The effect of CB₁ receptor antagonism in the right basolateral amygdala on conditioned fear and associated analgesia in rats

Abstract

The endocannabinoid system mediates analgesia expressed following exposure to conditioned or unconditioned aversive stimuli, and controls the extinction of conditioned aversive behaviour. The present study investigated the effects of administration of the cannabinoid(1) (CB(1)) receptor antagonist SR141716A into the right basolateral amygdala (BLA) on expression of conditioned fear, formalin-evoked nociceptive behaviour, fear-conditioned analgesia and associated alterations in monoamine levels in discrete rat brain areas. Re-exposure to a context previously paired with footshock significantly reduced formalin-evoked nociceptive behaviour. Intra-BLA administration of SR141716A did not attenuate fear-conditioned analgesia, but reduced formalin-evoked nociceptive behaviour and attenuated the formalin-induced decrease in freezing and 22-kHz ultrasonic vocalizations in the early part of the trial. Furthermore, intra-BLA SR141716A significantly prolonged the duration of these fear-related behaviours in fear-conditioned rats not receiving formalin. Fear-conditioned analgesia was accompanied by increased homovanillic acid (HVA) : dopamine (DA) ratio and reduced serotonin (5-HT) in the cerebellum, an effect not altered by SR141716A. SR141716A-induced analgesia was accompanied by reduced DA, increased HVA : DA ratio and reduced 5-HT levels in the cerebellum, increased hippocampal HVA levels and increased 5-hydroxyindole-3-acetic acid (5-HIAA) in the amygdaloid cortex. The SR141716A-induced prolongation of contextually induced aversive behaviour was accompanied by reduced DA and 3,4-dihydroxyphenylacetic acid (DOPAC), levels in the hippocampus, and increased DA and 5-HIAA in the periaqueductal grey. These data suggest an important role for CB(1) receptors in the right BLA in mediating short-term extinction of conditioned aversive behaviour but not fear-conditioned analgesia. The results also enhance our understanding of endocannabinoid-monoamine interactions of relevance to conditioned fear and associated analgesia.

A role for the ventral hippocampal endocannabinoid system in fear-conditioned analgesia and fear responding in the presence of nociceptive tone in rats

“…The antinociception observed here following re-exposure to an aversively conditioned context corroborates previous studies demonstrating robust expression of FCA in rats following Pavlovian fear conditioning [11,20,25,31,69,70,80]. Our previous work has demonstrated that this form of potent endogenous analgesia is mediated by the endocannabinoid system [11,25].…”

“…The role of the endocannabinoid system in fear responding per se (i.e. in the absence of nociceptive tone) has been examined extensively and the weight of evidence suggests that endocannabinoid-CB 1 receptor signalling serves to facilitate and enhance the extinction of conditioned fear responding (for review see [15,48,79]), while genetic deletion or pharmacological blockade of the CB 1 receptor attenuates short-and long-term extinction of conditioned fear responding [14,18,25,39,44,48,52,59,62,70,76]. The results of the present study, however, suggest that in the presence of formalin-evoked nociceptive tone, increased endocannabinoid signalling in the vHip (as a consequence of URB597 administration) in fact serves to inhibit rather than enhance the short-term, within-trial extinction of fear responding.…”

“…The experimental procedure was identical to that described previously [10,11,25,60,70]. In brief, it consisted of two phases, conditioning and testing, occurring 24h apart.…”

“…A bat detector (Batbox Duet, Batbox Ltd, West Sussex, UK) was positioned above the arena to detect ultrasonic vocalisation in the 22 kHz range and behaviours were recorded for 15 minutes from a video camera positioned under the observation chamber. The 30-45 minutes post-formalin interval was chosen on the basis of previous studies demonstrating that formalin-evoked nociceptive behaviour is stable over this time period and that fearconditioned analgesia and conditioned fear expressed during this period are regulated by the endocannabinoid system [11,25,70]. Immediately following completion of the 15 minute test trial, rats were removed from the arena, decapitated, and 2% fast green dye (dissolved in DMSO) was immediately microinjected into the vHip (1µL) or off-target into the medial geniculate nucleus (300nL) to mark the injection sites.…”

“…A rater blind to experimental conditions assessed fear-related behaviours (duration of freezing and 22 kHz ultrasonic vocalisation), formalin-evoked nociceptive behaviours and motor behaviours (rearing, grooming and walking) for the total duration of the trial as described previously [10,11,25,26,69,70]. Freezing was defined as cessation of all visible movement except that necessary for respiration.…”