Spinal transection restores sexual reflexes of rats following suppression by penile sheath removal

Modification of sexual behavior of Long-Evans male rats by drugs acting on the 5-HT1A receptor

Modulation of the sexual behavior of male rats by the anxiolytic buspirone (S-20499) and its analog gepirone were compared to the effects of 8-OH-DPAT (or DPAT, a selective 5-HT1A reference agonist), and BMY-7378 (a selective 5-HT1A partial agonist). Long-Evans rats were used; modulation of copulatory behavior and alteration of penile reflexes were examined. Modulation of copulatory behavior was assessed by three indices: frequency and length of intromission, and latency of ejaculation. DPAT, at doses of 1-8 mg/kg, reduced these three indices in a time dependent manner such that the effects peaked at 45 min and normalized at 90 min. The dose-effect relationship (assessed 45 min after DPAT injection) is bell-shaped with an ED50 approximately 1 mg/kg on the ascending limb of the curve. The effects of buspirone (2 mg/kg) and gepirone (2 mg/kg) on copulatory behavior were indistinguishable from control. BMY-7378 alone and in combination with these other 5-HT1A agonists reduced copulatory behavior, though not statistically significant. Penile reflexes, including number of erections, cups and flips, were inhibited by these agents: DPAT>buspirone>gepirone (inactive at 2 mg/kg). Furthermore, the latency period to erection was at least doubled by DPAT (2 mg/kg). Buspirone and gepirone, however, reduced the latency period to erection. BMY-7378 inhibited penile reflexes when administered alone and even more in combination with DPAT or buspirone. Two butyrophenone analogs, spiperone (a 5-HT1A and dopamine D2 antagonist) and haloperidol (a D2 antagonist), were also tested for their interaction with DPAT. Both of these drugs (at 0.25 mg/kg, 60 min after administration) reduced all indices of penile reflexes and copulation. Furthermore, in combination with DPAT (2 mg/kg, 45 min), the effects were synergistic such that sexual activity came nearly to a standstill. These opposing effects on putatively brain originated copulatory behavior and spinal mediated penile reflexes indicate that the effects of buspirone and DPAT on sexual behavior in the male rat may be possible at different parts of the central nervous system. If a tentative shared target site by DPAT and buspirone is the 5-HT1A receptor, than the same 5-HT receptor sub-type at different locations (brain, raphe nuclei, spinal cord and autonomic ganglia) may modulate rat sexual behavior in opposing ways.

Mounting and brief noncontact exposure of males to receptive females facilitate reflexive erection in rats, even after hypogastric nerve section

In three experiments, reflexive erection in male rats was facilitated by housing the males for 2 min with inaccessible sexually receptive females. In Experiment 1, males were sexually naive or experienced and received two reflexive erection tests, 1 week apart, immediately after the males were exposed to receptive females, to unreceptive females, or to no females (n = 8 per group). In both tests, experienced males exposed to estrous females had the shortest reflexive erection latencies; in Test 1 the differences among groups were of borderline significance (p = 0.057), but in Test 2 the differences among groups were highly reliable (p<0.01). Further analysis indicated that only experienced males exposed to receptive females were significantly different from other groups. In Experiment 2, sexually experienced males (n = 11) received four reflexive erection tests: after being with no female, and 0, 5, or 10 min after exposure to estrous females. As the interval between exposure and test increased, the males had progressively shorter erection latencies (p<0.01) and more intense glans erections (p<0.03). Experiments 1 and 2 may be viewed as demonstrating the psychogenic facilitation of reflexive erections. In Experiment 3, males underwent sham surgery (sham, n = 10) or bilateral transection of the hypogastric nerves (HgNx, n = 10), which are conventionally viewed as mediating psychogenic erection. After males mounted a receptive female for 5 min without intromission or had 2 min of non-contact exposure to receptive females, the males had shorter erection latencies (p<0.001) and more erections (p<0.02). These facilitative effects of pretest stimulation were unaffected by HgN transection. During copulation tests, HgNx males had longer ejaculation latencies (p<0.05) and lower intromission ratios (p<0.05), possible signs of impaired erectile function. However, in Experiment 4, other males were tested twice for reflexive erection and copulation after sham (n = 8) or HgNx (n = 9) surgery, and there were no significant effects of surgery on reflexive erection or copulatory behavior. Collectively, these experiments indicate (a) that brief noncontact exposure of sexually experienced males to estrous females facilitates reflexive erection, (b) that this facilitation increases for at least 10 min after the females are removed, and (c) that the hypogastric nerves do not mediate these facilitative effects. The evidence for a role for the HgN in copulation was inconclusive.

