Inflammation-associated regulation of RGS in astrocytes and putative implication in neuropathic pain

Regulators of G-protein signaling (RGS) are major physiological modulators of G-protein-coupled receptors (GPCR) signaling. Several GPCRs expressed in both neurons and astrocytes participate in the central control of pain processing, and the reduced efficacy of analgesics in neuropathic pain conditions may rely on alterations in RGS function. The expression and the regulation of RGS in astrocytes is poorly documented, and we herein hypothesized that neuroinflammation which is commonly observed in neuropathic pain could influence RGS expression in astrocytes.

In a validated model of neuropathic pain, the spared nerve injury (SNI), the regulation of RGS2, RGS3, RGS4, and RGS7 messenger RNA (mRNA) was examined up to 3 weeks after the lesion. Changes in the expression of the same RGS were also studied in cultured astrocytes exposed to defined activation protocols or to inflammatory cytokines.

We evidenced a differential regulation of these RGS in the lumbar spinal cord of animals undergoing SNI. In particular, RGS3 appeared upregulated at early stages after the lesion whereas expression of RGS2 and RGS4 was decreased at later stages. Decrease in RGS7 expression was already observed after 3 days and outlasted until 21 days after the lesion. In cultured astrocytes, we observed that changes in the culture conditions distinctly influenced the constitutive expression of these RGS. Also, brief exposures (4 to 8 h) to either interleukin-1β, interleukin-6, or tumor necrosis factor α caused rapid changes in the mRNA levels of the RGS, which however did not strictly recapitulate the regulations observed in the spinal cord of lesioned animals. Longer exposure (48 h) to inflammatory cytokines barely influenced RGS expression, confirming the rapid but transient regulation of these cell signaling modulators.

Changes in the environment of astrocytes mimicking the inflammation observed in the model of neuropathic pain can affect RGS expression. Considering the role of astrocytes in the onset and progression of neuropathic pain, we propose that the inflammation-mediated modulation of RGS in astrocytes constitutes an adaptive mechanism in a context of neuroinflammation and may participate in the regulation of nociception.