Comparative effects of scopolamine and quinpirole on the striatal fos expression induced by stimulation of D(1) dopamine receptors in the rat

Dopamine Agonist Cotreatment Alters Neuroplasticity and Pharmacology of Levodopa-Induced Dyskinesia

BACKGROUND

Current models of levodopa (L-dopa)-induced dyskinesia (LID) are obtained by treating dopamine-depleted animals with L-dopa. However, patients with LID receive combination therapies that often include dopamine agonists.

OBJECTIVE

Using 6-hydroxydopamine-lesioned rats as a model, we aimed to establish whether an adjunct treatment with the D2/3 agonist ropinirole impacts on patterns of LID-related neuroplasticity and drug responses.

METHODS

Different regimens of L-dopa monotreatment and L-dopa-ropinirole cotreatment were compared using measures of hypokinesia and dyskinesia. Striatal expression of ∆FosB and angiogenesis markers were studied immunohistochemically. Antidyskinetic effects of different drug categories were investigated in parallel groups of rats receiving either L-dopa monotreatment or L-dopa combined with ropinirole.

RESULTS

We defined chronic regimens of L-dopa monotreatment and L-dopa-ropinirole cotreatment inducing overall similar abnormal involuntary movement scores. Compared with the monotreatment group, animals receiving the L-dopa-ropinirole combination exhibited an overall lower striatal expression of ∆FosB with a distinctive compartmental distribution. The expression of angiogenesis markers and blood-brain barrier hyperpermeability was markedly reduced after L-dopa-ropinirole cotreatment compared with L-dopa monotreatment. Moreover, significant group differences were detected upon examining the response to candidate antidyskinetic drugs. In particular, compounds modulating D1 receptor signaling had a stronger effect in the L-dopa-only group, whereas both amantadine and the selective NMDA antagonist MK801 produced a markedly larger antidyskinetic effect in L-dopa-ropinirole cotreated animals.

CONCLUSIONS

Cotreatment with ropinirole altered LID-related neuroplasticity and pharmacological response profiles. The impact of adjuvant dopamine agonist treatment should be taken into consideration when investigating LID mechanisms and candidate interventions in both clinical and experimental settings. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Locomotor activating effects of cocaine and scopolamine combinations in rats: isobolographic analysis

Muscarinic cholinergic receptors are currently receiving renewed interest as viable targets for treating various psychiatric disorders. Dopaminergic and muscarinic systems interact in complex ways. The goal of this study was to quantify the interaction between a systemically administered psychomotor stimulant and muscarinic antagonist at the behavioral level. Through isobolographic analysis of locomotor activity data, we assessed the effects of three cocaine/scopolamine mixtures in terms of deviation from simple dose addition (additivity), at four effect levels. All three mixtures produced some more-than-additive (synergistic) effects, as lower doses were needed to produce the given effects relative to the calculated effect of additive doses. A mixture with comparable contributions from cocaine and scopolamine produced significantly more-than-additive effects at all but the lowest effect level examined. A mostly-cocaine mixture was more-than-additive only at low effect levels, whereas a mostly-scopolamine mixture produced effects more consistent with additivity, with only the highest effect level barely reaching significant synergism. Our study confirms and quantifies previous findings that suggested synergistic effects of stimulants and muscarinic antagonists. The synergism implies that cocaine and scopolamine stimulate locomotor activity through nonidentical pathways, and was most pronounced for a mixture containing cocaine and scopolamine in comparable proportions.

Nucleus accumbens neurotransmission and effort-related choice behavior in food motivation: effects of drugs acting on dopamine, adenosine, and muscarinic acetylcholine receptors

Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Although nucleus accumbens (NAc) DA depletions or antagonism leave aspects of appetite and primary food motivation intact, rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements, and instead select less effortful food-seeking behaviors. Previous work showed that adenosine A2A antagonists can reverse the effects of DA D2 antagonists on effort-related choice, and that stimulation of adenosine A2A receptors produces behavioral effects that are similar to those induced by DA antagonism. The present review summarizes the literature on the role of NAc DA and adenosine in effort-related processes, and also presents original data on the effects of local stimulation of muscarinic acetylcholine receptors in NAc core. Local injections of the muscarinic agonist pilocarpine directly into NAc core produces shifts in effort-related choice behavior similar to those induced by DA antagonism or A2A receptor stimulation, decreasing lever pressing but increasing chow intake in rats responding on a concurrent fixed ratio/chow feeding choice task. In contrast, injections into a neostriatal control site dorsal to the NAc were ineffective. The actions of pilocarpine on this task were attenuated by co-administration of the muscarinic antagonist scopolamine. Thus, drugs that act on DA, adenosine A2A, and muscarinic receptors regulate effort-related choice behavior, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, fatigue or anergia that can be observed in depression and other disorders.

