Evidence for up-regulation of purinergic receptor genes associating with TRPV1 receptors and neurotrophic factors in the inflamed human esophagus.

Purinergic receptors are implicated in nociceptive signaling in small primary afferents via activation of adenosine triphosphate (ATP). ATP appears to mediate HCl-induced transient receptor potential vanilloid receptor 1 (TRPV1) activation in esophageal mucosa. Up-regulation of TRPV1 expression in gastroesophageal reflux disease (GERD) is associated with increased nerve growth factor (NGF) and glial derived neurotrophic factor (GDNF). This study aims to genetically determine the expression of purinergic receptors in severe inflamed human esophagus. Distal esophageal biopsies from the subjects with erosive GERD, asymptomatic patients (AP) and healthy ones were examined. Using real-time qPCR for detecting purinergic receptors (P2X2, P2X3, P2X7, P2Y1, P2Y2, P2Y4, P2Y6 and P2Y12), TRPV1, TRPV4, NGF, and GDNF was done in this study. Both P2X3 and P2X7 mRNA expressions in GERD patients significantly increased than those in healthy controls (P < 0.001) and AP (P < 0.001), but P2X2, P2Y1, P2Y2, P2Y4, P2Y6, P2Y12 or P2Y12 had no difference within the control, AP or GERD subjects. The well correlated expression in P2X3 gene with TRPV1 (r = 0.46, P = 0.002), NGF (r = 0.54, P = 0.0002), and GDNF (r = 0.64, P = 0.0001) was found. The P2X7 gene expressions also well correlated with TRPV1 (r = 0.47, P = 0.002), NGF (r = 0.32, P = 0.037), and GDNF (r = 0.42, P = 0.005). These results suggest that chronic esophagitis increases mRNA expressions of P2X3 and P2X7 receptors accompanied by up-regulation of TRPV1 and neurotrophic factors (NGF and GDNF). These genetical alterations in esophageal mucosa might mediate sensitization of inflamed human esophagus.

Evidence for neurotrophic factors associating with TRPV1 gene expression in the inflamed human esophagus.

BACKGROUND: Transient receptor potential vanilloid-1 (TRPV1) receptor has been implicated in the mechanism of acid induced inflammation in gastro-esophageal reflux disease (GERD). It has been demonstrated that the increase in nerve growth factor (NGF) and glial derived neurotrophic factor (GDNF) was associated with the increased expression of TRPV1. We aimed to determine whether expression of TRPV1 was increased in severe inflamed human esophagus, and to test the hypothesis whether the expression of TRPV1 was mediated by neurotrophic factors such as NGF and GDNF. METHODS: We compared biopsies taken from the distal esophagus of 15 patients with erosive GERD, 16 asymptomatic patients (AP), and 10 healthy controls. We assessed the biopsies with reverse transcription polymerase chain reaction (RT-PCR) and real-time quantitative polymerase chain reaction (qPCR) for TRPV1, NGF, and GDNF. Immunohistochemical analysis of TRPV1 protein expression was also determined. KEY RESULTS: Transient receptor potential vanilloid-1 mRNA level and its protein expression were significantly greater in patients with erosive esophagitis than AP (P < 0.001) and healthy controls (P < 0.001). Nerve growth factor and GDNF gene levels in the esophageal mucosa were also significantly increased in patients with erosive esophagitis compared with AP and healthy controls (all P < 0.001). Transient receptor potential vanilloid-1 mRNA correlated well with NGF (r = 0.61, P < 0.001) and GDNF (r = 0.58, P < 0.001). CONCLUSIONS & INFERENCES: These results support the association of NGF and GDNF in the up-regulation of TRPV1 receptors in patients with erosive esophagitis.

Abnormal expressions of circadian-clock and circadian clock-controlled genes in the livers and kidneys of long-term, high-fat-diet-treated mice.

OBJECTIVES: Physiological and behavioral circadian rhythmicities are exhibited by all mammals and are generated by intracellular levels of circadian oscillators, which are composed of transcriptional/translational feedback loops involving a set of circadian-clock genes, such as Clock, Per1-3, Cry1-2, Bmal1, Dbp, E4BP4 and CK1varepsilon. These circadian-clock genes play important roles in regulating circadian rhythms and also energy homeostasis and metabolism. Determining whether obesity induced by high-fat diet affected the expressions of circadian-clock genes and their related genes in peripheral tissues, was the main focus of this study. To address this issue, we fed male C57BL/6 mice a high-fat diet for 11 months to induce obesity, hyperglycemic, hypercholesterolemic and hyperinsulinemic symptoms, and used quantitative real-time reverse transcription-PCR to measure gene expression levels. RESULTS: We found that the expressions of circadian-clock genes and circadian clock-controlled genes, including Per1-3, Cry1-2, Bmal1, Dbp, E4BP4, CK1varepsilon, PEPCK, PDK4 and NHE3, were altered in the livers and/or kidneys. CONCLUSIONS: These results indicate that obesity induced by high-fat diet alters the circadian-clock system, and obesity and metabolic syndrome are highly correlated with the expressions of circadian-clock genes and their downstream, circadian clock-controlled genes.

