Axis In Male Rats Research Articles

Leptin Stimulates LH Secretion in Peripubertal Male Rats Through NMDA Receptors

Leptin, a peptide hormone secreted by adipocytes that has been proposed as a metabolic signal in the reproductive system, appears to be linked to the different neuroendocrine processes involved in the onset of puberty. We studied the ontogenic effect of administration of leptin (30 mg/kg i.p) on serum LH levels during different stages of sexual development (7, 30, and 45 days of age) in male rats and on the hypothalamic content of glutamate (GLU) and γ-aminobutyric acid (GABA) in 30-day-old rats.Leptin induced a significant increase (p < 0.01) in LH levels in 30 days old rats. This hormone stimulatory effect was accompanied by a significant enhancement (p < 0.01) of the hypothalamic content of glutamate, the hypothalamic excitatory aminoacid involved in N-methyl-D-aspartate (NMDA) neurotransmission. No changes in the LH plasma levels were observed in 7- and 45-day‐old male rats treated with leptin.MK 801 (0.1 and 0.3 mg/kg i.p.), an antagonist of NMDA receptors of excitatory amino acid system (EAAs), antagonized the stimulatory effect of leptin on LH secretion and on the hypothalamic content of GLU. These results demonstrate that leptin stimulates the reproductive axis in male rats during a determined period of sexual maturation and that NMDA receptors are involved in the facilitatory action of leptin on the gonadal axis of male rats during sexual maturation.

Unilateral paramedian-sagittal brain cut extending from the level of the anterior commissure to the midlevel of the third ventricle above the amygdala affects gonadal function in male rat: a lateralized effect.

The aim of the present investigations was to study involvement of fiber systems to and from the insular cortex above the amygdala in the neural control of the hypophysio-testicular axis in male rats. Animals were subjected to a unilateral paramedian-sagittal brain cut above the amygdala, extending from the level of the anterior commissure to the midlevel of the third ventricle and causing among others partial deafferentation of the insular cortex. Right-sided cut induced a significant rise in basal testosterone secretion in vitro of both testes as compared to intact or sham-operated controls without affecting serum testosterone level. By contrast, left-sided cut slightly suppressed testicular steroidogenesis and significantly decreased serum testosterone concentration. In animals underwent sham or actual cut on either side, serum luteinizing hormone levels were similar, but significantly lower than those in intact controls. No change was observed in serum FSH concentration of any experimental group. The results indicate that afferent and efferent connections of the partially deafferented cortical regions including among others the insular cortex are involved in the control of testosterone secretion. The data further suggest functional laterality of the interrupted structures.

Cadmium effects on hypothalamic-pituitary-testicular axis in male rats.

This study analyzes cadmium effects at the hypothalamic-pituitary-testicular axis. Male rats were given cadmium during puberty or adulthood. Cadmium exposure through puberty increased norepinephrine content in all hypothalamic areas studied, but not in the median eminence. Metal exposure increased serotonin turnover in median eminence and the anterior hypothalamus, while decreased it in mediobasal hypothalamus. Also, decreased plasma levels of testosterone were found. Cadmium exposure during adulthood increased norepinephrine content in posterior hypothalamus and decreased the neuro-transmitter content in anterior and mediobasal hypothalamus. Decreased circulating levels of luteinizing hormone (LH) and testosterone and increased plasma follicle stimulating hormone (FSH) levels were also observed. Cadmium accumulated in all analyzed tissues. Various parameters showed age-dependent changes. These data suggest that cadmium globally effects hypothalamic-pituitary-testicular axis function by acting at the three levels analyzed and that an interaction between cadmium exposure and age emerge.

