Parvalbumin interneuron cell-to-network plasticity: mechanisms and therapeutic avenues.

Alzheimer's disease (AD) and schizophrenia (SCZ) represent two major neuropathological conditions with a high disease burden. Despite their distinct etiologies, patients suffering from AD or SCZ share a common burden of disrupted memory functions unattended by current therapies. Recent preclinical analyses highlight cell-type-specific contributions of parvalbumin interneurons (PVIs), particularly the plasticity of their cellular excitability, towards intact neuronal network function (cell-to-network plasticity) and memory performance. Here we argue that deficits of PVI cell-to-network plasticity may underlie memory deficits in AD and SCZ, and we explore two therapeutic avenues: the targeting of PVI-specific neuromodulation, including by neuropeptides, and the recruitment of network synchrony in the gamma frequency range (40 Hz) by external stimulation. We finally propose that these approaches be merged under consideration of recent insights into human brain physiology.

Estimation of persistent sodium-current density in rat hippocampal mossy fibre boutons: Correction of space-clamp errors.

We used whole-cell patch clamp to estimate the stationary voltage dependence of persistent sodium-current density (iNaP) in rat hippocampal mossy fibre boutons. Cox's method for correcting space-clamp errors was extended to the case of an isopotential compartment with attached neurites. The method was applied to voltage-ramp experiments, in which iNaP is assumed to gate instantaneously. The raw estimates of iNaP led to predicted clamp currents that were at variance with observation, hence an algorithm was devised to improve these estimates. Optionally, the method also allows an estimate of the membrane specific capacitance, although values of the axial resistivity and seal resistance must be provided. Assuming that membrane specific capacitance and axial resistivity were constant, we conclude that seal resistance continued to fall after adding TTX to the bath. This might have been attributable to a further deterioration of the seal after baseline rather than an unlikely effect of TTX. There was an increase in the membrane specific resistance in TTX. The reason for this is unknown, but it meant that iNaP could not be determined by simple subtraction. Attempts to account for iNaP with a Hodgkin-Huxley model of the transient sodium conductance met with mixed results. One thing to emerge was the importance of voltage shifts. Also, a large variability in previously reported values of transient sodium conductance in mossy fibre boutons made comparisons with our results difficult. Various other possible sources of error are discussed. Simulations suggest a role for iNaP in modulating the axonal attenuation of EPSPs. KEY POINTS: We used whole-cell patch clamp to estimate the stationary voltage dependence of persistent sodium-current density (iNaP) in rat hippocampal mossy fibre boutons, using a KCl-based internal (pipette) solution and correcting for the liquid junction potential (2 mV). Space-clamp errors and deterioration of the patch-clamp seal during the experiment were corrected for by compartmental modelling. Attempts to account for iNaP in terms of the transient sodium conductance met with mixed results. One possibility is that the transient sodium conductance is higher in mossy fibre boutons than in the axon shaft. The analysis illustrates the need to account for various voltage shifts (Donnan potentials, liquid junction potentials and, possibly, other voltage shifts). Simulations suggest a role for iNaP in modulating the axonal attenuation of excitatory postsynaptic potentials, hence analog signalling by dentate granule cells.

Directed and acyclic synaptic connectivity in the human layer 2-3 cortical microcircuit.

The computational capabilities of neuronal networks are fundamentally constrained by their specific connectivity. Previous studies of cortical connectivity have mostly been carried out in rodents; whether the principles established therein also apply to the evolutionarily expanded human cortex is unclear. We studied network properties within the human temporal cortex using samples obtained from brain surgery. We analyzed multineuron patch-clamp recordings in layer 2-3 pyramidal neurons and identified substantial differences compared with rodents. Reciprocity showed random distribution, synaptic strength was independent from connection probability, and connectivity of the supragranular temporal cortex followed a directed and mostly acyclic graph topology. Application of these principles in neuronal models increased dimensionality of network dynamics, suggesting a critical role for cortical computation.

