Interplay of multiple pathways and activity-dependent rules in STDP

Making time and space for calcium control of neuron activity

Calcium directly controls or indirectly regulates numerous functions that are critical for neuronal network activity. Intracellular calcium concentration is tightly regulated by numerous molecular mechanisms because spatial domains and temporal dynamics (not just peak amplitude) are critical for calcium control of synaptic plasticity and ion channel activation, which in turn determine neuron spiking activity. The computational models investigating calcium control are valuable because experiments achieving high spatial and temporal resolution simultaneously are technically unfeasible. Simulations of calcium nanodomains reveal that specific calcium sources can couple to specific calcium targets, providing a mechanism to determine the direction of synaptic plasticity. Cooperativity of calcium domains opposes specificity, suggesting that the dendritic branch might be the preferred computational unit of the neuron.

Frontoparietal paired associative stimulation versus single-site stimulation for generalized anxiety disorder: a pilot rTMS study

BACKGROUND

At present, the use of repetitive transcranial magnetic stimulation (rTMS) for generalized anxiety disorder (GAD) is limited to single-site interventions. We investigated whether dual-site frontoparietal stimulation delivered using cortical-cortical paired associative stimulation (ccPAS) had stronger clinical efficacy than single-site stimulation in patients with GAD.

METHODS

We randomized 50 patients with GAD to 1 Hz rTMS (10 sessions) using 1 of the following protocols: single-site stimulation over the right dorsolateral prefrontal cortex (dlPFC; 1500 pulses per session); single-site stimulation over the right posterior parietal cortex (PPC; 1500 pulses per session); repetitive dual-site ccPAS (rds-ccPAS) over the right dlPFC and right PPC with 1500 pulses per session (rd-ccPAS-1500); or rds-ccPAS over the right dlPFC and right PPC with 750 pulses per session (rd-ccPAS-750). Both rds-ccPAS treatments used a between-site interval of 100 ms.

RESULTS

Clinical scores for anxiety, depression and insomnia were reduced in all 4 groups after treatment. We found greater improvements in anxiety symptoms in the rds-ccPAS-1500 group compared to the rds-ccPAS-750 and single-site groups. We found greater improvements in depression symptoms and insomnia in the rds-PAS-1500 group compared to the single-site groups. The rds-ccPAS-1500 group also showed significant or trend-level improvements in anxiety symptoms and insomnia at 10-day and 1-month followup. More patients responded to treatment with rds-ccPAS-1500 than with single-site stimulation. The between-group differences in response rates persisted to the 3-month follow-up. Treatment using rds-ccPAS with a between-site interval of 100 ms induced a more significant improvement than the between-site interval of 50 ms we evaluated in a previous study.

LIMITATIONS

These results need to be replicated in a larger sample using sham control and equal-pulse single-site stimulation.

CONCLUSION

Frontoparietal rds-ccPAS may be a better treatment option for GAD.

A data-informed mean-field approach to mapping of cortical parameter landscapes

Constraining the many biological parameters that govern cortical dynamics is computationally and conceptually difficult because of the curse of dimensionality. This paper addresses these challenges by proposing (1) a novel data-informed mean-field (MF) approach to efficiently map the parameter space of network models; and (2) an organizing principle for studying parameter space that enables the extraction biologically meaningful relations from this high-dimensional data. We illustrate these ideas using a large-scale network model of the Macaque primary visual cortex. Of the 10-20 model parameters, we identify 7 that are especially poorly constrained, and use the MF algorithm in (1) to discover the firing rate contours in this 7D parameter cube. Defining a "biologically plausible" region to consist of parameters that exhibit spontaneous Excitatory and Inhibitory firing rates compatible with experimental values, we find that this region is a slightly thickened codimension-1 submanifold. An implication of this finding is that while plausible regimes depend sensitively on parameters, they are also robust and flexible provided one compensates appropriately when parameters are varied. Our organizing principle for conceptualizing parameter dependence is to focus on certain 2D parameter planes that govern lateral inhibition: Intersecting these planes with the biologically plausible region leads to very simple geometric structures which, when suitably scaled, have a universal character independent of where the intersections are taken. In addition to elucidating the geometry of the plausible region, this invariance suggests useful approximate scaling relations. Our study offers, for the first time, a complete characterization of the set of all biologically plausible parameters for a detailed cortical model, which has been out of reach due to the high dimensionality of parameter space.

Tonic GABAA Conductance Favors Spike-Timing-Dependent over Theta-Burst-Induced Long-Term Potentiation in the Hippocampus

Synaptic plasticity is triggered by different patterns of network activity. Here, we investigated how LTP in CA3-CA1 synapses induced by different stimulation patterns is affected by tonic GABA_A conductances in rat hippocampal slices. Spike-timing-dependent LTP was induced by pairing Schaffer collateral stimulation with antidromic stimulation of CA1 pyramidal neurons. Theta-burst-induced LTP was induced by theta-burst stimulation of Schaffer collaterals. We mimicked increased tonic GABA_A conductance by bath application of 30 μm GABA. Surprisingly, tonic GABA_A conductance selectively suppressed theta-burst-induced LTP but not spike-timing-dependent LTP. We combined whole-cell patch-clamp electrophysiology, two-photon Ca²⁺ imaging, glutamate uncaging, and mathematical modeling to dissect the mechanisms underlying these differential effects of tonic GABA_A conductance. We found that Ca²⁺ transients during pairing of an action potential with an EPSP were less sensitive to tonic GABA_A conductance-induced shunting inhibition than Ca²⁺ transients induced by EPSP burst. Our results may explain how different forms of memory are affected by increasing tonic GABA_A conductances under physiological or pathologic conditions, as well as under the influence of substances that target extrasynaptic GABA_A receptors (e.g., neurosteroids, sedatives, antiepileptic drugs, and alcohol).SIGNIFICANCE STATEMENT Brain activity is associated with neuronal firing and synaptic signaling among neurons. Synaptic plasticity represents a mechanism for learning and memory. However, some neurotransmitters that escape the synaptic cleft or are released by astrocytes can target extrasynaptic receptors. Extrasynaptic GABA_A receptors mediate tonic conductances that reduce the excitability of neurons by shunting. This results in the decreased ability for neurons to fire action potentials, but when action potentials are successfully triggered, tonic conductances are unable to reduce them significantly. As such, tonic GABA_A conductances have minimal effects on spike-timing-dependent synaptic plasticity while strongly attenuating the plasticity evoked by EPSP bursts. Our findings shed light on how changes in tonic conductances can selectively affect different forms of learning and memory.

Theories of rhythmogenesis

Rhythmogenesis is the process that develops the capacity for rhythmic activity in a non-rhythmic system. Theoretical works suggested a wide array of possible mechanisms for rhythmogenesis ranging from the regulation of cellular properties to top-down control. Here we discuss theories of rhythmogenesis with an emphasis on spike timing-dependent plasticity. We argue that even though the specifics of different mechanisms vary greatly they all share certain key features. Namely, rhythmogenesis can be described as a flow on the phase diagram leading the system into a rhythmic region and stabilizing it on a specific manifold characterized by the desired rhythmic activity. Functionality is retained despite biological diversity by forcing the system into a specific manifold, but allowing fluctuations within that manifold.