Neurological Bases of Male Sexual Behavior

ASAS-SSR Triennial Reproduction Symposium: Looking Back And Moving Forward-How Reproductive Physiology Has Evolved: Male reproductive behavior: sensory signaling in the brain of low-performing domestic rams

Rams are selected for genetic traits a producer desires to propagate in his flock. Even though practically all sheep are naturally bred, rams are rarely evaluated for expression of sexual interest or behavior. Research at the U.S. Sheep Experiment Station reported that the proportion of rams with limited interest in ewes was nearly 30% of the total number of breeding rams. Breeding soundness tested rams with low sexual interest sire less than 16% of the lamb crop. Although producers recognize the problem, their usual solution is to increase the number of rams in breeding flocks decreasing the risk of open ewes. Increased costs and a lack of genetic progress are clearly important considerations, but the biological question as to what controls sexual interest remains intriguing. Circulating concentrations of testosterone do not differ by sexual interest among rams. However, following exposure to estrous ewes, sexually active, but not inactive, rams exhibit an increase in LH pulsatile activity, a biological response to sexual stimuli. This begs the question as to whether sexually inactive rams recognize sexual cues. Using c fos activity as an indicator of neural activity, differences in the olfactory pathway were compared among sexually active and inactive rams. Differences in fos activity were not detected in the olfactory bulb or medial amygdala. However, sexually inactive rams had lower fos activity in the central amygdala, bed nucleus of the stria terminalis, and medial preoptic area of the hypothalamus following exposure to sexual evocative olfactory stimuli. This suggests sexually inactive rams detect olfactory cues but have decreased vigilance to sensory stimuli and a muted response to sexual signals in areas of the brain required for the execution of sexual performance. With the amygdala receiving and integrating sensory signals from many areas of the brain, sexually inactive rams may lack normal reward mechanisms contributing to their lack of sexual interest. Rams with limited sexual interest have decreased dopamine synthesis and a lower expression of dopamine D2 receptors following exposure to sexual stimuli. Thus, a tempered reward pathway in combination with decreased vigilance and sensory processing in the amygdala may reduce stimulation and/or responsiveness in areas of the brain required for expression of sexual behavior.

Sexual selection predicts brain structure in dragon lizards

Phenotypic traits such as ornaments and armaments are generally shaped by sexual selection, which often favours larger and more elaborate males compared to females. But can sexual selection also influence the brain? Previous studies in vertebrates report contradictory results with no consistent pattern between variation in brain structure and the strength of sexual selection. We hypothesize that sexual selection will act in a consistent way on two vertebrate brain regions that directly regulate sexual behaviour: the medial preoptic nucleus (MPON) and the ventromedial hypothalamic nucleus (VMN). The MPON regulates male reproductive behaviour whereas the VMN regulates female reproductive behaviour and is also involved in male aggression. To test our hypothesis, we used high-resolution magnetic resonance imaging combined with traditional histology of brains in 14 dragon lizard species of the genus Ctenophorus that vary in the strength of precopulatory sexual selection. Males belonging to species that experience greater sexual selection had a larger MPON and a smaller VMN. Conversely, females did not show any patterns of variation in these brain regions. As the volumes of both these regions also correlated with brain volume (BV) in our models, we tested whether they show the same pattern of evolution in response to changes in BV and found that the do. Therefore, we show that the primary brain nuclei underlying reproductive behaviour in vertebrates can evolve in a mosaic fashion, differently between males and females, likely in response to sexual selection, and that these same regions are simultaneously evolving in concert in relation to overall brain size.

Convergence and cross talk in urogenital neural circuitries

Despite common comorbidity of sexual and urinary dysfunctions, the interrelationships between the neural control of these functions are poorly understood. The medullary reticular formation (MRF) contributes to both mating/arousal functions and micturition, making it a good site to test circuitry interactions. Urethane-anesthetized adult Wistar rats were used to examine the impact of electrically stimulating different nerve targets [dorsal nerve of the penis (DNP) or clitoris (DNC); L6/S1 trunk] on responses of individual extracellularly recorded MRF neurons. The effect of bladder filling on MRF neurons was also examined, as was stimulation of DNP on bladder reflexes via cystometry. In total, 236 MRF neurons responded to neurostimulation: 102 to DNP stimulation (12 males), 64 to DNC stimulation (12 females), and 70 to L6/S1 trunk stimulation (12 males). Amplitude thresholds were significantly different at DNP (15.0 ± 0.6 μA), DNC (10.5 ± 0.7 μA), and L6/S1 trunk (54.2 ± 4.6 μA), whereas similar frequency responses were found (max responses near 30–40 Hz). In five males, filling/voiding cycles were lengthened with DNP stimulation (11.0 ± 0.9 μA), with a maximal effective frequency plateau beginning at 30 Hz. Bladder effects lasted ∼2 min after DNP stimulus offset. Many MRF neurons receiving DNP/DNC input responded to bladder filling (35.0% and 68.3%, respectively), either just before (43%) or simultaneously with (57%) the voiding reflex. Taken together, MRF-evoked responses with neurostimulation of multiple nerve targets along with different responses to bladder infusion have implications for the role of MRF in multiple aspects of urogenital functions.