Gamma oscillation plasticity is mediated via parvalbumin interneurons.

Understanding the plasticity of neuronal networks is an emerging field of (patho-) physiological research, yet the underlying cellular mechanisms remain poorly understood. Gamma oscillations (30 to 80 hertz), a biomarker of cognitive performance, require and potentiate glutamatergic transmission onto parvalbumin-positive interneurons (PVIs), suggesting an interface for cell-to-network plasticity. In ex vivo local field potential recordings, we demonstrate long-term potentiation of hippocampal gamma power. Gamma potentiation obeys established rules of PVI plasticity, requiring calcium-permeable AMPA receptors (CP-AMPARs) and metabotropic glutamate receptors (mGluRs). A microcircuit computational model of CA3 gamma oscillations predicts CP-AMPAR plasticity onto PVIs critically outperforms pyramidal cell plasticity in increasing gamma power and completely accounts for gamma potentiation. We reaffirm this ex vivo in three PVI-targeting animal models, demonstrating that gamma potentiation requires PVI-specific signaling via a Gq/PKC pathway comprising mGluR5 and a Gi-sensitive, PKA-dependent pathway. Gamma activity-dependent, metabotropically mediated CP-AMPAR plasticity on PVIs may serve as a guiding principle in understanding network plasticity in health and disease.

Persistent synaptic inhibition of the subthalamic nucleus by high frequency stimulation.

BACKGROUND: Deep brain stimulation (DBS) provides symptomatic relief in a growing number of neurological indications, but local synaptic dynamics in response to electrical stimulation that may relate to its mechanism of action have not been fully characterized. OBJECTIVE: The objectives of this study were to (1) study local synaptic dynamics during high frequency extracellular stimulation of the subthalamic nucleus (STN), and (2) compare STN synaptic dynamics with those of the neighboring substantia nigra pars reticulata (SNr). METHODS: Two microelectrodes were advanced into the STN and SNr of patients undergoing DBS surgery for Parkinson's disease (PD). Neuronal firing and evoked field potentials (fEPs) were recorded with one microelectrode during stimulation from an adjacent microelectrode. RESULTS: Inhibitory fEPs could be discerned within the STN and their amplitudes predicted bidirectional effects on neuronal firing (p = .013). There were no differences between STN and SNr inhibitory fEP dynamics at low stimulation frequencies (p > .999). However, inhibitory neuronal responses were sustained over time in STN during high frequency stimulation but not in SNr (p < .001) where depression of inhibitory input was coupled with a return of neuronal firing (p = .003). INTERPRETATION: Persistent inhibitory input to the STN suggests a local synaptic mechanism for the suppression of subthalamic firing during high frequency stimulation. Moreover, differences in the resiliency versus vulnerability of inhibitory inputs to the STN and SNr suggest a projection source- and frequency-specificity for this mechanism. The feasibility of targeting electrophysiologically-identified neural structures may provide insight into how DBS achieves frequency-specific modulation of neuronal projections.

High-throughput microcircuit analysis of individual human brains through next-generation multineuron patch-clamp.

Comparing neuronal microcircuits across different brain regions, species and individuals can reveal common and divergent principles of network computation. Simultaneous patch-clamp recordings from multiple neurons offer the highest temporal and subthreshold resolution to analyse local synaptic connectivity. However, its establishment is technically complex and the experimental performance is limited by high failure rates, long experimental times and small sample sizes. We introduce an in vitro multipatch setup with an automated pipette pressure and cleaning system facilitating recordings of up to 10 neurons simultaneously and sequential patching of additional neurons. We present hardware and software solutions that increase the usability, speed and data throughput of multipatch experiments which allowed probing of 150 synaptic connections between 17 neurons in one human cortical slice and screening of over 600 connections in tissue from a single patient. This method will facilitate the systematic analysis of microcircuits and allow unprecedented assessment of inter-individual variability.

Connectivity and Dynamics Underlying Synaptic Control of the Subthalamic Nucleus.

