GPCR Intracellular Loop Regulation of Beta-Arrestin-Mediated Endosomal Signaling Dynamics.

G protein-coupled receptors (GPCRs) are currently appreciated to be routed to diverse cellular platforms to generate both G protein-dependent and -independent signals. The latter has been best studied with respect to ß-arrestin-associated receptor internalization and trafficking to signaling endosomes for extracellular signal-regulated kinase (ERK) activation. However, how GPCR structural and conformational variants regulate endosomal ERK signaling dynamics, which can be central in neural development, plasticity, and disease processes, is not well understood. Among class B GPCRs, the PACAP-selective PAC1 receptor is unique in the expression of variants that can contain intracellular loop 3 (ICL3) cassette inserts. The nervous system expresses preferentially the PAC1Null (no insert) and PAC1Hop (28-amino acid Hop insert) receptor variants. Our molecular modeling and signaling studies revealed that the PAC1Null and PAC1Hop receptor variants can associate with ß-arrestin differentially, resulting in enhanced receptor internalization and ERK activation for the PAC1Hop variant. The study amplifies our understandings of GPCR intracellular loop structure/function relationships with the first example of how the duration of endosomal ERK activation can be guided by ICL3. The results provide a framework for how changes in GPCR variant expression can impact developmental and homeostatic processes and may be contributory to maladaptive neuroplasticity underlying chronic pain and stress-related disorders.

PAC1 Receptor Internalization and Endosomal MEK/ERK Activation Is Essential for PACAP-Mediated Neuronal Excitability.

Pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1) activation of PAC1 receptors (Adcyap1r1) can significantly increase the excitability of diverse neurons through differential mechanisms. For guinea pig cardiac neurons, the modulation of excitability can be mediated in part by PAC1 receptor plasma membrane G protein-dependent activation of adenylyl cyclase and downstream signaling cascades. By contrast, PAC1 receptor-mediated excitability of hippocampal dentate gyrus granule cells appears independent of membrane-delimited AC/cAMP/PKA and PLC/PKC signaling. For both neuronal types, there is mechanistic convergence demonstrating that endosomal PAC1 receptor signaling has prominent roles. In these models, neuronal exposure to Pitstop2 to inhibit ß-arrestin/clathrin-mediated PAC1 receptor internalization eliminates PACAP modulation of excitability. ß-arrestin is a scaffold for a number of effectors especially MEK/ERK and notably, paradigms that inhibit PAC1 receptor endosome formation and ERK signaling also blunt the PACAP-induced increase in excitability. Detailed PAC1 receptor internalization and endosomal ERK signaling mechanisms have been confirmed in HEK PAC1R-EGFP cells and shown to be long lasting which appear to recapitulate the sustained electrophysiological responses. Thus, PAC1 receptor internalization/endosomal recruitment efficiently and efficaciously activates MEK/ERK signaling and appears to represent a singular and critical common denominator in regulating neuronal excitability by PACAP.

Pituitary adenylate cyclase-activating polypeptide-induced PAC1 receptor internalization and recruitment of MEK/ERK signaling enhance excitability of dentate gyrus granule cells.

Pituitary adenylate cyclase activating polypeptide (PACAP; ADCYAP1) is a pleiotropic neuropeptide widely distributed in both the peripheral and central nervous systems. PACAP and its specific cognate PAC1 receptor (ADCYAP1R1) play critical roles in the homeostatic maintenance of multiple physiological and behavioral systems. Notably, maladaptations in the PACAPergic system have been associated with several psychopathologies related to fear and anxiety. PAC1 receptor transcripts are highly expressed in granule cells of the dentate gyrus (DG). Here, we examined the direct effects of PACAP on DG granule cells in brain slices using whole cell patch recordings in current clamp mode. PACAP significantly increased the intrinsic excitability of DG granule cells via PAC1 receptor activation. This increased excitability was not mediated by adenylyl cyclase/cAMP or phospholipase C/PKC activation, but instead via activation of an extracellular signal-regulated kinase (ERK) signaling pathway initiated through PAC1 receptor endocytosis/endosomal signaling. PACAP failed to increase excitability in DG granule cells pretreated with the persistent sodium current blocker riluzole, suggesting that the observed PACAP effects required this component of the inward sodium current.

