Primary afferent neurons are functionally heterogeneous. To determine whether this functional heterogeneity reflects, in part, heterogeneity in the regulation of the concentration of intracellular Ca²⁺ ([Ca²⁺]_i), the magnitude and decay of evoked Ca²⁺ transients were assessed in subpopulations of dorsal root ganglion (DRG) neurons with voltage clamp and fura‐2 ratiometric imaging. To determine whether differences in evoked Ca²⁺ transients among subpopulations of DRG neurons reflected differences in the contribution of Ca²⁺ regulatory mechanisms, pharmacological techniques were employed to assess the contribution of influx, efflux, release and uptake pathways. Subpopulations of DRG neurons were defined by cell body size, binding of the plant lectin IB₄ and responsiveness to the algogenic compound capsaicin (CAP). Ca²⁺ transients were evoked with 30 mm K⁺ or voltage steps to 0 mV. There were marked differences between subpopulations of neurons with respect to both the magnitude and decay of the Ca²⁺ transient, with the largest and most slowly decaying Ca²⁺ transients in small‐diameter, IB₄‐positive, CAP‐responsive neurons. The smallest and most rapidly decaying transients were in large‐diameter, IB₄‐negative and CAP‐unresponsive DRG neurons. These differences were not due to a differential distribution of voltage‐gated Ca²⁺ currents. However, these differences did appear to reflect a differential contribution of other influx, efflux, release and uptake mechanisms between subpopulations of neurons. These results suggest that electrical activity in subpopulations of DRG neurons will have a differential influence on Ca²⁺‐regulated phenomena such as spike adaptation, transmitter release and gene transcription. Significantly more activity should be required in large‐diameter non‐nociceptive afferents than in small‐diameter nociceptive afferents to have a comparable influence on these processes.

Background: Inflammation-induced sensitization of primary afferents is associated with a decrease in K⁺ current. However, the type of K⁺ current and basis for the decrease varies as a function of target of innervation. Because glabrous skin of the rat hindpaw is used often to assess changes in nociception in models of persistent pain, the purpose of the present study was to determine the type and extent to which K⁺ currents contribute to the inflammation-induced sensitization of cutaneous afferents. Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion neurons from naïve and inflamed (3 days post complete Freund's adjuvant injection) rats were studied with whole cell and perforated patch techniques. Results: Inflammation-induced sensitization of small diameter cutaneous neurons was associated with an increase in action potential duration and rate of decay of the afterhyperpolarization. However, no changes in voltage-gated K⁺ currents were detected. In contrast, Ca²⁺ modulated iberiotoxin sensitive and paxilline sensitive K⁺ (BK_Ca) currents were significantly smaller in small diameter IB4+ neurons. This decrease in current was not associated with a detectable change in total protein levels of the BK_Ca channel α or β subunits. Single cell PCR analysis revealed a significant change in the pattern of expression of α subunit splice variants and β subunits that were consistent, at least in part, with inflammation-induced changes in the biophysical properties of BK_Ca currents in cutaneous neurons. Conclusions: Results of this study provide additional support for the conclusion that it may be possible, if not necessary to selectively treat pain arising from specific body regions. Because a decrease in BK_Ca current appears to contribute to the inflammation-induced sensitization of cutaneous afferents, BK_Ca channel openers may be effective for the treatment of inflammatory pain.