Adaptive motor control critically depends on the interconnected nuclei of the basal ganglia in the CNS. A pivotal element of the basal ganglia is the subthalamic nucleus (STN), which serves as a therapeutic target for deep brain stimulation (DBS) in movement disorders, such as Parkinson's disease. The functional connectivity of the STN at the microcircuit level, however, still requires rigorous investigation. Here we combine multiple simultaneous whole-cell recordings with extracellular stimulation and post hoc neuroanatomical analysis to investigate intrinsic and afferent connectivity and synaptic properties of the STN in acute brain slices obtained from rats of both sexes. Our data reveal an absence of intrinsic connectivity and an afferent innervation with low divergence, suggesting that STN neurons operate as independent processing elements driven by upstream structures. Hence, synchrony in the STN, a hallmark of motor processing, exclusively depends on the interactions and dynamics of GABAergic and glutamatergic afferents. Importantly, these inputs are subject to differential short-term depression when stimulated at high, DBS-like frequencies, shifting the balance of excitation and inhibition toward inhibition. Thus, we present a mechanism for fast yet transient decoupling of the STN from synchronizing afferent control. Together, our study provides new insights into the microcircuit organization of the STN by identifying its neurons as parallel processing units and thus sets new constraints for future computational models of the basal ganglia. The observed differential short-term plasticity of afferent inputs further offers a basis to better understand and optimize DBS algorithms.SIGNIFICANCE STATEMENT The subthalamic nucleus (STN) is a pivotal element of the basal ganglia and serves as target for deep brain stimulation, but information on the functional connectivity of its neurons is limited. To investigate the STN microcircuitry, we combined multiple simultaneous patch-clamp recordings and neuroanatomical analysis. Our results provide new insights into the synaptic organization of the STN identifying its neurons as parallel processing units and thus set new constraints for future computational models of the basal ganglia. We further find that synaptic dynamics of afferent inputs result in a rapid yet transient decoupling of the STN when stimulated at high frequencies. These results offer a better understanding of deep brain stimulation mechanisms, promoting the development of optimized algorithms.

Spike sorting of synchronous spikes from local neuron ensembles.

Synchronous spike discharge of cortical neurons is thought to be a fingerprint of neuronal cooperativity. Because neighboring neurons are more densely connected to one another than neurons that are located further apart, near-synchronous spike discharge can be expected to be prevalent and it might provide an important basis for cortical computations. Using microelectrodes to record local groups of neurons does not allow for the reliable separation of synchronous spikes from different cells, because available spike sorting algorithms cannot correctly resolve the temporally overlapping waveforms. We show that high spike sorting performance of in vivo recordings, including overlapping spikes, can be achieved with a recently developed filter-based template matching procedure. Using tetrodes with a three-dimensional structure, we demonstrate with simulated data and ground truth in vitro data, obtained by dual intracellular recording of two neurons located next to a tetrode, that the spike sorting of synchronous spikes can be as successful as the spike sorting of nonoverlapping spikes and that the spatial information provided by multielectrodes greatly reduces the error rates. We apply the method to tetrode recordings from the prefrontal cortex of behaving primates, and we show that overlapping spikes can be identified and assigned to individual neurons to study synchronous activity in local groups of neurons.

Sparse but highly efficient Kv3 outpace BKCa channels in action potential repolarization at hippocampal mossy fiber boutons.

