Effect of pituitary transplants on the LH-RH concentrations in the medial basal hypothalamus and hypophyseal portal blood

The effects of hyperprolactinemia on catecholamine turnover in the medial basal hypothalamus (MBH) and on the luteinizing hormone-releasing hormone (LH-RH) concentrations in MBH and hypophyseal portal blood were investigated in female Wistar rats. Chronic endogenous hyperprolactinemia was produced by implantation of anterior pituitary glands under the kidney capsule. Catecholamine turnover in the MBH was studied by inhibiting monoamine oxidase and then measuring the accumulation of catecholamines by high-performance liquid chromatography with electrochemical detection. Rats bearing pituitary transplants exhibited: (1) persistent vaginal diestrus within 3–6 days of the implantation; (2) increased serum concentrations of prolactin (PRL); (3) decreased serum concentrations of luteinizing hormone (LH); (4) increased pituitary concentrations of LH and follicle-stimulating hormone (FSH); (5) increased turnovers of dopamine in the MBH; and (6) decreased concentrations of LH-RH in the MBH and in plasma of hypophyseal portal blood.These findings suggest that chronic hyperprolactinemia may increase dopaminergic tone in the MBH that may inhibit LH-RH secretion from the MBH, and LH release from the pituitary. These processes may be responsible for disturbances of cyclic pituitary-ovarian activity.     

Ontogeny of catecholamine turnover rates in limbic and hypothalamic structures in relation to serum prolactin and gonadotropin levels

Norepinephrine (NE) and dopamine (DA) concentrations and turnover rates have been studied in the n. accumbens, medial preoptic area (MPO) and anterior and posterior parts of the mediobasal hypothalamus of developing rats. Turnover rates are determined by injection of -methyl-p-tyrosine 30 and 90 min prior to decapitation. NE concentrations and turnover in the n. accumbens were low in all age groups with slightly increased values between days 20 and 35 after birth whereas DA concentrations and turnover rates were low at day 15 and 20 and at high adult values by day 25 after birth. Medial preoptic and anterior mediobasal hypothalamic catecholamines exhibited a rather unique pattern. Concentrations and turnover rates were low in 15-day-old animals and increased between days 20 and 30 to very high values. Such high values were never observed in adult diestrous animals. The same pattern was also observed in the posterior mediobasal hypothalamus for NE concentrations and turnover rates whereas the respective values for DA showed relatively large fluctuations. On the basis of catecholamine measurements 30 and 90 min after blockade of tyrosine hydroxylase an attempt was also made to differentiate turnover rates of the functional and of the storage pool.

Serum LH levels in the 15-day-old animals showed large fluctuations. FSH levels were high and prolactin levels were low. At the time of increased preoptic and hypothalamic NE and DA turnover rates, serum prolactin levels were also high whereas serum LH levels were lowest between days 20 and 30 and then slightly increased. Serum FSH levels were uniformly low. The possibility is discussed that high NE turnover may stimulate pituitary LH and prolactin release by hypothalamic mechanisms. High serum prolactin levels may stimulate DA turnover which is inhibitory to pituitary LH release, thus counteracting the stimulatory effect of NE on LH-RH release. The dopaminergic inhibition of LH may be relieved at the time of puberty partially because the DA receptors become desensitized to the action of DA.     

Involvement of catecholamines in eliciting LH peaks in 15-day-old female rats: effects of treatment with prolactin

In the serum of 15-day-old female rats, treated either with -methyl-p-tyrosine (-MPT) or saline, serum LH levels were determined. From the brains of all animals the medial preoptic areas (MPO) and the anterior and posterior mediobasal hypothalami (AMBH and PMBH) were punched out and stored frozen in perchloric acid. In the punches of those animals with very high or basal LH levels catecholamine concentrations were measured radioenzymatically and on the basis of the degree of catecholamine depletion after -MPT the turnover rates of dopamine (DA) and norepinephrine (NE) were calculated. NE turnover in the MPO was significantly higher in the animal with high LH levels as compared to those with low LH values. In the PMBH an inverse correlation between LH levels and NE turnover rates could be demonstrated. NE turnover in the AMBH, and DA turnover in all 3 structures did not correlate with serum LH levels. Serum FSH and prolactin levels were higher in the animals with high LH levels. These changes are similar to those observed in adult proestrous rats and it is concluded that the LH peaks in 15-day-old animals reflect a positive feedback action of estradiol.

