My studies described in this thesis have documented aspects of the comparative in vivo pharmacology of oxycodone and morphine in rats, as well as examining the in vitro binding profile of oxycodone in rat brain homogenate preparations. Specifically, my studies have documented the temporal profile for the development of antinociceptive tolerance to oxycodone as well as the extent of cross-tolerance with morphine in rats. The comparative potency and efficacy of oxycodone and morphine administered by each of the subcutaneous and spinal routes, for the relief of tactile (mechanical) allodynia in a rat model of neuropathic pain was also addressed. My final series of experiments documented the radioligand binding profile of oxycodone in rat brain homogenate that had been depleted of µ- and δ-opioid receptors relative to that determined in rat brain homogenate with its full complement of µ- and δ-opioid receptors.
In Chapter 1, I have reviewed previously published studies relevant to the aspects of the pharmacology of oxycodone and morphine that have been addressed by my experiments described in Chapters 2-7 of this thesis. In particular, my literature review provides a brief overview of the nociceptive signalling system and the endogenous pain relief system. I have also reviewed the pharmacology of morphine and oxycodone as well as studies that have addressed the aetiology of antinociceptive tolerance and cross-tolerance to opioids. In the final section, I have reviewed neuropathic pain and the use of opioids for its symptomatic relief.
In Chapter 2, my investigation of the temporal profile for the development of antinociceptive tolerance to oxycodone administered by continuous intravenous (i.v.) infusion to adult male Dark Agouti (DA) rats, relative to that for equipotent doses of morphine, is described. Following continuous i.v. infusion of either oxycodone in doses of 2.5 and 5.0 mg/24 or morphine in doses of 10 to 20 mg/24 h, all rats developed complete antinociceptive tolerance (assessed using the tail flick test), in 48 and 72 h, respectively. Based on the magnitude of antinociception produced by each of oxycodone and morphine, together with their respective time courses for tolerance development, oxycodone was found to be ≈ 4 times more potent than morphine for the relief of acute pain induced by the application of noxious heat to the rat's tail.
In Chapter 3, my experiments in rats showed that there is incomplete and asymmetric cross-tolerance between parenteral oxycodone and morphine such that the extent of cross-tolerance observed between these two opioids was dependent upon the order of drug administration. Specifically, the degree of antinociceptive cross-tolerance (% CT) for i.v. morphine administered to oxycodone-tolerant rats (≈ 71 %CT) was found to be ≈ 3 times greater than that for i.v. oxycodone administered to morphine-tolerant rats (≈ 24 %CT). Comparison of the ED50 values for bolus doses of each of i.v. oxycodone (285.3 nmol) and i.v. morphine (1191 nmol) in opioidnaive rats shows that i.v. oxycodone is approximately four times more potent than i.v. morphine in adult male DA rats, similar to the relative potency observed between these two opioids following administration by continuous i.v. infusion. By contrast, comparison of the ED50 values estimated for icv oxycodone (74.9 nmol) and icv morphine (37.3 nmol) indicate that icv oxycodone is approximately half as potent as icv morphine when administered to opioid-naive adult male DA rats. This finding is similar to that reported in a previous study from our laboratory (Leow and Smith, 1994) in which icv oxycodone was found to be ≈ 0.44 times as potent as icv morphine in adult male Sprague-Dawley (SD) rats.
My investigations described in Chapter 4 showed that there was a complete absence of cross-tolerance between supraspinally administered oxycodone and systemically administered morphine in the adult male DA rat, providing additional evidence to support earlier work from our laboratory (Ross and Smith, 1997) that the intrinsic antinociceptive effects of oxycodone are mediated by a distinctly different population of CNS opioid receptors (putative κ-opioid receptors) relative to morphine, the prototypic µ-agonist. By contrast with the lack of crosstolerance between icv oxycodone and i.v. morphine, there was a relatively high degree of crosstolerance (≈ 54 %CT) between icv morphine and i.v. oxycodone in oxycodone-tolerant rats. Collectively, these findings suggest that after parenteral administration, oxycodone is metabolized to a µ-opioid agonist metabolite, thereby producing cross-tolerance with subsequently administered morphine.
