Deep brain stimulation of the ventral hippocampus restores deficits in processing of auditory evoked potentials in a rodent developmental disruption model of schizophrenia

Advances in the study of phencyclidine-induced schizophrenia-like animal models and the underlying neural mechanisms

Schizophrenia (SZ) is a chronic, severe mental disorder with heterogeneous clinical manifestations and unknown etiology. Research on SZ has long been limited by the low reliability of and ambiguous pathogenesis in schizophrenia animal models. Phencyclidine (PCP), a noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist, rapidly induces both positive and negative symptoms of SZ as well as stable SZ-related cognitive impairment in rodents. However, the neural mechanism underlying PCP-induced SZ-like symptoms is not fully understood. Nondopaminergic pathophysiology, particularly excessive glutamate release induced by NMDAR hypofunction in the prefrontal cortex (PFC), may play a key role in the development of PCP-induced SZ-like symptoms. In this review, we summarize studies on the behavioral and metabolic effects of PCP and the cellular and circuitary targets of PCP in the PFC and hippocampus (HIP). PCP is thought to target the ventral HIP-PFC pathway more strongly than the PFC-VTA pathway and thalamocortical pathway. Systemic PCP administration might preferentially inhibit gamma-aminobutyric acid (GABA) neurons in the vHIP and in turn lead to hippocampal pyramidal cell disinhibition. Excitatory inputs from the HIP may trigger sustained, excessive and pathological PFC pyramidal neuron activation to mediate various SZ-like symptoms. In addition, astrocyte and microglial activation and oxidative stress in the cerebral cortex or hippocampus have been observed in PCP-induced models of SZ. These findings perfect the hypoglutamatergic hypothesis of schizophrenia. However, whether these effects direct the consequences of PCP administration and how about the relationships between these changes induced by PCP remain further elucidation through rigorous, causal and direct experimental evidence.

Dopaminergic dysfunction and excitatory/inhibitory imbalance in treatment-resistant schizophrenia and novel neuromodulatory treatment

Antipsychotic drugs are the mainstay in the treatment of schizophrenia. However, one-third of patients do not show adequate improvement in positive symptoms with non-clozapine antipsychotics. Additionally, approximately half of them show poor response to clozapine, electroconvulsive therapy, or other augmentation strategies. However, the development of novel treatment for these conditions is difficult due to the complex and heterogenous pathophysiology of treatment-resistant schizophrenia (TRS). Therefore, this review provides key findings, potential treatments, and a roadmap for future research in this area. First, we review the neurobiological pathophysiology of TRS, particularly the dopaminergic, glutamatergic, and GABAergic pathways. Next, the limitations of existing and promising treatments are presented. Specifically, this article focuses on the therapeutic potential of neuromodulation, including electroconvulsive therapy, repetitive transcranial magnetic stimulation, transcranial direct current stimulation, and deep brain stimulation. Finally, we propose multivariate analyses that integrate various perspectives of the pathogenesis, such as dopaminergic dysfunction and excitatory/inhibitory imbalance, thereby elucidating the heterogeneity of TRS that could not be obtained by conventional statistics. These analyses can in turn lead to a precision medicine approach with closed-loop neuromodulation targeting the detected pathophysiology of TRS.

Using cortical non-invasive neuromodulation as a potential preventive treatment in schizophrenia - A review

BACKGROUND

Evidence suggests that schizophrenia constitutes a neurodevelopmental disorder, characterized by a gradual emergence of behavioral and neurobiological abnormalities over time. Therefore, applying early interventions to prevent later manifestation of symptoms is appealing.

OBJECTIVE

This review focuses on the use of cortical neuromodulation in schizophrenia and its potential as a preventive treatment approach. We present clinical and preclinical findings investigating the use of neuromodulation in schizophrenia, including the current research focusing on cortical non-invasive stimulation and its possibility as a future preventive treatment.