Parallel signaling pathways of pituitary adenylate cyclase activating polypeptide (PACAP) regulate several intrinsic ion channels.

Pituitary adenylate cyclase activating polypeptide (PACAP), acting through its cognate receptors PAC1, VPAC1, and VPAC2, is a pleiotropic signaling neuropeptide of the vasoactive intestinal peptide/secretin/glucagon family. PACAP has known functions in neuronal growth, development, and repair, and central PACAP signaling has acute behavioral consequences. One of the ways in which PACAP may affect neuronal function is through the modulation of intrinsic membrane currents to control neuronal excitability. Here, we review the evidence of PACAP-dependent modulation of calcium- and voltage-gated potassium currents, hyperpolarization-activated cation currents, calcium currents, and voltage-gated sodium currents. Interestingly, PACAP signaling pathways diverge into parallel pathways to target different ionic currents for modulation, though single pathways are not limited to modulating just one target ionic current. Despite the various targets of modulation, the weight of the evidence suggests that PACAP signaling most commonly leads to a net increase in neuronal excitability. We discuss possible mechanisms by which PACAP signaling leads to the modulation of intrinsic membrane currents that may contribute to changes in behavior.

PACAP-Induced PAC1 Receptor Internalization and Recruitment of Endosomal Signaling Regulate Cardiac Neuron Excitability.

Pituitary adenylate cyclase-activating polypeptide (PACAP, Adcyap1) activation of PAC1 receptors (Adcyap1r1) significantly increases excitability of guinea pig cardiac neurons. This modulation of excitability is mediated in part by plasma membrane G protein-dependent activation of adenylyl cyclase and downstream signaling cascades, as well as by endosomal signaling mechanisms. PACAP/PAC1 receptor-mediated activation of plasma membrane adenylyl cyclase (AC) and the resulting increase in cellular cAMP enhances a hyperpolarization-induced nonselective cationic current Ih, which contributes to the PACAP-induced increase in cardiac neuron excitability. Further, PACAP-mediated AC/cAMP/PKA downstream signaling also appears to enhance cardiac neuron IT to facilitate the excitatory responses. PACAP activation of PAC1 receptors rapidly stimulates receptor internalization, and reducing ambient temperature or treatments with the clathrin inhibitor Pitstop2 or the dynamin I/II inhibitor dynasore to block endocytic events can suppress PACAP-enhanced neuronal excitability. Thus, endocytosis inhibitors essentially eliminate PACAP-enhanced excitability suggesting that endosomal platforms represent a primary signaling mechanism. Endosomal signaling is associated canonically with ERK activation and in accord, PACAP-enhanced cardiac neuron excitability is reduced by MEK inhibitor pretreatments. PACAP activation of MEK/ERK signaling can enhance currents through voltage-dependent Nav1.7 channels. Hence, PACAP-induced PAC1 receptor internalization/endosomal signaling, recruitment of MEK/ERK signaling, and modulation of Nav1.7 are implicated as key mechanisms contributing to the PACAP-enhanced neuronal excitability. PACAP/PAC1 receptor-mediated endosomal ERK signaling in central circuits can play key roles in development of chronic pain and anxiety-related responses; thus, PAC1 endosomal signaling likely participates in a variety of homeostatic responses within neuronal circuits in the CNS.

Src family kinase inhibitors blunt PACAP-induced PAC1 receptor endocytosis, phosphorylation of ERK, and the increase in cardiac neuron excitability.

Pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1) activation of PAC1 receptors ( Adcyap1r1) significantly increases excitability of guinea pig cardiac neurons. This modulation of excitability is mediated in part by plasma membrane G protein-dependent activation of adenylyl cyclase and downstream signaling cascades. However, additional mechanisms responsible for the enhanced excitability are activated following internalization of the PAC1 receptor and endosomal signaling. Src family kinases play critical roles mediating endocytosis of many trophic factor and G protein-coupled receptors. The present study investigated whether Src family kinases also support the PACAP-induced PAC1 receptor internalization, phosphorylation of ERK, and enhanced neuronal excitability. Using human embryonic kidney cells stably expressing a green fluorescent protein-tagged PAC1 receptor, treatment with the Src family kinase inhibitor PP2 (10 µM) markedly reduced the PACAP-induced PAC1 receptor internalization, and in parallel, both PP2 and Src inhibitor 1 (Src-1, 2 µM) reduced ERK activation determined by Western blot analysis. In contrast, Src family kinase inhibitors did not eliminate a PACAP-induced rise in global calcium generated by inositol (1,4,5)-trisphosphate-induced release of calcium from endoplasmic reticulum stores. From confocal analysis of phosphorylated ERK immunostaining, PP2 treatment significantly attenuated PACAP activation of ERK in neurons within cardiac ganglia whole mount preparations. Intracellular recordings demonstrated that PP2 also significantly blunted a PACAP-induced increase in cardiac neuron excitability. These studies demonstrate Src-related kinase activity in PAC1 receptor internalization, activation of MEK/ERK signaling, and regulation of neuronal excitability. The present results provide further support for the importance of PAC1 receptor endosomal signaling as a key mechanism regulating cellular function.

Recruitment of endosomal signaling mediates the forskolin modulation of guinea pig cardiac neuron excitability.

Forskolin, a selective activator of adenylyl cyclase (AC), commonly is used to establish actions of G protein-coupled receptors (GPCRs) that are initiated primarily through activation of AC/cAMP signaling pathways. In the present study, forskolin was used to evaluate the potential role of AC/cAMP, which is a major signaling mechanism for the pituitary adenylate cyclase-activating polypeptide (PACAP)-selective PAC1 receptor, in the regulation of guinea pig cardiac neuronal excitability. Forskolin (5-10 µM) increases excitability in ~60% of the cardiac neurons. The forskolin-mediated increase in excitability was considered related to cAMP regulation of a cyclic nucleotide gated channel or via protein kinase A (PKA)/ERK signaling, mechanisms that have been linked to PAC1 receptor activation. However, unlike PACAP mechanisms, forskolin enhancement of excitability was not significantly reduced by treatment with cesium to block currents through hyperpolarization-activated nonselective cation channels (Ih) or by treatment with PD98059 to block MEK/ERK signaling. In contrast, treatment with the clathrin inhibitor Pitstop2 or the dynamin inhibitor dynasore eliminated the forskolin-induced increase in excitability; treatments with the inactive Pitstop analog or PP2 treatment to inhibit Src-mediated endocytosis mechanisms were ineffective. The PKA inhibitor KT5702 significantly suppressed the forskolin-induced change in excitability; further, KT5702 and Pitstop2 reduced the forskolin-stimulated MEK/ERK activation in cardiac neurons. Collectively, the present results suggest that forskolin activation of AC/cAMP/PKA signaling leads to the recruitment of clathrin/dynamin-dependent endosomal transduction cascades, including MEK/ERK signaling, and that endosomal signaling is the critical mechanism underlying the forskolin-induced increase in cardiac neuron excitability.

G Protein-Coupled Receptor Endosomal Signaling and Regulation of Neuronal Excitability and Stress Responses: Signaling Options and Lessons From the PAC1 Receptor.

