This thesis is comprised of three parts. In the first part, we investigated zyklophin, a novel selective short-acting kappa opioid receptor (KOPR) antagonist, and its effects on scratching behaviors in Swiss-Webster mice. We investigated whether zyklophin was able to induce scratching in a dose-dependent fashion, and whether this scratching behavior could be blocked by pretreatment with nor-binaltorphimine (norBNI). We also used KOPR -/- mice to further clarify the role of the KOPR in this behavior. In the second part, we examined the role of the divergent amino acid at position 6.58 in the mu opioid receptor (MOPR) and the KOPR on the binding of beta-funaltrexamine ß-FNA). ß-FNA is an irreversible antagonist at the MOPR and a reversible agonist at the KOPR. Utilizing the recently published crystal structures of the MOPR and KOPR, we collaborated with Dr. Lei Shi, who employed molecular modeling to choose a residue in transmembrane helix 6 (TM6) to mutate at the same position in MOPR and KOPR. We then characterized the mutants by performing [3H]diprenorphine binding, competition binding by unlabeled β-FNA, irreversible ß-FNA binding and [35S]GTPγS binding. In the third part, we investigated the concept of functional selectivity, or ligand bias, at the KOPR. We studied 23 different KOPR agonists in vitro using [35S]GTPγS binding as a measure of G protein activation and the on-cell Western (OCW) as a measure of ß-arrestin-mediated receptor internalization at the human KOPR (hKOPR), and from the results, chose 13 ligands to study at the mouse KOPR (mKOPR). We then selected biased ligands from the in vitro mKOPR results and studied their effects on scratching behavior, inhibition of pain behaviors and dysphoria as measured by the conditioned place aversion (CPA) test. We predicted that the G biased ligand would produce analgesia and anti-scratching effects at lower doses than would produce aversion in the CPA test, since analgesia has been shown to be G protein mediated and CPA has been shown to be arrestin mediated. Our first set of studies revealed that zyklophin (0.1, 0.3 and 1 mg/kg, s.c., behind the neck), induced vigorous scratching in a dose-dependent manner. 0.3 mg/kg zyklophin induced 150 scratches over a 30 minute period. The scratching was not blocked by pretreatment with 20 mg/kg norBNI (i.p.) 18-20 hours before injection of 0.3 mg/kg zyklophin s.c. in the nape of the neck. The scratching also persisted in KOPR -/- mice, in which the absence of the KOPR was confirmed by [3H]U69,593 binding (2 nM). In our second set of studies, we mutated the lysine at position 303 in the MOPR to glutamic acid (K303E), and the glutamic acid at the equivalent position in the KOPR to lysine (E297K). We transfected these mutant receptors into mouse neuroblastoma (N2A) cells. We found that the mutations had no effect on [3H]diprenorphine binding affinity or competition binding with [3H]diprenorphine and β-FNA indicating a functional intact opioid receptor. The mutations also did not affect [35S]GTPγS binding EC50 or Emax values. The mutation K303E in the MOPR reduced irreversible binding by 2/3 compared to the wildtype MOPR. Finally, we found that there were several ligands that displayed bias at the hKOPR and the mKOPR. At the hKOPR, using dynorphin A as the reference ligand to calculate bias, ICI-199441 was the only G biased ligand, while enadoline, nalbuphine, pentazocine, salvinorin A, tifluadom and butorphanol were arrestin-biased. At the mKOPR, only salvinorin B methoxymethyl ether (MOM-SalB) was G-biased, and salvinorin B ethoxymethyl ether (EOM-SalB), ICI-199441, U50,488H, nalfurafine and 12-epi-salvinorin A (12epiSalA) were ß-arrestin-biased. Enadoline and salvinorin A were slightly arrestin biased with respect to dynorphin A. From the in vitro data at the mKOPR, we selected MOM-SalB as our G biased ligand, U50,488H as our arrestin biased ligand and additionally chose to investigate nalfurafine due to its use in clinical studies. We hypothesized that U50,488H and nalfurafine would produce aversion at lower doses than analgesia or anti-pruritic effects. We found that nalfurafine was the only ligand studied to have a separation between doses that produced analgesia and anti-scratching effects, with A50 values of 5.8 and 8 μg/kg, respectively, and only produced significant dysphoria at a dose of 20 μg/kg. U50,488H and MOM-SalB produced dysphoria at all doses tested (0.25-10 mg/kg and 0.01-0.3 mg/kg, respectively). U50,488H produced a dose-dependent analgesia and anti-scratching with A50 values of 0.58 mg/kg and 2.07 mg/kg, respectively. MOM-SalB was more potent than U50,488H in producing dose-dependent analgesia and anti-scratching, with A50 values of 0.017 mg/kg and 0.070 mg/kg, respectively. Therefore, we concluded that the in vitro bias is not able to accurately predict in vivo behaviors, and nalfurafine is the first selective full agonist at the KOPR to show ligand bias in vivo. / Pharmacology