My studies described in Chapters 5 and 6 compared the anti-allodynic potency and efficacy of oxycodone and morphine administered by each of the subcutaneous (s.c. Chapter 5) and intrathecal (i.t.. Chapter 6) routes in a rat model of neuropathic pain, viz rats with a chronic constriction injury (CCI) of the sciatic nerve. The potency of s.c. oxycodone, but not s.c. morphine, was increased ≈ 2-fold in the CCI-rat model of neuropathic pain relative to that determined in opioid-naive control (non-injured) rats for the alleviation of tactile allodynia, such that the anti-allodynic potency of oxycodone (ED50 = 2.1 µmol/kg) was ≈ 5 times higher than that of morphine (ED50 ≈ 11.2 µmol/kg). Following i.t. administration, both oxycodone and morphine produced dose-dependent alleviation of tactile allodynia in CCI-rats, such that i.t. morphine (ED50 ≈ 10.0 nmol) was ≈ twice as potent as oxycodone (ED50 = 17.2 nmol). However, both opioids were only partially effective, with i.t. morphine being ≈ 20% more efficacious than i.t. oxycodone.
As expected, i.t. naloxone completely reversed the anti-allodynic and antinociceptive effects of i.t. morphine in the ipsilateral and the contralateral hindpaws of the CCI-rat, respectively. Unexpectedly however, i.t. naloxone administered at the time of i.t. oxycodone's peak effects, did not significantly attenuate oxycodone's anti-allodynic actions, in contrast to a previous study from our laboratory (Leow and Smith, 1994) whereby icv naloxone completely reversed icv oxycodone's antinociceptive effects in non-injured rats, when assessed using the tail flick test. However, as i.t. administration of the muscarinic antagonist, atropine, resulted in a marked attenuation of the anti-allodynic effects of oxycodone in the ipsilateral hindpaw of the CCI-rat, these findings implicate a cholinergic mechanism, possibly involving an enhancement of acetylcholine (ACh) release in the spinal cord, in the anti-allodynic effects of i.t. oxycodone. Clearly, additional studies beyond the scope of this thesis, are required to more fully address this issue.
In Chapter 7,1 undertook a series of radioligand binding experiments designed to gain additional insight into the opioid receptor mechanism underpinning oxycodone's pain-relieving effects. Collectively, my results show that oxycodone binds to two populations of opioid receptors in whole DA rat brain homogenate containing its full complement of µ, δ and κ-opioid receptors, irrespective of whether [3H]-bremazocine, [3H]-DAMGO or [3H]-naloxone was used as the radioligand. Indeed, the higher affinity population of oxycodone binding sites was present in approximately twice the density of the lower affinity population, irtespective of the radioligand utilized.
In DA rat brain homogenate that had been depleted of µ- and δ-opioid receptors, oxycodone also bound to two populations of opioid receptors, with one population being relatively high affinity (Ki-1 = 20-30 nM) and the other being low affinity (Ki-2 > 4 µM). The high affinity binding site for oxycodone was similar to that of the high affinity, high density, binding site for the putative κ2-opioid agonist, GR89,696, (Ki-1 = 25-30 nM), suggesting that oxycodone may also be a putative κ2-opioid ligand. Furthermore, based on the reportedly high affinity of leu-enkephalin (LE) for putative κ2b-opioid receptors in the rat CNS (Rothman et al, 1992) and the fact that LE completely blocked all high affinity oxycodone binding in depleted DA rat brain membranes herein, my results add to the previous findings of Ross (1997) in our laboratory, to suggest that oxycodone binds with high affinity to κ2b-opioid binding sites in the rat brain. When these findings are considered together with previous results from our laboratory showing the nor-BNI sensitivity of oxycodone's intrinsic antinociceptive effects, it is certainly plausible that oxycodone may elicit its intrinsic antinociceptive effects via activation of putative κ2b-opioid receptors in the rat brain.