Depression- and anxiety-like behaviors of a rat model with absence epileptic discharges.

Depression and/or anxiety are major comorbidities of epilepsy. However, the contribution of absence epileptic discharges in psychiatric syndromes is inconclusive. This study aimed to clarify the influence of absence seizure in anxiety- and depression-like behaviors using normal Wistar rats and Long-Evans rats with spontaneous spike-wave discharges (SWDs). Anxiety-like behaviors were evaluated by the open field (OF) and elevated plus maze (EPM) tests, and depression-like behaviors by the forced swimming (FS) and sucrose consumption (SC) tests. Long-Evans rats displayed significantly higher frequency and longer duration in the open arms of the EPM and in the center zone of the OF than did Wistar rats. Normalized behavioral indexes by movement also were significantly higher in Long-Evans rats. An excess of SWD numbers was associated with lower indexes and worse movement in the two behavioral tests. Ethosuximide eliminated the seizure frequency-dependent relationship and also significantly increased all indexes of the EPM test. Additionally, Long-Evans rats revealed significantly longer immobility in the FS test and lower consumption of sucrose solution in the SC test than did Wistar rats. Meanwhile, no relationship was found between immobility of the FS test and SWD number. Ethosuximide ameliorated depression-like behavior of Long-Evans rats that was equal to that of Wistar rats. Thus, Long-Evans rats showed seizure frequency-related exacerbation in anxiety-like behavior; and they displayed a depressive propensity. Our data suggest that generalized SWDs may have distinct influences in anxious and depressive behaviors.

Effects of estradiol on the stimulation of dopamine turnover in mesolimbic and nigrostriatal systems by cocaine- and amphetamine-regulated transcript peptide in female rats.

The present studies aimed to determine whether estradiol (E(2)) modulates the stimulation of cocaine- and amphetamine-regulated transcript (CART) peptide in the mesolimbic and nigrostriatal dopaminergic systems. I.c.v. administration of the CART peptide (55-102, 1 microg/3 microl) increased dopamine turnover (3,4-dihydroxyphenylacetic acid, DOPAC) in the nucleus accumbens (NA) and striatum (ST) in ovariectomized (OVX) female Sprague-Dawley rats with E(2)-priming. This stimulation of NA and ST DOPAC contents by CART peptide was found in OVX+E(2) female rats, but not in OVX only female rats, suggesting E(2) is an important factor in modulating the stimulatory effect of CART in the regulation of NA and ST DOPAC contents. This stimulation by CART peptide was also restored by treatment with the water-soluble form of E(2), but not by treatment with the membrane-impermeable form of E(2) in OVX female rats, suggesting that E(2) acts through intracellular rather than extracellular mechanisms to modulate the effects of CART peptide. Furthermore, the effects of water-soluble form of E(2) were blocked by E(2) antagonist, tamoxifen, but not by testosterone antagonist, flutamide. Our findings are the first to demonstrate that that E(2) plays a regulatory role in stimulation of CART peptide in mesolimbic and nigrostriatal dopaminergic systems in female rats, and E(2) acts through its own receptor(s) and intracellular mechanisms.

Cocaine- and amphetamine-regulated transcript in the nucleus accumbens participates in the regulation of feeding behavior in rats.

The present studies aimed to determine whether cocaine- and amphetamine-regulated transcript (CART) peptide in the nucleus of accumbens shell (AcbSh) is implicated in the regulation of food intake. Bilateral intranuclear injections of CART peptide (55-102, 1 microg/microl/side) into the AcbSh decreased food intake with no change in locomotion activity and attenuated the orexigenic effect of the GABA(A) agonist muscimol (100 ng/microl/side) in male Sprague-Dawley rats. Decreased food intake after bilateral intranuclear injections of CART was more sustained in freely fed rats than in food-deprived rats, suggesting fuel availability is an important factor in modulating the function of CART in the regulation of feeding. Our anatomical findings indicate that in addition to the perifornical region and the arcuate nucleus, some neurons within the AcbSh also project within the AcbSh. Moreover, many of these efferent cells contain CART immunoreactivity, including those which reside within the AcbSh, suggesting that accumbal CART circuitry is involved in the central function of the nucleus accumbens. Furthermore, fasting suppressed CART mRNA levels in the AcbSh, paraventricular nucleus of the hypothalamus, arcuate nucleus, and the perifornical region, indicating that the Acb is sensitive to fuel availability to an extent similar to those regions in the hypothalamus. Our findings are the first to demonstrate that CART mRNA in the AcbSh is sensitive to metabolic challenges and that injection of CART peptide into the AcbSh has an inhibitory effect on food intake.

