Effects of acute and chronic clozapine on dopaminergic function in medial prefrontal cortex of awake, freely moving rats

Neuregulin-2 ablation results in dopamine dysregulation and severe behavioral phenotypes relevant to psychiatric disorders

Numerous genetic and functional studies implicate variants of Neuregulin-1 (NRG1) and its neuronal receptor ErbB4 in schizophrenia and many of its endophenotypes. Although the neurophysiological and behavioral phenotypes of NRG1 mutant mice have been investigated extensively, practically nothing is known about the function of NRG2, the closest NRG1 homolog. We found that NRG2 expression in the adult rodent brain does not overlap with NRG1 and is more extensive than originally reported, including expression in the striatum and medial prefrontal cortex (mPFC), and therefore generated NRG2 knockout mice (KO) to study its function. NRG2 KOs have higher extracellular dopamine levels in the dorsal striatum but lower levels in the mPFC; a pattern with similarities to dopamine dysbalance in schizophrenia. Like ErbB4 KO mice, NRG2 KOs performed abnormally in a battery of behavioral tasks relevant to psychiatric disorders. NRG2 KOs exhibit hyperactivity in a novelty-induced open field, deficits in prepulse inhibition, hypersensitivity to amphetamine, antisocial behaviors, reduced anxiety-like behavior in the elevated plus maze and deficits in the T-maze alteration reward test-a task dependent on hippocampal and mPFC function. Acute administration of clozapine rapidly increased extracellular dopamine levels in the mPFC and improved alternation T-maze performance. Similar to mice treated chronically with N-methyl-d-aspartate receptor (NMDAR) antagonists, we demonstrate that NMDAR synaptic currents in NRG2 KOs are augmented at hippocampal glutamatergic synapses and are more sensitive to ifenprodil, indicating an increased contribution of GluN2B-containing NMDARs. Our findings reveal a novel role for NRG2 in the modulation of behaviors with relevance to psychiatric disorders.

In rats chronically treated with clozapine, tyrosine depletion attenuates the clozapine-induced in vivo increase in prefrontal cortex dopamine and norepinephrine levels

We previously reported that depletion of brain tyrosine attenuated the acute clozapine (CLZ)-induced increase in medial prefrontal cortex (MPFC) dopamine (DA) levels. This effect was now examined after chronic CLZ treatment. Male rats received CLZ (10 mg kg(-1) day(-1)) in drinking water for 21 days. On day 18, a cannula was stereotaxically implanted over the MPFC. A microdialysis probe was inserted on day 20. On day 21 after a stable baseline was reached, rats received an acute injection of vehicle (VEH) or a tyrosine- and phenylalanine-free mixture of neutral amino acid [NAA(-)] (total 1 g kg(-1), i.p., two injections, 1 h apart) followed by CLZ (10 mg kg(-1), i.p.) or VEH. Basal tyrosine or norepinephrine (NE) levels were not different between the groups, but basal DA was higher in the group treated chronically with CLZ (p<0.05). Acute CLZ (10 mg kg(-1), i.p.) increased MPFC DA and NE levels to 370% and 510% of baseline, respectively, and similarly in rats chronically pretreated with CLZ or VEH. NAA(-) did not affect basal MPFC DA or NE levels but significantly attenuated acute CLZ-induced DA (220% of baseline) and NE (330% of baseline) levels (p<0.01) in rats pretreated chronically with CLZ or with VEH. These data demonstrate that even after chronic CLZ administration, the acute CLZ-induced increases in MPFC DA and NE levels depend on the availability of brain tyrosine. Judicious manipulation of brain tyrosine levels may provide a useful probe as well as a mechanism for enhancing psychotropic drug actions.

A meta-analysis of the response to chronic L-dopa in patients with schizophrenia: therapeutic and heuristic implications

RATIONALE

While it is generally believed that administration of the dopamine precursor levodopa ( L-dopa) exacerbates symptoms of schizophrenia, numerous reports suggest that adjunctive L-dopa may be beneficial. This body of literature has not been critically reviewed.

OBJECTIVES

On the basis of published studies, to determine whether L-dopa administered concomitantly with antipsychotic drugs provides a beneficial response in patients with schizophrenia.

METHODS

This review examined 30 studies involving 716 patients. Due to wide methodological variability and limited statistical information, only five studies encompassing 160 patients could be included in a meta-analysis. The others were evaluated qualitatively.

RESULTS

When L-dopa was added to antipsychotic drugs, the overall improvement was moderate ( d=0.71) and highly significant ( P<0.0001). There were 16 other studies in which L-dopa was added to antipsychotic drugs, but which did not meet criteria for inclusion in the meta-analysis. In these, worsening occurred in less than 20% of patients; the percentage of improved patients varied widely but had a central tendency around 50%.

