tetrodotoxin and Schizophrenia

Schizophrenia is a complex and devastating neuropsychiatric disease thought to result from impaired connectivity between several integrative regions, stemming from developmental failures. In particular, the left prefrontal cortex of schizophrenia patients seems to be targeted by such early developmental disturbances. Data obtained over the last three decades support the hypothesis of a dopaminergic dysfunction in schizophrenia. Striatal dopaminergic dysregulation in schizophrenia may result from a dysconnection between the prefrontal cortex and the striatum (dorsal and ventral) involving glutamatergic N-methyl-d-aspartate (NMDA) receptors. In the context of animal modeling of the pathophysiology of schizophrenia, the present study was designed to investigate the effects of MK 801 (dizocilpine) on locomotor activity and dopaminergic responses in the left core part of the nucleus accumbens (ventral striatum) in adult rats following neonatal tetrodotoxin inactivation of the left prefrontal cortex (infralimbic/prelimbic region) at postnatal day 8. Dopaminergic variations were recorded in the nucleus accumbens by means of in vivo voltammetry in freely moving adult animals. Following MK 801 administration, and in comparison to control (PBS) animals, animals microinjected with tetrodotoxin display locomotor hyperactivity and increased extracellular dopamine levels in the core part of the nucleus accumbens. These findings suggest neonatal functional inactivation of the prefrontal cortex may lead to a dysregulation of dopamine release in the core part of the nucleus accumbens involving NMDA receptors. The results obtained may provide new insight into the involvement of NMDA receptors in the pathophysiology of schizophrenia and suggest that future studies should look carefully at the core of the nucleus accumbens.

Topics: Animals; Animals, Newborn; Corpus Striatum; Disease Models, Animal; Dizocilpine Maleate; Dopamine; Dopamine Agents; Locomotion; Male; Neostriatum; Nucleus Accumbens; Prefrontal Cortex; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Schizophrenia; Tetrodotoxin

Cognitive deficits in schizophrenia (SZ) reflect maturational disruptions within a neural system that includes the ventral hippocampus (VH), nucleus accumbens (NAc), basal forebrain, and prefrontal cortex (PFC). A better understanding of these changes may reveal drug targets for more efficacious cognition enhancers. We have utilized an animal model in which the above distributed system is altered, during a sensitive period of development, by transiently inactivating the VH and its efferent projections. We determined the ability of NAc shell activation to evoke prefrontal glutamate release in adult male Wistar rats that had received saline (Sal) or tetrodotoxin (TTX) as neonates (PD7) or as adolescents (PD32). The nucleus accumbens shell (NAcSh) was activated by NMDA infusions (0.05-0.30 μg/0.5 μL). Basal and evoked glutamate levels were measured amperometrically using a glutamate-sensitive microelectrode. There were no differences in basal glutamate levels among the groups tested (overall 1.41 ± 0.26 uM). However, the dose-related stimulation of prefrontal glutamate levels seen in control rats treated with saline on PD7 (4.31 ± 0.22 μM after 0.15 μg) was markedly attenuated in rats treated with TTX on PD7 (0.45 ± 0.12 μM after 0.15 μg). This effect was age-dependent as infusions of TTX on PD32 did not alter the NMDA-induced increases in glutamate release (4.10 ± 0.37 μM after 0.15 μg). Collectively, these findings reveal that transient inactivation of VH transmission, during a sensitive period of development, leads to a functional mesolimbic-cortical disconnection that produces neurochemical and ultimately cognitive impairments resembling those seen in SZ.

Topics: Animals; Animals, Newborn; Catheters, Indwelling; Disease Models, Animal; Electrodes, Implanted; Glutamic Acid; Hippocampus; Male; Microelectrodes; N-Methylaspartate; Nucleus Accumbens; Prefrontal Cortex; Rats, Wistar; Schizophrenia; Sodium Channel Blockers; Tetrodotoxin

