In an attempt to develop preclinical models of ejaculatory disorders, previous studies in our laboratory have suggested that there are different endogenous phenotypes of male Wistar rats in copulatory behaviour, with rats displaying 'normal', 'sluggish'or 'rapid' ejaculatory behaviour. The present studies were conducted to further explore these putative endogenous phenotypes as an approach to study the neurobiology and pharmacology of ejaculation disorders. Several large groups of male Wistar rats (200-250 g; n=100) were tested once weekly for several weeks against an estrus Wistar female rat (175-200 g) during a 30-min sexual behaviour test (methods: Mos et al., 1999). Copulatory behaviour of the males could be described in terms of a Gaussian distribution of their sexual performance, measured by the number of ejaculations per test and consisting of 'sluggish' (0-1 ejaculation/ test), 'normal' (2-3 ejaculations/ test) and 'rapid' (>3 ejaculations/ test) ejaculators (n=12 per group). Importantly, this distribution was present in the first test and did not change over time. Univariate analyses of variance and subsequent Bonferroni post hoc tests revealed that compared to 'sluggish' ejaculators a 525±22% decrease in mount frequencies (p<0.001) and 687±18% decrease in ejaculation latencies (p<0.001) was observed in 'rapid' ejaculators.

In addition, we tested the prosexual effect of 8-OH-DPAT (Ahlenius et al., 1981; 0.8 mg/kg, s.c.,30 min pretreatment) in the three groups. In all groups, including the 'sluggish' ejaculators, 8-OH-DPAT strongly facilitated sexual behaviour by increasing the number of ejaculations ('sluggish' 697±7% increase; 'normal' 200±7% increase; 'rapid' 157±3% increase, all p< 0.005) and decreasing ejaculation latencies ('sluggish' 6621 ±65% decrease; 'normal' 10163±47% decrease; rapid' 1881±5% decrease, all p<0.001) indicating that 'sluggish' rats were physically able to perform all aspects of sexual behaviour. Interestingly, upon subsequent retesting without drug treatment animals showed their initial sexual behavioural phenotype, indicating that in normal male rat populations, different endogenous phenotypes for sexual behaviour are present. Neuroanatomical results indicated increased Fos-immunoreactivity in 'rapid' compared to 'sluggish' ejaculators in regions of the medial preoptic area, bed nucleus of the stria terminalis, medial amygdala and subparafascicular nucleus of the thalamus (131±3%, 325±6%, 167±5%, 289±6% respectively; all p<0.05), presumably related to the observed differences in sexual behaviour.

Taken together, behavioural results strongly suggest endo-phenotypic stable differences in male rat ejaculatory and sexual behaviour in a normal population of male Wistar rats. Currently studies are being performed to further unravel putative neurobiological differences underlying these endophenotypic differences.

Ahlenius et al., 1981. Pharmacol Biochem Behav. 15:785-792.
Mos et al., 1999. Eur Neuropsychopharmacol. 9: 123-135.