Anni-Maija Linden

Expanding knowledge about the cellular composition of subcortical brain regions demonstrates large heterogeneity and differences from the cortical architecture. Recently, we described three subtypes of somatostatin-expressing (Sst) neurons in the mouse ventral tegmental area (VTA) and showed their local inhibitory action on the neighbouring dopaminergic neurons (Nagaeva et al., 2020). Here, we report that VTA Sst neurons also project far outside the VTA and innervate several forebrain regions that are mainly involved in the regulation of emotional behaviour. When we deleted these VTA Sst neurons several behaviours and drug effects were affected, such as home cage activity, sensitization of locomotor activity to morphine, fear conditioning responses, and reactions to inescapable stress of forced swimming, often in a sex-dependent manner. Interestingly, the majority of these Sst projecting neurons was found to express Vglut2 and Th, but not Dat. Together, these data demonstrate that VTA Sst neurons have their selective projection targets, which are distinct from the main targets of VTA dopamine neurons and involved in the regulation of a variety of behaviours mostly associated with the stress response. This, in turn, makes these VTA neurons a meaningful addition to the somatostatinergic system of the brain.

The PSEN1 ΔE9 mutation causes a familial form of Alzheimer’s disease (AD) by shifting the processing of amyloid precursor protein (APP) towards the generation of highly amyloidogenic Aβ42 peptide. We have previously shown that the PSEN1 ΔE9 mutation in human-induced pluripotent stem cell (iPSC)-derived astrocytes increases Aβ42 production and impairs cellular responses. Here, we injected PSEN1 ΔE9 mutant astrosphere-derived glial progenitors into newborn mice and investigated mouse behavior at the ages of 8, 12, and 16 months. While we did not find significant behavioral changes in younger mice, spatial learning and memory were paradoxically improved in 16-month-old PSEN1 ΔE9 glia-transplanted male mice as compared to age-matched isogenic control-transplanted animals. Memory improvement was associated with lower levels of soluble, but not insoluble, human Aβ42 in the mouse brain. We also found a decreased engraftment of PSEN1 ΔE9 mutant cells in the cingulate cortex and significant transcriptional changes in both human and mouse genes in the hippocampus, including the extracellular matrix-related genes. Overall, the presence of PSEN1 ΔE9 mutant glia exerted a more beneficial effect on aged mouse brain than the isogenic control human cells likely as a combination of several factors.

Background: Psychedelics, like lysergic acid diethylamide (LSD), are again being studied as potential therapies for many neuropsychiatric disorders, including addictions. At the same time, the acute effects of psychedelics on rewarding behaviours have been scarcely studied. Aims: The current study aimed to clarify if LSD decreases binge-like ethanol drinking in mice, and whether the observed acute effects on ethanol consumption are generalizable to a natural reinforcer, sucrose, and if the effects resulted from aversive or reward-attenuating effects caused by LSD. Methods: The effects of acute LSD were examined using 2-bottle choice intermittent ethanol (20%) and sucrose drinking (10%), discrete-trial current-intensity threshold method of intracranial self-stimulation and short-term feeding behaviour assay in C57BL/6 male mice. Results: The results showed that acute 0.1 mg/kg, but not 0.05 mg/kg, dose (i.p.) of LSD reduced 2-h intermittent ethanol drinking transiently without any prolonged effects. No effects were seen in intermittent 2-h sucrose drinking. The tested LSD doses had neither effect on the intracranial self-stimulation current-intensity thresholds, nor did LSD affect the threshold-lowering, or rewarding, effects of simultaneous amphetamine treatment. Furthermore, LSD had small, acute diminishing effects on 2-h food and water intake. Conclusions: Based on these results, LSD decreases binge-like ethanol drinking in mice, but only acutely. This effect is not likely to stem from reward-attenuating effects but could be in part due to reduced consummatory behaviour.

The cellular architecture of the ventral tegmental area (VTA), the main hub of the brain reward system, remains only partially characterized. To extend the characterization to inhibitory neurons, we have identified three distinct subtypes of somatostatin (Sst)-expressing neurons in the mouse VTA. These neurons differ in their electrophysiological and morphological properties, anatomical localization, as well as mRNA expression profiles. Importantly, similar to cortical Sst-containing interneurons, most VTA Sst neurons express GABAergic inhibitory markers, but some of them also express glutamatergic excitatory markers and a subpopulation even express dopaminergic markers. Furthermore, only some of the proposed marker genes for cortical Sst neurons were expressed in the VTA Sst neurons. Physiologically, one of the VTA Sst neuron subtypes locally inhibited neighboring dopamine neurons. Overall, our results demonstrate the remarkable complexity and heterogeneity of VTA Sst neurons and suggest that these cells are multifunctional players in the midbrain reward circuitry.

The cellular architecture of the ventral tegmental area (VTA), the main hub of the brain reward system, remains only partially characterized. To extend the characterization to inhibitory neurons, we have identified three distinct subtypes of somatostatin (Sst)-expressing neurons in the mouse VTA. These neurons differ in their electrophysiological and morphological properties, anatomical localization, as well as mRNA expression profiles. Importantly, similar to cortical Sst-containing interneurons, most VTA Sst neurons express GABAergic inhibitory markers, but some of them also express glutamatergic excitatory markers and a subpopulation even express dopaminergic markers. Furthermore, only some of the proposed marker genes for cortical Sst neurons were expressed in the VTA Sst neurons. Physiologically, one of the VTA Sst neuron subtypes locally inhibited neighboring dopamine neurons. Overall, our results demonstrate the remarkable complexity and heterogeneity of VTA Sst neurons and suggest that these cells are multifunctional players in the midbrain reward circuitry.

