Our laboratories have been investigating the reactivity of central dopamine D1 and D2/D3 receptors by their corresponding dopamine agonists (SKF-38393 and quinpirole), during development and aging in rats. By evaluating the number of oral movements (a parameter for D1 receptor activation after SKF-38393) and the number of yawns (as a parameter for D3 activation after quinpirole), we demonstrated that not only was there a dose-response activity for both drugs in the two parameters tested, but that the D3 activity was enhanced with the rats' development and aging, due to life-long persisting D3 supersensitivity. In the present study we checked whether D2 and D1 receptors were also sensitized at old age, by measuring behavioral parameters characteristic to D2 (locomotor activity and rearings) and to D1 (grooming time). In the long-term study, male Wistar rats were challenged for 18 months with increasing doses of either SKF-38393, quinpirole or saline. At the age of 19 months they were given a single injection of either drug or saline. In the short-term study, male and female rats were given four single injections of either SKF-38393, quinpirole or saline, with one week intervals, and locomotor time and number of rearings recorded. Long-term quinpirole was found to induce supersensitivity of the D2 receptor complex, demonstrated by both enhanced locomotor time and rearing behavior, while long-term SKF-38393 treatment activated the D1 receptors, as evaluated by grooming time. Short-term quinpirole enhanced supersensitivity of the D2 receptors only in female rats, as assessed by increasing both locomotor time and rearing behavior, reiterating previous results on sex-dependent monoaminergic reactivity.

Male and Female newborn rats were primed with either quinpirole 0.05 mg/kg IP or SKF-38393 0.1 mg/kg IP on days 1-11, 12-22 and 23-33 of their lives. When the rats reached the age of 13 weeks, they were placed on metal rods in an activity avoidance chamber, and light and electric current of 30V/0.8 mA were used on them as conditioning stimuli. Avoidance of the electric shock was considered a positive conditioned reaction. Training and memorizing the conditioned avoidance reflex consisted of a series of ten trials, 60 seconds apart, once a week for ten weeks. The mean number of positive responses after quinpirole was more profound in all priming intervals tested, as compared to SKF-38393, and was higher as the priming started later in life. Significantly higher scores were obtained by the female rats primed with quinpirole, as compared to the male rats primed with the same drug. These differences were much weaker with SKF-38393. These findings confirm that the central D2 receptor system is involved in learn ing and memorizing of Conditioned Avoidance Reflex much more than the D1 receptors do, and that female rats are more sensitive and retentive to this reflex.

Repeated treatments with a dopamine (DA) D2 receptor agonist result in the induction of DA D2 receptor supersensitivity, as evidenced by enhanced behavioral responses to subsequent D2 agonist treatments - a phenomenon known as priming of receptors. Priming of D2 receptors has been well-studied in otherwise intact (non-lesioned) rats. In contrast to D2 priming, repeated treatments with a DA D1 agonist are unable to prime D1 receptors unless nigrostriatal DA fibers are largely destroyed in early postnatal ontogeny. In order to determine if D2 receptors could be primed in rats in which nigrostriatal DA fibers were largely destroyed in early postnatal ontogeny, rats were (a) lesioned at 3 days after birth with 6-hydroxydopamine (67 μg in each lateral ventricle; desipramine, 20 mg/kg IP, 1 h; 6-OHDA), (b) treated daily for the first 28 days after birth with the D2 agonist quinpirole HCl (3.0 mg/kg IP), and (c) observed in adulthood for both quinpirole-induced and SKF 38393- (D1 agonist-) induced locomotor activity and stereotyped activities. In 6-OHDA-lesioned rats in which endogenous striatal DA was reduced by 99%, quinpirole did not produce enhanced locomotor or stereotyped activities. However, SKF 38393 produced increased locomotor and stereotyped activities even after the first dose of SKF 38393. These findings demonstrate that D2 receptors are not primed by ontogenetic quinpirole treatments of neonatally 6-OHDA-lesioned rats, although D2 agonist treatments do at least partially prime D1 receptors in 6-OHDA-lesioned rats.