Effects of the adenosine A 2A antagonist KW 6002 (istradefylline) on pimozide-induced oral tremor and striatal c-Fos expression: comparisons with the muscarinic antagonist tropicamide

Typical antipsychotic drugs, including haloperidol and pimozide, have been shown to produce parkinsonian motor effects such as akinesia and tremor. Furthermore, there is an antagonistic interaction between adenosine A(2A) and dopamine D(2) receptors in the basal ganglia, which is important for motor functions related to the production of parkinsonian symptoms. Several experiments were conducted to assess the effects of the selective adenosine A(2A) antagonist KW 6002 on both the motor and cellular effects of subchronic administration of pimozide. The motor test employed was tremulous jaw movements, which is used as a model of parkinsonian tremor. In addition, c-Fos expression in the ventrolateral neostriatum, which is the striatal area most associated with tremulous jaw movements, was used as a marker of striatal cell activity in animals that were tested in the behavioral experiments. Repeated administration of 1.0 mg/kg pimozide induced tremulous jaw movements and increased ventrolateral striatal c-Fos expression, while administration of 20.0 mg/kg of the atypical antipsychotic quetiapine did not. The tremulous jaw movements induced by pimozide were significantly reduced by co-administration of either the adenosine A(2A) antagonist KW 6002 or the muscarinic antagonist tropicamide. Pimozide-induced increases in ventrolateral striatal c-Fos expression were reduced by a behaviorally effective dose of KW 6002, but c-Fos expression in pimozide-treated rats was actually increased by tropicamide. These results indicate that two different drug manipulations that act to reduce tremulous jaw movements can have different effects on DA antagonist-induced c-Fos expression, suggesting that adenosine A(2A) antagonism and muscarinic receptor antagonism exert their motor effects by acting on different striatal circuits.

Rotation and immediate-early gene expression in rats treated with the atypical D1 dopamine agonist SKF 83822

Classical agonists of the dopamine D1 receptor activate both adenylyl cyclase and phospholipase C (PLC) signaling pathways. As a result, the extent to which these two pathways are essentially involved in various effects produced by D1 receptor agonists is currently uncertain. In the present report we examined the effects of SKF 83822, a dopamine D1 agonist which has been reported to activate adenylyl cyclase, but not PLC, on behavior and immediate early gene (IEG) expression in rats with unilateral 6-hydroxydopamine lesions. SKF 83822 (25-100 microg/kg) induced dose dependent contralateral rotation in these subjects, and, additionally, stimulated strong expression of the IEG products c-Fos, Fra2, Zif/268 and Arc in the deinnervated striatum. All of these effects could be antagonized by pretreatment with the selective D1 dopamine antagonist SCH 23390 (0.5 mg/kg). Although PLC may be involved in many effects mediated through dopamine D1 receptors, these results suggest that direct activation of PLC is not necessary for the induction of either rotation or IEG expression in dopamine depleted rats.

Neuropharmacology of cognition and memory: A unifying theory of neuromodulator imbalance in psychiatry and amnesia

The case of HM, a man with intractable epilepsy who became amnesic following bilateral medial temporal lobe surgery nearly half a century ago has instigated ongoing research and theoretical speculation on the nature of memory and the role of the hippocampus. Neuropsychological testing showed that although HM had extensive anterograde memory loss he could still acquire motor and cognitive skills implicitly, but could not remember the context of this learning. This has lead to declarative and procedural descriptions of the memory process. Cholinergic and monoaminergic neurotransmitter systems have also been implicated in the memory process and anticholinergic drugs traditionally have been associated with impairment of declarative memory. The cholinergic hypothesis of Alzheimer's disease is a classic example of an application of these neuropharmacological findings. In schizophrenia, preattentive deficits have been amply demonstrated by unconscious priming studies. Memory processes are also impaired in these patients. Dopamine, glutamate and even cholinergic dysfunction has been implicated in the clinical picture of schizophrenia. The present paper will attempt to bring together both the anatomical and pharmacological data from these disparate fields of research under a cohesive theory of cognition and memory. A hypothesis is presented for an inverse relationship between monoaminergic and cholinergic systems in the modulation of implicit (unconscious) and explicit (conscious) cognitive processes. It is postulated that muscarinic cholinergic receptors and monoaminergic systems facilitate unconscious and conscious processes, respectively, and they disfacilitate conscious and unconscious processes, respectively (the purported inverse relationship). In fact, the muscarinic and monoaminergic modulations of a neural network are proposed to be finely balanced such that, if, the activity of one receptor system is modified then this by necessity has effects on the other system. It takes into account receptor subtypes and their effects mediated through excitatory and inhibitory G-protein complexes. For example, m1/D2 and D1/m4 paired receptor subtypes, colocalized on separate neurons would have opposing functional effects. A theory is then presented that the critical underlying pathophysiology of schizophrenia involves a hypofunctional muscarinic cholinergic system, which induces abnormal facilitation of monoaminergic subsystems such as dopamine (e.g., a decrease in m1R function would potentiate D2R function). This extends the idea of an inverted U function for optimal monoaminergic concentrations. Not only would this impair unconscious preattentive processes, but according to the hypothesis, explicit cognition as well including memory deficits and would underlie the mechanism of psychosis. Contrary to current thinking a different view is also presented for the role of the hippocampus in the memory process. It is postulated that long-term explicit memory traces in the neocortex are laid down by phasic coactivation of forebrain projecting monoaminergic systems above some basal firing rate, such as the rostral serotonergic raphe, which projects diffusely to the cortex and according to a modified Hebbian principle. This is the proposed principal function of the hippocampal theta rhythm. The phasic activation of the cholinergic basal forebrain is mediated by projections from a separate cortical structure, possibly the lateral prefrontal cortex. Phasic muscarinic receptor activation is proposed to strengthen implicit memory traces (at a synaptic level) in the neocortex. Thus, the latter are spared by medial temporal surgery explaining the dissociation of explicit from implicit memory.