CONCLUSIONS

. In patients already on antipsychotic drugs, the addition of L-dopa can be beneficial. Dopamine agonists merit further consideration as adjuncts to antipsychotic drugs in the treatment of schizophrenia.

Noradrenaline transporter blockers raise extracellular dopamine in medial prefrontal but not parietal and occipital cortex: differences with mianserin and clozapine

This study compared the interaction between noradrenaline (NA) and dopamine (DA) mechanisms in the prefrontal (PFCX) and in the parietal (ParCX) and occipital (OccCX) cortex. The effect of reboxetine and desipramine, two NA transporter blockers, of mianserin, an antagonist of alpha2 and 5-HT2 receptors, and of clozapine, an atypical antipsychotic, on dialysate DA in the medial PFCX, ParCX and OccCX was studied. We also assessed the influence of a prior 6-hydroxydopamine (6-OHDA) lesion of the dorsal noradrenergic bundle (DNAB) on the effect of reboxetine and clozapine on dialysate DA in the PFCX and ParCX. Systemic administration of reboxetine and desipramine dose-dependently increased dialysate DA in the PFCX but not in the ParCX and OccCX. In contrast, mianserin and clozapine raised dialysate DA in the ParCX and OccCX to an even larger extent than in the PFCX. 6-OHDA lesions of DNAB abolished the increase of dialysate DA elicited by reboxetine in the PFCX and by clozapine both in the PFCX and in the ParCX. It is concluded that, although PFCX and ParCX/OccCX share the presence of a strong control of DA transmission by NA through alpha2 receptors, they differ in the extent to which DA is cleared from the extracellular compartment by uptake through the NA transporter. This process, although extensive in the PFCX, appears insignificant in the ParCX and OccCX, probably as a result of the higher ratio of NA to DA resulting in exclusion of DA from NA transporter.

Co-release of noradrenaline and dopamine from noradrenergic neurons in the cerebral cortex induced by clozapine, the prototype atypical antipsychotic

RATIONALE

Clozapine has been shown to increase extracellular dopamine (DA) and noradrenaline (NA) in the medial prefrontal cortex (mPFC). A recent study of ours suggested that extracellular DA in the PFC originates not only from dopaminergic but also from noradrenergic terminals, its release being controlled by alpha(2)-adrenoceptors.

OBJECTIVES

Since clozapine binds to alpha(2)-adrenoceptors, the possibility that it might co-release DA and NA was studied.

METHODS

By means of microdialysis coupled to HPLC with electrochemical detection, the effect of clozapine on extracellular DA and NA in the mPFC, densely innervated by DA and NA, was compared to that in the occipital cortex, equally innervated by NA but receiving few DA projections.

RESULTS

Extracellular NA was found to be the same in the two cortices, consistent with homogeneous NA innervation. On the other hand, extracellular DA in the occipital cortex was only 29% lower than in the mPFC, in spite of the scarce dopaminergic innervation in the occipital cortex. Clozapine (10 mg/kg IP) increased extracellular DA and NA not only in the mPFC (by about 320% and 290%, respectively) but also in the occipital cortex (by 560% and 230%, respectively). Administration of the alpha(2)-agonist clonidine (0.15 mg/kg) reversed the effect of clozapine in both cortices, while the D(2)-agonist quinpirole (0.1 mg/kg IP) was ineffective.

CONCLUSIONS

The results suggest that clozapine, by inhibiting alpha(2)-adrenoceptors, co-releases DA and NA from noradrenergic terminals in the occipital cortex and that the same mechanism might be responsible for the concomitant increase of the two monoamines in the mPFC.

Monoamine neurotransmitters in resected hippocampal subparcellations from neocortical and mesial temporal lobe epilepsy patients: in situ microvoltammetric studies

