Effects of metabotropic glutamate receptor 5 on latent inhibition in conditioned taste aversion

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Antipsychotic drug-like facilitation of latent inhibition by a brain-penetrating neurotensin-1 receptor agonist

Latent inhibition (LI) is a measure of cognitive gating and refers to reduced conditioned learning when there is pre-exposure to the conditioned stimulus (CS) before it is paired with the unconditioned stimulus (US). Dysregulation of LI is associated with some neuropsychiatric disorders, including schizophrenia, and the ability to facilitate LI in rodents is a reasonably good predictive test for antipsychotic drugs. Converging evidence supports neurotensin-1 receptor (NTS1) agonists as novel drugs for schizophrenia. Therefore, we investigated the ability of a brain-penetrating, selective NTS1 agonist, PD149163, to facilitate LI in heterozygous Brattleboro rats, a strain that exhibits naturally low LI. Conditioned taste aversion to flavored water (FW; 0.1% saccharin) was induced by pairing it with malaise-inducing injections of lithium chloride (LiCl). Prior to LiCl-FW pairing, rats received subcutaneous injections of saline, or PD149163 (100 µg/kg or 200 µg/kg). Half the rats in each drug group had been allowed to drink FW the day before the LiCl-FW pairing (pre-exposed rats). Two days after pairing, the amount of FW each rat consumed was recorded. LI, defined as significantly greater FW drinking in the pre-exposed group compared with the non pre-exposed group, was exhibited only among rats that received 200 µg/kg of PD149163. These results further support NTS1 agonists as potentially novel drugs for the treatment of schizophrenia.

Behavioral side effects of prophylactic therapies against soman-induced seizures and lethality in rats

Four medical therapies previously shown to exert varying degrees of protection against a convulsant dose of soman were assessed for potential behavioral side effects in a novelty test. In Experiment 1, HI-6 (1-[([4-(aminocarbonyl)pyridino] methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium) (125 mg/kg), scopolamine (1 mg/kg), physostigmine (0.1 mg/kg), levetiracetam (50 mg/kg), and procyclidine (20 mg/kg) were tested separately. In Experiment 2, the combination of HI-6, scopolamine, and physostigmine (termed the physostigmine regimen) or HI-6, levetiracetam, and procyclidine (termed the procyclidine regimen) were tested. In Experiment 3, the metabotropic glutamate modulators DCG-IV ((2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine) (4 mg/kg) and MPEP (2-Methyl-6-(phenylethynyl)pyridine hydrochloride) (30 mg/kg) were tested separately or each drug in combination with HI-6 and procyclidine (termed the DCG-IV regimen and the MPEP regimen, respectively). The results showed that the physostigmine and procyclidine regimens both produced severe cognitive impairment (lack of preference for novelty) and reduced locomotor and rearing activities. The DCG-IV and MPEP regimens caused milder deficits on the same behavioral measures. Some relations were seen between prophylactic capacity and degree of behavioral side effects. Only HI-6 or levetiracetam had no adverse effects on behavior. DCG-IV or MPEP produced some impairment, whereas the detrimental effects of scopolamine or procyclidine were pronounced. The relatively high dose of procyclidine (anticholinergic and antiglutamatergic) needed for prophylactic efficacy may have played a major role for the side effects of the regimens in which the drug was used. It was concluded that behavioral side effects are inevitable for potent prophylactic therapies against soman intoxication.

An updated overview of animal models in neuropsychiatry

Animal models are vital tools to study the genetic, molecular, cellular, and environmental parameters involved in several neuropsychiatric disorders. Over the years, these models have expanded our understanding of the pathogenesis of many neuropsychiatric disorders and neurodegenerative diseases. Although animal models have been widely used in psychiatry, and despite several years of extensive research with these models, their validity is still being investigated and presents a challenge to both investigators and clinicians as well. In this concise review, we will describe the most common animal models utilized in neuropsychiatry, including animal models of depression, anxiety, and psychosis. In addition, we will also discuss the validity and reliability of these models and current challenges in this domain. Furthermore, this work will discuss the role of gene-environment interaction as an additional contributing factor that modulates neuropsychological outcome and its implication on animal models. This overview will give a succinct summary of animal models in psychiatry which will be useful both to the seasoned researcher, as well as novices in the field.

