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.
| Identifer | oai:union.ndltd.org:ADTP/279273 |
| Creators | Kathleen Otto |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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