Abnormal behavior, striatal dopamine turnover and opioid peptide gene expression in histamine-deficient mice

Hypothalamic histaminergic neurons regulate a variety of homeostatic, metabolic and cognitive functions. Recent data have suggested a modulatory role of histamine and histamine receptors in shaping striatal activity and connected the histaminergic system to neuropsychiatric disorders. We characterized exploratory behavior and striatal neurotransmission in mice lacking the histamine producing enzyme histidine decarboxylase (Hdc). The mutant mice showed a distinct behavioral pattern during exploration of novel environment, specifically, increased frequency of rearing seated against the wall, jumping and head/body shakes. This behavioral phenotype was associated with decreased levels of striatal dopamine and serotonin and increased level of dopamine metabolite DOPAC. Gene expression levels of dynorphin and enkephalin, opioids released by medium spiny neurons of striatal direct and indirect pathways respectively, were lower in Hdc mutant mice than in control animals. A low dose of amphetamine led to similar behavioral and biochemical outcomes in both genotypes. Increased striatal dopamine turnover was observed in Hdc KO mice after treatment with dopamine precursor l-Dopa. Overall, our study suggests a role for striatal dopamine and opioid peptides in formation of distinct behavioral phenotype of Hdc KO mice.Peer reviewe

Mice Lacking GABA(A) Receptor delta Subunit Have Altered Pharmaco-EEG Responses to Multiple Drugs

In the brain, extrasynaptically expressed ionotropic, delta subunit-containing gamma-aminobutyric acid A-type receptors (delta-GABA(A)Rs) have been implicated in drug effects at both neuronal and behavioral levels. These alterations are supposed to be caused via drug-induced modulation of receptor ionophores affecting chloride ion-mediated inhibitory tonic currents. Often, a transgenic mouse model genetically lacking the delta-GABA(A)Rs (delta-KO) has been used to study the roles of delta-GABA(A)Rs in brain functions, because a specific antagonist of the delta-GABA(A)Rs is still lacking. We have previously observed with these delta-KO mice that activation of delta-GABA(A)Rs is needed for morphine-induced conditioning of place preference, and others have suggested that delta-GABA(A)Rs act as targets selectively for low doses of ethanol. Furthermore, activation of these receptors via drug-mediated agonism induces a robust increase in the slow-wave frequency bands of electroencephalography (EEG). Here, we tested delta-KO mice (compared to littermate wild-type controls) for the pharmaco-EEG responses of a broad spectrum of pharmacologically different drug classes, including alcohol, opioids, stimulants, and psychedelics. Gaboxadol (THIP), a known superagonist of delta-GABA(A)Rs, was included as the positive control, and as expected, delta-KO mice produced a blunted pharmaco-EEG response to 6 mg/kg THIP. Pharmaco-EEGs showed notable differences between treatments but also differences between delta-KO mice and their wild-type littermates. Interestingly mephedrone (4-MMC, 5 mg/kg), an amphetamine-like stimulant, had reduced effects in the delta-KO mice. The responses to ethanol (1 g/kg), LSD (0.2 mg/kg), and morphine (20 mg/kg) were similar in delta-KO and wild-type mice. Since stimulants are not known to act on delta-GABA(A)Rs, our findings on pharmaco-EEG effects of 4-MMC suggest that delta-GABA(A)Rs are involved in the secondary indirect regulation of the brain rhythms after 4-MMC.Peer reviewe

Increased Sensitivity of Mice Lacking Extrasynaptic delta-Containing GABA(A) Receptors to Histamine Receptor 3 Antagonists

Histamine/gamma-aminobutyric acid (GABA) neurons of posterior hypothalamus send wide projections to many brain areas and participate in stabilizing the wake state. Recent research has suggested that GABA released from the histamine/GABA neurons acts on extrasynaptic GABA(A) receptors and balances the excitatory effect of histamine. In the current study, we show the presence of vesicular GABA transporter mRNA in a majority of quantified hypothalamic histaminergic neurons, which suggest vesicular release of GABA. As histamine/GABA neurons form conventional synapses infrequently, it is possible that GABA released from these neurons diffuses to target areas by volume transmission and acts on extrasynaptic GABA receptors. To investigate this hypothesis, mice lacking extrasynaptic GABA(A) receptor delta subunit (Gabrd KO) were used. A pharmacological approach was employed to activate histamine/GABA neurons and induce histamine and presumably, GABA, release. Control and Gabrd KO mice were treated with histamine receptor 3 (Hrh3) inverse agonists ciproxifan and pitolisant, which block Hrh3 autoreceptors on histamine/GABA neurons and histamine-dependently promote wakefulness. Low doses of ciproxifan (1 mg/kg) and pitolisant (5 mg/kg) reduced locomotion in Gabrd KO, but not in WT mice. EEG recording showed that Gabrd KO mice were also more sensitive to the wake-promoting effect of ciproxifan (3 mg/kg) than control mice. Low frequency delta waves, associated with NREM sleep, were significantly suppressed in Gabrd KO mice compared with the WT group. Ciproxifan-induced wakefulness was blocked by histamine synthesis inhibitor alpha-fluoromethylhistidine (alpha FMH). The findings indicate that both histamine and GABA, released from histamine/GABA neurons, are involved in regulation of brain arousal states and delta-containing subunit GABA(A) receptors are involved in mediating GABA response.Peer reviewe

Histamine/GABA neurons in the regulation of striatal signaling and brain arousal.

