Comparisons of the potential utility of LHRH agonists and antagonists for fertility control

Suppression of fertility in pre-pubertal dogs and cats

Pre-pubertal gonadectomy in dogs and cats is still controversially discussed because some consequences cause health problems. Nevertheless, postponement of puberty, that is, prevention of an increase in sexual hormones and thereby prevention of their manifold effects, is of major importance, not only in controlling overpopulation but also to preserve the genetic base for future breeding stock and pets. Therefore, alternatives for surgical suppression of fertility in pre-pubertal animals were critically reviewed. As a promising alternative, the slow-release GnRH agonist deslorelin and other GnRH analogues have been investigated. In female dogs and cats, puberty could be significantly postponed without initial flare-up effect and without disturbance of body development. First trials to delay puberty in female and male cats by application of a 4.7-mg deslorelin implant 24 h after birth so far are promising. In female dogs, a previous investigation showed that when the implant was inserted at the age of 4 months, the initial flare-up effect was prevented. Body development was normal in the studies reviewed here, and with the 9.4-mg implant, puberty was significantly delayed until the age of 21 months or older. In one study, bitches either received a 4.7- or a 9.4-mg implant at the age of 4 months and the epiphyses were mostly closed before the time of first oestrus. Using a 4.7-mg deslorelin implant in pre-pubertal male dogs significantly postponed puberty, and age at puberty was >2 years when a 9.4-mg implant was used. However, further investigations are required, especially concerning the effect of different GnRH agonist dosages and resorption rates on the duration of postponement of puberty as well as long-term effects in both dogs and cats.

Genetic approach for intracerebroventricular delivery

Administration of synthetic or purified peptides directly into the brain ventricles is a method commonly used by neuroscientists for exploring physiological and behavioral functions of gene products. i.v. administration is controlled by the blood-brain barrier, which limits its effectiveness, and current approaches for acute or chronic intracerebroventricular delivery have significant technical drawbacks resulting from both the chemical properties of the delivered substance and the experimental procedures. Here we describe a genetic approach for the delivery of secreted peptides or proteins into the cerebrospinal fluid (CSF). Using a choroid plexus-specific promoter, we established a lentiviral-based system, which offers inducible and reversible delivery of a gene product into the CSF. The functionality of this system was demonstrated by using the overexpression of the two established neuropeptides, corticotropin-releasing factor and gonadotropin-releasing hormone, modulating anxiety-like behavior and estrus cycle, respectively. We show that this choroid plexus-specific lentiviral-based system is a reliable, effective, and adaptable research tool for intracerebroventricular delivery.

New GnRH analogs in canine reproduction

Gonadotropin-releasing hormone (GnRH) analogs, which include agonists and antagonists, have been produced by amino acid substitutions of the native GnRH molecule to create greater potency and longer duration of effectiveness. The aim of this article was to review the pharmacological effects and the existing clinical literature of new GnRH analogs, namely agonists released from long-term formulations and third generation antagonists, in domestic dogs. Long-term administration of agonists functions through desensitization and down-regulation of GnRH pituitary receptors inhibiting gonadotropin production and release after an initial stimulation. Conversely, GnRH antagonists bind to gonadotrope GnRH receptors and compete successfully with endogenous GnRH for occupancy, thereby inhibiting the pituitary-gonadal axis immediately. There is a promising place for both agonists and antagonists in future canine reproduction. They can be used in the control of estrous cycle, hormone-dependent diseases as well as in contraception. Some information on the effectiveness and safety of these new analogs in canine reproduction is already available, yet further work is needed before they could be widely recommended. The increase in gonadotropins and gonadal steroids following administration of agonists might have adverse effects when used on hormone-dependent diseases. This initial "flare" should also be carefully managed in anestrous and prepubertal bitches. At present, the main application of antagonists seems limited to situations where an acute endocrine, inhibitory effect is required, e.g. proestrus or pregnancy termination. Future commercial availability of long-acting, single-dose antagonists could go far towards controlling pet population.