METHODS

We performed a search in Medline (PubMed) in September 2020 using a combination of relevant medical subject headings (MeSH) and text words. The search included human and preclinical trials as well as existing systematic reviews and meta-analysis. There were no restrictions on language or the date of publication.

RESULTS

Neurodevelopmental animal models may be used to investigate how the disease progresses and thus which brain areas ideally should be targeted at a given time point. Here, abnormalities of the prefrontal cortex have been often identified as an early and persistent impairment in schizophrenia. Currently there is insufficient evidence to either support or refute the use of neuromodulation to the cortex in adult patients with already manifested symptoms. However, preclinical results show that early non-invasive neuromodulation to the prefrontal cortex of adolescent animals, sufficiently prevents later psychosis-relevant abnormalities in adulthood. This points to the promising potential of cortical non-invasive neuromodulation as a preventive treatment when applied early in the course of the disease.

CONCLUSION

Preclinical translational-oriented findings indicate, that neuromodulation to cortical areas offers the possibility of targeting early neuropathology and through this diminish the progression of a later schizophrenic profile. Further studies are needed to investigate whether such early cortical stimulation may serve as a future preventive treatment in schizophrenia.

Effects of repetitive transcranial magnetic and deep brain stimulation on long-range synchrony of oscillatory activity in a rat model of developmental schizophrenia

Aberrant neuronal network activity likely resulting from disturbed interactions of excitatory and inhibitory systems may be a major cause of cognitive deficits in neuropsychiatric diseases, like within the spectrum of schizophrenic phenotypes. In particular, the synchrony and pattern of oscillatory brain activity appears to be disturbed within limbic networks, e.g. between prefrontal cortex and hippocampus. In a rat model of maternal immune activation (MIA), we compared the acute effects of deep brain stimulation within either medial prefrontal cortex or ventral hippocampus with the effects of repetitive transcranial magnetic stimulation (rTMS), using the intermittent theta-burst protocol (iTBS), on oscillatory activity within limbic structures. Simultaneous local field potential recordings were made from medial prefrontal cortex, ventral hippocampus, nucleus accumbens and rostral part of ventral tegmental area before and after deep brain stimulation in anaesthetized rats previously (~3 h) treated with sham or verum rTMS. We found a waxing and waning pattern of theta and gamma activity in all structures which was less synchronous in particular between medial prefrontal cortex and ventral hippocampus in MIA offspring. Deep brain stimulation in medial prefrontal cortex and pre-treatment with iTBS-rTMS partly improved this pattern. Gamma-theta cross-frequency coupling was stronger in MIA offspring and could partly be reduced by deep brain stimulation in medial prefrontal cortex. We can confirm aberrant limbic network activity in a rat MIA model, and at least acute normalizing effects of the neuromodulatory methods. It has to be proven whether these procedures can have chronic effects suitable for therapeutic purposes.

Modular, Circuit-Based Interventions Rescue Hippocampal-Dependent Social and Spatial Memory in a 22q11.2 Deletion Syndrome Mouse Model

BACKGROUND

22q11.2 deletion syndrome (22qDS) manifests with myriad symptoms, including multiple neuropsychiatric disorders. Complications associated with the polygenic haploinsufficiency make 22qDS symptoms particularly difficult to manage with traditional therapeutic approaches. However, the varying mechanistic consequences often culminate to generate inappropriate regulation of neuronal circuit activity. We explored whether managing this aberrant activity in adults could be a therapeutically beneficial strategy.

METHODS

To assess and dissect hippocampal circuit function, we performed functional imaging in acute slices and targeted eloquent circuits (specific subcircuits tied to specific behavioral tasks) to provide relevant behavioral outputs. For example, the ventral and dorsal CA1 regions critically support social and spatial discrimination, respectively. We focally introduced chemogenetic constructs in 34 control and 24 22qDS model mice via adeno-associated viral vectors, driven by excitatory neuron-specific promoter elements, to manipulate circuit recruitment in an on-demand fashion.

