Effect of λ-carrageenan-induced inflammatory pain on brain uptake of codeine and antinociception

Abstract

This study investigated the potential clinical implications of λ-carrageenan-induced inflammatory pain on brain uptake of a commonly used analgesic, codeine, in relation to the fundamental properties of the blood–brain barrier (BBB) correlated to its antinociceptive profile over a 168-h time course. BBB uptake of [¹⁴C]sucrose (a membrane impermeant marker) and [³H]codeine were investigated using an in situ brain perfusion model in the rat. Results demonstrated a significantly increased brain uptake of [¹⁴C]sucrose at 1, 3, 6 and 48 h (139±9%, 166±19%, 138±13% and 146±7% compared with control, respectively) and [³H]codeine at 3 and 48 h (179±6% and 179±12% compared with control, respectively). Capillary depletion analyses ensured that increased radioisotope associated with the brain was due to increased uptake rather than trapping in the cerebral vasculature. Antinociception studies using a radiant-heat tail flick analgesia method demonstrated that λ-carrageenan-induced inflammatory pain enhanced the in vivo antinociceptive profile of i.p.-administered codeine (7 mg/kg) at 3 and 48 h (144±11% and 155±9% compared with control, respectively). This study demonstrated that brain uptake and antinociception of codeine are increased during λ-carrageenan-induced inflammatory pain, suggesting that the presence of inflammatory pain may be an important consideration in therapeutic drug dosing, potential adverse effects and/or neurotoxicity.

Introduction

The blood–brain barrier (BBB) is a selective partition between the central nervous system (CNS) and peripheral circulation that limits the passage of blood-borne substances into the CNS on the basis of proteolytic stability, molecular size, charge, lipophilicity and/or solubility [1]. The BBB is characterized by tight junctions (TJs) between brain capillary endothelial cells and a lack of fenestrations, which help the BBB maintain brain homeostasis. The TJs, which severely restrict paracellular diffusion of drugs and solutes into the brain, are composed of a network of several transmembrane and cytoplasmic proteins linked to an actin cytoskeleton [11], [18], [30]. BBB dysregulation can result in loss of maintenance of brain homeostasis, resulting in altered neuronal function and potential neurotoxicity [37]. Pathological states such as human immunodeficiency virus-1 encephalitis [10], multiple sclerosis [33], non-occlusive hypoxia [36], Alzheimer's disease [21], cerebral malaria [7] and bacterial meningitis [22] have all been associated with decreased BBB function. Recent studies indicate peripheral inflammation also changes BBB structure and permeability [18], [19]. These alterations can lead to nutritional and ionic imbalances in the CNS, resulting in dysregulation of neuronal function, and altered delivery of therapeutic agents potentially causing improper dosing regimens and neurotoxicity. In this study, we investigated the effect of peripheral inflammatory pain on the BBB in terms of therapeutic drug delivery of codeine, as altered delivery of therapeutic agents may lead to ineffectual dosing and potential neurotoxicity. Codeine was chosen for this examination because it is an analgesic drug commonly used for the treatment of inflammatory pain.

Previous studies from our laboratory have shown that peripheral inflammatory pain alters BBB function and structure, resulting in increased permeability of sucrose associated with cytostructural changes in endothelial cell TJs and the protein cytoskeleton [18], [19]. Formalin-, λ-carrageenan- and complete Freund's adjuvant-induced inflammatory pain models were shown to increase BBB permeability to sucrose and alter TJ protein expression of occludin, zonula occludens-1 (ZO-1) and cytoskeletal protein expression of actin during peak inflammation [18]. When λ-carrageenan-induced inflammatory pain was investigated over a 72-h time course, there was a biphasic increase in BBB permeability to [¹⁴C]sucrose (membrane impermeant marker), at 1–6 and 48 h. This functional change correlated with modified protein–protein interactions between ZO-1/occludin and ZO-2/actin. Furthermore, increased BBB permeability correlated with decreased occludin expression [19]. We have also shown that λ-carrageenan, when administered intravenously into the peripheral circulation, had no direct effect on BBB function or structural integrity [20]. These findings suggest that λ-carrageenan-induced inflammatory pain causes alterations to the BBB TJs. Based on these observations, previously observed peak time points of increased BBB permeability (biphasic: 3 and 48 h) were the primary focus in this investigation of codeine uptake and antinociception.

Codeine, a moderate agonist for the mu opioid receptor, is prescribed for its antinociceptive and antitussive properties [14]. Therapeutic properties are centrally mediated; thus, it is essential that codeine cross the BBB for it to reach its site of action. Due to the physicochemical properties of codeine, BBB transport occurs via passive diffusion and is primarily dependent on BBB transcellular permeability and blood-flow [4], [13], [38]. As with most opioids, high doses of codeine in the CNS produce a wide spectrum of CNS-mediated side effects, including respiratory depression, nausea, vomiting, dizziness and mental clouding [14]. It is well known that BBB integrity and P-glycoprotein expression of the opioid efflux pump affects sensitivity to opioids in terms of these side effects. For example, pre-delivery administration of a nontoxic level of morphine to an expecting mother has resulted in the newborn infant exhibiting profound respiratory depression and analgesia because of immature BBB development [34]. In addition, opioids have been associated with acute tolerance and addiction, both of which may be exacerbated by BBB alterations resulting in higher than expected CNS doses. Thus, increased codeine uptake into the brain due to BBB disruptions may have serious implications. BBB perturbations may manifest clinically through functional changes in codeine uptake into the brain with increased analgesia and/or adverse side effects, thus, establishing the clinical significance of inflammatory pain-induced alterations of the BBB.