Multiple classes of pharmaceuticals, including acetaminophen, aspirin, and other nonsteroidal anti-inflammatory drugs (NSAIDs), are used to relieve mild to moderate pain; however, one of the oldest classes of pharmaceuticals, opioids, remains the primary class of drugs used in the management of severe pain. For decades, the unique pharmacological profiles of opioid compounds have suggested the existence of multiple opioid receptor subtypes and, accordingly, four opioid receptors have been cloned to date; the mu (μ)-opioid receptor, the kappa (κ)-opioid receptor, the delta (δ)-opioid receptor, and the nociceptin/orphanin FQ receptor. Additionally, each receptor is encoded by its own distinct gene; the OPRM1, OPRK1, OPRD1, and OPRL1, respectively. Despite the identification and characterization of these four opioid receptor subtypes, pharmacological data, particularly from opioid receptor knockout mice, does not conform to the predications of a four opioid receptor model and instead suggests the existence of additional receptor subtypes. Additional opioid receptors have since been proposed but corresponding genes have either been unidentified or found to be genetically unrelated. Interestingly, this problem is not unique to opioid receptors, as there is a large discrepancy between the number of protein encoding genes and the repertoire of mRNA transcripts and encoded proteins they produce, with gene products far more numerous than estimates would predict. It is now understood that this discrepancy is due to the generation of multiple RNA transcripts from a single gene. Several mechanisms are utilized in order to generate mRNA transcript variants, or isoforms, from a single gene; however, the primary mechanism, known as alternative splicing, involves a complex macromolecular machine, referred to as the spliceosome, through which specific portions of the precursor mRNA (pre-mRNA) sequence are selectively removed and the remaining nucleotide sequences are ligated to form a unique mRNA transcript. Recently, multiple opioid receptor isoforms, particularly for the μ-opioid receptor, have been identified; however, both their regulation and their functional significance are poorly characterized. As such, multiple studies are needed to more precisely describe alternatively spliced μ-opioid receptor isoforms, particularly the regulation of spliceosome components that determine the splicing specificity of particular isoforms as well as the distinct signaling pathways utilized by particular isoforms both constitutively and following agonist binding. Using a model of dopaminergic neurons, this study sought to examine these questions and found that expression of a particular splice variant, MOR-1X, was up-regulated by morphine through a mechanism involving the essential splicing factor ASF/SF2. Structural comparison of this isoform to the prototypical variant MOR-1 found that the unique distal portion of C-terminal domain contains two additional PKA phosphorylation sites as well as a second agonist-induced phosphorylation motif highly conserved among opioid receptors. Functional comparison of MOR-1 and MOR-1X found distinct signaling differences, both constitutively and following morphine treatment, in MAPK signaling cascades, particularly ERK1/2. While the pharmacological significance of MOR-1X expression and signaling remains unclear, the clinical importance of this finding extends beyond a mechanism of opioid analgesic variability, as the physiological roles of opioids also include immunomodulation and have been implicated specifically in the exacerbation of HIV viral replication and pathology, particularly neurocognitive dysfunction. Accordingly, the HIV viral protein Tat was found to block morphine-mediated increases in MOR-1X expression by similarly blocking morphine-mediated increases in ASF/SF2 expression. Consequently, MOR-1X and HIV viral proteins were found to have a unique and synergistic role in the regulation of intrinsic apoptotic signaling cascades, specifically Bax expression, and in cell proliferation. Therefore, the regulation of alternative splicing events by both opioids and HIV viral proteins involves, in part, the inverse regulation of ASF/SF2 protein expression, through which the expression of the MOR-1X isoform is subsequently and significantly altered. This, in turn, may lead to functional consequences in opioid pharmacokinetics as well as in opioid-related pathology, such as the exacerbation of HIV associated neurocognitive dysfunction, as MOR-1X contains unique functional regions which may be responsible for the observed differences in MAPK and intrinsic apoptotic signaling and cellular proliferation. Collectively, these findings support previous studies that suggest alternative splicing of the MOR is altered by exogenous factors, such as morphine and HIV, identify unique signaling pathways for various opioid receptor isoforms, and are the first to suggest a potential mechanism through which pharmacological interventions could be utilized to alter opioid receptor isoform expression, thereby altering the pharmacological and physiological effects of opioids. / Biomedical Neuroscience