Functional interaction of mGlu5 and NMDA receptors in aversive learning in rats

Metabotropic glutamate receptor 5 (mGlu5) has been implicated in a variety of learning processes and is important for inhibitory avoidance and conditioned taste aversion learning. MGlu5 receptors are physically connected with NMDA receptors and they interact with, and modulate, the function of one another in several brain regions. The present studies used systemic co-administration of an mGlu5 receptor positive allosteric modulator, 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and an NMDA receptor antagonist dizocilpine maleate (MK-801) to characterize the interactions of these receptors in two aversive learning tasks. Male Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance or conditioned taste aversion task. CDPPB (3 or 10mg/kg, s.c.), delivered by itself prior to the conditioning trial, did not have any effect on performance in either task 48 h after training. However, CDPPB (at 3mg/kg) attenuated the MK-801 (0.2mg/kg, i.p.) induced learning deficit in both tasks. CDPPB also reduced MK-801-induced hyperactivity. These results underlie the importance of mGlu5 and NMDA receptor interactions in modulating memory processing, and are consistent with findings showing the efficacy of positive allosteric modulators of mGlu5 receptors in reversing the negative effects of NMDA receptor antagonists on other behaviors such as stereotypy, sensorimotor gating, or working, spatial and recognition memory.

Metabotropic glutamate receptor subtype 5 antagonism in learning and memory

The role of the metabotropic glutamate receptor 5 (mGlu(5) receptor) in learning and memory and other behaviors are reviewed by examining the influence of selective antagonists and genetic knockout on performance. This receptor is involved in spatial learning, contextual fear conditioning, inhibitory avoidance, fear potentiated startle, and conditioned taste aversion. However, mGlu(5) receptor antagonists have proven to be ineffective in other learning tasks, such as the delayed-match-to-position test and a three-hole spatial learning task. Locomotion is often decreased by mGlu(5) receptor antagonists; and other behaviors such as social interaction and consummatory responses can also be affected. In mGlu(5) receptor knockout mice, performance in contextual fear conditioning and spatial water maze tasks is impaired. Although the available evidence is suggestive of an important contribution of mGlu(5) receptors to cognitive functions, further studies are needed, particularly those with in vivo evaluation of the role of mGlu(5) receptors in selective brain regions in different stages of memory formation.

Effect of MPEP on rat's behavioral activity in experimental episodes of hypoxia

PURPOSE

The influence of the selective antagonism of metabotropic glutamate receptor subtype 5 (mGluR5) by MPEP (2-methyl-6-(phenylethynyl)-pyridine) on some behaviors was tested in control groups of rats and in rats exposed to short-term hypoxia once or to repeated episodes of hypoxia.

MATERIAL AND METHODS

We used the following methods: the open field test, the passive avoidance test and the object recognition test. Experimental hypoxia was produced by placing rats in a glass chamber flushed with a mixture of 2% O2 in N2.

RESULTS

MPEP applied intravenously (IV) at the dose of 1 mg kg-1 significantly enhanced locomotor and exploratory activity, impaired acquisition, but improved consolidation and retrieval in the passive avoidance situation and did not alter rats' activity in the object recognition test. The single short-term hypoxia significantly inhibited motility of rats and profoundly impaired acquisition, consolidation and retrieval processes, but the positive effect of MPEP on retrieval was preserved. Hypoxia also did not influence the activity of rats in the object recognition object. The repeated episodes of short-term hypoxia were induced for five consecutive days and it also inhibited motility of rats, but did not change consolidation and retrieval processes. The episodes of hypoxia significantly diminished the beneficial effect of MPEP on consolidation and retrieval, and also the enhancement of locomotor and exploratory activity. MPEP, used in rats subjected to the single or the repeated episodes of short-term hypoxia, did not change recognition memory.

CONCLUSION

MPEP used before the single episode of hypoxia only, had beneficial effect on retrieval.