Schizophrenia is a debilitating disorder that affects 1% of the US population. While the exogenous administration of cannabinoids such as tetrahydrocannabinol is reported to exacerbate psychosis in schizophrenia patients, augmenting the levels of endogenous cannabinoids has gained attention as a possible alternative therapy to schizophrenia due to clinical and preclinical observations. Thus, patients with schizophrenia demonstrate an inverse relationship between psychotic symptoms and levels of the endocannabinoid anandamide. In addition, increasing endocannabinoid levels (by blockade of enzymatic degradation) has been reported to attenuate social withdrawal in a preclinical model of schizophrenia. Here we examine the effects of increasing endogenous cannabinoids on dopamine neuron activity in the sub-chronic phencyclidine (PCP) model. Aberrant dopamine system function is thought to underlie the positive symptoms of schizophrenia.. Using in vivo extracellular recordings in chloral hydrate-anesthetized rats, we now demonstrate an increase in dopamine neuron population activity in PCP-treated rats.. Interestingly, endocannabinoid upregulation, induced by URB-597, was able to normalize this aberrant dopamine neuron activity. Furthermore, we provide evidence that the ventral pallidum is the site where URB-597 acts to restore ventral tegmental area activity.. Taken together, we provide preclinical evidence that augmenting endogenous cannabinoids may be an effective therapy for schizophrenia, acting in part to restore ventral pallidal activity.

Topics: Animals; Basal Forebrain; Benzamides; Carbamates; Central Nervous System Agents; Disease Models, Animal; Dopaminergic Neurons; Endocannabinoids; Hippocampus; Male; Microelectrodes; Phencyclidine; Rats, Sprague-Dawley; Schizophrenia; Sodium Channel Blockers; Tetrodotoxin; Ventral Tegmental Area

Schizophrenia is a complex psychiatric disorder that may result from defective connectivity, of neurodevelopmental origin, between several integrative brain regions. Different anomalies consistent with brain development failures have been observed in patients' left prefrontal cortex (PFC). A striatal dopaminergic functional disturbance is also commonly acknowledged in schizophrenia and could be related to a dysfunctioning of dopamine-glutamate interactions. Non-competitive NMDA antagonists, such as ketamine, can induce psychotic symptoms in healthy individuals and worsen these symptoms in patients with schizophrenia. Our study set out to investigate the consequences of neonatal functional blockade of the PFC for dopaminergic and behavioral reactivity to ketamine in adult rats. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, dopaminergic responses induced by ketamine (5 mg/kg, 10 mg/kg, 20 mg/kg sc) were monitored using in vivo voltammetry in the left part of the dorsal striatum in freely moving adult rats. Dopaminergic responses and locomotor activity were followed in parallel. Compared to PBS animals, in rats microinjected with TTX, ketamine challenge induced a greater release of dopamine in the dorsal striatum for the highest dose (20 mg/kg sc) and the intermediate dose (10mg/kg sc). A higher increase in locomotor activity in TTX animals was observed only for the highest dose of ketamine (20 mg/kg sc). These data suggest transient inactivation of the PFC during early development results in greater behavioral and striatal dopaminergic reactivity to ketamine in adulthood. Our study provides an anatomo-functional framework that may contribute toward a better understanding of the involvement of NMDA glutamatergic receptors in schizophrenia.

Topics: Animals; Animals, Newborn; Corpus Striatum; Dopamine; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Ketamine; Male; Motor Activity; Prefrontal Cortex; Rats, Sprague-Dawley; Schizophrenia; Tetrodotoxin; Time Factors

Deficient transmission at the glutamate NMDA receptor is considered a key component of the pathophysiology of schizophrenia. However, the effects of antipsychotic drugs on the release of the endogenous NMDA receptor partial agonist, D-serine, remain to be clarified.. We determined the interaction between antipsychotic drugs (clozapine and haloperidol) and transmission-modulating toxins (tetanus toxin, fluorocitrate, tetrodotoxin) on the release of L-glutamate and D-serine in the medial prefrontal cortex (mPFC) of freely moving rats, using microdialysis, and primary cultures of astrocytes using extreme high-pressure liquid chromatography.. Release of L-glutamate and D-serine in the mPFC and in cultured astrocytes was inhibited by tetanus toxin (a synaptobrevin inhibitor) and fluorocitrate (a glial toxin), whereas tetrodotoxin (a voltage-sensitive Na(+) blocker) inhibited depolarization-induced L-glutamate release in the mPFC without affecting that of D-serine. Clozapine (1 and 5 mg·kg(-1)), but not haloperidol (0.5 and 1 mg·kg(-1)), dose-dependently increased L-glutamate and D-serine release from both astrocytes and mPFC. Clozapine-induced release of L-glutamate and D-serine was also reduced by tetanus toxin and fluorocitrate. Tetrodotoxin reduced clozapine-induced mPFC L-glutamate release but not that of D-serine. Clozapine-induced L-glutamate release preceded clozapine-induced D-serine release. MK-801 (a NMDA receptor antagonist) inhibited the delayed clozapine-induced L-glutamate release without affecting that of D-serine.. Clozapine predominantly activated glial exocytosis of D-serine, and this clozapine-induced D-serine release subsequently enhances neuronal L-glutamate release via NMDA receptor activation. The enhanced D-serine associated glial transmission seems a novel mechanism of action of clozapine but not haloperidol.

