Effects of intracaudate microinjections of 6-hydroxydopamine upon the suppression of lever pressing and upon passive avoidance conditioning in cats

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Facilitation of the suppressing effect of dopamine upon a motor conditioned response by 6-hydroxydopamine applied into the caudate nucleus in cats

It has been described that the application of DA in the Caudate Nucleus improves the learning of an inhibitory motor conditioned response (SMCR). However, the application of 6-hydroxydopamine (6-OHDA) at low doses of 5, 10 and 20 micrograms produced a clear increment of the inhibition, while higher doses of 80 or 160 micrograms diminished markedly the suppression of the MCR. We postulated that although low doses of 6-OHDA produce lesions in the dopaminergic structures they cause hypersensitization by denervation. To prove this hypothesis we observed the effect of DA application into the CN before and after applying one of the lower doses of 6-OHDA. Cats were trained to press a lever (MCR) to obtain 0.5 ml of milk when a conditioned discriminative stimulus (light) was on, and to suppress the response when the light was off (SMCR). The various applications of 10 micrograms of DA in both CNs, always produced a decrement in the lever pressing rate only in the SMCR situation. The effects of the DA applications after the 6-OHDA application were more significant (p < 0.01) than when applied before the 6-OHDA. These findings further support the postulation that catecholamines in the CN improve the inhibitory actions required for the suppression of motor responses.

Differential effects of cholinergic blockade of anterior and posterior caudate nucleus on avoidance behaviors

Groups of rats were trained in a one-trial passive avoidance task and then tested for retention 24 and 48 h later. They were also trained, in a single session, according to a one-way active avoidance paradigm. The effects of microinjections of atropine or of saline into the anterior caudate nucleus (CN) and of atropine into the posterior CN were assessed on these conditioned responses. Only those rats injected with atropine in the anterior CN showed a retention deficit in passive avoidance, while no effects on active avoidance became evident in any of the groups. These results suggest that cholinergic activity of the anterior CN is critically involved in memory consolidation of passive avoidance, but not in the processes mediating the acquisition of relatively simple active avoidance learning.

Facilitation of conditioned motor suppression by microinjections of dopamine in the caudate nucleus of cats

The effects of Caudate Nucleus (CN) injections of catecholamines on the suppression of a motor conditioned response (MCR), lever pressing, was investigated. Cats were trained to press a lever to obtain 0.5 ml of milk when a conditioned discriminative stimulus was on (CS-on-reward on, MCR) and to suppress the response when the light was off (CS-off-reward off, suppression of motor conditioned response, SMCR). The bilateral application of 5 or 10 micrograms of Dopamine (DA) through chronically implanted cannulae in the CN significantly decreased the lever pressing in the non rewarding situation without changing the MCR. Injection of 5 micrograms of L-DOPA caused very small effects assessed during the following 10 min. However, 10 micrograms of L-DOPA produced a significant decrement of lever pressing (CS-off) in two out of four injections. These findings further support the postulation that catecholamines in the CN have a behavioral inhibitory action upon a motor conditioned response.