Our understanding of G protein coupled receptor (GPCR) mechanisms and functions have evolved considerably. Among the many conceptual realignments, GPCRs can exist in an ensemble of active microstates that have the potential to differentially engage specific downstream signaling events. Furthermore, among GPCR dynamics, GPCR internalization and vesicular trafficking are no longer solely mechanisms for desensitization, but now appreciated to form intricate endosomal signaling complexes that can potentially target second messengers to intracellular compartments with high temporal and spatial resolution. The PACAPergic system is important in the maintenance of physiological homeostasis in the central and peripheral nervous systems and activation of the PACAP-selective PAC1 receptor can generate differential but coordinate plasma membrane and endosomal signals for cellular responses. The integration of these signals can modulate PACAP-induced changes in ionic conductances that gate neuronal excitability. PACAP/PAC1 receptor generation of endosomal ERK signals participate in chronic pain and anxiety-like responses which can be attenuated with endocytosis inhibitors. From the abilities of ligands to stabilize the different GPCR microstates for biased downstream signaling, the development of biased PAC1 receptor agonists and antagonists may provide opportunities to dissociate the homeostatic regulatory signals of PACAP from the maladaptive effects. In particular, the development of biased antagonists to PAC1 receptor-mediated endosomal signaling may offer therapeutic options for chronic pain and stress-related disorders. J. Cell. Physiol. 232: 698-706, 2017. © 2016 Wiley Periodicals, Inc.

Activation of MEK/ERK signaling contributes to the PACAP-induced increase in guinea pig cardiac neuron excitability.

Pituitary adenylate cyclase (PAC)-activating polypeptide (PACAP) peptides (Adcyap1) signaling at the selective PAC1 receptor (Adcyap1r1) participate in multiple homeostatic and stress-related responses, yet the cellular mechanisms underlying PACAP actions remain to be completely elucidated. PACAP/PAC1 receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, and as these neurons are readily accessible, this neuronal system is particularly amenable to study of PACAP modulation of ionic conductances. The present study investigated how PACAP activation of MEK/ERK signaling contributed to the peptide-induced increase in cardiac neuron excitability. Treatment with the MEK inhibitor PD 98059 blocked PACAP-stimulated phosphorylated ERK and, in parallel, suppressed the increase in cardiac neuron excitability. However, PD 98059 did not blunt the ability of PACAP to enhance two inward ionic currents, one flowing through hyperpolarization-activated nonselective cationic channels (Ih) and another flowing through low-voltage-activated calcium channels (IT), which support the peptide-induced increase in excitability. Thus a PACAP- and MEK/ERK-sensitive, voltage-dependent conductance(s), in addition to Ih and IT, modulates neuronal excitability. Despite prior work implicating PACAP downregulation of the KV4.2 potassium channel in modulation of excitability in other cells, treatment with the KV4.2 current blocker 4-aminopyridine did not replicate the PACAP-induced increase in excitability in cardiac neurons. However, cardiac neurons express the ERK target, the NaV1.7 sodium channel, and treatment with the selective NaV1.7 channel inhibitor PF-04856264 decreased the PACAP modulation of excitability. From these results, PACAP/PAC1 activation of MEK/ERK signaling may phosphorylate the NaV1.7 channel, enhancing sodium currents near the threshold, an action contributing to repetitive firing of the cardiac neurons exposed to PACAP.

Activation of MEK/ERK Signaling by PACAP in Guinea Pig Cardiac Neurons.

Pituitary adenylate cyclase-activating polypeptide (PACAP) signaling can increase guinea pig cardiac neuron excitability in part through extracellular signal-regulated kinase (ERK) activation. The present study examined the PACAP receptors and signaling cascades that stimulate guinea pig cardiac neuron ERK signaling using confocal microscopy to quantify PACAP-induced neuronal phosphorylated ERK (pERK) immunoreactivity. PACAP and maxadilan, but not vasoactive intestinal polypeptide (VIP), increased cardiac neuron pERK, implicating primary roles for PACAP-selective PAC1 receptor (Adcyap1r1) signaling rather than VPAC receptors (Vipr1 and Vipr2) in the generation of cardiac neuron pERK. The adenylyl cyclase (AC) activator forskolin, but not the protein kinase C (PKC) activator phorbol myristate acetate (PMA), increased pERK. Also, Bim1 did not blunt PACAP activation of pERK. Together, the results suggest PAC1 receptor signal transduction via Gs/adenylyl cyclase (AC)/cAMP rather than Gq/phospholipase C (PLC) generated neuronal pERK. Activator and inhibitor studies suggested that the PACAP-mediated pERK activation was PKA-dependent rather than an exchange protein directly activated by a cAMP (EPAC), PKA-independent mechanism. The PACAP-induced pERK was inhibited by the clathrin inhibitor Pitstop2 to block receptor internalization and endosomal signaling. We propose that the PACAP-mediated MEK/ERK activation in cardiac neurons involves both AC/cAMP/PKA signaling and PAC1 receptor internalization/activation of signaling endosomes.