Effects of the cocaine- and amphetamine-regulated transcript peptide on the turnover of central dopaminergic neurons.

The effects of the cocaine- and amphetamine-regulated transcript (CART) peptide on central dopaminergic (DA) neurons were examined in ovariectomized, estrogen-primed Sprague-Dawley rats in both the morning and afternoon. Intracerebroventricular administration of 1 microg, but not lower doses of the CART peptide (55-102), either in the morning or afternoon produced a prolonged increase in the 3,4-dihydroxyphenylacetic acid (DOPAC) level in the median eminence (ME) and a corresponding decrease of serum prolactin (PRL) levels, which resulted from stimulation of tuberoinfundibular dopaminergic neurons. The CART peptide stimulated DOPAC levels in the striatum (ST), nucleus accumbens (NA), hypothalamic paraventricular nucleus (PVN), and periventricular (A14), but had no effect in the medial prefrontal cortex (MPFC) or suprachiasmatic nucleus (SCN). These effects of the CART peptide on stimulation of central DA systems and inhibition of PRL levels are specific because the inactive form of the CART peptide (0.1 and 1 microg) could not induce a similar response. Stimulatory effects of the CART peptide on different central DA systems displayed differential time-response profiles in the NA and ST, ME, and PVN and A14. These findings indicate that the CART peptide may selectively regulate certain central DA neuronal activities.

Distribution of the rhythm-related genes rPERIOD1, rPERIOD2, and rCLOCK, in the rat brain.

High densities of mRNAs for three rhythm-related genes, rPeriod1 (rPer1), rPer2, and rClock, which share high homology in Drosophila and mice, were found in the hypothalamic suprachiasmatic nucleus (SCN). The SCN, however, is not the only brain region that expresses these genes. To understand the distributions and possible physiological roles of these rhythm-related genes, we examined the gene expressions of rPer1, rPer2, and rClock in different brain regions by serial coronal, sagittal, and horizontal brain sections in Sprague-Dawley male rats. Animals were housed in a light-controlled room (lights on from 0600 to 1800 h) and killed at 1000 or 1200 h, which corresponds to Zeitgeber time 4 or 6. Semi-quantitative in situ hybridization with (35)S-riboprobes was used to evaluate mRNA levels. The mRNAs of rPer1, rPer2, and rClock were widely distributed in the rat CNS, including the olfactory bulb, cortex, piriform cortex, SCN, ventromedial hypothalamus, arcuate nucleus, hippocampus, mammillary nucleus, pontine nucleus, superior and inferior colliculus, cerebellum, median eminence/pars tuberalis, pineal gland, and pituitary. The expression patterns of mRNAs for rPer1 and rPer2 were almost identical. In contrast, different expression patterns were observed between rClock and rPer1 or rPer2 in several brain regions, including the hypothalamic supraoptic and suprachiasmatic nuclei, the paraventricular zone of the caudate putamen, the superior olivary nucleus, and anterior and intermediate lobes of the pituitary. These findings suggest that the different expression patterns observed for rPer1, rPer2, and rClock might be due to their different physiological role(s) in those brain regions.

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 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.

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 melatonin's 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.

Distribution of Mahogany/Attractin mRNA in the rat central nervous system.

The Mahogany/Attractin gene (Atrn) has been proposed as a downstream mediator of Agouti signaling because yellow hair color and obesity in lethal yellow (A(y)) mice are suppressed by the mahogany (Atrn(mg)) mutation. The present study examined the distribution of Atrn mRNA in the brain and spinal cord by in situ hybridization. Atrn mRNA was found widely distributed throughout the central nervous system, with high levels in regions of the olfactory system, some limbic structures, regions of the brainstem, cerebellum and spinal cord. In the hypothalamus, Atrn mRNA was found in specific nuclei including the suprachiasmatic nucleus, the supraoptic nucleus, the medial preoptic nucleus, the paraventricular hypothalamic nucleus, the ventromedial hypothalamic nucleus, and the arcuate nucleus. These results suggest a broad spectrum of physiological functions for the Atrn gene product.

Stimulatory role of prolactin on the development of tuberoinfundibular dopaminergic neurones in prepubertal female rats: studies with cysteamine and somatostatin.

