Doctor of Philosophy (PhD) / The investigation into the relationship between neural plasticity in the rat forebrain associated with opiate-induced behaviours yielded two major results. The major finding of the functional neuroanatomy of acute morphine dependence was that doses of naloxone that induced hyperalgesia following a brief exposure to morphine, in previously drug-naïve rats, caused a specific induction of the inducible transcription factor (itf) proteins c-Fos and zif268 in the extended amygdala. Moreover, doses of naloxone that caused a simple reversal in morphine analgesia failed to induce itf proteins in these same brain regions. This increase in itf proteins was specific to regions of the extended amygdala that receive and process nociceptive information relayed via the spino-parabrachio-amygdaloid pathway and was not observed in other regions that are involved in supraspinal pain modulation such as the rostral ventromedial medulla and the periaqueductal gray. We also found that acute morphine increased c-Fos protein in the basolateral amygdala and the major output nucleus of the central amygdala the medial subdivision. Acute morphine also up-regulated c-Fos protein in striatal, midbrain, and hypothalamic nuclei. A unique finding of the current study was that prolonged exposure to morphine was required to induce c-Fos in these brain regions, as the subsequent administration of naloxone 30-minutes after morphine either reversed or blocked this induction. These results indicate the potential role of the amygdala in analgesia following systemic morphine and in pain facilitation during acute morphine abstinence. Investigation into the neurons and circuitry that undergo long-term neuroplasticity in response to repeated morphine exposure revealed that network-level changes in the distribution of Fos protein in the nucleus accumbens and striatum predicted both tolerance to catalepsy and psychomotor sensitisation. Drug-naïve rats became profoundly cataleptic following morphine, an effect that rats with a drug-history became tolerant. Rats with a history of morphine exposure showed an increase in stereotyped behaviours compared to drug-naïve rats. The major finding of this study was that a shift in the induction of c-Fos protein from a matrix predominance in drug-naïve rats toward a patch predominance in drug-sensitised rats in the accumbens core predicted both tolerance to catalepsy and sensitisation of oral stereotyped behaviours. Acute injection of morphine in a drug-naïve rat induced catalepsy and increased the number of c-Fos-positive neurons in matrix striatopallidal projection neurons of the rostral accumbens core. An increase in activity of striatopallidal projection neurons, which give rise to the indirect pathway, could potentially increase inhibitory drive to the pedunculopontine nucleus (PPN). The PPN, long known as a site of termination for basal ganglia output, is thought to direct the outflow of incentive-motivational and sensorimotor information from the nucleus accumbens to pons, medullary, and spinal cord nuclei translating the incentive impact of the stimuli into appropriate motor, autonomic and emotive responses (Winn et al., 1997). Inhibition of this nucleus would cause the animal to be unable to initiate a movement and in effect lock up, which is precisely what cataleptic postures look like. In contrast c-Fos-positive neurons were decreased in the rostral matrix and increased in patch striatonigral projection neurons along the rostro-caudal extent of the accumbens core when morphine was administered to drug-sensitised rats. Striatonigral neurons located in the patch give rise to the direct pathway innervating the dopaminergic neurons in both substantia pars compacta and the dopamine rich islands in the substantia nigra pars reticulata (Berendse et al., 1992; Gerfen, 1992; Furuta et al., 2002). Activity of this pathway is thought to be involved in the initiation of movement (Gerfen, 1992; Gerfen and Wilson, 1996), however, when this pathway is overstimulated as is the case when morphine is injected in drug-sensitised rats this could potentially cause increased activity of PPN neurons leading to repetitive psychomotor behaviours or stereotypy. This data adds to the growing body of evidence that suggests that long-term neuroadaptations induced by drugs of abuse including morphine that lead to behavioural sensitisation involves the circuitry that includes the nucleus accumbens.

In searching for new human pain models that more closely resemble clinical pain states, the capsaicin pain model has emerged as a viable model for both inflammatory and neuropathic pain states. A principal benefit of the capsaicin model is that it allows study of two different pain processes, primary and secondary hyperalgesia. Primary hyperalgesia is characterized by spontaneous pain and both heat and mechanical hyperalgesia. In addition, it is likely the result of activation and sensitization of both peripheral and central nociceptors. In contrast, secondary hyperalgesia is characterized by only mechanical hyperalgesia and is caused by the sensitization of central nociceptive neurons. Previous research utilizing the capsaicin pain model has primarily focused on the neural properties with little focus on the impact of affective states on capsaicin-related pain processes. The present study examined the impact of a noise stressor on both primary and secondary hyperalgesia. Results indicated that the effects of the noise stressor impacted secondary hyperalgesia, but not primary hyperalgesia.

