Neurobiological Correlates of Pain Avoidance-Like Behavior in Morphine-Dependent and Non-Dependent Rats

Abstract

Repeated use of opioids can lead to the development of analgesic tolerance and dependence. Additionally, chronic opioid exposure can cause a paradoxical emergence of heightened pain sensitivity to noxious stimuli, termed hyperalgesia, which may drive continued or escalated use of opioids to manage worsening pain symptoms. Opioid-induced hyperalgesia has traditionally been measured in rodents via reflex-based assays, including the von Frey method. To better model the cognitive/motivational dimension of pain in a state of opioid dependence and withdrawal, we employed a recently developed non-reflex-based method for measuring pain avoidance-like behavior in animals (mechanical conflict avoidance test). Adult male Wistar rats were administered an escalating dose regimen of morphine (opioid-dependent group) or repeated saline (control group). Morphine-dependent rats exhibited significantly greater avoidance of noxious stimuli during withdrawal. We next investigated individual relationships between pain avoidance-like behavior and alterations in protein phosphorylation in central motivation-related brain areas. We discovered that pain avoidance-like behavior was significantly correlated with alterations in phosphorylation status of protein kinases (ERK, CaMKII), transcription factors (CREB), presynaptic markers of neurotransmitter release (Synapsin), and the rate-limiting enzyme for dopamine synthesis (TH) across specific brain regions. Our findings suggest that alterations in phosphorylation events in specific brain centers may support cognitive/motivational responses to avoid pain.

Measuring Pain Avoidance‐Like Behavior in Drug‐Dependent Rats

“…*The experimental procedures described in this unit occur during the dark phase of a reverse 12-hr light/dark cycle, however, it is possible to test animals during the light cycle of a 12-hr light/dark cycle (see Harte et al, 2016). There were no evident differences in behavior between control animals tested under dark conditions during the dark phase (see Pahng et al, 2017) compared to control animals tested under light conditions during the light phase (see Harte et al, 2016). …”

“…This is the order of probe presentation that was used in Pahng et al, 2017. The criteria for this semi-randomized order of probe presentation is (1) the first probe height is 0 mm, (2) the second probe height must not be greater than 2mm (as recommended by the manufacturer), and (3) the presentation of probes must be completed in non-sequential order (e.g.…”

“…Some measures for induction of alcohol intoxication in rats include intragastric intubation (Unit 9.28) (Faingold, 2008), operant ethanol self-administration with 0.1 ml fluid delivery per lever press of 10% (w/v) ethanol (Edwards et al, 2012a), and intraperitoneal administration of 1g/kg of 15% (w/v) ethanol alcohol (McGinn et al, 2016). Common procedures for the induction of opioid dependence include intravenous self-administration of opioids (Unite 9.20) (Thomsen & Caine, 2005) and subcutaneous administration (Park et al, 2015; Pahng et al, 2017). Intravenous self-administration (12-hr access) of fentanyl (2.5μg/kg/infusion; 5mg/kg/infusion), oxycodone (150μg/kg/infusion; 300μg/kg/infusion) , and heroin ( 60μg/kg/infusion) , but not buprenorphine produces a robust escalation of opioid intake (Wade et al, 2015).…”

“…Intravenous self-administration (12-hr access) of heroin ( 0.06mg/kg/infusion) produces hyperalgesia (Edwards et al, 2012b). Subcutaneous administration of an escalating dose of morphine (10mg/kg to 20mg/kg) can be used to produce hyperalgesia (Craft et al, 1999) and pain avoidance (Pahng et al, 2017). Similarly, subcutaneous administration of heroin (1.25mg/kg) and fentanyl (20–100μg/kg) produces hyperalgesia (Celerier et al, 2000; Park et al, 2015).…”

“…morphine, pregabalin) on pain avoidance behavior (Harte et al, 2016). More recently, the mechanical conflict-avoidance system was used to measure the motivation to avoid noxious stimuli during withdrawal from chronic morphine administration (Pahng et al, 2017). Compared to traditional reflex-based pain measures (e.g.…”