Dopamine D1- and D2-dependent catalepsy in the rat requires functional NMDA receptors in the corpus striatum, nucleus accumbens and substantia nigra pars reticulata

Influence of aversive stimulation on haloperidol-induced catalepsy in rats

Catalepsy - an immobile state in which individuals fail to change imposed postures - can be induced by haloperidol. In rats, the pattern of haloperidol-induced catalepsy is very similar to that observed in Parkinson's disease (PD). As some PD symptoms seem to depend on the patient's emotional state, and as anxiety disorders are common in PD, it is possible that the central mechanisms regulating emotional and cataleptic states interplay. Previously, we showed that haloperidol impaired contextual-induced alarm calls in rats, without affecting footshock-evoked calls. Here, we evaluated the influence of distinct aversive stimulations on the haloperidol-induced catalepsy. First, male Wistar rats were subjected to catalepsy tests to establish a baseline state after haloperidol or saline administration. Next, distinct cohorts were exposed to open-field; elevated plus-maze; open-arm confinement; inescapable footshocks; contextual conditioned fear; or corticosterone administration. Subsequently, catalepsy tests were performed again. Haloperidol-induced catalepsy was verified in all drug-treated animals. Exposure to open-field, elevated plus-maze, open-arm confinement, footshocks, or administration of corticosterone had no significant effect on haloperidol-induced catalepsy. Contextual conditioned fear, which is supposed to promote a more intense fear, increased catalepsy over time. Our findings suggest that only specific defensive circuitries modulate the nigrostriatal system mediating the haloperidol-induced cataleptic state.

Activation of Dopamine Receptors in the Nucleus Accumbens Promotes Sucrose-Reinforced Cued Approach Behavior

Dopamine receptor activation in the nucleus accumbens (NAc) promotes vigorous environmentally-cued food-seeking in hungry rats. Rats fed ad libitum, however, respond to fewer food-predictive cues, particularly when the value of food reward is low. Here, we investigated whether this difference could be due to differences in the degree of dopamine receptor activation in the NAc. First, we observed that although rats given ad libitum access to chow in their home cages approached a food receptacle in response to reward-predictive cues, the number of such approaches declined as animals accumulated food rewards. Intriguingly, cued approach to food occurred in clusters, with several cued responses followed by successive non-responses. This pattern suggested that behavior was dictated by transitions between two states, responsive and non-responsive. Injection of D1 or D2 dopamine receptor agonists into the NAc dose-dependently increased cue responding by promoting transitions to the responsive state and by preventing transitions to the non-responsive state. In contrast, antagonists of either D1 or D2 receptors promoted long bouts of non-responding by inducing transitions to the non-responsive state and by preventing transitions to the responsive state. Moreover, locomotor behavior during the inter-trial interval was correlated with the responsive state, and was also increased by dopamine receptor agonists. These results suggest that activation of NAc dopamine receptors plays an important role in regulating the probability of approach to food under conditions of normative satiety.

Dopamine invigorates reward seeking by promoting cue-evoked excitation in the nucleus accumbens

Approach to reward is a fundamental adaptive behavior, disruption of which is a core symptom of addiction and depression. Nucleus accumbens (NAc) dopamine is required for reward-predictive cues to activate vigorous reward seeking, but the underlying neural mechanism is unknown. Reward-predictive cues elicit both dopamine release in the NAc and excitations and inhibitions in NAc neurons. However, a direct link has not been established between dopamine receptor activation, NAc cue-evoked neuronal activity, and reward-seeking behavior. Here, we use a novel microelectrode array that enables simultaneous recording of neuronal firing and local dopamine receptor antagonist injection. We demonstrate that, in the NAc of rats performing a discriminative stimulus task for sucrose reward, blockade of either D1 or D2 receptors selectively attenuates excitation, but not inhibition, evoked by reward-predictive cues. Furthermore, we establish that this dopamine-dependent signal is necessary for reward-seeking behavior. These results demonstrate a neural mechanism by which NAc dopamine invigorates environmentally cued reward-seeking behavior.

