Prolactin release during exercise in normal and adrenodemedullated untrained rats submitted to central cholinergic blockade with atropine

The median eminence detects and responds to circulating prolactin in the male mouse

In addition to its established lactational roles, prolactin acts on multiple target tissues and its circulating levels are responsive to a range of physiological stimuli. The present study used immunohistochemistry to demonstrate that systemic administration of prolactin activates target cells in the arcuate nucleus and median eminence of the male mouse. Prolactin receptor stimulation results in the phosphorylation and thus activation of the signal transducer and activator of transcription (STAT)5 pathway. Interestingly, although, in the arcuate nucleus, this response was localised to cell nuclei, the median eminence displayed both nuclear and diffuse, non-nuclear, phospho-STAT5 (pSTAT5) staining. Dual-label immunostaining demonstrated that, although the majority of nuclear pSTAT5 within the median eminence was located within vimentin-positive tanycytes, the non-nuclear staining occurred primarily in neuronal (βIII tubulin immunoreactive) elements. This conclusion was supported by the marked reduction of this signal in prolactin-treated mice lacking neuronal prolactin receptors. A smaller reduction was also seen in animals lacking prolactin receptors on GABAergic but not glutamatergic neurones. These findings identify a new prolactin target tissue and, in doing so, support the proposal that the median eminence has a sensory role in addition to its established secretory function. The physiological significance of this prolactin response is unknown, although its rapidity (maximum within 2 minutes of i.p. injection) suggests that it may enable the early detection of an increase in circulating prolactin. It is also possibile that non-nuclear prolactin-generated pSTAT5 in the median eminence may have a local, non-transcriptional, action. To this end, we used Evans Blue dye to demonstrate that elevated prolactin appears to reduce median eminence permeability and also that this effect is lost in animals lacking neuronal prolactin receptors.

Behavioral and endocrine evaluation of the stressfulness of single-pen housing compared to group-housing and social isolation conditions

Regulation of neuroendocrine responses is often studied in animals housed indoors in individual contiguous pens. In sheep, these housing conditions are used to control the environment, facilitate biological sampling and limit social stress. However, this type of housing also prevents exploratory behaviors and could induce stereotypies, non-compliant with welfare and possibly associated with a state of stress. In this context, we investigated the impact of housing in a single-pen, with other familiar conspecifics, on emotional state by evaluating behavioral, hormonal and neuronal measures in adult ewes. We hypothesized that emotional state would be more negative in animals housed in a single-pen for one week (Pen) than in freely moving animals (Free) but less negative than in socially isolated subjects (Isol). We tested our hypothesis in ovariectomized ewes to avoid the interaction with sexual steroid variations. Our behavioral, endocrine and neuronal (Fos activation of the corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus) measures confirmed that withdrawing familiar conspecifics was sufficient to induce strong stress responses in Isol ewes, but there was no indication that Pen ewes were stressed. However, the latter showed less mastication activity than Free ewes, probably due to limited accessibility to straw. The highest plasma prolactin levels were observed in Isol and Free animals, which might result from stress and physical activity, respectively. In Free ewes, plasma dopamine was low, consistent with its inhibitory control of prolactin. However, Isol animals had both high levels of prolactin and dopamine, suggesting a dysregulated balance in socially stressed ewes. As in other species, we suggest that the regulation of prolactin by dopamine varies with stress and/or social context. Overall, this study shows that the impact of housing conditions on different neuroendocrine systems should be considered more in the future.

Neonatal maternal separation affects endocrine and metabolic stress responses to ether exposure but not to restraint exposure in adult rats

We investigated prolactin secretion and metabolic changes in stress response in adult male rats submitted to periodic maternal separation (MS; 180 min/day) at 2 weeks of life. Restraint and ether exposure were randomly performed when the animals were 10-12 weeks of age. Restraint exposure: the animals were placed into plastic tubes (21 cm long, 4.5 cm diameter) for 20 min. Ether exposure: the rats were exposed to ether for 10 min. Atrial cannulation for blood sampling was performed through the jugular vein 5 days before the experiments. In both protocols, blood samples were taken immediately before (0), and 5, 15 and 20 min after the beginning of stress exposure. Ours results showed attenuated endocrine and metabolic responses to ether exposure in the maternal separation (MS) group compared to the control group. The measured metabolic parameters, plasma glucose, prolactin, lactate, and insulin secretion, were 32%, 55%, 41%, 73% lower (P < 0.01), respectively, in MS than in control animals. On the other hand, the endocrine and metabolic stress responses to restraint exposure were not affected by maternal separation. There was no difference between the MS and the control groups in any of the parameters studied. Our data demonstrated that early life experiences affect the hormonal systems beyond the hypothalamic-pituitary-adrenal axis, such as the central neuronal pathways, and their activities related to hormonal and metabolic responses to stress in adulthood. More importantly, these modifications were specific, but dependent on stress situation affecting mainly the circuitry related to the stress response to ether exposure.

