Effects of systemically and intrastriatally injected haloperidol and apomorphine on grooming, feeding and locomotion in the rat

Simulated Dopamine Modulation of a Neurorobotic Model of the Basal Ganglia

The vertebrate basal ganglia play an important role in action selection-the resolution of conflicts between alternative motor programs. The effective operation of basal ganglia circuitry is also known to rely on appropriate levels of the neurotransmitter dopamine. We investigated reducing or increasing the tonic level of simulated dopamine in a prior model of the basal ganglia integrated into a robot control architecture engaged in a foraging task inspired by animal behaviour. The main findings were that progressive reductions in the levels of simulated dopamine caused slowed behaviour and, at low levels, an inability to initiate movement. These states were partially relieved by increased salience levels (stronger sensory/motivational input). Conversely, increased simulated dopamine caused distortion of the robot's motor acts through partially expressed motor activity relating to losing actions. This could also lead to an increased frequency of behaviour switching. Levels of simulated dopamine that were either significantly lower or higher than baseline could cause a loss of behavioural integration, sometimes leaving the robot in a 'behavioral trap'. That some analogous traits are observed in animals and humans affected by dopamine dysregulation suggests that robotic models could prove useful in understanding the role of dopamine neurotransmission in basal ganglia function and dysfunction.

Anatomical localization of SKF-38393-induced behaviors in rats using the irreversible monoamine receptor antagonist EEDQ

This study was designed to localize the population of dopamine D1-like receptors involved in grooming and oral movements elicited by systemic administration of the D1-selective agonist SKF-38393. Receptors in specific dopamine terminal regions were inactivated by intracranial injection of the nonselective irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The effect of these injections on behaviors induced by systemic administration of SKF-38393 (10 mg/kg) was measured 48 hours later. The specific populations of D1-like receptors inactivated by the EEDQ injections were identified as a loss of 3H-SCH-23390 binding in a given region using quantitative autoradiography. EEDQ (1.5 micrograms/microliters/side) injected into the nucleus accumbens (NAc) did not alter SKF-38393-induced behaviors. Similarly, injection of EEDQ into the medial caudate-putamen (CPu) failed to alter these behaviors. In contrast, EEDQ (0.15-1.5 micrograms/microliters/side) injected into the lateral CPu decreased both SKF-38393-induced grooming and oral movements, with complete blockade of grooming observed at the highest dose. To determine whether this effect of EEDQ was due to inactivation of D1-like receptors, separate groups of animals were pretreated with SCH-23390 (3 mg/kg, S.C.) 15 min prior to injection with EEDQ. Pretreatment with SCH-23390 prevented the disruption of SKF-38393-induced behaviors, as well as the loss of 3H-SCH-23390-labeled binding sites observed after injection of EEDQ into the lateral CPu. EEDQ injections that produced disruption of SKF-38393-induced behaviors were associated with a greater loss of binding in the lateral CPu relative to other regions examined including the NAc, medial CPu, and globus pallidus. Furthermore, EEDQ injections that produced the greatest loss of 3H-SCH-23390 binding in the latter three regions did not disrupt SKF-38393-induced behavior. These results demonstrate that stimulation of D1-like receptors in the lateral CPu is necessary for behaviors induced by systemic administration of SKF-38393. The results also demonstrate the utility of this "receptor lesion" technique to localize receptor-mediated behaviors.

Differential behavioural and neurochemical effects of competitive and non-competitive NMDA receptor antagonists in rats

The behavioural and biochemical effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine and memantine, and the competitive NMDA receptor antagonist, CGP 39551, were investigated in rats. Systemic injections of dizocilpine (0.33 mg/kg) increased locomotion and rearing in an open field, whereas memantine (20 mg/kg) increased only locomotor activity. CGP 39551 (10 and 20 mg/kg) did not change open field activity. Dopamine (DA) metabolism--as measured by the ratio of dihydroxyphenylacetic acid/dopamine (DOPAC/DA)--increased in response to dizocilpine in the prefrontal cortex and the nucleus accumbens. Memantine enhanced DOPAC/DA in the prefrontal cortex, the nucleus accumbens and to a lesser degree in the posterior striatum. In contrast to non-competitive NMDA receptor antagonists, CGP 39551 did not increase DA metabolism of subcortical structures and even decreased DOPAC/DA in the prefrontal cortex. These results indicate that competitive and non-competitive NMDA receptor antagonists affect spontaneous locomotion differentially in rats. The biochemical data imply that the stimulant actions non-competitive NMDA receptor antagonists are at least partially due to activation of ascending dopaminergic systems. Potential mechanisms involved in the differential effects of both types of NMDA receptor antagonists are discussed.

Changes in locomotion and dopamine neurotransmission following amphetamine, haloperidol, and exposure to novel environmental stimuli

Locomotor behavior and dopamine (DA) neurotransmission were assessed in rats exposed to either a novel or familiar stimulus environment while under the influence of amphetamine, haloperidol or saline. The behavioral results indicated that, as expected, amphetamine increased horizontal locomotor activity in a dose-dependent manner. Exposure to novelty also increased horizontal activity, and this behavioral effect was disrupted by both amphetamine and haloperidol. Regardless of whether the animals were exposed to the novel or familiar stimulus environment, amphetamine increased DA synthesis in the nigrostriatal system, but not in the mesolimbic system, whereas haloperidol increased DA synthesis in both the nigrostriatal and mesolimbic systems. Amphetamine also decreased DA metabolism and haloperidol increased DA metabolism in both the nigrostriatal and mesolimbic systems. In contrast, exposure to novelty alone was without effect on DA synthesis or metabolism in any region examined, suggesting that novelty-induced hyperactivity and amphetamine-induced hyperactivity involve different neurochemical mechanisms. However, exposure to novelty while under the influence of haloperidol produced a significant increase in DA metabolism in both the nigrostriatal and mesolimbic systems. These latter results suggest that exposure to novelty may produce a measurable activation of DA systems when the autoreceptors involved in the negative feedback loop are blocked.

The involvement of the nucleus accumbens in the ability of rats to switch to cue-directed behaviours

Previous studies have shown that the dorsal striatum is involved in switching arbitrarily behaviour (switching to non cue-directed behaviours). These experiments also revealed that switching behaviour with the help of currently available sensory information (switching to cue-directed behaviours) was not influenced when striatal function was blocked. A number of studies suggested that the nucleus accumbens could mediate the latter type of switching. For testing this hypothesis we used the swimming-test as it allows for studying separately both cue-directed and non cue-directed behaviours. Rats with cannulae aimed at the nucleus accumbens were forced to swim after injection of distilled water (AD) or d-amphetamine (1-10 micrograms/0.5 microliter) in a circular water-tank from which there was no escape. Their behaviour was recorded and analysed according to the presence of cue-directed and non cue-directed behaviours. After six minutes a rope was introduced into the water-tank to determine whether the rats were able to change their behaviour with the help of this external stimulus as well. Rats treated with d-amphetamine showed an enhanced ability to switch to cue-directed behaviours in comparison with rats treated with AD. There was no effect on the ability to switch to non cue-directed behaviours. Furthermore there was a dose-dependent increase in the ability to escape along the rope. The results are taken as evidence that the nucleus accumbens is involved in switching to cue-directed behaviours.