Effects of amphetamine, morphine and dizocilpine (MK-801) on spontaneous alternation in the 8-arm radial maze

MK-801 increases the baseline level of anxiety in mice introduced to a spatial memory task without prior habituation

C57BL/6J mice were introduced to a nine arm radial maze without prior habituation and trained in the acquisition of a working memory task in 16 sessions, one session per day. In this maze mice need to climb onto an upward inclined bridge in order to reach and cross onto an arm. They received in each session an i.p. injection of MK-801 (0.1 mg/kg) 30 min before training or immediately after training. MK-801 pre-treated mice made significantly more entries onto the bridges, fewer entries onto the arms and took significantly longer time to make a first arm visit compared to saline and MK-801 post-treated mice during the first 3 session blocks (4 sessions per block). These results indicate that MK-801 induced anxiety which was extended throughout the first 3 session blocks. MK-801 pre-treated mice made also significantly more errors and required more sessions to reach the criterion compared to saline and MK-801 post-treated mice. Administration of MK-801 after training did not affect the acquisition of the task. The present results indicate that MK-801 pre-treatment impaired the acquisition of a spatial task and this can be accounted for by its effect on the baseline level of anxiety which was elevated. The introduction of mice to the acquisition of the task without prior habituation demonstrates that a drug treatment can affect learning and memory by increasing and/or prolonging anxiety. Such effect may be confounded with learning and memory performance and not detected with pre-habituation training procedures, particularly when the number of sessions is determined a-priori.

Metabolic and type 1 cannabinoid receptor imaging of a transgenic rat model in the early phase of Huntington disease

Several lines of evidence imply early alterations in metabolic and endocannabinoid neurotransmission in Huntington disease (HD). Using [(18)F]MK-9470 and small animal PET, we investigated for the first time cerebral changes in type 1 cannabinoid (CB1) receptor binding in vivo in pre-symptomatic and early symptomatic rats of HD (tgHD), in relation to glucose metabolism, morphology and behavioral testing for motor and cognitive function. Twenty-three Sprague-Dawley rats (14 tgHD and 9 wild-types) were investigated between the age of 2 and 11 months. Relative glucose metabolism and parametric CB1 receptor images were anatomically standardized to Paxinos space and analyzed voxel-wise. Volumetric microMRI imaging was performed to assess HD neuropathology. Within the first 10 months, bilateral volumes of caudate-putamen and lateral ventricles did not significantly differ between genotypes. Longitudinal- and genotype evolution showed that relative [(18)F]MK-9470 binding progressively decreased in the caudate-putamen and lateral globus pallidus of tgHD rats (-8.3%, p≤1.1×10(-5) at 5 months vs. -10.9%, p<1.5×10(-5) at 10 months). In addition, relative glucose metabolism increased in the bilateral sensorimotor cortex of 2-month-old tgHD rats (+8.1%, p≤1.5×10(-5)), where it was positively correlated to motor function at that time point. TgHD rats developed cognitive deficits at 6 and 11 months of age. Our findings point to early regional dysfunctions in endocannabinoid signalling, involving the lateral globus pallidus and caudate-putamen. In vivo CB1 receptor measurements using [(18)F]MK-9470 may thus be a useful early biomarker for HD. Our results also provide evidence of subtle motor and cognitive deficits at earlier stages than previously described.

Selective cognitive deficits and reduced hippocampal brain-derived neurotrophic factor mRNA expression in small-conductance calcium-activated K+ channel deficient mice

Small-conductance calcium-activated K(+) channels 1-3 (SK1-3) are important for neuronal firing regulation and are considered putative CNS drug targets. For instance non-selective SK blockers improve performance in animal models of cognition. The SK subtype(s) involved herein awaits identification and the question is difficult to address pharmacologically due to the lack of subtype-selective SK-channel modulators. In this study, we used doxycycline-induced conditional SK3-deficient (T/T) mice to address the cognitive consequences of selective SK3 deficiency. In T/T mice SK3 protein is near-eliminated from the brain following doxycycline treatment. We tested T/T and wild type (WT) littermate mice in five distinct learning and memory paradigms. In Y-maze spontaneous alternations and five-trial inhibitory avoidance the performance of T/T mice was markedly inferior to WT mice. In contrast, T/T and WT mice performed equally well in passive avoidance, object recognition and the Morris water maze. Thus, some aspects of working/short-term memory are disrupted in T/T mice. Using in situ hybridization, we further found the cognitive deficits in T/T mice to be paralleled by reduced brain-derived neurotrophic factor (BDNF) mRNA expression in the dentate gyrus and CA3 of the hippocampus. BDNF mRNA levels in the frontal cortex were not affected. BDNF has been crucially implicated in many cognitive processes. Hence, the biological substrate for the cognitive impairments in T/T mice could conceivably entail reduced trophic support of the hippocampus.

Behavioral abnormalities precede neuropathological markers in rats transgenic for Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG repeat leading to the synthesis of an aberrant protein and to the formation of polyglutamine (polyQ)-containing inclusions and aggregates. Limited information is available concerning the association of neuropathological markers with the development of behavioral markers in HD. Using a previously generated transgenic rat model of HD (tgHD rat), we performed association studies on the time-course of behavioral symptoms (motor function, learning, anxiety) and the appearance of striatal atrophy, 1C2 immunopositive aggregates and polyQ recruitment sites, a precursor to these aggregates. At the age of 1 month, tgHD rats exhibited reduced anxiety and improved motor performance, while at 6 months motor impairments and at 9 months cognitive decline occurred. In contrast, polyQ recruitment sites appeared at around 6-9 months of age, indicating that HD-like behavioral markers preceded the appearance of currently detectable neuropathological markers. Interestingly, numerous punctate sites containing polyQ aggregates were also seen in areas receiving afferents from the densely recruiting regions suggesting either transport of recruitment-competent aggregates to terminal projections where initially 1C2 positive aggregates were formed or different internal properties of neurons in different regions. Furthermore, striatal atrophy was observed at the age of 12 months. Taken together, our findings support the hypothesis of a dynamic process leading to region- and age-specific polyQ recruitment and aggregation. The dissociation of onset between behavioral and neuropathological markers is suggestive of as yet undetected processes, which contribute to the early phenotype of these HD transgenic rats.

