Tim S. Heistek

Diacylglycerol Lipase Is Not Involved in Depolarization-Induced Suppression of Inhibition at Unitary Inhibitory Connections in Mouse Hippocampus

Endocannabinoids control hippocampal inhibitory synaptic transmission through activation of presynaptic CB1 receptors. During depolarization-induced suppression of inhibition (DSI), endocannabinoids are synthesized upon postsynaptic depolarization. The endocannabinoid 2-arachidonoylglycerol (2-AG) may mediate hippocampal DSI. Currently, the best studied pathway for biosynthesis of 2-AG involves the enzyme diacylglycerol lipase (DAGL). However, whether DAGL is necessary for hippocampal DSI is controversial and was not systematically addressed. Here, we investigate DSI at unitary connections between CB1 receptor-containing interneurons and pyramidal neurons in CA1. We found that the novel DAGL inhibitor OMDM-188, as well as the established inhibitor RHC-80267, did not affect DSI. As reported previously, effects of the DAGL inhibitor tetrahydrolipstatin depended on the application method: postsynaptic intracellular application left DSI intact, while incubation blocked DSI. We show that all DAGL inhibitors tested block slow self-inhibition in neocortical interneurons, which involves DAGL. We conclude that DAGL is not involved in DSI at unitary connections in hippocampus.

Flexible spike timing of layer 5 neurons during dynamic beta oscillation shifts in rat prefrontal cortex.

Human brain oscillations occur in different frequency bands that have been linked to different behaviours and cognitive processes. Even within specific frequency bands such as the beta‐ (14–30 Hz) or gamma‐band (30–100 Hz), oscillations fluctuate in frequency and amplitude. Such frequency fluctuations most probably reflect changing states of neuronal network activity, as brain oscillations arise from the correlated synchronized activity of large numbers of neurons. However, the neuronal mechanisms governing the dynamic nature of amplitude and frequency fluctuations within frequency bands remain elusive. Here we show that in acute slices of rat prefrontal cortex (PFC), carbachol‐induced oscillations in the beta‐band show frequency and amplitude fluctuations. Fast and slow non‐harmonic frequencies are distributed differentially over superficial and deep cortical layers, with fast frequencies being present in layer 3, while layer 6 only showed slow oscillation frequencies. Layer 5 pyramidal cells and interneurons experience both fast and slow frequencies and they time their spiking with respect to the dominant frequency. Frequency and phase information is encoded and relayed in the layer 5 network through timed excitatory and inhibitory synaptic transmission. Our data indicate that frequency fluctuations in the beta‐band reflect synchronized activity in different cortical subnetworks, that both influence spike timing of output layer 5 neurons. Thus, amplitude and frequency fluctuations within frequency bands may reflect activity in distinct cortical neuronal subnetworks that may process information in a parallel fashion.

Prelimbic and Infralimbic Prefrontal Cortex Interact during Fast Network Oscillations

Background The medial prefrontal cortex has been implicated in a variety of cognitive and executive processes such as decision making and working memory. The medial prefrontal cortex of rodents consists of several areas including the prelimbic and infralimbic cortex that are thought to be involved in different aspects of cognitive performance. Despite the distinct roles in cognitive behavior that have been attributed to prelimbic and infralimbic cortex, little is known about neuronal network functioning of these areas, and whether these networks show any interaction during fast network oscillations. Methodology/Principal Findings Here we show that fast network oscillations in rat infralimbic cortex slices occur at higher frequencies and with higher power than oscillations in prelimbic cortex. The difference in oscillation frequency disappeared when prelimbic and infralimbic cortex were disconnected. Conclusions/Significance Our data indicate that neuronal networks of prelimbic and infralimbic cortex can sustain fast network oscillations independent of each other, but suggest that neuronal networks of prelimbic and infralimbic cortex are interacting during these oscillations.

Astrocyte lipid metabolism is critical for synapse development and function in vivo

