Electrical properties of neurons in the intact rat major pelvic ganglion

Spinal cord injury is associated with changes in synaptic properties of the mouse major pelvic ganglion

Spinal cord injury (SCI) has substantial impacts on autonomic function. In part, SCI results in loss of normal autonomic activity that contributes to injury-associated pathology such as neurogenic bladder, bowel, and sexual dysfunction. Yet little is known of the impacts of SCI on peripheral autonomic neurons that directly innervate these target organs. In this study, we measured changes in synaptic properties of neurons of the mouse major pelvic ganglion (MPG) associated with acute and chronic SCI. Our data show that functional and physiological properties of synapses onto MPG neurons are altered after SCI, and differ between acute and chronic injury. After acute injury, excitatory post-synaptic potentials (EPSPs) show increased rise and decay time constants leading to overall broader and longer EPSPs, while in chronic injured animals EPSPs are reduced in amplitude and show faster rise and decay leading to shorter EPSPs. Synaptic depression and low pass filtering are also altered in injured animals. Lastly, cholinergic currents are smaller in acute injured animals, but larger in chronic injured animals relative to controls. These changes in synaptic properties are associated with differences in nicotinic receptor subunit expression as well. MPG CHRNA3 mRNA levels decreased after injury, while CHRNA4 mRNAs increased. Further, changes in the correlations of alpha- and beta-subunit mRNAs suggests that nicotinic receptor subtype composition is altered after injury. Taken together, our data demonstrate that peripheral autonomic neurons are fundamentally altered after SCI, suggesting that longer-term therapeutic approaches could target these neurons directly to potentially help ameliorate neurogenic target organ dysfunction.

Changes in excitability and ion channel expression in neurons of the major pelvic ganglion in female type II diabetic mice

Bladder cystopathy and autonomic dysfunction are common complications of diabetes, and have been associated with changes in ganglionic transmission and some measures of neuronal excitability in male mice. To determine whether type II diabetes also impacts excitability of ganglionic neurons in females, we investigated neuronal excitability and firing properties, as well as underlying ion channel expression, in major pelvic ganglion (MPG) neurons in control, 10-week, and 21-week Lepr^db/db mice. Type II diabetes in Lepr^db/db animals caused a non-linear change in excitability and firing properties of MPG neurons. At 10 weeks, cells exhibited increased excitability as demonstrated by an increased likelihood of firing multiple spikes upon depolarization, decreased rebound spike latency, and overall narrower action potential half-widths as a result of increased depolarization and repolarization slopes. Conversely, at 21 weeks MPG neurons of Lepr^db/db mice reversed these changes, with spiking patterns and action-potential properties largely returning to control levels. These changes are associated with numerous time-specific changes in calcium, sodium, and potassium channel subunit mRNA levels. However, Principal Components Analysis of channel expression patterns revealed that rectification of excitability is not simply a return to control levels, but rather a distinct ion channel expression profile in 21-week Lepr^db/db neurons. These data indicate that type II diabetes can impact the excitability of post-ganglionic, autonomic neurons of female mice, and suggest that the non-linear progression of these properties with diabetes may be the result of compensatory changes in channel expression that act to rectify disrupted firing patterns of Lepr^db/db MPG neurons.

Electrophysiological identification of tonic and phasic neurons in sensory dorsal root ganglion and their distinct implications in inflammatory pain

In the mammalian autonomic nervous system, tonic and phasic neurons can be differentiated on firing patterns in response to long depolarizing current pulse. However, the similar firing patterns in the somatic primary sensory neurons and their functional significance are not well investigated. Here, we identified two types of neurons innervating somatic sensory in rat dorsal root ganglia (DRG). Tonic neurons fire action potentials (APs) in an intensity-dependent manner, whereas phasic neurons typically generate only one AP firing at the onset of stimulation regardless of intensity. Combining retrograde labeling of somatic DRG neurons with fluorescent tracer DiI, we further find that these neurons demonstrate distinct changes under inflammatory pain states induced by complete Freund's adjuvant (CFA) or bee venom toxin melittin. In tonic neurons, CFA and melittin treatments significantly decrease rheobase and AP durations (depolarization and repolarization), enhance amplitudes of overshoot and afterhyperpolarization (AHP), and increase the number of evoked action potentials. In phasic neurons, however, the same inflammation treatments cause fewer changes in these electrophysiological parameters except for the increased overshoot and decreased AP durations. In the present study, we find that tonic neurons are more hyperexcitable than phasic neurons after peripheral noxious inflammatory stimulation. The results indicate the distinct contributions of two types of DRG neurons in inflammatory pain.

Excitatory GABAA receptor in autonomic pelvic ganglion neurons innervating bladder

Major pelvic ganglia (MPG) are relay centers for autonomic reflexes such as micturition and penile erection. MPG innervate the urogenital system, including bladder. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system, and may also play an important role in some peripheral autonomic ganglia, including MPG. However, the electrophysiological properties and function of GABAA receptor in MPG neurons innervating bladder remain unknown. This study examined the electrophysiological properties and functional roles of GABAA receptors in bladder-innervating neurons identified by retrograde Dil tracing. Neurons innervating bladder showed previously established parasympathetic properties, including small membrane capacitance, lack of T-type Ca(2+) channel expression, and tyrosine-hydroxylase immunoreactivity. GABAA receptors were functionally expressed in bladder innervating neurons, but GABAC receptors were not. GABA elicited strong depolarization followed by increase of intracellular Ca(2+) in neurons innervating bladder, supporting the hypothesis GABA may play an important role in bladder function. These results provide useful information about the autonomic function of bladder in physiological and pathological conditions.

