Different MK-801 administration schedules induce mild to severe learning impairments in an operant conditioning task: Role of buspirone and risperidone in ameliorating these cognitive deficits

Uncovering the complex genetics of human temperament

Experimental studies of learning suggest that human temperament may depend on the molecular mechanisms for associative conditioning, which are highly conserved in animals. The main genetic pathways for associative conditioning are known in experimental animals, but have not been identified in prior genome-wide association studies (GWAS) of human temperament. We used a data-driven machine learning method for GWAS to uncover the complex genotypic-phenotypic networks and environmental interactions related to human temperament. In a discovery sample of 2149 healthy Finns, we identified sets of single-nucleotide polymorphisms (SNPs) that cluster within particular individuals (i.e., SNP sets) regardless of phenotype. Second, we identified 3 clusters of people with distinct temperament profiles measured by the Temperament and Character Inventory regardless of genotype. Third, we found 51 SNP sets that identified 736 gene loci and were significantly associated with temperament. The identified genes were enriched in pathways activated by associative conditioning in animals, including the ERK, PI3K, and PKC pathways. 74% of the identified genes were unique to a specific temperament profile. Environmental influences measured in childhood and adulthood had small but significant effects. We confirmed the replicability of the 51 Finnish SNP sets in healthy Korean (90%) and German samples (89%), as well as their associations with temperament. The identified SNPs explained nearly all the heritability expected in each sample (37-53%) despite variable cultures and environments. We conclude that human temperament is strongly influenced by more than 700 genes that modulate associative conditioning by molecular processes for synaptic plasticity and long-term memory.

The complex genetics and biology of human temperament: a review of traditional concepts in relation to new molecular findings

Recent genome-wide association studies (GWAS) have shown that temperament is strongly influenced by more than 700 genes that modulate associative conditioning by molecular processes for synaptic plasticity and long-term learning and memory. The results were replicated in three independent samples despite variable cultures and environments. The identified genes were enriched in pathways activated by behavioral conditioning in animals, including the two major molecular pathways for response to extracellular stimuli, the Ras-MEK-ERK and the PI3K-AKT-mTOR cascades. These pathways are activated by a wide variety of physiological and psychosocial stimuli that vary in positive and negative valence and in consequences for health and survival. Changes in these pathways are orchestrated to maintain cellular homeostasis despite changing conditions by modulating temperament and its circadian and seasonal rhythms. In this review we first consider traditional concepts of temperament in relation to the new genetic findings by examining the partial overlap of alternative measures of temperament. Then we propose a definition of temperament as the disposition of a person to learn how to behave, react emotionally, and form attachments automatically by associative conditioning. This definition provides necessary and sufficient criteria to distinguish temperament from other aspects of personality that become integrated with it across the life span. We describe the effects of specific stimuli on the molecular processes underlying temperament from functional, developmental, and evolutionary perspectives. Our new knowledge can improve communication among investigators, increase the power and efficacy of clinical trials, and improve the effectiveness of treatment of personality and its disorders.

Effects of N-methyl-D-aspartate receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure

RATIONALE

The N-methyl-D-aspartate (NMDA) receptor has been recently identified as an important mediator of impulsive choice, as assessed in delay discounting. Although discounting is independently influenced by sensitivity to reinforcer magnitude and delayed reinforcement, few studies have examined how NMDA receptor ligands differentially affect these parameters.

OBJECTIVES

The current study examined the effects of various NMDA receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure.

METHODS

Following behavioral training, rats received treatments of the following NMDA receptor ligands: the uncompetitive antagonists ketamine (0, 1.0, 5.0, or 10.0 mg/kg; i.p.), MK-801 (0, 0.003, 0.01, or 0.03 mg/kg; s.c.), and memantine (0, 2.5, 5.0, or 10.0 mg/kg; i.p.), the competitive antagonist CGS 19755 (0, 5.0, 10.0, or 20.0 mg/kg; s.c.), the non-competitive NR2B subunit-selective antagonist ifenprodil (0, 1.0, 3.0, or 10.0 mg/kg; i.p), and the partial agonist D-cycloserine (0, 3.25, 15.0, or 30.0 mg/kg; s.c.).

RESULTS

When an exponential model was used to describe discounting, CGS 19755 (5.0 mg/kg) increased impulsive choice without altering sensitivity to reinforcer magnitude. Conversely, ketamine (10.0 mg/kg), memantine (5.0 mg/kg), and ifenprodil (10.0 mg/kg) decreased sensitivity to reinforcer magnitude without altering impulsive choice. MK-801 and D-cycloserine did not alter delay-discounting performance, although two-way ANOVA analyses indicated D-cycloserine (15.0 mg/kg) decreased impulsive choice.

CONCLUSIONS

The behavioral changes observed in delay discounting following administration of NMDA receptor antagonists do not always reflect an alteration in impulsive choice. These results emphasize the utility in employing quantitative methods to assess drug effects in delay discounting.