In a second experiment prolactin was administered to determine if it could prevent the occurrence of these LH peaks. Ovine prolactin (2 × 0.5 μg/g body weight) injected from day 8–15 suppressed serum LH levels to almost undetectable values. Catecholamine turnover measurements in these animals revealed increased DA turnover in the MPO, AMBH and PMBH, whereas NE turnover was not affected. These results indicate that DA turnover of the incertohypothalamic and of the tuberoinfundibular DA neurons is accelerated by prolactin and that an increased DA turnover in one of these systems is responsible for inhibition of LH release.     

Ontogeny of tyrosine hydroxylase and dopamine-beta-hydroxylase activity in discrete limbic and hypothalamic structures of female rats

Tyrosine hydroxylase (TOH) and dopamine-ß-hydroxylase (DBH) activities were measured in the mediocortical amygdala (AMY), hippocampus (HPC), nucleus accumbens (ACB), medial preoptic area (MPO), and anterior and posterior parts of the mediobasal hypothalamus (AMBH and PMBH) in female rats of various postnatal ages. Serum LH, FSH and prolactin levels were also measured. In a previous study it was shown that dopamine (DA) and norepinephrine (NE) turnover rates in the MPO and the MBH increase between days 20 and 30 after birth and decrease thereafter. The cause for increased DA turnover was shown to be increasing serum prolactin levels. The activity of TOH in the ACB, MPO, AMBH and PMBH did not parallel DA turnover rates. Around day 20 after birth, when serum prolactin levels and preoptic and hypothalamic DA turnover increase, TOH activity also increased in the ACB and PMBH. In adult, diestrous animals, however, which have low serum prolactin levels and low preoptic and hypothalamic DA turnover, TOH activity was higher than in immature rats. The activity of DBH in the MPO, AMBH, AMY and HPC was high in 15-day-old animals, decreased between days 20 and 30 and then increased again to adult values. This pattern is just opposite to the one observed for serum prolactin levels and for preoptic and hypothalamic NE turnover. It is concluded that activity of both enzymes is not a good measure for neuronal activity of those NE and DA cells which innervate limbic and hypothalamic structures.     

Blockade of progresterone-induced increase in hypothalamic luteinizing hormone-releasing hormone levels and serum gonadotropins by intrahypothalamic implantation of 6-hydroxydopamine

The present study was undertaken to identify the hypothalamic locus where norepinephrine (NE) nerve terminals communicate with leteinizing hormone-releasing hormone (LH-RH)-containing neurons involved in the stimulatory feedback action of gonadal steroids on LH and FSH release. Ovariectomized rats received estradiol benzoate (10 μg/rat s.c.) on day 0. Intracranial implants containing either 6-hydroxydopamine (6-OHDA), to destroy NE terminals, or cocoa butter (controls) were placed bilaterally in the suprachiasmatic nucleus (SCN), medial basal hypothalamus (MBH) or olfactory tubercle (OT) on day 1. Progesterone (P, 5 mg/rat s.c.) was administered at 10.00 h on day 2 to elicit increases in serum LH and FSH and the MBH LH-RH levels in the afternoon. Implantation of 6-OHDA in the SCN resulted in a marked depletion of NE in and around the region of the SCN in the preoptic-anterior hypothalamic area (POA-AH) without adversely affecting dopamine (DA) concentrations, and blocked the P-induced afternoon increase in the MBH LH-RH and serum gonadotropin levels. Similar reduction in the MBH NE concentrations occurred following placement of 6-OHDA in the MBH; however, these as well as implants in the OT were ineffective in suppressing the P-induced effects. These studies show that functional integrity of the SCN regions is critical in manifestation of the P-induced rise in the MBH LH-RH activity, and this region in the POA-AH, therefore, may be the primary locus of synaptic communication between NE terminals and LH-RH neurons.     

Blockade of progesterone-induced increase in hypothalamic luteinizing hormone-releasing hormone levels and serum gonadotropins by intrahypothalamic implantation of 6-hydroxydopamine.

