Neuropathic pain (NP) is one of the most common and debilitating comorbidities of spinal cord injury (SCI). Current therapies are often ineffective due in part to an incomplete understanding of underlying pathogenic mechanisms. In particular, it remains unclear how SCI leads to dysfunction in the excitability of nociceptive circuitry. The immediate early gene c-Fos has long been used in pain processing locations as a marker of neuronal activation. We employed a mouse reporter line with fospromoter driven Cre-recombinase to define neuronal activity changes in relevant pain circuitry locations following cervical spinal cord level (C)5/6 contusion (using both females and males), a SCI model that results in multiple forms of persistent NP-related behavior. SCI significantly increased activation of cervical dorsal horn (DH) projection neurons, as well as induced a selective reduction in the activation of a specific DH projection neuron subpopulation that innervates the periaqueductal gray (PAG), an important brain region involved in descending inhibitory modulation of DH pain transmission. SCI also increased the activation of both protein kinase C (PKC)c and calretinin excitatory DH interneuron populations. Interestingly, SCI promoted a significant decrease in the activation selectively of neuronal nitric oxide synthase (nNOS)-expressing inhibitory interneurons of cervical DH. In addition, SCI altered activation of various supraspinal neuron populations associated with pain processing, including a large increase in thalamus and a significant decrease in PAG. These findings reveal a complex and diverse set of SCI-induced neuron activity changes across the pain circuitry neuraxis. Moving forward, these results can be used to inform therapeutic targeting of defined neuronal populations in NP.

No abstract

The mu opioid receptor has a distinct place in the opioid receptor family, since it mediates the actions of most opioids used clinically (e.g., morphine and fentanyl), as well as drugs of abuse (e.g., heroin). The single-copy mu opioid receptor gene, OPRM1, goes through extensive alternative pre-mRNA splicing to generate numerous splice variants that are conserved from rodents to humans. These OPRM1 splice variants can be classified into three structurally distinct types: (1) full-length 7 transmembrane (TM) carboxyl (C)-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Distinct pharmacological functions of these splice variants have been demonstrated by both in vitro and in vivo studies, particularly by using several unique gene-targeted mouse models. These studies provide new insights into our understanding of the complex actions of mu opioids with regard to OPRM1 alternative splicing. This review provides an overview of the studies that used these gene-targeted mouse models for exploring the functional importance of OPRM1 splice variants.

ID 53836 Poster Board 135Opioid overdose deaths have skyrocketed by 3.5x in the last decade, mainly due to "opioid-induced respiratory depression (OIRD)." Understanding the mechanisms underpinning this is essential towards prevention and the development of new effective interventions. Mu opioid actions, including analgesia, tolerance, addiction and OIRD, are primarily mediated through mu opioid receptors (MOR). The single-copy MOR gene, Oprm1, undergoes extensive alternative splicing to generate two classes of MOR splice variants: Exon 1-(E1) and Exon 11-(E11) associated variants. The functional relevance of these splice variants has been demonstrated in mediating the actions of various mu opioids, including analgesia, tolerance, physical dependence, and reward. However, it remains unknown how the two sets of Oprm1 splice variants influence OIRD. To address this question, we used two Oprm1 gene targeted rat models, in which E1-or E11-associated variants are floxed with loxPs and can be conditionally disrupted by Cre-loxP recombination. We measured OIRD induced by morphine and fentanyl in these rat models under awake and unrestricted conditions using Whole-Body Plethysmography (WBP) technology that can define specific changes in respiration from opioids. The results demonstrated that E1-and E11-associated variants differentially influence OIRD, with varied responses from morphine and fentanyl administrations, as well as between male and female rats. Additionally, OIRD responses in rats differed from those in mice, suggesting species differences in OIRD. Together, this study not only provides a deeper understanding of the unique role of E1-and E11-associated variants on OIRD, but also enables the development of potential therapeutic strategies to reduce unnecessary opioid-related deaths.

No abstract