Topics: Animals; Antipsychotic Agents; Astrocytes; Cells, Cultured; Citrates; Clozapine; Glutamic Acid; Haloperidol; Male; Prefrontal Cortex; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Schizophrenia; Serine; Tetanus Toxin; Tetrodotoxin

Impaired α7 nicotinic acetylcholine receptor (nAChR) function and GABAergic transmission in the hippocampus and elevated brain levels of kynurenic acid (KYNA), an astrocyte-derived metabolite of the kynurenine pathway, are key features of schizophrenia. KYNA acts as a noncompetitive antagonist with respect to agonists at both α7 nAChRs and N-methyl-D-aspartate receptors. Here, we tested the hypothesis that in hippocampal slices tonically active α7 nAChRs control GABAergic transmission to CA1 pyramidal neurons and are sensitive to inhibition by rising levels of KYNA. The α7 nAChR-selective antagonist α-bungarotoxin (α-BGT; 100 nM) and methyllycaconitine (MLA; 10 nM), an antagonist at α7 and other nAChRs, reduced by 51.3 ± 1.3 and 65.2 ± 1.5%, respectively, the frequency of GABAergic postsynaptic currents (PSCs) recorded from CA1 pyramidal neurons. MLA had no effect on miniature GABAergic PSCs. Thus, GABAergic synaptic activity in CA1 pyramidal neurons is maintained, in part, by tonically active α7 nAChRs located on the preterminal region of axons and/or the somatodendritic region of interneurons that synapse onto the neurons under study. L-Kynurenine (20 or 200 μM) or KYNA (20-200 μM) suppressed concentration-dependently the frequency of GABAergic PSCs; the inhibitory effect of 20 μM L-kynurenine had an onset time of approximately 35 min and could not be detected in the presence of 100 nM α-BGT. These results suggest that KYNA levels generated from 20 μM kynurenine inhibit tonically active α7 nAChR-dependent GABAergic transmission to the pyramidal neurons. Disruption of nAChR-dependent GABAergic transmission by mildly elevated levels of KYNA can be an important determinant of the cognitive deficits presented by patients with schizophrenia.

Topics: Aconitine; alpha7 Nicotinic Acetylcholine Receptor; Animals; Bungarotoxins; CA1 Region, Hippocampal; GABAergic Neurons; In Vitro Techniques; Kynurenic Acid; Kynurenine; Male; Nicotinic Antagonists; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Receptors, Nicotinic; Schizophrenia; Sodium Channel Blockers; Synaptic Transmission; Tetrodotoxin

Growing evidence suggests schizophrenia may arise from abnormalities in early brain development. The prefrontal cortex (PFC) stands out as one of the main regions affected in schizophrenia. Latent inhibition, an interesting cognitive marker for schizophrenia, has been found in some studies to be reduced in acute patients. It is generally widely accepted that there is a dopaminergic dysfunctioning in schizophrenia. Moreover, several authors have reported that the psychostimulant, D-amphetamine (D-AMP), exacerbates symptoms in patients with schizophrenia. We explored in rats the effects in adulthood of neonatal transient inactivation of the PFC on behavioral and neurochemical anomalies associated with schizophrenia. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, latent inhibition-related dopaminergic responses and dopaminergic reactivity to D-AMP were monitored using in vivo voltammetry in the left core part of the nucleus accumbens in adult freely moving rats. Dopaminergic responses and behavioral responses were followed in parallel. Prefrontal neonatal inactivation resulted in disrupted behavioral responses of latent inhibition and latent inhibition-related dopaminergic responses in the core subregion. After D-AMP challenge, the highest dose (1.5 mg/kg i.p.) induced a greater dopamine increase in the core in rats microinjected with TTX, and a parallel increase in locomotor activity, suggesting that following prefrontal neonatal TTX inactivation animals display a greater behavioral and dopaminergic reactivity to D-AMP. Transitory inactivation of the PFC early in the postnatal developmental period leads to behavioral and neurochemical changes in adulthood that are meaningful for schizophrenia modeling. The data obtained may help our understanding of the pathophysiology of this disabling disorder.

