effects of constant light and melatonin.

TWENTY-four hour profiles of tuberoinfundibular (TI), nigrostriatal and mesolimbic dopaminergic (DA) neuronal activities were assessed in estrogen-primed ovariectomized rats using DOPAC and DOPA levels in terminal regions of DA neurons. Significant decreases in DOPAC and DOPA levels in the median eminence were observed at 17.00 and 21.00 h, which corresponded with higher serum prolactin levels. DOPAC or DOPA levels in the striatum and nucleus accumbens were, however, significantly higher during the dark (21.00–05.00 h) phase. In rats kept under conditions of continuous light, no late afternoon decline in median eminence DOPA was observed; this decline could be reinstated by repeated injections of melatonin between 18.00 and 01.30 h for 3 days. In summary, circadian rhythms of central DA neurons were shown and melatonin may play an entraining role.

An endogenous serotonergic rhythm acting on 5-HT(2A) receptors may be involved in the diurnal changes in tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in female rats.

The central serotonergic system has long been known to have a stimulatory role on the secretion of prolactin (PRL). The integrity of serotonergic neurotransmission is essential for the expression of the estrogen-induced afternoon PRL surge. Whether its effect on PRL involves change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons has not been ascertained. In adult ovariectomized rats treated with estrogen, depletion of central serotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT, 200 µg/rat, i.c.v.) effectively prevented the afternoon fall in TIDA neuronal activity (using the levels of 3,4-dihydroxyphenylalanine and 3,4-dihydroxyphenylacetic acid (DOPAC), and the ratio of DOPAC/dopamine in the median eminence as indices), and blunted the afternoon PRL surge. A single injection of a 5-HT2A receptor antagonist, ketanserin (5 mg/kg, i.p. at 12.00 h), also had the same effects on the diurnal changes in TIDA neuronal activity and PRL secretion as the treatment with 5,7-DHT did. Intracerebroventricular injection of a 5-HT2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) in the morning inhibited the TIDA neuronal activity and stimulated PRL secretion in a dose-dependent manner; while injection of a 5-HT1 agonist, 8-hydroxy-dipropylaminotetralin, was without effect. Injection of DOI in 5,7-DHT-pretreated rats at 14.30 h also lowered the TIDA neuronal activity and reinstated the PRL surge. In all, endogenous 5-HT, acting through the 5-HT2A receptor, appears to exhibit an inhibitory effect on TIDA neuronal activity during the afternoon, which is essential for the PRL surge.

Nicotinic control of tuberoinfundibular dopaminergic neuron activity and prolactin secretion: diurnal rhythm and involvement of endogenous opioidergic system.

The possible involvement of cholinergic and opioidergic neurons in the control of diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activity was reported. Adult Sprague-Dawley rats ovariectomized and treated with estrogen were used. All drugs were administered centrally through preimplanted intracerebroventricular cannula, and both TIDA neuronal activity and serum prolactin level were determined. Nicotine (10 ng/3 microl/rat) given at 10:00 h significantly inhibited TIDA neuronal activity from 5 to 30 min and stimulated serum PRL levels at 5 and 15 min. Co-administration of either mecamylamine (1 microg) or naloxone (2.5 microg) prevented both nicotines effects. A dose-related (0.1-100 ng) effect of nicotine on TIDA neuronal activity and serum PRL level was also observed in the morning when TIDA neuronal activity is high and serum PRL level is low, but not in the afternoon when the former activity is low and the latter is high. When atropine (20 microg), naloxone (25 microg) or Nor-BNI (20 microg) was given at 14:00 h all increased the lowered TIDA neuronal activity in the afternoon. When atropine was co-administered with either naloxone or Nor-BNI, however, no additive effect was observed. Submaximal doses of atropine (0.2 microg), mecamylamine (0.1 microg) or naloxone (0.25 microg) was also effective in stimulating the afternoon levels of TIDA neuronal activity and inhibiting serum PRL, and no additive effect was observed either. Moreover, simultaneous injection of morphine (15 microg) prevented atropines effect in the afternoon. These results indicate that cholinergic neurons may act through activating the endogenous opioidergic neurons to exhibit an inhibitory effect on TIDA neuronal activity and a stimulatory one on prolactin secretion. A diurnal difference in its endogenous activity between morning and afternoon was also implicated.

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence dopamine concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatments with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.

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 microg/3 microl) 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 microg each, i.v.) 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.

Dopamine and 7-OH-DPAT may act on D3 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 microl) 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 microl) 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.