Nickel suppresses the PACAP-induced increase in guinea pig cardiac neuron excitability.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent intercellular signaling molecule involved in multiple homeostatic functions. PACAP/PAC1 receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, making them a unique system to establish mechanisms underlying PACAP modulation of neuronal function. Calcium influx is required for the PACAP-increased cardiac neuron excitability, although the pathway is unknown. This study tested whether PACAP enhancement of calcium influx through either T-type or R-type channels contributed to the modulation of excitability. Real-time quantitative polymerase chain reaction analyses indicated transcripts for Cav3.1, Cav3.2, and Cav3.3 T-type isoforms and R-type Cav2.3 in cardiac neurons. These neurons often exhibit a hyperpolarization-induced rebound depolarization that remains when cesium is present to block hyperpolarization-activated nonselective cationic currents (Ih). The T-type calcium channel inhibitors, nickel (Ni(2+)) or mibefradil, suppressed the rebound depolarization, and treatment with both drugs hyperpolarized cardiac neurons by 2-4 mV. Together, these results are consistent with the presence of functional T-type channels, potentially along with R-type channels, in these cardiac neurons. Fifty micromolar Ni(2+), a concentration that suppresses currents in both T-type and R-type channels, blunted the PACAP-initiated increase in excitability. Ni(2+) also blunted PACAP enhancement of the hyperpolarization-induced rebound depolarization and reversed the PACAP-mediated increase in excitability, after being initiated, in a subset of cells. Lastly, low voltage-activated currents, measured under perforated patch whole cell recording conditions and potentially flowing through T-type or R-type channels, were enhanced by PACAP. Together, our results suggest that a PACAP-enhanced, Ni(2+)-sensitive current contributes to PACAP-induced modulation of neuronal excitability.

PACAP-induced ERK activation in HEK cells expressing PAC1 receptors involves both receptor internalization and PKC signaling.

The pituitary adenylate cyclase-activating polypeptide (PACAP)-selective PAC1 receptor (Adcyap1r1) is a G protein-coupled receptor (GPCR) that activates adenylyl cyclase and PLC. Similar to many other GPCRs, our previous studies showed that the PAC1 receptor is internalized after ligand binding to form signaling endosomes, which recruit additional second messenger pathways. Using a human embryonic kidney (HEK 293) PAC1Hop1-EGFP receptor cell line, we have examined how different PAC1 receptor signaling mechanisms contribute to MEK/ERK activation. Unlike PAC1 receptor-stimulated adenylyl cyclase/cAMP production in the plasma membrane, PACAP-mediated ERK phosphorylation was partly dependent on receptor internalization, as determined by treatment with pharmacological inhibitors of endocytosis or temperature reduction, which also suppressed receptor internalization. Stimulation of cAMP generation by forskolin or exposure to the cell-permeable cAMP analogs 8-bromo-cAMP and dibutyryl cAMP had minimal effects on ERK phosphorylation in this system. The ability of reduced temperature (24°C) to consistently suppress ERK activation to a greater extent than the endocytosis inhibitors Pitstop 2 and dynasore indicated that other mechanisms, in addition to PAC1 internalization/endosome activation, were involved. Inhibition of PAC1 receptor-stimulated PLC/diacylglycerol/PKC signaling by bisindoylmaleimide I also attenuated ERK phosphorylation, and direct PKC activation with phorbol ester increased ERK phosphorylation in a temperature-dependent manner. Inhibition of PAC1 receptor endocytosis and PKC activation completely blocked PACAP-stimulated ERK activation. PACAP augmented phosphorylated ERK staining uniformly over the cytoplasm and nucleus, and PKC signaling facilitated nuclear phosphorylated ERK translocation. In sum, our results show that PACAP/PAC1 receptor endocytosis and PLC/diacylglycerol/PKC activation represent two complementary mechanisms contributing to PACAP-induced ERK activation.