RESULTS

22qDS model mice exhibited CA1 excitatory ensemble hyperexcitability and concomitant behavioral deficits in both social and spatial memory. Remarkably, acute chemogenetic inhibition of pyramidal cells successfully corrected memory deficits and did so in a regionally specific manner: ventrally targeted constructs rescued only social behavior, while those expressed dorsally selectively affected spatial memory. Additionally, manipulating activity in control mice could recapitulate the memory deficits in a regionally specific manner.

CONCLUSIONS

These data suggest that retuning activity dysregulation can rescue function in disease-altered circuits, even in the face of a polygenetic haploinsufficiency with a strong developmental component. Targeting circuit excitability in a focal, modular manner may prove to be an effective therapeutic for treatment-resistant symptoms of mental illness.

Long-term potentiation prevents ketamine-induced aberrant neurophysiological dynamics in the hippocampus-prefrontal cortex pathway in vivo

N-methyl-D-aspartate receptor (NMDAr) antagonists such as ketamine (KET) produce psychotic-like behavior in both humans and animal models. NMDAr hypofunction affects normal oscillatory dynamics and synaptic plasticity in key brain regions related to schizophrenia, particularly in the hippocampus and the prefrontal cortex. It has been shown that prior long-term potentiation (LTP) occluded the increase of synaptic efficacy in the hippocampus-prefrontal cortex pathway induced by MK-801, a non-competitive NMDAr antagonist. However, it is not clear whether LTP could also modulate aberrant oscillations and short-term plasticity disruptions induced by NMDAr antagonists. Thus, we tested whether LTP could mitigate the electrophysiological changes promoted by KET. We recorded HPC-PFC local field potentials and evoked responses in urethane anesthetized rats, before and after KET administration, preceded or not by LTP induction. Our results show that KET promotes an aberrant delta-high-gamma cross-frequency coupling in the PFC and an enhancement in HPC-PFC evoked responses. LTP induction prior to KET attenuates changes in synaptic efficiency and prevents the increase in cortical gamma amplitude comodulation. These findings are consistent with evidence that increased efficiency of glutamatergic receptors attenuates cognitive impairment in animal models of psychosis. Therefore, high-frequency stimulation in HPC may be a useful tool to better understand how to prevent NMDAr hypofunction effects on synaptic plasticity and oscillatory coordination in cortico-limbic circuits.

Biomarkers and neuromodulation techniques in substance use disorders

Addictive disorders are a severe health concern. Conventional therapies have just moderate success and the probability of relapse after treatment remains high. Brain stimulation techniques, such as transcranial Direct Current Stimulation (tDCS) and Deep Brain Stimulation (DBS), have been shown to be effective in reducing subjectively rated substance craving. However, there are few objective and measurable parameters that reflect neural mechanisms of addictive disorders and relapse. Key electrophysiological features that characterize substance related changes in neural processing are Event-Related Potentials (ERP). These high temporal resolution measurements of brain activity are able to identify neurocognitive correlates of addictive behaviours. Moreover, ERP have shown utility as biomarkers to predict treatment outcome and relapse probability. A future direction for the treatment of addiction might include neural interfaces able to detect addiction-related neurophysiological parameters and deploy neuromodulation adapted to the identified pathological features in a closed-loop fashion. Such systems may go beyond electrical recording and stimulation to employ sensing and neuromodulation in the pharmacological domain as well as advanced signal analysis and machine learning algorithms. In this review, we describe the state-of-the-art in the treatment of addictive disorders with electrical brain stimulation and its effect on addiction-related neurophysiological markers. We discuss advanced signal processing approaches and multi-modal neural interfaces as building blocks in future bioelectronics systems for treatment of addictive disorders.

