Comparison of tolerance characteristics in the guinea pig following chronic in-vivo exposure to opioid versus cannabinoid receptor agonists

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)
Hercules Maguma (Creator)
East Carolina University (ECU )
Web Site:
David Taylor

Abstract: Few studies have compared the nature of tolerance that develops following chronic in vivo opioid treatment with that which develops after chronic cannabinoid exposure in the same tissue and species. Based on similarities in signaling and overlapping neuroanatomical receptor localization the candidate determined whether the tolerance that develops to hypothermic analgesic and inhibitory action on neurogenic contractions of the longitudinal muscle-myenteric plexus (LM/MP) in the guinea pig is qualitatively similar regardless of the agonist employed. Since previous in vitro drug exposure studies using the LM/MP model have reported bidirectional heterologous tolerance it was hypothesized: 1) that in vivo exposure to either agonist would result in heterologous tolerance 2) that the type of tolerance could be used to define the underlying cellular mechanisms; 3) that homologous tolerance would employ a mechanism that involved receptor regulation; 4) that co-localization of opioid and cannabinoid receptors may provide a basis for some cross-tolerance between agonists; and 5) that the mechanisms that underlie the development of tolerance can vary among different tissues or models. Specific aim 1 assessed the effect of chronic cannabinoid or opioid exposure on the sensitivity of the LM/MP to inhibitory agonists (WIN-55 212-2-2 CADO or morphine) or an excitatory agent (nicotine). Animals pretreated with morphine in vivo developed an increased responsiveness to nicotine and tolerance to all inhibitory agonists tested: the magnitude of rightward shift (i.e. ratio of mean IC50 values) or loss of sensitivity of the treated compared to the control group was 4.8-fold for DAMGO 3.5-fold for CADO and 5.2-fold for WIN-55 212-2. In contrast in vivo WIN-55 212-2 pretreatment resulted in subsensitivity to WIN-55 212-2 only (factor of rightward shift of the IC50 values was 9.8) and reduced maximum responses to WIN-55 212-2 and DAMGO; no shift was observed in the dose response curves for DAMGO CADO and nicotine. Specific aim 2 sought to determine the effect of chronic WIN-55 212-2 or morphine exposure on the levels of both mu-opioid receptor (MOR) and cannabinoid receptor 1 (CB1) protein in homogenates of the LM/MP. WIN-55 212-2 treatment resulted in a selective reduction in CB1 receptor protein levels by 35% while MOR levels remained unchanged whereas morphine exposure altered neither MOR nor CB1 receptor protein levels. Specific aim 3 sought to determine the qualitative nature of tolerance that develops in analgesic (thermal and mechanical) and hypothermic models. Chronic morphine treatment resulted in heterologous tolerance to the thermal analgesic effect of morphine and WIN-55 212-2 but did not alter the sensitivity to the hypothermic effect of WIN-55 212-2. The nature of tolerance observed in the hot plate test corresponds closely to that observed in the LM/MP studies where chronic morphine treatment produced heterologous tolerance and WIN-55 212-2 pretreatment resulted in homologous tolerance. In contrast to the results in the LM/MP studies WIN-55 212-2 pretreatment resulted in tolerance to the analgesic effect of morphine in the paw pressure model despite the fact that WIN-55 212-2 did not produce analgesia in this model. Unlike chronic treatment with WIN-55 212-2 chronic morphine treatment did not induce tolerance to the hypothermic effect of WIN-55 212-2. However since only a very modest hypothermia was observed in response to a morphine challenge tolerance to this effect was difficult to assess and may not be pharmacologically relevant. For specific aim 4 the candidate explored the distribution of MOR and CB1 receptor expressing neurons in the LM/MP and hypothalamus. Immunofluorescence assessment of the distribution of neurons expressing MOR and CB1 receptors in the LM/MP revealed significant co-localization of receptors on myenteric plexus neurons thus raises the possibility of intracellular crosstalk between the two receptor systems. Furthermore neither opioid nor cannabinoid treatment altered the density or distribution pattern of neurons expressing MOR or CB1 receptors. Assessment of neurons expressing MOR and CB1 receptors in the preoptic anterior hypothalamus revealed extensive co-localization suggesting possible interaction of the two receptor systems in the regulation of body temperature. In conclusion the variable tolerance expression observed in different models affirms the notion that nature and potential cellular mechanisms of tolerance can vary depending on the model system the drug the species and regimen used to establish the phenomenon. The data also suggest that multiple cellular effects may play a role in the induction of functional tolerance in different model systems. 

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Date: 2012
Health Sciences, Pharmacology, Biology, Neuroscience

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