Formosan wood mice (Apodemus semotus) exhibit more exploratory behaviors and central dopaminergic activities than C57BL/6 mice in the open field test

Three-quarters of the lands in Taiwan are over 1000 m above sea level. Formosan wood mice (Apodemus semotus), also called Taiwanese field mice, are largely found at altitudes of 1400 ~ 3700 m and are the dominant rodents in these areas. Notably, Formosan wood mice show high levels of exploratory behaviors, not only in the wild but also in laboratory situations. Therefore, in this study, we examined the behavioral responses and central dopaminergic activities of male C57BL/6J mice and Formosan wood mice in the open field test. Dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid were used as indices of dopaminergic activities. Formosan wood mice showed higher levels of exploration and locomotor activity than C57BL/6J mice in the open field test. Higher central dopaminergic activities in the nucleus accumbens, striatum, and medial prefrontal cortex were found in Formosan wood mice than in C57BL/6J mice in the open field test. Higher levels of locomotion and central dopaminergic activities in Formosan wood mice were consistent after two exposures to the open field test; however, dramatic decreases in levels of locomotion and central dopaminergic activities in C57BL/6J mice were found after two exposures to the open field test. The present study found that Formosan wood mice exhibited higher levels of locomotor activity and exploration and central dopaminergic activities than C57BL/6J mice after one or two exposures to the open field test.

Intermittent Hypoxia Induces Autophagy to Protect Cardiomyocytes From Endoplasmic Reticulum Stress and Apoptosis

Intermittent hypoxia (IH), characterized as cyclic episodes of short-period hypoxia followed by normoxia, occurs in many physiological and pathophysiological conditions such as pregnancy, athlete, obstructive sleep apnea, and asthma. Hypoxia can induce autophagy, which is activated in response to protein aggregates, in the proteotoxic forms of cardiac diseases. Previous studies suggested that autophagy can protect cells by avoiding accumulation of misfolded proteins, which can be generated in response to ischemia/reperfusion (I/R) injury. The objective of the present study was to determine whether IH-induced autophagy can attenuate endoplasmic reticulum (ER) stress and cell death. In this study, H9c2 cell line, rat primary cultured cardiomyocytes, and C57BL/6 male mice underwent IH with an oscillating O₂ concentration between 4 and 20% every 30 min for 1-4 days in an incubator. The levels of LC3, an autophagy indicator protein and CHOP and GRP78 (ER stress-related proteins) were measured by Western blotting analyses. Our data demonstrated that the autophagy-related proteins were upregulated in days 1-3, while the ER stress-related proteins were downregulated on the second day after IH. Treatment with H₂O₂ (100 μM) for 24 h caused ER stress and increased the level of ER stress-related proteins, and these effects were abolished by pre-treatment with IH condition. In response to the autophagy inhibitor, the level of ER stress-related proteins was upregulated again. Taken together, our data suggested that IH could increase myocardial autophagy as an adaptive response to prevent the ER stress and apoptosis.

Exploratory and agile behaviors with central dopaminergic activities in open field tests in Formosan wood mice (Apodemus semotus)

Taiwan is a mountainous island, and nearly 75% of its lands are 1000 m above sea level. Formosan wood mice, Apodemus semotus, are endemic rodents and are broadly distributed at altitudes between 1400 m and 3700 m in Taiwan. Interestingly, Formosan wood mice show similar locomotor activity in the laboratory as they do in the wild. Hence, we are interested in studying whether exploratory behaviors and central dopaminergic activity are changed in the open field test. We used male C57BL/6J mice as the control, comparing their behavioral responses in the open field, step-down inhibitory avoidance discrimination and novel object recognition tests with those of male Formosan wood mice. We also examined dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid in the medial prefrontal cortex, striatum and nucleus accumbens. In open field tests, Formosan wood mice revealed higher levels of locomotion and exploration than C57BL/6J mice. Learning and memory performance in the novel object recognition test was similar in both Formosan wood mice and C57BL/6J mice, but more agile responses in the inhibitory avoidance discrimination task were found in Formosan wood mice. There was no difference in behavioral responses in the open field test between new second-generation Formosan wood mice and Formosan wood mice that were inbred for more than ten generations. After repeated exposure to the open field test, high levels of locomotion and exploration as well as central dopaminergic activities were markedly persistent in Formosan wood mice, but these activities were significantly reduced in C57BL/6J mice. Diazepam (anxiolytic) treatment reduced the higher exploratory activity and central dopaminergic activities in Formosan wood mice, but this treatment had no effect in C57BL/6J mice. This study provides comparative findings, as two phylogenetically related species showed differences in behavioral responses.