Optogenetic inhibition of ventral hippocampal neurons alleviates associative motor learning dysfunction in a rodent model of schizophrenia

Schizophrenia (SZ) is a serious and incurable mental disorder characterized by clinical manifestations of positive and negative symptoms and cognitive dysfunction. High-frequency deep brain stimulation (DBS) of the ventral hippocampus (VHP) has been recently applied as a therapeutic approach for SZ in both experimental and clinical studies. However, little is known about the precise mechanism of VHP-DBS treatment for SZ and the role of hippocampal cell activation in the pathogenesis of SZ. With optogenetic technology in this study, we tried to inhibit neuronal activity in the VHP which has dense projections to the prefrontal cortex, before measuring long stumulus-induced delay eyeblink conditioning (long-dEBC) in a rodent model of SZ. Rats were administrated with phencyclidine (PCP, 3 mg/kg, 1/d, ip) for successive 7 days before optogenetic intervention. The current data show that PCP administration causes significant impairment in the acquisition and timing of long-dEBC; the inhibition of bilateral VHP neurons alleviates the decreased acquisition and impaired timing of longd-dEBC in PCP-administered rats. The results provide direct evidence at the cellular level that the inhibition of VHP neuronal cells may be a prominent effect of hippocampal DBS intervention, and increased activity in the hippocampal network play a pivotal role in SZ.

Approaches to neuromodulation for schizophrenia

Based on the success of deep brain stimulation (DBS) for treating movement disorders, there is growing interest in using DBS to treat schizophrenia (SZ). We review the unmet needs of patients with SZ and the scientific rationale behind the DBS targets proposed in the literature in order to guide future development of DBS to treat this vulnerable patient population. SZ remains a devastating disorder despite treatment. Relapse, untreated psychosis, intolerable side effects and the lack of effective treatment for negative and cognitive symptoms contribute to poor outcome. Novel therapeutic interventions are needed to treat SZ and DBS is emerging as a potential intervention. Convergent genetic, pharmacological and neuroimaging evidence implicating neuropathology associated with psychosis is consistent with SZ being a circuit disorder amenable to striatal modulation with DBS. Many of the DBS targets proposed in the literature may modulate striatal dysregulation. Additional targets are considered for treating tardive dyskinesia and negative and cognitive symptoms. A need is identified for the concurrent development of neurophysiological biomarkers relevant to SZ pathology in order to inform DBS targeting. Finally, we discuss the current clinical trials of DBS for SZ, and their ethical considerations. We conclude that patients with severe symptoms despite treatment must have the capacity to consent for a DBS clinical trial in which risks can be estimated, but benefit is not known. In addition, psychiatric populations should have access to the potential benefits of neurosurgical advances.

Targeted neural network interventions for auditory hallucinations: Can TMS inform DBS?

The debilitating and refractory nature of auditory hallucinations (AH) in schizophrenia and other psychiatric disorders has stimulated investigations into neuromodulatory interventions that target the aberrant neural networks associated with them. Internal or invasive forms of brain stimulation such as deep brain stimulation (DBS) are currently being explored for treatment-refractory schizophrenia. The process of developing and implementing DBS is limited by symptom clustering within psychiatric constructs as well as a scarcity of causal tools with which to predict response, refine targeting or guide clinical decisions. Transcranial magnetic stimulation (TMS), an external or non-invasive form of brain stimulation, has shown some promise as a therapeutic intervention for AH but remains relatively underutilized as an investigational probe of clinically relevant neural networks. In this editorial, we propose that TMS has the potential to inform DBS by adding individualized causal evidence to an evaluation processes otherwise devoid of it in patients. Although there are significant limitations and safety concerns regarding DBS, the combination of TMS with computational modeling of neuroimaging and neurophysiological data could provide critical insights into more robust and adaptable network modulation.

