Immunocytochemical localization of the gonadotropin-releasing hormone-associated peptide of the LHRH precursor

Effect of maternal deprivation on the gonadotrophin-releasing hormone (GnRH) and GnRH-associated peptide neurobiology in lambs during the transition from infancy to prepuberty

Using morphological criteria we describe the effect of maternal deprivation on the gonadotrophin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) of the GnRH prohormone (proGnRH) in the preoptic area (POA)-hypothalamus during the weaning period. The immunohistochemical GnRH- and GAP-neuroanatomy was investigated in female 12-week-old weanling and maternally deprived lambs and 15-week-old weaned lambs. The GnRH-immunoreactive (ir) nerve elements in the POA were more numerous in weanling and weaned lambs in comparison with maternally deprived lambs, whereas the nerve elements ir for GAP were numerous in weanlings and scarce in remaining lambs. In the hypothalamus, GnRH-ir fibers were more numerous in weaned lambs in comparison with others. Immunoreactive GnRH in the median eminence was scarce in weanlings and comparable greater in maternally deprived and weaned lambs. In contrast to ir GnRH, the GAP-ir fibers and nerve terminals in the hypothalamus and median eminence were numerous in weanlings and maternally deprived lambs and scarce in weaned lambs. In conclusion, maternal deprivation affects the intraneuronal locations involved in the maturation of GnRH from proGnRH in the POA-hypothalamus of weanlings. The described effect involves the increase in the GnRH posttranslational processing and terminal accumulation in the median eminence, which reflects the maturational increase from the low infantile terminal storage to the high prepubertal one.

Biology of hypothalamic neurons and pituitary cells

Results obtained by examining hypothalamic neurons producing precursors to neurohormones, and pituitary cells synthesizing peptide and glycoprotein families of hormones, and recent advances in comparative endocrinology, have been summarized and considered from the following viewpoints: species specificity in the organization and communication of the hypothalamic neurons with different brain areas lying inside the BBB and with CVOs; sensitivity of hypothalamic neurons and pituitary cells to the environmental stimuli; gonadal steroids as modulators of gene expression needed for neuronal differentiation and synaptogenesis; dose(s)-dependent pituitary cell proliferation and differentiation; an inverse relationship between PRL and GH synthesis and release and also between degree of hyperplasia and hypertrophy of PRL cells and retardation of GTH cell differentiation; and responsiveness of neurons producing CRH, and of neurons and pituitary cells synthesizing POMC hormones, to stress and glucocorticosteroids. These data show that growth of the animals may be stimulated, retarded, or inhibited; reproductive properties and behavior may be under hormonal control; and character of responsiveness in reaction to stress, and ability for adaptation and other related functions, may be controlled.

Distribution of luteinizing hormone-releasing hormones I and II (LHRH-I and -II) in the quail and chicken brain as demonstrated with antibodies directed against synthetic peptides

Polyclonal antibodies were raised in rabbits against polypeptides corresponding to the N-terminal part (heptapeptides) of the two avian gonadotropin-releasing hormones, chicken (c) LHRH-I and -II. These peptides, which were synthesized by the continuous-flow technique, were selected because they contained the smallest number of common amino acid residues. The pGlu-His-Trp-Ser sequence at the C-terminal was suppressed to avoid possible cross-reactions between the antisera. The antisera generated in this way were tested for specificity by solid and liquid phase absorption as well as by antigen spot tests. The antiserum raised against cLHRH-I recognized this peptide preferentially though not exclusively. Some cross-reaction with cLHRH-II was observed in the absorption test, although spotting tests suggested a total specificity. The anti cLHRH-II appeared to be completely specific in all tests. These two antibodies were then used to study the distribution of cLHRH-I and -II immunoreactive structures in the quail and chicken brain. cLHRH-I immunoreactive perikarya were observed in a fairly wide area covering the preoptic-anterior hypothalamic and septal region. By contrast, cLHRH-II cells were confined to a single group located in the dorsal aspects of the occulomotor nuclei, at the junction of the di- and mesencephalon. A sex difference in the number of cLHRH-I cells was detected in the anterior lateral preoptic region of the quail. Fibers immunoreactive for either cLHRH-I or cLHRH-II were widely distributed in the telencephalon, diencephalon, and mesencephalon but showed a specific pattern of anatomical localization. In particular, a high density of cLHRH-I fibers were seen in the external layer of the median eminence, while cLHRH-II fibers were less prominent at this level. Contrary to previous reports, a significant amount of cLHRH-II fibers were however seen throughout the median eminence (mostly external layer). The extensive distribution of both cLHRH-I and -II fibers in the quail and chicken brain is consistent with the potential role played by these peptides in the gonadotropin secretion and in the control of reproductive behavior. The specific role of cLHRH-II remains however elusive at present.