Electrical stimulation of the dorsal nerve of the penis evokes reflex tonic erections of the penile body and reflex ejaculatory responses in the spinal rat

An animal model using the spinal rat was characterized. Electrical stimulation of the dorsal nerve of the penis elicited reflex tonic erections of the penile body and reflex bulbospongiosus muscle activity, flips and ejaculations. The tonic erections of the penile body are independent from contractions of the bulbospongiosus muscle and appear to be the result of a neurovascular process. Our observations suggest that reflex bulbospongiosus muscle activity, flips and ejaculations are a single complex reflex response, which we define as reflex ejaculatory response. Two parameters predicted the occurrence and type of reflex response. The visualization of bulbospongiosus muscle activity during surgical isolation of the dorsal nerve of the penis was sufficient to anticipate the elicitability of reflex ejaculatory responses. The latter, together with a systemic systolic pressure > or = 73 mmHg., warranted the elicitability of reflex tonic erections. The similarities found in the physiology of rat tonic penile body erections and of human erections make this model promising for further elucidation of sexual function. Moreover, the present model may prove useful for the investigation of neurogenic erectile dysfunction, and of neurogenic ejaculatory disorders.

Higher CNS control of penile responses in rats: the effect of hypothalamic stimulation

The effect of electrical stimulation of the medial preoptic area (MPOA) of the hypothalamus on penile responses in rats was investigated. No females were present and penile responses were observed through the use of an inclined mirror. MPOA stimulation was found to elicit two types of penile responses: penile movement within the sheath and protrusion of the penis outside the penile sheath. While these responses also occurred spontaneously during the nonstimulation control sessions, they occurred in significantly higher frequencies following MPOA stimulation. The results also produced an unexpected finding. While studies on the effect of hypothalamic stimulation on copulation show stimulation-bound effects, MPOA stimulation in the absence of a receptive female consistently triggered penile responses as a poststimulation effect.

The neuroanatomical basis for the protopathic sensibility of the human glans penis

The human glans penis is covered by stratified squamous epithelium and a dense layer of connective tissue equivalent to the dermis of typical skin. Rete ridges of the epidermis are irregular and vary in height depending on location, age, and presence or absence of a foreskin. The papillary layer of the dermis blends into and is continuous with the dense connective tissue forming the tunica albuginea of the corpus spongiosum of the glans penis. The most numerous nerve terminals are free nerve endings (FNEs) present in almost every dermal papilla, as well as scattered throughout the deeper dermis. FNEs are characterized by an incomplete Schwann cell investment, and contain irregularly scattered neurofilaments and neurotubules, clusters of mitochondria, vesicles of variable size and various inclusions. The ratio of FNEs to corpuscular receptors is approximately 10:1 and a similar ratio of small to large axons is seen in dermal nerves. Genital end bulbs are present throughout the glans, but are most numerous in the corona and near the frenulum. The unique corpuscular receptor of the glans penis consists of axon terminals that at an ultrastructural level resemble a tangled skein of FNEs. Simple, Pacinian and Ruffini corpuscles were occasionally identified predominantly in the corona glandis. Epidermal Merkel nerve endings and other types of mechanoreceptors typically found in primate glabrous skin (lip or digit) are not present. Rarely, dermal Merkel cells have been identified associated with genital end bulbs. The abundance of FNEs in isolated as well as corpuscular form can be correlated with the embryogenesis and known neurophysiologic and psychophysical parameters of sensory function of the glans penis. Finally, the divergence in reported values for the threshold of tactile and painful stimuli when applied to glabrous skin of fingertip and glans penis can be considered as an example of dissociated sensibility. The anatomical basis for this dissociation is the abundance of FNEs and absence of Merkel terminals and typical Meissner corpuscles in the covering of the glans, and the converse in glabrous skin of the digit.

Penile reflexes in intact rats following anesthetization of the penis and ejaculation

Application of the topical anesthetic pontocaine (tetracaine hydrochloride) to the penis and preputial sheath virtually abolished the display of penile reflexes by intact rats. When pontocaine was applied immediately following ejaculation in copula, however, normal genital reflexes were observed in the majority of rats, albeit with a prolonged latency to the first erection relative to the control test. These same animals were, nonetheless, unable to achieve intromission when returned to the mating arena despite repeated mounting attempts. These data suggest that sensory input to the genitalia is generally necessary for the display of penile reflexes in the rat. However, stimuli associated with copulation can override the suppressive effect of sensory loss of these reflexes, presumably due to removal of supraspinal inhibition.