Effects of methylphenidate and morphine on delay-discount functions obtained within sessions

Four rats responded under a "self-control" procedure designed to obtain delay-discount functions within sessions. Each session consisted of seven blocks, with seven trials within each block. Each block consisted of two initial forced-choice trials followed by five free-choice trials. On choice trials, the rats could press either of two retractable levers. A press on one lever was followed by presentation of a smaller reinforcer (a single dipper presentation of a sucrose solution); a press on the other lever was followed by presentation of a larger reinforcer (four consecutive dipper presentations). The delay associated with the smaller reinforcer always was 0 s, whereas the signaled delay associated with the larger reinforcer increased across blocks (from 0 to 50 s). Under these conditions, the percentage of choices of the larger reinforcer decreased across blocks, and relatively reliable delay-discount functions were obtained within sessions. Doses of methylphenidate (1.0 to 17.0 mg/kg) and morphine (0.3 to 17.0 mg/kg) were then administered prior to selected sessions. Typically, intermediate doses of methylphenidate shifted the discount functions to the right (increased choices of the larger reinforcer). For 2 of the rats, this effect was pronounced; for the other 2 rats, this effect occurred after the range of delays for the larger reinforcer was decreased (0 to 20 s). On the other hand, in most cases morphine produced a slight leftward shift in the discount function (decreased choices of the larger reinforcer). The present procedure appears to be a useful and efficient method to characterize drug effects on an entire delay-discount function. As with many procedures used to study self-control choices, however, sources of control other than reinforcement delay and amount may have been operating in the present study, and these sources must be considered when interpreting drug effects.

Integrative Proteomics: Functional and Molecular Characterization of a Particular Glutamate-Related Neuregulin Isoform

Glutamate is the major excitatory neurotransmitter in the mammalian brain and is related to memory by calcium-conducting receptors. Neuregulins have emerged as long-term modulating molecules of synaptic signaling by glutamate receptors, playing a role in some cognition/memory-related disorders and moreover being part of transient functional microdomains, called lipid rafts. Here we characterize one specific isoform of neuregulin as a central biomarker for glutamate-related signaling, integrating results from in vitro and in vivo models by a differential functional and proteomic approach.

Effects of NMDA receptor channel blockers, MK-801 and memantine, on locomotor activity and tolerance to delay of reward in Wistar-Kyoto and spontaneously hypertensive rats

N-Methyl-D-aspartate (NMDA) receptor blockade enhances motor activity and stimulates dopamine metabolism, effects shared with classical psychostimulant drugs. The present study aimed to characterize behavioral effects of two NMDA receptor channel blockers, MK-801 and memantine, in both Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In Experiment 1, SHR rats demonstrated higher spontaneous locomotor activity and spent more time in the central area of the open field apparatus compared with WKY rats. Rats of both strains pre-treated with MK-801 (0.01-0.3 mg/kg) or memantine (1-32 mg/kg) demonstrated dose-dependent increases in the total distance traveled and time spent in the central area. Experiment 2 was based on the two-lever discrete-trial delayed reinforcement task in which rats could press one lever to obtain one pellet immediately or another lever for four pellets delivered after a variable delay (0-60 s). Tolerance to delay of reward did not differ between strains. MK-801 (0.03-0.3 mg/kg) and memantine (1-10 mg/kg) produced small, but significant, facilitation of the large-reward lever responding and markedly impaired operant performance at higher dose levels (increased number of missed trials). For both experiments, effects of MK-801 and memantine were more pronounced in WKY compared with SHR rats. Additional studies are needed to address the utility of noncompetitive NMDA receptor blockers in the treatment of attention deficit and hyperactivity disorder.

Evidence for disrupted NMDA receptor function in tissue plasminogen activator knockout mice

Tissue plasminogen activator (tPA), a serine protease immediate-early gene product expressed in brain areas important in learning and memory, has been shown to cleave the NR1 subunit of the NMDA receptor leading to a potentiated Ca(2+) influx. Mice lacking tPA (tPA-/- mice) have disrupted late phase-LTP in the hippocampus, possibly as a consequence of reduced Ca(2+) flux through NMDA receptors. In the present experiments, we investigated whether the NMDA antagonist dizocilpine might alter performance in tPA-/- mice in behavioural tasks shown to be sensitive to hippocampal lesions. tPA-/- mice and wild-type controls (WT) showed similar rates of acquisition and performance of a spatial working memory task (eight-arm radial maze). Dizocilpine (0.03-0.3 mg/kg, i.p.), given acutely, disrupted performance by increasing the number of errors equally across both genotypes. At asymptotic performance of a differential reinforcement of low response rate operant task (DRL), acute dizocilpine (0.03-0.3 mg/kg) impaired performance, but no differences between genotypes were observed. However, dizocilpine (0.1 mg/kg), given repeatedly during acquisition of a signalled-DRL15" task, retarded acquisition in tPA-/- but not WT mice. This treatment regime had no effect on locomotor activity in either genotype. tPA-/- mice showed no spatial learning deficits, but were more sensitive to dizocilpine during acquisition (though not expression) of a DRL task. This supports a role for tPA in modification of the NMDA receptor, although absence of tPA does not have consequences for all forms of NMDA-dependent mediated learning.