Presynaptic elements of axons, in which action potentials (APs) cause release of neurotransmitter, are sites of high densities and complex interactions of proteins. We report that the presence of K(v)3 channels in addition to K(v)1 at glutamatergic mossy fiber boutons (MFBs) in rat hippocampal slices considerably limits the number of fast, voltage-activated potassium channels necessary to achieve basal presynaptic AP repolarization. The ∼ 10-fold higher repolarization efficacy per K(v)3 channel compared with presynaptic K(v)1 results from a higher steady-state availability at rest, a better recruitment by the presynaptic AP as a result of faster activation kinetics, and a larger single-channel conductance. Large-conductance calcium- and voltage-activated potassium channels (BK(Ca)) at MFBs give rise to a fast activating/fast inactivating and a slowly activating/sustained K(+) current component during long depolarizations. However, BK(Ca) contribute to MFB-AP repolarization only after presynaptic K(v)3 have been disabled. The calcium chelators EGTA and BAPTA are equally effective in preventing BK(Ca) activation, suggesting that BK(Ca) are not organized in nanodomain complexes with presynaptic voltage-gated calcium channels. Thus, the functional properties of K(v)3 channels at MFBs are tuned to both promote brevity of presynaptic APs limiting glutamate release and at the same time keep surface protein density of potassium channels low. Presynaptic BK(Ca) channels are restricted to limit additional increases of the AP half-duration in case of K(v)3 hypofunction, because rapid membrane repolarization by K(v)3 combined with distant calcium sources prevent BK(Ca) activation during basal APs.

Role of microcircuit structure and input integration in hippocampal interneuron recruitment and plasticity.

The proper operation of cortical neuronal networks depends on the temporally precise recruitment of GABAergic inhibitory interneurons. Inhibitory cells receive convergent excitatory inputs from afferent pathways, as well as local collaterals of principal cells, and provide feedforward or feedback inhibition within the circuitry. Accumulating evidence indicates that recruitment of GABAergic cells is highly diverse among interneuron types. Differences in the properties of input synapses, dendritic architecture and membrane properties, as well as the rich repertoire of plasticity mechanisms contribute to this diversity. Efficient and precise recruitment of interneurons is thought to depend on the coincident occurrence of rapid synaptic responses and their faithful propagation to the action potential initiation site. However, slow inputs can also play important roles by facilitating the activation of interneurons by rapid synaptic inputs and supporting associative synaptic plasticity. Here we review how the diversity in the synaptic and integrative properties as well as dendritic geometry of hippocampal inhibitory cells impact on their activation. We further discuss how the various modes of interneuron recruitment can support the versatile cell type- and input-specific computational functions which appear to be adapted to the structure and the function of the network they are embedded in. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Presynaptic glycine receptors on hippocampal mossy fibers.

Presynaptic glycine receptors (GlyRs) have been implicated in the regulation of glutamatergic synaptic transmission. Here, we characterized presynaptic GlyR-mediated currents by patch-clamp recording from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs, focal puff-application of glycine-evoked chloride currents that were blocked by the GlyR antagonist strychnine. Their amplitudes declined substantially during postnatal development, from a mean conductance per MFB of approximately 600 pS in young to approximately 130 pS in adult animals. Single-channel analysis revealed multiple conductance states between approximately 20 and approximately 120 pS, consistent with expression of both homo- and hetero-oligomeric GlyRs. Accordingly, estimated GlyRs densities varied between 8-17 per young, and 1-3 per adult, MFB. Our results demonstrate that functional presynaptic GlyRs are present on hippocampal mossy fiber terminals and suggest a role of these receptors in the regulation of glutamate release during the development of the mossy fiber--CA3 synapse.

Energy-efficient action potentials in hippocampal mossy fibers.

Action potentials in nonmyelinated axons are considered to contribute substantially to activity-dependent brain metabolism. Here we show that fast Na+ current decay and delayed K+ current onset during action potentials in nonmyelinated mossy fibers of the rat hippocampus minimize the overlap of their respective ion fluxes. This results in total Na+ influx and associated energy demand per action potential of only 1.3 times the theoretical minimum, in contrast to the factor of 4 used in previous energy budget calculations for neural activity. Analysis of ionic conductance parameters revealed that the properties of Na+ and K+ channels are matched to make axonal action potentials energy-efficient, minimizing their contribution to activity-dependent metabolism.

Interactions between short-interval intracortical inhibition and short-latency afferent inhibition in human motor cortex.