Neurotransmitter signaling pathways required for normal development in Xenopus laevis embryos: a pharmacological survey screen

Neurotransmitters are not only involved in brain function but are also important signaling molecules for many diverse cell types. Neurotransmitters are widely conserved, from evolutionarily ancient organisms lacking nervous systems through man. Here, results are reported from a loss- and gain-of-function survey, using pharmacological modulators of several neurotransmitter pathways to examine possible roles for these pathways in normal embryogenesis. Applying reagents targeting the glutamatergic, adrenergic and dopaminergic pathways to embryos of Xenopus laevis from gastrulation to organogenesis stages, we observed and quantified numerous malformations, including craniofacial defects, hyperpigmentation, muscle mispatterning and miscoiling of the gut. These data implicate several key neurotransmitters in new embryonic patterning roles, reveal novel earlier stages for processes involved in eye development, suggest new targets for subsequent molecular-genetic investigation, and highlight the necessity for in-depth toxicology studies of psychoactive compounds to which human embryos might be exposed during pregnancy.

Effects of enriched environment on alterations in the prefrontal cortex GFAP- and S100B-immunopositive astrocytes and behavioral deficits in MK-801-treated rats

A plethora of studies have indicated that enriched environment (EE) paradigm provokes plastic and morphological changes in astrocytes with accompanying increments of their density and positively affects the behavior of rodents. We also previously documented that EE could be employed to preclude several behavioral abnormalities, mainly cognitive deficits, attributed to postnatal N-methyl-d-aspartate (NMDA) receptor antagonist (MK-801) treatment, as a rodent model of schizophrenia (SCH) aspects. Given this, the current study quantitatively investigated the number of cells, presumed to be astrocytes, expressing two astroglia-associated proteins (S100B and glial fibrillary acidic protein (GFAP)) by immunohistochemistry in the prefrontal cortex (PFC), along with anxiety and passive avoidance (PA) learning behaviors by utilizing elevated plus maze (EPM) and shuttle-box tests, in MK-801-treated male wistar rats submitted to EE and non-EE rats. Following a treatment regime of sub-chronic MK-801 (1.0mg/kg i.p. daily for five consecutive days from postnatal day (P) 6), S-100B-positive cells and anxiety level were markedly increased, while the GFAP-positive cells and PA learning were notably attenuated. The trend of diminished GFAP-immunopositive cells and elevated S100B-immunostained cells in the PFC was reversed in the SCH-like rats by exposure of animals to EE, commencing from birth up to the time of experiments on P28-85. Additionally, EE exhibited an ameliorating effect on the behavioral abnormalities evoked by MK-801. Overall, present findings support that improper astrocyte functioning and behavioral changes, reminiscent of the many facets of SCH, occur consequential to repetitive administration of MK-801 and that raising rat pups in an EE mitigates these alterations.

Modulatory effects of an NMDAR partial agonist in MK-801-induced memory impairment

RATIONALE

Acute administration of the N-methyl-d-aspartate (NMDA) non-competitive antagonist, MK-801, impairs novel object recognition (NOR), locomotor activity in open field (OF) and conditioned taste aversion (CTA) in rodents. NMDAR partial agonist d-cycloserine (DCS) reverses these effects in NOR and CTA via modulation of glutamatergic, cholinergic and dopaminergic systems.

OBJECTIVES AND METHODS

To test this hypothesis, we investigated the effects of DCS, a partial NMDAR agonist, on NOR memory, locomotor activity, and CTA memory in Wistar rats on NMDA-glutamate receptor antagonism by MK-801. The potential involvement of dopaminergic and cholinergic systems in improving cognitive functions was explored. MK-801-induced cognitive deficits were assessed using NOR, OF and CTA paradigms. MK-801-induced dopamine release increase in acetylcholinesterase (AChE), mono amine oxidase (MAO) activity and increase in c-fos expression were also investigated.

RESULTS

The effects caused by MK-801 (0.2 mg/kg) were inhibited by administration of the NMDA receptor agonist DCS (15 mg/kg). NOR and CTA paradigms inhibited by MK-801 were attenuated by DCS administration. Moreover, DCS also blocked the MK-801-induced abnormal increase in dopamine content, AChE activity and MAO activity. However, c-fos overexpression was controlled to some extent only.

CONCLUSIONS

Based on the NMDAR hypo function hypothesis in some neuropsychiatric disorders, our finding suggests that improving NMDAR hypo function by agonist DCS may play a significant role.

Dopamine bioavailability in the mPFC modulates operant learning performance in rats: an experimental study with a computational interpretation

Dopamine encodes reward and its prediction in reinforcement learning. Catechol-O-methyltransferase (COMT) activity in the medial prefrontal cortex (mPFC) has been shown to influence cognitive abilities by modifying dopamine clearance. Nevertheless, it is unknown how COMT in the mPFC influences operant learning. Systemic entacapone (50mg/kg), as well as local entacapone (3 pg) and recombinant COMT (17 μg) in the mPFC were administered to male Long Evans rats prior to training in an operant conditioning task. We found that systemic and local administration of the COMT inhibitor entacapone significantly improves learning performance. Conversely, recombinant COMT administration totally impaired learning. These data have been interpreted through a computational model where the phasic firing of dopaminergic neurons was computed by means of a temporal difference algorithm and dopamine bioavailability in the mPFC was simulated with a gating window. The duration of this window was selected to simulate the effects of inhibited or enhanced COMT activity (by entacapone or recombinant COMT respectively). The model accounts for an improved performance reproducing the entacapone effects, and a detrimental impact on learning when the clearance is increased reproducing the recombinant COMT effects. The experimental and computational results show that learning performance can be deeply influenced by COMT manipulations in the mPFC.