Chemokines, immune proteins that induce chemotaxis and adhesion, and their G-protein coupled receptors distribute throughout the central nervous system (CNS), regulate neuronal patterning, and mediate neuropathology. These chemo-attractant molecules may provide a neuro-immune "link" by regulating CNS systems. The purpose of this study was to investigate the interactions of specific chemokines, stromal cell-derived factor (SDF)-1a/CXCL12, and fractalkine/CX3CL1, and their receptors, CXCR4 and CX3CR1, with the serotonin (5-hydroxytryptamine; 5-HT) and opioid systems using anatomical and electrophysiological techniques in the rat brain. In the serotonin dense midbrain raphe nuclei (RN), SDF-1a, CXCR4, fractalkine and CX3CR1 co-localize over 70% with 5-HT neurons. CX3CR1 also localizes to microglia in the RN and hippocampus. Functionally, SDF-1a (10 nM) increases spontaneous inhibitory postsynaptic current (sIPSC) frequency and evoked IPSC (eIPSC) amplitude, while decreasing paired-pulse ratio (PPR) selectively in 5-HT neurons, thus stimulating presynaptic GABA release at these neurons. Alternatively, fractalkine (10 nM) increases sIPSC and eIPSC amplitude without changing PPR selectively in 5-HT neurons, thereby elevating the postsynaptic GABA receptor number or sensitivity. These results are dose-dependent and receptor-mediated. Chemokine interactions with serotonin, a neurotransmitter regulating mood, may lead to therapies for depression comorbid with immune diseases. Additional immunohistochemical analysis in the brain shows CXCR4 and CX3CR1 neuronal co-localization with the mu-opioid receptor (MOR) in the hippocampus, cingulate cortex, periaqueductal grey (PAG), nucleus accumbens, ventral tegmental area, globus pallidus, but not in the striatum or habenular nuclei, suggesting region specific receptor interactions. Electrophysiological recordings following morphine, SDF-1?? or fractalkine in vitro treatment reveal morphine (10 ?M)-mediated hyperpolarization of the membrane potential and reduction of the input resistance of PAG neurons, however, SDF-1??and fractalkine at 10 nM do not impact either parameter. In combination, SDF-1? inhibits morphine's actions in all PAG neurons tested, and fractalkine blocks morphine-mediated changes in 60% of PAG neurons examined. Thus, CXCR4 as well as CX3CR1, although less consistently, both appear to desensitize MOR at the neuronal level. Chemokine-opioid receptor interactions may mediate novel mechanisms to treat neuro-inflammatory pain and opiate abuse. The combined anatomical and electrophysiological results support chemokines as neuromodulatory proteins that may provide communication between the nervous and immune systems. / Anatomy

No / The aim of the present study was to investigate an opioid receptor involvement in the adaptation response to motion sickness in Suncus murinus. Different groups of animals were treated intraperitoneally with either saline, morphine (0.1 and 1.0 mg/kg), naloxone (1.0, 10.0 and 5.0 mg/kg) or a combination of naloxone plus morphine in the absence or 30 min prior to a horizontal motion stimulus of I Hz and 40 mm amplitude. For the study of adaptation, different groups received saline on the first trial, and in subsequent trials (every 2 days) they received either saline, naloxone (1.0 and 10.0 mg/kg, ip) or morphine (0.1 mg/kg, ip) 30 min prior to the motion stimulus. Pretreatment with morphine caused a dose-related reduction in emesis induced by a single challenge to a motion stimulus. Pretreatment with naloxone alone did not induce emesis in its own right nor did it modify emesis induced by a single challenge to a motion stimulus. However, pretreatment with naloxone (5.0 mg/kg, ip) revealed an emetic response to morphine (P<.001) (1.0 mg/kg, ip) and antagonised the reduction of motion sickness induced by morphine. In animals that received saline or naloxone (1.0 mg/kg), a motion stimulus inducing emesis decreased the responsiveness of animals to a second and subsequent motion stimulus challenge when applied every 2 days for 11 trials. However, the animals receiving naloxone 10.0 mg/kg prior to the second and subsequent challenges showed no significant reduction in the intensity of emesis compared to the first trial. The data are revealing of an emetic potential of morphine when administered in the presence of a naloxone pretreatment. The administration of naloxone is also revealing of an additional inhibitory opioid system whose activation by endogenous opioid(s) may play a role in the adaptation to motion sickness on repeated challenge in S. murinus.

