The early excitatory action of striatal cholinergic-GABAergic microcircuits conditions the subsequent GABA inhibitory shift.

Cholinergic interneurons of the striatum play a role in action selection and associative learning by activating local GABAergic inhibitory microcircuits. We investigated whether cholinergic-GABAergic microcircuits function differently and fulfill a different role during early postnatal development, when GABAA actions are not inhibitory and mice pups do not walk. We focused our study mainly on dual cholinergic/GABAergic interneurons (CGINs). We report that morphological and intrinsic electrophysiological properties of CGINs rapidly develop during the first post-natal week. At this stage, CGINs are excited by the activation of GABAA receptors or GABAergic synaptic inputs, respond to cortical stimulation by a long excitation and are linked by polysynaptic excitations. All these excitations are replaced by inhibitions at P12-P15. Early chronic treatment with the NKCC1 antagonist bumetanide to evoke premature GABAergic inhibitions from P4 to P8, prevented the GABA polarity shift and corticostriatal pause response at control postnatal days. We propose that early excitatory cholinergic-GABAergic microcircuits are instrumental in the maturation of GABAergic inhibition.

[Experimental study on developmental changes of synaptic currents in V1B cortical layer â £ pyramidal neurons in C57BL/6J mice].

Objective: To investigate the developmental changes of miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) of layer Ⅳ pyramidal neurons in the primary visual cortex binocular zone (V1B) of C57BL/6J wild-type mice at different developmental stages. Methods: Sixteen male C57BL/6J mice of specific-pathogen-free grade were selected and divided into 4 groups according to their postnatal age: P14 group (before and after eye opening), P28 group (the peak of the critical period), P35 group (the end of the critical period), and P130 group (fully adult). Whole-cell patch-clamp technique was used to record the frequency and amplitude of mEPSCs and mIPSCs of layer Ⅳ pyramidal neurons in V1B of each group, and to analyze their differences and changes. Results: The frequency of mEPSCs of layer Ⅳ pyramidal neurons in V1B of mice in the four groups was statistically different (F=9.46, P<0.001), with the P35 group being higher than the P28 group [P28 and P35 groups were (8.72±1.34) and (13.28±4.05) Hz, t=3.39, P=0.012], and the P130 group being lower than the P35 group [P35 and P130 groups were (13.28±4.05) and (5.82±1.98) Hz, t=5.21, P<0.001]; the amplitude of mEPSCs of layer Ⅳ pyramidal neurons in V1B of mice in the four groups was not statistically different (F=2.84, P=0.055). The frequency of mIPSCs of layer Ⅳ pyramidal neurons in V1B of mice in the four groups was statistically different (F=8.14, P<0.001), with the P130 group being higher than the P14 group [P14 and P130 groups were (5.22±1.33) and (12.03±3.94) Hz, t=4.678, P<0.001]; the amplitude of mIPSCs of layer Ⅳ pyramidal neurons in V1B of mice in the four groups was statistically different (F=7.06, P=0.001), with the P35 group being higher than the P28 group [P28 and P35 groups were (20.07±3.56) and (28.47±5.98) pA, t=3.66, P=0.006], and the P130 group being lower than the P35 group [P35 and P130 groups were (28.47±5.98) and (20.32±3.55) pA, t=3.33, P=0.014]. Conclusions: The excitatory synaptic development of layer Ⅳ pyramidal neurons in V1B of mice is in a vigorous growth state during development and gradually weakens with age, while the inhibitory synaptic development gradually strengthens with increasing postnatal age, and both of them rapidly develop during the critical period.

GABAB - and GABAA -receptor-mediated regulation of Up and Down states across development.

