Schizophrenia; from structure to function with special focus on the mediodorsal thalamic prefrontal loop.

OBJECTIVE: To describe structural and biochemical evidence from postmortem brains that implicates the reciprocal connections between the mediodorsal thalamic nucleus and the prefrontal cortex in cognitive symptoms of schizophrenia. METHOD: The estimation of the regional volumes and cell numbers was obtained using stereological methods. The biochemical analyses of molecular expression in postmortem brain involve quantitative measurement of transcripts and proteins by in-situ (RNA) or Western blot/autoradiography in brains from patients with schizophrenia and comparison subjects. RESULTS: Stereological studies in postmortem brain from patients with schizophrenia have reported divergent and often opposing findings in the total number of neurons and volume of the mediodorsal (MD) thalamic nucleus, and to a lesser degree in its reciprocally associated areas of the prefrontal cortex. Similarly, quantitative molecular postmortem studies have found large inter-subject and between-study variance at both the transcript and protein levels for receptors and their interacting molecules of several neurotransmitter systems in these interconnected anatomical regions. Combined, large variation in stereological and molecular studies indicates a complex and heterogeneous involvement of the MD thalamic-prefrontal loop in schizophrenia. CONCLUSION: Based on a considerable heterogeneity in patients suffering from schizophrenia, large variation in postmortem studies, including stereological and molecular postmortem studies of the MD thalamus and frontal cortex, might be expected and may in fact partly help to explain the variable endophenotypic traits associated with this severe psychiatric illness.

The role of alpha1- and alpha2-adrenoreceptors on venlafaxine-induced elevation of extracellular serotonin, noradrenaline and dopamine levels in the rat prefrontal cortex and hippocampus.

The role of adrenergic alpha1- and alpha2-adrenoreceptors in augmentation of venlafaxine-induced elevation of extracellular serotonin (5-HT),noradrenaline (NA) and dopamine (DA) levels in the rat prefrontal cortex (PFC) and hippocampus (HIPP) was studied by in vivo microdialysis in anaesthetized rats. The alpha1-adrenoreceptor antagonist prazosin given alone (0.3 mg/kg, s.c.) induced only a moderate reduction of hippocampal 5-HT and NA levels. The alpha2-adrenoreceptor antagonist idazoxan (1.5 mg/kg, s.c.) causes moderate increases in the levels of 5-HT and DA in the PFC. The mixed 5-HT and NA reuptake inhibitor venlafaxine (10 mg/kg, i.p.) increased the efflux of 5-HT, NA and DA almost equally, to approximately 200% of the control levels in the PFC. The levels of 5-HT increased to 310%, an effect approximately twice the effect on NA in the HIPP. Venlafaxine also produced a moderate increase in DA levels in the PFC but had no effect in the HIPP. Pre-treatment with prazosin caused a significant attenuation of the venlafaxine induced 5-HT effect in the PFC, and a moderate increase in DA levels in the HIPP. Prazosin had no significant effect on the venlafaxine-induced increase of the NA levels in PFC or HIPP. A combined treatment of venlafaxine with idazoxan increased the venlafaxine NA and DA effects in PFC by a factor of two and resulted in a very robust five-fold augmentation of NA and DA concentrations in the HIPP. In summary, idazoxan was found to produce a potent enhancement of the venlafaxine effect to increase extracellular NA and DA levels in the PFC and, in particular, in the HIPP. Idazoxan had no effect on venlafaxine-induced elevation of extracellular 5-HT levels in either PFC or HIPP and prazosin induced a decrease of 5-HT in the PFC. The present data suggest that blockade of alpha2-adrenoreceptors may play an important role in augmentation of the effects of mixed monoamine reuptake inhibitors.

Striatal plasticity at the network level. Focus on adenosine A2A and D2 interactions in models of Parkinson's Disease.