Brain oxytocin inhibits the (re)activity of the hypothalamo–pituitary–adrenal axis in male rats: involvement of hypothalamic and limbic brain regions

In response to various stressors, oxytocin is released not only into blood, but also within hypothalamic and extrahypothalamic limbic brain regions. Here, we describe the involvement of intracerebrally released oxytocin in the regulation of the activity of the hypothalamo–pituitary–adrenal (HPA) axis by infusion of the oxytocin receptor antagonist (des Gly-NH 2 d(CH 2) 5 [Tyr(Me) 2, Thr 4] OVT; pH 7.4; Dr. M. Manning, Toledo, OH, USA) either into the lateral cerebral ventricle (icv[0.75 μg/5 μl,]) or via retrodialysis (10 μg/ml, 3.3 μl/min, 15 min) into the hypothalamic paraventricular nuclei (PVN), the medio-lateral septum or the amygdala. Male Wistar rats fitted with a chronic jugular vein catheter and an icv guide cannula or a microdialysis probe targeting the respective brain region 4 days prior to the experiment were blood sampled under basal as well as stressful conditions. Rats were exposed to the elevated platform (emotional stressor) and/or to forced swimming (combined physical and emotional stressor). Blockade of the receptor-mediated action of endogenous oxytocin within the PVN resulted in an enhanced basal secretion of ACTH whereas, in response to forced swimming, ACTH secretion was rather reduced, indicating a tonic inhibitory effect of OXT on basal HPA axis activity, but a potentiating action under conditions of stress. Within the medio-lateral septum, antagonist treatment did not alter basal ACTH secretion, but significantly disinhibited ACTH secretion in response to the elevated platform, but not to forced swimming. Within the amygdala, no significant effects either on basal or stress-induced HPA axis activity could be found. The results indicate a differential involvement of brain oxytocin in the regulation of the HPA axis activity which depends both on the site of intracerebral oxytocin release and the stressor the animals are exposed to.

Interleukin 6 modulates interleukin-1-and stress-induced activation of the hypothalamic-pituitary-adrenal axis in male rats.

This study examined the interaction between interleukin 6 (IL-6) and IL-1 and between IL-6 and stress on the activation of the hypothalamic-pituitary-adrenal (HPA) axis. Coadministration of IL-6 (100 ng/rat) with IL-1 (20 or 100 mg/ rat) resulted in synergistic stimulation of the HPA axis, as determined by increased plasma levels of ACTH and corticosterone (CORT) which were greater in rats that received both cytokines than in rats receiving either cytokine alone. Concomitant administration of IL-6 (100 ng/rat) with exposure to a novelty stressor also synergistically stimulated the activation of the HPA axis, as IL-6-treated rats subjected to novelty stress had greater increases in plasma levels of ACTH and CORT than vehicle-treated rats exposed to novelty stress or rats receiving IL-6 alone. However, concomitant administration of IL-6 (100 ng/rat) did not significantly affect restraint stress induced elevation of plasma levels of ACTH and CORT, although IL-6 tended to prolong restraint stress induced elevation of plasma levels of CORT. These findings indicate a modulatory role for IL-6 stimulated HPA axis activity in response to IL-1 or a novelty psychological stressor, but not for restraint stress.

Effects of HSV-1, a neurotropic virus, on the hypothalamic-pituitary-adrenocortical axis in rats

It is well established that the hypothalamic-pituitary-adrenocortical (HPA) axis is activated during systemic viral diseases. In this study we examined the effects of a neurotropic virus, herpes simplex virus type 1 (HSV-1), on the HPA axis in male rats. Following corneal inoculation with HSV-1, the virus invaded the nervous system and replicated in the brainstem without clinical signs of disease. During this asymptomatic brainstem infection with HSV-1, significant changes were found in the function of the HPA axis: On days 3, 7 and 14 post-infection (p.i.) basal ACTH and corticosterone (CS) levels were markedly elevated, and photic stressful stimulation failed to further increase the levels of these hormones. In addition, the elevated basal serum levels of ACTH and CS could not be suppressed by pretreatment with dexamethasone. The content of CRF-41 in the paraventricular nucleus of the hypothalamus and in the median eminence measured at 6 days p.i. was similar to that of vehicle inoculated rats. By 4 weeks p.i. the basal levels of ACTH and CS returned to normal and these animals responded to photic stimulation and dexamethasone similar to vehicle inoculated rats. Systemic (intraperitoneal) inoculation of HSV-1 did not induce any changes in the HPA axis responses. We therefore suggest that asymptomatic acute infection of the brainstem with HSV-1 may affect brain regions involved in the regulation of the HPA axis, and that those effects are mediated centrally and not by a systemic mechanism.

Effects of toxic levels of lead on gene regulation in the male axis: increase in messenger ribonucleic acids and intracellular stores of gonadotrophs within the central nervous system.

