The partial D2-like dopamine receptor agonist terguride acts as a functional antagonist in states of high and low dopaminergic tone: evidence from preweanling rats

Intraamygdaloid Oxytocin Increases Time Spent on Social Interaction in Valproate-Induced Autism Animal Model

Autism spectrum disorder (ASD) is a pervasive neurodevelopmental disorder that affects about 1.5% of children worldwide. One of the core symptoms is impaired social interaction. Since proper treatment has not been found yet, an investigation of the exact pathophysiology of autism is essential. The valproate (VPA)-induced rat model can be an appropriate way to study autism. Oxytocin (OT) may amend some symptoms of ASD since it plays a key role in developing social relationships. In the present study, we investigated the effect of the intraamygdaloid OT on sham and intrauterine VPA-treated rats' social interaction using Crawley's social interaction test. Bilateral guide cannulae were implanted above the central nucleus of the amygdala (CeA), and intraamygdaloid microinjections were carried out before the test. Our results show that male Wistar rats prenatally exposed to VPA spent significantly less time on social interaction. Bilateral OT microinjection increased the time spent in the social zone; it also reached the level of sham-control animals. OT receptor antagonist blocked this effect of the OT but in itself did not significantly influence the behavior of the rats. Based on our results, we can establish that intraamygdaloid OT has significantly increased time spent on social interaction in the VPA-induced autism model, and its effect is receptor-specific.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Development of a peptide targeting dopamine transporter to improve ADHD-like deficits

Attention-deficit hyperactivity disorder (ADHD) is a neurocognitive disorder characterized by hyperactivity, inattention, working memory deficits and impulsivity. Its worldwide prevalence is estimated to be 3–5% in children and adolescents. The mainstay treatment for ADHD is stimulant medications (e.g. methylphenidate), which increase synaptic dopamine by directly blocking dopamine transporter (DAT). Although these pharmacological agents are effective, they are often associated with various side effects including risks for future substance use disorders in ADHD patients. Here, we investigated an interaction between DAT and dopamine D2 receptor (D2R) as a novel target to develop potential therapeutics for the treatment of ADHD by using an interfering peptide (TAT-DAT_NT) to dissociate this protein complex. We found that TAT-DAT_NT promotes locomotor behavior in Sprague-Dawley rats. Furthermore, using in vivo microdialysis and high-performance liquid chromatography, we found that the disruption of D2R-DAT elevates extracellular dopamine level. More importantly, the interfering peptide, TAT-DAT_NT, attenuates hyperactivity and improves spontaneous alternation behavior in spontaneously hypertensive rats (SHR) ------ a common animal model of ADHD. This work presents a different means (i.e. other than direct blockade by a DAT inhibitor) to regulate the activity of DAT and dopaminergic neurotransmission, and a potential target site for future development of ADHD treatments.

Ontogenetic differences in sensitivity to LiCl- and amphetamine-induced taste avoidance in preweanling rats

When amphetamine is associated with a tastant conditioned stimulus, rats learn to avoid the taste even when employing doses that promote conditioned place preference. One hypothesis raised to account for this effect proposes that taste avoidance induced by amphetamine may be motivated by fear. A sensitive period has been identified in the rat (until postnatal day 10) in which infants learn conditioned appetitive effects to stimuli to which aversions are conditioned after this period. Exogenous administration of corticosterone within this period reverses this effect, generating aversive conditioning. In the present study, we tested conditioning of aversions to amphetamine or LiCl, within and after the sensitive period (Experiments 1 and 2). A third experiment evaluated unconditioned rejection of an aversive quinine solution within the sensitive period. Finally, we tested whether corticosterone administration before conditioning modulates amphetamine-induced taste avoidance. After the sensitive period, infant rats rejected the solution paired with amphetamine or LiCl after 2 conditioning trials, but within the sensitive period, aversions were conditioned only by LiCl and after 4 conditioning trials. Amphetamine-induced taste avoidance was not observed even when corticosterone was administered before conditioning. Additionally, during the sensitive period, a low LiCl dose promoted conditioned taste preference. According to Experiment 3, parameters employed in this study were suitable to yield rejection of aversive solutions within the sensitive period. These results suggest that during the sensitive period, there is a notable resistance to the acquisition of taste avoidance induced by amphetamine. The present experimental framework may represent a useful tool for studying mechanisms underlying taste avoidance and aversion effects.