Association between dopamine-related polymorphisms and plasma concentrations of prolactin during risperidone treatment in schizophrenic patients

Hyperprolactinemia is an inevitable consequence of treatment with antipsychotic agents to some extent because prolactin response to antipsychotics is related to dopamine blockade. Recent studies have suggested that polymorphisms of the dopamine receptors are associated with therapeutic response to antipsychotics. Thus, we studied the effects of major polymorphisms of dopamine-related genes on plasma concentration of prolactin. Subjects were 174 schizophrenic patients (68 males, 106 females) receiving 3 mg twice daily of risperidone for at least 4 weeks. Sample collections were conducted 12 h after the bedtime dosing. Five dopamine-related polymorphisms (Taq1A, -141C ins/del for DRD2, Ser9Gly for DRD3, 48 bp VNTR for DRD4, Val158Met for COMT) were identified. The mean (+/-SD) plasma concentration of prolactin in females was significantly higher than males (54.3+/-27.2 ng/ml versus 126.8+/-70.2 ng/ml, p<0.001). No dopamine-related polymorphisms differed the plasma concentration of prolactin in males or females. Multiple regression analyses including plasma drug concentration and age revealed that plasma concentration of prolactin correlated with gender (standardized partial correlation coefficients (beta)=0.551, p<0.001) and negatively with age (standardized beta=-0.202, p<0.01). No correlations were found between prolactin concentration and dopamine-related polymorphisms. These findings suggest that plasma prolactin concentrations in females are much higher than in males but the dopamine-related variants are not predominantly associated with plasma concentration of prolactin.

Central AT(1) receptor blockade increases metabolic cost during exercise reducing mechanical efficiency and running performance in rats

The effect of central angiotensin AT(1) receptor blockade on metabolic rate and running performance in rats during exercise on a treadmill (18 m x min(-1), 5% inclination) was investigated. Oxygen consumption (VO(2)) was measured, using the indirect calorimetry system, while the animals were exercising until fatigue after injection of 2 microL of losartan (Los; 60 nmol, n=9), an angiotensin II AT(1) receptor antagonist, or 2 microL of 0.15 M NaCl (Sal, n=9) into the right lateral cerebral ventricle. Mechanical efficiency (ME) and workload (W) were calculated. The W performance by Los-treated animals was 29% lesser than in Sal-treated animals (p<0.02). During the first 10 min of exercise (dynamic state of exercise), there was a similar increase in VO(2), while ME remained the same in both groups. Thereafter (steady state of exercise), VO(2) remained stable in the Sal group but continued to increase and stabilized at a higher level in Los-treated animals until fatigue. During the steady state of exercise there was a sharper reduction in ME in Los-treated rats compared to Sal-treated animals (p<0.01) that was closely correlated to W (r=0.74; p<0.01). Our data showed that AT(1) receptor blockade increases metabolic cost during exercise, reducing mechanical efficiency. These results indicate that central angiotensinergic transmission modulates heat production, improving ME during the steady state of exercise.

Muscarinic cholinoceptors in the ventromedial hypothalamic nucleus facilitate tail heat loss during physical exercise

The aim of this study was to evaluate the participation of ventromedial hypothalamic nucleus (VMH) muscarinic cholinoceptors in heat balance and central fatigue during treadmill exercise (24 m min(-1), 5% inclination). The animals were anesthetized with pentobarbital sodium (50 mg/kg body weight i.p.) and fitted with bilateral cannulae into the VMH 1 week prior to the experiments. Tail skin (T(tail)) and core body temperatures (T(b)) were measured after the injection of 0.2 microL of 5 x 10(-9) mol methylatropine (Matr) or 0.15 M NaCl solution (Sal) into the hypothalamus. Methylatropine injection into the VMH greatly increased heat storage rate (HSR) measured until fatigue (19.7+/-4.6 cal min(-1) Matr versus 9.7+/-3.3 cal min(-1) Sal; P<0.05) and attenuated the exercise-induced tail vasodilation as seen by T(tail) (23.98+/-0.43 degrees C Matr versus 25.52+/-0.85 degrees C Sal; at 6.5 min; P<0.05), indicating inhibition of the heat loss process. The 2 min delay and the increased DeltaT(b), which triggered the heat loss mechanisms observed in Matr-treated rats, are associated with increased HSR and may be responsible for the decreased running performance of these animals (21.0+/-2.9 min Matr versus 33.5+/-3.4 min Sal; P<0.001). In fact, a close negative correlation was observed between HSR and time to fatigue (r=-0.61; P<0.01). In conclusion, VMH muscarinic cholinoceptors facilitate tail heat loss mechanisms, and a delay in this adjustment would lead to a decrease in physical exercise performance due to excess heat storage.