Transcranial magnetic stimulation (TMS) allows the testing of various inhibitory processes in human motor cortex. Here we aimed at gaining more insight into the underlying physiology by studying the interactions between short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI). SICI and SAI were examined in a slightly contracting hand muscle of healthy subjects by measuring inhibition of a test motor-evoked potential conditioned by a sub-threshold motor cortical magnetic pulse (S1) or an electrical pulse (P) applied to the ulnar nerve at the wrist, respectively. SICI alone and SAI alone had similar magnitude when S1 intensity was set to 90% active motor threshold and P intensity to three times the perceptual sensory threshold. SICI was reduced or even disinhibited when P was co-applied, and SAI was reduced or disinhibited when S1 was co-applied. These interactions did not depend on the exact timing of arrival of P and S1 in motor cortex. A control experiment with a S1 intensity lowered to 70% active motor threshold excluded a contribution by short-interval intracortical facilitation. Finally, SICI with co-applied P correlated linearly with SICI alone with a slope of the regression line close to 1 whereas SAI did not correlate with SAI when S1 was co-applied with a slope of the regression line close to zero. Data indicate that S1 largely eliminates the effects of P when applied together, suggesting dominance of S1 over P. Findings strongly support the idea that SICI and SAI are mediated through two distinct and reciprocally connected subtypes of GABAergic inhibitory interneurons with convergent projections onto the corticospinal neurons. Furthermore, dominance of S1 over P is compatible with the notion that the SICI interneurons target the corticospinal neurons closer to their axon initial segment than the SAI interneurons.

Analog signalling in mammalian cortical axons.

In the mammalian cortex, the classic view assumes that the output information of a neuron is encoded in rather stereotyped action potentials, which provide an all-or-none or digital way of communication between cell body and axonal boutons. A role for subthreshold signal propagation within cortical axons has largely been ignored. Recent achievements of direct recordings from axonal structures in the hippocampus and neocortex extended the classic view by the observation that subthreshold-graded signals propagate down the axon over distances of up to 1 mm. At certain synapses, these analog axonal signals modulate action-potential-dependent transmitter release, thereby enabling a hybrid code of information transmission in local cortical circuits.

GABAergic spill-over transmission onto hippocampal mossy fiber boutons.

Presynaptic ionotropic GABA(A) receptors have been suggested to contribute to the regulation of cortical glutamatergic synaptic transmission. Here, we analyzed presynaptic GABA(A) receptor-mediated currents (34 degrees C) recorded from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs from young and adult animals, GABA puff application activated currents that were blocked by GABA(A) receptor antagonists. The conductance density of 0.65 mS x cm2 was comparable to that of other presynaptic terminals. The single-channel conductance was 36 pS (symmetrical chloride), yielding an estimated GABA(A) receptor density of 20-200 receptors per MFB. Presynaptic GABA(A) receptors likely contain alpha2-subunits as indicated by their zolpidem sensitivity. In accordance with the low apparent GABA affinity (EC50 = 60 microM) of the receptors and a tight control of ambient GABA concentration by GABA transporters, no tonic background activation of presynaptic GABA(A) receptors was observed. Instead, extracellular high-frequency stimulation led to transient presynaptic currents, which were blocked by GABA(A) receptor antagonists but were enhanced by block of GAT 1 (GABA transporter 1), indicating that these currents were generated by GABA spill-over and subsequent presynaptic GABA(A) receptor activation. Presynaptic spill-over currents were depressed by pharmacological cannabinoid 1 (CB1) receptor activation, suggesting that GABA was released predominantly by a CB1 receptor-expressing interneuron subpopulation. Because GABA(A) receptors in axons are considered to act depolarizing, high activity of CB1 receptor-expressing interneurons will exert substantial impact on presynaptic membrane potential, thus modulating action potential-evoked transmitter release at the mossy fiber-CA3 synapse.

Combined analog and action potential coding in hippocampal mossy fibers.