Circadian Patterns of Rats in Their Home Cages Detected Using a Video Tracking System

The diurnal rhythm is the common event in nature and specially shows in the behavioral patterns. Using the infrared sensor or photo beam detector to detect this 24-h rhythmicity in behaviors of mammalian, including in the rats and mice, is also the common way. The photo-sensory detecting mean is friendly and its advantage is unrestricted by light density and light-dark transition. However, this kind of equipment is cost-expensive and uneasy to fit for home cage in rodents. In this study, we tried to use the video-tracking system to detect the rhythmic activity of rats in their home cages. Adult male Sprague-Dawley rats, weighing 250-280 g, were used in this study and individual was kept in its own cage. Combined with the infrared sensitive charge-coupled device (CCD) camera and with automatically lights-off sensitive infrared illuminants as the accessory device, we found that animals exhibited the circadian locomotor activity in either light-dark cycles or constant darkness conditions. Moreover, the rhythmic patterns of locomotion in animals were affected by the one-hour exposure of white light under the constant darkness condition. The phase-advanced effects were found by the video tracking system. In summary, the video tracking system is the useful way to detect the rhythmic activity, especially in long-term circadian rhythmicity, in rats.

Short-term pre- and post-operative stress prolongs incision-induced pain hypersensitivity without changing basal pain perception

BACKGROUND

Chronic stress has been reported to increase basal pain sensitivity and/or exacerbate existing persistent pain. However, most surgical patients have normal physiological and psychological health status such as normal pain perception before surgery although they do experience short-term stress during pre- and post-operative periods. Whether or not this short-term stress affects persistent postsurgical pain is unclear.

RESULTS

In this study, we showed that pre- or post-surgical exposure to immobilization 6 h daily for three consecutive days did not change basal responses to mechanical, thermal, or cold stimuli or peak levels of incision-induced hypersensitivity to these stimuli; however, immobilization did prolong the duration of incision-induced hypersensitivity in both male and female rats. These phenomena were also observed in post-surgical exposure to forced swimming 25 min daily for 3 consecutive days. Short-term stress induced by immobilization was demonstrated by an elevation in the level of serum corticosterone, an increase in swim immobility, and a decrease in sucrose consumption. Blocking this short-term stress via intrathecal administration of a selective glucocorticoid receptor antagonist, RU38486, or bilateral adrenalectomy significantly attenuated the prolongation of incision-induced hypersensitivity to mechanical, thermal, and cold stimuli.

CONCLUSION

Our results indicate that short-term stress during the pre- or post-operative period delays postoperative pain recovery although it does not affect basal pain perception. Prevention of short-term stress may facilitate patients' recovery from postoperative pain.