Ontogeny of the Diurnal Rhythm of Tuberoinfundibular Dopaminergic Neuronal Activity in Peripubertal Female Rats: Possible Involvement of Cholinergic and Opioidergic Systems

The ontogeny of the diurnal change of tuberoinfundibular dopaminergic (TIDA) neuronal activity in female rats and its control mechanism were the foci of this study. Intact pre (28- and 35-day-old)-, peri (39- and 42-day-old)- and post (49- and 56-day-old)-pubertal female, as well as pre (28-day-old)- and post (56-day-old)-pubertal male Sprague-Dawley rats were used. Basal TIDA neuronal activity, determined by measuring 3,4-dihydroxyphenylalanine or 3,4-dihydroxyphenylacetic acid in the median eminence, increased 3- to 4-fold in the female and less than 2-fold in the male rats from day 28 to 49; so did serum prolactin levels. The diurnal rhythm of TIDA neuronal activity was not evident until day 42 in the female, but not male, rats. Neither nigrostriatal, nor mesolimbic DA neurons exhibited any significant change between pre- and postpubertal stages. In postpubertal (56-day-old) female rats, injection of nicotine (10 µg/kg) in the morning, but not in the afternoon, inhibited TIDA neuronal activity while similar injections of mecamylamine (1 mg/kg) or naloxone (2.5 mg/kg) stimulated these neurons in the afternoon, but not in the morning. Serum prolactin levels changed accordingly. In contrast, none of the injections had any effect in prepubertal (28-day-old) female rats. These results indicate that the genesis of the diurnal rhythm of TIDA neuronal activity in female rats occurs during the peripubertal stage, and endogenous cholinergic and opioidergic neurons may play a significant role.

Pretreatment with antisense oligodeoxynucleotide against D2 or D3 receptor mRNA diminished dopamine’s inhibitory effect on dorsomedial arcuate neurons in brain slices of estrogen-treated ovariectomized rats

A high percentage of dopamine (DA)-responsive neurons has been repeatedly shown in hypothalamic dorsomedial arcuate nucleus (dmARN) using single-unit recording in brain slices. Both D(2) and D(3) receptors may be involved in the inhibitory action of DA as indicated by results obtained from using specific DA agonists and antagonists. To further delineate the DA receptor types involved, ovariectomized, estrogen-primed Sprague-Dawley rats pretreated with antisense oligodeoxynucleotide (ODN, 10 microg/3 microl, i.c.v.) against either D(2) or D(3) receptor mRNA for 2 days were used in this study for brain slice preparation. Rats pretreated with aCSF, random antisense or sense ODNs were used as controls. DA (5-50 nmol) inhibited a majority of dmARN neurons in brain slices prepared from rats pretreated with aCSF (71.4% of 35 U), random antisense/sense ODN for D(2) (67.6%, n=34), D(3) (59.5%, n=42), or D(2) plus D(3) (60.5%, n=38) mRNAs. In contrast, DA inhibited 43.6% (n=39) of dmARN neurons in slices prepared from D(2), and 38.5% (n=39) from D(3) antisense ODN-pretreated rats. Furthermore, in brain slices prepared from rats pretreated with combined D(2) and D(3) antisense ODNs, DA only inhibited 18.4% (n=38) of dmARN neurons. We conclude that both D(2) and D(3) receptors are involved in the action of DA on dmARN neurons in ovariectomized, estrogen-treated rats.

Stimulatory and entraining effect of melatonin on tuberoinfundibular dopaminergic neuron activity and inhibition on prolactin secretion

The aims of the present study were to determine if melatonin exerts an effect on prolactin (PRL) secretion via the tuberoinfundibular dopaminergic (TIDA) neurons and if endogenous or exogenous melatonin has an entraining effect on the rhythmic changes of TIDA neuronal activity and PRL secretion. Melatonin given in the morning (10:00 h), dose‐ (0.01–1 mg/kg, ip) and time‐ (at 15 and 60 min, but not at 30 min) dependently stimulated TIDA neuronal activity in ovariectomized (OVX), estrogen‐treated rats as determined by 3,4‐dihydroxyphenylacetic acid (DOPAC) levels in the median eminence (ME). Serum PRL was concurrently inhibited by the injection. Melatonin administered in the afternoon (15:00 h) was even more effective in stimulating the lowered TIDA neuronal activity and inhibiting the increased PRL level than that given in the morning (10:00 h). S‐20098, a melatonin agonist was also effective in stimulating the TIDA neurons. In contrast, S‐20928, a putative melatonin antagonist, while it had no effect by itself, blocked the effect of S‐20098. Although S‐20928 failed to prevent melatonins effect on ME DOPAC levels, six interspaced injections of S‐20928, from 18:00 to 01:30 h, significantly blocked the increase of ME DOPAC levels at 03:00 h, indicating that the endogenous melatonin may play a role. We further used rats that received daily injection of melatonin (1 mg/kg, ip) at 18:00 h for 10 days and found that the injection augmented basal TIDA neuronal activity at 11:00 h and blunted the afternoon PRL surge. In all, melatonin can have an inhibitory effect on PRL secretion by stimulating the TIDA neurons, and it may help to entrain the circadian rhythms of both TIDA neuronal activity and PRL secretion.