The Mechanism of Functional Up-Regulation of P2X3 Receptors of Trigeminal Sensory Neurons in a Genetic Mouse Model of Familial Hemiplegic Migraine Type 1 (FHM-1)

Abstract

A knock-in (KI) mouse model of FHM-1 expressing the R192Q missense mutation of the Cacna1a gene coding for the α1 subunit of CaV2.1 channels shows, at the level of the trigeminal ganglion, selective functional up-regulation of ATP -gated P2X3 receptors of sensory neurons that convey nociceptive signals to the brainstem. Why P2X3 receptors are constitutively more responsive, however, remains unclear as their membrane expression and TRPV1 nociceptor activity are the same as in wildtype (WT) neurons. Using primary cultures of WT or KI trigeminal ganglia, we investigated whether soluble compounds that may contribute to initiating (or maintaining) migraine attacks, such as TNFα, CGRP, and BDNF, might be responsible for increasing P2X3 receptor responses. Exogenous application of TNFα potentiated P2X3 receptor-mediated currents of WT but not of KI neurons, most of which expressed both the P2X3 receptor and the TNFα receptor TNFR2. However, sustained TNFα neutralization failed to change WT or KI P2X3 receptor currents. This suggests that endogenous TNFα does not regulate P2X3 receptor responses. Nonetheless, on cultures made from both genotypes, exogenous TNFα enhanced TRPV1 receptor-mediated currents expressed by a few neurons, suggesting transient amplification of TRPV1 nociceptor responses. CGRP increased P2X3 receptor currents only in WT cultures, although prolonged CGRP receptor antagonism or BDNF neutralization reduced KI currents to WT levels. Our data suggest that, in KI trigeminal ganglion cultures, constitutive up-regulation of P2X3 receptors probably is already maximal and is apparently contributed by basal CGRP and BDNF levels, thereby rendering these neurons more responsive to extracellular ATP.

In situ imaging reveals properties of purinergic signalling in trigeminal sensory ganglia in vitro

“…Purinergic signalling and pore formation R192Q KI mice express mutated Ca V 2.1 channels that exhibit gain of function [13]. As a consequence, this genetic model shows enhanced excitability of TG neurons [14,15] and larger release of neuroinflammatory modulators [16,18]. However, the time course and the impact of soluble mediators on neuron-glia crosstalk or their role in amplifying cell responses remain incompletely understood.…”

“…Our previous work was concerned with R192Q Ca V 2.1 knock-in mice (R192Q KI), which express voltage-gated Ca V 2.1 Ca 2+ channels containing α 1A subunits harbouring the R192Q missense mutation that leads to familial hemiplegic migraine type 1 (FHM-1) in patients [12,13]. This model suggests a direct link between mutated Ca V 2.1 channels, potentiated neuronal responses and an inflammatory basal state [14,15] due to a larger release of CGRP [16], ATP and TNF-α [17,18]. The present study aimed at comparing the occurrence, time course and intensity of responses of wild-type (WT) and R192Q KI trigeminal ganglion (TG) cultures.…”

The nicotinic α6 subunit gene determines variability in chronic pain sensitivity via cross-inhibition of P2X2/3 receptors

“…Purinergic receptors are important pain processing molecules known to be expressed on nociceptive small diameter neurons in the DRG (46), with important roles having been demonstrated for P2X2/3 (23, 24, 47), P2X4 (48), and P2X7 (49). P2X2/3 receptors have been shown to contribute to multiple pain modalities, including inflammatory pain (23, 24, 47, 50), neuropathic pain (51), visceral pain (52), musculoskeletal pain (53), cancer pain (54), and migraine (55). Presumably the α6* nicotinic receptors interacting in the periphery with P2X2/3 receptors are activated endogenously by acetylcholine, which exists abundantly in mammals both neuronally and non-neuronally, for example in keratinocytes (56).…”

The Purinergic System and Glial Cells: Emerging Costars in Nociception

“…On the other hand, P2X3 has never been found to be expressed by glial cells, by either immunohistochemical analysis or calcium imaging [29]. P2X3 immunoreactivity has also been observed in other structures indirectly involved in nociceptive transmission, such as the nucleus of the solitary tract, and the spinal trigeminal nucleus [30].…”