Cysteamine, a potent depletor of prolactin and somatostatin, was used to determine the role of prolactin and somatostatin in the control of central dopamine neurones in prepubertal rats. Cysteamine (100 mg/kg, i.p., twice daily) was injected for 7, 14 or 21 days in 28-day-old Sprague-Dawley female rats in one study and for 3 days in 35-day-old rats in another. In control rats, the 3, 4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence increased threefold from day 35 to day 49, and serum prolactin concentration increased about 50%. Cysteamine lowered serum prolactin concentrations to 20%, and median eminence DOPAC and dopamine levels to 32-50% of control levels in both studies. The DOPAC levels in the nucleus accumbens and striatum were also lowered, while both DOPAC and dopamine in the paraventricular nucleus and periventricular nucleus (A14) were increased by cysteamine. A single injection of rat prolactin (0.01, 0.1 or 1 mg/kg) significantly increased DOPAC or DOPA levels in the median eminence, nucleus accumbens and striatum, but not in the paraventricular nucleus or A14 at 14 h later in 28-day old female rats or in 40-day-old rats pretreated with cysteamine. In contrast, central injection of somatostatin dose (0.001-1 microg/rat) and time (30-90 min) dependently decreased the DOPAC levels in the median eminence, paraventricular nucleus and A14 and increased those in the nucleus accumbens and striatum of adult female rats. These results indicate that serum prolactin is important for the maturation and maintenance of dopamine systems in the median eminence, nucleus accumbens and striatum, while somatostatin exhibits inhibitory and stimulatory effects on hypothalamic and midbrain dopamine systems, respectively.

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.

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 nicotine's 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 atropine's 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.

Sexual differences in the diurnal changes of tuberoinfundibular dopaminergic neuron activity in the rat: role of cholinergic control.

A diurnal change of the tuberoinfundibular dopaminergic (TIDA) neuron activity in ovariectomized rats treated with or without estrogen was recently reported, and the endogenous cholinergic system may be responsible for its induction. Whether a similar rhythm exists in intact female or male rats was the focus of this study. TIDA neuron activity was assessed by measurement of the precursor or metabolite of dopamine in the median eminence by HPLC with electrochemical detection. In intact female Sprague-Dawley rats, diurnal changes in TIDA neuron activity were observed during all stages of the estrous cycle, i.e., proestrus, estrus, and diestrus 1, and they were nearly identical. No such rhythm, however, was observed in intact male rats. Castration alone decreased and increased basal TIDA neuron activity in female and male rats, respectively, and estrogen treatment increased the activity in both sexes. The diurnal changes in TIDA neuron activity, however, were observed only in the female rats, not the male rats, regardless of castration or estrogen treatment. Serum prolactin levels in the male rats exhibited no diurnal changes either, irrespective of whether the animals had been castrated or treated with estrogen. Central administration of mecamylamine (1 micrograms/3 microliters per rat, i.c.v.), a nicotinic receptor antagonist, in the morning (at 1000 h) or afternoon (at 1500 and 1700 h) induced a differential effect on the TIDA neuron activity of intact female rats, i.e., no effect in the morning and stimulation in the afternoon; it had no effect on the TIDA neuron activity in the male rats regardless of the injection time. In castrated male rats treated with estrogen, however, mecamylamine treatment further increased TIDA neuron activity, but still with no diurnal difference. In summary, a sexual difference in the diurnal changes of TIDA neuron activity was observed, and these changes may be under differential control by the cholinergic system.

An endogenous cholinergic rhythm may be involved in the circadian changes of tuberoinfundibular dopaminergic neuron activity in ovariectomized rats treated with or without estrogen.

We recently reported that a circadian change in the activities of hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons exists in ovariectomized (OVX) rats treated with or without estrogen. The involvement of an endogenous cholinergic control mechanism was the focus of this study. Adult female Sprague-Dawley rats OVX for 2 weeks and treated with or without a long-acting estrogen (poly-estradiol phosphate, 0.1 mg/rat, sc) were used in the study. An intracerebroventricular cannula for drug injection was implanted into the lateral cerebroventricle of each rat 6 days before experiment. TIDA neuron activity was determined by measuring the concentrations of 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxyphenylalanine in the median eminence by HPLC plus electrochemical detection. Serum PRL levels were determined by RIA. Neither atropine nor mecamylamine, two cholinergic receptor antagonists, had any effect on TIDA neuron activity in the morning before 1200 h, when endogenous TIDA neuron activity is high. Both drugs, however, exhibited a dose-related stimulating effect on the TIDA neuron activity in the afternoon, when endogenous activity is low. The estrogen-induced afternoon PRL surge was also blocked by a single injection of atropine or mecamylamine at 1300 h. The rhythmic changes in endogenous TIDA neuron activity and their responses to atropine were also observed in OVX rats with no estrogen replacement. In conclusion, a daily change in endogenous cholinergic neuron activity may be responsible for the change in TIDA neuron activity in female rats, which is a prerequisite for the PRL surge.

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.