The brain is considered to be autonomous in lipid synthesis with astrocytes producing lipids far more efficiently than neurons. Accordingly, it is generally assumed that astrocyte‐derived lipids are taken up by neurons to support synapse formation and function. Initial confirmation of this assumption has been obtained in cell cultures, but whether astrocyte‐derived lipids support synapses in vivo is not known. Here, we address this issue and determined the role of astrocyte lipid metabolism in hippocampal synapse formation and function in vivo. Hippocampal protein expression for the sterol regulatory element‐binding protein (SREBP) and its target gene fatty acid synthase (Fasn) was found in astrocytes but not in neurons. Diminishing SREBP activity in astrocytes using mice in which the SREBP cleavage‐activating protein (SCAP) was deleted from GFAP‐expressing cells resulted in decreased cholesterol and phospholipid secretion by astrocytes. Interestingly, SCAP mutant mice showed more immature synapses, lower presynaptic protein SNAP‐25 levels as well as reduced numbers of synaptic vesicles, indicating impaired development of the presynaptic terminal. Accordingly, hippocampal short‐term and long‐term synaptic plasticity were defective in mutant mice. These findings establish a critical role for astrocyte lipid metabolism in presynaptic terminal development and function in vivo. GLIA 2017;65:670–682

GABAergic synapse properties may explain genetic variation in hippocampal network oscillations in mice

Cognitive ability and the properties of brain oscillation are highly heritable in humans. Genetic variation underlying oscillatory activity might give rise to differences in cognition and behavior. How genetic diversity translates into altered properties of oscillations and synchronization of neuronal activity is unknown. To address this issue, we investigated cellular and synaptic mechanisms of hippocampal fast network oscillations in eight genetically distinct inbred mouse strains. The frequency of carbachol-induced oscillations differed substantially between mouse strains. Since GABAergic inhibition sets oscillation frequency, we studied the properties of inhibitory synaptic inputs (IPSCs) received by CA3 and CA1 pyramidal cells of three mouse strains that showed the highest, lowest and intermediate frequencies of oscillations. In CA3 pyramidal cells, the frequency of rhythmic IPSC input showed the same strain differences as the frequency of field oscillations. Furthermore, IPSC decay times in both CA1 and CA3 pyramidal cells were faster in mouse strains with higher oscillation frequencies than in mouse strains with lower oscillation frequency, suggesting that differences in GABA(A)-receptor subunit composition exist between these strains. Indeed, gene expression of GABA(A)-receptor β2 (Gabrb2) and β3 (Gabrb2) subunits was higher in mouse strains with faster decay kinetics compared with mouse strains with slower decay kinetics. Hippocampal pyramidal neurons in mouse strains with higher oscillation frequencies and faster decay kinetics fired action potential at higher frequencies. These data indicate that differences in genetic background may result in different GABA(A)-receptor subunit expression, which affects the rhythm of pyramidal neuron firing and fast network activity through GABA synapse kinetics.

Inbred mouse strains differ in multiple hippocampal activity traits

A major challenge in neuroscience is to identify genes that influence specific behaviors and to understand the intermediary neuronal mechanisms. One approach is to identify so‐called endophenotypes at different levels of neuronal organization from synapse to brain activity. An endophenotype is a quantitative trait that is closer to the gene action than behavior, and potentially a marker of neuronal mechanisms underlying behavior. Hippocampal activity and, in particular, hippocampal oscillations have been suggested to underlie various cognitive and motor functions. To identify quantitative traits that are potentially useful for identifying genes influencing hippocampal activity, we measured gamma oscillations and spontaneous activity in acute hippocampal slices from eight inbred mouse strains under three experimental conditions. We estimated the heritability of more than 200 quantitative traits derived from this activity. We observed significant differences between the different mouse strains, particularly in the amplitude of the activity and the correlation between activities in different hippocampal subregions. Interestingly, these traits had a low genetic correlation between the three experimental conditions, which suggests that different genetic components influence the activity in different conditions. Our findings show that several traits of hippocampal gamma oscillations and spontaneous activity are heritable and could thus be potentially useful in gene‐finding strategies based on endophenotypes.

Sustained Attentional States Require Distinct Temporal Involvement of the Dorsal and Ventral Medial Prefrontal Cortex

Attending the sensory environment for cue detection is a cognitive operation that occurs on a time scale of seconds. The dorsal and ventral medial prefrontal cortex (mPFC) contribute to separate aspects of attentional processing. Pyramidal neurons in different parts of the mPFC are active during cognitive behavior, yet whether this activity is causally underlying attentional processing is not known. We aimed to determine the precise temporal requirements for activation of the mPFC subregions during the seconds prior to cue detection. To test this, we used optogenetic silencing of dorsal or ventral mPFC pyramidal neurons at defined time windows during a sustained attentional state. We find that the requirement of ventral mPFC pyramidal neuron activity is strictly time-locked to stimulus detection. Inhibiting the ventral mPFC 2 s before or during cue presentation reduces response accuracy and hampers behavioral inhibition. The requirement for dorsal mPFC activity on the other hand is temporally more loosely related to a preparatory attentional state, and short lapses in pyramidal neuron activity in dorsal mPFC do not affect performance. This only occurs when the dorsal mPFC is inhibited during the entire preparatory period. Together, our results reveal that a dissociable temporal recruitment of ventral and dorsal mPFC is required during attentional processing.

