Intrathecal methiothepin and thyrotropin-releasing hormone effects on penile erection

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

5-Hydroxytryptamine2C receptors on pudendal motoneurons innervating the external anal sphincter

The aim of this study was to determine the localization of 5-hydroxytryptamine2C (5-HT2C) receptors on the motoneurons innervating the external anal sphincter (EAS) of male rats. Motoneurons were retrogradely labeled after percutaneous intramuscular injection of Fluorogold (FG) into the EAS. Using fluorescent immunohistochemistry, FG-positive EAS motoneurons that were immunoreactive for the 5-HT2C receptor (5-HT2C-IR) were targeted for specific examination with widefield microscopy or confocal laser scanning microscopy with spectral separation. Widefield microscopy revealed distributions of FG-positive EAS motoneurons in the L5-S1 gray matter corresponding to the dorsomedial cell group. 5-HT2C-IR positive cells were distributed in the intermediolateral cell column and the ventral horn. Ventral horn 5-HT2C-IR labeling included the dorsomedial cell group as well as the dorsolateral, ventromedial and ventrolateral areas. Confocal analysis of FG-positive EAS motoneurons and 5-HT2C-IR positive motoneuron profiles adjacent to EAS motoneurons that were not labeled with FG but presumably innervate the bulbospongiosus muscle confirmed that EAS motoneurons were immunopositive for the 5-HT2C receptor. These data suggest that previously identified descending serotonergic immunopositive fibers observed terminating on EAS motoneurons might mediate their input through the activation of 5-HT2C receptors.

Alterations in eliminative and sexual reflexes after spinal cord injury: defecatory function and development of spasticity in pelvic floor musculature

Spinal cord injury often results in loss of normal eliminative and sexual functions. This chapter is focused on defecatory function, although aspects of micturition and erectile function will be covered as well due to the overlap in anatomical organization and response to injury. These systems have both autonomic and somatic components, and are organized in the thoracolumbar (sympathetic), lumbosacral (somatic), and sacral (parasympathetic) spinal cord. Loss of supraspinal descending control and plasticity-mediated alterations at the level of the spinal cord, result in loss of voluntary control and in abnormal functioning of these systems including the development of dyssynergies and spasticity. There are several useful models of spinal cord injury in rodents that exhibit many of the autonomic dysfunctions observed after spinal cord injury in humans. Numerous studies involving these animal models have demonstrated development of abnormalities in bladder, external anal sphincter, and erectile function, such as detrusor-sphincter-dyssynergia and external anal sphincter hyperreflexia. Here we review many of these studies and show some of the anatomical alterations that develop within the spinal cord during the development of these hyperreflexias. Furthermore, we show that spasticity develops in other pelvic floor musculature as well, such as the bulbospongiosus muscle, which results in increased duration and magnitude of pressures developed during erectile events and increased duration of micturition. Advances and continued improvement in the use of current animal models of spinal cord injury should encourage and increase the laboratory work devoted to this relatively neglected area of experimental spinal cord injury.

Telemetric monitoring of corpus spongiosum penis pressure in conscious rats for assessment of micturition and sexual function following spinal cord contusion injury

Disruption of bladder function and sexual reflexes are major complications following spinal cord injury (SCI). We examined the use of telemetric monitoring of corpus spongiosum penis (CSP) pressures for assessment of micturition and erectile events following SCI in rats. Pressure catheters were implanted in the bulb of the CSP of seven male Long-Evans hooded rats, subjected to a standardized weight drop SCI (10 g x 12.5 mm) at T10. CSP pressures were analyzed for spontaneously occurring micturition and erectile events, and during ex copula reflex erection tests until 25 days after SCI. Urine volume was determined until 21 days after SCI. Results show initial loss of bladder function after SCI with gradual return of reflex micturition. When compared to baseline (BL), micturition pressure characteristics after SCI included prolonged duration, increased area under the curve (AUC), increased mean pressures, increased number of pressure peaks, and increased peak frequency. At 21 days after SCI, the urine volume per micturition was significantly increased. The number of full erectile events decreased significantly following SCI. Pressure wave analyses demonstrated increased AUC, increased maximum pressures, increased suprasystolic peak duration, increased AUC of the suprasystolic peaks, and increased maximum pressures of the suprasystolic peaks during recovery. The number of partial erectile events decreased significantly following SCI. Ex copula reflex erection testing demonstrated significantly decreased latency. The study demonstrates that telemetric monitoring of CSP pressures in conscious rats is a valuable and reliable method for assessing recovery of autonomic function following SCI.