Effect of the GnRH antagonist, acyline, on canine testicular characteristics

The primary objective was to assess the effects and clinical safety of a single high-dose of the third generation GnRH antagonist, acyline, on testicular characteristics in male dogs. Seven dogs were followed up weekly for six, 2-week periods (-2, -1, 1, 2, 3 and 4). At the end of the second period, they were given acyline (330 microg/kg sc). Responses to treatment varied among individual animals. Testicular consistency and scrotal diameter were slightly diminished (P>0.05) in Periods 1, 2, and 3. Libido and erection decreased during Periods 1 and 2 (P<0.05). Second and third fractions of the ejaculate volume, sperm count and motility varied throughout periods (P<0.01); there was a clear impairment of these parameters (< or =0.2 mL, < or =0.6 mL, < or =0.5 million/mL and < or =30%, respectively) around the second week of Period 1, followed by slow improvement (to the end of the study). Semen volumes and motility diminished during Period 1 (P<0.05). Sperm count decreased during Periods 1, 2, and 3, relative to Periods -2 and -1 (22.7+/-11.7, 62.8+/-19.9, and 51.0+/-25.4 versus 235.7+/-63.3 and 315.5+/-27.3, respectively; P<0.05; (L.S.M.+/-S.E.M.). Morphologically abnormal sperm increased during Periods 2 and 3 (up to 64.9+/-2.6%; P<0.05). Throughout the study, no dog had hematological, biochemical, local, or systemic side effects. In conclusion, a single high-dose acyline treatment severely decreased semen quality with no adverse effects.

Does a gonadotropin-releasing hormone analogue prevent cisplatin-induced spermatogenic impairment? An experimental study in the mouse

The cytotoxic effect of cis-diamminedichloroplatinum II (cisplatin, CDDP) on spermatogenesis in BALB/C mice, and possible protection of the testes by leuprolide acetate (D-Leu-6 LHRH(1-9)-ethylamide, TAP-144, leuprolide), a synthetic gonadotropin-releasing hormone analogue, were examined. Temporary interruption of the pituitary-gonadal axis by the analogue and amelioration of the gonadal toxicity of cisplatin by reducing the cell division rate in spermatogenesis were expected. The results showed cisplatin to have a cytotoxic effect on spermatogenic cells in BALB/C mice. The administration of leuprolide had no effect on testicular weight or histological findings in the mouse testes. Pretreatment and simultaneous administration of leuprolide did not reduce the damaging effect of cisplatin on the testes in mice. This is at variance with a previously published report.

Prolonged suppression of rat testis function by a depot formulation of Zoladex, a GnRH agonist

A sustained-release formulation of a potent gonadotropin-releasing hormone (GnRH) agonist, Zoladex (D-Ser(But),6 Aza Gly10-GnRH; ICI 118,630; goserelin), was administered subcutaneously (3.6 mg/depot) to male rats once every 28 days for 2-24 wk to determine the extent to which pituitary-testis function could be suppressed and whether suppression was maintained throughout the period of treatment. Administration of Zoladex resulted in sustained decreases in weight of the testis, epididymis, seminal vesicles and prostate gland. The decreases were apparent within 2 wk of initiating treatment. Patchy degeneration of the seminiferous tubules and atrophy of the Leydig cells were observed, but did not progress beyond the degree observed after 1 month of treatment. Serum and testis testosterone were markedly depressed after 2 wk of treatment, as was testis [125I]hCG binding. Serum gonadotropins were also reduced by treatment. Serum androgen binding protein (ABP) was elevated, testis ABP content remained unchanged, and epididymal ABP content was reduced. The changes are consistent with the hypothesis that this compound affects both the anterior pituitary gland and the testis. These findings indicate that depot delivery systems are a convenient way to administer GnRH analogs for sustained treatment schedules.