Novel molecular correlates of endocannabinoid-mediated fear-conditioned analgesia in rats.

BACKGROUND: Fear-conditioned analgesia (FCA) is the profound suppression of pain during exposure to conditioned aversive stimuli and is mediated at spinal and supraspinal levels. The endocannabinoid system plays a key role in FCA. This study investigated brain and spinal cord expression of genes implicated in pain- and fear-related plasticity (Zif268 and Sgk1), following expression of formalin-evoked nociception, contextual fear or endocannabinoid-mediated FCA. METHODS: Adult male Lister-Hooded rats received intra-plantar injection of formalin or saline, with or without administration of the CB1 receptor antagonist AM251 (3 mg/kg, i.p.) or vehicle, 30 min prior to re-exposure to an arena paired 24 h previously with footshock. Real time quantitative polymerase chain reaction was used to measure expression of Zif268 and Sgk1 mRNA in the dorsal horn of the spinal cord (DHSC) and rostral ventromedial medulla (RVM) 30 min following arena re-exposure. RESULTS: Intra-plantar injection of formalin resulted in an increase in Zif268 and Sgk1 mRNA expression in the ipsilateral DHSC of non-fear-conditioned rats, effects not observed in rats expressing FCA. Systemic administration of the CB1 receptor antagonist/inverse agonist AM251 attenuated both FCA and the FCA-associated suppression of Zif268 expression in the ipsilateral DHSC without affecting expression of Sgk1. Conditioned fear was associated with an increase in Zif268 mRNA expression in the RVM of saline-, but not formalin-treated rats. CONCLUSIONS: The present findings suggest that Zif268 in the DHSC is an important molecular correlate of endocannabinoid-mediated FCA, and that fear-related expression of Zif268 in the RVM is influenced by the presence of nociceptive tone.

A role for PPARα in the medial prefrontal cortex in formalin-evoked nociceptive responding in rats.

BACKGROUND AND PURPOSE: The nuclear hormone receptor, PPARα, and its endogenous ligands, are involved in pain modulation. PPARα is expressed in the medial prefrontal cortex (mPFC), a key brain region involved in both the cognitive-affective component of pain and in descending modulation of pain. However, the role of PPARα in the mPFC in pain responding has not been investigated. Here, we investigated the effects of pharmacological modulation of PPARα in the rat mPFC on formalin-evoked nociceptive behaviour and the impact of formalin-induced nociception on components of PPARα signalling in the mPFC. EXPERIMENTAL APPROACH: The effects of intra-mPFC microinjection of a PPARα agonist (GW7647) or a PPARα antagonist (GW6471) on formalin-evoked nociceptive behaviour in rats were studied. Quantitative real-time PCR and LC-MS/MS were used to study the effects of intraplantar injection of formalin on PPARα mRNA expression and levels of endogenous ligands, respectively, in the mPFC. KEY RESULTS: Intra-mPFC administration of GW6471, but not GW7647, resulted in delayed onset of the early second phase of formalin-evoked nociceptive behaviour. Furthermore, formalin-evoked nociceptive behaviour was associated with significant reductions in mPFC levels of endogenous PPARα ligands (N-palmitoylethanolamide and N-oleoylethanolamide) and a 70% reduction in PPARα mRNA but not protein expression. CONCLUSIONS AND IMPLICATIONS: These data suggest that endogenous ligands may act at PPARα in the mPFC to play a facilitatory/permissive role in second phase formalin-evoked nociceptive behaviour in rats. LINKED ARTICLES: This article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6.

The endocannabinoid system in the rat dorsolateral periaqueductal grey mediates fear-conditioned analgesia and controls fear expression in the presence of nociceptive tone.

BACKGROUND AND PURPOSE: Endocannabinoids in the midbrain periaqueductal grey (PAG) modulate nociception and unconditioned stress-induced analgesia; however, their role in fear-conditioned analgesia (FCA) has not been examined. The present study examined the role of the endocannabinoid system in the dorsolateral (dl) PAG in formalin-evoked nociceptive behaviour, conditioned fear and FCA in rats. EXPERIMENTAL APPROACH: Rats received intra-dlPAG administration of the CB(1) receptor antagonist/inverse agonist rimonabant, or vehicle, before re-exposure to a context paired 24 h previously with foot shock. Formalin-evoked nociceptive behaviour and fear-related behaviours (freezing and 22 kHz ultrasonic vocalization) were assessed. In a separate cohort, levels of endocannabinoids [2-arachidonoyl glycerol (2-AG) and N-arachidonoyl ethanolamide (anandamide; AEA)] and the related N-acylethanolamines (NAEs) [N-palmitoyl ethanolamide (PEA) and N-oleoyl ethanolamide (OEA)] were measured in dlPAG tissue following re-exposure to conditioned context in the presence or absence of formalin-evoked nociceptive tone. KEY RESULTS: Re-exposure of rats to the context previously associated with foot shock resulted in FCA. Intra-dlPAG administration of rimonabant significantly attenuated FCA and fear-related behaviours expressed in the presence of nociceptive tone. Conditioned fear without formalin-evoked nociceptive tone was associated with increased levels of 2-AG, AEA, PEA and OEA in the dlPAG. FCA was specifically associated with an increase in AEA levels in the dlPAG. CONCLUSIONS AND IMPLICATIONS: Conditioned fear to context mobilises endocannabinoids and NAEs in the dlPAG. These data support a role for endocannabinoids in the dlPAG in mediating the potent suppression of pain responding which occurs during exposure to conditioned aversive contexts. LINKED ARTICLES: This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.