Serotonergic fiber sprouting to external anal sphincter motoneurons after spinal cord contusion

The present study analyzed the anatomical plasticity of serotonergic immunoreactive projections to external anal sphincter (EAS) motoneurons, and the behavioral plasticity of EAS reflexes, penile erection, and locomotion in rats with spinal contusion injury (SCI) or complete spinal cord transection (TX). Electromyographic activity of the EAS, penile erection latency, and BBB locomotor score exhibited parallel recovery over the 6-week recovery period after contusion SCI. This pattern of recovery was not observed in TX animals. While locomotor scores demonstrated a small increase after TX, erectile and anorectal function remained at abnormal levels established immediately after injury. Serotonergic immunofluorescent (5-HT-IF) staining at the lesion site identified a small number of fibers spared after SCI that may provide a substrate for functional recovery. Pixel density measurements of 5-HT-IF in the vicinity of retrogradely labeled EAS and unlabeled pudendal motoneurons necessary for penile erection provide indirect evidence of serotonergic sprouting that parallels the observed functional recovery in animals with SCI. No 5-HT-IF was detected caudal to the injury site in TX animals. These studies indicate: (1) lumbosacral eliminative and reproductive reflexes provide a valid means of studying the mechanisms of post-SCI plasticity; (2) the similar recovery curves suggest similar return of descending control, perhaps through sprouting of descending serotonergic fibers; (3) the observed deficits after TX likely represent the permanent removal of descending inhibition and reflect reorganization of segmental circuitry.

Methiothepin attenuates gastric secretion and motility effects of vagal stimulants at the dorsal vagal complex

Methiothepin, a nonselective 5-HT receptor antagonist was utilized to explore the 5-HT modulation of dorsal vagal complex-TRH (thyrotropin releasing hormone) analogue stimulated gastric functional parameters. Intracisternal methiothepin pretreatment (200, 0.1 nmol) produced significant inhibition (70%, 44%, respectively) of the TRH analogue [p-Glu-His-(3,3'-dimethyl)-Pro NH2; RX 77368 (12 pmol)]-induced gastric acid output compared to vehicle pretreatment. Intracisternal pretreatment with methysergide (nonspecific 5-HT receptor antagonist) or combined cyanopindolol (5-HT(1A and 1B) receptor antagonist)+ritanserin (receptor antagonist of the 5-HT(2) family) did not alter the dorsal vagal complex-RX 77368 response. Unilateral dorsal vagal complex pretreatment with methiothepin (50 nmol/50 nl) attenuated ipsilateral dorsal vagal complex-TRH analog (12 pmol) induced gastric secretory response by 57%. The gastric secretagogue response to stimulation of the raphe obscurus (mediated by TRH release into the dorsal vagal complex) was inhibited 50% by pretreatment with intracisternal dorsal medullary methiothepin (0.1 nmol/10 microl). Intracisternal methiothepin (200 nmol/20 microl) also attenuated (a) dorsal vagal complex-glutamate (60 nmol/30 nl) stimulated gastric acid secretion and (b) gastric motility stimulated by dorsal vagal complex-RX 77368 (12 pmol/30 nl). The data suggest that other properties of methiothepin, alone or in addition to its 5-HT receptor antagonist effect, mediate its inhibitory actions at the dorsal vagal complex.

Inhibition of pudendal reflexes in spinal rats. Reassessing the role of serotonin

The effects of serotonin (5-HT) and thyrotropin-releasing hormone (TRH) on penile reflexes were investigated in intact and spinally transected male rats. Doses of intrathecal 5-HT (0.0, 1.13, 2.26, 11.3, 22.6, and 113.0 nmol), in a range previously shown to inhibit pudendal reflexes in anesthetized spinal preparations, prolonged the latency to the first penile erection in awake intact rats. However, these doses also provoked hyperreactivity and vocalization. Doses of intrathecal TRH (100 and 500 pmol) that effectively inhibited penile erection in intact animals were less effective in spinalized animals. Finally, a combination of subthreshold doses of TRH (100 pmol) and 5-HT (4.0 nmol) at a ratio known to affect other TRH/5-HT-mediated circuits significantly extended erection latency in animals with spinal transections. These data suggest that 5-HT and TRH are both involved in the inhibitory circuits regulating penile erection, either through corelease onto the same population of cells or through independent release onto different populations of neurons.