Behavioral and microdialysis studies have been performed on antagonistic A(2A)/D(2) interactions in animal models of Parkinson's Disease. The behavioral analysis involved studies on locomotor activity in reserpinized mice, haloperidol-induced catalepsy in rats and rotational behavior in rats with unilateral 6-OHDA lesions of the ascending DA pathways (Ungerstedt model). Dual probe microdialysis studies were indirectly performed on the striatopallidal GABA neurons by studying extracellular glutamate levels in the striatum and globus pallidus of the awake freely moving rat. The striatum was perfused with A(2A) and/or D(2) agonists via reverse microdialysis. The results show that the A(2A) antagonists SCH58261 and KF17837 can increase locomotor activity in reserpinized mice and produce contralateral rotational behavior only after administration of subthreshold doses of l-DOPA or the D(2) like agonist quinpirole. Furthermore, antagonizing the A(2A) receptor (R) reduced haloperidol induced catalepsy. The behavioral results underline the view that A(2A) antagonists act by blocking A(2A) R in A(2A)/D(2) heterodimers where A(2A) R inhibits the D(2) R transduction and D(2) inhibits the adenylate cyclase (AC) activated by A(2A) R. The microdialysis studies show that the A(2A) agonist CGS21680 striatally coperfused with the D(2) agonist quinpirole more potently counteract the D(2) agonist (quinpirole) induced reduction of pallidal glutamate levels in the DA denervated vs the control striatum indicating an enhancement of the inhibitory A(2A)/D(2) interaction. In the DA denervated but not in the control striatum the A(2A) agonist CGS21680 could strongly increase striatal glutamate levels, indicating an increased receptor signaling in the A(2A) R located on the striatal glutamate terminals, where also D(2) like R exist, here probably as D(4). Thus, the signaling of this A(2A) R may be set free by the loss of D(4) tone on the AC activated by A(2A) in this postulated A(2A)/D(4) heteromer on the glutamate terminals. Taken together, the results indicate that the antiparkinsonian actions of A(2A) antagonists probably are produced by blockade of A(2A) R in the A(2A)/D(2) heterodimers mainly located in the striatopallidal GABA neurons.

Synthesis and evaluation of racemic [(11)C]NS2456 and enantiomers as selective serotonin reuptake radiotracers for PET.

Positron emission tomography (PET) radiotracers are needed for quantifying serotonin uptake sites in the living brain. Therefore, we evaluated a new selective serotonin reuptake inhibitor, NS2456, to determine whether it is suited for use in PET. Racemic NS2456 [(1RS,5SR)-8-methyl-3-[4-trifluoromethoxyphenyl]-8-azabicyclo [3.2.1]oct-2-ene] and its N-demethylated analog, racemic NS2463, selectively inhibited serotonin uptake in rat brain synaptosomes; their IC(50) values were 3000-fold lower for [(3)H]serotonin than for either [(3)H]dopamine or [(3)H]noradrenaline. The enantiomers of NS2463 were also potent inhibitors of serotonin uptake in vitro, but they failed to show stereoselectivity. Racemic NS2463 as well as its enantiomers were radiolabelled by N-methylation with C-11, yielding [(11)C]NS2456 for use in PET of the living porcine brain. The compounds crossed the blood-brain barrier rapidly and accumulated preferentially in regions rich in serotonin uptake sites (e.g., brainstem, subthalamus and thalamus). However, their binding potentials were relatively low and no stereoselectivity was found. Thus, neither racemic [(11)C]NS2456 nor its [(11)C]-labelled enantiomers are ideal for PET neuroimaging of neuronal serotonin uptake sites.

Uptake and distribution of a new SSRI, NS2381, studied by PET in living porcine brain.

This study tests the utility of a new selective serotonin reuptake inhibitor (SSRI), [11C]NS2381 {(+/-)-(8-[11C]methyl-3-(4-trifluoromethyl-phenyl)-8-azabicyclo[3.2.1]oc t-2-ene)}, as positron-emitting radioligand for labelling serotonin (5-HT) reuptake sites in living brain. Studies of monoamine uptake were carried out initially in vitro using rat brain synaptosomes. They showed that NS2381 and its precursor NS2435 are selective inhibitors of serotonin (5-HT) uptake. Then, studies were carried out in vivo on the uptake and distribution of [11C]NS2381 in living porcine brain. They showed that the radiotracer accumulates readily in brain, and binds reversibly in regions rich in serotonin uptake sites (e.g. raphe, basal ganglia and thalamus). In addition, [11C]NS2381 was displaced from brain tissue by the potent SSRI citalopram. The enantiomers of [11C]NS2381 were, in general, found to be similar to the racemate in terms of their uptake and distribution in living pig brain. Thus, [11C]NS2381 fulfilled several criteria of a PET radioligand for studying 5-HT uptake sites in the living brain.

No loss in total neuron number in the thalamic reticular nucleus and neocortex in the genetic absence epilepsy rats from Strasbourg.

The thalamic reticular nucleus (nRt) as well as the neocortex are involved in the bilateral spike- and wave-discharge loop in genetic absence epilepsy rats from Strasbourg (GAERS). Neuron loss in different brain areas has been described in relation to epilepsy with convulsive seizures. We have previously investigated the ventrolateral/posterior nucleus of thalamus in GAERS and found no neuron loss. We applied the same efficient and unbiased stereological methods to nRt and to neocortex and again found no loss of neurons. The oscillatory properties of nRt are not related to neurons loss.

Nitric oxide promotes seizure activity in kainate-treated rats.