Ketamine induced converged synchronous gamma oscillations in the cortico-basal ganglia network of nonhuman primates

N-methyl-d-aspartate (NMDA) antagonists are widely used in anesthesia, pain management, and schizophrenia animal model studies, and recently as potential antidepressants. However, the mechanisms underlying their anesthetic, psychotic, cognitive, and emotional effects are still elusive. The basal ganglia (BG) integrate input from different cortical domains through their dopamine-modulated connections to achieve optimal behavior control. NMDA antagonists have been shown to induce gamma oscillations in human EEG recordings and in rodent cortical and BG networks. However, network relations and implications to the primate brain are still unclear. We recorded local field potentials (LFPs) simultaneously from the primary motor cortex (M1) and the external globus pallidus (GPe) of four vervet monkeys (26 sessions, 97 and 76 cortical and pallidal LFPs, respectively) before and after administration of ketamine (NMDA antagonist, 10 mg/kg im). Ketamine induced robust, spontaneous gamma (30-50 Hz) oscillations in M1 and GPe. These oscillations were initially modulated by ultraslow oscillations (~0.3 Hz) and were highly synchronized within and between M1 and the GPe (mean coherence magnitude = 0.76, 0.88, and 0.41 for M1-M1, GPe-GPe, and M1-GPe pairs). Phase differences were distributed evenly around zero with broad and very narrow distribution for the M1-M1 and GPe-GPe pairs (-3.5 ± 31.8° and -0.4 ± 6.0°), respectively. The distribution of M1-GPe phase shift was skewed to the left with a mean of -18.4 ± 20.9°. The increased gamma coherence between M1 and GPe, two central stages in the cortico-BG loops, suggests a global abnormal network phenomenon with a unique spectral signature, which is enabled by the BG funneling architecture.NEW & NOTEWORTHY This study is the first to show spontaneous gamma oscillations under NMDA antagonist in nonhuman primates. These oscillations appear in synchrony in the cortex and the basal ganglia. Phase analysis refutes the confounding effects of volume conduction and supports the funneling and amplifying architecture of the cortico-basal ganglia loops. These results suggest an abnormal network phenomenon with a unique spectral signature that could account for pathological mental and neurological states.

Cellular and circuit models of increased resting-state network gamma activity in schizophrenia

Schizophrenia (SCZ) is a disorder characterized by positive symptoms (hallucinations, delusions), negative symptoms (blunted affect, alogia, reduced sociability, and anhedonia), as well as persistent cognitive deficits (memory, concentration, and learning). While the biology underlying subjective experiences is difficult to study, abnormalities in electroencephalographic (EEG) measures offer a means to dissect potential circuit and cellular changes in brain function. EEG is indispensable for studying cerebral information processing due to the introduction of techniques for the decomposition of event-related activity into its frequency components. Specifically, brain activity in the gamma frequency range (30-80Hz) is thought to underlie cognitive function and may be used as an endophenotype to aid in diagnosis and treatment of SCZ. In this review we address evidence indicating that there is increased resting-state gamma power in SCZ. We address how modeling this aspect of the illness in animals may help treatment development as well as providing insights into the etiology of SCZ.

Neurosurgery for schizophrenia: an update on pathophysiology and a novel therapeutic target

The main objectives of this review were to provide an update on the progress made in understanding specific circuit abnormalities leading to psychotic symptoms in schizophrenia and to propose rational targets for therapeutic deep brain stimulation (DBS). Refractory schizophrenia remains a major unsolved clinical problem, with 10%-30% of patients not responding to standard treatment options. Progress made over the last decade was analyzed through reviewing structural and functional neuroimaging studies in humans, along with studies of animal models of schizophrenia. The authors reviewed theories implicating dysfunction in dopaminergic and glutamatergic signaling in the pathophysiology of the disorder, paying particular attention to neurosurgically relevant nodes in the circuit. In this context, the authors focused on an important pathological circuit involving the associative striatum, anterior hippocampus, and ventral striatum, and discuss the possibility of targeting these nodes for therapeutic neuromodulation with DBS. Finally, the authors examined ethical considerations in the treatment of these vulnerable patients. The functional anatomy of neural circuits relevant to schizophrenia remains of great interest to neurosurgeons and psychiatrists and lends itself to the development of specific targets for neuromodulation. Ongoing progress in the understanding of these structures will be critical to the development of potential neurosurgical treatments of schizophrenia.

