Neuropeptide Y innervation in the spinal nucleus of bulbocavernosus of the rat

Testosterone prevents synaptic loss in the perineal motoneuron pool in the spinal cord in male rats exposed to chronic stress

Chronic stress is known to induce disorders of reproductive neuroendocrine functions. Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in male rats play an important role in copulatory behavior. In the present study, it was examined whether chronic stress would alter synaptic organization of the SNB motoneurons and whether androgen would modify the changes under chronic stress. Five male rats were under restraint stress for 5 days per week for 3 weeks, and five males implanted subcutaneously with Silastic capsules containing testosterone were also exposed to stress. Five males served as unstressed controls. After 3 weeks of restraint stress, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the SNB were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 SNB motoneurons (total for three groups) were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts. The mean percentage of somatic membranes covered by synapses in males exposed to chronic stress was significantly less than that in controls or stressed males treated with testosterone. Size and number of synaptic contacts per unit length of somatic membranes in males exposed to stress were also significantly less than those in controls or stressed males treated with testosterone. There was no significant difference in any of the parameters between controls and stressed males treated with testosterone. Changes in plasma levels of testosterone showed the same profile as changes in the synaptic contacts. These results suggest that the SNB motoneurons of male rats exposed to chronic stress retain a considerable synaptic plasticity in response to androgen, and that androgen treatment can rescue the SNB system in male rats when under chronic restraint stress.

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.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Synaptic changes in the perineal motoneurons of aged male rats

Cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles of young (2 months of age) and old (19-20 months of age) male rats, 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 120 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 young and old SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in old rats was significantly smaller than that in young ones. Size and number of synaptic contacts per unit length of somatic membranes in old animals were also significantly reduced. Plasma levels of testosterone in old males were significantly smaller than those in young ones. These age-related changes in synaptic inputs to SNB motoneurons and plasma levels of androgen seem to correlate with aging of the SNB system.

Spinal control of penile erection

Smooth muscle relaxation of penile arteries, the corpus cavernosum, and the corpus spongiosum, leading to penile erection, results from parasympathetic neural pathway activation and, likely, simultaneous inhibition of sympathetic outflow. Proerectile parasympathetic outflow is reflexively activated by sensory information of peripheral origin, conveyed by the dorsal penile nerve, and reflexive erections are supported by an intraspinal circuitry. Supraspinal influences modulate the reflex. Information integrated at or originating from supraspinal structures may also elicit penile erection. Several neurotransmitters are involved in either the modulation of the spinal reflex or the mediation of supraspinal influences. Spinal cord injury differently alters reflexive penile erection or erection from a central origin, depending on the neurologic level of injury.