Effect of treatment with GnRH antagonist, GnRH antiserum and bromocriptine on pituitary-testicular function of adult rats.

Gonadotropin secretion of adult male rats was inhibited by one-week treatment with a GnRH antagonist analogue (N-acetyl-Ala1,D-p-Cl-Phe2,D-Trp3,6-GnRH, Ant.) or a GnRH antiserum (A/S). Since Ant. but not A/S binds to GnRH receptors (R), it was expected that comparison of the two treatment regimes would elucidate a physiological role for testicular GnRH-R. Furthermore, the effect of combined gonadotropin and prolactin deficiency was studied in rats treated with Ant. and bromocriptine (BR). Available pituitary GnRH-R were decreased by both Ant. and A/S (P less than 0.01), but not by BR. Testicular content of free GnRH-R was decreased by Ant. by 70-80% (P less than 0.01), probably owing to occupancy, but A/S and BR had no effect on these binding sites. Serum LH and FSH fell by about 75% with Ant. and A/S treatments (P less than 0.01), and Prl by 90% with BR (P less than 0.01). Intratesticular testosterone (T) decreased significantly with Ant. and A/S treatment (P less than 0.01-0.05), but was unaffected by BR. Only Ant. and BR treatments together, but neither alone, were able to suppress Leydig cell capacity to produce T in vitro. Relative blockade of C21 steroid side-chain cleavage by Ant. and A/S was suggested by elevated progesterone (P) and/or P/T ratios in the serum and testis tissues. Ant. and A/S had no effect on testicular LH and FSH-R, but both decreased testicular Prl-R by about 50% (P less than 0.01-0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary-testicular function in immature rats after treatment with GnRH antagonist, GnRH antiserum and bromocriptine.

The effects of a GnRH antagonist analogue (N-acetyl-Ala1,D-p-Cl-Phe2,D-Trp3,6-GnRH, Ant.) and a GnRH antiserum (A/S) on the development of pituitary-testicular function were studied in immature (23/24-31/32-day-old) rats. In another experiment the Ant. treatment was combined with bromocriptine (BR)-induced hypoprolactinaemia. Ant. and A/S decreased serum and pituitary levels of LH and FSH, and BR those of Prl (P less than 0.01-0.05). Testicular testosterone (T) and progesterone (P) contents were significantly decreased only by Ant. (P less than 0.01). Ant. decreased the weights of the testes, ventral prostates and seminal vesicles, as well as testicular LH, FSH and Prl receptors (R) (P less than 0.01-0.05). BR decreased LH-R but had no effect on Prl-R. Both Ant. and A/S decreased available pituitary GnRH-R (P less than 0.01), but free testicular GnRH-R were reduced only by Ant. BR increased GnRH receptors in the pituitaries. It is concluded that Ant.-induced low gonadotropin levels in immature animals inhibit the developmental increase of testicular weight, gonadotropin and Prl-R, steroidogenesis and androgen action on accessory sex glands. Hypoprolactinaemia had an additive inhibitory effect to the antigonadal effects of Ant. The testis tissue of immature (23/24-day-old) animals already contains GnRH-R. In general, developing animals are clearly very sensitive to the antigonadal actions of Ant. and BR, whereas the effect of GnRH-A/S is less pronounced than in adults.

Counteractive effects of agonistic and antagonistic gonadotropin-releasing hormone analogs on spermatogenesis: sites of action.

Both gonadotropin-releasing hormone (GnRH) agonistic and antagonistic analogs have been shown to inhibit reproductive hormonal function. While predictable and complete suppression of spermatogenesis is the ultimate goal of a number of clinical studies aimed at developing male contraceptive agents based on GnRH analogs, neither class of analog has been shown to completely inhibit spermatogenesis in man. The potential for a synergistic interaction of submaximal doses of these two classes of GnRH analogs was investigated in the present studies. In these studies 200 ng/day of a potent agonist (D-Leu6des-Gly10GnRH ethylamide) and 100 micrograms/day of a potent antagonist (NAc-L-Ala1, pCl-D-Phe2, D-Trp3,6GnRH) were administered subcutaneously, both alone and in combination, to adult male rats for 21 days. Serum gonadotropins and testosterone, pituitary GnRH receptor content, gonadal gonadotropin receptors, and intratesticular sperm counts were quantitated in each treatment group. Despite the ability of both GnRH agonists and antagonists to inhibit reproductive function when administered as single agents in this study, combined treatment with the two classes of GnRH analogs was less effective than either agent alone at these doses in the pharmacologic suppression of spermatogenesis.