GnRH-Associated peptide (GAP) is present in the rat striatum and affects the synthesis and release of dopamine

A possible interference of the GnRH-associated peptide (GAP) with the metabolism of dopamine in the rat striatum was investigated. The presence of the precursor of the peptide in this central region of dopaminergic terminals was first evidenced using specific RIA. The action of GAP on dopamine release was investigated in the caudate nucleus using the local superfusion with a push-pull cannula supplied with an artificial CSF containing the tritiated precursor of dopamine [( 3H]tyrosine). Addition of GAP (1 microM) to the superfusing fluid resulted in an increase of the release of the newly synthesized dopamine without a significant modification of the total amine release. In situ neutralization of GAP by addition in the CSF of a rabbit serum containing antibodies directed against the GAP produced opposite effects evidencing a tonic function for this peptide. In addition to the increased specific activity of the dopamine released during GAP treatment, the alterations observed in the efflux (and the specific activity) of dihydroxyphenyl acetic acid and the activation of dopamine synthesis obtained in vitro in striatal slices in the presence of GAP led us to conclude that the GAP system could be considered as a positive control of dopamine synthesis.

Immunocytochemical demonstration of GAP-like immunoreactive neuronal elements in the human hypothalamus and pituitary

GnRH-associated peptide (GAP)-like immunonreactive elements located in the human hypothalamus were investigated by PAP immunocytochemistry using specific antiserum against [pro-GnRH (14-69) OH]. Immunoreactive neuronal perikarya were distributed in the MPOA, PVN and infundibular nucleus, with the largest numbers of GAP-like immunoreactive perikarya found in the infundibular nucleus. We also detected the coexistence of GAP-like and GnRH-like immunoreactivities in the same neuronal perikarya in the MPOA by using a double immunolabelling procedure. In addition to the above regions immunoreactive neuronal perikarya were present in the region dorsal to the medial mammillary nucleus. GAP-like immunoreactive fibers were distributed in same areas that immunoreactive perikarya were observed. Many immunoreactive terminals were found adjacent to capillaries in the infundibulum. Immunoreactive dots, presumably terminals, were observed in the posterior pituitary and these were particularly evident along the margin adjacent to the anterior pituitary. The distribution pattern and density of GAP-like immunoreactive neuronal elements are compared with those of other mammalian species. We also compared GAP-like immunoreactive elements with that of GnRH as has been previously observed in the human hypothalamus.

Estradiol stimulates preoptic area-anterior hypothalamic proGnRH-GAP gene expression in ovariectomized rats

The decapeptide gonadotropin-releasing hormone (GnRH) and the 56-amino acid GnRH-associated peptide (GAP) are derived from a common precursor translated from the proGnRH-GAP mRNA. Studies using solid-phase hybridization techniques (i.e., Northern blot analysis, dot blot analysis, or in situ hybridization autoradiography) have yielded a controversy as to whether estradiol stimulates, inhibits, or has any effect on proGnRH-GAP gene expression in the preoptic area-anterior hypothalamus (POA-AH) of the ovariectomized (OVX) rat. Using a sensitive and quantitative solution hybridization-nuclease protection assay, which ensures complete hybridization of target RNA to probe RNA, we examined the effects of OVX and estradiol replacement on the amount of proGnRH-GAP mRNA in individual POA-AH dissections. Rats sacrificed at different intervals after OVX showed a significant time-dependent decrease (34-60%) in the levels of POA-AH proGnRH-GAP mRNA relative to sham-operated animals; OVX rats treated with estradiol, however, had proGnRH-GAP mRNA levels comparable to those of sham-OVX animals. To verify these observations, levels of the proGnRH-GAP peptide, measured by radioimmunoassay with antibodies directed against the cleavage and amidation site between the GnRH and the GAP portions fo the precursor molecular, were also found to decrease (37%) after OVX and increase (63-85%) following estradiol replacement, relative to intact rats. These data support the view that estradiol stimulates the levels of both proGnRH-GAP mRNA and its primary translation product in the POA-AH region of the OVX rat.