L-Arginine-derived nitrogen monoxide (NO) formation was determined in different regions of the rat brain during kainate-induced seizures. NO was trapped in vivo as a paramagnetic mononitrosyl-iron diethyldithiocarbamate complex, the concentration of which was determined ex vivo by cryogenic electron spin resonance spectroscopy. Basal NO formation (0.3-0.8 nmol g-1 tissue 30 min-1) was detected in the brain of control rats. In kainate-injected rats NO formation was increased six-fold within 30-60 min in the amygdala/temporal cortex region, and up to 12-fold, though more slowly, in the remaining cortex. The kainate-elicited convulsions and NO formation were attenuated in animals pretreated with either 7-nitroindazole, a specific inhibitor of neuronal NO synthase, or diazepam. These findings identify NO as a proconvulsant mediator in kainate-evoked seizures.

Relation of spatial learning of rats in the Morris water maze task to the number of viable CA1 neurons following four-vessel occlusion.

Male Wistar rats were tested in the Morris water maze task 1 week after 6, 9, or 12 min of transient global ischemia. The 9-min and 12-min ischemia groups were significantly impaired in the acquisition and the reversal experiment. A systematic counting of CA1 neurons in the whole hippocampal formation revealed a unilateral number of CA1 neurons of 286,000 in the sham group, of which 2/3 were located in the dorsal hippocampus. The ischemia groups showed a significant decline in the number of dorsal CA1 neurons, whereas only the 12-min ischemia group showed a significant but minor decline (10%-15%) in the number of ventral CA1 neurons. A correlation analysis showed that the escape distance declined with increasing number of viable CA1 neurons, but poor correlation coefficients were obtained. Thus, some of the ischemic rats with even very few viable CA1 neurons in the dorsal hippocampus were capable of performing this spatial learning task at sham-group level.

Does neuronal damage of CA1 relate to spatial memory performance of rats subjected to transient forebrain ischemia?

The effect of ischemia-induced hippocampal neuronal damage on acquisition and performance in the Morris water maze task was investigated in male Wistar rats, subjected to 8 min of transient forebrain ischemia, induced by the 4-vessel occlusion (4-VO) method. After a morphological scoring of the neuronal damage within the CA1, CA2, and CA3 subfields of the anterior-dorsal part of hippocampus we found that rats with a total neuronal cell loss of the anterior-dorsal CA1 region showed memory performance impairments in the acquisition trials, in a probe trial, and in a reversal experiment. However, rats with only partial damage to the CA1 region did not exhibit significant impairments during the acquisition trials of the water maze test or in the probe trial and the reversal experiment. In conclusion, these results suggest that it is possible to relate the histological damage score of CA1 in the anterior-dorsal hippocampus to impaired memory performance in the present water maze setup.

Spatial navigation learning in spontaneously hypertensive, renal hypertensive and normotensive Wistar rats.

The relation between blood pressure and cognitive performance was assessed in the spatial navigation task. Spatial learning by rats with spontaneous hypertension (SHR) and Goldblatt renal hypertension (RHR) was compared with that of normotensive Wistar rats (NR). The task required the rats to escape from water by finding a submerged and hidden platform. It was found that SHR rats showed improved learning capacity in the maze task in acquisition compared to the RHR and NR groups already on Day 1 and Day 2. The performances of all tested groups reached almost similar asymptotic level on Day 4 and in the probe trial on Day 5. After a reversal training the SHR rats did not show preference to swim in the new platform position quadrant. The present results confirm earlier reports on different behavioural characteristics associated with hypertension.

Enhanced disruptive spatial learning effect after sufentanil in renal hypertensive rats versus normotensive rats.

The effects of the peripherally administered sufentanil citrate (S), a potent opioid agonist with high affinity for mu receptors on the spatial navigation task, were tested in normotensive Wistar (NR) and renal hypertensive rats (RHR). Rats were injected subcutaneously once daily in doses of 0.25 or 1 microgram/kg S before the water maze training. In NR rats, weak effects of 0.25 micrograms/kg S and impairments after 1 microgram/kg S were seen, whereas in RHR 0.25 and 1 microgram/kg S showed clearcut impairments. These data from the Morris water maze task support previous reports that RHR have an increased sensitivity for opioid agonists.

Effect of imipramine in the "learned helplessness" model of depression in rats is not mimicked by combinations of specific reuptake inhibitors and scopolamine.