The atypical dopamine D1 receptor agonist SKF 83959 induces striatal Fos expression in rats

The effects of dopamine D1 receptor agonists are often presumed to result from an activation of adenylyl cyclase, but dopamine D1 receptors may also be linked to other signal transduction cascades and the relative importance of these various pathways is currently unclear. SKF 83959 is an agonist at dopamine D1 receptors linked to phospholipase C, but has been reported to be an antagonist at receptors linked to adenylyl cyclase. The current report demonstrates that SKF 83959 induces pronounced, nonpatchy, expression of the immediate-early gene product Fos in the striatum of intact rats which can be converted to a patchy pattern by pretreatment with the dopamine D2-like receptor agonist quinpirole. In rats with unilateral 6-hydroxydopamine lesions SKF 83959 induces strong behavioral rotation and a greatly potentiated Fos response. All of the responses to SKF 83959, in both intact and dopamine-depleted animals, can be blocked by pretreatment with the dopamine D1 receptor antagonist SCH-23390. In intact subjects, SKF 83959 induced Fos expression less potently than the standard dopamine D1 receptor agonist SKF 82958, but the two drugs were approximately equipotent in deinnervated animals. These results demonstrate for the first time that possession of full efficacy at dopamine D1 receptors linked to adenylyl cyclase is not a necessary requirement for the induction of striatal Fos expression in intact animals and suggest that alternative signal transduction pathways may play a role in dopamine agonist induced Fos expression, especially in dopamine-depleted subjects.

Cholinergic involvement in the cortical and hippocampal Fos expression induced in the rat by placement in a novel environment

Placing rats into a series of novel environments induced vigorous c-fos expression in the infralimbic, anterior cingulate and retrosplenial cortices, and in the hippocampus. Pretreatment with the antimuscarinic drugs scopolamine and atropine was able to greatly suppress novelty-induced Fos expression at these sites. Placement into the novel environments also induced Fos expression in the habenula and the paraventricular thalamic nucleus, but the response at these sites did not appear to be sensitive to cholinergic blockade. These findings suggest that cholinergic mechanisms play an important role in ability of behavioral events to influence cortical and hippocampal immediate-early gene expression and are consistent with the possibility that some of the effects of anticholinergic drugs on placticity and learning may be mediated through alterations in the expression of these genes.

Muscarinic receptor blockade attenuates reserpine-mediated Fos induction in the rat striatopallidal pathway

Acute administration of the dopamine-depleting agent reserpine (10 mg/kg) induces Fos expression in striatopallidal neurons of intact rats-an effect that is blocked by pretreatment with the D2 agonist quinpirole (0.5 mg/kg). Systemic administration of the muscarinic antagonist scopolamine (50 mg/kg) partially attenuates reserpine-mediated striatal Fos expression. These data suggest that muscarinic receptors, either within the striatum or in extrastriatal sites, regulate D2 receptor-mediated Fos expression in rat striatopallidal neurons.

Could nursery rhymes cause violent behaviour? A comparison with television viewing

AIMS

To assess the rates of violence in nursery rhymes compared to pre-watershed television viewing.

METHODS

Data regarding television viewing habits, and the amount of violence on British television, were obtained from Ofcom. A compilation of nursery rhymes was examined for episodes of violence by three of the researchers. Each nursery rhyme was analysed by number and type of episode. They were then recited to the fourth researcher whose reactions were scrutinised.