Immunohistochemical localization of gonadotropin-releasing-hormone-associated peptide in the brain of the frog

Gonadotropin-releasing-hormone (GnRH)-associated peptide (GnRH-AP) is a 56 amino acid neuropeptide derived from the GnRH prohormone. GnRH-AP corresponds to the C-terminal fragment flanking the GnRH peptide. Using an antiserum raised against human GnRH-AP [1-56], or against human GnRH, we have investigated the neuronal systems containing either peptide in the central nervous system of the frog Rana ridibunda by immunohistological techniques. A main group of GnRH-AP-containing perikarya was found in a dorsoventral orientation of the supra anterior preoptic area (SAPA) just in front of the preoptic recess. Fibers originating from these perikarya projected rostrally toward the medial septal nucleus and the diagonal band of Broca. A network of GnRH-AP-immunoreactive (ir) fibers runs caudally from the SAPA toward the ventral hypothalamus. A high density of GnRH-AP-ir terminals was found in the median eminence. A few positive fibers were detected in the neural lobe of the pituitary, particularly in the region bordering the pars intermedia. Labelling of consecutive sections by either GnRH-AP or GnRH antibodies showed that GnRH and GnRH-AP-like irs were contained in the same cells of the SAPA. The double-staining technique with electrophoretic elution confirmed the colocalization of GnRH and GnRH-AP within the same neurons. Such a coexistence indicates that frog GnRH originates from a high molecular weight precursor which is closely related to rat pro-GnRH. The relative preservation of the C-terminal sequence of the pro-GnRH during evolution suggests that GnRH-AP may possess intrinsic biological activity. The high density of GnRH-AP-containing neurons projecting through the external zone of the median eminence would support the concept that GnRH-AP is involved in the modulation of pituitary hormone secretion.

Assessment of the effects of a synthetic gonadotropin-releasing hormone associated peptide on hormone release from the in situ and ectopic pituitaries in adult male rats

The in vivo effects of synthetic human gonadotropin-releasing hormone associated peptide (GAP) were evaluated in adult male rats. In normal rats, intracerebroventricular (III ventricle) injection of 5 ng GAP significantly increased plasma LH levels after 60 minutes. Intracerebroventricular administration of 5, 25 or 125 ng of GAP elevated circulating LH levels also at 120 minutes of injection but did not alter plasma FSH, prolactin or testosterone concentrations. In hypophysectomized-pituitary-grafted rats injection of 125 ng GAP directly into the ectopic pituitary induced no changes in peripheral hormone levels. However, intrapituitary graft injection of 25 ng of GnRH significantly elevated circulating levels of LH and testosterone. These results indicate that the ectopic pituitary graft can respond to acute exogenous GnRH stimulation and that the commercially available synthetic GAP fails to inhibit prolactin release in adult male rats.

The Pro-LHRH system of the rat brain. Effects of changes in the endocrine background

The gonadotropin-releasing hormone-associated peptide (GAP) and luteinizing hormone-releasing hormone (LHRH) portions of the LHRH precursor were localized by immunocytochemistry in prepubertal female rats, in adult female rats at different stages of the estrous cycle, and in ovariectomized rats. Our results indicate that GAP is present in the same population of neurons as LHRH in the rat brain. These results confirm the specificity of previous immunocytochemical studies which used antisera to LHRH alone. The endocrine status of the animal was demonstrated to affect the immunocytochemical appearance of the GAP system. The number of GAP immunopositive cells and terminals is highest during diestrus II and lowest on the day of estrus, suggesting either a role in and/or a dependence upon the endocrine changes associated with the estrous cycle. Ovariectomy results in a gradual decrease in GAP immunoreactivity in the median eminence. This observation, in concert with other recent studies, suggests that ovarian factors may be acting to maintain the LHRH system and that ovariectomy may result in decreased synthesis and/or processing of the LHRH system and that ovariectomy may result in decreased synthesis and/or processing of the LHRH precursor.

Differential secretion of proLHRH fragments in response to [K+], prostaglandin E2 and C kinase activation

ProLHRH contains the luteinizing hormone-releasing hormone (LHRH) decapeptide and a 56 amino acid peptide designated gonadotropin-releasing hormone-associated peptide (GAP). We studied the effects of various known secretagogues of LHRH on the in vitro release of proLHRH fragments from the median eminence (ME) and subsequently characterized these immunoreactive products according to molecular weight (MW). GAP- and LHRH-like immunoreactive (LI) materials were secreted simultaneously into the media under basal conditions. Prostaglandin E2 stimulated release of both peptides by approximately 2-fold. Both phorbol ester and [K+] stimulated release of GAP-LI by 4-fold and LHRH-LI by 9-fold over baseline levels. When materials from [K+]-stimulated media were separated according to MW by high performance size-exclusion chromatography, a single peak eluted at 1300 MW in the same position as synthetic LHRH. Two GAP-LI peaks were observed. One eluted in the void volume, while the predominant peak co-eluted with synthetic rat GAP1-56 at approximately 6500 MW. These results indicate that GAP and LHRH are co-secreted, that they are released as intact peptides, and that activation of different intracellular pathways may cause their differential secretion. These results emphasize the importance of using both in vitro and chromatographic methodologies to evaluate the changes which may occur in LHRH prohormone processing and secretion.