Metabotropic glutamate receptor 5 in conditioned taste aversion learning

In conditioned taste aversion (CTA), animals learn to avoid a flavored solution (conditioned stimulus, CS) previously paired with internal malaise (unconditioned stimulus, US). Metabotropic glutamate receptor 5 (mGlu5) has been implicated in learning and memory processes and is necessary for CTA. In the present study, local microinjections of a mGlu5-selective antagonist, 3-[2-methyl-1,3-thiazol-4yl)ethynyl]pyridine (MTEP, 0, 1 or 5 microg) into the insular cortex and basolateral amygdala were used in male, Sprague-Dawley rats to examine the role of mGlu5 receptors in the encoding of taste memory. MTEP was infused 20 min before saccharin intake during CTA conditioning. MTEP injection into the basolateral amygdala resulted in robust CTA, similar to the vehicle-treated animals but slowed extinction; that is, MTEP enhanced CTA. MTEP injection into the insular cortex resulted in an increased saccharin intake on the conditioning trial, which potentially influenced the performance on the test trials; MTEP had no effect on CTA learning when controlled access to saccharin was used on the conditioning trial. These results indicate that mGlu5 receptors are involved in taste memories in a region-specific manner.

MGlu5 antagonism impairs exploration and memory of spatial and non-spatial stimuli in rats

Metabotropic glutamate receptor subtype 5 (mGlu5) has been implicated in memory processing in some but not all learning tasks. The reason why this receptor is involved in some tasks but not in others remains to be determined. The present experiments using rats examined effects of the mGlu5-antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP)--applied systemically i.p. (1-10mg/kg) or bilaterally into the prelimbic cortex (1-10 microg)---on the ability of rats to explore and remember new stimuli. A cross-maze, open field, and object recognition task were used to evaluate exploration and memory and it was found that: (1) locomotion during exploration of spatial environments and exploration time at novel objects were reduced by i.p. but not by prelimbic administration of MPEP, (2) spatial short-term memory was impaired in cross-maze and object discrimination was reduced after both types of administration, (3) long-term retention of spatial conditioning in the cross-maze was inhibited after i.p. applications which (4) also inhibited spontaneous alternation performance during maze-exploration. Reduced exploratory locomotion and exploration time after i.p. injections may have contributed to the observed retention impairments. However, the fact that prelimbic administration of MPEP inhibited retention without reducing exploration shows that memory formation was also impacted directly by prelimbic mGlu5 in both spatial and non-spatial learning.

Glutamate receptor metabotropic 7 is cis-regulated in the mouse brain and modulates alcohol drinking

Alcoholism is a heritable disease that afflicts about 8% of the adult population. Its development and symptoms, such as craving, loss of control, physical dependence, and tolerance, have been linked to changes in mesolimbic, mesocortical neurotransmitter systems utilizing biogenic amines, GABA, and glutamate. Identification of genes predisposing to alcoholism, or to alcohol-related behaviors in animal models, has been elusive because of variable interactions of multiple genes with relatively small individual effect size and sensitivity of the predisposing genotype to lifestyle and environmental factors. Here, using near-isogenic advanced animal models with reduced genetic background interactions, we integrate gene mapping and gene mRNA expression data in segregating and congenic mice and identify glutamate receptor metabotropic 7 (Grm7) as a cis-regulated gene for alcohol consumption. Traditionally, the mesoaccumbal dopamine reward hypothesis of addiction and the role of the ionotropic glutamate receptors have been emphasized. Our results lend support to an emerging direction of research on the role of metabotropic glutamate receptors in alcoholism and drug addiction. These data suggest for the first time that Grm7 is a risk factor for alcohol drinking and a new target in addiction therapy.

The ampakine CX546 restores the prepulse inhibition and latent inhibition deficits in mGluR5-deficient mice

In order to test the possible role of mGluR5 signaling in the behavioral endophenotypes of schizophrenia and other psychiatric disorders, we used genetic engineering to create mice carrying null mutations in this gene. Compared to their mGluR5(+/+) littermates, mGluR5(-/-) mice have disrupted latent inhibition (LI) as measured in a thirst-motivated conditioned emotional response procedure. Administration of the positive modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPAR), CX546, during the conditioning phase only, improved the disrupted LI in mGluR5 knockout mice and facilitated LI in control C57BL/6J mice, given extended number of conditioning trails (four conditioning stimulus-unconditioned stimulus). Prepulse inhibition (PPI) was impaired in mGluR5(-/-) mice to a level that could not be disrupted further by the antagonist of N-methyl-D-aspartate receptors - MK-801. PPI deficit of mGluR5(-/-) mice was effectively reversed by CX546, whereas aniracetam had a less pronounced effect. These data provide evidence that a potent positive AMPAR modulator can elicit antipsychotic action and represents a new approach for treatment of schizophrenia.