Changes in Gene Expression Patterns of Circadian-Clock, Transient Receptor Potential Vanilloid-1 and Nerve Growth Factor in Inflamed Human Esophagus

Circadian rhythm is driven by the molecular circadian-clock system and regulates many physiological functions. Diurnal rhythms in the gastrointestinal tract are known to be related to feeding pattern, but whether these rhythms are also related to the gastrointestinal damage or injuries; for example, gastroesophageal reflux disease (GERD), is unclear. This study was conducted to determine whether expression of circadian-clock genes or factors involved in vagal stimulation or sensitization were altered in the esophagus of GERD patients. Diurnal patterns of PER1, PER2, BMAL1, CRY2, TRPV1, and NGF mRNA expression were found in patient controls, and these patterns were altered and significantly correlated to the GERD severity in GERD patients. Although levels of CRY1, TIM, CB1, NHE3, GDNF, and TAC1 mRNA expression did not show diurnal patterns, they were elevated and also correlated with GERD severity in GERD patients. Finally, strong correlations among PER1, TRPV1, NGF and CRY2 mRNA expression, and among PER2, TRPV1 and CRY2 expression were found. Expression levels of CRY1 mRNA highly correlated with levels of TIM, CB1, NHE3, GDNF and TAC1. This study suggests that the circadian rhythm in the esophagus may be important for the mediation of and/or the response to erosive damage in GERD patients.

Short-Term Sleep Disturbance-Induced Stress Does not Affect Basal Pain Perception, but Does Delay Postsurgical Pain Recovery

Chronic sleep disturbance-induced stress is known to increase basal pain sensitivity. However, most surgical patients frequently report short-term sleep disturbance/deprivation during the pre- and postoperation periods and have normal pain perception presurgery. Whether this short-term sleep disturbance affects postsurgical pain is elusive. Here, we report that pre- or postexposure to rapid eye movement sleep disturbance (REMSD) for 6 hours daily for 3 consecutive days did not alter basal responses to mechanical, heat, and cold stimuli, but did delay recovery in incision-induced reductions in paw withdrawal threshold to mechanical stimulation and paw withdrawal latencies to heat and cold stimuli on the ipsilateral side of male or female rats. This short-term REMSD led to stress shown by an increase in swim immobility time, a decrease in sucrose consumption, and an increase in the level of corticosterone in serum. Blocking this stress via intrathecal RU38486 or bilateral adrenalectomy abolished REMSD-caused delay in recovery of incision-induced reductions in behavioral responses to mechanical, heat, and cold stimuli. Moreover, this short-term REMSD produced significant reductions in the levels of mu opioid receptor and kappa opioid receptor, but not Kv1.2, in the ipsilateral L4/5 spinal cord and dorsal root ganglia on day 9 after incision (but not after sham surgery).

PERSPECTIVE

Our findings show that short-term sleep disturbance either pre- or postsurgery does not alter basal pain perception, but does exacerbate postsurgical pain hypersensitivity. The latter may be related to the reductions of mu and kappa opioid receptors in the spinal cord and dorsal root ganglia caused by REMSD plus incision. Prevention of short-term sleep disturbance may help recovery from postsurgical pain in patients.

Hepatic circadian-clock system altered by insulin resistance, diabetes and insulin sensitizer in mice

Circadian rhythms are intrinsic rhythms that are coordinated with the rotation of the Earth and are also generated by a set of circadian-clock genes at the intracellular level. Growing evidence suggests a strong link between circadian rhythms and energy metabolism; however, the fundamental mechanisms remain unclear. In the present study, neonatal streptozotocin (STZ)-treated mice were used to model the molecular and physiological progress from insulin resistance to diabetes. Two-day-old male C57BL/6 mice received a single injection of STZ and were tested for non-obese, hyperglycemic and hyperinsulinemic conditions in the early stage, insulin resistance in the middle stage, and diabetes in the late stage. Gene expression levels of the hepatic circadian-clock system were examined by real-time quantitative PCR. Most of the components of the hepatic circadian-clock gene expression system, such as the mRNAs of Bmal1 (brain and muscle Arnt-like protein-1), Per2 (period 2) and Cry1 (cryptochrome 1), were elevated, and circadian patterns were retained in the early and middle stages of insulin-resistant conditions. The insulin sensitizer, rosiglitazone, returns the physiological and molecular changes associated with the diabetic phenotype to normal levels through peroxisome proliferator-activated receptor γ (PPARγ) rather than PPARα. Early and chronic treatment with rosiglitazone has been shown to be effective to counter the diabetic condition. Over time, this effect acts to attenuate the increased gene expression levels of the hepatic circadian-clock system and delay the severity of diabetic conditions. Together, these results support an essential role for the hepatic circadian-clock system in the coordinated regulation and/or response of metabolic pathways.