Topics: Animals; Animals, Newborn; Dextroamphetamine; Disease Models, Animal; Dopamine; Dopamine Uptake Inhibitors; Inhibition, Psychological; Male; Motor Activity; Neural Inhibition; Nucleus Accumbens; Prefrontal Cortex; Rats; Rats, Sprague-Dawley; Schizophrenia; Sodium Channel Blockers; Tetrodotoxin

Latent inhibition has been found to be disrupted in patients with acute schizophrenia. Striatal dopaminergic dysregulation is commonly acknowledged in schizophrenia. This disease may be consecutive to a functional disconnection between integrative regions, stemming from neurodevelopmental failures. Various anomalies suggesting early abnormal brain development have been described in the entorhinal cortex (ENT) and ventral subiculum (SUB) of patients. This study examines the consequences of a neonatal transitory blockade of the left ENT or left SUB for latent inhibition-related dopamine responses in the anterior part of the dorsal striatum using in-vivo voltammetry in freely moving adult rats. Reversible inactivation of both structures in different animals was achieved by local microinjection of tetrodotoxin (TTX) at postnatal day 8. Results obtained during the retention session of a three-stage latent inhibition protocol showed that the functional neonatal disconnection of the ENT or SUB caused the behavioural latent inhibition expression in pre-exposed (PE)-TTX-conditioned adult rats to disappear. After postnatal inactivation of the SUB, PE-TTX-conditioned rats displayed a reversal of the latent inhibition-related striatal dopamine responses, whereas after neonatal blockade of the ENT, dopamine changes in PE-TTX-conditioned rats monitored in the anterior striatum were between those observed in PE-phosphate-buffered-saline-conditioned and non-PE-TTX-conditioned animals. These data suggest that neonatal functional inactivation of the SUB disrupts latent inhibition-related striatal dopamine responses in adult animals more than that of the ENT. They may help improve understanding of the pathophysiology of schizophrenia.

Topics: Animals; Behavior, Animal; Corpus Striatum; Dopamine; Entorhinal Cortex; Female; Hippocampus; Inhibition, Psychological; Male; Neurogenesis; Poisons; Rats; Rats, Sprague-Dawley; Schizophrenia; Tetrodotoxin

Latent inhibition (LI) has been found to be disrupted in non-treated patients with schizophrenia. Dopaminergic (DAergic) dysfunctioning is generally acknowledged to occur in schizophrenia. Various abnormalities in the entorhinal cortex (ENT) have been described in patients with schizophrenia. Numerous data also suggest that schizophrenia has a neurodevelopmental origin. The present study was designed to test the hypothesis that reversible inactivation of the ENT during neonatal development results in disrupted DA responses characteristic of LI in adult rats. Tetrodotoxin (TTX) was microinjected locally in the left ENT at postnatal day 8 (PND8). DA variations were recorded in the dorsomedial shell and core parts of the nucleus accumbens (Nacc) using in vivo voltammetry in freely-moving grown-up rats in a LI paradigm. In the first session the animals were pre-exposed (PE) to the conditional stimulus (banana odour) alone. In the second they were aversively conditioned to banana odour. In the third (test) session the following results were obtained in PE animals subjected to temporary inactivation of the ENT at PND8: (1) aversive behaviour was observed in TTX-PE conditioned animals; (2) DA variations in the dorsomedial shell and core parts of the Nacc were similar in TTX-PE and non-pre-exposed conditioned rats. These findings strongly suggest that neonatal disconnection of the ENT disrupts LI in adult animals. They may further our understanding of the pathophysiology of schizophrenia.

Topics: Animals; Animals, Newborn; Behavior, Animal; Conditioning, Classical; Dopamine; Electrochemistry; Entorhinal Cortex; Female; Humans; Inhibition, Psychological; Male; Nucleus Accumbens; Poisons; Random Allocation; Rats; Rats, Sprague-Dawley; Schizophrenia; Tetrodotoxin