Slow oscillations, the hallmark of non-REM sleep, and their cellular counterpart, Up and Down states (UDSs), are considered a signature of cortical dynamics that reflect the intrinsic network organization. Although previous studies have explored the role of inhibition in regulating UDSs, little is known about whether this role changes with maturation. This is surprising since both slow oscillations and UDSs exhibit significant age-dependent alterations. To elucidate the developmental impact of GABAB and GABAA receptors on UDS activity, we conducted simultaneous local field potentials and intracellular recordings ex vivo, in brain slices of young and adult male mice, using selective blockers, CGP55845 and a non-saturating concentration of gabazine, respectively. Blockade of both GABAB and GABAA signalling showed age-differentiated functions. CGP55845 caused an increase in Down state duration in young animals, but a decrease in adults. Gabazine evoked spike and wave discharges in both ages; however, while young networks became completely epileptic, adults maintained the ability to generate UDSs. Furthermore, voltage clamp recordings of miniature inhibitory postsynaptic currents revealed that gabazine selectively blocks phasic currents, particularly involving postsynaptic mechanisms. The latter exhibit clear maturational changes, suggesting a different subunit composition of GABAA receptors in young vs. adult animals. Indeed, subsequent local field potential recordings under diazepam (nanomolar or micromolar concentrations) revealed that mechanisms engaging the drug's classical binding site, mediated by α1-subunit-containing GABAA receptors, make a bigger contribution to Up state initiation in young networks compared to adults. Taken together, these findings help clarify the mechanisms that underlie the maturation of cortical network activity and enhance our understanding regarding the emergence of neurodevelopmental disorders. KEY POINTS: Slow oscillations, the EEG hallmark of non-REM sleep, and their cellular counterpart, Up and Down states (UDSs), are considered the default activity of the cerebral cortex and reflect the underlying neural connectivity. GABAB - and GABAA -receptor-mediated inhibition play a major role in regulating UDS activity. Although slow oscillations and UDSs exhibit significant alterations as a function of age, it is unknown how developmental changes in inhibition contribute to the developmental profile of this activity. In this study, we reveal for the first time age-dependent effects of GABAB and GABAA signalling on UDSs. We also document the differential subunit composition of postsynaptic GABAA receptors in young and adult animals, highlighting the α1-subunit as a major component of the age-differentiated regulation of UDSs. These findings help clarify the mechanisms that underlie the maturation of cortical network activity, and enhance our understanding regarding the emergence of neurodevelopmental disorders.

Gq neuromodulation of BLA parvalbumin interneurons induces burst firing and mediates fear-associated network and behavioral state transition in mice.

Patterned coordination of network activity in the basolateral amygdala (BLA) is important for fear expression. Neuromodulatory systems play an essential role in regulating changes between behavioral states, however the mechanisms underlying this neuromodulatory control of transitions between brain and behavioral states remain largely unknown. We show that chemogenetic Gq activation and α1 adrenoreceptor activation in mouse BLA parvalbumin (PV) interneurons induces a previously undescribed, stereotyped phasic bursting in PV neurons and time-locked synchronized bursts of inhibitory postsynaptic currents and phasic firing in BLA principal neurons. This Gq-coupled receptor activation in PV neurons suppresses gamma oscillations in vivo and in an ex vivo slice model, and facilitates fear memory recall, which is consistent with BLA gamma suppression during conditioned fear expression. Thus, here we identify a neuromodulatory mechanism in PV inhibitory interneurons of the BLA which regulates BLA network oscillations and fear memory recall.

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina.

Amacrine cells constitute a large and heterogeneous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signalling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABAA receptors of A17 amacrines using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10%-90% rise time ~740 µs) and double-exponential decays (τfast ~4.5 ms [43% of amplitude]; τslow ~22 ms). Ultra-fast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ1 ~5.3 ms [55% of amplitude]; τ2 ~48 ms [32% of amplitude]; τ3 ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1ßγ2 subunit composition and the somatic receptors have an α2ßγ2 and/or α3ßγ2 composition. These results demonstrate the presence of synaptic GABAA receptors on A17s, which may play an important role in signal integration in these cells.

Differential Activity-Dependent Increase in Synaptic Inhibition and Parvalbumin Interneuron Recruitment in Dentate Granule Cells and Semilunar Granule Cells.