Opium, the dried resin obtained from the unripe seedpods of the poppy flower, has been used for medicinal and euphoric purposes since ancient times. Morphine, the main active ingredient of opium, and other clinically useful opioid analgesics all mediate their effects through activating the mu opioid receptor. Studies involving the mu opioid receptor knockout mice showed that the interaction with the mu opioid receptor is also responsible for many notorious side effects associated with these drugs including dependence and addiction. Therefore, selective antagonists for the mu opioid receptor are needed to study its function in drug abuse and addiction. Previously, based on molecular modeling studies and the “message-address” concept, a series of 14-O-substituted naltrexone derivatives were designed and synthesized. These compounds carried an ester-linked heteroaromatic substitution at the 14-position of naltrexone which was designed to interact with the putative “address” site, that was identified in the mu opioid receptor through molecular modeling studies. The lead compound of this series was determined to have a high affinity and selectivity for the mu opioid receptor. Because the 14-O-substituted naltrexone derivatives were not very stable, the ester linkage in these compounds was replaced by an amide one and a series of 14-N-substituted naltrexone derivatives were synthesized. The affinity and selectivity of these novel naltrexone derivatives were determined in a competitive radioligand binding assay. Interestingly, the 14-N-substituted naltrexone derivatives did not maintain the high selectivity of the 14-O-substituted series. It was hypothesized that the conformational constraint introduced by the amide linker was detrimental to the mu opioid receptor selectivity. Therefore, three 14-N-substituted naltrexone derivatives which carried more flexible linkages were synthesized and evaluated. The mu opioid receptor selectivity was not recovered by introducing rotational freedom into the linker. Some of these 14-N-substitued naltrexone derivatives were determined to be mu-kappa opioid receptor dual selective antagonists. Since the mu opioid receptor antagonists are effective at treating drug addiction, while growing evidence suggests that the kappa opioid receptor antagonists may be beneficial in lowering drug cravings, these novel mu-kappa opioid receptor dual selective antagonists may find unique clinical utility in the treatment of opioid dependence.

While the classical function of myelin is to facilitate saltatory conduction, this membrane and the myelin-making oligodendrocytes (OLGs) are now recognized as regulators of plasticity and remodeling in the central nervous system (CNS). Thus, OLG maturation and myelination are highly vulnerable processes along CNS development. We previously showed that rat brain myelination is altered by perinatal exposure to buprenorphine, an opioid analogue in clinical trials for the treatment of pregnant opioid addicts. We now found that the in vivo effects on myelination could result from direct alteration in the balance between μ-opioid receptor (MOR) and nociceptin/orphanin FQ receptor (NOPR) activities in the OLGs. Furthermore, we found that myelination could also be affected by the FDA-approved methadone. A delicate balance between MOR and NOPR signaling may play a crucial role timing OLG maturation and myelin formation and exogenous opioids may disrupt this interplay, altering the developmental pattern of brain myelination.

