Microinjection of 2-arachidonoyl glycerol into the rat ventral hippocampus differentially modulates contextually induced fear, depending on a persistent pain state

• Published in: The European journal of neuroscience Vol. 39; no. 3; pp. 435 - 443
• Main Authors: Rea, Kieran, Ford, Gemma K., Olango, Weredeselam M., Harhen, Brendan, Roche, Michelle, Finn, David P.
• Format: Journal Article
• Language: English
• Published: France Blackwell Publishing Ltd 01.02.2014
• Subjects: Amygdala; Animals; Arachidonic Acids - administration & dosage; Arachidonic Acids - pharmacology; aversion; cannabinoid 1 (CB1) receptor; Cannabinoid Receptor Agonists - administration & dosage; Cannabinoid Receptor Agonists - pharmacology; Cannabinoid Receptor Antagonists - pharmacology; Conditioning, Classical; endocannabinoid; Endocannabinoids - administration & dosage; Endocannabinoids - pharmacology; Fear - drug effects; formalin; Freezing Reaction, Cataleptic; Glycerides - administration & dosage; Glycerides - pharmacology; Hippocampus - drug effects; Hippocampus - physiopathology; Injections, Intraventricular; Mice; Nociception; Pain - metabolism; Pain - physiopathology; periaqueductal grey; Piperidines - pharmacology; Polyunsaturated Alkamides - pharmacology; Pyrazoles - pharmacology; Rats; Receptor, Cannabinoid, CB1 - agonists; Receptor, Cannabinoid, CB1 - antagonists & inhibitors; aversion; endocannabinoid; periaqueductal grey; cannabinoid 1 (CB1) receptor; formalin
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/ejn.12452

The endogenous cannabinoid (endocannabinoid) system plays a key role in the modulation of aversive and nociceptive behaviour. The components of the endocannabinoid system are expressed throughout the hippocampus, a brain region implicated in both conditioned fear and pain. In light of evidence that pain can impact on the expression of fear‐related behaviour, and vice versa, we hypothesised that exogenous administration of the endocannabinoid 2‐arachidonoyl glycerol (2‐AG) into the ventral hippocampus (vHip) would differentially regulate fear responding in the absence vs. the presence of formalin‐evoked nociceptive tone. Fear‐conditioned rats showed significantly increased freezing and a reduction in formalin‐evoked nociceptive behaviour upon re‐exposure to a context previously paired with footshock. Bilateral microinjection of 2‐AG into the vHip significantly reduced contextually induced freezing in non‐formalin‐treated rats, and reduced formalin‐evoked nociceptive behaviour in non‐fear‐conditioned rats. In contrast, 2‐AG microinjection had no effect on fear responding in formalin‐treated rats, and no effect on nociceptive behaviour in fear‐conditioned rats. The inhibitory effect of 2‐AG on fear‐related behaviour, but not pain‐related behaviour, was blocked by co‐administration of the cannabinoid receptor 1 (CB1) antagonist/inverse agonist rimonabant. Tissue levels of the endocannabinoids N‐arachidonoylethanolamide (anandamide, AEA) and 2‐AG were similar in the vHip of fear‐conditioned rats receiving formalin injection and the vHip of fear‐conditioned rats receiving saline injection. However, the levels of AEA and 2‐AG were significantly lower in the contralateral ventrolateral periaqueductal grey of formalin‐treated fear‐conditioned rats than in that of their saline‐treated counterparts. These data suggest that 2‐AG–CB1 receptor signalling in the vHip has an anti‐aversive effect, and that this effect is abolished in the presence of a persistent pain state. Schematic depicting the role of the vHip in both contextual fear conditioning and pain processing and the putative neural circuitry and mechanisms mediating the effects of intra‐vHip 2‐AG injection observed in the present study. The vHip shares connectivity with the lateral amygdala (LA), basolateral amygdala (BLA) and central nucleus of the amygdala (CeA), as well as cortical regions, from which neurons project to the periaqueductal gray (PAG) to regulate the behavioural expression of pain and fear. The re‐exposure of an animal to an environment where it has previously been exposed to footshock recruits neurons in the vHip, amygdala and cortical regions to engage neurons in the PAG, resulting in the expression of conditioned fear. Upon activation of peripheral nociceptors by a noxious stimulus, nociceptive information is relayed through the dorsal horn of the spinal cord, and reaches higher centres in the brain. The ventrolateral aspect of the periaqueductal gray (vlPAG) is critically involved in the expression of contextual fear and formalin‐evoked nociception. AEA (anandamide) and 2‐AG (2‐arachidonoyl glycerol) levels were significantly decreased in left vlPAG of fear‐conditioned, formalin‐treated rats i.e. the vlPAG side contralateral to the hindpaw into which formalin was injected. The suppression of conditioned freezing as a consequence of CB1 receptor activation in the vHip by 2‐AG was not apparent in formalin‐treated rats. These differential effects of intra‐vHip 2‐AG on conditioned fear responding in the absence versus presence of formalin‐evoked nociceptive tone do not appear to be due to formalin‐evoked alterations in levels of endocannabinoids locally in the vHip but may be related to the formalin‐evoked reduction in endocannabinoid levels observed in the contralateral vlPAG. CB1R: cannabinoid receptor type 1, AEA: anandamide, 2‐AG: 2‐arachidonoyl glycerol, vHip: ventral hippocampus, CeA: central nucleus of the amygdala, LA: lateral amygdala, BLA: basolateral amygdala, vlPAG: ventrolateral periaqueductal gray, RVM: rostral ventromedial medulla, FC: fear‐conditioned, Form: formalin.