The neuropeptide nociceptin/orphanin FQ (OFQ) was recently identified as the endogenous agonist of the orphan opioid receptor ORL<SUB>1</SUB>. In this thesis pharmacological studies on the ORL<SUB>1</SUB> receptor were advanced by the comparison of the potencies and efficacies of several ligands at the rat ORL<SUB>1</SUB> receptor stably transfected into chinese hamster ovary (CHO) cells, with those at the native receptor in the central (frontal cortex) and peripheral (anococcygeus muscle) nervous systems. Most notably, the work involved a close examination of the pharmacology of the first selective non-peptide antagonist for the ORL<SUB>1</SUB> receptor, J-113397, and in this case the work extended to the use of <I>in-vivo</I> techniques measuring effects on locomotor activity and food consumption. The work on the pharmacology of the ORL<SUB>1</SUB> receptor in the anococcygeus followed our discovery of the presence of the receptor in this tissue. Under conditions where electrical-field stimulation produced a selective activation of the sympathetic nerves, nociceptin/OFQ fully inhibited the pure adrenergic motor response. The hexapeptides Ac-RYYRWK-NH<SUB>2</SUB> (RWK) and Ac-RYYRIK-NH<SUB>2</SUB> (RIK) were potent partial-agonists whereas [Phe<SUP>1</SUP>ψ(CH<SUB>2</SUB>-NH)Gly<SUP>2</SUP>]nociceptin(1-13)NH<SUB>2</SUB> (FGNC13) and J-113397 had little or no efficacy and were competitive antagonists. This thesis reflects on the various pharmacological actions of nociceptin/OFQ, concentrating on establishing discriminatory effects for selected compounds and possible functional role of the ORL<SUB>1</SUB> receptor in the brain.

The role of the neural specific vesicular monoamine transporter (<I>Vmat2</I>) in presynaptic dopamine handling and postsynaptic dopamine transmission was investigated using a <I>Vmat2</I> mutant mouse. Expression studies using <I>in situ</I> hybridization showed that <I>Vmat2 </I>mRNA was absent in the dopaminergic cells of the <I>Vmat2</I> homozygous mutant whereas dopamine transporter expression was unchanged. Furthermore, no <I>Vmat2</I> immunopositive cells were found in the midbrain and striatum of the <I>Vmat2</I> mutant. Neurochemical analysis showed that levels of dopamine and its metabolite homovanillic acid were reduced in the homozygous mice. Following MPTP treatment, the number of remaining dopaminergic cells in the midbrain was much reduced in the <I>Vmat2</I> mutant as compared with the normal wild type suggesting impaired functioning of <I>Vmat2. </I>Presynaptic dopamine handling in the <I>Vmat2</I> mutant was investigated by <I>in vivo</I> microdialysis which revealed much lower levels of basal an amphetamine stimulated release of dopamine in the striatum, as compared to the wild type. The consequences of this hypodopaminergic transmission on gene expression in striatal neurons was investigated. The results showed that the prolonged dopamine depletion due to impaired presynaptic vesicular monoamine storage resulted in down-regulation of the expression of substance P and dynorphin and up-regulation of enkephalin mRNA expression, consistent with loss of tonic dopamine release in the striatum. The expression of the adenosine A2a receptor which is preferentially expressed in the striatopallidal output neurons was also significantly upregulated.

This thesis investigates the role of central 5-hydroxytryptamine (5-HT) in the process of adaptation to chronic stress. Central 5-HT of male rats was depleted by bilateral intraventricular (i.c.v.) infusion of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) with the use of specific re-uptake blockers to protect the dopamine (DA) and noradrenaline (NA) system. They were then exposed to different chronic stress paradigms. The effect of 5-HT depletion on social defeat was studied. Lesioned or sham-operated rats were either exposed daily for 10 days to a second larger aggressive male in the latter's home cage, or simply transferred to an empty cage (control procedure). Lesioned rats failed to show the increased freezing behaviour during the pre-defeat phase of the social interaction test characteristics of sham-operated animals. Core temperature increased during aggressive interaction in sham-operated rats, and this did not adapt with repeated stress. By contrast, stress-induced hyperthermia was accentuated in 5-HT-reduced rats as the number of defeat sessions increased. Basal core temperature was unaffected by 5-HT depletion. Heart rate increased during social defeat, but this did not adapt with repeated stress: 5-HT depletion had no effect on this cardiovascular response. Basal corticosterone was increased in lesioned rats, but the progressive reduction in stress response over days was not altered. <I>C-fos</I> expression patterns in lateral preoptic area (LPOA) and medial amygdala (Me-AMY) in rats with repeated defeat were altered by 5-HT depletion. These results suggest that 5-HT is important in the modulation of overall adaptation process in exposure to chronic stress.

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