The present study was undertaken to identify the hypothalamic locus where norepinephrine (NE) nerve terminals communicate with luteinizing hormone-releasing hormone (LH-RH)-containing neurons involved in the stimulatory feedback action of gonadal steroids on LH and FSH release. Ovariectomized rats received estradiol benzoate (10 microgram/rat s.c.) on day 0. Intracranial implants containing either 6-hydroxydopamine (6-OHDA), to destroy NE terminals, or cocoa butter (controls) were placed bilaterally in the suprachiasmatic nucleus (SCN), medial basal hypothalamus (MBH) or olfactory tubercle (OT) on day 1. Progesterone (P, 5 mg/rat s.c.) was administered at 10.00 h on day 2 to elicit increases in serum LH and FSH and the MBH LH-RH levels in the afternoon. Implantation of 6-OHDA in the SCN resulted in a marked depletion of NE in and around the region of the SCN in the preoptic-anterior hypothalamic area (POA-AH) without adversely affecting dopamine (DA) concentrations, and blocked the P-induced afternoon increase in the MBH LH-RH and serum gonadotropin levels. Similar reduction in the MBH NE concentrations occurred following placement of 6-OHDA in the MBH; however, these as well as implants in the OT were ineffective in suppressing the P-induced effects. These studies show that functional integrity of the SCN regions is critical in manifestation of the P-induced rise in the MBH LH-RH activity, and this region in the POA-AH, therefore, may be the primary locus of synaptic communication between NE terminals and LH-RH neurons.     

Effects of suckling on serum prolactin levels and catecholamine concentrations and turnover in discrete brain regions

The effects of suckling on serum prolactin levels and catecholamine concentrations and turnover were examined in several discrete brain regions. Turnover rates were assessed by using the synthesis inhibitor alpha-methyltyrosine (alpha-MT) in combination with microdissection techniques for the removal of individual brain regions and sensitive radioenzymatic assays for norepinephrine (NE) and dopaime (DA). Prolactin secretion was induced by mothers experiencing 6 h of pup removal with subsequent pup replacement. Suckling or the administration of alpha-MT to mothers resulted in a marked increase in circulating titers of prolactin. A decrease in steady-state NE concentrations in the anterior hypothalamus and a decrease in steady-state DA concentrations in the ventromedial nucleus were noted in suckled mothers. The comparison of relative rates of NE depletion after alpha-MT treatment revealed a suckling-induced increase in turnover in the ventromedial nucleus and a suckling-induced decrease in turnover in the anterior hypothalamus. Neither suckling nor alpha-MT treatment produced any changes in NE or DA turnover rates in the arcuate nucleus or median eminence. These findings demonstrate that suckling-induced activation of prolactin results in changes in noradrenergic processes in the ventromedial and anterior hypothalamic nuclei. This suggests an involvement of noradrenergic systems in suckling-induced prolactin release.     

Age-related alteration in catecholamine activity within microdissected brain regions of ovariectomized fischer 344 rats

The effects of increasing age on catecholamine (CA) metabolism in microdissected brain regions and on serum and pituitary hormone levels were examined in ovariectomized Fischer 344 rats. Young (4 to 5 months old) and middle-aged (9 to 10 months old) normally cycling and old repeated pseudopregnant rats (21–22 months old, PP) were ovariectomized to eliminate the complicating effects of cyclic gonadal steroid fluctuations. CA metabolism was examined 2 weeks later. To determine CA turnover rates, each agegroup was subdivided into three groups, which were killed by decapitation 0, 45, or 90 min after administration of α-methyl-para-tyrosine (α-mpt). Dopamine (DA) and norepinephrine (NE) concentrations were determined in microdissected brain regions by radioenzymatic assay, and turnover rates were estimated. Steady-state concentrations of NE were not altered in middle-aged rats, but NE turnover rates increased in middle-aged rats in five of the six areas examined. While NE concentrations did not change with age in the median eminence (ME), NE turnover rates increased significantly in the two older age groups. These data indicate that the age-related decline in NE concentrations in several ventral diencephalic nuclei is preceded by a period of hyperactivity in noradrenergic neurons. DA concentrations were generally decreased in most areas examined in old versus young rats, with dramatic DA depletions (42–78%) observed in five regions. However, no consistent relationship between DA concentrations and turnover rates was seen either in regions with stable DA levels or in those which showed an age-associated decrease in DA concentrations. In the ME, a 42% decline in DA concentration was associated with an increase in the DA turnover rate in the oldest group of rats. Serum luteinizing hormone (LH) levels were similar in all three age groups of ovariectomized rats, while serum prolactin was elevated four-fold in old compared to younger animals. These data indicate that a complex pattern of regional alterations in CA metabolism accompanies the aging process and these may be related to the pseudopregnant state and hormone secretory capacity of aging Fischer 344 rats.     