Research into the mechanisms of hyperalgesia is ongoing with the goal of improving clinical management of chronic pain. One animal model of chronic musculoskeletal pain uses two injections of acidic saline into a lateral gastrocnemius muscle to induce a long-lasting bilateral decrease in paw withdrawal thresholds. This study tested whether the two injections need to occur in the same muscle. Male Sprague-Dawley rats were injected with acidic saline (pH 4.0) in either the lateral or medial head of gastrocnemius or the contralateral gastrocnemius (lateral head). All animals received a second injection in the ipsilateral gastrocnemius (lateral head). Mechanical withdrawal thresholds were reduced in all groups when tested 24 hours after the second injection. Animals in which the first muscle injection was substituted with a non-specific treatment (intraperitoneal injection of lipopolysaccharide) developed bilateral hyperalgesia after a single acidic saline injection. Thus, the mechanism of hyperalgesia in this model is not restricted to the injected tissues and may include central nervous system structures. Consistent with this, an inhibitor of glia cell activation (minocycline) blocked the development of bilateral hyperalgesia. These data indicate that the central nervous system may play a large role in mediating chronic musculoskeletal pain. / October 2007

No description available.

Pain is the most common and debilitating sign of a medical problem, with nearly 15 million patients suffering from chronic pain, including neuropathic pain. Widely used therapies for treating neuropathic pain include tri-cyclic antidepressants, opioids, anticonvulsants, non-steroidal anti-inflammatory agents and combinations thereof. Despite the abundance of treatments, the management of chronic pain remains difficult due to an inability for many patients to achieve appropriate pain relief at doses which are tolerable over long periods of time.Opiates (natural products), or opioids (synthetic derivatives), are considered the gold standard of analgesic care, though with little efficacy for neuropathic pain. Opioids are associated with unwanted side effects, including paradoxical pain and abuse liability that may result from several nervous system adaptations within the pain modulating neural network. These dose related side effects become more prevalent as clinicians try to overcome analgesic tolerance.Molecular mechanisms underlying these unwanted side effects have been studied extensively, and the literature purports a variety of contributing factors and neurobiological adaptations. The studies herein describe additional molecular adaptations and novel pharmacological approaches to counteract these changes. First, the contributions of neurobiological remodeling within a single receptor system (the opioid system) were investigated in the spinal dorsal horn after peripheral nerve ligation and chronic exposure to an opioid agonist in combination with an ultra-low-dose of opioid antagonist. The effects of the ultra-low-dose opioid antagonist naltrexone on the efficacy of oxycodone for neuropathic pain were investigated after both central and systemic administration.Secondly, molecular remodeling occurs across different receptor systems in the pain network, including altered regulation of pronociceptive molecules (e.g. substance P; SP). Previous studies have reported that opioid-induced hyperalgesia, tolerance and reward can be prevented by a blockade or ablation of SP activity at the neurokinin 1 receptor (NK1). We have characterized single compounds, rationally designed to act as opioid agonists and an NK1 antagonist using in vitro assays and the efficacy in vivo using rodent models of pain, antinociceptive tolerance and reward. Collectively, these studies validate the concept of targeting multiple neurobiological adaptations as a therapeutic option for neuropathic pain and reducing opioid- mediated side effects.