Androgen stimulates neuronal plasticity in the perineal motoneurons of aged male rats

Ten aged male rats (24 months of age) were castrated and implanted subcutaneously with Silastic capsules containing testosterone (T)(5 males) or nothing (5 males). Five sham-castrated males (25 months of age) served as controls. Four weeks after castration, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the control and experimental SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in castrated, aged males treated with T was significantly greater than that in control or castrated animals. Size and number of synaptic contacts per unit length of somatic membranes in castrated, aged males treated with T were also significantly greater than those in control or castrated animals. Plasma levels of T in castrated, aged males treated with T were significantly greater than that in controls. These results suggest that the SNB motoneurons of aged male rats retain a considerable synaptic plasticity in response to androgen, and that androgen may be, at least in part, involved in the process of aging of the SNB system in male rats.

Progesterone receptor isoforms expression in the prepuberal and adult male rat brain

Progesterone receptor (PR) isoforms expression was determined in several regions of the prepuberal and adult male rat brain by using reverse transcription coupled to polymerase chain reaction. Rats under a 14:10-h light-dark cycle, with lights on at 0600 h were used. We found that in the hypothalamus of prepuberal animals the expression of both PR isoforms was similar, whereas PR-A expression was higher than that of PR-B in adults. In the cerebellum PR-B expression was predominant in both prepuberal and adult rats. In both ages PR-A and PR-B exhibited a non-significant tendency to be predominant in the hippocampus and the preoptic area respectively. In the frontal cortex and the olfactory bulb PR isoforms were expressed at a similar level. These results indicate a differential expression pattern of PR isoforms in the male rat brain and suggest that the tissue-specific expression of PR-A and PR-B is important for the appropriate response of each cerebral region to progesterone.

Lesions of the medial preoptic area/anterior hypothalamus (MPOA/AH) modify partner preference in male rats

When given the choice, male rats will interact with a receptive female while female rats will interact with a sexually active male. In the present experiment partner preference was tested in male and female rats before and after lesions of the medial preoptic area of the anterior hypothalamus (MPOA/HA). Subjects were gonadectomized, treated for 10 days with 5 microgram/kg of estradiol benzoate (EB) and tested for male coital behavior with receptive females and for partner preference in a three compartment box with free access to either a sexually receptive female or a sexually active male. The same tests were repeated after 10 days of treatment with 5 mg/kg of testosterone propionate (TP). The subjects then received a bilateral electrolytic lesion aimed at the MPOA/AH. Two weeks after the lesion the hormonal treatments and behavioral tests were repeated in the same sequence. Prior to the lesion, females showed a clear preference to interact with the stimulus male while male subjects showed a preference to interact with the receptive female regardless of the hormonal treatment they received. After lesions the females preference for the opposite sex was not modified, they spent more time in the chamber with the stimulus male regardless of whether they had an extensive bilateral destruction of the MPOA/AH or a sham lesion. Males with bilateral destruction of the MPOA/AH changed their partner preference after the lesion. They spent significantly more time in the chamber with the stimulus male than in the chamber with the receptive female. As well, the coital behavior of males with bilateral destruction of the MPOA/AH was significantly reduced after the lesion. The change of preference was observed when the lesioned animals were treated either with EB or TP. The results of the present experiment further support the notion that the MPOA/AH is a crucial structure in the integration of sensory cues that determine partner preference.

Neuronal cell death during sexual differentiation and lateralisation of vocal communication

A rodent analogy has been established to investigate the neural mechanisms occurring during sexual differentiation and lateralization. A sexually dimorphic hypothalamic nucleus (SDApc) is closely associated with a stereotyped, courtship vocalisation in male gerbils. Stereological analysis of SDApc cytoarchitecture reveals that neuron number and nuclear volume are asymmetric in male adults. Strikingly, neuron number on the left side of the SDApc correlates significantly with the rate of the courtship call in males. Exogenous testosterone treatment in female neonates masculinises and lateralises SDApc structure and function. Neuronal programmed cell death (apoptosis), manifested in SDApcs of neonates, is more frequent in females. Significantly, apoptosis in males is lateralised, as revealed by lateral asymmetry of neuron number at postnatal day 16. It is concluded that neuroendocrine-dependent, sexual differentiation and lateralization are concurrent and influenced by apoptotic mechanisms. It is suggested that apoptosis is the result of a genetically-driven device, inherent in postmitotic, undifferentiated cells which may have recently migrated into the SDApc. The genomic mechanism inducing lateralised apoptosis is apparently activated only neonatally in males.

