Presence of NMDA receptor subunits in the male lower urogenital tract.

Abstract

Some sexual responses in the male rat, specifically penile erection, are controlled by neural circuits in the brain and spine that are stimulated by the binding of excitatory amino acids (EAAs) to the postsynaptic N-methyl-D-aspartate receptor (NMDAR). In the hypothalamus, EAA/NMDAR interaction triggers the activation of neuronal nitric oxide synthase (nNOS) to produce nitric oxide (NO). The local synthesis of this neurotransmitter in the penile nerve terminals causes corpora cavernosal relaxation and erection. During sexual activity, NO is assumed to participate in seminal emission and ejaculation in the prostate and to inhibit voiding reflexes in the bladder. This study aimed to determine in vitro whether NMDAR is present in these organs and whether it affects the tone of tissue strips through an NO-dependent mechanism. We obtained penile, urinary bladder, and ventral prostate tissues from adult male rats and homologous surgical tissues from human male patients. We detected the NMDAR protein by Western blot and determined the binding of the NMDA antagonist, 3H-CGP. The NMDAR messenger ribonucleic acid (mRNA) was detected by reverse transcription/polymerase chain reaction and identified by cloning and sequencing. The in vitro response to NMDAR antagonists was measured in tissue strips that were precontracted with bethanechol or electrical field stimulation (in rat and human bladder), phenylephrine (in human corpora cavernosa), or norepinephrine (in human prostate). The NMDAR2B protein; ligand-binding activity; and NMDAR1, 2A, and 2B mRNAs were detected in all tissues studied. We found an NMDAR1 variant in rat prostate and penis and in human prostate that is larger than its cerebellar counterpart, but it encodes a 767-amino acid truncated protein (NMDAR1-T). The in vitro contraction of tissue strips was inhibited by NMDAR antagonists against the following sites: polyamine (with ifenprodil); ion channnel high affinity (with dizocilpine); ion channel low affinity (with memantidine, dextrometorphan, and ketamine); and, in an NO-independent, nonadrenergic-noncholinergic pathway that was only partially affected by EAAs. We conclude that, in vitro, all the essential NMDAR subunits are present in the lower urogenital tract, a novel variant of subunit 1 is expressed, the tissues bind an NMDAR ligand, and the NMDAR antagonists induce relaxation of tissue strips. Further work is necessary to determine whether the NMDAR subunits form a fully active receptor and participate in the control of organ tone that is relevant to male sexual activity.