Judith V. Schweimer

Genetic mouse models relevant to schizophrenia: taking stock and looking forward.

Genetic mouse models relevant to schizophrenia complement, and have to a large extent supplanted, pharmacological and lesion-based rat models. The main attraction is that they potentially have greater construct validity; however, they share the fundamental limitations of all animal models of psychiatric disorder, and must also be viewed in the context of the uncertain and complex genetic architecture of psychosis. Some of the key issues, including the choice of gene to target, the manner of its manipulation, gene-gene and gene-environment interactions, and phenotypic characterization, are briefly considered in this commentary, illustrated by the relevant papers reported in this special issue.

Spike-timing relationship of neurochemically-identified dorsal raphe neurons during cortical slow oscillations.

The firing activity of dorsal raphe neurons is related to arousal state. However, it is unclear how this firing activity is precisely related to cortical activity, in particular oscillations occurring during sleep rhythms. Here we conducted single-cell extracellular recordings and juxtacellular labelling while monitoring electrocorticogram (ECoG) activity in urethane anaesthetised rats, to relate activity in neurochemically identified groups of neurons to cortical slow-wave activity (SWA). We observed that electrophysiological heterogeneity in dorsal raphe neurons revealed different neurochemical groups of DRN neurons and was mirrored by significant differences in the phase and strength of coupling to the cortical slow oscillations. Spike firing relationship of clock-like neurons, identified as 5-HT (5-hydroxytryptamine) or serotonin neurons, was higher during the inactive component of the oscillations. In contrast, half of the identified bursting 5-HT neurons did not exhibit strong cortical entrainment; those that did fired most during the inactive component of the SWA. Two groups of putatively non-5-HT neurons (irregular slow-firing and fast-firing) exhibited significant coherence and fired most during the active component of the SWA. These findings indicate that within the DRN electrophysiologically and neurochemically discrete neuronal groups exhibit distinct relations to cortical activity.

Increased burst-firing of ventral tegmental area dopaminergic neurons in D-amino acid oxidase knockout mice in vivo.

d‐Amino acid oxidase (DAO) degrades the N‐methyl‐d‐aspartate (NMDA) receptor co‐agonist d‐serine, and is implicated in schizophrenia as a risk gene and therapeutic target. In schizophrenia, the critical neurochemical abnormality affects dopamine, but to date there is little evidence that DAO impacts on the dopamine system. To address this issue, we measured the electrophysiological properties of dopaminergic (DA) and non‐DA neurons in the ventral tegmental area (VTA) of anaesthetised DAO knockout (DAO−/−) and DAO heterozygote (DAO+/−) mice as compared with their wild‐type (DAO+/+) littermates. Genotype was confirmed at the protein level by western blotting and immunohistochemistry. One hundred and thirty‐nine VTA neurons were recorded in total, and juxtacellular labelling of a subset revealed that neurons immunopositive for tyrosine hydroxylase had DA‐like electrophysiological properties that were distinct from those of neurons that were tyrosine hydroxylase‐immunonegative. In DAO−/− mice, approximately twice as many DA‐like neurons fired in a bursting pattern than in DAO+/− or DAO+/+ mice, but other electrophysiological properties did not differ between genotypes. In contrast, non‐DA‐like neurons had a lower firing rate in DAO−/− mice than in DAO+/− or DAO+/+ mice. These data provide the first direct evidence that DAO modulates VTA DA neuron activity, which is of interest for understanding both the glutamatergic regulation of dopamine function and the therapeutic potential of DAO inhibitors. The increased DA neuron burst‐firing probably reflects increased availability of d‐serine at VTA NMDA receptors, but the site, mechanism and mediation of the effect requires further investigation, and may include both direct and indirect processes.

Pharmacological Evidence for 5-HT6 Receptor Modulation of 5-HT Neuron Firing in Vivo

5-Hydroxytryptamine (5-HT) neurons in the midbrain dorsal raphe nucleus (DRN) are implicated in the drug treatment and pathophysiology of a wide variety of neuropsychiatric disorders. Accumulating evidence suggests that 5-HT6 receptors may be located and functional in the DRN; therefore, 5-HT6 receptor ligands may have potential as novel modulators of 5-HT neurotransmission. The current study investigated the effect of intravenous (i.v.) administration of the selective 5-HT6 receptor agonist, WAY-181187, and antagonist, SB-399885, on the firing of 5-HT neurons in the DRN in vivo. Extracellular recordings were made in the DRN of anesthetized rats, and single 5-HT neurons were identified on the basis of electrophysiological properties combined with juxtacellular labeling and postmortem immunohistochemical analysis. WAY-181187 (1-4 mg/kg i.v.) caused a dose-dependent increase in 5-HT neuron firing rate. In comparison, SB-399885 (0.125-1 mg/kg i.v.) caused a dose-dependent decrease in 5-HT neuron firing rate, an effect reversed by WAY-181187 (3 mg/kg i.v.). These effects of WAY-181187 and SB-399885 were observed in two separate sets of experiments. In summary, the current data show the modulation of 5-HT neuronal firing by the 5-HT6 ligands WAY-181187 and SB-399885 and are consistent with the presence of 5-HT6 receptor-mediated positive feedback control of 5-HT neurons.

D-amino acid oxidase is expressed in the ventral tegmental area and modulates cortical dopamine.

D-amino acid oxidase (DAO, DAAO) degrades the NMDA receptor co-agonist D-serine, modulating D-serine levels and thence NMDA receptor function. DAO inhibitors are under development as a therapy for schizophrenia, a disorder involving both NMDA receptor and dopaminergic dysfunction. However, a direct role for DAO in dopamine regulation has not been demonstrated. Here, we address this question in two ways. First, using in situ hybridization and immunohistochemistry, we show that DAO mRNA and immunoreactivity are present in the ventral tegmental area (VTA) of the rat, in tyrosine hydroxylase (TH)-positive and -negative neurons, and in glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes. Second, we show that injection into the VTA of sodium benzoate, a DAO inhibitor, increases frontal cortex extracellular dopamine, as measured by in vivo microdialysis and high performance liquid chromatography. Combining sodium benzoate and D-serine did not enhance this effect, and injection of D-serine alone affected dopamine metabolites but not dopamine. These data show that DAO is expressed in the VTA, and suggest that it impacts on the mesocortical dopamine system. The mechanism by which the observed effects occur, and the implications of these findings for schizophrenia therapy, require further study.

Phosphodiesterase 4 inhibition affects both the direct and indirect pathway : an electrophysiological study examining the tri-phasic response in the substantia nigra pars reticulata

Fronto-striatal circuits constitute the neurobiological basis of many neuropsychiatric disorders. Part of the intracellular signaling within these circuits, including its dopaminergic modulation, is regulated by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling cascade. Based on the overall expression in human fronto-striatal circuitry, we tested the effects of a cAMP selective phosphodiesterase 4 (PDE4) inhibitor on the tri-phasic response in the dorsomedial substantia nigra pars reticulata (SNr) upon stimulation of the infralimbic cortex in rats. Our results show for the first time that stimulation of the cognitive infralimbic cortex leads to a tri-phasic response in SNr neurons. In addition and in line with previous biochemical and behavioral studies, PDE4 inhibition by roflumilast affects the direct pathway as well as the indirect pathway of which the latter appears more sensitive than the former.