Georg Köhr

The rat delta-1 and delta-2 subunits extend the excitatory amino acid receptor family

We have characterized a second member (delta‐2) of a new class of subunits for the ligand‐gated excitatory amino acid receptor superfamily. The sequence of delta‐2 exhibits an average identity of 25% and 18.5% to the non‐NMDA and NMDA receptor subunits, respectively. The rat delta‐2 gene is expressed predominantly in Purkinje cells of the cerebellum whereas only low levels of delta‐1 transcripts are found in the adult brain. However, delta‐1 gene expression undergoes a pronounced developmental peak, with particularly high mRNA levels in the caudate putamen of late embryonic/early postnatal stages.

A systems medicine research approach for studying alcohol addiction

According to the World Health Organization, about 2 billion people drink alcohol. Excessive alcohol consumption can result in alcohol addiction, which is one of the most prevalent neuropsychiatric diseases afflicting our society today. Prevention and intervention of alcohol binging in adolescents and treatment of alcoholism are major unmet challenges affecting our health‐care system and society alike. Our newly formed German SysMedAlcoholism consortium is using a new systems medicine approach and intends (1) to define individual neurobehavioral risk profiles in adolescents that are predictive of alcohol use disorders later in life and (2) to identify new pharmacological targets and molecules for the treatment of alcoholism. To achieve these goals, we will use omics‐information from epigenomics, genetics transcriptomics, neurodynamics, global neurochemical connectomes and neuroimaging (IMAGEN; Schumann et al. ) to feed mathematical prediction modules provided by two Bernstein Centers for Computational Neurosciences (Berlin and Heidelberg/Mannheim), the results of which will subsequently be functionally validated in independent clinical samples and appropriate animal models. This approach will lead to new early intervention strategies and identify innovative molecules for relapse prevention that will be tested in experimental human studies. This research program will ultimately help in consolidating addiction research clusters in Germany that can effectively conduct large clinical trials, implement early intervention strategies and impact political and healthcare decision makers.

Convergent evidence from alcohol-dependent humans and rats for a hyperdopaminergic state in protracted abstinence.

Significance A major hypothesis in the addiction field suggests there are deficits in dopamine signaling during abstinence. This hypodopaminergic state is considered a driving mechanism for the relapsing course of the disorder. Experimental support for this view comes mostly from human PET studies that found reduced striatal D2-like receptors in alcoholics. Here we report on surprising findings from postmortem brains of deceased alcoholics and alcohol-dependent rats that show no differences in D2-like receptor binding during withdrawal and prolonged abstinence. Instead we observe a dynamic regulation of D1 receptors, dopamine transporter, dopamine release properties, and phenotypic characteristics that all are in line with a hyperdopaminergic state during protracted abstinence. We propose that both hypo- and hyperdopaminergia are states of vulnerability to relapse. A major hypothesis in addiction research is that alcohol induces neuroadaptations in the mesolimbic dopamine (DA) system and that these neuroadaptations represent a key neurochemical event in compulsive drug use and relapse. Whether these neuroadaptations lead to a hypo- or hyperdopaminergic state during abstinence is a long-standing, unresolved debate among addiction researchers. The answer is of critical importance for understanding the neurobiological mechanism of addictive behavior. Here we set out to study systematically the neuroadaptive changes in the DA system during the addiction cycle in alcohol-dependent patients and rats. In postmortem brain samples from human alcoholics we found a strong down-regulation of the D1 receptor- and DA transporter (DAT)-binding sites, but D2-like receptor binding was unaffected. To gain insight into the time course of these neuroadaptations, we compared the human data with that from alcohol-dependent rats at several time points during abstinence. We found a dynamic regulation of D1 and DAT during 3 wk of abstinence. After the third week the rat data mirrored our human data. This time point was characterized by elevated extracellular DA levels, lack of synaptic response to D1 stimulation, and augmented motor activity. Further functional evidence is given by a genetic rat model for hyperdopaminergia that resembles a phenocopy of alcohol-dependent rats during protracted abstinence. In summary, we provide a new dynamic model of abstinence-related changes in the striatal DA system; in this model a hyperdopaminergic state during protracted abstinence is associated with vulnerability for relapse.

Phasic Dopaminergic Activity Exerts Fast Control of Cholinergic Interneuron Firing via Sequential NMDA, D2, and D1 Receptor Activation