Strong inhibitory synaptic gating of dentate gyrus granule cells (GCs), attributed largely to fast-spiking parvalbumin interneurons (PV-INs), is essential to maintain sparse network activity needed for dentate dependent behaviors. However, the contribution of PV-INs to basal and input-driven sustained synaptic inhibition in GCs and semilunar granule cells (SGCs), a sparse morphologically distinct dentate projection neuron subtype, is currently unknown. In studies conducted in hippocampal slices from mice, we find that although basal IPSCs are more frequent in SGCs and optical activation of PV-INs reliably elicited IPSCs in both GCs and SGCs, optical suppression of PV-INs failed to reduce IPSC frequency in either cell type. Amplitude and kinetics of IPSCs evoked by perforant path (PP) activation were not different between GCs and SGCs. However, the robust increase in sustained polysynaptic IPSCs elicited by paired afferent stimulation was lower in SGCs than in simultaneously recorded GCs. Optical suppression of PV-IN selectively reduced sustained IPSCs in SGCs but not in GCs. These results demonstrate that PV-INs, while contributing minimally to basal synaptic inhibition in both GCs and SGCs in slices, mediate sustained feedback inhibition selectively in SGCs. The temporally selective blunting of activity-driven sustained inhibitory gating of SGCs could support their preferential and persistent recruitment during behavioral tasks.SIGNIFICANCE STATEMENT Our study identifies that feedback inhibitory regulation of dentate semilunar granule cells (SGCs), a sparse and functionally distinct class of projection neurons, differs from that of the classical projection neurons, GCs. Notably, we demonstrate relatively lower activity-dependent increase in sustained feedback inhibitory synaptic inputs to SGCs when compared with GCs which would facilitate their persistent activity and preferential recruitment as part of memory ensembles. Since dentate GC activity levels during memory processing are heavily shaped by basal and feedback inhibition, the fundamental differences in basal and evoked sustained inhibition between SGCs and GCs characterized here provide a framework to reorganize current understanding of the dentate circuit processing.

Brainstem Circuits Triggering Saccades and Fixation.

Omnipause neurons (OPNs) in the nucleus raphe interpositus have tonic activity while the eyes are stationary ("fixation") but stop firing immediately before and during saccades. To locate the source of suppression, we analyzed synaptic inputs from the rostral and caudal superior colliculi (SCs) to OPNs by using intracellular recording and staining, and investigated pathways transmitting the inputs in anesthetized cats of both sexes. Electrophysiologically or morphologically identified OPNs received monosynaptic excitation from the rostral SCs with contralateral dominance, and received disynaptic inhibition from the caudal SCs with ipsilateral dominance. Cutting the tectoreticular tract transversely between the contralateral OPN and inhibitory burst neuron (IBN) regions eliminated inhibition from the caudal SCs, but not excitation from the rostral SCs in OPNs. In contrast, a midline section between IBN regions eliminated disynaptic inhibition in OPNs from the caudal SCs but did not affect the monosynaptic excitation from the rostral SCs. Stimulation of the contralateral IBN region evoked monosynaptic inhibition in OPNs, which was facilitated by preconditioning SC stimulation. Three-dimensional reconstruction of HRP-stained cells revealed that individual OPNs have axons that terminate in the opposite IBN area, while individual IBNs have axon collaterals to the opposite OPN area. These results show that there are differences in the neural circuit from the rostral and caudal SCs to the brainstem premotor circuitry and that IBNs suppress OPNs immediately before and during saccades. Thus, the IBNs, which are activated by caudal SC saccade neurons, shut down OPN firing and help to trigger saccades and suppress ("latch") OPN activity during saccades.SIGNIFICANCE STATEMENT Saccades are the fastest eye movements to redirect gaze to an object of interest and bring its image on the fovea for fixation. Burst neurons (BNs) and omnipause neurons (OPNs) which behave reciprocally in the brainstem, are important for saccade generation and fixation. This study investigated unsolved important questions about where these neurons receive command signals and how they interact for initiating saccades from visual fixation. The results show that the rostral superior colliculi (SCs) excite OPNs monosynaptically for fixation, whereas the caudal SCs monosynaptically excite inhibitory BNs, which then directly inhibit OPNs for the initiation of saccades. This inhibition from the caudal SCs may account for the omnipause behavior of OPNs for initiation and maintenance of saccades in all directions.

REST/NRSF drives homeostatic plasticity of inhibitory synapses in a target-dependent fashion.