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.

Transcranial magnetic stimulation for auditory hallucination in severe schizophrenia: partial efficacy and acute elevation of sympathetic modulation

Repetitive transcranial magnetic stimulation (rTMS) has been reported to be an effective treatment for auditory hallucination (AH) in schizophrenia patients. The efficacy of rTMS and immediate changes in cardiac autonomic function (CAF) after rTMS in severe schizophrenia patients with AH (n = 8) were investigated. Three patients reported a >or=50% reduction of AH after rTMS. The ratio of low-frequency power to high-frequency power, an index of sympathetic modulation, increased significantly after rTMS. Further replication studies with larger sample sizes are indicated.

Different patterns of food consumption and locomotor activity among Taiwanese native rodents, Formosan wood mice (Apodemus semotus), and common laboratory mice, C57BL/6 (Mus musculus)

The comparisons of food consumption and locomotor activity among Taiwan native rodents, Formosan wood mice (Apodemus semotus), and laboratory mice, C57BL/6, were examined in this study. The food consumption exhibited the circadian rhythmicity, e.g. higher in the lights-off period and lower in the lights-on period, in either Formosan wood mice (WM) or C57BL/6 mice. We also found that Formosan WM ate more food than C57BL/6 mice in the lights-off period and the whole day in males, but not in females. Similarly, the male Formosan WMs had more locomotor activities than the male C57BL/6 mice in the lights-off period, but this phenomenon did not appear in female mice. These results indicated that even though the Formosan WMs have been successfully inbred in the laboratory, they still keep more native paradigm than the laboratory C57BL/6 mice do. This study is the first report to provide basic physiological comparisons on native and common laboratory mice.

Gonadal hormones-mediated effects on the stimulation of dopamine turnover in mesolimbic and nigrostriatal systems by cocaine- and amphetamine-regulated transcript (CART) peptide in male rats

Estradiol and testosterone modulated behavioral and neurochemical activities in the mesolimbic and nigrostriatal dopaminergic systems have been reported. We examined whether estradiol and testosterone affect stimulation of cocaine- and amphetamine-regulated transcript (CART) peptide in the mesolimbic and nigrostriatal dopaminergic systems in this study. Intracerebroventricular administration of CART peptide increased dopamine turnover in the nucleus accumbens and striatum in male rats. Stimulation of dopamine turnover in nucleus accumbens and striatum by CART peptide were found in intact male rats, but not in castrated male rats. This stimulation was restored in castrated male rats by testosterone or estradiol priming, or by treatment with the water-soluble form of estradiol, but not by treatment with the membrane-impermeable form of estradiol. Estradiol and testosterone antagonists blocked testosterone's effects, but only estradiol antagonist blocked estradiol's effects. Moreover, treatment of dihydrotestosterone also restored the stimulation in castrated male rats. This dihydrotestosterone's effect was blocked by a testosterone antagonist, but not by an estradiol antagonist. All of these findings indicate that gonadal hormones play a regulatory role in stimulation of CART peptide in mesolimbic and nigrostriatal dopaminergic systems, and suggest that acts through intracellular rather than extracellular mechanisms.