Phasic increases in dopamine (DA) are involved in the detection and selection of relevant sensory stimuli. The DAergic and cholinergic system dynamically interact to gate and potentiate sensory inputs to striatum. Striatal cholinergic interneurons (CINs) respond to relevant sensory stimuli with an initial burst, a firing pause, or a late burst, or a combination of these three components. CIN responses coincide with phasic firing of DAergic neurons in vivo. In particular, the late burst of CINs codes for the anticipated reward. To examine whether DAergic midbrain afferents can evoke the different CIN responses, we recorded from adult olfactory tubercle slices in the mouse ventral striatum. Olfactory inputs to striatal projection neurons were gated by the cholinergic tone. Phasic optogenetic activation of DAergic terminals evoked combinations of initial bursts, pauses, and late bursts in subsets of CINs by distinct receptor pathways. Glutamate release from midbrain afferents evoked an NMDAR-dependent initial burst followed by an afterhyperpolarization-induced pause. Phasic release of DA itself evoked acute changes in CIN firing. In particular, in CINs without an initial burst, phasic DA release evoked a pause through D2-type DA receptor activation. Independently, phasic DA activated a slow depolarizing conductance and the late burst through a D1-type DA receptor pathway. In summary, DAergic neurons elicit transient subsecond firing responses in CINs by sequential activation of NMDA, D2-type, and D1-type receptors. This fast control of striatal cholinergic tone by phasic DA provides a novel dynamic link of two transmitter systems central to the detection and selection of relevant stimuli.

Heteromeric channels formed by TRPC1, TRPC4 and TRPC5 define hippocampal synaptic transmission and working memory

Canonical transient receptor potential (TRPC) channels influence various neuronal functions. Using quantitative high‐resolution mass spectrometry, we demonstrate that TRPC1, TRPC4, and TRPC5 assemble into heteromultimers with each other, but not with other TRP family members in the mouse brain and hippocampus. In hippocampal neurons from Trpc1/Trpc4/Trpc5‐triple‐knockout (Trpc1/4/5−/−) mice, lacking any TRPC1‐, TRPC4‐, or TRPC5‐containing channels, action potential‐triggered excitatory postsynaptic currents (EPSCs) were significantly reduced, whereas frequency, amplitude, and kinetics of quantal miniature EPSC signaling remained unchanged. Likewise, evoked postsynaptic responses in hippocampal slice recordings and transient potentiation after tetanic stimulation were decreased. In vivo, Trpc1/4/5−/− mice displayed impaired cross‐frequency coupling in hippocampal networks and deficits in spatial working memory, while spatial reference memory was unaltered. Trpc1/4/5−/− animals also exhibited deficiencies in adapting to a new challenge in a relearning task. Our results indicate the contribution of heteromultimeric channels from TRPC1, TRPC4, and TRPC5 subunits to the regulation of mechanisms underlying spatial working memory and flexible relearning by facilitating proper synaptic transmission in hippocampal neurons.

Phasic Dopamine Modifies Sensory-Driven Output of Striatal Neurons through Synaptic Plasticity

Animals are facing a complex sensory world in which only few stimuli are relevant to guide behavior. Value has to be assigned to relevant stimuli such as odors to select them over concurring information. Phasic dopamine is involved in the value assignment to stimuli in the ventral striatum. The underlying cellular mechanisms are incompletely understood. In striatal projection neurons of the ventral striatum in adult mice, we therefore examined the features and dynamics of phasic dopamine-induced synaptic plasticity and how this plasticity may modify the striatal output. Phasic dopamine is predicted to tag inputs that occur in temporal proximity. Indeed, we observed D1 receptor-dependent synaptic potentiation only when odor-like bursts and optogenetically evoked phasic dopamine release were paired within a time window of <1 s. Compatible with predictions of dynamic value assignment, the synaptic potentiation persisted after the phasic dopamine signal had ceased, but gradually reversed when odor-like bursts continued to be presented. The synaptic plasticity depended on the sensory input rate and was input specific. Importantly, synaptic plasticity amplified the firing response to a given olfactory input as the dendritic integration and the firing threshold remained unchanged during synaptic potentiation. Thus, phasic dopamine-induced synaptic plasticity can change information transfer through dynamic increases of the output of striatal projection neurons to specific sensory inputs. This plasticity may provide a neural substrate for dynamic value assignment in the striatum.

Dichotomy in the anxiolytic versus antidepressant effect of C-terminal truncation of the GluN2A subunit of NMDA receptors

The glutamate system is thought to play an important role in modulating mood and anxiety. Ionotropic NMDA receptors critically influence neuronal circuits regulating emotional behaviour. Their pharmacological blockade triggers fast antidepressant and anxiolytic effects. In line with this concept, ablation of the GluN2A subunit of NMDA receptors induces antidepressant and anxiolytic effects. However, it is not known if absence of the GluN2A-containing NMDA channel or of the GluN2A-mediated intracellular signalling is responsible for these effects. To further investigate the contribution of the GluN2A-containing NMDA receptors in mood disorders we analysed mice lacking the intracellular C-terminus of the GluN2A subunit (GluN2AΔC/ΔC) in tests relevant for anxiety and depression. Interestingly, GluN2AΔC/ΔC mice showed decreased anxiety, but no anti-depressive-like phenotype, indicating a predominant role of the intracellular signalling of the GluN2A subunit in anxiety. These data suggest distinct roles of the GluN2A subunit as whole vs. its intracellular domain in modulating anxiety and depression-like symptoms and reveal differential molecular targets for the therapy of mood and anxiety disorders.