Sex differences in estrogenic regulation of preproenkephalin mRNA levels in the medial preoptic area of prepubertal rats

Opioids have been implicated in sexual differentiation of the brain and in the regulation of reproductive behavior and endocrinology of mammals. Previous studies have indicated that estrogen administration in adults regulates preproenkephalin MRNA levels in several hypothalamic brain nuclei. We have determined preproenkephalin mRNA levels in estrogen-treated juvenile male and female rats to investigate the developmental pattern of estrogenic regulation of enkephalinergic neurons in the medial preoptic area. Rats were treated with estradiol benzoate (20 microgram/kg/day) or oil from day 21 to 23. Sections of the medial preoptic area (mPOA) were studied by in situ hybridization histochemistry at the single cell level and quantified with the assistance of an image analysis system. Our data indicate that males contain higher levels of preproenkephalin mRNA per neuron than females. In addition, our results indicate that estrogen causes an upward shift in the amount of mRNA expressed per cell, females demonstrating a greater response to estrogen than males. An increase in soma cell area following estrogen treatment was observed only in female mPOA enkephalinergic neurons. Taken together, these results indicate a sex difference in total preproenkephalin levels and in estrogenic regulation of preproenkephalin mRNA in the POA of juvenile rats. These results are discussed in relation to the differential role opioids may play in male and female reproductive physiology.

Intracranial androgenic activation of male-typical behaviours in house mice: concurrent stimulation of the medial preoptic area and medial nucleus of the amygdala

This experiment examined whether testosterone proprionate (T) action in the medial preoptic area (MPO) would synergize with T action in the medial nucleus of the amygdala (AME) for the expression of androgen-dependent behaviors in house mice. Cannulae containing T were bilaterally implanted into the MPO, the AME, or both areas concurrently (MPO/AME) of castrated males. In addition, other castrates were implanted subcutaneously with empty Silastic capsules (BSIL) or Silastic capsules containing T (TSIL). All subjects were examined for the following androgen-dependent, male-typical behaviors: mounting, urinary scent marking, preference for female urine over male urine, preference for female over male conspecifics and ultrasonic mating vocalizations. MPO implants restored ultrasonic vocalizations and preference for females, but had little or no effect upon urine marking, mounting or preference for female urine. In contrast, AME implants were ineffective at restoring any of these male-typical behaviors. The combined MPO/AME implants were not more effective in restoring male-typical behaviors than MPO implants alone, thus providing no evidence for synergy in hormone action between these two brain areas. In general, castration (BSIL) resulted in low levels of all behaviors whereas systemic T replacement (TSIL) resulted in high levels of behavior, verifying the androgen-dependence of these behaviors. Group differences in male-typical behavior could not be accounted for by differences in general activity levels. Moreover, none of the brain-implanted groups had larger seminal vesicles than those of the BSIL. Thus, when the brain implants affected behavior, they most probably did so through their effects within the brain. Although the AME is a target for steroid hormones and is an important area for the expression of male-typical behaviors, intracranial T implants into the AME did not demonstrate a role for androgen in the AME in restoring male-typical behaviors in castrated mice.

Hormonally induced neuronal plasticity in the adult motoneurons

Sex steroids are known to play a crucial role in reproductive neuroendocrine functions in adulthood. A number of neurons in the neuroendocrine brain contain sex steroid receptors, and are thought to be a key element of functional neural circuits that are regulated by sex steroids. Motoneurons in the spinal nucleus of the bulbocavernosus in adult male rodents are one of the androgen-sensitive neural substrates. In the spinal nucleus of the bulbocavernosus, castration of adult male rats results in a significant decrease in the somatic size and dendritic length of the motoneurons, and in the number and size of chemical and electrical (gap junction) synapses onto these motoneurons. Androgen treatment of castrates reverses these changes. Furthermore, androgen has been reported to be involved in regulation of androgen receptor expression and gene expression of structural proteins such as beta-actin, beta-tubulin and gap junction channels in these motoneurons. The findings suggest that androgen induces morphological and molecular changes in the motoneurons that reflect their neural functions, and may provide evidence for the mechanisms of hormonally induced neuronal plasticity in the motoneurons in adulthood.

