Blockade of Cannabinoid CB1 Receptors in the Dorsal Periaqueductal Gray Unmasks the Antinociceptive Effect of Local Injections of Anandamide in Mice.

Diego C Mascarenhas,Tatiani Sorregotti,Karina S Gomes,Ricardo L Nunes-De-Souza

doi:10.3389/fphar.2017.00695

Abstract

Divergent results in pain management account for the growing number of studies aiming at elucidating the pharmacology of the endocannabinoid/endovanilloid anandamide (AEA) within several pain-related brain structures. For instance, the stimulation of both Transient Receptor Potential Vanilloid type 1 (TRPV1) and Cannabinoid type 1 (CB1) receptors led to paradoxical effects on nociception. Here, we attempted to propose a clear and reproducible methodology to achieve the antinociceptive effect of exogenous AEA within the dorsal periaqueductal gray (dPAG) of mice exposed to the tail-flick test. Accordingly, male Swiss mice received intra-dPAG injection of AEA (CB1/TRPV1 agonist), capsaicin (TRPV1 agonist), WIN (CB1 agonist), AM251 (CB1 antagonist), and 6-iodonordihydrocapsaicin (6-IODO) (TRPV1 selective antagonist) and their nociceptive response was assessed with the tail-flick test. In order to assess AEA effects on nociception specifically at vanilloid or cannabinoid (CB) substrates into the dPAG, mice underwent an intrinsically inactive dose of AM251 or 6-IODO followed by local AEA injections and were subjected to the same test. While intra-dPAG AEA did not change acute pain, local injections of capsaicin or WIN induced a marked TRPV1- and CB1-dependent antinociceptive effect, respectively. Regarding the role of AEA specifically at CB/vanilloid substrates, while the blockade of TRPV1 did not change the lack of effects of intra-dPAG AEA on nociception, local pre-treatment of AM251, a CB1 antagonist, led to a clear AEA-induced antinociception. It seems that the exogenous AEA-induced antinociception is unmasked when it selectively binds to vanilloid substrates, which might be useful to address acute pain in basic and perhaps clinical trials.

Highlights

Pain-related diseases have been extensively investigated in order to unmask its complex neurobiology and underlying mechanisms, and/or to provide novel treatment options
Firstly we investigated the effects of intra-dorsal periaqueductal gray (dPAG) injections of AEA, capsaicin, WIN 55,212-2, AM251, or 6-iodonordihydrocapsaicin (6-IODO) on acute nociceptive response assessed through the tail-flick test (Experiments 1A–E)
The main results of this study point out that AEA (0.5–50 pmol), a TRPV1/Cannabinoid type 1 (CB1) agonist, injected into the mouse dPAG does not produce any intrinsic effect on acute pain as assessed through the tail-flick test (Exp. 1A)

Summary

Introduction

Pain-related diseases have been extensively investigated in order to unmask its complex neurobiology and underlying mechanisms, and/or to provide novel treatment options In this context, several neurotransmitters have been implicated mediating nociception, for instance, opioids The TRPV1 channels were found to be expressed in primary afferent neurons and implicated in transmitting noxious stimuli to the spinal cord [for a review, see Salat et al (2013)] Besides their role in pain modulation on peripheral nervous system (Gewehr et al, 2011), TRPV1 are found in brainstem areas including the periaqueductal gray matter (PAG) (Cristino et al, 2006). This midbrain structure is highly involved in the modulation of defensive reactions such as fear/anxiety states as well as nociception (e.g., Starowicz et al, 2007; Lisboa and Guimaraes, 2012; Mascarenhas et al, 2013, 2015)

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