Activation of Mu or Delta Opioid Receptors in the Lumbosacral Spinal Cord Is Essential for Ejaculatory Reflexes in Male Rats

Abstract

Ejaculation is controlled by a spinal ejaculation generator located in the lumbosacral spinal cord, consisting in male rats of lumbar spinothalamic (LSt) cells and their inter-spinal projections to autonomic and motor centers. LSt cells co-express several neuropeptides, including gastrin releasing peptide (GRP) and enkephalin. We previously demonstrated in rats that GRP regulates ejaculation by acting within the lumbosacral spinal cord. In the present study, the hypothesis was tested that enkephalin controls ejaculation by acting on mu (MOR) or delta opioid receptors (DOR) in LSt target areas. Adult male rats were anesthetized and spinalized and received intrathecal infusions of vehicle, MOR antagonist CTOP (0.4 or 4 nmol), DOR antagonist (TIPP (0.4, 4 or 40 nmol), MOR agonist DAMGO (0.1 or 10 nmol), or DOR agonist deltorphin II (1.3 or 13 nmol). Ejaculatory reflexes were triggered by stimulation of the dorsal penile nerve (DPN) and seminal vesicle pressure and rhythmic contractions of the bulbocavernosus muscle were analyzed. Intrathecal infusion of MOR or DOR antagonists effectively blocked ejaculatory reflexes induced by DPN stimulation. Intrathecal infusion of DAMGO, but not deltorphin II triggered ejaculation in absence of DPN stimulation. Both MOR and DOR agonists facilitated ejaculatory reflexes induced by subthreshold DPN stimulation in all animals. Overall, these results support the hypothesis that enkephalin plays a critical role in the control of ejaculation in male rats. Activation of either MOR or DOR in LSt target areas is required for ejaculation, while MOR activation is sufficient to trigger ejaculation in the absence of sensory stimulation.

Activation of galanin and cholecystokinin receptors in the lumbosacral spinal cord is required for ejaculation in male rats

“…In contrast, galanin or CCK infusions in the absence of DPN stimulation were not sufficient to trigger ejaculatory reflexes. This finding is in contrast with our previous observations that agonists for GRP or mu opioid receptors do trigger ejaculatory reflexes in the absence of DPN stimulation (Kozyrev et al ., ; Kozyrev & Coolen, ). It is therefore possible that galanin and CCK act synergistically with the endogenous release of neuropeptides from LSt axon terminals, including GRP and enkephalin, upon sensory stimulation of the DPN.…”

“…Infusions of galanin significantly reduced the numbers of BCM events, bursts, and SVP increases following threshold DPN stimulation (60 Hz). This finding is in agreement with previous reports of intrathecal infusions of GRP, DAMGO, and deltorphin II that have all significantly reduced the numbers of BCM events, bursts, and SVP increases following 60 Hz DPN stimulation (Kozyrev & Coolen, ). This consistent suppression of ejaculatory reflexes to threshold DPN stimulation (60 Hz) may be due to the effects of intrathecal infusions of neuropeptides combined with the endogenous release of opioids following DPN stimulation that cumulatively act to inhibit or desensitize the G‐protein‐coupled receptors in autonomic and motor regions involved in ejaculation and suppress the transmission of sensory inputs to the LSt cells which are required to trigger ejaculation.…”

“…Following the saline infusion, the DPN was stimulated at 30 and 60 Hz in a counterbalanced order, with 5‐min rest periods between stimulations. These stimulation parameters reliably trigger ejaculatory reflexes in control male rats (Staudt et al ., , , ; Kozyrev et al ., , ; Kozyrev & Coolen, ). After each DPN stimulation, BCM EMG and SVP were recorded for a period of 90 s, corresponding to the duration of a representative ejaculatory reflex triggered by DPN stimulation in male rats (Staudt et al ., , , ; Kozyrev et al ., , ; Kozyrev & Coolen, ).…”

“…Conversely, intrathecal infusion of the GRP agonist GRP 2029 or mu opioid receptor agonist DAMGO triggered ejaculatory reflexes in male rats. In addition, intrathecal infusion of GRP 2029 , DAMGO, or delta receptor agonist deltorphin II, facilitated ejaculatory reflexes following subthreshold levels of DPN stimulation insufficient to trigger ejaculatory reflexes in control animals (Kozyrev et al ., ; Kozyrev & Coolen, ). However, the roles of galanin and CKK for ejaculation are currently unknown, even though LSt cells were first described based on the expression of these neuropeptides (Truitt & Coolen, ; Coolen et al ., ) Moreover, galanin immunoreactivity has been demonstrated in the human spinal ejaculation generator (Chéhensse et al ., ).…”

“…Therefore, we hypothesize that these neuropeptides are released from LSt cell axon terminals onto preganglionic sympathetic (IML and CAN), parasympathetic (SPN), and motor (SNB) neurons to modulate ejaculatory reflexes. Indeed, our laboratory has recently revealed a critical role for GRP and enkephalin within the spinal ejaculation generator in male rats (Kozyrev et al, 2012;Kozyrev & Coolen, 2015). It was discovered that intrathecal infusions of the GRP antagonist RC-3095, mu opioid receptor antagonists CTOP, and delta opioid receptor TIPP, severely disrupted ejaculatory reflexes in anesthetized and spinalized male rats (Kozyrev et al, 2012;Kozyrev & Coolen, 2015).…”