Year

2012

Degree Name

Doctor of Philosophy

Department

School of Health Sciences

Abstract

Second generation antipsychotics (SGAs) are a key pharmacotherapy for the treatment of schizophrenia but can cause serious metabolic side-effects, including obesity and type II diabetes mellitus. Clinical studies have associated the high risk SGA olanzapine with hyperphagia,increased abdominal adiposity, reduced physical activity and altered circulating metabolic hormone levels, i.e. leptin, ghrelin and insulin; however, the underlying mechanisms remain unclear. The hypothalamus and caudal brainstem are well-documented for their involvement in appetite and energy homeostasis. Altered neurotransmission in these regions during olanzapine treatment may contribute to metabolic dysfunction, however, the ability to examine drug effects on neural signalling in humans in-vivo is limited. Therefore, an animal model is required that mimics numerous aspects of clinical metabolic side-effects caused by olanzapine treatment. Previous pre-clinical studies have utilised a range of olanzapine dosages and treatment regimes, however the dose that best resembles the clinic is unclear.

Chapter 2 aimed to test the validity of the female rat model through its ability to mimic clinical olanzapine-induced metabolic dysfunction. Female Sprague Dawley rats were treated orally, three times daily with olanzapine (0.25mg/kg, 0.5 mg/kg, 1.0 mg/kg, 2.0mg/kg), self-administered in a sweet cookie dough pellet at 8-hourly intervals or vehicle (n=12/group) for 14-days. The dosage response in each parameter was examined in order to identify the most appropriate dose to use in the rat model. Olanzapine increased body weight (0.5, 1.0, and 2.0mg/kg olanzapine treatment groups), food intake (2.0mg/kg olanzapine) and feeding efficiency (0.5-2.0mg/kg olanzapine), but did not affect water intake. Subcutaneous inguinal (1.0-2.0mg/kg olanzapine) and intra-abdominal perirenal fat were increased (2.0mg/kg), and a trend for an increase in periovary fat mass was observed in the 2.0mg/kg olanzapine treatment group (p=0.07). Interscapula brown adipose tissue mass was unchanged. Olanzapine decreased insulin in all dosage groups (0.25mg/kg-2.0mg/kg), increased circulating ghrelin and CCK, but had no effect on peptide YY (3-36). Locomotor activity in the open field arena was reduced following olanzapine treatment (0.5-2.0mg/kg olanzapine). Together, these findings demonstrate that multiple aspects of clinically-reported metabolic side-effects associated with olanzapine treatment can be modelled in the rat. In addition, this chapter demonstrated that low, clinically-relevant dosages of olanzapine can cause metabolic effects when administered in-line with the half life of the drug (i.e. 8-hourly). Therefore, this model is a valid foundation that can give insight into the in-vivo effects of olanzapine on neurotransmission in the brain.

Following the demonstration in Chapter 2 of the metabolic side-effects of olanzapine, Chapters 3 and 4 aimed to elucidate the mechanisms underlying olanzapine-induced weight gain and insulin dysregulation by examining the central in-vivo effects of olanzapine on key neurotransmitter signals that regulate metabolic homeostasis in the hypothalamus and brainstem; including the muscarinic, melanocortinergic, GABAergic and cannabinoid systems. In Chapter 3, the dosage effects of olanzapine on muscarinic M3 receptor (M3R) binding density in the hypothalamic arcuate (Arc) and ventromedial hypothalamic nucleus (VMH), and the dorsal vagal complex (DVC) of the caudal brainstem were investigated, and relationships between changes in M3R density and metabolic hormone levels were examined. Olanzapine increased M3R binding density in the Arc and DVC (0.5-2.0mg/kg olanzapine), and the VMH (0.25-2.0mg/kg olanzapine), and decreased blood glucose levels. Changes in M3R binding density significantly correlated with plasma insulin, ghrelin and CCK, as well as food intake and body weight. Olanzapine is a potent M3R antagonist, therefore the increase in M3R density was likely to be a compensatory upregulation. Inhibition of the cholinergic pathway for insulin secretion by olanzapine’s M3R blockade may explain the hypoinsulinaemia observed in treated rats, an effect that was irrespective of dosage. The data shows that M3R blockade by olanzapine was also associated with altered levels of ghrelin and CCK. The results show for the first time that olanzapine acts on M3Rs in regions of the brain that regulate appetite and insulin secretion, and support a role for M3Rs in modulating insulin, ghrelin and CCK during olanzapine treatment, possibly via cholinergic vagal innervation of the GI tract. This study provides a novel mechanism for olanzapine’s diabetogenic and weight gain liability that can also apply independent of obesity; as olanzapine can promote the onset of diabetes in normal-weight individuals in the clinic.

Several other key neurotransmitter systems involved in energy homeostasis may also play a role in olanzapine-induced metabolic side-effects, including the potent orexigen neuropeptide Y (NPY) and anorexigenic pro-opiomelanocortin (POMC) of the melanocortinergic system. The GABAergic and endogenous cannabinoid systems are also documented for their effects on appetite and form part of the hypothalamic microcircuitry that regulates POMC and NPY neurotransmission. Chapter 4 revealed an olanzapine-induced increase in NPY (1.0-2.0mg/kg olanzapine) and reduction in POMC mRNA expression (0.5-2.0mg/kg olanzapine) in the Arc but not in the DVC. Cannabinoid CB1 receptor (CB1R) binding density decreased (Arc: 0.25-2.0mg/kg olanzapine, DVC: 0.5-2.0mg/kg olanzapine) and GAD65 mRNA expression increased (Arc and DVC: 1.0- 2.0mg/kg olanzapine) in both the Arc and DVC during olanzapine treatment. Taken together, these results demonstrate that olanzapine alters the balance of major neuronal regulators of energy homeostasis in a dose-sensitive manner that favours body weight gain. These data provide evidence to support a novel mechanism for olanzapine-induced weight gain, whereby increased NPY and enhanced inhibitory GABAergic output, through reduced CB1R density, contribute to POMC inhibition.

Collectively, this thesis is novel in revealing that changes to major neurotransmission systems involved in controlling energy homeostasis in the hypothalamus, and to a lesser extent, the brainstem, contribute to olanzapine-induced metabolic dysfunction. High expression of orexigenic NPY and GABA, together with low expression of anorexigenic POMC and CB1R density may favour positive energy balance during olanzapine treatment. The data also suggests that blockade of the M3R in the hypothalamus and brainstem by olanzapine contributes to its diabetogenic liability by causing insulin dysregulation, and to its weight gain risk by altering other metabolic hormones including ghrelin and CCK levels, possibly via disruption to vagal cholinergic innervation of the GI tract. Together, metabolic hormone imbalance, due to M3R antagonism by olanzapine, may lead to alterations in neurotransmission through interactions between the braingut axis that regulates energy homeostasis. In addition, this thesis reveals that a dosage of 1.0mg/kg olanzapine (t.i.d.) is sufficient to induce changes in most parameters measured. Overall, this thesis presents considerations for the discovery of new antipsychotic drugs with low risk of metabolic side-effects and provides direction for the experimental design of future animal modelling studies of antipsychotic effects.

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