Activation of the central cholinergic pathway increases post-exercise tail heat loss in rats

The aim of this study was to evaluate the effects of stimulation of the central cholinergic pathway on the regulation of post-exercise tail heat loss in rats. Either 2.0microL of 25x10(-3)M physostigmine (Phy) or 0.15M NaCl solution (Sal) were injected into the right lateral cerebral ventricle of both resting (n=8) and post-exercising rats (n=6; 24mmin(-1); 25min; 5% inclination). Tail temperature (Ttail) was measured using a thermistor taped to the tail, and intraperitoneal temperature, an index of core temperature (Tc), was recorded using a telemetry sensor implanted into the peritoneal cavity. In resting rats, Phy induced an increase in both Ttail (26.8+/-0.3 degrees C Phy versus 25.2+/-0.6 degrees C Sal; P<0.05) and in heat loss index (0.26+/-0.03 Phy versus 0.14+/-0.05 Sal; P<0.05; 30min after injection), and a decrease in Tc compared to the Sal injection group (36.6+/-0.2 degrees C Phy versus 37.0+/-0.2 degrees C Sal; P<0.05). In post-exercising rats, Phy injection attenuated the decrease in both T(tail) (28.3+/-0.8 degrees C Phy versus 26.4+/-0.6 degrees C Sal; P<0.05) and heat loss index (0.37+/-0.07 Phy versus 0.19+/-0.02 Sal; P<0.05) without altering Tc. We conclude that activation of the central cholinergic pathway increases post-exercise tail heat loss in rats.

Tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area

To assess the effects of increased hypothalamic tryptophan (TRP) availability on 5-HT content in preoptic area on thermoregulation and work production during exercise on treadmill, 20.3 microM of L-TRP (n=7) or 0.15M NaCl (n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18 m min(-1) 5% inclination). Exercise time to fatigue (min), and workload (kgm) were analysed. Core temperature was measured by telemetry. At fatigue, brains were quickly removed and preoptic area (POA), hypothalamus (HP), frontal cortex (FC), hippocampi (HC) were rapidly dissected and frozen immediately in dry ice. Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured by HPLC. TRP-exercised rats showed the highest content of 5-HT in the POA and the lowest in the hippocampi compared to the rested and SAL-exercised rats. An inverse relationship between TF and a direct correlation with body temperature changes and POA-5HT levels were observed. A correlation between HC 5-HT content and TF was also found. However, there was no correlation between HC 5-HT content and changes in Tb at fatigue. Finally, our results bring further evidences that increased 5-HT content in POA is involved with an increase in heat production during exercise. In addition, the direct correlation of 5-HT level in hippocampi and TF of TRP-exercised rats suggests that this brain area is also related to motor activity control during exercise. In conclusion, our data indicated that tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area.

Central nitric oxide inhibition modifies metabolic adjustments induced by exercise in rats

The influence of the central nervous system on metabolic function is of interest in situations deviating from basal states, such as during exercise. Our previous study in rats demonstrated that central nitric oxide (NO) blockade increases metabolic rate, reducing mechanical efficiency during exercise. To assess the role of brain nitric oxide in the plasma glucose, lactate and free fatty acids (FFAs) concentrations of rats submitted to an incremental exercise protocol on a treadmill until fatigue, 1.43 micromol (2 microl) of N(omega)-nitro-l-arginine methyl ester (L-NAME, n=6), a NO synthase inhibitor, or 2 microl of 0.15M NaCl (SAL, n=6) was injected into the lateral cerebral ventricle (icv) of male Wistar rats immediately before exercise (starting at 10 m/min, with increments of 1m/min every 3 min until fatigue, 10% inclination). Blood samples were collected through a chronic jugular catheter at rest and during exercise until fatigue. During exercise, the L-NAME-treated animals had the following metabolic response compared to controls: (1) an increased hyperglycemic response during the first 60% of time to fatigue; (2) higher plasma lactate levels; and (3) a significant transitory increase in plasma free fatty acids during the dynamic phase of exercise that returned to basal levels earlier than controls during the steady state phase of exercise. In addition L-NAME-treated rats fatigued earlier than controls. The data indicate that the inhibition of the brain nitrergic system induced by icv L-NAME treatment disrupted the accuracy of the neural mechanism that regulates plasma glucose and free fatty acids mobilization during exercise in rats.