The effect of intraventricular gamma aminobutyric acid (GABA) on hypothalamic catecholamines and LHRH and on pituitary hormone release

The present experiments were designed to evaluate the role of catecholamines in mediating the actions of GABA on pituitary hormone release following its intraventricular injection. GABA was administered intraventricularly, at doses of 0.1 and 4 μMol, to chronically ovariectomized rats. Five and 15 min after intraventricular injection, the animals were sacrificed by decapitation. Blood was collected for LH and PRL determination by RIA, and brains were frozen for subsequent determination of LHRH by RIA, and dopamine (DA) and norepinephrine (NE) by radioenzymatic assay. Brain areas analyzed included the median eminence (ME), medial basal hypothalamus (MBH) and suprachiasmatic-medial preoptic area (Sch-PO). Controls received an equal volume (2 μl) of saline. As previously observed, GABA had opposite effects on prolactin (PRL) release depending on the dose employed. A decline in serum PRL was induced by a low (0.1 μMol) dose, whereas a larger (4 μMol) dose induced an increase in PRL levels. LH levels were increased only with the 4 μMol dose. The elevated LH values were paralleled by an increase in LHRH levels in the Sch-PO region 15 min after GABA. Marked changes in catecholamines, particularly in DA, were seen after GABA injection. Significant increments in DA levels in the ME were seen 5 and 15 min after the 0.1 μMol dose of GABA. Similarly, this low dose of GABA induced a marked increase in DA levels in the anterior-pituitary (AP) gland at 15 min. No changes in DA were seen in either MBH or Sch-PO areas. The 4 μMol dose of GABA induced only a small increase in AP levels of DA, without altering the amine levels in any of the brain areas examined. NE levels in the ME were elevated 5 min after the administration of either dose of GABA. No changes in NE were seen in either the MBH or Sch-PO areas. These results indicate that intraventricular GABA injection not only alters pituitary hormone release but also release of DA and NE from terminals in the ME. The released catecholamines may be important in mediating the effects of GABA on releasing factor discharge. In addition, the DA released may have acted directly on the AP to inhibit release of PRL.     

Effect of independent deafferentation of each border of the medial basal hypothalamus on catecholamine content of the median eminence and on serum LH and prolactin levels in ovariectomized rats

The purpose of this work was to correlate the norepinephrine (NE) and dopamine (DA) content in the region of the median eminence (ME) with serum levels of LH and prolactin in ovariectomized rats after independent deafferentation of each one of the borders of the medial basal hypothalamus (MBH). Surgical transections of the MBH or sham operations, followed by ovariectomy, were performed in adult female Holtzman rats. The following types of transections were performed: bilateral, dorsal or rostral and caudal. One week later, the animals were decapitated, trunk blood was collected for LH and prolactin quantification by RIA and the region of the ME was dissected out for catecholamine assay. Both NE and DA were measured in each individual sample by a highly sensitive enzymatic-isotopic method.Bilateral transections of the MBH induced a significant decrease in NE in the region of the ME, without altering DA levels in the same area or LH and prolactin values. Cuts of the rostral and caudal or dorsal borders of the MBH did not modify any of the parameters evaluated. These results indicate that: (a) Most noradrenergic fibers innervating the ME enter the MBH through the lateral borders. However, some residual NE activity in the ME can be detected, suggesting other sources of noradrenergic innervation of the ME. (b) Normal levels of NE in the ME are not required for the LH post-castration rise to take place.