Different Forms of AMPA Receptor Mediated LTP and Their Correlation to the Spatial Working Memory Formation

Spatial working memory (SWM) and the classical, tetanus-induced long-term potentiation (LTP) at hippocampal CA3/CA1 synapses are dependent on L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) containing GluA1 subunits as demonstrated by knockout mice lacking GluA1. In GluA1 knockout mice LTP and SWM deficits could be partially recovered by transgenic re-installation of full-length GluA1 in principle forebrain neurons. Here we partially restored hippocampal LTP in GluA1-deficient mice by forebrain-specific depletion of the GluA2 gene, by the activation of a hypomorphic GluA2(Q) allele and by transgenic expression of PDZ-site truncated GFP-GluA1(TG). In none of these three mouse lines, the partial LTP recovery improved the SWM performance of GluA1-deficient mice suggesting a specific function of intact GluA1/2 receptors and the GluA1 intracellular carboxyl-terminus in SWM and its associated behavior.

Subunit-selective N-Methyl-d-aspartate (NMDA) Receptor Signaling through Brefeldin A-resistant Arf Guanine Nucleotide Exchange Factors BRAG1 and BRAG2 during Synapse Maturation.

The maturation of glutamatergic synapses in the CNS is regulated by NMDA receptors (NMDARs) that gradually change from a GluN2B- to a GluN2A-dominated subunit composition during postnatal development. Here we show that NMDARs control the activity of the small GTPase ADP-ribosylation factor 6 (Arf6) by consecutively recruiting two related brefeldin A-resistant Arf guanine nucleotide exchange factors, BRAG1 and BRAG2, in a GluN2 subunit-dependent manner. In young cortical cultures, GluN2B and BRAG1 tonically activated Arf6. In mature cultures, Arf6 was activated through GluN2A and BRAG2 upon NMDA treatment, whereas the tonic Arf6 activation was not detectable any longer. This shift in Arf6 regulation and the associated drop in Arf6 activity were reversed by a knockdown of BRAG2. Given their sequential recruitment during development, we examined whether BRAG1 and BRAG2 influence synaptic currents in hippocampal CA1 pyramidal neurons using patch clamp recordings in acute slices from mice at different ages. The number of AMPA receptor (AMPAR) miniature events was reduced by depletion of BRAG1 but not by depletion of BRAG2 during the first 2 weeks after birth. In contrast, depletion of BRAG2 during postnatal weeks 4 and 5 reduced the number of AMPAR miniature events and compromised the quantal sizes of both AMPAR and NMDAR currents evoked at Schaffer collateral synapses. We conclude that both Arf6 activation through GluN2B-BRAG1 during early development and the transition from BRAG1- to BRAG2-dependent Arf6 signaling induced by the GluN2 subunit switch are critical for the development of mature glutamatergic synapses.

Persistent strengthening of the prefrontal cortex - nucleus accumbens pathway during incubation of cocaine-seeking behavior.

Graphical abstract Figure. No Caption available. HighlightsPFC‐evoked field potentials in NAc core are suitable for longitudinal in vivo studies.Chronic contingent exposure to cocaine strengthens the PFC‐NAc pathway.Decrease of paired‐pulse ratios persists throughout withdrawal.Degree of incubation correlates with potentiated FPs at chronic time point. Abstract High rates of relapse after prolonged abstinence are often triggered by exposure to drug‐associated cues that induce drug craving. Incubation of drug craving is a phenomenon that consists of time‐dependent increases in cue‐induced drug craving during withdrawal. Plasticity mechanisms in the nucleus accumbens (NAc) underlie drug‐seeking responses and involve changes in excitatory synaptic transmission’s efficacy. In particular, the prefrontal cortex (PFC) glutamatergic input to the NAc core has been well characterized regarding cocaine‐evoked plasticity following non‐contingent versus contingent exposure to cocaine or alternatively after protracted abstinence. Still, the synaptic strength during the course of withdrawal compared to drug‐naïve condition is unknown, since electrophysiological characterizations are mainly performed in brain slices or focus on distinct time points during cocaine‐evoked plasticity in vivo. Here we used an incubation paradigm, in which rats had extended accessed to cocaine self‐administration, and underwent cue‐induced reinstatement at withdrawal day 1 and 30. Longitudinal in vivo field potential recordings in awake rats showed that chronic contingent exposure to cocaine strengthened the prelimbic PFC to NAc core pathway when compared to pre‐cocaine condition. This strengthening was associated with decreased paired‐pulse ratios (PPR), indicative of presynaptic enhancement of glutamate release, which persisted throughout withdrawal. Moreover, both field potential increase and PPR reduction after chronic cocaine exposure correlated with the number of cocaine infusions received during training. The present results together with previous findings of withdrawal‐dependent postsynaptic enhancement of the PFC‐NAc core pathway, suggest an additional presynaptic strengthening that is initiated during self‐administration and maintained throughout abstinence in drug‐seeking rats. These cocaine‐driven neuroadaptations may provide a neural substrate for maladaptive processing of cues that can ultimately trigger craving and relapse.