Ingram, S. L. | Search bioRxiv

bioRxiv2022

Persistent inflammation promotes endocannabinoid release and presynaptic cannabinoid 1 receptor desensitization

Pain therapies targeting the cannabinoid system are increasing with expansion of cannabis legalization but adaptations in the endogenous cannabinoid system during inflammatory pain could limit their efficacy. Presynaptic inhibition of GABA release mediated by cannabinoid 1 receptor (CB1R) agonists in the ventrolateral periaqueductal gray (vlPAG) is markedly reduced in male and female Sprague Dawley rats after persistent inflammation induced by Complete Freunds Adjuvant (CFA). Inflammation results in increased endocannabinoid (eCB) synthesis and desensitization of presynaptic CB1Rs that is reversed by a GRK2/3 inhibitor, Compound 101. Despite CB1R desensitization, eCB activation of CB1Rs is maintained after inflammation. Depolarization-induced suppression of inhibition (DSI) in naive animals is rapid and transient, but is prolonged in recordings after inflammation. Prolonged DSI is mediated by 2-arachidonoylglycerol (2-AG) indicating reduced monoacylglycerol lipase (MAGL) activity. These adaptations within the endogenous cannabinoid system have important implications for the development of future pain therapies targeting CB1Rs or MAGL.

neuroscience↗

bioRxiv2021

Physiologically distinct neurons within the ventrolateral periaqueductal gray are not defined by mu-opioid receptor expression but are differentially activated by persistent inflammation

The ventrolateral periaqueductal gray (vlPAG) is a key structure within the descending pain modulatory pathway and an important target for opioid-induced analgesia. This area contains heterogeneous neurons with respect to neurotransmitter and receptor expression so it is difficult to define vlPAG neurons that contribute to pain and analgesia. Characterization of intrinsic membrane properties of 371 vlPAG neurons from female and male Long-Evans rats identified 4 neuron types with distinct intrinsic firing patterns: Phasic, Tonic, Onset, and Random. Phasic and Tonic neurons comprise the majority of the neurons sampled. Mu-opioid receptor (MOR) expression was determined by the ability of the selective MOR agonist DAMGO to activate G protein-coupled inwardly-rectifying potassium channel (GIRK) currents. Opioid-sensitive and -insensitive neurons were observed within each neuron type in naive rats and in rats pretreated with Complete Freunds adjuvant in a hindpaw to produce persistent inflammation. The presence of low threshold spikes (LTS) did not correlate with MOR-mediated GIRK currents indicating that MOR expression alone does not define a physiologically distinct neuron type in the vlPAG. MOR activation inhibited firing in nearly all spontaneously active neurons, both in naive and persistent inflammation conditions. CFA-induced inflammation increased Fos expression at both acute (2 h) and persistent inflammation (5-7 d) time points. However, persistent, but not acute, inflammation selectively enhanced spontaneous firing and lowered firing thresholds of Phasic neurons which was maintained in the absence of synaptic inputs. Taken together, persistent inflammation selectively activates Phasic neurons, of which only a subset was opioid-sensitive. Significance statementIntrinsic membrane properties define separate vlPAG neurons types that are functionally important. Persistent, and not acute, inflammation selectively activates Phasic firing vlPAG neurons that are not defined by MOR expression. Although the vlPAG is known to contribute to the descending inhibition of pain, the activation of a single physiologically-defined neuron type in the presence of inflammation may represent a mechanism by which the vlPAG participates in descending facilitation of pain.

neuroscience↗