Organizational actions of sex hormones on sexual partner preference

Sexual dimorphism in copulatory behavior results from organizational actions of sex steroids (permanent effects of sex steroids occurring during early development). Reproductive success depends not only on copulatory behavior, but also on mate choice, which is often sexually dimorphic as well. The clearest example is sexual partner preference: the preference of males for female sexual partners and females for males. Are organizational hormone actions responsible for sexual differentiation of sexual partner preference? The zebra finch (Taeniopygia guttata) is a potentially valuable species for addressing this question, because the birds form life-long socially monogamous pair bonds. In one experiment, both early estrogen treatment (injection with estradiol benzoate-EB-for the first 2 weeks posthatch) and unisex housing during juvenile development independently resulted in a preference for females over males in two-choice tests, and only females that experienced both EB treatment and unisex living were more likely than controls to pair with other females in colony tests. In a second experiment, females injected with an estrogen synthesis inhibitor for the first week posthatch preferred to spend time near females instead of males in two-choice tests, unlike control females. These experiments suggest that sexual partner preference may result from organizational hormone actions in this pair-bonding species. Possible neural mechanisms or sites that could underly hormonal organization of sexual partner preference in birds and mammals include the anterior hypothalamic/preoptic area, the corticomedial amygdala, and its avian homologue nucleus taeniae of the archistriatum, the septum, and peripheral sensory processes.

Postnatal development of a sexually dimorphic, hypothalamic nucleus in gerbils: a stereological study of neuronal number and apoptosis

Steroid-sensitive, vocal courtship behavior is a function of a specific, hypothalamic nucleus, the sexually dimorphic area pars compacta (SDApc) in the male adult gerbil. Gender-related differences in the number of neurons in this nucleus are evident immediately after birth. By using unbiased stereological estimates of cell numbers in Nissl-stained, paraffin-wax sections of brain, we investigated the mechanisms differentiating cell number between the sexes in the SDApc on postnatal days 0, 3, 6, and 15. Cell death, identified by pyknosis, was greatest in the SDApc between days 0-3 in males, whereas in females, maximum values were reached between days 3-6. Similarly, the ratio of pyknotic to normal neurons peaked between days 0-3 in males and 3-6 in females but then declined in both sexes. Pyknotic cells were seldom seen in either sex by day 15. Morphological characteristics of apoptosis including chromatin condensation, cell fragmentation, and ingestion of apoptic bodies by macrophages were all demonstrated by transmission electron microscopy. Macrophages showed specific morphological characteristics of microglia. Cell division (mitosis) was identified in the SDApc during postnatal days 0, 3, and 6 but the numbers of mitotic figures were low, negligible on day 15, and similar between the sexes. These results demonstrate that cell death and proliferation occur simultaneously in the neonatal gerbil brain. The stereological estimates of cell death in the developing SDApc indicated a lower incidence of neuronal death occurring earlier in males than in females.

Mapping of neurochemical markers in quail central nervous system: VIP- and SP-like immunoreactivity

The distribution of cells and fibres containing vasoactive intestinal polypeptide (VIP) and substance P (SP) was investigated in the brain of Japanese quail focussing on the centers involved in reproductive activities. SP-immunoreactive (ir) structures were chiefly present within the ventral telencephalic regions, the periventricular hypothalamus and the dorsal aspects of thalamus. VIP immunopositive structures were rarely associated with recognizable nuclei and they were observed in the organum septi laterale (LSO), the lobus paraolfactorius (LPO), the eminentia mediana (ME), the nucleus striae terminalis (nST) and the area ventralis of Tsai (AVT). SP- and VIP-ir structures were both associated with regions implicated in the control of reproduction. SP was mainly distributed within regions that control male copulatory behavior (the preoptic region, the anterior hypothalamus and the central gray), whereas VIP was prevalently located in the mediobasal hypothalamus that is implicated in the control of female reproductive activities.

Effects of yohimbine and idazoxan on motor behaviors in male rats

Yohimbine, an alpha 2 adrenergic antagonist, facilitates copulatory behaviors in male rats. This facilitation may reflect nonspecific activation of behavior rather than a more selective activation of copulatory behaviors. The present experiments assessed the effects of yohimbine on locomotor behaviors at a dose (2.0 mg/kg) known to facilitate sexual behaviors. Experiment 1 used a computer pattern-recognition system to classify motor behaviors into specific acts and act groups. Male albino rats were tested in three conspecific conditions: estrous female, anestrous female, or no conspecific. Yohimbine decreased locomotor activity in all three conspecific conditions. Experiment 2 examined the effects of yohimbine (2.0 mg/kg) and amphetamine (1.0 mg/kg) on locomotor behavior in a photocell-equipped activity measurement system. Amphetamine increased and yohimbine decreased locomotor activity. Experiment 3 used the computer pattern-recognition system to compare the effects of yohimbine and idazoxan, another alpha 2 adrenergic antagonist, on motor behaviors. Yohimbine and idazoxan both decreased activity but produced different patterns of behavioral change. The facilitatory effects of yohimbine on copulatory behaviors at a dose of 2.0 mg/kg are apparently not mediated by nonspecific activation of behavior.