Lead is a male reproductive toxicant. Lead exposure results in a general suppression of the hypothalamic-pituitary-testicular (HPT) axis in male rats. The mechanism(s) for this disruption by lead is unknown. Toxic lead levels seem to disrupt central nervous system (CNS) control of the HPT system, resulting in a decrease in serum testosterone levels and sperm concentrations. A study designed to elucidate the mechanisms accounting for the disruption of the normal function of the male axis by toxic lead levels at the molecular level demonstrated a 2-3-fold enhancement of mRNA levels of GnRH and the tropic hormone LH. A 3-fold increase of intracellular stores of LH was also found. Because mRNA levels of LH and GnRH and pituitary levels of stored LH are proportional to blood levels of lead, we hypothesize that lead interferes with the normal release of tropic hormones and disrupts hormonal feedback mechanisms. The observed pleiotropic effects of lead upon the male axis and other systems may be explained by simple and unique competition by lead with normal metal ion binding sites that govern genetic control of specific genes.

Lead Exposure in Vivo Alters the Fertility Potential of Sperm in Vitro

Lead is a male reproductive toxicant. We previously reported that in vivo lead exposure results in a suppression of the hypothalamic-pituitary-testicular axis in male rats. This study was designed to assess if lead exposure in vivo alters (1) the ability of sperm to fertilize ova in vitro, (2) the morphology of the spermatozoa, and (3) the relationship of cell types in the testes as evaluated by DNA flow cytometry. Male Sprague-Dawley rats were given access to either lead-free (0.0%) or 0.3% lead acetate-containing water for 14, 30, or 60 days starting at 100 days of age. On the day of termination, sperm harvested from the caudae epididymis were incubated with eggs harvested from superovulated non-lead-treated females and were scored for stages of penetration. Sperm were also studied by electron microscopy. The testes of animals treated for 60 days were processed for DNA flow cytometry. The overall distribution of the stages of fertilization was significantly different between control and lead-treated animals. The lead-treated groups fertilized significantly fewer eggs than did sperm from the control group. Increased duration of exposure to lead acetate did not result in a more significant percentage of eggs not fertilized. No ultrastructural changes were noted in the spermatozoa of animals treated with lead compared to control animals. There were no differences in the histogram patterns of testicular cells collected from lead-treated animals and control animals. We conclude that lead alters sperm function by altering the hormonal control of spermatogenesis rather than by direct toxic action on spermatozoa.

Differences in effects of spironolactone and phenobarbital on the hepatic-thyroid axis in male rats.

Treatment with 150mg/kg of spironolactone (SPL) and 100mg/kg of phenobarbital (PB) for 13 weeks increased thyroid weight, numbers of apical vesicles and colloid droplets in thyroid follicular cells, and thyroid follicular hypertrophy. The patterns of elevated serum thyrotropin (TSH) and transiently decreased serum thyroxine (T4) and triiodothyronine (T3) suggest that thyroid follicular hypertrophy is a compensatory reaction to lowered thyroid hormone levels. Treatment with PB or SPL induced increased liver weight, hepatocellular hypertrophy, and smooth endoplasmic reticulum proliferation in hepatocytes. PB-treatment increased Uridine diphosphate glucuronosyl transferase (UDP-GT) activity toward p-nitrophenol and P-450 content but SPL-treatment had no such effect on hepatic microsomal enzymes. PB and SPL have been shown not to directly affect thyroid hormone synthesis. It is concluded that the effect of PB on thyroid function in male rats is secondary to increased hepatic UDP-GT and P-450 and that in SPL-treated male rats a distinct spectrum of isozymes of cytochrome P-450 and UDP-GT and/or other enzymes may play an important role for induction of thyroid hypertrophy.

Effects of tylosin on the pituitary-gonadal axis in male rats.