Morphine is an opioid agonist, which can exert cardioprotective effects under certain conditions. Lipid rafts are considered important platforms for membrane organization of signaling proteins and, therefore, these structures could play a role in the effects of morphine, which acts through the opioid receptors. The aim of this thesis was to investigate the distribution of the main components of the opioid receptor and Gi/o-mediated signaling pathway in lipid rafts isolated from rat myocardium, which was affected by various doses of morphine. Because we used different isolation techniques with different solubilization agents (Triton X-100, CHAPS, cholate and sodium carbonate) for preparation of lipid rafts, it was of interest to characterize more closely these preparations. Another aim of this study was to investigate how different methods of isolating these structures affect activity of the key target enzyme of the opioid signaling pathway, i.e. adenylyl cyclase. The presence of signaling molecules of the Gi/o/AC pathway of the opioid system in membrane rafts was confirmed and the distribution of selected proteins was dependent on the type of extractant. We also observed the effect of morphine on the localization of proteins in lipid rafts. Different extractants provided different degree of...

Selective Mu Opioid Receptor (MOR) antagonists possess immense potential in the treatment of opioid abuse/addiction. Utilizing the “message-address” concept, our laboratory reported a novel, reversible, non-peptide MOR selective antagonist 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4՛-pyridyl)carboxamido]morphinan (NAP). Molecular modeling studies revealed that the selectivity of NAP for the MOR is because of a π-π stacking interaction of its pyridine ring with the Trp318residue in theMOR. Pharmacological characterization showed that NAP is a P-glycoprotein substrate, thereby limiting its use in the treatment of opioid abuse/addiction. Thus, to modify NAP, we replaced the pyridine ring with its isosteric counterpart thiophene. Isosteric replacement could lead to development of compounds with different pharmacologic properties. Additionally, exploring other ring systems would diversify and enrich our library of compounds and aid in establishing a comprehensive structure-activity relationship. Therefore, newly synthesized compounds included thiophene derivatives of 6α/β-naltrexamine with potential to be used in the treatment of opioid abuse/addiction. Preliminary in vivo screening revealed that compounds 8 and 11 could be acting as antagonists. To aid in the design and synthesis of newer generation of MOR selective analogs, a 3-Dimensional Quantitative Structure-Activity Relationship (3D-QSAR) Comparative Molecular Field Analysis (CoMFA) on 6β-N-heterocyclic substituted naltrexamine derivatives was conducted. After rigorous optimizations, the best CoMFA model possessed low predictive power. Results obtained suggested that small structural changes could lead to significant change in binding modes of these ligands. To further validate this observation, molecular docking studies were performed which revealed that these ligands indeed possessed multiple distinct binding modes thereby offering rationale for the CoMFA results. Thus, overall this study furnished useful information about the complexity of protein-ligand interactions which will aid in designing more potent and selective MOR ligands.

Proper autonomic regulation of mammalian cardiac function is dependent upon very complex and precise communication among the intracardiac ganglia and individual neurons within the ganglia. An array of neuromodulators is found within the ganglia that direct neuronal activity by modulating the movement of calcium. The current study determines that opioidergic agonists, which have been found to contribute to severe cardiac disease states and intracellular calcium mobilization, are also responsible for changes in the function of the intracardiac neuron via their effects on store-operated calcium channels (SOCs). Previous studies suggest that phosphorylation plays a role in SOC regulation. Using Fura-2 calcium fluorometry, we determined that protein kinase A (PKA), protein kinase C (PKC), and cyclic adenosine monophosphate (cAMP) had no effect on store-operated calcium entry in the presence of antagonists, phorbol 12, 13 dibutyrate (PDBu), forskolin, and 8-Br cAMP, respectively. We also found pharmacologically that using both electrophysiology and calcium imaging that μ-opioid agonists, met-enkephalin (ME) and endomorphin (EM) depress SOC activity in intracardiac neurons. Arachidonic acid (AA), which has been found to depress SOC function in rat liver cells and μ-opioid receptor activation (MOR), blocked both store-operated calcium