The repressor-element 1-silencing transcription/neuron-restrictive silencer factor (REST/NRSF) controls hundreds of neuron-specific genes. We showed that REST/NRSF downregulates glutamatergic transmission in response to hyperactivity, thus contributing to neuronal homeostasis. However, whether GABAergic transmission is also implicated in the homeostatic action of REST/NRSF is unknown. Here, we show that hyperactivity-induced REST/NRSF activation, triggers a homeostatic rearrangement of GABAergic inhibition, with increased frequency of miniature inhibitory postsynaptic currents (IPSCs) and amplitude of evoked IPSCs in mouse cultured hippocampal neurons. Notably, this effect is limited to inhibitory-onto-excitatory neuron synapses, whose density increases at somatic level and decreases in dendritic regions, demonstrating a complex target- and area-selectivity. The upscaling of perisomatic inhibition was occluded by TrkB receptor inhibition and resulted from a coordinated and sequential activation of the Npas4 and Bdnf gene programs. On the opposite, the downscaling of dendritic inhibition was REST-dependent, but BDNF-independent. The findings highlight the central role of REST/NRSF in the complex transcriptional responses aimed at rescuing physiological levels of network activity in front of the ever-changing environment.

Dopamine D2 receptor regulates cortical synaptic pruning in rodents.

Synaptic pruning during adolescence is important for appropriate neurodevelopment and synaptic plasticity. Aberrant synaptic pruning may underlie a variety of brain disorders such as schizophrenia, autism and anxiety. Dopamine D2 receptor (Drd2) is associated with several neuropsychiatric diseases and is the target of some antipsychotic drugs. Here we generate self-reporting Drd2 heterozygous (SR-Drd2+/-) rats to simultaneously visualize Drd2-positive neurons and downregulate Drd2 expression. Time course studies on the developing anterior cingulate cortex (ACC) from control and SR-Drd2+/- rats reveal important roles of Drd2 in regulating synaptic pruning rather than synapse formation. Drd2 also regulates LTD, a form of synaptic plasticity which includes some similar cellular/biochemical processes as synaptic pruning. We further demonstrate that Drd2 regulates synaptic pruning via cell-autonomous mechanisms involving activation of mTOR signaling. Deficits of Drd2-mediated synaptic pruning in the ACC during adolescence lead to hyper-glutamatergic function and anxiety-like behaviors in adulthood. Taken together, our results demonstrate important roles of Drd2 in cortical synaptic pruning.

Synchronous inhibitory synaptic inputs to layer II/III pyramidal neurons in the murine barrel cortex.

The barrel cortex exhibits obvious columnar organization. Although GABAergic inhibition plays a critical role in regulating neural excitation in response to mechanical stimuli applied to whiskers, the profiles of synchronous events for inhibitory synaptic transmission in intracolumnar and transcolumnar pyramidal neurons remain unknown. To explore a functional mechanism of synchronous inhibition of pyramidal neurons, we performed paired whole-cell patch-clamp recordings and recorded spontaneous inhibitory postsynaptic currents (sIPSCs) from layer II/III pyramidal neurons. A cross-correlogram of sIPSCs (1 ms bin) was used to detect synchronous sIPSCs. Synchronous neuron pairs were defined as those whose peak number of sIPSCs between -3 and 3 ms exceeded the mean + 2 SD of the number of sIPSCs in the period of -50 to 50 ms minus the number in that of -3 to 3 ms period. In the recording of pyramidal neurons located in the same column (intracolumn), 61.5% of neuron pairs were classified as synchronous neuron pairs, while 52.6% of pyramidal neuron pairs in adjacent columns (transcolumn) were defined as synchronous neuron pairs. The amplitude of synchronous sIPSCs was comparable to that of asynchronous sIPSCs in asynchronous neuron pairs, whereas that of synchronous sIPSCs was larger than that of asynchronous sIPSCs in synchronous neuron pairs. Synchronicity of sIPSCs did not depend on the distance of neuron pairs. These results suggest that layer II/III pyramidal neurons receive synchronous inhibitory synaptic inputs generated by a certain type of GABAergic interneuron that induces large IPSCs in pyramidal neurons, likely to be fast-spiking cells.

Tonotopic Specializations in Number, Size, and Reversal Potential of GABAergic Inputs Fine-Tune Temporal Coding at Avian Cochlear Nucleus.