Differential effects of melanin concentrating hormone on the central dopaminergic neurons induced by the cocaine- and amphetamine-regulated transcript peptide

Stimulatory effects of cocaine- and amphetamine-regulated transcript (CART) peptide on central mesolimbic, nigrostriatal and mesocortical dopaminergic (DA) neurons were examined in female Sprague-Dawley rats. We also determined the different blocking effects of melanin concentrating hormone (MCH) on the stimulation by CART peptide in central DA systems. Intracerebroventricular administration of 1 microg CART peptide (55-102) produced increases in 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the nucleus accumbens (NA) at 15 and 45 min, and in the striatum (ST) at 15 min, but not in the medial prefrontal cortex (MPFC). We found that the agonist of alpha-melanocyte stimulating hormone (alpha-MSH), MT II, at 10 microg had a stimulatory effect on the NA and ST DOPAC levels similar to the CART peptide. In contrast, 1 microg MCH and the antagonist of alpha-MSH, HS014, significantly decreased NA and ST DOPAC levels. However, only MCH prevented the stimulatory effect of CART peptide on DOPAC levels in the NA, but not in the ST. These results indicate that the stimulation of CART peptide on central DA neurons is region-specific, and that this effect can be blocked by MCH but not by the antagonist of alpha-MSH.

Diurnal rhythmic expression of the rhythm-related genes, rPeriod1, rPeriod2, and rClock, in the rat brain

High densities of the mRNA of three rhythm-related genes, rPeriod1 (rPer1), rPer2, and rClock, which share high homology in Drosophila and mammals, are found in the rat hypothalamic suprachiasmatic nucleus (SCN). The SCN, however, is not the only brain region that expresses these genes. To understand the possible physiological roles of these rhythm-related genes, we examined expression of these genes in different brain regions at various time points in male Sprague--Dawley rats. Using semi quantitativein situ hybridization with 35S-riboprobes to evaluate mRNA levels, the diurnal rhythmicity of rPer1, and rPer2 mRNA levels was found in the SCN, arcuate nucleus, and median eminence/pars tuberalis. Expression patterns of mRNA for rPer1 and rPer2, however, were not similar in these brain regions. The rhythmicity in these brain regions was specific, because it was not observed in the cerebellum or hippocampus. Moreover, diurnal changes in rClock mRNA expression were not detected in any of the brain regions examined. These findings suggest that the different expression patterns observed for rPer1, rPer2, and rClock mRNAs may be attributed to their different physiological roles in these brain regions, and support previous work indicating that circadian rhythms in the brain are widespread.

Effects of the cocaine- and amphetamine-regulated transcript peptide on the turnover of dopamine in tuberoinfundibular neurons and serum prolactin levels: studies using estrogen, melanin concentrating hormone, and melanocortin

Effects of the cocaine- and amphetamine-regulated transcript (CART) peptide on tuberoinfundibular dopaminergic (TIDA) neurons were examined in female and male Sprague-Dawley rats in the morning and afternoon. We also examined the blocking effects of melanin concentrating hormone (MCH) and the antagonists of alpha-melanocyte stimulating hormone (alpha-MSH), SHU9119 and HS014, on stimulation induced by the CART peptide in TIDA systems. Intracerebroventricular administration of 1 mug CART peptide (55-102) at 45 min, either in the morning or afternoon, produced an increase in the median eminence (ME) DOPAC (3,4-dihydroxyphenylacetic acid) level and a corresponding decrease in serum prolactin (PRL) levels. This resulted from stimulation of TIDA neurons regardless of castration, and whether or not male and female rats were estrogen-primed. The stimulatory effects of the CART peptide on ME DOPAC levels were similar in the morning and afternoon in both male and female rats. Central treatment with 1 microg SHU9119, HS014, or MCH significantly decreased the ME DOPAC levels and elevated serum PRL levels in female rats. However, only MCH prevented the stimulatory effect of the CART peptide on TIDA neurons. These results indicate that stimulation by the CART peptide on TIDA neurons is gender-independent; and this stimulatory effect can be blocked by MCH, but not the antagonists of alpha-MSH.