entry (SOCE) and the calcium release-activated current (ICRAC) significantly. Contrastingly, AA metabolites, prostaglandin E2)(PGE2) and prostaglandin D2 (PGD2), do not significantly influence SOCE which suggests that the effects of AA may be direct. The block elicited by EM was partially reversed by pertussis toxin (PTX), indicative of activation of a PTX-sensitive G-protein following MOR activation. Similarly, PLA2 inhibitors, OBAA and AACOCF3, decreased the percent block of SOCE due to opioid agonist-induced inhibition. Using the perforated-patch method of I-clamp electrophysiology, we demonstrated that gadolinium, at low micromolar concentrations, reversibly reduced action potential firing. Importantly, these results suggest that SOCs may influence action potential firing in mammalian intracardiac neurons. Similarly, AA and EM depressed action potential firing. Taken together, these experiments suggest that a pathway involving EM and AA influences repetitive firing through SOC inhibition. The importance of SOCs in the maintenance of action potential firing and more specifically, the expression and biophysical functionality of the individual pore-forming subunits (Orai1, 2, and 3) in any neuronal cell type has previously not been explored. Quantitative RT-PCR along with I-clamp electrophysiology revealed that Orai3 was exclusive to repetitively firing neurons. As a result, we hypothesize that robust Ca2+-dependent fast inactivation, also associated Orai3, is a factor in the maintenance of repetitive action potential firing. Using Fura-2 calcium fluorometry and patch-clamp electrophysiology, we determined pharmacologically that μ-opioid receptor activation precedes an intracellular cascade that is dependent on a PTX-sensitive G-protein and AA but independent of prostaglandin and protein kinase activity. Finally, we used RT-PCR to determine the Orai subunits expressed in the intracardiac neurons and their influence on neuronal firing patterns. This study is the first to determine the role expressed subunits has in the maintenance of the electrical activity of the neuron.

My PhD research studies, described in this thesis, were designed to document the temporal development of mechanical allodynia, a hallmark symptom of painful diabetic neuropathy (PDN), as well as opioid hyposensitivity using two different rat models of diabetes mellitus (DM). Specifically, the studies were conducted using the streptozotocin (STZ)-diabetic rat model of chemically-induced Type 1 diabetes in two different rat strains, as well as the Zucker Diabetic Fatty (ZDF) rat genetic model of Type 2 diabetes. Additionally, a longitudinal investigation of the effect of basal insulin replacement therapy to restore euglycaemia from 7-days post-STZ administration, on the development of mechanical allodynia in the hindpaws of the STZ-diabetic Wistar rat model of PDN, was conducted. The studies herein also included a longitudinal study to document the temporal development of mechanical allodynia and opioid hyposensitivity in the ZDF rat, which also examined the influence of dietary composition on the time course for the development of mechanical allodynia in the hindpaws, together with opioid hyposensitivity in these animals. In the final section of this thesis, the experiments were designed to examine possible mechanisms that may contribute to the development of opioid hyposensitivity in ZDF diabetic rats. These experiments involved the quantification of opioid receptor messenger ribonucleic acid (mRNA) gene expression as well as μ-opioid receptor (MOP-r) functional responses in tissues collected from 29-wk old diabetic ZDF rats relative to 7-wk old pre-diabetic control ZDF animals. In Chapter One, diabetes mellitus and more specifically its longterm complication, PDN, the focus of this doctoral research program, has been reviewed. Specifically, possible pathogenic mechanisms underlying mechanical allodynia, the relevant diabetic rodent models of PDN, use of insulin replacement therapy in diabetic rodents and its impact on hallmark symptoms of PDN, role of opioid pharmacology, the comparative efficacy of opioids in the treatment of PDN, and possible mechanisms that may underpin the development of opioid hyposensitivity in PDN, including the impact of altered excitatory neurotransmitters, have been reviewed. In Chapter Two, a preliminary study was conducted to investigate the efficacy of 4-wks treatment with Linplants (subcutaneous (s.c.) sustained-release bovine insulin implants) alone and in combination with ActRapid® (s.c. human insulin; 0.05 U to 3.5 U/100 g/day) with respect to glycaemic control in STZ-diabetic Wistar rats, and on acute diabetes characteristics for a 5-wk post-STZ administration period. Briefly, STZ-diabetic rats were divided into