Spinal astrocytic activation contributes to both induction and maintenance of pituitary adenylate cyclase-activating polypeptide type 1 receptor-induced long-lasting mechanical allodynia in mice

Abstract

BackgroundPituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are present in the spinal dorsal horn and dorsal root ganglia, suggesting an important role of PACAP–PACAP receptors signaling system in the modulation of spinal nociceptive transmission. We have previously reported that a single intrathecal injection of PACAP or a PACAP specific (PAC1) receptor selective agonist, maxadilan, in mice induced dose-dependent aversive behaviors, which lasted more than 30 min, and suggested that the maintenance of the nociceptive behaviors was associated with the spinal astrocytic activation.ResultsWe found that a single intrathecal administration of PACAP or maxadilan also produced long-lasting hind paw mechanical allodynia, which persisted at least 84 days without affecting thermal nociceptive threshold. In contrast, intrathecal application of vasoactive intestinal polypeptide did not change mechanical threshold, and substance P, calcitonin gene-related peptide, or N-methyl-D-aspartate induced only transient mechanical allodynia, which disappeared within 21 days. Western blot and immunohistochemical analyses with an astrocytic marker, glial fibrillary acidic protein, revealed that the spinal PAC1 receptor stimulation caused sustained astrocytic activation, which also lasted more than 84 days. Intrathecal co-administration of L-α-aminoadipate, an astroglial toxin, with PACAP or maxadilan almost completely prevented the induction of the mechanical allodynia. Furthermore, intrathecal treatment of L-α-aminoadipate at 84 days after the PAC1 stimulation transiently reversed the mechanical allodynia accompanied by the reduction of glial fibrillary acidic protein expression level.ConclusionOur data suggest that spinal astrocytic activation triggered by the PAC1 receptor stimulation contributes to both induction and maintenance of the long-term mechanical allodynia.

Spinal astrocyte-neuron lactate shuttle contributes to the PACAP/PAC1 receptor-induced nociceptive behaviors

“…Exposure of PACAP (0.0001 ~ 10 nM) or Max (0.1 ~ 10 nM) dose-dependently decreased glycogen amount ( Figure 3A, B ), but VIP failed to decrease the glycogen amount even at the concentration of 10 nM ( Figure 3C ), suggesting that PAC1 receptor is primarily involved in the glycogenolysis. Interestingly, this pharmacological characteristic of glycogenolysis was very similar to that of the PACAP-induced nociceptive behaviors in mice reported previously ( Ohnou et al, 2016; Yokai et al, 2016 ). This PACAP-induced glycogenolysis was completely abolished by the DAB (1 mM) ( Figure 3D ), indicating that the glycogenolysis by PACAP would be mediated by PYGB.…”

“…Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally isolated from ovine hypothalamic extracts based on its ability to stimulate adenylate cyclase in rat anterior pituitary cell cultures ( Miyata et al, 1989; Miyata et al, 1990 ). In normal state, PACAP specific receptor, PAC1 receptor, is particularly abundant in central nervous system (CNS) including spinal dorsal horn ( Dickinson et al, 1999;Jongsma et al, 2000;Sakashita et al, 2001;Vaudry et al, 2009; Yokai et al, 2016 ), where PACAP-immunoreactive fibers are also considerably localized ( Moller et al, 1993; Dun et al, 1996a; Dun et al, 1996b; Narita et al, 1996 ), and PACAP mRNA/immunoreactivity in rat dorsal root ganglia is markedly upregulated in peripheral nerve injury or inflammation ( Zhang et al, 1995; Zhang et al, 1998; Jongsma et al, 2003; Mabuchi et al, 2004 ). These observations coupled with other lines of evidence propose that PACAP/PAC1 receptor system could play an important role in the modulation of spinal nociceptive transmission.…”

“…injection of PACAP or a PAC1 receptor specific agonist, maxadilan (Max) ( Moro and Lerner, 1997 ), induced spontaneous aversive behaviors, such as licking, biting, and scratching directed toward the caudal part of the body for more than 30 min ( Shimizu et al, 2004; Ohnou et al, 2016 ), and the aversive behaviors were followed by a induction of the mechanical allodynia, which lasted for more than 84 days ( Yokai et al, 2016 ). However, vasoactive intestinal polypeptide (VIP), which share VIP/PACAP (VPAC) 1 or VPAC2 receptors with PACAP, failed to induce these nociceptive behaviors ( Ohnou et al, 2016; Yokai et al, 2016 ). Co-treatment of a PAC1 receptor antagonist, max.d.4 with PACAP, almost completely inhibited the induction of both aversive behaviors and the mechanical allodynia ( Ohnou et al, 2016; Yokai et al, 2016 ).…”

“…However, vasoactive intestinal polypeptide (VIP), which share VIP/PACAP (VPAC) 1 or VPAC2 receptors with PACAP, failed to induce these nociceptive behaviors ( Ohnou et al, 2016; Yokai et al, 2016 ). Co-treatment of a PAC1 receptor antagonist, max.d.4 with PACAP, almost completely inhibited the induction of both aversive behaviors and the mechanical allodynia ( Ohnou et al, 2016; Yokai et al, 2016 ). These results suggested critical role of PAC1 receptor in nociceptive transmission.…”

“…Immunohistochemical and immunoblotting studies revealed that spinal application of PACAP or Max induced upregulation of an astrocyte marker, glial fibrillary acidic protein (GFAP) at least for 84 days, and L-α-aminoadipate, an astroglial toxin, attenuated the induction of both aversive behaviors and mechanical allodynia. Interestingly, L-α-aminoadipate were still effective to reverse the mechanical allodynia even if it was i.t.- administered 84 days after spinal PAC1 receptor stimulation ( Yokai et al, 2016 ). These results suggest that long-lasting spinal astocytic activation underlies the PACAP/PAC1 receptor-induced aversive behaviors and mechanical allodynia.…”