RESULTS

There were 1045 violent scenes on pre-watershed television over two weeks, of which 61% showed the act and the result; 51% of programmes contained violence. The 25 nursery rhymes had 20 episodes of violence, with 41% of rhymes being violent in some way; 30% mentioned the act and the result, with 50% only the act. Episodes of law breaking and animal abuse were also identified. Television has 4.8 violent scenes per hour and nursery rhymes have 52.2 violent scenes per hour. Analysis of the reactions of the fourth researcher were inconclusive.

CONCLUSIONS

Although we do not advocate exposure for anyone to violent scenes or stimuli, childhood violence is not a new phenomenon. Whether visual violence and imagined violence have the same effect is likely to depend on the age of the child and the effectiveness of the storyteller. Re-interpretation of the ancient problem of childhood and youth violence through modern eyes is difficult, and laying the blame solely on television viewing is simplistic and may divert attention from vastly more complex societal problems.

Role of dopamine D1 receptors in the striatal and cortical fos expression induced by the muscarinic agonist pilocarpine

Injections of the muscarinic cholinergic receptor agonist pilocarpine (50 mg/kg) induced pronounced expression of the immediate early gene (IEG) product Fos in the striatum and cortex of rats. Pretreatment with the dopamine D1 receptor antagonist 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-H-3-benzazepine hydrochloride (SCH-23390; 0.2-2.0 mg/kg) drastically attenuated the pilocarpine response in the striatum, but had no effect in the cortex. In contrast, the muscarinic receptor antagonist scopolamine (0.75-3.00 mg/kg) virtually abolished the Fos response at both sites. These results suggest that stimulation of dopamine D1 receptors may mediate the effects of muscarinic agonists on Fos expression in the striatum, but not the cortex.

Role of dopamine D2 receptors in the striatal immediate early gene response to amphetamine in reserpinized rats

The indirect dopamine agonist amphetamine has been shown to induce a patchy pattern of immediate early gene (IEG) expression in the rostral striatum of both pharmacologically intact and reserpinized rats. The available data suggest that stimulation of D(2) dopamine receptors may play a role in the patterning of amphetamine-induced IEG expression, but direct evidence is lacking. In the current study of reserpinized animals, we found that pretreatment with the selective D(2) dopamine antagonist raclopride did not block the induction of the IEGs Fos and Arc by amphetamine, but greatly reduced the "patchiness" of the induced expression. Raclopride did not induce Fos or Arc expression by itself under the conditions studied here. These findings suggest that although stimulation of D(2) receptors is not necessary for amphetamine to induce IEG expression in reserpinized animals, these receptors do play a critical role in the spatial patterning of the resulting response.

Dopamine D1 receptor agonists induce penile erections in rats

The dopamine receptor agonist apomorphine has been recently introduced in the treatment of erectile dysfunction. While it is well established that dopamine D2-like receptors play a crucial role in this effect, conflicting result are reported in the literature as for the role of dopamine D1-like receptors. The aim of this study was to determine the effect of systemic administration of dopamine D1-like receptor agonists on penile erection in rats. Male Wistar rats were treated with three different, and not structurally related, dopamine D1-like receptor agonists: the partial agonists SKF38393 ((+) 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine) and CY 208-243 ((-)-4,6,6a,7,8,12b-exahydro-7-methylindole [4,3-ab]fenantridine), and the full agonist A 77636 ((-)-(1R,3S)-3-Adamantyl-1-(aminomethyl)-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyran hydrochloride). All three compounds dose-dependently increased the number of penile erections, with the full agonist A77636 showing a more pronounced effect with respect to the other two. Moreover, the dopamine D1-like receptor antagonist SCH 23390 ((R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) dose-dependently antagonised A77636 effect. These results show that systemic administration of dopamine D1-like receptor agonists induce penile erection in rats. This observation suggests that dopamine D1-like receptor agonists might be considered as a possible alternative to apomorphine in the treatment of erectile dysfunction, thus avoiding the typical side effects related to the stimulation of dopamine D2-like receptors such as nausea.

Molecular dissection of dopamine receptor signaling

The use of genetically engineered mice has provided substantial new insights into the functional organization of the striatum. Increasing evidence suggests that specific genes expressed within the striatum contribute to its functional activity. We studied the dopamine (DA) D1 receptor gene and one of its downstream targets, the transcription factor c-Fos. We have evaluated the functional interaction between the D1 and D2 DA receptor subtypes at the cellular and behavioral levels. Our results show that haloperidol, a DA D2-class receptor antagonist, activates c-Fos predominantly in enkephalin-positive striatal neurons, which project to the globus pallidus and are thought to mediate motor inhibition. Deletion of the DA D1 receptor increased the responsiveness of enkephalin neurons to haloperidol, in that haloperidol-induced increases in c-Fos and catalepsy were enhanced in D1 receptor knockout mice. These results suggest a functionally opposing role of the D1 receptor against the D2 DA-class receptors in the striatum.