title: Inhibitory effects of endomorphin-2 on excitatory synaptic transmission and the neuronal excitability of sacral parasympathetic preganglionic neurons in young rats.
abstract: The function of the urinary bladder is partly controlled by parasympathetic preganglionic neurons (PPNs) of the sacral parasympathetic nucleus (SPN). Our recent work demonstrated that endomorphin-2 (EM-2)-immunoreactive (IR) terminals form synapses with μ-opioid receptor (MOR)-expressing PPNs in the rat SPN. Here, we examined the effects of EM-2 on excitatory synaptic transmission and the neuronal excitability of the PPNs in young rats (24-30 days old) using a whole-cell patch-clamp approach. PPNs were identified by retrograde labeling with the fluorescent tracer tetramethylrhodamine-dextran (TMR). EM-2 (3 μM) markedly decreased both the amplitude and the frequency of the spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) of PPNs. EM-2 not only decreased the resting membrane potentials (RMPs) in 61.1% of the examined PPNs with half-maximal response at the concentration of 0.282 μM, but also increased the rheobase current and reduced the repetitive action potential firing of PPNs. Analysis of the current-voltage relationship revealed that the EM-2-induced current was reversed at -95 ± 2.5 mV and was suppressed by perfusion of the potassium channel blockers 4-aminopyridine (4-AP) or BaCl2 or by the addition of guanosine 5'-[β-thio]diphosphate trilithium salt (GDP-β-S) to the pipette solution, suggesting the involvement of the G-protein-coupled inwardly rectifying potassium (GIRK) channel. The above EM-2-invoked inhibitory effects were abolished by the MOR selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), indicating that the effects of EM-2 on PPNs were mediated by MOR via pre- and/or post-synaptic mechanisms. EM-2 activated pre- and post-synaptic MORs, inhibiting excitatory neurotransmitter release from the presynaptic terminals and decreasing the excitability of PPNs due to hyperpolarization of their membrane potentials, respectively. These inhibitory effects of EM-2 on PPNs at the spinal cord level may explain the mechanism of action of morphine treatment and morphine-induced bladder dysfunction in the clinic.
authors.affiliation: Department of Anatomy, Histology and Embryology, Fujian Medical University Fuzhou, China.
authors.forename: Ying-Biao
authors.initials: YB
authors.lastname: Chen
authors.affiliation: Department of Anatomy, Histology and Embryology and K. K. Leung Brain Research Centre, The Fourth Military Medical University Xi'an, China ; Division of Medical Biophysics, Institute of Neuroscience and Physiology, Göteborg University Göteborg, Sweden.
authors.forename: Fen-Sheng
authors.initials: FS
authors.lastname: Huang
authors.affiliation: Department of Anatomy, Histology and Embryology and K. K. Leung Brain Research Centre, The Fourth Military Medical University Xi'an, China.
authors.forename: Ban
authors.initials: B
authors.lastname: Fen
authors.affiliation: Department of Anatomy, Histology and Embryology and K. K. Leung Brain Research Centre, The Fourth Military Medical University Xi'an, China.
authors.forename: Jun-Bin
authors.initials: JB
authors.lastname: Yin
authors.affiliation: Department of Anatomy, Histology and Embryology, Fujian Medical University Fuzhou, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.affiliation: Department of Anatomy, Histology and Embryology, Fujian Medical University Fuzhou, China ; Department of Anatomy, Histology and Embryology and K. K. Leung Brain Research Centre, The Fourth Military Medical University Xi'an, China ; Collaborative Innovation Center for Brain Science, Fudan University Shanghai, China.
authors.forename: Yun-Qing
authors.initials: YQ
authors.lastname: Li
keywords: endomorphin-2; neuronal excitability; parasympathetic preganglionic neurons; sacral parasympathetic nucleus; spontaneous excitatory postsynaptic current; urinary retention