Administration of imipramine, which blocks noradrenergic, serotonergic and cholinergic reuptake, to rats for 4 days counteracts the shuttlebox escape failures otherwise seen in rats which have been exposed to inescapable shock (the "learned helplessness" model of depression). The effects of the more selective reuptake inhibitors talsupram (noradrenergic), citalopram (serotonergic) and the anticholinergic compound scopolamine were assessed alone and in combination after acute or 4 days' administration on escape behavior. Their possible synergistic effects when combined with imipramine were also assessed. Talsupram and citalopram were ineffective, whereas scopolamine counteracted the escape failures. Combinations of talsupram, citalopram and a subeffective dose of scopolamine were ineffective. A synergistic effect was only seen when scopolamine was combined with a suboptimal dose of imipramine. Thus, the effect of imipramine on "learned helplessness" might rely partly on its anticholinergic component. However, as an acute high dose of imipramine (25 mg/kg) was ineffective [unlike the acute administration of scopolamine (0.12 mg/kg)], this drug retains a pharmacological effect which is not mimicked by scopolamine alone or by combining the specific reuptake inhibitors with scopolamine.

Central nervous system stimulants: neuropharmacological mechanisms.

The mechanisms underlying CNS-stimulant drug discrimination are discussed. Although different doses of CNS stimulants may produce qualitatively different cues, it appears that a relatively low dose of d-amphetamine (e.g., 1 mg/kg) elicits a "general" CNS-stimulant cue. Presynaptically, this cue may primarily depend on release of endogenous dopamine whereas inhibition of dopamine reuptake, per se, is insufficient to elicit the cue. Postsynaptically, the involvement of both dopamine D-1 and D-2 receptors is implicated. Furthermore, in the drug discrimination situation, D-1/D-2 receptors may be coupled differently than in dopamine-dependent locomotor activation. Anatomically, CNS-stimulant drug discrimination may depend primarily on mesolimbic dopamine systems.

Different behavioral responses of rats to kainate injections into the dorsal and median raphe nuclei.

A local injection of kainic acid (KA) into the dorsal raphe nucleus (NRD) increased the motor activity and produced head shakes, hind limb abduction, forepaw treading and sniffing. This syndrome was antagonized partly by cyproheptadine and completely by naloxone. An injection of KA into the median raphe nucleus (NRM) produced sedation, catalepsy and analgesia, which were accompanied by a decrease in the beta-endorphin immunoreactivity in the mesencephalon. Naloxone completely reversed the behavioral inhibition after KA injections into the NRM, while a pretreatment with cyproheptadine augmented the catalepsy. KA injected into the NRD and NRM depressed the forebrain level of serotonin and slightly elevated that of 5-hydroxyindoleacetic acid. The results demonstrate that besides serotonin the opioid system is also involved in various effects induced by activation of the raphe nuclei.

Cueing effects of amphetamine and LSD: elicitation by direct microinjection of the drugs into the nucleus accumbens.

Two groups of rats were trained to discriminate either d-amphetamine sulphate (AMPH; 1 mg/kg) or d-lysergic acid diethylamide bitartrate (LSD; 0.16 mg/kg). Microinjection of AMPH into the nucleus accumbens elicited a cueing effect which was similar to that of systemically administered AMPH in AMPH-trained animals (ED50 was 0.24 micrograms). Co-injection of (-)-sulpiride (50 or 100 ng) into the accumbens antagonized the effect of a fixed dose of AMPH (1 microgram) which, alone, produced 76% of the systemic AMPH cue effect. Microinjections of AMPH (1-5 micrograms) into either the anterior dorsomedial or the anterior ventrolateral striatum failed to elicit the cueing effect of AMPH. In LSD-trained animals a dose of 1 microgram LSD injected into the accumbens produced 84% of the systemic cueing effect of LSD. These results suggest that dopamine (DA) receptors in the nucleus accumbens are involved in AMPH discrimination. Furthermore, since both classical and atypical antipsychotic drugs block the AMPH cue, the results provide indirect evidence for involvement of mesolimbic DA in antipsychotic drug action.

Evidence that the anticonflict effect of midazolam in amygdala is mediated by the specific benzodiazepine receptors.

The benzodiazepine midazolam produced an anticonflict effect in rats measured in a water lick paradigm following local injection into the basolateral and lateral complex of the amygdala. This effect of midazolam seems to involve specific benzodiazepine receptors, since the systemic injection of benzodiazepine antagonists Ro 15-1788, ZK 93426, FG 7142 and CGS 8216 produced strong antagonism of the effect of midazolam in doses not affecting the non-punished drinking behaviour.

New aspects on the role of dopamine, acetylcholine, and GABA in the development of tardive dyskinesia.

In this paper various new findings on the possible anatomical substrates of tardive dyskinesia will be presented. The results show that the striatum is heterogeneously organized, and the syndromes of biting, gnawing, and licking activities in the rat model involve a complex balance between various dopamine (DA), cholinergic, and GABAergic systems within the striatum and the mesolimbic and mesocortical systems.