Central angiotensin AT1-receptor blockade affects thermoregulation and running performance in rats

The effect of central angiotensin AT1-receptor blockade on thermoregulation in rats during exercise on a treadmill (18 m/min, 5% inclination) was investigated. Core (Tb) and skin tail temperatures were measured in rats while they were exercising until fatigue after injection of 2 microl of losartan (Los; 20 nmol, n = 4; 30 nmol, n = 4; 60 nmol, n = 7), an angiotensin II AT1-receptor antagonist, or 2 microl of 0.15 mol/l NaCl (Sal; n = 15) into the right lateral cerebral ventricle. Body heat rate (BHR), heat storage rate, threshold Tb for tail vasodilation (TTbV), time to fatigue, and workload were calculated. During exercise, the BHR and heat storage rate of Los-treated animals were, respectively, 40 and 53% higher (P < 0.01) than in Sal-treated animals. Additionally, rats injected with Los showed an increased TTbV (38.59 +/- 0.19 degrees C for Los vs. 38.12 +/- 0.1 degrees C for Sal, P < 0.02), a higher Tb at fatigue point (39.07 +/- 0.14 degrees C Los vs. 38.66 +/- 0.07 degrees C Sal, P < 0.01), and a reduced running performance (27.29 +/- 4.48 min Los vs. 52.47 +/- 6.67 min Sal, P < 0.01), which was closely related to the increased BHR. Our data suggest that AT1-receptor blockade attenuates heat dissipation during exercise due to the higher TTbV, leading to a faster exercise-induced increase in Tb, thus decreasing running performance.

Evidence that brain nitric oxide inhibition increases metabolic cost of exercise, reducing running performance in rats

To assess the role of nitric oxide (NO) in the metabolic rate and running performance of rats submitted to exercise on a treadmill, 1.43 micromol (2 microL) of Nomega-nitro-L-arginine methyl ester (L-NAME, n=6), a NO synthase inhibitor, or 2 microL of 0.15M NaCl (SAL, n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18m min(-1), 5% inclination). Oxygen consumption (VO2) was measured at rest, during the exercise until fatigue and thereafter during the 30 min of recovery using the indirect calorimetry system. Mechanical efficiency (ME) was also calculated during the running period. During the first 11 min of exercise, there was a similar increase in VO2 while ME remained the same in both groups. Thereafter, VO2 remained stable in the SAL group but continued to increase and remained higher in the L-NAME group until fatigue. The L-NAME-treated rats also showed a sharper decrease in ME than controls. In addition, there was a significant reduction in workload performance by L-NAME-treated animals compared to SAL-treated animals. This suggests that central blockage of nitric oxide increases metabolic cost during exercise, reduces mechanical efficiency and decreases running performance in rats.

Nitric oxide pathway is an important modulator of heat loss in rats during exercise

To assess the role of nitric oxide (NO) in central thermoregulatory mechanisms during exercise, 1.43 micromol (2 microL) of N(omega)-nitro-L-arginine methyl ester (L-NAME, n=6), a NO synthase inhibitor, or 2 microL of 0.15M NaCl (SAL, n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18 m min(-1), 5% inclination). Core (Tb) and skin tail (Ttail) temperatures were measured. Body heating rate (BHR), threshold Tb for tail vasodilation (TTbV), and workload (W) were calculated. During the first 11 min of exercise, there was a greater increase in Tb in the L-NAME group than in the SAL group (BRH=0.17+/-0.02 degrees C min(-1), L-NAME, versus 0.09+/-0.01 degrees C min(-1), SAL, p<0.05). Following the first 11 min until approximately 40 min of exercise, Tb levels remained stable in both groups, but levels remained higher in the L-NAME group than in the SAL group (39.16+/-0.04 degrees C, L-NAME, versus 38.33+/-0.02 degrees C, SAL, p<0.01). However, exercise went on to induce an additional rise in Tb in the SAL group prior to fatigue. These results suggest that the reduced W observed in L-NAME-treated rats (10.8+/-2.0 kg m, L-NAME, versus 25.0+/-2.1 kg m, SAL, p<0.01) was related to the increased BHR in L-NAME-treated animals observed during the first 11 min of exercise (r=0.74, p<0.01) due to the change in TTbV (39.12+/-0.24 degrees C, L-NAME, versus 38.27+/-0.10 degrees C, SAL, p<0.05). Finally, our data suggest that the central nitric oxide pathway modulates mechanisms of heat dissipation during exercise through an inhibitory mechanism.