Converging data suggest a dysfunction of prefrontal cortical GABAergic interneurons in schizophrenia. Morphological and physiological studies indicate that cortical GABA cells are modulated by a variety of afferents. The peptide transmitter neurotensin may be one such modulator of interneurons. In the rat prefrontal cortex (PFC), neurotensin is exclusively localized to dopamine axons and has been suggested to be decreased in schizophrenia. However, the effects of neurotensin on cortical interneurons are poorly understood. We used in vivo microdialysis in freely moving rats to assess whether neurotensin regulates PFC GABAergic interneurons. Intra-PFC administration of neurotensin concentration-dependently increased extracellular GABA levels; this effect was impulse dependent, being blocked by treatment with tetrodotoxin. The ability of neurotensin to increase GABA levels in the PFC was also blocked by pretreatment with 2-[1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazole-3-yl)carbonylamino]tricyclo(3.3.1.1 [EC] .3.7)decan-2-carboxylic acid (SR48692), a high-affinity neurotensin receptor 1 (NTR1) antagonist. This finding is consistent with our observation that NTR1 was localized to GABAergic interneurons in the PFC, particularly parvalbumin-containing interneurons. Because neurotensin is exclusively localized to dopamine axons in the PFC, we also determined whether neurotensin plays a role in the ability of dopamine agonists to increase extracellular GABA levels. We found that D2 agonist-elicited increases in PFC GABA levels were blocked by pretreatment with SR48692, consistent with data indicating that D2 autoreceptor agonists increase neurotensin release from dopamine-neurotensin axons in the PFC. These findings suggest that neurotensin plays an important role in regulating prefrontal cortical interneurons and that it may be useful to consider neurotensin agonists as an adjunct in the treatment of schizophrenia.

Topics: Animals; Axons; Dopamine; Dopamine Agonists; gamma-Aminobutyric Acid; Interneurons; Male; Microdialysis; Neurotensin; Parvalbumins; Prefrontal Cortex; Pyrazoles; Quinolines; Quinpirole; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D2; Receptors, Neurotensin; Schizophrenia; Tetrodotoxin

The neural regulation of sensorimotor gating, as measured by prepulse inhibition (PPI) of the startle reflex, has been a focus of interest based on the consistent deficits in PPI reported in schizophrenia patients. While dorsomedial thalamus (MD) dysfunction has been implicated in the clinical 'gating' deficits of schizophrenia patients, relatively little is known regarding the regulation of PPI by the MD. We previously reported that PPI in rats is reduced after intra-MD infusion of the GABA agonist muscimol, or after excitotoxic lesions of the MD. In the present study, we tested the regulation of PPI by D2 receptors in the MD. PPI was measured after intra-MD infusion of the D2 agonist quinpirole (0, 1 or 10 microg/side) in a within-subject design. Infusion placement was confirmed functionally in later tests by reversible inactivation of the MD via intra-MD infusion of tetrodotoxin (TTX; 10 ng/side), and subsequently by direct histological examination. Intra-MD infusion of quinpirole had no significant effect on PPI, using doses that significantly disrupt PPI after infusion into the ventral forebrain (nucleus accumbens). TTX infusion into the MD caused a significant loss of PPI; this effect was not reversed by pretreatment with the atypical antipsychotic quetiapine (7.5 mg/kg). The MD regulation of PPI in rats is not mediated via D2 receptors, but is clearly manifested via PPI deficits after reversible MD inactivation via TTX.

Topics: Animals; Antipsychotic Agents; Dibenzothiazepines; Dopamine; Dopamine Agonists; Male; Mediodorsal Thalamic Nucleus; Quetiapine Fumarate; Quinpirole; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D2; Reflex, Startle; Schizophrenia; Sodium Channel Blockers; Tetrodotoxin

Rats with neonatal excitotoxic damage of the ventral hippocampus display in adulthood a variety of abnormalities reminiscent of schizophrenia and are used as an animal model of this disorder. In the present study, we hypothesized that transient inactivation of ventral hippocampal activity during a critical developmental period may be sufficient to disrupt normal maturation of relevant brain systems and produce similar lasting behavioral changes. We infused tetrodotoxin (TTX) or artificial CSF into the ventral hippocampus on postnatal day 7 (P7) and assessed behavioral changes in response to stress, amphetamine, and (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate in juvenile (P35) and young adult (P56) rats. In adulthood, rats infused neonatally with TTX displayed motor hyperactivity after pharmacological stimulation and after stress compared with sham controls. Analogous TTX infusions in adult animals did not alter these behaviors later in life. These data suggest that transient loss of ventral hippocampal function during a critical time in maturation of intracortical connections permanently changes the development of neural circuits mediating certain dopamine- and NMDA-related behaviors. These results represent a potential new model of aspects of schizophrenia without involving a gross anatomic lesion.

Topics: Age Factors; Aging; Amphetamine; Analysis of Variance; Animals; Animals, Newborn; Behavior, Animal; Central Nervous System Stimulants; Cohort Studies; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Hippocampus; Interpersonal Relations; Male; Microinjections; Motor Activity; Psychomotor Agitation; Rats; Rats, Sprague-Dawley; Schizophrenia; Tetrodotoxin