three groups: (1) rats which received no insulin treatment, (2) rats which were implanted with one s.c. Linplant at Day 7 post-STZ administration, and (3) rats which received one s.c. Linplant plus a once-daily injection of ActRapid® once diabetes was confirmed at 7-days post-STZ administration. The findings were that following implantation of a single Linplant at Day 7 post-STZ administration, euglycaemia was achieved in 50% of STZ-diabetic rats, with glycaemic control maintained for up to 4-wks post-implantation. Furthermore, once-daily injection of ActRapid™ to animals whose blood glucose levels (BGLs) were not well-controlled through use of Linplants alone, failed to achieve euglycaemia. It is possible that the ActRapid™ doses administered were not sufficient to achieve euglycaemia, and that increasing the doses may provide more effective glycaemic control. However, doubling the mean ActRapid™ dose from 1.63 (+ 0.3) U administered at Day 28 to 2.56 (+ 0.6) U administered at Day 34 post-STZ administration effectively only reduced BGLs by 1.3 mM to 11.6 + 1.6 mM. This suggests that although administering additional large doses of ActRapid™ to STZ-diabetic rats may eventually achieve euglycaemia, this method would presumably not be a more efficient method in achieving euglycaemia compared with the use of dosage-adjustable s.c. Linplants. Group (1) STZ-diabetic rats which were not treated with insulin developed diabetic signs including polydipsia, hyperphagia, decreased rate of body weight gain, and mechanical allodynia. Group (2) rats in which insulin treatment from 7-days post-STZ administration restored euglycaemia and reversed polydipsia and hyperphagia, were protected against the development of mechanical allodynia and reduced weight gain for the 5-wk study duration, while rats from Group (3) with incomplete glycaemic control developed levels of polydipsia, hyperphagia, reduced weight gain and mechanical allodynia intermediate between rats in Groups (1) and (2). These findings collectively suggest a direct correlation between the level of glycaemic control and the extent to which mechanical allodynia, a defining symptom of PDN, develops. In Chapter Three, the findings from the preliminary 5-wk study in Chapter Two were used to design a 24-wk longitudinal study of the temporal development of mechanical allodynia and opioid hyposensitivity in STZ-diabetic Wistar rats for comparison with the findings of a similar study previously undertaken by our laboratory using STZ-diabetic Dark Agouti rats (Nielsen et al, 2007). Additionally, this study examined the effects of tight glycaemic control achieved through the use of insulin implants as a means of potentially preventing the development of mechanical allodynia and opioid hyposensitivity for up to 24 weeks in STZ-diabetic Wistar rats. Briefly, STZ-diabetic rats were divided into 3 groups: (1) non-insulin treated STZ-diabetic Dark Agouti rats to provide comparison data with our laboratory’s previously published data in this rat strain (Nielsen et al, 2007), (2) non-insulin treated STZ-diabetic Wistar rats to examine possible between-species differences, and (3) STZ-diabetic Wistar rats which were treated with adjustable-dose s.c. Linplants from Day 7 post-STZ administration to maintain euglycaemia for the remainder of the 24-wk study period. In this 24-wk longitudinal study in STZ-diabetic rats, body weight, 24-hr water intake, paw withdrawal thresholds (PWTs) and BGLs were monitored at fortnightly intervals in all animals in order to document possible temporal changes in the development of diabetic signs and mechanical allodynia in the hindpaws respectively. STZ-diabetic rats underwent 6-wkly opioid antinociceptive testing, using single bolus doses of each of morphine and oxycodone with a 2-3 day washout period between individual opioids in order to assess the potential influence of both diabetes and glycaemic control on opioid potency in these animals. The findings demonstrate that non-insulin treated STZ-diabetic rats of both strains exhibited a decreased rate of body weight gain and polydipsia, as well as progressive development of mechanical allodynia in the hindpaws and loss of morphine potency. Importantly, STZ-diabetic Wistar rats which were treated with insulin to maintain euglycaemia from Day 7 post-STZ administration failed to develop these diabetic symptoms for the duration of the 24-wk study period, highlighting the importance of chronic hyperglycaemia in the development of mechanical allodynia and morphine hyposensitivity in the STZ-diabetic rodent model of PDN. The research described in Chapter Four involved a 22-wk longitudinal study of the development of diabetes and its longterm sensory nerve complications, viz mechanical allodynia and opioid