GABAergic inhibition in neurons plays a critical role in determining the output of neural circuits. Neurons in avian nucleus magnocellularis (NM) use several tonotopic-region-dependent specializations to relay the timing information of sound in the auditory nerve to higher auditory nuclei. Previously, we showed that feedforward GABAergic inhibition in NM has a different dependence on the level of auditory nerve activity, with the low-frequency region having a low-threshold and linear relationship, while the high-frequency region has a high-threshold and step-like relationship. However, it remains unclear how the GABAergic synapses are tonotopically regulated and interact with other specializations of NM neurons. In this study, we examined GABAergic transmission in the NM of chickens of both sexes and explored its contributions to the temporal coding of sound at each tonotopic region. We found that the number and size of unitary GABAergic currents and their reversal potential were finely tuned at each tonotopic region in the NM. At the lower-frequency region, unitary GABAergic currents were larger in number but smaller in size. In addition, their reversal potential was close to the resting potential of neurons, which enabled reliable inhibition despite the smaller potassium conductance. At the higher-frequency region, on the other hand, unitary GABAergic currents were fewer, larger, and highly depolarizing, which enabled powerful inhibition via activating the large potassium conductance. Thus, we propose that GABAergic synapses are coordinated with the characteristics of excitatory synapses and postsynaptic neurons, ensuring the temporal coding for wide frequency and intensity ranges.SIGNIFICANCE STATEMENT We found in avian cochlear nucleus that the number and size of unitary GABAergic inputs differed among tonotopic regions and correlated to respective excitatory inputs; it was larger in number but smaller in size for neurons tuned to lower-frequency sound. Furthermore, GABAergic reversal potential also differed among the regions in accordance with the size of Kv1 current; it was less depolarized in the lower-frequency neurons with smaller Kv1 current. These differentiations of GABAergic transmission maximized the effects of inhibition at each tonotopic region, ensuring precise and reliable temporal coding across frequencies and intensities. Our results emphasize the importance of optimizing characteristics of GABAergic transmission within individual neurons for proper neural circuit function.

Influence of BDNF Val66Met polymorphism on excitatory-inhibitory balance and plasticity in human motor cortex.

OBJECTIVE: While previous studies showed that the single nucleotide polymorphism (Val66Met) of brain-derived neurotrophic factor (BDNF) can impact neuroplasticity, the influence of BDNF genotype on cortical circuitry and relationship to neuroplasticity remain relatively unexplored in human. METHODS: Using individualised transcranial magnetic stimulation (TMS) parameters, we explored the influence of the BDNF Val66Met polymorphism on excitatory and inhibitory neural circuitry, its relation to I-wave TMS (ITMS) plasticity and effect on the excitatory/inhibitory (E/I) balance in 18 healthy individuals. RESULTS: Excitatory and inhibitory indexes of neurotransmission were reduced in Met allele carriers. An E/I balance was evident, which was influenced by BDNF with higher E/I ratios in Val/Val homozygotes. Both long-term potentiation (LTP-) and depression (LTD-) like ITMS plasticity were greater in Val/Val homozygotes. LTP- but not LTD-like effects were restored in Met allele carriers by increasing stimulus intensity to compensate for reduced excitatory transmission. CONCLUSIONS: The influence of BDNF genotype may extend beyond neuroplasticity to neurotransmission. The E/I balance was evident in human motor cortex, modulated by BDNF and measurable using TMS. Given the limited sample, these preliminary findings warrant further investigation. SIGNIFICANCE: These novel findings suggest a broader role of BDNF genotype on neurocircuitry in human motor cortex.

Analyses of the Mode of Action of an Alpha-Adrenoceptor Blocker in Substantia Gelatinosa Neurons in Rats.

To elucidate why naftopidil increases the frequency of spontaneous synaptic currents in only some substantia gelatinosa (SG) neurons, post-hoc analyses were performed. Blind patch-clamp recording was performed using slice preparations of SG neurons from the spinal cords of adult rats. Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs, respectively) were recorded. The ratios of the frequency and amplitude of the sIPSCs and sEPSCs following the introduction of naftopidil compared with baseline, and after the application of naftopidil, serotonin (5-HT), and prazosin, compared with noradrenaline (NA) were evaluated. First, the sIPSC analysis indicated that SG neurons reached their full response ratio for NA at 50 µM. Second, they responded to 5-HT (50 µM) with a response ratio similar to that for NA, but prazosin (10 µM) did not change the sEPSCs and sIPSCs. Third, the highest concentration of naftopidil (100 µM) led to two types of response in the SG neurons, which corresponded with the reactions to 5-HT and prazosin. These results indicate that not all neurons were necessarily activated by naftopidil, and that the micturition reflex may be regulated in a sophisticated manner by inhibitory mechanisms in these interneurons.

