Endocannabinoids control vesicle release mode at midbrain periaqueductal grey inhibitory synapses.

J Physiol

Pain Management Research Institute, Kolling Institute of Medical Research, Northern Clinical School, The University of Sydney and Royal North Shore Hospital, St. Leonards, New South Wales, Australia.

Published: January 2017

Key Points: The midbrain periaqueductal grey (PAG) forms part of an endogenous analgesic system which is tightly regulated by the neurotransmitter GABA. The role of endocannabinoids in regulating GABAergic control of this system was examined in rat PAG slices. Under basal conditions GABAergic neurotransmission onto PAG output neurons was multivesicular. Activation of the endocannabinoid system reduced GABAergic inhibition by reducing the probability of release and by shifting release to a univesicular mode. Blockade of endocannabinoid system unmasked a tonic control over the probability and mode of GABA release. These findings provides a mechanistic foundation for the control of the PAG analgesic system by disinhibition.

Abstract: The midbrain periaqueductal grey (PAG) has a crucial role in coordinating endogenous analgesic responses to physiological and psychological stressors. Endocannabinoids are thought to mediate a form of stress-induced analgesia within the PAG by relieving GABAergic inhibition of output neurons, a process known as disinhibition. This disinhibition is thought to be achieved by a presynaptic reduction in GABA release probability. We examined whether other mechanisms have a role in endocannabinoid modulation of GABAergic synaptic transmission within the rat PAG. The group I mGluR agonist DHPG ((R,S)-3,5-dihydroxyphenylglycine) inhibited evoked IPSCs and increased their paired pulse ratio in normal external Ca , and when release probability was reduced by lowering Ca . However, the effect of DHPG on the coefficient of variation and kinetics of evoked IPSCs differed between normal and low Ca . Lowering external Ca had a similar effect on evoked IPSCs to that observed for DHPG in normal external Ca . The low affinity GABA receptor antagonist TPMPA ((1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid) inhibited evoked IPSCs to a greater extent in low than in normal Ca . Together these findings indicate that the normal mode of GABA release is multivesicular within the PAG, and that DHPG and lowering external Ca switch this to a univesicular mode. The effects of DHPG were mediated by mGlu5 receptor engagement of the retrograde endocannabinoid system. Blockade of endocannabinoid breakdown produced a similar shift in the mode of release. We conclude that endocannabinoids control both the mode and the probability of GABA release within the PAG.

Download full-text PDF

Source
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5199746PMC
http://dx.doi.org/10.1113/JP272292DOI Listing

Publication Analysis

Top Keywords

gaba release
16
evoked ipscs
16
midbrain periaqueductal
12
periaqueductal grey
12
endocannabinoid system
12
release
9
pag
9
endocannabinoids control
8
grey pag
8
endogenous analgesic
8

Similar Publications

Want AI Summaries of new PubMed Abstracts delivered to your In-box?

Enter search terms and have AI summaries delivered each week - change queries or unsubscribe any time!