Calcium influx, but not intracellular calcium release, supports PACAP-mediated ERK activation in HEK PAC1 receptor cells.

In HEK cells expressing GFP-tagged PAC1Hop1 receptors, PACAP augments ERK phosphorylation through two parallel pathways: one through PACAP/PAC1 receptor internalization/endosome MEK/ERK signaling and the other through PLC/DAG/PKC activation. We examined whether elevation of intracellular calcium ([Ca(2+)]i) was required for either of the PACAP/PAC1 receptor-mediated ERK activation mechanisms. The PACAP (25 nM)-induced elevation of [Ca(2+)]i was greater with cells maintained in Ca(2+)-containing than in Ca(2+)-deficient solution, suggesting that both calcium release from internal stores and calcium influx contributed to the rise in [Ca(2+)]i. A thapsigargin-induced increase in [Ca(2+)]i also was greater with calcium in the external solution. OAG, the cell permeable analogue of DAG, increased [Ca(2+)]i, but only in Ca(2+)-containing solution. Decreasing external calcium or depleting internal calcium stores did not block PACAP-induced PAC1 receptor internalization. Omission of calcium from the external solution, but not thapsigargin pretreatment, significantly blunted PACAP-stimulated ERK phosphorylation. The PKC inhibitor BimI decreased PACAP-mediated ERK activation in both Ca(2+)-containing or Ca(2+)-deficient solutions. In contrast, following Pitstop 2 pretreatment to block endocytic mechanisms, PACAP activated ERK only when calcium was present in the external solution. We conclude that the endosome signaling pathway is largely calcium-independent whereas calcium influx appears necessary for the PLC/DAG/PKC component of PACAP-induced ERK activation.

Decrease in neuronal nicotinic acetylcholine receptor subunit and PSD-93 transcript levels in the male mouse MPG after cavernous nerve injury or explant culture.

Quantitative real-time PCR was used to test whether cavernous nerve injury leads to a decrease in major pelvic ganglia (MPG) neuronal nicotinic ACh receptor (nAChR) subunit and postsynaptic density (PSD)-93 transcript levels. Subunits α3, ß4, and α7, commonly expressed in the MPG, were selected for analysis. After 72 h in explant culture, MPG transcript levels for α3, ß4, α7, and PSD-93 were significantly depressed. Three days after cavernous nerve axotomy or crush in vivo, transcript levels for α3, ß4, and PSD-93, but not for α7, were significantly depressed. Three days after dissection of the cavernous nerve free of underlying tissue and application of a 5-mm lateral stretch (manipulation), transcript levels for α3 and PSD-93 were also significantly decreased. Seven days after all three surgical procedures, α3 transcript levels remained depressed, but PSD-93 transcript levels were still decreased only after axotomy or nerve crush. At 30 days postsurgery, transcript levels for the nAChR subunits and PSD-93 had recovered. ACh-induced currents were significantly smaller in MPG neurons dissociated from 3-day explant cultured ganglia than from those recorded in neurons dissociated from acutely isolated ganglia; this observation provides direct evidence showing that a decrease in nAChR function was coincident with a decrease in nAChR subunit transcript levels. We conclude that a downregulation of nAChR subunit and PSD-93 expression after cavernous nerve injury, or even manipulation, could interrupt synaptic transmission within the MPG and thus contribute to the loss of neural control of urogenital organs after pelvic surgeries.

PAC1 receptors mediate positive chronotropic responses to PACAP-27 and VIP in isolated mouse atria.