Pharmacological treatment to augment hole board habituation in prenatal Vitamin D-deficient rats

Neurocognitive impairment has consistently been considered a central and stable feature of schizophrenia. There is much controversy about the effects of neuroleptics on neurocognitive deficits. Thus, further investigations are needed to clarify the pathological substrate of cognitive deficits in schizophrenia as well as to identify pharmacological tools for treatment. Transient prenatal Vitamin D deficiency is considered a developmental model in schizophrenia research. Recently, it was reported that prenatal Vitamin D-depleted rats showed a habituation deficit in the hole board. Here, we tested the effect on hole board habituation of haloperidol (Hal, 0.075 mg/kg, i.p.), risperidone (Ris, 0.2 mg/kg, i.p.) and the mGluR5 agonist CHPG (0.1 mg, i.c.v.) after subchronic treatment. Hal was found to impair habituation in control animals, Ris restored hole board habituation, whereas Hal and CHPG normalised hole board habituation in the deplete animals completely. The results of the study demonstrate that (i) the Vitamin D model might be a valuable tool in the study of neurodevelopmental aspects of schizophrenia, (ii) the model is sensitive in detecting the effect of antipsychotic drugs and (iii) the model appears to be sensitive in differentiating between typical and atypical antipsychotic drug.

Latent inhibition of conditioned taste aversion is not disrupted, but can be enhanced, by selective nucleus accumbens shell lesions in rats

Latent inhibition is a form of negative priming in which repeated non-reinforced pre-exposures to a stimulus retard subsequent learning about the predictive significance of that stimulus. The nucleus accumbens shell and the anatomical projection it receives from the hippocampal formation have been attributed a pivotal role in the control or regulation of latent inhibition expression. A number of studies in rats have demonstrated the efficacy of selective shell lesions to disrupt latent inhibition in different associative learning paradigms, including conditioned active avoidance and conditioned emotional response. Here, we extended the test to the conditioned taste aversion paradigm, in which the effect of direct hippocampal damage on latent inhibition remains controversial. We demonstrated the expected effect of selective shell lesions on latent inhibition of conditioned emotional response and of conditioned active avoidance, before evaluating in a separate cohort of rats the effect of comparable selective lesions on latent inhibition of conditioned taste aversion: a null effect of the lesions was first obtained using parameters known to be sensitive to amphetamine treatment, then an enhancement of latent inhibition was revealed with a modified conditioned taste aversion procedure. Our results show that depending on the associative learning paradigm chosen, shell lesions can disrupt or enhance the expression of latent inhibition; and the pattern is reminiscent of that seen following hippocampal damage.

Metabotropic glutamate receptor antagonists but not NMDA antagonists affect conditioned taste aversion acquisition in the parabrachial nucleus of rats

The effect of glutamate receptor antagonists on conditioned taste aversion (CTA) was studied in rats. The association of the short-term memory of a gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in the CTA paradigm takes place in the parabrachial nuclei (PBN) of the brainstem. The first direct evidence of participation of glutamatergic neurotransmission in the PBN during CTA demonstrated that the extracellular level of glutamate rises during saccharin drinking (Bielavska et al. in Brain Res 887:413-417, 2000). Our results show an effect of microdialysis administration of selective GluR antagonists into the PBN on the formation of CTA engram. We used four glutamate receptor (GluR) antagonists of different types (D-AP5, MK-801 as antagonists of ionotropic GluR and L-AP3, MSPG as antagonists of metabotropic GluR). The disruptive effect of MK-801 on CTA formation in the PBN is concentration-dependent, with the greatest inhibition under the higher concentrations eliciting significant disruption. The application of D-AP5 (0.1, 1, 5 mM) did not elicit a statistically significant blockade of CTA acquisition. This indicates that the association of the US-CS in the PBN is not dependent on NMDA receptors. On the contrary, application of L-AP3 (0.1, 1, 5 mM) blocked the CS-US association.