Blockade of BK channels attenuates chronic visceral hypersensitivity in an IBS-like rat model.

BACKGROUND: Visceral hypersensitivity in irritable bowel syndrome (IBS) is still poorly understood, despite that chronic abdominal pain is the most common symptoms in IBS patients. To study effects of BK channels on visceral hypersensitivity in IBS rats and the underlying mechanisms, IBS rats were established by colorectal distention (CRD) in postnatal rats. The expression of large-conductance calcium and voltage-dependent potassium ion channels (BK channels) of the thoracolumbar spinal cord was examined in IBS and control rats. The effects of BK channel blockade on visceral hypersensitivity were evaluated. The interaction of BK channels and N-methyl-D-aspartate acid (NMDA) receptors was explored, and synaptic transmission at superficial dorsal horn (SDH) neurons of the thoracolumbar spinal cord was recorded by whole-cell patch clamp in IBS rats. RESULTS: The expression of the BK channels of the thoracolumbar spinal cord in IBS rats was significantly reduced. The blockade of BK channels could reduce the visceral hypersensitivity in IBS rats. There was an interaction between BK channels and NMDA receptors in the spinal cord. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in SDH neurons is significantly reduced in IBS rats. The blockade of BK channels depolarizes the inhibitory interneuron membrane and increases their excitability in IBS rats. CONCLUSIONS: BK channels could interact with NMDA receptors in the thoracolumbar spinal cord of rats and regulate visceral hypersensitivity in IBS rats.

Increased inhibitory activity in the basolateral amygdala and decreased anxiety during estrus: A potential role for ASIC1a channels.

The amygdala is central to emotional behavior, and the excitability level of the basolateral nucleus of the amygdala (BLA) is associated with the level of anxiety. The excitability of neuronal networks is significantly controlled by GABAergic inhibition. Here, we investigated whether GABAergic inhibition in the BLA is altered during the rat estrous cycle. In rat amygdala slices, most principal BLA neurons display spontaneous IPSCs (sIPSCs) in the form of "bursts" of inhibitory currents, occurring rhythmically at a frequency of about 0.5 Hz. The percentage of BLA neurons displaying sIPSC bursts, along with the inhibitory charge transferred by sIPSCs and the frequency of sIPSC bursts, were significantly increased during the estrus phase; increased inhibition was accompanied by reduced anxiety in the open field, the light-dark box, and the acoustic startle response tests. sIPSC bursts were blocked by ibuprofen, an antagonist of acid-sensing-1a channels (ASIC1a), whose activity is known to increase by decreasing temperature. A transient reduction in the temperature of the slice medium, strengthened the sIPSCs bursts; this effect was blocked in the presence of ibuprofen. Further analysis of the sIPSC bursts during estrus showed significantly stronger rhythmic inhibitory activity in early estrus, when body temperature drops, compared with late estrus. To the extent that these results may relate to humans, it is suggested that "a calmer amygdala" due to increased inhibitory activity may underlie the positive affect in women around ovulation time. ASIC1a may contribute to increased inhibition, with their activity facilitated by the body-temperature drop preceding ovulation.

A subthreshold synaptic mechanism regulating BDNF expression and resting synaptic strength.

Recent studies have demonstrated that protein translation can be regulated by spontaneous excitatory neurotransmission. However, the impact of spontaneous neurotransmitter release on gene transcription remains unclear. Here, we study the effects of the balance between inhibitory and excitatory spontaneous neurotransmission on brain-derived neurotrophic factor (BDNF) regulation and synaptic plasticity. Blockade of spontaneous inhibitory events leads to an increase in the transcription of Bdnf and Npas4 through altered synaptic calcium signaling, which can be blocked by antagonism of NMDA receptors (NMDARs) or L-type voltage-gated calcium channels (VGCCs). Transcription is bidirectionally altered by manipulating spontaneous inhibitory, but not excitatory, currents. Moreover, blocking spontaneous inhibitory events leads to multiplicative downscaling of excitatory synaptic strength in a manner that is dependent on both transcription and BDNF signaling. These results reveal a role for spontaneous inhibitory neurotransmission in BDNF signaling that sets excitatory synaptic strength at rest.