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 δ 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 α-fluoromethylhistidine (αFMH). The findings indicate that both histamine and GABA, released from histamine/GABA neurons, are involved in regulation of brain arousal states and δ-containing subunit GABA_A receptors are involved in mediating GABA response.

Objective: Human TNKS, encoding tankyrase 1 (TNKS1), localizes to a susceptibility locus for obesity and type 2 diabetes mellitus (T2DM). Here, we addressed the therapeutic potential of G007-LK, a TNKS-specific inhibitor, for obesity and T2DM. Methods: We administered G007-LK to diabetic db/db mice and measured the impact on body weight, abdominal adiposity, and serum metabolites. Muscle, liver, and white adipose tissues were analyzed by quantitative RT-PCR and western blotting to determine TNKS inhibition, lipolysis, beiging, adiponectin level, mitochondrial oxidative metabolism and mass, and gluconeogenesis. Protein interaction and PARylation analyses were carried out by immunoprecipitation, pull-down and in situ proximity ligation assays. Results: TNKS inhibition reduced body weight gain, abdominal fat content, serum cholesterol levels, steatosis, and proteins associated with lipolysis in diabetic db/db mice. We discovered that TNKS associates with PGC-1α and that TNKS inhibition attenuates PARylation of PGC-1α, contributing to increased PGC-1α level in WAT and muscle in db/db mice. PGC-1α upregulation apparently modulated transcriptional reprogramming to increase mitochondrial mass and fatty acid oxidative metabolism in muscle, beiging of WAT, and raised circulating adiponectin level in db/db mice. This was in sharp contrast to the liver, where TNKS inhibition in db/db mice had no effect on PGC-1α expression, lipid metabolism, or gluconeogenesis. Conclusion: Our study unravels a novel molecular mechanism whereby pharmacological inhibition of TNKS in obesity and diabetes enhances oxidative metabolism and ameliorates lipid disorder. This happens via tissue-specific PGC-1α-driven transcriptional reprogramming in muscle and WAT, without affecting liver. This highlights inhibition of TNKS as a potential pharmacotherapy for obesity and T2DM.

Background: Alcohol use associates with environmental cues that can later reinstate drinking patterns without any alcohol exposure. Alcohol-induced reward, when combined with contextual signals of various sensory modalities in the central synapses of mesolimbic reward circuitries, can lead to the formation of conditioned responses. Aims: As the activation of glutamatergic synapses is pivotal in such processes, we aimed to investigate whether the metabotropic glutamate receptor subtype 3 plays a role in alcohol-induced behaviours including place preference. Methods: The metabotropic glutamate receptor subtype 3 knockout (mGluR3-KO) mouse line was used to study alcohol-induced place preference, locomotor activating and ataxic effects, limited access alcohol drinking, and preference for sucrose and saccharin. Results: Alcohol-induced horizontal locomotor stimulation and reduced rearing behaviour remained unchanged in the mGluR3-KO mice. However, alcohol-induced place conditioning in an unbiased paradigm setup was lacking in the mGluR3-KO mice, but clearly present in wildtype mice. Locomotor activity was not different between the mGluR3-KO and wildtype mice during the acquisition and expression trials. Alcohol consumption, studied through the ‘drinking in the dark’ model, remained unchanged in the mGluR3-KO mice, although low consumption in both wildtype and knockout mice hampers interpretation. The mGluR3-KO mice also showed normal sucrose and saccharin preference. Conclusions: These studies indicate a role for metabotropic glutamate receptor subtype 3 in the conditioned contextual alcohol responses, but not in stimulatory, and ataxic alcohol effects.

Drugs of abuse induce widespread synaptic adaptations in the mesolimbic dopamine (DA) neurons. Such drug‐induced neuroadaptations may constitute an initial cellular mechanism eventually leading to compulsive drug‐seeking behavior. To evaluate the impact of GABA_B receptors on addiction‐related persistent neuroplasticity, we tested the ability of orthosteric agonist baclofen and two positive allosteric modulators (PAMs) of GABA_B receptors to suppress neuroadaptations in the ventral tegmental area (VTA) and reward‐related behaviors induced by ethanol and cocaine. A novel compound (S)‐1‐(5‐fluoro‐2,3‐dihydro‐1H‐inden‐2‐yl)‐4‐methyl‐6,7,8,9‐tetrahydro‐[1,2,4]triazolo[4,3‐a]quinazolin‐5(4H)‐one (ORM‐27669) was found to be a GABA_B PAM of low efficacy as agonist, whereas the reference compound (R,S)‐5,7‐di‐tert‐butyl‐3‐hydroxy‐3‐trifluoromethyl‐3H‐benzofuran‐2‐one (rac‐BHFF) had a different allosteric profile being a more potent PAM in the calcium‐based assay and an agonist, coupled with potent PAM activity, in the [³⁵S] GTPγS binding assay in rat and human recombinant receptors. Using autoradiography, the high‐efficacy rac‐BHFF and the low‐efficacy ORM‐27669 potentiated the effects of baclofen on [³⁵S] GTPγS binding with identical brain regional distribution. Treatment of mice with baclofen, rac‐BHFF, or ORM‐27669 failed to induce glutamate receptor neuroplasticity in the VTA DA neurons. Pretreatment with rac‐BHFF at non‐sedative doses effectively reversed both ethanol‐ and cocaine‐induced plasticity and attenuated cocaine i.v. self‐administration and ethanol drinking. Pretreatment with ORM‐27669 only reversed ethanol‐induced neuroplasticity and attenuated ethanol drinking but had no effects on cocaine‐induced neuroplasticity or self‐administration. These findings encourage further investigation of GABA_B receptor PAMs with different efficacies in addiction models to develop novel treatment strategies for drug addiction.