There is pharmacological evidence Group II and III metabotropic glutamate receptors (mGluRs) function as activity-dependent autoreceptors, inhibiting transmission in supraspinal sites. These receptors are expressed by peripheral nociceptors. We investigated whether mGluRs function as activity-dependent autoreceptors inhibiting pain transmission to the rat CNS, particularly TRPV1-induced activity. Blocking peripheral mGluR activity by intraplantar injection of antagonists LY341495 (LY, 20, 100 μM, Group II/III ), APICA (100 μM, Group II) or UBP1112 (30 μM, Group III) increased capsaicin (CAP)-induced nociceptive behaviors and nociceptor activity. In contrast, Group II agonist APDC (0.1 μM) or Group III agonist L-AP4 (10 μM) blocked the LY-induced increase. Ca2+ imaging in dorsal root ganglion (DRG) cells confirmed LY enhanced CAP-induced Ca2+ mobilization which was blocked by APDC and L-AP4. We hypothesized that excess glutamate (GLU), released by high intensity and/or prolonged stimulation endogenously activated Group II/III, dampening nociceptor activation. In support of this, intraplantar GLU+LY produced heat hyperalgesia and exogenous GLU+LY applied to nociceptors produced enhanced nociceptor activity and thermal sensitization. Intraplantar formalin known to elevate extracellular GLU, enhanced pain behaviors in the presence of LY. LY alone produced no pain behaviors, no change in nociceptor discharge rate or heat-evoked responses and no change in cytosolic Ca2+ in DRG cells, demonstrating a lack of tonic inhibitory control. Group II/III mGluRs maintain an activity-dependent autoinhibition, capable of significantly reducing TRPV1-induced activity. They are endogenously activated following high frequency and/or prolonged nociceptor stimulation, acting as built-in negative modulators of TRPV1 and nociceptor function, reducing pain transmission to the CNS.

Several lines of evidence indicate group III metabotropic glutamate receptors (mGluRs) have systemic anti-hyperalgesic effects. We hypothesized this could occur through modulation of TRPV1 receptors on nociceptors. To address this question we performed anatomical studies to determine if group III mGluRs were expressed on cutaneous axons and if they co-localized with TRPV1. Immunostaining at the electron microscopic level demonstrated that 22% of unmyelinated axons labeled for mGluR8. Immunostaining at the light microscopic level in lumbar dorsal root ganglia (DRG) demonstrated that 80% and 28% of neurons labeled for mGluR8 or TRPV1, respectively. Of those neurons labeled for mGluR8, 25% labeled for TRPV1; of those labeled for TRPV1, 71% labeled for mGluR8. In behavior studies intraplantar injection of the group III mGluR agonist, L-AP-4 (0.1 1.0, 10.0 µM) had no effect on paw withdrawal latency (PWL) to heat in naïve rats but administration of 10 µM L-AP-4 prior to 0.05% capsaicin (CAP), significantly attenuated CAP-induced lifting/licking and reduced flinching behavior. The L-AP-4 effect was specific since administration of a group III antagonist UBP1112 (100 µM) blocked the L-AP-4 effect on CAP, resulting in behaviors similar to CAP alone. Intraplantar injection of UBP1112 alone did not result in nociceptive behaviors, indicating group III mGluRs are not tonically active. Finally, the anti-hyperalgesic effect of group III in this paradigm was local and not systemic since intraplantar administration of L-AP-4 in one hind paw did not attenuate nociceptive behaviors following CAP injection in the contralateral hind paw. Adenyl cyclase/cAMP/PKA may be the second messenger pathway linking these two receptor families because intraplantar injection of forskolin (FSK, 10 µM) reduced PWL to heat and L-AP-4 reversed this FSK effect. Taken together, these results suggest group III mGluRs can negatively modulate TRPV1 through inhibition of AC and downstream intracellular activity, blocking TRPV1-induced activation of nociceptors.

We have previously shown that nerve inflammation (neuritis) and transient vinblastine application lead to axonal mechanical sensitivity in nociceptors innervating deep structures. We also have shown that these treatments reduce axonal transport and have proposed that this leads to functional accumulation of mechanically sensitive channels in the affected part of the axons. Though informing the etiology of mechanically induced pain, axonal mechanical sensitivity does not address the common report of ongoing radiating pain during neuritis, which could be secondary to the provocation of axonal chemical sensitivity. We proposed that neuritis and vinblastine application would induce sensitivities to noxious chemicals and that the number of chemo-sensitive channels would be increased at the affected site. In adult female rats, nerves were either untreated or treated with complete Freund's adjuvant (to induce neuritis) or vinblastine. After 3-7 days, dorsal root teased fiber recordings were taken from group IV neurons with axons within the sciatic nerve. Sciatic nerves were injected intraneurally with a combination of noxious inflammatory chemicals. Whereas no normal sciatic axons responded to this stimulus, 80% and 38% of axons responded in the neuritis and vinblastine groups, respectively. In separate experiments, sciatic nerves were partially ligated and treated with complete Freund's adjuvant or vinblastine (with controls), and after 3-5 days were immunolabeled for the histamine H receptor. The results support that both neuritis and vinblastine treatment reduce transport of the histamine H receptor. The finding that nociceptor axons can develop ectopic chemical sensitivity is consistent with ongoing radiating pain due to nerve inflammation. Many patients suffer ongoing pain with no local pathology or apparent nerve injury. We show that nerve inflammation and transient application of vinblastine induce sensitivity of group IV nociceptor axons to a mixture of endogenous inflammatory chemicals. We also show that the same conditions reduce the axonal transport of the histamine H receptor. The results provide a mechanism for ongoing nociception from focal nerve inflammation or pressure without overt nerve damage.

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