Behavioral and electrophysiological effects of endocannabinoid and dopaminergic systems on salient stimuli

Rewarding effects have been related to enhanced dopamine (DA) release in corticolimbic and basal ganglia structures. The DAergic and endocannabinoid interaction in the responses to reward is described. This study investigated the link between endocannabinoid and DAergic transmission in the processes that are related to response to two types of reward, palatable food and novelty. Mice treated with drugs acting on endocannabinoid system (ECS) (URB597, AM251) or DAergic system (haloperidol) were submitted to approach-avoidance conflict tasks with palatable food or novelty. In the same mice, the cannabinoid type-1 (CB₁)-mediated GABAergic transmission in medium spiny neurons of the dorsomedial striatum was analyzed. The endocannabinoid potentiation by URB597 magnified approach behavior for reward (food and novelty) and in parallel inhibited dorsostriatal GABAergic neurotransmission. The decreased activity of CB₁ receptor by AM251 (alone or with URB597) or of DAergic D₂ receptor by haloperidol had inhibitory effects toward the reward and did not permit the inhibition of dorsostriatal GABAergic transmission. When haloperidol was coadministered with URB597, a restoration effect on reward and reward-dependent motor activity was observed, only if the reward was the palatable food. In parallel, the coadministration led to restoring inhibition of CB₁-mediated GABAergic transmission. Thus, in the presence of simultaneous ECS activation and inhibition of DAergic system the response to reward appears to be a stimulus-dependent manner.

Dopaminergic mechanisms underlying catalepsy, fear and anxiety: do they interact?

Haloperidol is a dopamine D2 receptor antagonist that induces catalepsy when systemically administered to rodents. The haloperidol-induced catalepsy is a state of akinesia and rigidity very similar to that seen in Parkinson's disease. There exists great interest in knowing whether or not some degree of emotionality underlies catalepsy. If so, what kind of emotional distress would permeate such motor disturbance? This study is an attempt to shed some light on this issue through an analysis of ultrasound vocalizations (USVs) of 22 kHz, open-field test, and contextual conditioned fear in rats with some degree of catalepsy induced by haloperidol. Systemic administration of haloperidol caused catalepsy and decreased exploratory activity in the open-field. There was no difference in the emission of USVs between groups during the catalepsy or the exploratory behavior in the open-field test. In the contextual conditioned fear, when administered before training session, haloperidol did not change the emission of USVs or the freezing response. When administered before testing session, haloperidol enhanced the freezing response and decreased the emission of USVs on the test day. These findings suggest that the involvement of dopaminergic mechanisms in threatening situations depends on the nature of the aversive stimulus. Activation of D2 receptors occurs in the setting up of adaptive responses to conditioned fear stimuli so that these mechanisms seem to be important for the emission of 22 kHz USVs during the testing phase of the contextual conditioned fear, but not during the training session or the open-field test (unconditioned fear stimuli). Catalepsy, on the other hand, is the result of the blockage of D2 receptors in neural circuits associated to motor behavior that appears to be dissociated from those directly linked to dopamine-mediated neural mechanisms associated to fear.

Role of dopamine receptors in the ventral tegmental area in conditioned fear

The increased startle reflex in the presence of a stimulus that has been previously paired with footshock has been termed fear-potentiated startle (FPS) and is considered a reliable index of anxiety. Some studies have suggested an association between stressful situations and alterations in dopaminergic (DA) transmission. Many studies converge on the hypothesis that the mesocorticolimbic pathway, originating from DA neurons in the ventral tegmental area (VTA), is particularly sensitive to fear-arousing stimuli. The present study explored the involvement of VTA DA receptors in the acquisition and expression of conditioned fear to a light conditioned stimulus (CS). We evaluated the effects of intra-VTA administration of SKF 38393 (D(1) agonist), SCH 23390 (D(1) antagonist), quinpirole (D(2) agonist), and sulpiride (D(2) antagonist) on FPS. All drugs were administered bilaterally into the VTA (1.0 microg/0.2 microl/site). Locomotor activity/exploration and motor coordination were evaluated in the open-field and rotarod tests. None of the drugs produced significant effects on FPS when injected before conditioning, indicating that VTA DA receptors are not involved in the acquisition of conditioned fear to a light-CS. In contrast, when injected before the test session, quinpirole significantly reduced FPS, whereas the other drugs had no effect. Quinpirole's ability to decrease FPS may be the result of an action on VTA D(2) presynaptic autoreceptors that decrease dopamine levels in terminal fields of the mesocorticolimbic pathway. Altogether, the present results suggest the importance of VTA DA neurons in the fear-activating effects of Pavlovian conditioning. In addition to demonstrating the importance of dopaminergic mechanisms in the motivational consequences of footshock, the present findings also indicate that these neural circuits are mainly involved in the expression, rather than acquisition, of conditioned fear.