Glucocorticoid receptor overexpression in forebrain: a mouse model of increased emotional lability

The molecular mechanisms that control the range and stability of emotions are unknown, yet this knowledge is critical for understanding mood disorders, especially bipolar illness. Here, we show that the glucocorticoid receptor (GR) modulates these features of emotional responsiveness. We generated transgenic mice overexpressing GR specifically in forebrain. These mice display a significant increase in anxiety-like and depressant-like behaviors relative to wild type. Yet, they are also supersensitive to antidepressants and show enhanced sensitization to cocaine. Thus, mice overexpressing GR in forebrain have a consistently wider than normal range of reactivity in both positive and negative emotionality tests. This phenotype is associated, in specific brain regions, with increased expression of genes relevant to emotionality: corticotropin-releasing hormone, serotonin, norepinephrine and dopamine transporters, and 5-hydroxytryptamine(1A) receptor. Thus, GR overexpression in forebrain causes higher "emotional lability" secondary to a unique pattern of molecular regulation. This finding suggests that natural variations in GR gene expression can contribute to the fine-tuning of emotional stability or lability and may play a role in bipolar disorder.

Colocalization of estrogen beta-receptor messenger RNA with orphanin FQ, vasopressin and oxytocin in the rat hypothalamic paraventricular and supraoptic nuclei

The functional significance of the novel estrogen receptor beta in brain areas that exclusively contain the ERbeta receptor subtype such as the paraventricular (PVN) and the supraoptic (SON) nuclei of the hypothalamus is not yet fully understood. The present study attempts to characterize the peptidergic nature of the ERbeta-containing neuronal population in the PVN and the SON using the double in situ histochemistry method in the female rat. Using this method, the ERbeta mRNA coexpressions with the novel opioid neuropeptide (orphanin FQ and its receptor ORL1) mRNA in addition to the previously reported neuropeptide (arginine vasopressin-AVP, oxytocin-OXY, corticotropin releasing hormone-CRH, enkephalin-ENK) mRNAs were assessed. In the PVN, roughly half of the ERbeta expression was colocalized with the prepro-orphanin FQ mRNA, which was comparable to the colocalization observed between the ERbeta and AVP mRNAs in the same region. In addition, there was 20% overlap between the ERbeta and ORL1 receptor mRNAs, and 10% overlap between the ERbeta and OXY mRNAs in the PVN. By contrast, the coexpression between the prepro-orphanin FQ and ERbeta mRNAs was less striking in the SON. Potential interactions between the ERbeta and the well-characterized AVP-OXY neurosecretory system as well as the novel OFQ-ORL1 opioid neuropeptide system may provide new leads for the functional significance of ERbeta, specifically in stress/autonomic responses.

Diurnal rhythm of agouti-related protein and its relation to corticosterone and food intake

In the present study we examined the diurnal patterns of agouti-related protein (AGRP) and proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus and their relation to circulating glucocorticoids and food intake. Animals were killed at 4-h intervals throughout the 24-h diurnal cycle, and the expression of AGRP and POMC mRNA was evaluated by semiquantitative in situ hybridization analysis. We observed a significant diurnal rhythm in AGRP mRNA expression, with a marked peak at 2200 h (4 h after lights off) and a trough at 1000 h (4 h after lights on), consistent with the overall day-night rhythm of food intake. In contrast, POMC mRNA levels did not show a significant fluctuation across the diurnal cycle, although there was a tendency for levels to decrease after the onset of the dark cycle. Corticosterone secretion temporally coincided with the rising phase of AGRP mRNA expression. Depletion of corticosterone by adrenalectomy abolished the AGRP diurnal rhythm by suppressing the nighttime expression, but did not alter the feeding rhythm. Exposure of adrenalectomized rats to constant corticosterone replacement (10 or 50 mg continuous release corticosterone pellet) resulted in fixed AGRP mRNA expression throughout the 12-h light, 12-h dark cycle. A relatively high level of corticosterone (50 mg) significantly increased AGRP mRNA expression, with a positive correlation between these two measures. These results indicate that 1) the diurnal expression of AGRP mRNA is regulated by corticosterone independently of the light/dark cue; and 2) a normal endogenous corticosterone rhythm is required for generating the diurnal AGRP rhythm.

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.

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.