Background: Artemin (Artn), a member of the glial cell line-derived growth factor (GDNF) family, supports the development and function of a subpopulation of peptidergic, TRPV1-positive sensory neurons. Artn (enovin, neublastin) is elevated in inflamed tissue and its injection in skin causes transient thermal hyperalgesia. A genome wide expression analysis of trigeminal ganglia of mice that overexpress Artn in the skin (ART-OE mice) showed elevation in nicotinic acetylcholine receptor (nAChR) subunits, suggesting these ion channels contribute to Artn-induced sensitivity. Here we have used gene expression, immunolabeling, patch clamp electrophysiology and behavioral testing assays to investigate the link between Artn, nicotinic subunit expression and thermal hypersensitivity.Results: Reverse transcriptase-PCR validation showed increased levels of mRNAs encoding the nAChR subunits α3 (13.3-fold), β3 (4-fold) and β4 (7.7-fold) in trigeminal ganglia and α3 (4-fold) and β4 (2.8-fold) in dorsal root ganglia (DRG) of ART-OE mice. Sensory ganglia of ART-OE mice had increased immunoreactivity for nAChRα3 and exhibited increased overlap in labeling with GFRα3-positive neurons. Patch clamp analysis of back-labeled cutaneous afferents showed that while the majority of nicotine-evoked currents in DRG neurons had biophysical and pharmacological properties of α7-subunit containing nAChRs, the Artn-induced increase in α3 and β4 subunits resulted in functional channels. Behavioral analysis of ART-OE and wildtype mice showed that Artn-induced thermal hyperalgesia can be blocked by mecamylamine or hexamethonium. Complete Freund's adjuvant (CFA) inflammation of paw skin, which causes an increase in Artn in the skin, also increased the level of nAChR mRNAs in DRG. Finally, the increase in nAChRs transcription was not dependent on the Artn-induced increase in TRPV1 or TRPA1 in ART-OE mice since nAChRs were elevated in ganglia of TRPV1/TRPA1 double knockout mice.Conclusions: These findings suggest that Artn regulates the expression and composition of nAChRs in GFRα3 nociceptors and that these changes contribute to the thermal hypersensitivity that develops in response to Artn injection and perhaps to inflammation.

The transient receptor potential cation channel subfamily M member-3 (TRPM3) channel is a recently recognized noxious heat sensor that is involved in inflammatory thermal hyperalgesia. To examine its involvement in the development of hyperalgesia in interstitial cystitis/painful bladder syndrome (IC/PBS), rats with cyclophosphamide (CYP)-induced chronic cystitis were used as a model of IC/PBS. Mechanical and thermal hyperalgesia in lower abdominal region overlying the bladder in CYP rats were measured using von Frey filaments and radiant heat, respectively. Transient receptor potential cation channel subfamily M member-3 expression at the mRNA, protein, and functional levels in dorsal root ganglion neurons innervating the bladder was detected using RNA in situ hybridization (RNAscope), Western blotting, immunohistochemistry, and Ca²⁺ imaging, respectively. Transient receptor potential cation channel subfamily M member-3 channels were expressed on most of the bladder primary afferent nerve terminals containing calcitonin gene–related peptide and their cell bodies in L6-S1 dorsal root ganglion. Activation of TRPM3 in the bladder wall by its specific agonist pregnenolone sulphate or CIM0216 induced spontaneous bladder pain, calcitonin gene–related peptide release, and neurogenic inflammation that was evidenced by edema, plasma extravasation, inflammatory cell accumulation, and mast cell infiltration. In CYP rats, pretreatment with the TRPM3 antagonist primidone (2 mg/kg, i.p.) significantly alleviated the mechanical and thermal hyperalgesia, bladder submucosal edema, mast cell infiltration, and bladder hyperactivity. Cyclophosphamide-induced cystitis was associated with TRPM3 upregulation at the mRNA, protein, and functional levels in bladder afferent neurons. Our results suggest that upregulation of TRPM3 channels is involved in the development of chronic pain in CYP-induced cystitis, and targeting TRPM3 may be a pharmacological strategy for treating bladder pain in IC/PBS.

The present study in a rat model showed that oral administration of metformin alleviated inflammation following partial bladder outlet obstruction in the early phase and ameliorated bladder fibrosis as well as bladder dysfunction by long-term treatment. Our study indicated that metformin is a potential drug to inhibit bladder remodeling and alleviate bladder dysfunction. Clinical trials are needed to validate the effect of metformin on the bladder dysfunction and bladder fibrosis in the future.