Adenosine A2A Receptors Control Glutamatergic Synaptic Plasticity in Fast Spiking Interneurons of the Prefrontal Cortex

Adenosine A2A receptors (A2AR) are activated upon increased synaptic activity to assist in the implementation of long-term plastic changes at synapses. While it is reported that A2AR are involved in the control of prefrontal cortex (PFC)-dependent behavior such as working memory, reversal learning and effort-based decision making, it is not known whether A2AR control glutamatergic synapse plasticity within the medial PFC (mPFC). To elucidate that, we tested whether A2AR blockade affects long-term plasticity (LTP) of excitatory post-synaptic potentials in pyramidal neurons and fast spiking (FS) interneurons in layer 5 of the mPFC and of population spikes. Our results show that A2AR are enriched at mPFC synapses, where their blockade reversed the direction of plasticity at excitatory synapses onto layer 5 FS interneurons from LTP to long-term depression, while their blockade had no effect on the induction of LTP at excitatory synapses onto layer 5 pyramidal neurons. At the network level, extracellularly induced LTP of population spikes was reduced by A2AR blockade. The interneuron-specificity of A2AR in controlling glutamatergic synapse LTP may ensure that during periods of high synaptic activity, a proper excitation/inhibition balance is maintained within the mPFC.

Hippocampal extracellular matrix alterations contribute to cognitive impairment associated with a chronic depressive-like state in rats

Alterations in the hippocampal extracellular matrix contribute to cognitive deficits in rats exhibiting a chronic depressive-like state after exposure to social defeat–induced persistent stress. Netting a new understanding of hippocampal function A common feature of major depression is cognitive impairment, including difficulties in memory recall. The underlying mechanisms of these symptoms are unclear. In a new study, Riga and colleagues used social defeat–induced persistent stress to induce a depressive-like state in rats and then examined molecular changes in the hippocampus related to cognitive deficits associated with this state. They found increased expression of extracellular matrix proteins and decreased plasticity potential and inhibitory neurotransmission in the dorsal hippocampus in this rat model. Treatment with an antidepressant drug or a single injection into the hippocampus of an enzyme that breaks down the extracellular matrix resulted in improved hippocampal function and rescue of memory recall in this preclinical rat model. Patients with depression often suffer from cognitive impairments that contribute to disease burden. We used social defeat–induced persistent stress (SDPS) to induce a depressive-like state in rats and then studied long-lasting memory deficits in the absence of acute stressors in these animals. The SDPS rat model showed reduced short-term object location memory and maintenance of long-term potentiation (LTP) in CA1 pyramidal neurons of the dorsal hippocampus. SDPS animals displayed increased expression of synaptic chondroitin sulfate proteoglycans in the dorsal hippocampus. These effects were abrogated by a 3-week treatment with the antidepressant imipramine starting 8 weeks after the last defeat encounter. Next, we observed an increase in the number of perineuronal nets (PNNs) surrounding parvalbumin-expressing interneurons and a decrease in the frequency of inhibitory postsynaptic currents (IPSCs) in the hippocampal CA1 region in SDPS animals. In vivo breakdown of the hippocampus CA1 extracellular matrix by the enzyme chondroitinase ABC administered intracranially restored the number of PNNs, LTP maintenance, hippocampal inhibitory tone, and memory performance on the object place recognition test. Our data reveal a causal link between increased hippocampal extracellular matrix and the cognitive deficits associated with a chronic depressive-like state in rats exposed to SDPS.

α2‐containing GABAA receptors expressed in hippocampal region CA3 control fast network oscillations

Hippocampal oscillations are thought to be important for memory encoding and retrieval and depend on inhibition via GABA synapses. GABAA receptor subunits are differentially expressed throughout the hippocampal circuitry. Here we address which subunit controls cholinergically induced fast network oscillations and where it is expressed. By selectively increasing and decreasing the function of α1 and α2 subunits, we find that hippocampal oscillations are controlled by α2 subunits expressed in CA3. Synapses from fast spiking interneurons to pyramidal cells in CA3 that provide the perisomatic inhibition necessary for fast network oscillations contain GABAA receptors with the α2 subunit. Our data suggest that α2‐containing GABA receptors in CA3 have an important role in rhythmic hippocampal activity and thereby possibly in cognitive processing.