Pituitary receptor site blockade by a gonadotropin-releasing hormone antagonist in vivo: mechanism of action.

Administration of a potent gonadotropin-releasing hormone (GnRH) antagonist [Nac-L-Ala1,pCl-D-Phe2,D-Trp3,6]GnRH as a single subcutaneous injection to castrated adult male rats reduced, by more than 90 percent, both serum luteinizing hormone concentrations and specific pituitary GnRH receptor binding. This effect persisted for 24 hours. The dissociation rate of the antagonist from pituitary membrane homogenates was fourfold slower than the dissociation rate of a potent agonist. The prolonged in vivo inhibition of pituitary GnRH receptor binding and luteinizing hormone secretion by the GnRH antagonist may be mediated by the slower dissociation rate of the antagonist from its specific pituitary membrane receptor site.

The effects of peptides on the stimulus properties of ethanol.

Male Sprague-Dawley rats were trained to discriminate ethanol (2 g/kg, PO: EtOH) from saline (10 ml/kg, PO: SAL) in a two-bar positively reinforced operant task on a VI 15 sec schedule. After the rats reached criterion performance (greater than 90% correct responses on the appropriate lever), thyrotropin releasing hormone (pyroGlu-His-Pro-NH2: TRH), a metabolite of TRH (His-Pro diketopiperazine: HP), and a structural analog of TRH (HPCA-His-ThiaPro-NH2: OHT) were tested for their ability to antagonize the EtOH cue. These peptides were chosen for their reported ability to reverse ethanol-induced narcosis. However, at doses that did not disrupt performance, TRH, HP, and OHT did not affect the stimulus properties of ethanol at any dose tested, nor did they change the stimulus properties of saline. Naloxone and ACTH(1-10)-NH2 were also tested as ethanol antagonists of the training dose. Pretreatment with either of these compounds failed to alter ethanol-appropriate responding. In addition, (DA1a2-Met5)-enkephalin-ol, (DAla2-Met(O)5)-enkephalin-ol, substance P, delta sleep-inducing peptide, and bombesin were tested for their ability to elicit ethanol appropriate responding. The EtOH cue generalized to none of these peptides.

Circular dichroism study of the solution conformation of luteinizing hormone releasing hormone.

A systematic investigation has been made into the circular dichroic behavior of luteinizing hormone releasing hormone and its peptide fragments and deletion analogues. The results are interpreted to mean that the hormone exists in solution as an ensemble of conformers with different sensitivities to temperature and solvent composition. The far-ultraviolet circular dichroic spectra exhibited by the hormone under different experimental conditions can be simulated satisfactorily by the weighted addition of the spectra of its aliphatic- and aromatic-containing halves. However, the structure of the hormone is not simply the sum of its halves, since some conformational feature of the intact molecule perturbs the near-ultraviolet circular dichroism of its aromatic residues.

Evolutionary aspects of some neuropeptides.

The study of biologically active peptides is a study of structural homology and functional diversity. Some fundamental peptide structures used as chemical messengers by simple organisms were evidently maintained throughout evolution with little change. In higher organisms these structures may still be used for functions related to the original use as well as for very different purposes. Examples are given of the multifunctional roles of common peptides and of the mechanisms by means of which these functions are separated and regulated. The active core of ACTH appears to be used as a signal in regulation of reproduction throughout the range of eukaryotes and also appears in a baffling range of other important proteins. The ubiquitous nature of this peptide sequence suggests that it plays a universal role in recognition or activation of specific receptors. Peptides used in lower species as hormones or chemical defense substances play an important role in research as presagers of the discovery of corresponding mammalian peptides.

Fibrinopeptides. IV. Synthesis of human fibrinopeptide A.

Human fibrinopeptide A, a hexadecapeptide, released by the action of thrombin on fibrinogen during clotting of blood, has been synthesized by conventional methods. The synthetic peptide as well as some of the intermediates in the synthesis have been examined for anticoagulant activity. Though all of them were found to be active, the terminal carboxyl protected peptides are more potent inhibitors of clotting than the carboxyl free peptides.

Synthesis of the pentadecapeptide sequence of the active site of rabbit muscle triosephosphate isomerase.

The pentadecapeptide fragment, Trp-Val-Leu-Ala-Tyr-Glu-Pro-Val-Trp-Ala-Ile-Gly-Thr-Gly-Lys, which constitutes a part of the active site of rabbit muscle triosephosphate isomerase has been synthesized. It does not exhibit any catalytic activity typical of triosephosphate isomerase.