Dopamine D2 receptor mechanisms in the expression of conditioned fear

The increase in the startle reflex in the presence of a stimulus that has been previously paired to foot shock is taken as an index of anxiety and named fear potentiated startle (FPS). Freezing behavior, defined as a cessation of all observable movements except those associated with respiration, has also been used as an index of fear in rats. Recently, a growing body of evidence has suggested that dopaminergic mechanisms are significant for different aspects of affective memory, namely its formation, expression or retrieval. However, the results of studies that have directly examined the ability of the dopaminergic system to influence fear have been inconsistent. This work is aimed at examining the involvement of D1 and D2 receptors in the acquisition and expression of conditioned fear using the fear potentiated startle test and the freezing behavior. We evaluated the effects of systemic administration of the D1 antagonist SCH 23390, the D1 agonist SKF 38393, the D2 antagonist sulpiride and the D2 agonist quinpirole before and after conditioning on FPS and freezing as indices of fear memory. The motor activity of the animals was also evaluated in an open field test. Injections of SCH 23390, SKF 38393, sulpiride and quinpirole before conditioning sessions did not produce any significant effect on FPS, but SCH 23390 decreased freezing. Injections of SCH 23390, SKF 38393 and sulpiride before testing session did not produce any significant effect on FPS or freezing. Quinpirole caused significant reduction in FPS and freezing when injected before testing. Drugs' actions were not due to nonspecific impairments in the ability to respond to the CS because the identical drug treatments had no effect in an open field test. Our findings indicate that DA mechanisms mediated by D2 receptors are mainly involved in the expression rather than in the acquisition of fear.

Dopaminergic mechanisms in the conditioned and unconditioned fear as assessed by the two-way avoidance and light switch-off tests

The involvement of dopaminergic mechanisms in fear and anxiety is still unclear. Behavioral studies aimed to disclose the involvement of dopamine in anxiety have reported anxiolytic-like, anxiogenic-like and lack of effects with the use of dopaminergic agonists and antagonists in animal models of anxiety. This work was an attempt to contribute to this field by providing evidence that these discrepancies may be due to the kind of aversive situation the animals experience in these models. The present study examined the effects of a dopaminergic agonist apomorphine, a dopaminergic D(1) antagonist SCH 23390 and a D(2) receptor antagonist sulpiride on the two-way avoidance response test (CAR) and on the switch-off responses to light (SOR). In both tests, learning was assessed by the performance of the animals across four blocks of 10 trials in which light was paired to footshocks (CAR) or only light was presented to the animals (SOR). The obtained data show that rats learn to make a shuttling response to avoid the shock in the CAR test and maintain a regular pace of switch-off responses in the SOR. While sulpiride and SCH 23390 administrations prevented learning of the avoidance responses, apomorphine injections produced a dose-dependent enhancement in the conditioned learning in the CAR test. The number of escape responses was unchanged by these drugs. In the light-induced switch-off test, apomorphine reduced the number of switch-off responses whereas sulpiride increased these responses. These findings suggest that the involvement of dopaminergic mechanisms in threatening situations depends on the nature of the aversive stimulus. Activation of D(1) and D(2) receptors seems to be implicated in the heightened aversiveness to conditioned stressful situations, as assessed by the CAR test. Thus, blockade of D(1) and D(2) receptors may be necessary for attenuating the aversiveness triggered by these conditioned fear stimuli. In contrast, mechanisms mediated by D(2) receptors seem to be involved in the setting up of adaptive responses to innate fear reactions. Therefore, the signal of the modulatory dopaminergic mechanisms on defensive behavior will depend on the type of emotional stimuli triggering the coping reaction.

Reverse microdialysis of a dopamine D2 receptor antagonist alters extracellular adenosine levels in the rat nucleus accumbens

Recent evidence suggests that modulation of dopaminergic transmission alters striatal levels of extracellular adenosine. The present study used reverse microdialysis of the selective dopamine D(2) receptor antagonist raclopride to investigate whether a blockade of dopamine D(2) receptors modifies extracellular adenosine concentrations in the nucleus accumbens. Results reveal that perfusion of raclopride produced an increase of dialysate adenosine which was significant with a high (10 mM) and intermediate (1 mM) drug concentration, but not with lower drug concentrations (10 and 100 microM). Thus, the present study demonstrates that a selective blockade of dopamine D(2) receptors in the nucleus accumbens produced a pronounced increase of extracellular adenosine. The cellular mechanisms underlying this effect are yet unknown. It is suggested that the increase of extracellular adenosine might be related to a homeostatic modulatory mechanism proposed to be a key function of adenosine in response to neuronal metabolic challenges.