Double-blind study of pardoprunox, a new partial dopamine agonist, in early Parkinson's disease

This study examined the efficacy and safety of the partial dopamine agonist, pardoprunox (SLV308), in the treatment of patients with early Parkinson's disease (PD). Patients were randomized to receive pardoprunox (n = 69) or placebo (n = 70). Pardoprunox was titrated to each patient's optimal dose (9-45 mg/d) over 2 to 6 weeks and then maintained at this dose for a further 3 weeks. Concomitant anti-Parkinson treatment was not permitted. In the primary analysis, Unified PD Rating Scale (UPDRS)-Motor score was improved in pardoprunox-treated patients (overall mean dose 23.8 mg/d; -7.3 points), as compared with placebo (-3.0 points; P = 0.0001), from baseline to end point. At end point, there were more responders (> or = 30% reduction in UPDRS-Motor score) in the pardoprunox group (50.7%) than in the placebo group (15.7%; P < 0.0001). In other secondary analyses, UPDRS-activities of daily living (ADL) and -ADL+Motor scores were also significantly more improved in the pardoprunox group. Nausea was reported by 32 of 68 (47.1%) pardoprunox-treated patients (vs. 3/70, 4.3%, placebo-treated patients), with dizziness, somnolence, headache, and asthenia also reported by > or = 10 patients. In this exploratory proof-of-concept study, pardoprunox significantly improved motor function in patients with early PD. The efficacy and safety profile of pardoprunox justifies its further investigation in PD.

Preclinical and clinical pharmacology of alcohol dependence

In recent years, advances in neuroscience led to the development of new medications to treat alcohol dependence and especially to prevent alcohol relapse after detoxification. Whereas the earliest medications against alcohol dependence were fortuitously discovered, recently developed drugs are increasingly based on alcohol's neurobiological mechanisms of action. This review discusses the most recent developments in alcohol pharmacotherapy and emphasizes the neurobiological basis of anti-alcohol medications. There are currently three approved drugs for the treatment of alcohol dependence with quite different mechanisms of action. Disulfiram is an inhibitor of the enzyme aldehyde dehydrogenase and acts as an alcohol-deterrent drug. Naltrexone, an opiate antagonist, reduces alcohol craving and relapse in heavy drinking, probably via a modulation of the mesolimbic dopamine activity. Finally, acamprosate helps maintaining alcohol abstinence, probably through a normalization of the chronic alcohol-induced hyperglutamatergic state. In addition to these approved medications, many other drugs have been suggested for preventing alcohol consumption on the basis of preclinical studies. Some of these drugs remain promising, whereas others have produced disappointing results in preliminary clinical studies. These new drugs in the field of alcohol pharmacotherapy are also discussed, together with their mechanisms of action.

Pre- and postsynaptic actions of a partial D2 receptor agonist in reserpinized young rats: Longevity of agonistic effects

