Changes in cerebral neurotransmitters and metabolites induced by acute donepezil and memantine administrations: A microdialysis study

Effects of glutamate-related drugs on marble-burying behavior in mice: implications for obsessive-compulsive disorder

Clinical evidence demonstrates altered glutamatergic neurotransmission in patients suffering from obsessive-compulsive disorder (OCD). We examined the effects of glutamate-related drugs on marble-burying behavior, which is an animal model of OCD. The uncompetitive N-methyl-d-aspartate (NMDA) antagonists memantine (10 mg/kg, i.p.) and amantadine (30 mg/kg, i.p.) significantly inhibited marble-burying behavior without affecting locomotor activity in mice. Similarly, the uncompetitive NMDA receptor antagonist 5R,10S-(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801, 0.3 mg/kg, i.p.) inhibited marble-burying behavior. However, MK-801 at the same dose markedly increased locomotor activity. By contrast, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and the glutamate release inhibitor riluzole showed no effect on marble-burying behavior and significant suppression of locomotor activity. MK-801 (0.3 mg/kg, i.p.) and memantine (10 mg/kg, i.p.) significantly disrupted prepulse inhibition as an operational measure of sensorimotor gating. By contrast, amantadine (30 mg/kg, i.p.) did not affect prepulse inhibition. These findings suggest that amantadine could be a useful drug for the treatment of OCD.

Food reward-induced neurotransmitter changes in cognitive brain regions

Recent evidence indicates that mechanisms involved in reward and mechanisms involved in learning interact, in that reward includes learning processes and learning includes reward processes. In spite of such interactions, reward and learning represent distinct functions. In the present study, as part of an examination of the differences in learning and reward mechanisms, it was assumed that food principally affects reward mechanisms. After a brief period of fasting, we assayed the release of three neurotransmitters and their associated metabolites in eight brain areas associated with learning and memory as a response to feeding. Using microdialysis for the assay, we found changes in the hippocampus, cortex, amygdala, and the thalamic nucleus, (considered cognitive areas), in addition to those in the nucleus accumbens and ventral tegmental area (considered reward areas). Extracellular dopamine levels increased in the nucleus accumbens, ventral tegmental area, amygdala, and thalamic nucleus, while they decreased in the hippocampus and prefrontal cortex. Dopamine metabolites increased in all areas tested (except the dorsal hippocampus); changes in norepinephrine varied with decreases in the accumbens, dorsal hippocampus, amygdala, and thalamic nucleus, and increases in the prefrontal cortex; serotonin levels decreased in all the areas tested; although its metabolite 5HIAA increased in two regions (the medial temporal cortex, and thalamic nucleus). Our assays indicate that in reward activities such as feeding, in addition to areas usually associated with reward such as the mesolimbic dopamine system, other areas associated with cognition also participate. Results also indicate that several transmitter systems play a part, with several neurotransmitters and several receptors involved in the response to food in a number of brain structures, and the changes in transmitter levels may be affected by metabolism and transport in addition to changes in release in a regionally heterogeneous manner. Food reward represents a complex pattern of changes in the brain that involve cognitive processes. Although food reward elements overlap with other reward systems sharing some neurotransmitter compounds, it significantly differs indicating a specific reward to process for food consumption. Like in other rewards, both learning and cognitive areas play a significant part in food reward.

Combination of rimonabant and donepezil prolongs spatial memory duration

The observations that the cannabinoid(1)(CB(1)) receptor antagonist/inverse agonist, rimonabant, and the selective noncompetitive inhibitor of acetylcholinesterase (AChE), donepezil, improve performance in a variety of animal memory models, suggest that these neurochemical systems play integral roles in cognition. The present study tested whether each of these agents administered alone or in combination will prolong the duration of spatial memory. Rats were trained in a two-phase radial-arm maze procedure, consisting of acquisition and retrieval tests, which were separated by an 18 h delay. Each drug was administered 30 min before the acquisition phase, immediately after the acquisition phase, or 30 min before the retrieval test to assess acquisition/consolidation, consolidation, and retrieval mnemonic processes, respectively. Rimonabant or donepezil administered before the acquisition phase, but not immediately after acquisition or before retrieval, led to a significant decrease in the number of errors committed during the retrieval test. Combined administration of subthreshold doses of rimonabant and donepezil that had no discernable effects on performance when given alone, enhanced memory. These results taken together demonstrate that the delay radial-arm maze task is sufficiently sensitive to detect memory enhancing effects of these drugs. Moreover, these findings suggest that combined administration of subthreshold doses of rimonabant and donepezil can improve memory and may represent a novel approach to treat cognitive deficits associated with neurodegenerative disorders.

Abnormal neurotransmitter release underlying behavioral and cognitive disorders: toward concepts of dynamic and function-specific dysregulation

Abnormalities in the regulation of neurotransmitter release and/or abnormal levels of extracellular neurotransmitter concentrations have remained core components of hypotheses on the neuronal foundations of behavioral and cognitive disorders and the symptoms of neuropsychiatric and neurodegenerative disorders. Furthermore, therapeutic drugs for the treatment of these disorders have been developed and categorized largely on the basis of their effects on neurotransmitter release and resulting receptor stimulation. This perspective stresses the theoretical and practical implications of hypotheses that address the dynamic nature of neurotransmitter dysregulation, including the multiple feedback mechanisms regulating synaptic processes, phasic and tonic components of neurotransmission, compartmentalized release, differentiation between dysregulation of basal vs activated release, and abnormal release from neuronal systems recruited by behavioral and cognitive activity. Several examples illustrate that the nature of the neurotransmitter dysregulation in animal models, including the direction of drug effects on neurotransmitter release, depends fundamentally on the state of activity of the neurotransmitter system of interest and on the behavioral and cognitive functions recruiting these systems. Evidence from evolving techniques for the measurement of neurotransmitter release at high spatial and temporal resolution is likely to advance hypotheses describing the pivotal role of neurotransmitter dysfunction in the development of essential symptoms of major neuropsychiatric disorders, and also to refine neuropharmacological mechanisms to serve as targets for new treatment approaches. The significance and usefulness of hypotheses concerning the abnormal regulation of the release of extracellular concentrations of primary messengers depend on the effective integration of emerging concepts describing the dynamic, compartmentalized, and activity-dependent characteristics of dysregulated neurotransmitter systems.