Former investigations in rats showed a decrease of thyroid hormone concentrations after treatment with the antibiotic and growth promoter tylosin (Schäfer 1984). In the present study, the effects of tylosin on the pituitary-gonadal axis in adult rats were studied. The substance was administered in two concentrations to rats (0.1 and 5.0 mg tylosin/kg feed) for three different periods: 15, 29 and 65 days. At the end of each period the organ weights were determined and the hormone levels in serum and pituitary gland were measured by radioimmunoassay. After 15 days reduced levels of LH (luteinizing hormone) and FSH (follicle stimulating hormone) in the pituitary gland and LH in serum were found. Moreover, the weight of seminal vesicles was decreased and the weight of pituitary increased. After 29 days an equilibrium between effects of tylosin and endocrine contraregulation seemed to be achieved. The prolonged tylosin administration (65 days) depressed testosterone concentration and increased hypophyseal LH stores. The testing of the pituitary-testicular axis with acute LHRH (luteinizing hormone-releasing hormone) stimulation caused a reduced increase of LH in animals treated with 0.1 mg tylosin. In contrast, the LH responsiveness to LHRH in animals treated with 5.0 mg tylosin was unchanged, while the testosterone response to released LH was reduced. These findings demonstrate that tylosin acts on the pituitary as well as on peripheral functions of the pituitary-gonadal-axis and that its effects depends on the time interval of tylosin administration.

Endothelin 3 (ET3) stimulates the hypothalamic-pituitary-adrenal axis mainly by corticotropin-releasing-hormone (CRH)

Endothelin 3 (ET3) is a member of the novel vasoconstrictive peptide family, identified in the porcine central nervous system. The effect of ET3 on the hypothalamic-pituitary-adrenal axis in male rats was examined in vivo and in vitro. Intravenous bolus injection of 1000pmol/kg of ET3 in free moving rats caused significant increases in plasma ACTH and corticosterone levels, almost equivalent to those of 100pmol/kg of rat corticotropin-releasing hormone (rCRH). Since an iv bolus injection of ET3 1000pmol/kg did not cause significant changes in the blood pressure of anesthetized rats or the locomotor activity of free moving rats, it seems unlikely that ET3 1000pmol/kg acted as a nonspecific stressor. When ET3 (10(-11) greater than 10(-7)M) was added to cultured anterior pituitary cells, neither direct stimulation of ACTH release nor potentiation of rCRH action was noted. Although it has been shown that ET3 administered systemically probably does not cross the brain-blood-barrier, circulating ET3 may reach the brain tissues through regions lacking the tight barrier, circumventricular structures. The next studies included pretreatment of antagonists or blockers of ACTH stimulating hormones to elucidate the mechanisms of ET3 induced ACTH release. The action of ET3 was virtually abolished by pretreatment of CRH-antagonist alpha helical CRH (150 micrograms/rat icv). But pretreatment of catecholamine-blocker alpha methyl-tyrosine (100mg/kg iv), arginine vasopressin-antagonist dP-thy(Me)AVP (50 micrograms/rat iv) and prostaglandin-blocker indomethacin (3mg/rat iv) did not inhibit the action of ET3. The results indicate that ET3 may play the role of a neuropeptide and that the stimulation of the CRH-neurons is mainly responsible for activation of ACTH and corticosterone release.

The adrenal gland and progesterone stimulates testicular steroidogenesis in the rat in vivo

Administration of pharmacological doses of glucocorticoid to male rats in vivo suppresses adrenal steroidogenesis and inhibits testicular steroidogenesis by inhibiting the anterior pituitary secretion of LH. In contrast, administration of ACTH to these pharmacologically-suppressed rats stimulates the adrenal secretion of progesterone and testicular steroidogenesis. The mechanism by which ACTH increases testicular steroidogenesis is dependent on the presence of the adrenal gland and is reproduced by the administration of progesterone. The conclusion from these data is that the adrenal gland has an important role in generating external signals that modulate the hypothalamic-pituitary-gonadal axis in male rats. The adrenal secretion of glucocorticoid acts as a negative signal to testicular steroidogenesis whereas progesterone acts as a positive signal. The adrenal secretion of progesterone and its conversion to testosterone by steroidogenic enzymes in the cytoplasm of the Leydig cell may provide an alternative pathway for testosterone biosynthesis and may account for the increased plasma testosterone levels during the acute phase of stress and mating.

Morphological changes in the hypothalamic-hypophyseal-gonadal axis of male rats after twelve months of streptozotocin-induced diabetes.