PACAP and VIP have prominent effects on cardiac function in several species, but little is known about their influence on the murine heart. Accordingly, we evaluated the expression of PACAP/VIP receptors in mouse heart and the response of isolated atria to peptide agonists. Quantitative PCR demonstrated that PAC1, VPAC1, and VPAC2 receptor mRNAs are present throughout the mouse heart. Expression of all three receptor transcripts was low, PAC1 being the lowest. No regional differences in expression were detected for individual receptor mRNAs after normalization to L32. Pharmacological effects of PACAP-27, VIP, and the selective PAC1 agonist maxadilan were evaluated in isolated, spontaneously beating atria from C57BL/6 mice of either sex. Incremental additions of PACAP-27 at 1 min intervals caused a concentration-dependent tachycardia with a logEC50=-9.08 ± 0.15 M (n=7) and a maximum of 96.3 ± 5.9% above baseline heart rate. VIP and maxadilan also caused tachycardia but their potencies were about two orders of magnitude less. Increasing the dosing interval to 5 min caused a leftward shift of the concentration-response curve to maxadilan but no changes in the curves for PACAP-27 or VIP. Under this condition, neither the potency nor the efficacy of maxadilan differed from those of PACAP-27. Neither PACAP-27 nor maxadilan caused tachyphylaxis, and maximal responses to maxadilan were maintained for at least 2 h. We conclude that all three VIP/PACAP family receptors are expressed by mouse cardiac tissue, but only PAC1 receptors mediate positive chronotropic responses to PACAP-27 and VIP.

Pituitary adenylate cyclase 1 receptor internalization and endosomal signaling mediate the pituitary adenylate cyclase activating polypeptide-induced increase in guinea pig cardiac neuron excitability.

After G-protein-coupled receptor activation and signaling at the plasma membrane, the receptor complex is often rapidly internalized via endocytic vesicles for trafficking into various intracellular compartments and pathways. The formation of signaling endosomes is recognized as a mechanism that produces sustained intracellular signals that may be distinct from those generated at the cell surface for cellular responses including growth, differentiation, and survival. Pituitary adenylate cyclase activating polypeptide (PACAP; Adcyap1) is a potent neurotransmitter/neurotrophic peptide and mediates its diverse cellular functions in part through internalization of its cognate G-protein-coupled PAC1 receptor (PAC1R; Adcyap1r1). In the present study, we examined whether PAC1R endocytosis participates in the regulation of neuronal excitability. Although PACAP increased excitability in 90% of guinea pig cardiac neurons, pretreatment with Pitstop 2 or dynasore to inhibit clathrin and dynamin I/II, respectively, suppressed the PACAP effect. Subsequent addition of inhibitor after the PACAP-induced increase in excitability developed gradually attenuated excitability with no changes in action potential properties. Likewise, the PACAP-induced increase in excitability was markedly decreased at ambient temperature. Receptor trafficking studies with GFP-PAC1 cell lines demonstrated the efficacy of Pitstop 2, dynasore, and low temperatures at suppressing PAC1R endocytosis. In contrast, brefeldin A pretreatments to disrupt Golgi vesicle trafficking did not blunt the PACAP effect, and PACAP/PAC1R signaling still increased neuronal cAMP production even with endocytic blockade. Our results demonstrate that PACAP/PAC1R complex endocytosis is a key step for the PACAP modulation of cardiac neuron excitability.

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.

Effects of CYP-induced cystitis on PACAP/VIP and receptor expression in micturition pathways and bladder function in mice with overexpression of NGF in urothelium.