Synaptic transmission at parasympathetic neurons of the major pelvic ganglion from normal and diabetic male mice

Bladder and erectile dysfunction are common urologic complications of diabetes and are associated with reduced parasympathetic autonomic control. To determine whether disruption of ganglionic neurotransmission contributes to the loss of function, we investigated synaptic transmission at parasympathetic, major pelvic ganglion (MPG) neurons in control and chronically (20 wk) diabetic mice. In contrast to what has been reported for sympathetic neurons, diabetes did not cause an interruption of synaptic transmission at parasympathetic MPG neurons from streptozotocin-treated C57BL/6J (STZ) or db/db mice. Cholinergically mediated excitatory postsynaptic potentials (EPSPs) were suprathreshold during 5-s trains of 5-, 10-, and 20-Hz stimuli. Asynchronous neurotransmitter release, observed as miniature EPSPs (mEPSPs) during and after stimulation, permitted quantitative assessment of postganglionic, cholinergic receptor sensitivity. mEPSP amplitude following tetanic stimulation (recorded at -60 mV) was reduced in STZ (4.95 ± 0.4 vs. 3.71 ± 0.3 mV, P = 0.03), but not db/db mice. The number of posttetanic mEPSPs was significantly greater in db/db mice at all frequencies tested. Assessment of basic electrophysiological properties revealed that parasympathetic MPG neurons from db/db mice had less negative membrane potentials, lower input resistances, and shorter afterhyperpolarizations relative to their control. MPG neurons from STZ had longer afterhyperpolarizations but were otherwise similar to controls. Membrane excitability, measured by the membrane responsiveness to long-duration (1 s), suprathreshold depolarizing pulses, was unchanged in either model. The present study indicates that, while parasympathetic neurotransmission at the MPG is intact in chronically diabetic mice, obese, type 2 diabetic animals exhibit an altered presynaptic regulation of neurotransmitter release.

Src family kinase potentiates the activity of nicotinic acetylcholine receptor in rat autonomic ganglion innervating urinary bladder

Src family kinases (SFKs), one of the tyrosine kinase groups, are primary regulators of signal transductions that control cellular functions such as cell proliferation, differentiation, survival, metabolism, and other important roles of the cell. One of the crucial functions of SFKs is to regulate the activities of various neuronal channels. In this study, we investigated the modulatory action of SFK on nicotinic acetylcholine receptors (nAChRs) expressed in rat major pelvic ganglion (MPG) neurons innervating the urinary bladder. PP1 and PP2 (5 μM), selective Src-kinase inhibitors, attenuated ACh-induced ionic currents and [Ca²+](i) transients in MPG neurons, whereas PP3, an inactive analogue, had no effect. Blocking the tyrosine kinase activity of Src kinase by pp60 c-src inhibitory peptide also reduced the ACh-induced currents. Conversely, sodium orthovanadate (200 μM), a tyrosine phosphatase inhibitor, significantly augmented the ACh-induced currents. In the kinase assay, the activities of SFKs in MPG neurons were also inhibited by PP2, but not by PP3. These data suggests that SFKs may have a facilitative role on the synaptic transmission in rat pelvic autonomic ganglion.

Anatomical and physiological properties of pelvic ganglion neurons in female mice

Most neurons that regulate motility and blood flow in female pelvic organs are located within pelvic (paracervical) ganglia. In this study we investigated the anatomical and physiological properties of neurons within mouse (C57/Bl/6) paracervical ganglia. Most neurons showed immunoreactivity for choline acetyl transferase (CHAT) and were presumably cholinergic. Few neurons (approximately 5%) were tyrosine hydroxylase (TH) positive. Immunohistochemical labelling for microtubule associated protein 2 showed most neurons had small somata (cross sectional area approximately 300 microm(2)) and lacked dendrites. Action potential (AP) discharge characteristics, determined by depolarising current step injection, revealed most neurons (70%) adapted rapidly to depolarising current injection and were classified as "phasic". The remaining neurons discharged APs throughout the current step and were classified as "tonic". Membrane properties and current-voltage relationships were similar in phasic and tonic neurons, however the afterhyperpolarisation was significantly smaller in tonic neurons. Stimulation of preganglionic axons usually evoked a single strong preganglionic input (21/27 and 9/10 for pelvic and hypogastric nerves, respectively). In 19 preparations where we tested for inputs from both nerves pelvic inputs predominated (23/45 neurons) and inputs via the hypogastric nerve were rarely observed (3/45 neurons). Together, our data indicate that most neurons within mouse paracervical ganglia are cholinergic and parasympathetic. As there is little anatomical or functional evidence for integration of preganglionic inputs we propose that the role of paracervical neurons is restricted to one of spatial amplification or filtering of preganglionic inputs.