Partial D2 receptor agonists (e.g., terguride, preclamol, and aripiprazole) have antagonist-like effects at normosensitive D2 postsynaptic receptors and synthesis modulating autoreceptors. In reserpine-pretreated adult and young rats, however, partial D2 agonists function like high efficacy agonists at D2 postsynaptic receptors and autoreceptors (i.e., terguride increases locomotor activity and decreases dopamine synthesis). The purpose of the present study was to examine the time-course of these pharmacological effects. In all experiments, preweanling rats were given daily injections of reserpine (1 mg/kg, i.p.) or vehicle on postnatal day (PD) 16-PD 20. In the dopamine synthesis experiments, the ability of terguride (0.8 mg/kg) to reduce striatal DOPA accumulation (in NSD-1015 treated rats) was assessed either 5 h or 1, 2, 4, or 8 days (Experiment 1) or 4, 8, 12, 16, 20, or 24 days (Experiment 2) after reserpine pretreatment. In the behavioral experiments, locomotor activity of vehicle or terguride (0.8 mg/kg, i.p.) treated rats was assessed 5 h or 1, 2, 4, or 8 days after the 5-day reserpine regimen. Results from the dopamine synthesis experiments showed that terguride caused agonist-like effects (i.e., decreased DOPA accumulation) at only the 5 h and 1 day time points, although terguride did not induce its normal antagonist-like effects even 20 days after reserpine pretreatment. In the behavioral experiments, terguride stimulated locomotor activity for only the initial 2 days after reserpine pretreatment. The results of the present study show that the agonistic effects of terguride at pre- and postsynaptic receptors are short-lived, but terguride may not exhibit normal antagonistic effects, at least at synthesis modulating autoreceptors, until long after conclusion of reserpine pretreatment.

The partial dopamine D2-like receptor agonist terguride functions as an agonist in preweanling rats after a 5-day reserpine regimen

RATIONALE

Treating children and adolescents with partial D2-like agonists is becoming increasingly common, although few developmental animal studies have assessed the psychopharmacology of this class of drug. Contrary to results from adult rat studies, it has been reported that partial D2-like agonists may not induce agonist-like behavioral effects in preweanling rats during states of low dopaminergic tone.

OBJECTIVE

The purpose of the present study was to determine whether a partial D2-like agonist would act as an agonist in preweanling rats after a 5-day regimen of the dopamine-depleting agent reserpine or the tyrosine hydroxylase inhibitor alpha-methyl-DL-p-tyrosine (AMPT).

METHODS

Sprague-Dawley rats were pretreated with reserpine (1 mg kg(-1) per day) or AMPT (3 x 200 mg kg(-1) per day) on postnatal day (PD) 16-PD 20. Either 2 h (AMPT) or 5 h (reserpine) after the last pretreatment injection, rats were treated with saline, the partial D2-like agonist terguride, or the full D2-like agonist R(-)-propylnorapomorphine (NPA). Distance traveled and repetitive motor movements were measured for 60 min.

RESULTS

After repeated reserpine treatment, both terguride and NPA increased the distance-traveled scores of preweanling rats; however, only NPA, but not terguride, increased distance-traveled scores after a 5-day regimen of AMPT or an acute injection of reserpine.

CONCLUSIONS

It is now apparent that partial D2-like agonists are capable of inducing agonist-like behavioral effects in preweanling rats during a state of low dopaminergic tone. For agonistic actions to be observed, the pretreatment regimen must result in substantial and prolonged dopamine depletion.

Effects of a partial D2-like receptor agonist on striatal dopamine autoreceptor functioning in preweanling rats

There is evidence that partial D2-like dopamine agonists (e.g., terguride) may not affect D2-like postsynaptic receptors in an adult-typical manner during the preweanling period. To determine whether synthesis modulating dopamine autoreceptors are also affected in an adult atypical manner by partial D2-like agonists, preweanling rats were treated either acutely or repeatedly with reserpine (low dopaminergic tone) or vehicle (high dopaminergic tone). The ability of terguride, quinpirole (a full D2-like agonist), or haloperidol (a D2-like antagonist) to alter striatal DOPA accumulation was assessed after NSD-1015 treatment on postnatal day (PD) 21. In a separate set of experiments, terguride's ability to modulate dopamine synthesis was assessed in rats treated with the nerve impulse flow inhibitor gamma-butyrolactone (GBL). Results showed that both terguride and quinpirole reduced striatal DOPA accumulation during a state of low dopaminergic tone (i.e., after reserpine pretreatment). During a state of high dopaminergic tone (i.e., after vehicle pretreatment), terguride had similar effects as haloperidol and increased DOPA accumulation. Terguride, like quinpirole, partially inhibited the GBL-induced increase in striatal DOPA accumulation. When considered together, these results indicate that synthesis modulating D2-like autoreceptors are functional during the late preweanling period, and they respond in an adult-typical manner to a partial D2-like agonist.