Quinpirole-induced vertical jumping is a phenomenon first observed in rats treated from birth, once a day for 21 days or more, with the dopamine D 2 receptor agonist quinpirole. This quinpirole-induced behavioral sensitization is known as a priming process. To determine whether dopaminergic innervation influenced this priming phenomenon, groups of rats were lesioned at 3 days after birth with the neurotoxin 6-hydroxydopamine (6-OHDA; 67 μg in each lateral ventricle; desipramine pretreatment, 20 mg/kg ip, 1 h). Rats were additionally treated daily from birth with quinpirole HCl (3.0 mg/kg ip, salt form). Controls received saline vehicle in place of 6-OHDA and/or quinpirole. When rats were placed in individual observation cages (1 h acclimation) starting at 20 days after birth, acute quinpirole treatment produced vertical jumping in the quinpirole-primed group; and the effect persisted through the twenty-ninth day. In rats additionally lesioned with 6-OHDA, vertical jumping was enhanced at 20, 24, 26/27, and 28/29 day-with there being as much as a 32-fold increase in vertical jumping versus the group that was primed with quinpirole, but not lesioned with 6-OHDA. This finding indicates that an ontogenetic 6-OHDA lesion enhances quinpirole-induced vertical jumping in rats and that dopaminergic innervation may normally exert a suppressive effect on vertical jumping.

Repeated treatments of rats with the dopamine (DA) D2 receptor agonist quinpirole, consistently produce long-lived DA D2 receptor supersensitization, by the process that has been termed priming. Rats so-primed in ontogeny behaviorally demonstrate adulthood enhancement of low-dose quinpirole-induced yawning. Because 1) dopaminergic neurons originate in midbrain nuclei (substantia nigra and ventral tegmental area), and 2) noradrenergic neurons originate in pontine (locus coeruleus) and medullary areas, it might be presumed that these two monoaminergic systems are independent, not interdependent. However, in the present study we demonstrate that there was an attenuation of quinpirole-enhanced yawning at 8 weeks in rats that were 1) primed by repeated neonatal quinpirole HCl treatments (50 μg/kg per day SC) during the first ten days of postnatal ontogeny, and 2) lesioned at 3 days after birth with DSP-4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride, 50 mg/kg SC). Dose-effect curves indicated a 23-45% reduction in yawning by DSP-4 treatment of quinpirole-primed rats, acutely treated as adults with quinpirole (25, 50, or 100 μg/kg). Effectiveness of DSP-4 is reflected by the 95% and 99% reductions in norepinephrine contents of frontal cortex and hippocampus, respectively (HPLC/ED method). The findings are supportive of a modulatory role of noradrenergic fibers on dopamine receptor priming (supersensitization) in rat brain.

The 5-HT agonist drug mCPP contributed to a 5-HT hypothesis of obsessive-compulsive disorder (OCD), but the effects of the drug in human and animal studies have been inconsistent. The objective of this thesis was to shed light on the behavioural and neurobiological effects of mCPP using the quinpirole sensitization rat model of OCD and in a reciprocal manner, to use the drug to further reveal behavioural and neurobiological components of the animal model. The utility in using the quinpirole model is that the process of analysis by experimentation can be employed to observe effects of the drug on three separate behavioural components identified to underlie the model compulsive behaviour: vigor, focus and satiety. Four original studies were designed to address this objective, and the findings yielded novel contributions to the literature. We suggest that mCPP attenuates compulsive checking by attenuating the exacerbated vigor and satiety characteristic of compulsive behavior, but this effect may not have been captured in previous clinical studies because OCD was measured as a unitary phenomenon across different symptom subtypes. We also reveal that separate systems underlie the development and performance of compulsive behaviour in the animal model, and mCPP reduces its performance but not its development. Hence, the animal model findings suggest that mCPP can attenuate performance of OCD behavior but the drug does not reverse the pathology of OCD or arrest the pathogenesis of OCD. Neurobiologically, we hypothesize that the underlying mechanism mediating the response to mCPP is mediated downstream of the nucleus accumbens core (NAc), at the substantia nigra pars reticulata, based on the finding that the effects of mCPP on vigor and satiety are present in NAc lesioned animals. Finally, although findings of this thesis indicate that 5-HT2A/C receptors do not mediate the response to mCPP, an oppositional role for DA and 5-HT on the model of compulsive behaviour is proposed, consistent with a security motivation theory of OCD. Overall, this thesis shed new light on the effects of mCPP on OCD, and reveals novel neurobiological and behavioural correlates of the quinpirole model. / Thesis / Doctor of Philosophy (PhD)

Une injection intrastriatale de l'agoniste D2 quinpirole facilite l'apprentissage de la tâche de labyrinthe radial win-stay (Packard et White, 1991). Trois expériences sont effectuées pour approfondir l'étude de l'implication des récepteurs dopaminergiques D2 dans ce type l'apprentissage et aussi pour vérifier l'impact de cet effet sur l'expression du proto-oncogène c-fos. Dans la première étude nous parvenons à répliquer l'effet facilitateur du quinpirole mais il comporte des faiblesses et nous reprenons la procédure dans une deuxième étude. Dans cette deuxième étude un groupe supplémentaire reçoit préalablement une injection i.p. de l'antagoniste D2 sulpiride. Les résultats démontrent que l'injection de quinpirole n'améliore pas la performance et que le groupe recevant préalablement l'injection de sulpiride à étonnamment une meilleure performance. Une troisième étude est reconduite. De nouveau, l'effet facilitant du quinpirole n'est pas observé et le sulpiride induit une amélioration non significative de la performance. L'impact de ces procédures, sur l'expression du c-fos, n'a pas été effectué faute d'avoir réussi à maîtriser l'effet recherché. La discussion s'attarde à identifier les raisons pouvant expliquer ces difficultés à observer l'effet de facilitation induit par le quinpirole.

