Stimulant drug effects on touchscreen automated paired-associates learning (PAL) in rats

Effects of the T-type calcium channel antagonist Z944 on paired associates learning and locomotor activity in rats treated with the NMDA receptor antagonist MK-801

RATIONALE

Currently available antipsychotics are unsatisfactory given their side effects and limited efficacy for the cognitive symptoms of schizophrenia. Many currently available drugs, such as haloperidol, are T-type calcium channel antagonists in addition to their well-established antagonism of dopamine D2 receptors. Thus, preclinical research into the effects of T-type calcium channel antagonists/blockers in behavioral assays related to schizophrenia may inform novel therapeutic strategies.

OBJECTIVES

We explored the effects of a recently developed highly selective T-type calcium channel antagonist, Z944 (2.5, 5.0, 10.0 mg/kg), on the MK-801 (0.15 mg/kg) model of acute psychosis.

METHODS

To examine the effects of Z944 on behaviors relevant to schizophrenia, we tested touchscreen-based paired associates learning given its relevance to the cognitive symptoms of the disorder and locomotor activity given its relevance to the positive symptoms.

RESULTS

Acute treatment with Z944 failed to reverse the visuospatial associative memory impairments caused by MK-801 in paired associates learning. The highest dose of drug (10.0 mg/kg) given alone produced subtle impairments on paired associates learning. In contrast, Z944 (5.0 mg/kg) blocked the expected increase in locomotion following MK-801 treatment in a locomotor assay.

CONCLUSIONS

These experiments provide support that Z944 may reduce behaviors relevant to positive symptoms of schizophrenia, although additional study of its effects on cognition is required. These findings and other research suggest T-type calcium channel antagonists may be an alternative to currently available antipsychotics with less serious side effects.

Acute stress, but not corticosterone, facilitates acquisition of paired associates learning in rats using touchscreen-equipped operant conditioning chambers

Acute stress influences learning and memory in humans and rodents, enhancing performance in some tasks while impairing it in others. Typically, subjects preferentially employ striatal-mediated stimulus-response strategies in spatial memory tasks following stress, making use of fewer hippocampal-based strategies which may be more cognitively demanding. Previous research demonstrated that the acquisition of rodent paired associates learning (PAL) relies primarily on the striatum, while task performance after extensive training is impaired by hippocampal disruption. Therefore, we sought to explore whether the acquisition of PAL, an operant conditioning task involving spatial stimuli, could be enhanced by acute stress. Male Long-Evans rats were trained to a predefined criterion in PAL and then subjected to either a single session of restraint stress (30 min) or injection of corticosterone (CORT; 3 mg/kg). Subsequent task performance was monitored for one week. We found that rats subjected to restraint stress, but not those rats injected with CORT, performed with higher accuracy and efficiency, when compared to untreated controls. These results suggest that while acute stress enhances the acquisition of PAL, CORT alone does not. This dissociation may be due to differences between these treatments and their ability to produce sufficient catecholamine release in the amygdala, a requirement for stress effects on memory.

Effects of dorsal hippocampus catecholamine depletion on paired-associates learning and place learning in rats

Growing evidence suggests that the catecholamine (CA) neurotransmitters dopamine and noradrenaline support hippocampus-mediated learning and memory. However, little is known to date about which forms of hippocampus-mediated spatial learning are modulated by CA signaling in the hippocampus. Therefore, in the current study we examined the effects of 6-hydroxydopamine-induced CA depletion in the dorsal hippocampus on two prominent forms of hippocampus-based spatial learning, that is learning of object-location associations (paired-associates learning) as well as learning and choosing actions based on a representation of the context (place learning). Results show that rats with CA depletion of the dorsal hippocampus were able to learn object-location associations in an automated touch screen paired-associates learning (PAL) task. One possibility to explain this negative result is that object-location learning as tested in the touchscreen PAL task seems to require relatively little hippocampal processing. Results further show that in rats with CA depletion of the dorsal hippocampus the use of a response strategy was facilitated in a T-maze spatial learning task. We suspect that impaired hippocampus CA signaling may attenuate hippocampus-based place learning and favor dorsolateral striatum-based response learning.