Intracerebroventricular tryptophan increases heating and heat storage rate in exercising rats

The role of increased hypothalamic tryptophan (TRP) availability on thermoregulation and rates of core temperature increase and heat storage (HS) during exercise was studied in normal untrained rats running until fatigue. The rats were each anesthetized with 2.5% tribromoethanol (1.0 ml kg(-1) ip) and fitted with a chronic guiding cannula attached to the right lateral cerebral ventricle 1 week prior to the experiments. Immediately before exercise, they were randomly injected through these cannulae with 2.0 microl of 0.15 M NaCl (SAL; n=6) or 20.3 microM L-TRP solution (n=7). Exercise consisted of running on a treadmill at 18 m min(-1) and 5% inclination until fatigue. Body temperature was recorded before and during exercise with a thermistor probe implanted into the peritoneal area. Rates of core temperature increase (HR, degrees C min(-1)) and heat storage (HSR, cal min(-1)) were calculated. TRP-treated rats showed a rapid increase in body temperature which was faster than that observed in the saline-treated group during the exercise period. The TRP group also showed a higher rate of core temperature increase and HS. TRP-treated rats that presented higher HR and HSR also fatigued much earlier than saline-treated animals (16.8+/-1.1 min TRP vs. 40+/-3 min SAL). This suggests that the reduced running performance observed in TRP-treated rats is related to increased HR and HSR induced by intracerebroventricular injection of TRP in these animals.

Intracerebroventricular physostigmine facilitates heat loss mechanisms in running rats

The aim of this study was to evaluate the participation of central cholinergic transmission in the regulation of metabolic rate, core temperature, and heat storage in untrained rats submitted to exercise on a treadmill (20 m/min, 5% inclination) until fatigue. The animals were separated into eight experimental groups, and core temperature or metabolic rate was measured in the rats while they were exercising or while they were at rest after injection of 2 microl of 5 x 10(-3) M physostigmine (Phy) or 0.15 M NaCl solution (Sal) into the lateral cerebral ventricle. Metabolic rate was determined by the indirect calorimetry system, and colonic temperature was recorded as an index of core temperature. In resting animals, Phy induced only a small increase in metabolic rate compared with Sal injection, without having any effect on core temperature. During exercise, the Phy-treated animals showed a lower core heating rate (0.022 +/- 0.003 degrees C/min Phy vs. 0.033 +/- 0.003 degrees C/min Sal; P < 0.02), lower heat storage (285 +/- 37 cal Phy vs. 436 +/- 34 cal Sal; P < 0.02) and lower core temperature at fatigue point than the Sal-treated group (38.5 +/- 0.1 degrees C Phy vs. 39.0 +/- 0.1 degrees C Sal; P < 0.05). However, despite the lower core heating rate, heat storage, and core temperature at fatigue, the Phy-treated rats showed a similar running time compared with the Sal-treated group. We conclude that the activation of the central cholinergic system during exercise increases heat dissipation and attenuates the exercise-induced increase in core temperature without affecting running performance.

Evidence that tryptophan reduces mechanical efficiency and running performance in rats

It has been reported that exercise increases brain tryptophan (TRP), which is related to exhaustive fatigue. To study this further, the effect of increased TRP availability on the central nervous system (CNS) with regard to mechanical efficiency, oxygen consumption (VO(2)) and run-time to exhaustion was studied in normal untrained rats. Each rat was anesthetized with thiopental (30 mg/kg ip b. wt.) and fitted with a chronic guiding cannula attached to the right lateral cerebral ventricle 1 week prior to the experiments. Immediately before exercise, the rats were randomly injected through these cannulae with 2.0 microl of 0.15 M NaCl (n=6) or 20.3 microM L-TRP solution (n=6). Exercise consisted of running on a treadmill at 18 m min(-1) and 5% inclination until exhaustion. TRP-treated rats presented a decrease in their mechanical efficiency (21.25+/-0.84%, TRP group vs. 24.31+/-0.98%, saline-treated group; P< or =.05), and increased VO(2) at exhaustion (40.3+/-1.6 ml kg(-1) min(-1), TRP group vs. 36.0+/-0.8 ml kg(-1) min(-1), saline group; P< or =.05), indicating that the metabolic cost of exercise was higher in the former group. In addition, a highly significant reduction was also observed in run-time to exhaustion of TRP animals compared to those of the saline-treated group (15.2+/-1.52 min, TRP group vs. 50.6+/-5.4 min, saline group; P< or =.0001). It can be deduced from the data that intracerebroventricular TRP injection in rats increases O(2) consumption and reduces mechanical efficiency during exercise, diminishing running performance.