hyposensitivity, in the ZDF rodent model of Type 2 diabetes commencing at 7-wks of age. This study also examined the influence of four different diets fed to separate groups of ZDF rats from 7-wks age, on the time course for the development of diabetes, mechanical allodynia in the hindpaws and opioid hyposensitivity in these animals. Briefly, ZDF rats were sub-divided into four dietary groups, each of which was fed one of the four following diets for 22-wks commencing at 7-wks of age, viz: (a) Purina 5008™, (b) a domestically-produced rat chow of similar composition to Purina 5008 (termed Purina Composition diet), (c) a Diabetogenic diet, or (d) Standard Rat Chow. All rats underwent once-fortnightly measurement of BGLs, body weight, 24-hr water intake, and measurement of PWTs in the hindpaws. Additionally, ZDF rats underwent opioid antinociceptive testing, similar to that previously described for STZ-diabetic rats (Chapter Three), to investigate the influence of diabetes and dietary composition on the antinociceptive potency of single bolus doses of morphine and oxycodone administered at 6-weekly intervals over a 22-wk study period. The afore-mentioned data were compared with the respective data obtained from the pre-diabetic control group of ZDF rats that were euthanised at 7-wks of age prior to the development of hyperglycaemia. The results demonstrate that the ZDF rat develops mechanical allodynia in the hindpaws and opioid hyposensitivity in a temporal fashion, in a manner similar to that previously documented for the STZ-diabetic Wistar rat model of Type 1 diabetes (Chapter Three). For the four diets assessed, there did not appear to be significant differences between dietary groups with respect to the time course and extent of development of hyperglycaemia, mechanical allodynia or opioid hyposensitivity in the ZDF rat model of PDN. The study described in Chapter Five investigated the effect of both diabetes and dietary composition on opioid receptor mRNA expression in tissue samples collected from the five groups of ZDF rats used in the behavioural studies described in Chapter Four and outlined above. Briefly, mRNA expression for each of the - (MOP), - (DOP), and - (KOP) receptors were quantified in mid-brain and spinal cord tissues prepared from 29-wk old diabetic ZDF rats maintained on one of four diets from 7-wks age, and compared with the respective expression levels in samples prepared from pre-diabetic ZDF rats euthanised at 7-wks of age. Overall, the findings suggest that diabetes does not alter opioid receptor mRNA expression in the mid-brain or spinal cord of diabetic ZDF rats at 29-wks of age relative to the corresponding levels of mRNA expression in the mid-brain and spinal cord of pre-diabetic ZDF rats at 7-wks of age. Hence, the marked reduction in the anti-allodynic potency of morphine and oxycodone observed in diabetic ZDF rats at 29-wks of age relative to that observed in pre-diabetic ZDF rats at 7-wks of age (Chapter Four) does not appear to be associated with a decrease in opioid receptor mRNA expression. In Chapter Six, the effect of both advanced diabetes and dietary composition on opioid-agonist stimulated [35S]GTPγS binding was examined in spinal cord tissue membranes from the ZDF rat. Specifically, [35S]GTPγS binding assays were used to assess the ability of a -opioid ligand (DAMGO) to stimulate -opioid receptor coupling to inhibitory G proteins in homogenates prepared from spinal cord samples of 29-wk old ZDF rats maintained on one of four different diets from 7-wks age (Chapter Four), relative to [35S]GTPγS binding in homogenates prepared from spinal cord samples of pre-diabetic 7-wk old ZDF rats. As specific MOP agonist-stimulated [35S]GTPγS binding was significantly decreased in spinal cord homogenates from diabetic ZDF rats at 29-wks of age relative to that for pre-diabetic ZDF rats (7-wks), this may contribute, at least in part, to the morphine hyposensitivity observed in diabetic ZDF rats at 29-wks of age relative to the pre-diabetic ZDF group. However, closer examination of these data revealed that specific MOP agonist-stimulated [35S]GTPγS binding above basal did not differ significantly between the pre-diabetic group and the longterm diabetic group of ZDF rats. Instead, there was significantly lower basal [35S]GTPγS binding in the spinal cord of ZDF rats at 29-wks c.f. 7-wks of age. Together, the findings suggest that impaired basal G-protein function rather than impaired coupling of MOP-r to its inhibitory G-protein may, at least in part, underpin -opioid agonist hyposensitivity in 29-wk ZDF rats. Finally, Chapter 7 contains a brief description of the main conclusions and discussion of the relevance of this doctoral research project, including potential future research directions.