Dorsolateral PAG neurons: tonic immobility and morphine effect in freely moving rabbits

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.

Unit activity alterations induced in the mesencephalic periaqueductal gray by local electrical stimulation

In the framework of a series of investigations concerning the neural substrate of aversion, electrophysiological methods were used in order to specify, within the rat's periaqueductal gray (PAG), functional properties, viz. conduction velocity and refractory period, of PAG neurons already assessed in previous studies by means of behavioral methods, and to gather data on their local synaptic relationships. Unit activities were recorded from the periaqueductal gray with the aim of analyzing those alterations that would be induced by locally applying an electrical stimulation with parameters shown to elicit escape behavior. An implanted row of electrodes allowed the application of a stimulation to several sites aligned along a mediolateral or a rostrocaudal axis through the periaqueductal gray. The results indicate that an electrical stimulation applied to the periaqueductal gray may induce its effects through the activation of a number of dendrites and many slow conducting fibers running in a great variety of directions and branching within the periaqueductal gray. Their refractory period was surprisingly low (0.6 ms) for slow conducting fibers (below 1 m/s). The local circuitry appears to include many inhibitory connections. Their organization is assumed to be partly recurrent. Stimulation-induced inhibition becomes predominant when the stimulation is moved away from the recorded neuron along the mediolateral axis, but not along the rostrocaudal axis.