Dopamine D1 or D2 receptor blockade in the globus pallidus produces akinesia in the rat

In the present study, the involvement of dopamine D1 and D2 receptors in the dorsal globus pallidus (GP) in motor control was investigated in rats. Results show that bilateral microinfusions of the dopamine D1 receptor antagonist SCH23390 or the dopamine D2 antagonist S( - )-sulpiride into the GP induced akinesia determined by means of the catalepsy test. These findings indicate that pallidal dopamine D1 and D2 receptors are critically involved in the control of motor behaviour. The findings further imply that defective dopaminergic transmission in the GP might contribute to akinesia due to lesion- or drug-induced dopamine hypofunction in experimental animals and in neurodegenerative diseases, e.g. Parkinson's disease, affecting the nigrostriatal dopamine system.

Dopamine receptor blockade in the nucleus accumbens inhibits maternal retrieval and licking, but enhances nursing behavior in lactating rats

Maternal behaviors were recorded in rats after a 4-h dam-litter separation and intracranial microinfusion of saline on Day 6 postpartum or cis-flupenthixol (FLU), a dopamine (DA) receptor antagonist, on Days 7-9, within the nucleus accumbens (NA) or dorsomedial striatum (DMS) bilaterally (5, 10, or 20 micro/microL/side), or the lateral ventricle (LV) unilaterally (20 or 40 micro/microL). The number of pups retrieved was inhibited in a dosage-dependent manner by FLU within the NA, but not in other sites. Pup retrieval did not occur within 5 min after 20 microg FLU in five out of nine NA dams; only in these dams did infusions include the shell region of the NA. Duration of pup licking was dose dependently decreased by FLU, the most within the NA, and to a lesser extent within the DMS. Nursing behavior in the kyphotic (upright, dorsally arched) posture, initiated in the absence of pup retrieval by placing the dam over the gathered pups, was not inhibited by intracranial FLU in any site assessed, but rather lasted longer after FLU in NA dams. These various effects of FLU, especially in NA, may be related to modest increases in catalepsy.

Blockade of dopamine D2, but not of D1 receptors in the rat globus pallidus induced Fos-like immunoreactivity in the caudate-putamen, substantia nigra and entopeduncular nucleus

In the present study, we investigated Fos-like immunoreactivity (FLI) evoked by pallidal dopamine (DA) D1 and D2 receptor blockade in the caudate-putamen (CPu), substantia nigra (SN) and entopeduncular nucleus (EP), i.e. major target areas of pallidal efferents. Results demonstrate that infusion of the selective D1 antagonist SCH23390 (1 and 4 microg/0.5 microl) into the globus pallidus (GP) did not induce FLI in the CPu, SN and EP. In contrast, intrapallidal infusion of a low dose of the selective D2 antagonist S(-)-sulpiride (15 microg/0.5 microl) induced FLI restricted to the CPu. A higher dose of intrapallidal S(-)-sulpiride (25 microg/0.5 microl) induced FLI in the CPu as well as in the SN and EP. These findings add further evidence to notion that the GP plays a central role in the basal ganglia circuitry and demonstrate an involvement of extrastriatal DA via D2 receptors.

Motor effects and mapping of cerebral alterations in animal models of Parkinson's and Huntington's diseases

Changes in stimulant-induced behavioral effects and subcortical c-Fos expression were compared between rodent models of Parkinson's disease (PD) and Huntington's disease (HD). Rats received either a unilateral 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal dopamine pathway (PD model) or a unilateral infusion of antisense oligodeoxynucleotides targeting c-fos into the striatum (HD model). Dopamine-lesioned animals received intraperitoneal injections of either d-amphetamine (6-OHDAamp group) or apomorphine (6-OHDAapo group), whereas all animals that received antisense infusions received d-amphetamine (ASF group). All groups exhibited robust circling behavior upon stimulant challenge. Changes in subcortical activation, as assessed by the induction of Fos-like immunoreactivity (Fos-LI), were examined in several brain regions. The 6-OHDAamp and ASF groups exhibited robust, ipsiversive circling behavior, with similar changes in Fos-LI in the striatum, entopeduncular nucleus, superior colliculus, and ventromedial thalamus. The 6-OHDAapo group exhibited contraversive rotation and had reciprocal patterns of Fos-LI in these regions. Despite exhibiting the same direction of rotation, the 6-OHDAamp and ASF groups had markedly different patterns of Fos-LI in the globus pallidus and the pontine reticular formation. These results suggest that the globus pallidus may undergo distinct alterations in PD and HD and that the pontine reticular formation is particularly susceptible to changes in mesencephalic dopamine sources.