Aberrant high frequency oscillations recorded in the rat nucleus accumbens in the methylazoxymethanol acetate neurodevelopmental model of schizophrenia

BACKGROUND

Altered activity of the nucleus accumbens (NAc) is thought to be a core feature of schizophrenia and animal models of the disease. Abnormal high frequency oscillations (HFO) in the rat NAc have been associated with pharmacological models of schizophrenia, in particular the N-methyl-d-aspartate receptor (NMDAR) hypofunction model. Here, we tested the hypothesis that abnormal HFO are also associated with a neurodevelopmental rat model.

METHODS

Using prenatal administration of the mitotoxin methylazoxymethanol acetate (MAM) we obtained the offspring MAM rats. Adult MAM and Sham rats were implanted with electrodes, for local field potential recordings, in the NAc.

RESULTS

Spontaneous HFO (spHFO) in MAM rats were characterized by increased power and frequency relative to Sham rats. MK801 dose-dependently increased the power of HFO in both groups. However, the dose-dependent increase in HFO frequency found in Sham rats was occluded in MAM rats. The antipsychotic compound, clozapine reduced the frequency of HFO which was similar in both MAM and Sham rats. Further, HFO were modulated in a similar manner by delta oscillations in both MAM and Sham rats.

CONCLUSION

Together these findings suggest that increased HFO frequency represents an important feature in certain animal models of schizophrenia. These findings support the hypothesis that altered functioning of the NAc is a core feature in animal models of schizophrenia.

Brain circuit dysfunction in a distinct subset of chronic psychotic patients

OBJECTIVE

To identify the mechanism of unexplained hyponatremia and primary polydipsia in schizophrenia and its relationship to the underlying psychiatric illness.

METHODS

Briefly review previous studies that led to the conclusion the hyponatremia reflects altered hippocampal inhibition of peripheral neuroendocrine secretion. In greater detail, present the evidence supporting the hypothesis that circuit dysfunction associated with the hyponatremia and the polydipsia contributes to the underlying mental disorder.

RESULTS

Polydipsic patients with and without hyponatremia exhibit enhanced neuroendocrine responses to psychological stress in proportion to structural deformations on their anterior hippocampus, amygdala and anterior hypothalamus. Nonpolydipsic patients exhibit blunted responses and deformations on other hippocampal and amygdala surfaces. The deformations in polydipsic patients are also proportional to diminished peripheral oxytocin levels and impaired facial affect recognition that is reversed by intranasal oxytocin. The anterior hippocampus is at the hub of a circuit that modulates neuroendocrine and other responses to psychological stress and is implicated in schizophrenia. Preliminary data indicate that other measures of stress reactivity are also enhanced in polydipsics and that the functional connectivity of the hippocampus with the other structures in this circuitry differs in schizophrenia patients with and without polydipsia.

CONCLUSION

Polydipsia may identify a subset of schizophrenia patients whose enhanced stress reactivity contributes to their mental illness. Stress reactivity may be a symptom dimension of chronic psychosis that arises from circuit dysfunction that can be modeled in animals. Hence polydipsia could be a biomarker that helps to clarify the pathophysiology and heterogeneity of psychosis as well as identify novel therapies. Clinical investigators should consider obtaining indices of water balance, as these may help them unravel and more concisely interpret their findings. Basic researchers should assess if the polydipsic subset is a patient group particularly suitable to test hypotheses arising from their translational studies.