It is known that epilepsy patients diagnosed with neocortical temporal lobe epilepsy (NTLE), differ from those diagnosed with mesial temporal lobe epilepsy (MTLE), e.g., in hippocampal (HPC) pathology. In the present studies, we tested the hypothesis that NTLE and MTLE subtypes of human epilepsy might differ in regards to their HPC monoamine neurochemistry. Monoamine neurotransmitters were studied in separate signals and within s with semiderivative microvoltammetry, used in combination with stearate indicator, Ag-AgCl reference and stainless steel auxiliary microelectrodes. Anterior HPC specimens from the patients' epileptogenic zone, defined by electrocorticography, were resected neurosurgically from 13 consecutive patients with intractable temporal lobe epilepsy. Four patients were diagnosed with NTLE and nine with MTLE. The criteria for the diagnosis of NTLE versus MTLE was absence versus presence of HPC sclerosis, respectively, based on MRI examination of resected tissue. In addition, NTLE patients demonstrated seizure onset in anterolateral temporal neocortex on electroencephalography (EEG). HPC subparcellations studied were: (a) Granular Cells of the Dentate Gyrus (DG), (b) Polymorphic Layer of DG and (c) Pyramidal Layer: subfields, CA1 and CA2. Dopamine (DA), serotonin (5-HT), norepinephrine (NE) and ascorbic acid (AA) (co-factor in DA to NE synthesis), exhibited separate and characteristic half-wave potentials in millivolts. Each half-wave potential, i.e., the potential at which maximum current was generated, was experimentally established in vitro. Concentrations of neurotransmitters found in HPC subparcellations were interpolated from calibration curves derived in vitro from electrochemical detection of monoamines and AA in saline phosphate buffer. Significant differences between subtypes in concentration of monoamines were analyzed by the Mann Whitney rank sum test and those differences in probability distribution of monoamines were analyzed by the Fisher Exact test; in each case, P<0.01 was the criteria selected for determining statistical significance. DA concentrations were higher in NTLE compared with MTLE in each HPC subparcellation [P=0.037, 0.024 and 0.007, respectively (P<0.01)] and DA occurred more frequently in NTLE in the Pyramidal Layer [P=0.077 (P<0.01)]. AA was present in one NTLE patient. NE concentrations were higher in MTLE vs. NTLE in each subparcellation [P=0.012, 0.067 and 0.07, respectively (P<0.01)] and NE occurred more frequently in MTLE in Granular Cells of DG and Pyramidal Layer [P=0.052 and 0.014, respectively (P<0.01)]. In MTLE, NE concentrations in the CA1 subfield of the Pyramidal Layer were decreased vs. the CA2 subfield [P=0.063 (P<0.01)]. Serotonin was found in every HPC subparcellation of each subtype but 5-HT concentrations were higher in NTLE vs. MTLE in the Granular Cells of DG and the Pyramidal Layer (CA1 subfield) [P=0.076 and 0.095, respectively (P<0.01)]. Thus, this preliminary study showed that marked differences in HPC monoamine neurochemistry occurred in NTLE patients as compared with MTLE patients.

Effects of chronic neuroleptic treatment on dopamine release: insights from studies using 3-methoxytyramine

Antipsychotic medications appear to exert their therapeutic effects by blocking D2 receptors. While D2 blockade occurs rapidly, reduction in psychotic symptoms is often delayed. This time discrepancy has been attributed to the relatively slow development of depolarization inactivation (DI) of dopaminergic neurons. The reduced firing rates associated with DI has been hypothesized to reduce dopamine release and thus psychotic symptoms. Studies assessing changes in dopamine release during chronic neuroleptic treatment, using microdialysis and voltammetry, have been inconsistent. This may be due to methodological differences between studies, the invasive nature of these procedures, or other confounds. To investigate the effects of DI on dopamine release, 3-MT accumulation, an index of dopamine release that does not involve disruption of brain tissue, was measured during acute and chronic neuroleptic treatment. These results are compared with those using other techniques. 3-MT levels remained elevated after chronic treatment, suggesting that DI does not markedly reduce release. Regulation of dopamine release during DI was examined using two techniques known to block dopamine neuronal impulse flow. 3-MT levels were markedly reduced by both, implying that DI does not alter the portion of dopamine release mediated by neuronal impulse flow. Overall, studies to date suggest that the delayed therapeutic effects of neuroleptics are not due to reductions in impulse dependent dopamine release. Recent studies using a neurodevelopmental animal model of schizophrenia have pointed to altered pre- and post-synaptic indices of dopamine neurotransmission. The results suggest that neuroleptics may exert their therapeutic effects, in part, by limiting the fluctuations in dopamine release, and raise new issues for future research.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Chronic clozapine selectively decreases prefrontal cortex dopamine as shown by simultaneous cortical, accumbens, and striatal microdialysis in freely moving rats

We used microdialysis to study the acute and chronic effects of clozapine on the metabolism of dopamine (DA) in terminal areas of the mesocortical, mesolimbic, and nigrostriatal systems simultaneously. In the acute experiment, groups of four rats received the following doses: 0 (vehicle), 10, 20, and 40 mg/kg of clozapine subcutaneously, which resulted in a dose-related increase in extracellular DA, 3,4-dihydroxyphenalacetic acid (DOPAC), and homovanillic acid (HVA) in the prefrontal cortex (PFC). In the nucleus accumbens (NAC) and striatum (STR), no significant changes were observed at any dose. In the chronic experiment, six rats received 20 mg/kg of clozapine and a control group received vehicle daily for 30 days. After 30 days of treatment, DA, DOPAC, and HVA were significantly lower in the PFC, and unchanged in the NAC or STR. The 30th clozapine injection failed to increase DA, DOPAC, or HVA in any of the three regions. We conclude that clozapine acted selectively on the mesocortical system, and that this may underlie clozapine's therapeutic, antipsychotic effect.