The hypothalami, pituitaries and testes from streptozotocin-treated and control male Wistar rats were examined by light and electron microscopy 12 months after induction of diabetes. Light and electron microscopic immunohistochemical techniques were employed for the localization of luteinizing hormone-releasing hormone and luteinizing hormone in the hypothalami and the pituitaries. In the hypothalami of diabetic animals swollen neuronal processes containing anti-luteinizing hormone-releasing-hormone positive material were frequent. In the pituitaries of the same animals a large number of small luteinizing hormone-gonadotrophs was found. These cells contained numerous secretory granules and were deficient in endoplasmic reticulum. The average testicular weight of the diabetic rats was significantly reduced but with marked individual variations. Histologically, the testes with the highest weights appeared normal, those with the lowest weights atrophic with few degenerating Leydig cells. these hypothalamic-hypophyseal changes are probably responsible for the testicular lesions found in experimental diabetes mellitus and may have relevance to the problem of infertility in human diabetes.

Effects of a superactive analogue of LH-RH on the hypothalamo-pituitary-testicular axis in male rats pretreated with estrogens.

The effect of a highly active agonistic analogue analogue of LH-RH on testicular weight an pituitary gonadotropin levels was studied in male rats previously treated with estrogens. These steroids induced a considerable depression of testicular weight. In the first experiment the treatment with a high dose (4 microgram daily) of D-Ala-6-des-Gly-10-LH-RH-ethylamide, the superactive analogue used, further depressed testicular weight and pituitary gonadotropins. In a second experiment, the treatment with lower doses (100 ng daily) of the analogue in estrogen-treated rats induced a slight but significant stimulatory effect on testicular weight and pituitary FSH content. In the third experiment none of the three doses of the analogue used (1, 10, or 100 ng daily) modified testicular weight when the treatment was started shortly after the administration of estrogens. It is concluded that, according to the dose of the analogue used, it is possible to observe inhibitory or stimulatory effect on testes and pituitary gonadotropin levels.

Diurnal rhythm of pituitary-thyroid axis in male rats and the effect of adrenalectomy.

Levels of TSH, thyroxine (T4) and triiodothyronine (T3) were measured by radioimmunoassay in plasma (TSH, T4, T3) and pituitary (TSH) of 60-day old male Long-Evans rats. Definite diurnal rhythms were demonstrated in pituitary TSH, plasma TSH and T3 in intact rats, evidenced by the statistically significant differences between zenith and nadir for pituitary TSH, plasma TSH and T3. The zenith value of pituitary TSH and the nadir values of plasma TSH, T4 and T3 were observed at the same time (2400 h) as were the nadir value of pituitary TSH and the zenith values of plasma TSH, T4, and T3 (at 1200 h). Our results indicate that the rhythmicity of pituitary TSH content is a mirror image of that of plasma TSH. Adrenalectomy not only reduced plasma corticosterone levels to almost zero, but also decreased plasma T3 and T4 levels. In adrenalectomized rats, the absolute concentration of pituitary and plasma TSH increased at 1200 h and at 2400 h and, in both cases, the difference between values at 1200 h and at 2400 h persisted. The differences in plasma TSH, T3 and T4 between 1200 h and 2400 h were also observed in sham-adrenalectomized rats. These results suggest glucocorticoids do influence the pituitary-thyroid axis, but that the rhythmicity of the pituitary-thyroid axis does not seem to depend on the rhythmicity of glucocorticoid secretion.

Effects of Repeated Morphine Administration on Copulation and on the Hypothalamic-Pituitary-Gonadal Axis of Male Rats

Adult male rats were administered morphine twice a day for 45 days and the effects of morphine on the copulation rate, the weight of various organs, and on the hypothalamic-pituitary-gonadal axis were examined. Morphine administered rats showed a loss of weight, hypertrophy of the adrenals, decreased weight of accessory sex organs, low sperm count, and decreased copulation rate. The contents of the luteinizing hormone releasing hormone in the hypothalamus and the luteinizing hormone in the pituitary remained unchanged. Serum luteinizing hormone and testosterone levels decreased, but serum follicle-stimulating hormone levels increased. These results suggest that morphine inhibits the hypothalamic-pituitary-gonadal axis and causes a diminution in the number of fertilizations of the partner females.