Heat hypersensitivity is attenuated with altered expression level of spinal astrocytes after sciatic nerve injury in TRPV1 knockout mice

Highlights

• Heat sensitivity was attenuated in TRPV1 knockout mice after partial sciatic nerve ligation.

• Astrogliosis was impaired in TRPV1 knockout mice after partial sciatic nerve injury.

• Chemical inhibition of astrocytes induced heat hyposensitivity.

• TRPV1 was co-localized in astrocytic GFAP positive cells in the spinal dorsal horn.

• TRPV1 may modulate heat sensitivity via spinal astrogliosis.

Abstract

Transient receptor potential vanilloid 1 (TRPV1) modulates pain. Studies have indicated that TRPV1 is upregulated in the spinal dorsal horn in the neuropathic pain model, but its mechanism is unknown. Here, we examined the mechanism by which TRPV1 modulates neuropathic pain by employing partial sciatic nerve ligation (pSNL) in adult male C57BL/6 J (wild-type: WT) and TRPV1 knockout (Trpv1-/-) mice. We analyzed mechanical/heat sensitivities (von Frey test/hot plate test) and glial/neuronal activities (Iba-1/GFAP/FosB by immunofluorescence) in laminae I and II in the L5 ipsilateral dorsal horn of the spinal cord. Mechanical/heat sensitivities, expression levels of microglial Iba-1 and astrocytic GFAP, and the number of FosB-positive neurons were significantly increased on days 7 and 14 in the pSNL group compared with the sham-operated and non-operated groups of both WT and Trpv1-/- mice. While mechanical sensitivity was comparable between WT and Trpv1-/- mice, the threshold against heat sensitivity was markedly prolonged in Trpv1-/- than WT mice on day 14 after pSNL. Conversely, the increment of FosB positive neurons was significantly attenuated in Trpv1-/- than WT mice on days 7 and 14 after pSNL. These results suggest that TRPV1 may modulate thermal perception via increased astrocytes in the dorsal horn of the spinal cord.

Introduction

Neuropathic pain is defined as pain caused by a lesion or disease of the somatosensory nervous system (Jensen et al., 2011). It has been reported that 7–10 % of the general population suffer from neuropathic pain (Colloca et al., 2017). Neuropathic pain, classified as chronic pain, is characterized by persistent hyperalgesia and allodynia (Jensen and Finnerup, 2014). Since neuropathic pain has a complex pathophysiology, it is sometimes difficult to provide effective treatment, resulting in poor prognosis (Colloca et al., 2017). Thus, clarifying the pathophysiological mechanism of neuropathic pain would contribute to the identification of successful remedies.

The transient receptor potential (TRP) channels family, which is closely associated with nociception (Nilius et al., 2007), has six subfamilies: TRP ankyrin (TRPA), TRP canonical (TRPC), TRP melastatin (TRPM), TRP mucolipin (TRPML), TRP polycystin (TRPP), and TRP vanilloid (TRPV) subfamilies (Nilius and Owsianik, 2011). There are six TRPV subfamilies, TRPV1−6. TRPV1, a capsaicin receptor, mediates painful stimuli such as noxious heat from the primary sensory afferent nerves to the central nervous system (Caterina and Julius, 2001; Caterina et al., 1997). It plays a crucial role in the transmission of nociception under physiological and pathological conditions (Lappin et al., 2006; Spicarová and Palecek, 2008). Neuropathic pain and exaggerated TRPV1 expression in the dorsal spinal horn was induced by peripheral nerve injury (Kanai et al., 2005). In addition, TRPV1 plays a pivotal role in the development and maintenance of allodynia in rats (Kanai et al., 2005). Taken together, TRPV1 in the spinal cord might be involved in neuropathic pain, although detailed mechanisms remain unclear.

In the present study, to investigate the role of TRPV1 in mediating neuropathic pain, we evaluated the behavioral changes by von Frey test/hot plate test as well as glial/neuronal activities in the spinal dorsal horn by using a neuropathic pain model of TRPV1 knockout (Trpv1-/-) mice. We carried out von Frey test/hot plate test, both of which are widely prevailed to evaluate mechanical/heat sensitivity, respectively (Nishimura et al., 2020, 2019). We employed a partial sciatic nerve ligation (pSNL)-induced neuropathic pain model, known as a Seltzer model (Seltzer et al., 1990), as this model typifies the neuropathic pain characteristically. Since ionized calcium-binding adapter molecule-1 (Iba-1) and GFAP, which are the markers for microglia (Imai and Kohsaka, 2002) and astrocytes (Pekny and Pekna, 2004), respectively, are markedly increased in the neuropathic pain model, we evaluated these expressions, by immunofluorescence, in order to confirm whether the model was successfully developed. We also assessed the expression of Iba-1 and GFAP in Trpv1-/- mice compared to wild-type (WT). Expression of FosB, a neuronal marker expressed in persistently activated neurons, especially in the chronic stage, in the spinal dorsal horn was analyzed, since FosB was increased in an inflammatory pain model (Luis-Delgado et al., 2006) as well as a spinal cord injury model (Watson et al., 2014).