In vivo stimulated dopamine release in the nucleus accumbens: modulation by the prefrontal cortex

In vivo voltammetry was used to measure stimulated dopamine (DA) release in the nucleus accumbens following 6-hydroxydopamine (6-OHDA) lesions of the medial prefrontal cortex (PFC). Lesions of the PFC resulted in a significant potentiation of K(+)-stimulated DA release (383% of control). These data are in agreement with other studies which suggest DAergic activation of cortical sites can alter the activity of DAergic neurons at subcortical sites. Specifically these results suggest that DAergic nerve terminals in the nucleus accumbens are tonically inhibited by DAergic activity in the PFC and that alleviation of this tonic inhibition produces hyperresponsiveness in the nucleus accumbens.

Effects of systemically administered ceruletide on the in vivo release and metabolism of dopamine in the medial prefrontal cortex of awake, freely moving rats: an in vivo microdialysis study

The effects of the cholecystokinin octapeptide-related peptide, ceruletide (CER), on the in vivo release and metabolism of dopamine (DA) in the medial prefrontal cortex were examined in awake, freely moving rats, using in vivo microdialysis. Subcutaneously administered CER (200 micrograms/kg) increased extracellular levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), indicating that extracellular levels of DOPAC and HVA may reflect DA release in the medial prefrontal cortex. Bilateral subdiaphragmatic vagotomy markedly attenuated the CER-induced effect, but did not abolish it completely. CER (10(-7) and 10(-10) M), applied locally via the dialysis tube, had no effect on the extracellular levels of either DOPAC or HVA. The CCK-A receptor antagonist, L-364,718 (3 mg/kg, i.p.), completely prevented CER-induced increases in the extracellular levels of DOPAC and HVA. The CCK-B antagonist, L-365,260 (3 mg/kg, i.p.), however, given 1 h before the CER treatment, slightly attenuated the CER-induced increase in the extracellular levels of DOPAC, but not the CER-induced increase in HVA, 60-180 min after the treatment. These findings indicate that systemically administered CER modulates the in vivo release and metabolism of DA in the medial prefrontal cortex. We suggest that systemically administered CER exerts its action on both vagal afferent nerves and the area postrema via CCK-A receptors, thus enhancing the in vivo release and metabolism of DA in the medial prefrontal cortex.

Preferential activation of dopamine overflow in prefrontal cortex produced by chronic clozapine treatment

The effect of chronic treatment with clozapine on extracellular dopamine levels in the rat striatum, nucleus accumbens and medial prefrontal cortex (mPFC) was examined using intracerebral microdialysis. Clozapine (20 mg/kg/day x 21 days in drinking water) increased basal dopamine release in the mPFC but had no effect in the striatum or nucleus accumbens. After chronic treatment, an acute dose of clozapine (20 mg/kg i.p.) produced large and long-lasting increases in extracellular dopamine in all three brain regions. The data suggest that chronic clozapine produces a sustained enhancement in dopaminergic tone in the mPFC.

Clozapine's functional mesolimbic selectivity is not duplicated by the addition of anticholinergic action to haloperidol: a brain stimulation study in the rat

This study examined whether the anticholinergic potency of the clinically superior antipsychotic drug clozapine contributes to clozapine's anatomically-selective functional inhibition of the mesolimbic dopamine (DA) system, using an electrical brain-stimulation reward (BSR) paradigm in rats that has been previously shown to be highly sensitive to clozapine's mesolimbic functional selectivity. Rats were chronically administered saline, clozapine, haloperidol, or haloperidol plus the anticholinergic compound trihexyphenidyl, and threshold sensitivity of the mesolimbic and nigrostriatal DA systems was assessed using the BSR paradigm, to infer degree of functional DA blockade produced by the chronic drug regimens. Chronic saline produced no change in either DA system. Congruent with previous findings, chronic clozapine powerfully inhibited the mesolimbic DA system but spared the nigrostriatal DA system. Also congruent with previous findings, chronic haloperidol powerfully inhibited both DA systems. Compared to chronic haloperidol alone, chronic haloperidol plus chronic trihexyphenidyl exerted diminished anti-DA action in both the mesolimbic and nigrostriatal DA systems. These results suggest that clozapine's anticholinergic potency is not an adequate explanation for its functional mesolimbic selectivity.