In the mammalian cortex, it is generally assumed that the output information of neurons is encoded in the number and the timing of action potentials. Here, we show, by using direct patchclamp recordings from presynaptic hippocampal mossy fiber boutons, that axons transmit analog signals in addition to action potentials. Excitatory presynaptic potentials result from subthreshold dendritic synaptic inputs, which propagate several hundreds of micrometers along the axon and modulate action potential-evoked transmitter release at the mossy fiber-CA3 synapse. This combined analog and action potential coding represents an additional mechanism for information transmission in a major hippocampal pathway.

Cortico-motoneuronal excitation of three hand muscles determined by a novel penta-stimulation technique.

The cortico-motoneuronal system (CMS), i.e. the monosynaptic projection from primary motor cortex to motoneurons in lamina IX of the spinal cord is, among all mammals, best developed in humans. Increasing evidence suggests that the CMS is crucially important for skilled individuated finger movements. Little is known about to what extent the strength of the CMS differs between hand muscles. Here we measured CMS excitation to the first dorsal interosseus (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles in healthy subjects by using a novel penta-stimulation technique (PST) and single motor unit (SMU) recordings. The PST is an extension of the triple-stimulation technique. It applies two additional supramaximal electrical stimuli at the wrist to the 'peripheral nerve of no interest' (in the case of the FDI and ADM the median nerve, in the case of the APB the ulnar nerve) to collide with the descending volleys in that nerve elicited by transcranial magnetic stimulation of motor cortex and electrical stimulation of Erb's point. This eliminates volume conduction from neighbouring muscles innervated by the nerve of no interest and, therefore, allows accurate determination of the PST response. The PST response was significantly larger in the FDI compared with the ADM and APB. This was validated by the SMU recordings, which showed a higher estimated amplitude of the mean compound excitatory postsynaptic potential in spinal motoneurons of the FDI than in those of the APB and ADM. Finally, as a possible functional correlate, the maximum rate of repetitive voluntary finger movements was higher for index finger abduction (prime mover, FDI) than for little finger abduction (prime mover, ADM) and thumb abduction (prime mover, APB), and individual differences in maximum rate between the different movements correlated with individual differences in the corresponding PST responses. In conclusion, PST is a valuable novel method for accurate quantification of CMS excitation. The findings strongly suggest that CMS excitation differs between hand muscles and that these differences directly link to capability differences in individuated finger movements.

Estimated magnitude and interactions of cortico-motoneuronal and Ia afferent input to spinal motoneurones of the human hand.

Magnitude and interactions of cortico-motoneuronal (CM) and Ia afferent input to spinal alpha-motoneurones (MNs) of the human hand are largely unknown. This is, however, an important question, which bears on the cortical versus peripheral-segmental 'interest' in controlling alpha-MN excitation. Alpha-MN excitation can be quantified by estimating the amplitude of alpha-MN compound excitatory post-synaptic potentials (cEPSPs) from single motor unit (SMU) recordings, if certain assumptions about the membrane trajectory are made [Exp. Brain Res. 47 (1982) 33]. Here we recorded 29 SMUs from three different hand muscles (FDI, first dorsal interosseous; ADM, abductor digiti minimi; APB, abductor pollicis brevis) of healthy subjects. Each SMU was tested for CM input by transcranial magnetic stimulation of the contralateral motor cortex, for Ia afferent input by electrical peripheral nerve stimulation, and for the interaction between inputs by paired stimulation timed to arrive coincidently at the alpha-MN. Mean cEPSP amplitude elicited by CM input was larger in the alpha-MNs of the FDI than in those of the ADM or APB, whereas mean cEPSP amplitude elicited by Ia input was larger in the alpha-MNs of the APB than in those of the FDI. Generally, cEPSP amplitude evoked by paired input closely matched the arithmetic sum of the cEPSP amplitudes evoked by the single inputs. In conclusion, alpha-MNs of the human hand can be viewed as linear integrators of CM and Ia excitatory inputs. The weights of these inputs may relate to the specific functions of the different intrinsic hand muscles in skilled finger movements.