We have shown that repeated neonatal quinpirole (QUIN; a dopamine D2-like receptor agonist) treatment in rats produces long-lasting supersensitization of dopamine D2 receptors that persists into adulthood but without producing a change in receptor number. The current study was designed to analyze the effects of neonatal QUIN on auditory sensorimotor gating as measured through prepulse inhibition (PPI). Male and female Sprague–Dawley rats were neonatally treated with QUIN (1mg/kg) or saline from postnatal days (P)1–21. At P60, the number of yawns was recorded for a 1h period in response to an acute QUIN (1mg/kg) injection as yawning is a D2-like receptor mediated behavioral event. Five days later, rats began (PPI) behavioral testing in two phases. In phase I, three different prepulse intensities (73, 76, and 82dB) were administered 100-ms before a 115dB pulse on 10 consecutive days. In phase II, three different interstimulus intervals (ISI; 50, 100, and 150ms) were inserted between the 73 or 76dB prepulse and 115dB pulse over 10 consecutive days of testing. A PPI probe trial was administered at the end of each phase after an acute 100μg/kgi.p. injection of QUIN to all animals. Replicating previous work, neonatal QUIN enhanced yawning compared to controls, verifying D2 receptor supersensitization. Regarding PPI, neonatal QUIN resulted in deficits across both phases of testing persistent across all testing days. Probe trial results revealed that acute QUIN treatment resulted in more robust PPI deficits in neonatal QUIN animals, although this deficit was related to prepulse intensity and ISI. These findings provide evidence that neonatal QUIN treatment results in deficits of auditory sensorimotor gating in adulthood as measured through PPI.

The extended striatum is a large, dopamine-innervated forebrain structure comprising the caudate-putamen, nucleus accumbens and olfactory tubercle (OT). The OT remains largely unexplored, despite its potentially important role in behaviour and dopamine (DA)-mediated reward. One method of studying function is examining "supersensitive" behavioural responses to DA agonists in animals after striatal DA loss. We examined whether D1 or D2 receptor supersensitivity occurs in the OT and neighbouring islands of Calleja (ICj), after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle and medial OT (mOT). We also asked if the resulting DA receptor supersensitivity is anatomically heterogeneous. Our results showed D1-like receptor supersensitivity occurring in the OT with several DA agonists, and heterogeneously across the extended striatum. There is evidence of D2-like receptor supersensitivity in the ICj. Our focal mOT lesion failed to show DA receptor supersensitivity. Finally, there is little evidence for D2 supersensitivity as measured by [ 35S]GTPgammaS binding.

Neonatal quinpirole treatment has been shown to produce an increase in dopamine D2-like receptor sensitivity that persists throughout the subject's lifetime. The objective was to analyze the effects of neonatal quinpirole treatment on effects of amphetamine in adolescent rats using locomotor sensitization and conditioned place preference procedures. Sprague-Dawley rats were treated with quinpirole (1 mg/kg) or saline from postnatal days (P)1 to P11 and raised to adolescence. For locomotor sensitization, subjects were given amphetamine (1 mg/kg) or saline every second day from P35 to P47 and were placed into a locomotor arena. In female rats, neonatal quinpirole treatment enhanced amphetamine locomotor sensitization compared with quinpirole-free controls sensitized to amphetamine. Male rats demonstrated sensitization to amphetamine, although this was muted compared with female rats, and were unaffected by neonatal quinpirole. For conditioned place preference, subjects were conditioned for 8 consecutive days (P32-39) with amphetamine (1 mg/kg) or saline and a drug-free preference test was conducted at P40. Rats treated with neonatal quinpirole enhanced time spent in the amphetamine-paired context compared with quinpirole-free controls conditioned with amphetamine, but only female controls conditioned with amphetamine spent more time in the drug-paired context compared with saline-treated controls. Increased D₂-like receptor sensitivity appears to have enhanced the behavioral effects of amphetamine, but these effects were more prevalent in adolescent female rats compared with male rats.