The hypothalamus is one of the oldest brain structures and it is responsible for the regulation of vital homeostatic functions. A small population of the neurons containing neurotransmitters histamine and GABA, reside in the posterior hypothalamus. Activation of these histamine/GABA neurons promotes attentive wakefulness and vigilance state. In addition to its main role in supporting wake state, histamine released from histamine/GABA neurons is involved in controlling appetite, water intake and energy expenditure. Although the role of GABA released from the histamine/GABA neurons has been much less studied, it is suggested to provide tonic inhibition in the cortical and striatal regions. There are three major topics addressed in the current work. First, we aimed to further probe the hypothesis of the dual histamine/GABA neurotransmitter phenotype of the hypothalamic histaminergic neurons. We were especially interested in whether histamine/GABA neurons are able to release GABA into the synapse via the vesicular transport mechanism. Our second aim was to investigate the consequences of global histamine deficiency with a focus on the cortico-striatal system and brain dopamine-histamine interactions. Lastly, we studied the possible role of GABA released from histamine/GABA neurons in tonic extrasynaptic inhibition. It is well established that histaminergic neurons are also GABAergic, based on the presence of GABA producing enzyme - glutamic acid decarboxylase (GAD) and GABA itself in histaminergic neurons, as demonstrated by immunohistochemical methods. Contradictory findings have been reported on the presence of the vesicular GABA transporter (Vgat) in these neurons. We used double fluorescence in situ hybridization (dFISH) to simultaneously detect GABAergic markers (GAD67 or Vgat mRNA) with a marker for histaminergic neurons - histidine decarboxylase (Hdc) mRNA. We confirmed that histamine/GABA neurons express Vgat mRNA and are able to release GABA via a classical Vgat-dependent mechanism. Previous research has shown the interaction of histamine and dopamine systems at the level of the striatum and proposed involvement of these two systems in neuropsychiatric disorders such as Gilles de la Tourette syndrome. Using a mouse line lacking the histamine-synthesizing enzyme (Hdc KO mice), we investigated the role of histamine in the regulation of the striatal system and its interaction with dopamine at the striatal level. We measured striatal dopamine and its metabolites levels with high performance liquid chromatography (HPLC) at the baseline and after treatment with dopamine precursor l-3,4-dihydroxyphenylalanine (L-Dopa). By quantitative polymerase chain reaction (qPCR), we compared the levels of striatal prodynorphin and proenkephalin transcripts in Hdc WT and KO mice. The transcript level of prodynorphin and proenkephalin is tightly regulated by the activity of principal striatal neurons, medium spiny neurons (MSN) and various neurotransmitters such as dopamine and acetylcholine. Furthermore, we performed detailed analyses of exploratory open field behavior of Hdc KO mice and used a stereological approach to assess the morphology and cytoarchitecture of the corticostriatal system in these mice. We found that Hdc KO mice had increased dopamine turnover in the striatum and impaired expression of striatal prodynorphin and proenkephalin transcripts. We hypothesized that global deficiency of histamine leads to upregulation of the dopaminergic system in the striatum, which in turn leads to altered behavioral structure observed in the novel open field test. Impaired dynorphin/κ-opioid receptor inhibitory feedback on the dopaminergic terminals might be responsible for the increased striatal dopamine release. Finally, we provided new evidence suggesting that GABA released from the histamine/GABA neurons acts on the extrasynaptic δ subunit containing GABAA receptors and provides tonic inhibition. Pharmacological activation of the histamine/GABA neurons in the mice lacking GABAA δ subunit (Gabrd KO) led to a hypervigilant phenotype in these mice as was shown by EEG recordings. In conclusion, we showed that histamine together with dopamine regulates striatal circuits and that GABA released from the histamine/GABA neurons regulates brain arousal state at least partially through the extrasynaptic δ subunit containing GABAA receptors

Cholinergic basal forebrain structures are not essential for mediation of the arousing action of glutamate

The cholinergic basal forebrain contributes to cortical activation and receives rich innervations from the ascending activating system. It is involved in the mediation of the arousing actions of noradrenaline and histamine. Glutamatergic stimulation in the basal forebrain results in cortical acetylcholine release and suppression of sleep. However, it is not known to what extent the cholinergic versus non-cholinergic basal forebrain projection neurones contribute to the arousing action of glutamate. To clarify this question, we administered N-methyl-D-aspartate (NMDA), a glutamate agonist, into the basal forebrain in intact rats and after destruction of the cholinergic cells in the basal forebrain with 192 immunoglobulin (Ig)G-saporin. In eight Han-Wistar rats with implanted electroencephalogram/electromyogram (EEG/EMG) electrodes and guide cannulas for microdialysis probes, 0.23 mug 192 IgG-saporin was administered into the basal forebrain, while the eight control animals received artificial cerebrospinal fluid. Two weeks later, a microdialysis probe targeted into the basal forebrain was perfused with cerebrospinal fluid on the baseline day and for 3 h with 0.3 mmNMDA on the subsequent day. Sleep-wake activity was recorded for 24 h on both days. NMDA exhibited a robust arousing effect in both the intact and the lesioned rats. Wakefulness was increased and both non-REM and REM sleep were decreased significantly during the 3-h NMDA perfusion. Destruction of the basal forebrain cholinergic neurones did not abolish the wake-enhancing action of NMDA. Thus, the cholinergic basal forebrain structures are not essential for the mediation of the arousing action of glutamate