'I think therefore I am': improving cognition

PURPOSE OF REVIEW

In the absence of a specific treatment for dementia, the effective management of cognitive symptoms is a clinical priority.

RECENT FINDINGS

Although some differences have been observed in the profile of cognitive complaints observed in subtypes of dementia, there is increasing recognition of common, interacting neurobiological causes suggesting the need to seek a common treatment applicable to all causes of cognitive deterioration. There also exists increasing interest in intervening at the level of minor cognitive dysfunction by reducing risk factors for subclinical states.

SUMMARY

Pharmacological treatment of cognitive disorder is beneficial but has only temporary benefit for a subgroup of patients. Pharmacogenetics may have an important future role to play in deciding which patients may best benefit from the treatment. Low side effect therapies such as cognitive therapy and acupuncture show some benefits but their utility in combination with pharmacotherapies remains to be demonstrated. Prevention of milder forms of cognitive disorder by controlling risk factors such as hypertension and diabetes may reduce rates of more severe cognitive degeneration. Persons with cognitive dysfunction are commonly excluded from making decisions about the implementation of cognition-enhancing treatments although they wish to do so.

Behavioral effects of aminoadamantane class NMDA receptor antagonists on schedule-induced alcohol and self-administration of water in mice

RATIONALE

Aminoadamantanes represent a class of NMDA glutamate receptor antagonists that reduce alcohol consumption and may prevent alcohol-induced neuronal adaptations and side effects.

OBJECTIVE

Behavioral specificity of memantine and amantadine on alcohol drinking in a schedule-induced polydipsia (SIP) task was investigated in mice.

METHODS

Male C57BL/6J mice were food-deprived and divided into four groups: 5% alcohol SIP, water SIP, 1 h limited access regulatory water drinking, and a control group to determine if either drug altered ethanol drinking. Behavioral specificity of memantine (5, 10, and 25 mg/kg, ip) and amantadine (20, 40, and 60 mg/kg, ip) was determined by comparing alterations in alcohol or water consumption in SIP and regulatory water drinking. Drug effects on SIP drinking-specific measures (grams per kilogram consumption) were also compared to nondrinking measures (locomotion, head-entries for food, and lick efficiency).

RESULTS

Compared to saline, memantine reduced alcohol SIP drinking (10 and 25 mg/kg). Memantine increased locomotion during alcohol SIP (25 mg/kg) and during water SIP (5 and 25 mg/kg). In contrast, amantadine reduced both alcohol SIP (40 mg/kg) and water SIP (40 and 60 mg/kg). Both drugs reduced regulatory water consumption over the entire dose range tested. Blood alcohol concentrations indicated consumption of physiologically meaningful amounts of alcohol during SIP, and that changes in alcohol metabolism did not account for drug-induced reductions in alcohol drinking.

CONCLUSIONS

In addition to reducing alcohol drinking, both drugs had other behavioral effects that included reductions in regulatory drinking. These results suggest that the therapeutic utility of these drugs for ameliorating human alcohol addiction remains questionable.

The Effects of Glutamate and GABA Receptor Antagonists on Nicotine-induced Neurotransmitter Changes in Cognitive Areas*

In the present study, we tested the effects of glutamate and GABA receptor antagonists on nicotine-induced neurotransmitter changes in the hippocampal (dorsal and ventral) and cortical (medial temporal and prefrontal) brain areas of conscious freely moving rats via microdialysis. Both the antagonists and nicotine were administered intracerebrally. The antagonists tested were NMDA, AMPA-kainate, and metabotropic glutamate receptor subtype antagonists (MK801, CNQX, and LY 341495, respectively) and GABA(A) and GABA(B) receptor subtype antagonists (bicuculline and hydroxysaclofen, respectively). We assayed nicotine-induced changes in dopamine (DA), norepinephrine (NE), serotonin (5-HT), and their metabolites. We found with the antagonists, both decreases and increases in nicotine-induced neurotransmitter responses. In the presence of nicotine all the antagonists (except LY 341495) caused a decrease in DA levels in the regions tested. NE levels were decreased in the cortex by all antagonists. In the hippocampus, GABA antagonists decreased NE levels, as did the metabotropic glutamate antagonist, LY 341495, while the other glutamate antagonists increased NE levels. The results of the 5-HT assay were more variable and dependent on the region and antagonist examined; increases were found slightly more often than decreases. The changes in metabolites were not often parallel with changes in their associated neurotransmitters, indicating that the antagonists also affect the metabolism of the neurotransmitters. The effect of the antagonists in the absence of nicotine was mostly to decrease the level of neurotransmitters, although increases were seen in a few cases. The results suggest that the excitatory glutamatergic- and inhibitory GABAergic-amino acid receptors are both involved in mediating nicotine-induced neurotransmitter responses, and their inhibitory or stimulatory effects are receptor subtype and brain region dependent.