We have previously demonstrated nerve growth factor (NGF) regulation of pituitary adenylate cyclase-activating polypeptide (PACAP)/receptors in bladder reflex pathways using a transgenic mouse model of chronic NGF overexpression in the bladder using the urothelial-specific uroplakin II promoter. We have now explored the contribution of target-derived NGF in combination with cyclophosphamide (CYP)-induced cystitis to determine whether additional changes in neuropeptides/receptors are observed in micturition reflex pathways due to the presence of additional inflammatory mediators in the urinary bladder. Quantitative PCR was used to determine PACAP/vasoactive intestinal polypeptide (VIP), substance P, galanin, and receptor transcript expression in the urinary bladder (urothelium, detrusor) in mice with overexpression of NGF in the urothelium (NGF-OE) and wild-type (WT) mice with CYP-induced cystitis (4 h, 48 h, and chronic). With CYP-induced cystitis (4 h), WT and NGF-OE mice exhibited similar changes in galanin transcript expression in the urothelium (30-fold increase) and detrusor (threefold increase). In contrast, PACAP, VIP, and substance P transcripts exhibited differential changes in WT and NGF-OE with CYP-induced cystitis. PAC1, VPAC1, and VPAC2 transcript expression also exhibited differential responses in NGF-OE mice that were tissue (urothelium vs. detrusor) and CYP-induced cystitis duration-dependent. Using conscious cystometry, NGF-OE mice treated with CYP exhibited significant (p ≤ 0.01) increases in voiding frequency above that observed in control NGF-OE mice. In addition, no changes in the electrical properties of the major pelvic ganglia neurons of NGF-OE mice were detected using intracellular recording, suggesting that the urinary bladder phenotype in NGF-OE mice is not influenced by changes in the efferent limb of the micturition reflex. These studies are consistent with target-derived NGF and other inflammatory mediators affecting neurochemical plasticity and the reflex function of micturition pathways.

Galanin expression in the mouse major pelvic ganglia during explant culture and following cavernous nerve transection.

Autonomic neurons commonly respond to injury/axotomy with an increased expression of neuropeptides including galanin and pituitary adenylyl cyclase-activating polypeptide (PACAP). The increased peptide expression may enhance neuronal survival and axonal regeneration. Using quantitative (Q) PCR and immunocytochemistry, the present study tested whether galanin expression increased in male mouse major pelvic ganglia (MPG) neurons in response to injury. Galanin transcript expression increased significantly in MPG neurons following 72 h in explant culture and 72 h after unilateral transection of the cavernous nerve. Under both conditions, the increase in galanin transcript levels was greater than the increase in PACAP transcript levels. In control MPG, galanin-IR nerve fibers formed pericellular arrangements around MPG neurons although few galanin-IR cells were evident and many of the galanin-IR cells may be small intensely fluorescent (SIF) cells. In 3-day-cultured MPGs, many more galanin-IR cells and nerve fibers were noted. The increased galanin expression was most apparent in neurons that were also immunoreactive for neuronal nitric oxide synthase, rather than tyrosine hydroxylase. Some explant-cultured MPG neurons exhibited immunoreactivity to galanin and PACAP. As reported previously for PACAP, there is an injury-induced increase in MPG galanin expression, which occurs preferentially in the parasympathetic postganglionic neurons.

Pretreatment with nonselective cationic channel inhibitors blunts the PACAP-induced increase in guinea pig cardiac neuron excitability.

Calcium influx is required for the pituitary adenylyl cyclase activating polypeptide (PACAP)-induced increase in guinea pig cardiac neuron excitability, noted as a change from a phasic to multiple action potential firing pattern. Intracellular recordings indicated that pretreatment with the nonselective cationic channel inhibitors, 2-aminoethoxydiphenylborate (2-APB), 1-[ß-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96365), and flufenamic acid (FFA) reduced the 20-nM PACAP-induced excitability increase. Additional experiments tested whether 2-APB, FFA, and SKF 96365 could suppress the increase in excitability by PACAP once it had developed. The increased action potential firing remained following application of 2-APB but was diminished by FFA. SKF 96365 transiently depressed the PACAP-induced excitability increase. A decrease and recovery of action potential amplitude paralleled the excitability shift. Since semiquantitative PCR indicated that cardiac neurons express TRPC subunit transcripts, we hypothesize that PACAP activates calcium-permeable, nonselective cationic channels, which possibly are members of the TRPC family. Our results are consistent with calcium influx being required for the initiation of the PACAP-induced increase in excitability, but suggest that it may not be required to sustain the peptide effect. The present results also demonstrate that nonselective cationic channel inhibitors could have other actions, which might contribute to the inhibition of the PACAP-induced excitability increase.