Probing the polarity of spontaneous perisomatic GABAergic synaptic transmission in the mouse CA3 circuit in vivo.

The hypothesis that reversed, excitatory GABA may be involved in various brain pathologies, including epileptogenesis, is appealing but controversial because of the technical difficulty of probing endogenous GABAergic synaptic function in vivo. We overcome this challenge by non-invasive extracellular recording of neuronal firing responses to optogenetically evoked and spontaneously occurring inhibitory perisomatic GABAergic field potentials, generated by individual parvalbumin interneurons on their target pyramidal cells. Our direct probing of GABAergic transmission suggests a rather anecdotal participation of excitatory GABA in two specific models of epileptogenesis in the mouse CA3 circuit in vivo, even though this does not preclude its expression in other brain areas or pathological conditions. Our approach allows the detection of distinct alterations of inhibition during spontaneous activity in vivo, with high sensitivity. It represents a promising tool for the investigation of excitatory GABA in different pathological conditions that may affect the hippocampal circuit.

Ionic and signaling mechanisms involved in neurotensin-mediated excitation of central amygdala neurons.

Neurotensin (NT) serves as a neuromodulator in the brain where it regulates a variety of physiological functions. Whereas the central amygdala (CeA) expresses NT peptide and NTS1 receptors and application of NT has been shown to excite CeA neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of NTS1 receptors increased the neuronal excitability of the lateral nucleus (CeL) of CeA. Both phospholipase Cß (PLCß) and phosphatidylinositol 4,5-bisphosphate (PIP2) depletion were required, whereas intracellular Ca2+ release and PKC were unnecessary for NT-elicited excitation of CeL neurons. NT increased the input resistance and time constants of CeL neurons, suggesting that NT excites CeL neurons by decreasing a membrane conductance. Depressions of the inwardly rectifying K+ (Kir) channels including both the Kir2 subfamily and the GIRK channels were required for NT-elicited excitation of CeL neurons. Activation of NTS1 receptors in the CeL led to GABAergic inhibition of medial nucleus of CeA neurons, suggesting that NT modulates the network activity in the amygdala. Our results may provide a cellular and molecular mechanism to explain the physiological functions of NT in vivo.

Regulating synchronous oscillations of cerebellar granule cells by different types of inhibition.

Synchronous oscillations in neural populations are considered being controlled by inhibitory neurons. In the granular layer of the cerebellum, two major types of cells are excitatory granular cells (GCs) and inhibitory Golgi cells (GoCs). GC spatiotemporal dynamics, as the output of the granular layer, is highly regulated by GoCs. However, there are various types of inhibition implemented by GoCs. With inputs from mossy fibers, GCs and GoCs are reciprocally connected to exhibit different network motifs of synaptic connections. From the view of GCs, feedforward inhibition is expressed as the direct input from GoCs excited by mossy fibers, whereas feedback inhibition is from GoCs via GCs themselves. In addition, there are abundant gap junctions between GoCs showing another form of inhibition. It remains unclear how these diverse copies of inhibition regulate neural population oscillation changes. Leveraging a computational model of the granular layer network, we addressed this question to examine the emergence and modulation of network oscillation using different types of inhibition. We show that at the network level, feedback inhibition is crucial to generate neural oscillation. When short-term plasticity was equipped on GoC-GC synapses, oscillations were largely diminished. Robust oscillations can only appear with additional gap junctions. Moreover, there was a substantial level of cross-frequency coupling in oscillation dynamics. Such a coupling was adjusted and strengthened by GoCs through feedback inhibition. Taken together, our results suggest that the cooperation of distinct types of GoC inhibition plays an essential role in regulating synchronous oscillations of the GC population. With GCs as the sole output of the granular network, their oscillation dynamics could potentially enhance the computational capability of downstream neurons.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of µ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release.

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic µ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo1 and Gαi1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo1 inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi1-3 inhibitors had no effect. Both Gαo1 and Gαi1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo1 and Gαi1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo1 and Gαi1-3 inhibitor peptides, suggesting that GABAB receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic µ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABAB receptor coupling to G proteins that could be exploited for new pain therapies.