Sustained administration of morphine in humans and in animals induces a state of abnormal pain (i.e., hyperalgesia) which may be associated with the development of reduced analgesic efficacy (i.e., tolerance). Evidence suggests that opiate treatment may upregulate cholecystokinin (CCK), a pronociceptive peptide, in the brain and spinal cord. Therefore, we hypothesized that CCK may be upregulated by opiate treatment in the rostral ventromedial medulla (RVM) and to subsequently drive descending facilitation mechanisms to elicit hyperalgesia and antinociceptive tolerance in rats.CCK administered into the RVM of naive rats elicited hyperalgesia which was blocked by either RVM CCK2 receptor antagonist L365,260; or by bilateral lesion of dorsolateral funiculus, a major bulbospinal descending pain modulation pathway from the RVM to spinal cord.Sustained subcutaneous morphine induced hyperalgesia and spinal antinociceptive tolerance. Both effects were reversed by RVM CCK2 antagonist, suggesting that the up-regulation of the endogenous RVM CCK system played a critical role in the expression of these phenomena.Lesion of cells in the RVM which selectively express CCK2 receptors with a saporin construct (CCK-SAP) to inhibit ribosome activity, prevented morphine-induced hyperalgesia and spinal antinociceptive tolerance. These findings suggest that the integrity of the RVM CCK system is required for the development of hyperalgesia and antinociceptive tolerance induced by sustained morphine.The CCK system does not seem to play a role in setting the baseline sensory thresholds in normal rats because neither RVM L365,260 nor CCK-SAP treatment altered baseline sensory thresholds in naive rats.CCK appears to be present exclusively in nerve terminals of RVM neurons in naive rats. There was no obvious change in the levels of CCK-LI, CCK2 receptor, or CCK2 receptor mRNA in the RVM after sustained morphine treatment. However, microdialysis studies showed an approximately 5-fold increase in basal CCK levels in the RVM after sustained morphine treatment.Taken together, our results support the hypothesis that increased release of CCK in the RVM is induced by sustained morphine and drives descending facilitation to mediate morphine-induced paradoxical "pain" and spinal antinociceptive tolerance.

In searching for new human pain models that more closely resemble clinical pain states, the capsaicin pain model has emerged as a viable model for both inflammatory and neuropathic pain states. A principal benefit of the capsaicin model is that it allows study of two different pain processes, primary and secondary hyperalgesia. Primary hyperalgesia is characterized by spontaneous pain and both heat and mechanical hyperalgesia. In addition, it is likely the result of activation and sensitization of both peripheral and central nociceptors. In contrast, secondary hyperalgesia is characterized by only mechanical hyperalgesia and is caused by the sensitization of central nociceptive neurons. Previous research utilizing the capsaicin pain model has primarily focused on the neural properties with little focus on the impact of affective states on capsaicin-related pain processes. The present study examined the impact of a noise stressor on both primary and secondary hyperalgesia. Results indicated that the effects of the noise stressor impacted secondary hyperalgesia, but not primary hyperalgesia.

No description available.

Research into the mechanisms of hyperalgesia is ongoing with the goal of improving clinical management of chronic pain. One animal model of chronic musculoskeletal pain uses two injections of acidic saline into a lateral gastrocnemius muscle to induce a long-lasting bilateral decrease in paw withdrawal thresholds. This study tested whether the two injections need to occur in the same muscle. Male Sprague-Dawley rats were injected with acidic saline (pH 4.0) in either the lateral or medial head of gastrocnemius or the contralateral gastrocnemius (lateral head). All animals received a second injection in the ipsilateral gastrocnemius (lateral head). Mechanical withdrawal thresholds were reduced in all groups when tested 24 hours after the second injection. Animals in which the first muscle injection was substituted with a non-specific treatment (intraperitoneal injection of lipopolysaccharide) developed bilateral hyperalgesia after a single acidic saline injection. Thus, the mechanism of hyperalgesia in this model is not restricted to the injected tissues and may include central nervous system structures. Consistent with this, an inhibitor of glia cell activation (minocycline) blocked the development of bilateral hyperalgesia. These data indicate that the central nervous system may play a large role in mediating chronic musculoskeletal pain.

Research into the mechanisms of hyperalgesia is ongoing with the goal of improving clinical management of chronic pain. One animal model of chronic musculoskeletal pain uses two injections of acidic saline into a lateral gastrocnemius muscle to induce a long-lasting bilateral decrease in paw withdrawal thresholds. This study tested whether the two injections need to occur in the same muscle. Male Sprague-Dawley rats were injected with acidic saline (pH 4.0) in either the lateral or medial head of gastrocnemius or the contralateral gastrocnemius (lateral head). All animals received a second injection in the ipsilateral gastrocnemius (lateral head). Mechanical withdrawal thresholds were reduced in all groups when tested 24 hours after the second injection. Animals in which the first muscle injection was substituted with a non-specific treatment (intraperitoneal injection of lipopolysaccharide) developed bilateral hyperalgesia after a single acidic saline injection. Thus, the mechanism of hyperalgesia in this model is not restricted to the injected tissues and may include central nervous system structures. Consistent with this, an inhibitor of glia cell activation (minocycline) blocked the development of bilateral hyperalgesia. These data indicate that the central nervous system may play a large role in mediating chronic musculoskeletal pain.