Behavioral and frontal cortical metabolic effects of apomorphine and muscimol microinjections into the mediodorsal thalamic nucleus

Activation of mediodorsal thalamic dopamine receptors inhibited nicotine-induced anxiety in rats: A possible role of corticolimbic NMDA neurotransmission and BDNF expression

The present study aimed to evaluate the functional interaction between the dopaminergic and glutamatergic systems of the mediodorsal thalamus (MD), the ventral hippocampus (VH), and the prefrontal cortex (PFC) in nicotine-induced anxiogenic-like behaviors. Brain-derived neurotrophic factor (BDNF) level changes were measured in the targeted brain areas following the drug treatments. The percentage of time spent in the open arm (% OAT) and open arm entry (% OAE) were calculated in the elevated plus maze (EPM) to measure anxiety-related behaviors in adult male Wistar rats. Systemic administration of nicotine at a dose of 0.5 mg/kg induced an anxiogenic-like response associated with decreased BDNF levels in the hippocampus and the PFC. Intra-MD microinjection of apomorphine (0.1-0.3 μg/rat) induced an anxiogenic-like response, while apomorphine inhibited nicotine-induced anxiogenic-like behaviors associated with increased hippocampal and PFC BDNF expression levels. Interestingly, the blockade of the VH or the PFC NMDA receptors via the microinjection of D-AP5 (0.3-0.5 μg/rat) into the targeted sites reversed the inhibitory effect of apomorphine (0.5 μg/rat, intra-MD) on the nicotine response and led to the decrease of BDNF levels in the hippocampus and the PFC. Also, the microinjection of a higher dose of D-AP5 (0.5 μg/rat, intra-PFC) alone produced an anxiogenic effect. These findings suggest that the functional interaction between the MD dopaminergic D1/D2-like and the VH/PFC glutamatergic NMDA receptors may be partially involved in the anxiogenic-like effects of nicotine, likely via the alteration of BDNF levels in the hippocampus and the PFC.

The associative and limbic thalamus in the pathophysiology of obsessive-compulsive disorder: an experimental study in the monkey

Obsessive-compulsive disorder (OCD) is a frequent psychiatric disorder characterized by repetitive intrusive thoughts and severe anxiety, leading to compulsive behaviors. Although medical treatment is effective in most cases, resistance is observed in about 30% of patients. In this context, deep brain stimulation (DBS) of the caudate or subthalamic nuclei has been recently proposed with encouraging results. However, some patients were unimproved or exhibited awkward side effects. Therefore, exploration of new targets for DBS remains critical in OCD. In the latter, functional imaging studies revealed overactivity in the limbic and associative cortico-subcortical loops encompassing the thalamus. However, the role of the thalamus in the genesis of repetitive behaviors and related anxiety is unknown. Here, we tested the hypothesis that pharmacological-induced overactivity of the medial thalamus could give rise to abnormal behaviors close to that observed in OCD. We modulated the ventral anterior (VA) and medial dorsal (MD) nuclei activity by in situ bicuculline (GABA(A) antagonist) microinjections in subhuman primates and assessed their pharmacological-induced behavior. Bicuculline injections within the VA caused significant repetitive and time-consuming motor acts whereas those performed within the MD induced symptoms of dysautonomic dysregulation along with abnormal vocalizations and marked motor hypoactivity. These findings suggest that overactivation of the VA and MD nuclei of the thalamus provokes compulsive-like behaviors and neurovegetative manifestations usually associated with the feeling of anxiety in OCD patients. In further research, this translational approach should allow us to test the effectiveness and side effects of these thalamic nuclei DBS in monkey and perhaps, in a second step, to propose a transfer of this technique to severely disabled OCD patients.

Reduced number of mediodorsal and anterior thalamic neurons in schizophrenia

BACKGROUND

The thalamus is a brain region of interest in the study of schizophrenia because it provides critical input to brain regions such as the prefrontal, cingulate, and temporal cortices, where abnormalities have been repeatedly observed in patients with schizophrenia. Postmortem anatomic studies have rarely investigated the thalamus in this population.

METHODS

Postmortem tissue was obtained from the left hemisphere of eight male schizophrenic patients and eight male age-matched control subjects. The optical dissector stereologic procedure was used to count neurons in the mediodorsal (MD) and anteroventral/anteromedial (AV/AM) nuclei of the thalamus.

RESULTS

The number of neurons and volume of the MD were significantly reduced by 35% and 24%, respectively. The MD cell number reduction was a consistent finding; every control subject had more and every schizophrenic subject had fewer than 3.5 million neurons. Neuron number was also significantly reduced (16%) in the AV/AM nuclei.

CONCLUSIONS

The present data indicate that schizophrenia is associated with robust reductions in nerve cell numbers in nuclei that communicate with the prefrontal cortex and limbic system. These thalamic anatomic deficits may be responsible, in part, for previous reports of such prefrontal cortical abnormalities as reduced synaptic density, reduced volume, and metabolic hypofunction.

Modulatory role of catecholamines in the transsynaptic expression of c-fos in the rat medial prefrontal cortex induced by disinhibition of the mediodorsal thalamus: a study employing microdialysis and immunohistochemistry

We studied the interaction of catecholaminergic and thalamic afferents of the medial prefrontal cortex (PFC) by analyzing the effects of catecholamine depletion on thalamus-induced c-fos expression in the PFC of freely moving rats. Thalamic projections to the PFC were pharmacologically activated by perfusing the GABA-A receptor antagonist bicuculline (0.03 mM or 0.1 mM) through a dialysis probe implanted into the mediodorsal thalamic nucleus. Bicuculline perfusion induced Fos-like immunoreactivity in the thalamic projection areas, including the PFC, and in the thalamic nuclei surrounding the dialysis probe. 6-Hydroxydopamine lesions of the ventral tegmental area causing a 70-80% depletion of catecholamines in the PFC did not influence the increase in the number of Fos-like immunoreactive nuclei in the prefrontal cortex in response to thalamic stimulation. However, densitometric image analysis revealed that the intensity of Fos-like immunoreactivity in the PFC of lesioned rats perfused with 0.1 mM bicuculline was higher than in correspondingly treated controls. The behavioral activity to bicuculline perfusion, an increase of non-ambulatory activity (0.03 mM) followed by locomotion and rearing (0.1 mM), was not changed in 6-hydroxydopamine-lesioned rats. It is suggested that the thalamically induced c-fos response is directly mediated by excitatory, presumably glutamatergic, transmission and not indirectly by an activation of catecholaminergic afferents of the PFC. The increase in the intensity of Fos-like immunostaining in strongly stimulated, catecholamine-depleted rats suggests that catecholamines modulate the degree to which thalamic activity can activate the PFC of awake animals.