Deep brain stimulation of the medial septum or nucleus accumbens alleviates psychosis-relevant behavior in ketamine-treated rats

Deep brain stimulation (DBS) has been shown to be effective for relief of Parkinson's disease, depression and obsessive-compulsive disorder in humans, but the effect of DBS on psychosis is largely unknown. In previous studies, we showed that inactivation of the medial septum or nucleus accumbens normalized the hyperactive and psychosis-related behaviors induced by psychoactive drugs. We hypothesized that DBS of the medial septum or nucleus accumbens normalizes the ketamine-induced abnormal behaviors and brain activity in freely moving rats. Male Long-Evans rats were subcutaneously injected with ketamine (3 mg/kg) alone, or given ketamine and DBS, or injected with saline alone. Subcutaneous injection of ketamine resulted in loss of gating of hippocampal auditory evoked potentials (AEPs), deficit in prepulse inhibition (PPI) and hyperlocomotion, accompanied by increased hippocampal gamma oscillations of 70-100 Hz. Continuous 130-Hz stimulation of the nucleus accumbens, or 100-Hz burst stimulation of the medial septum (1s on and 5s off) significantly attenuated ketamine-induced PPI deficit and hyperlocomotion. Medial septal stimulation also prevented the loss of gating of hippocampal AEPs and the increase in hippocampal gamma waves induced by ketamine. Neither septal or accumbens DBS alone without ketamine injection affected spontaneous locomotion or PPI. The results suggest that DBS of the medial septum or nucleus accumbens may be an effective method to alleviate psychiatric symptoms of schizophrenia. The effect of medial septal DBS in suppressing both hippocampal gamma oscillations and abnormal behaviors induced by ketamine suggests that hippocampal gamma oscillations are a correlate of disrupted behaviors.

Evidence for impaired sound intensity processing during prepulse inhibition of the startle response in a rodent developmental disruption model of schizophrenia

A number of studies have implicated disruptions in prepulse inhibition (PPI) of the startle response in both schizophrenia patients and animal models of this disorder. These disruptions are believed to reflect deficits in sensorimotor gating and are ascribed to aberrant filtering of sensory inputs leading to sensory overload and enhanced "noise" in neural structures. Here we examined auditory evoked potentials in a rodent model of schizophrenia (MAM-GD17) during an auditory PPI paradigm to better understand this phenomenon. MAM rats exhibited reductions in specific components of auditory evoked potentials in the orbitofrontal cortex and an abolition of the graded response to stimuli of differing intensities indicating deficient intensity processing in the orbitofrontal cortex. These data indicate that aberrant sensory information processing, rather than being attributable to enhanced noise in neural structures, may be better attributed to diminished evoked amplitudes resulting in a reduction in the "signal-to-noise" ratio. Therefore, the ability for sensory input to modulate the ongoing background activity may be severely disrupted in schizophrenia yielding an internal state which is insufficiently responsive to external input.

Deep brain stimulation of the mediodorsal thalamic nucleus yields increases in the expression of zif-268 but not c-fos in the frontal cortex

This study explores the regions activated by deep brain stimulation of the mediodorsal thalamic nucleus through examination of immediate early genes as markers of neuronal activation. Stimulation was delivered unilaterally with constant current 100 μs duration pulses at a frequency of 130 Hz delivered at an amplitude of 200 μA for 3h. Brains were removed, sectioned and radio-labelled for the IEGs zif-268 and c-fos. In anaesthetised rats, deep brain stimulation of mediodorsal thalamic nucleus produced robust increases in the expression of zif-268 but not c-fos localised to regions that are reciprocally connected with the mediodorsal thalamic nucleus, including the prelimbic and orbitofrontal cortices, and the premotor cortex indicating an increase in synaptic activity in these regions. These findings map those brain regions that are persistently, rather than transiently, activated by high frequency electrical stimulation of the mediodorsal thalamic nucleus by a putatively antidromic mechanism which may be relevant to neuropsychiatric disorders such as schizophrenia in which thalamocortical systems are disrupted and in which DBS protocols are being considered.