The effects of the general anesthetic sevoflurane on neurotransmission: an experimental and computational study

The brain functions can be reversibly modulated by the action of general anesthetics. Despite a wide number of pharmacological studies, an extensive analysis of the cellular determinants of anesthesia at the microcircuits level is still missing. Here, by combining patch-clamp recordings and mathematical modeling, we examined the impact of sevoflurane, a general anesthetic widely employed in the clinical practice, on neuronal communication. The cerebellar microcircuit was used as a benchmark to analyze the action mechanisms of sevoflurane while a biologically realistic mathematical model was employed to explore at fine grain the molecular targets of anesthetic analyzing its impact on neuronal activity. The sevoflurane altered neurotransmission by strongly increasing GABAergic inhibition while decreasing glutamatergic NMDA activity. These changes caused a notable reduction of spike discharge in cerebellar granule cells (GrCs) following repetitive activation by excitatory mossy fibers (mfs). Unexpectedly, sevoflurane altered GrCs intrinsic excitability promoting action potential generation. Computational modelling revealed that this effect was triggered by an acceleration of persistent sodium current kinetics and by an increase in voltage dependent potassium current conductance. The overall effect was a reduced variability of GrCs responses elicited by mfs supporting the idea that sevoflurane shapes neuronal communication without silencing neural circuits

The effect of anesthesia on neuronal communication

One challenging aspect in the analysis of neuronal circuits is the lack of quantitative and objective measurements of network activity to be translated into functional states. For example, the clinical assessment of the consciousness state in a brain-injured, unresponsive patient can be hardly analyzed at the cellular and network level. General anesthesia employs different classes of molecules to modulate at various levels neuronal functional states. General anesthetics (GA) are known to progressively and selectively reduce consciousness, perception and motor control. In this work we have investigated in a simplified neuronal circuit the effect of GA on information transfer. The Shannon mutual information (MI) was used to evaluate how much the neuron response reflected the input stimuli versus its intrinsic variability, providing a statistical tool to dissect the contribution of spike timing to neural information transmission. The cerebellum granule cell (GrC), due to its limited number of excitatory inputs, can be used to calculate the Mutual Information (MI) and its variation during a perturbed state (e.g. under anesthesia). The MI was experimentally assessed by detecting action potentials elicited in response to specific inputs through whole-cell patch-clamp recordings in rat acute cerebellar slices (P18-24). In order to test the action of the application of GA, GABAergic currents elicited by inhibitory afferent connections were recorded. The action of GA (in particular Sevoflurane and Desflurane) increased (+120%) post-synaptic inhibitory currents (IPSCs) in less than 10 sec and was fully recovered in 30 sec. Furthermore, the action of GA was to markedly reduce the MI measured in control condition (-57.4%). This control condition was fully recovered after removal the anesthetics, therefore leaving unaltered neuronal activity. This approach will be applied to larger circuits and investigated with other techniques (e.g. Multielectrode array recordings or cellular imaging), moreover different concentration of anesthetics could lead to the identification of multiple functional states

COMPLEX DYNAMICS IN THE GRANULAR LAYER NETWORK OF THE CEREBELLUM: EXPERIMENTAL RESULTS AND COMPUTATIONAL RECONSTRUCTIONS

Aim: The cerebellum is classically depicted as a well defined circuit, whose basic function can be understood by considering the anatomical organization and the sign of neuronal interactions. However, local connectivity and neuronal and synaptic properties suggest the emergence of complex spatio-temporal dynamics. Here, experimental measurements are combined with computational models to investigate network interactions.Methods: Patch-clamp whole-cell recordings from granule and Golgi cells have been used to develop detailed models of the neurons and synapses. MEA and voltage-sensitive dye recordings in acute cerebellar slices were used to investigate the spatio-temporal structure of responses. Finally, we have developed a large-scale computational model of the cerebellar granular layer network (NEURON) and evaluated its ability to reproduce the spatio-temporal patterns of activity recorded in vitro and in vivo.Results: The granular layer network showed a series of remarkable properties in response to mossy fiber stimulation, that could be faithfully reproduced by the model:Granule cells emitted spikes for a limited time period (< 5 ms) before inhibition through the Golgi cell loop interrupted the output (time-window).Granule cells optimally transmitted spikes when the input frequency was higher than 100 Hz (high-pass filtering).Sustained and diffused mossy fiber activity generated oscillations through the reverberant Golgi cell loops (oscillation and resonance).Activity was organized with maximum excitation in the core and inhibition in the periphery of activated areas (center-surround).Conclusions: These results suggest that the granular layer can perform complex transformations on incoming signals behaving as an adaptable spatio-temporal filter. LTP and LTD at the mossy fiber – granule cells relay, by regulating the release machinery and postsynaptic responsiveness, could contribute to organize time-window, oscillation, filtering, and center-surround properties

The cerebellar network from structure to function and dynamics

Since the discoveries of Camillo Golgi and Ramon y Cajal, the precise cellular organization of the cerebellum has inspired major computational theories, which have then influenced the scientific thought not only on the cerebellar function but also on the brain as a whole. However, six major issues revealing a discrepancy between morphologically inspired hypothesis and function have emerged. (1) The cerebellar granular layer does not simply operate a simple combinatorial decorrelation of the inputs but performs more complex non-linear spatio-temporal transformations and is endowed with synaptic plasticity. (2) Transmission along the ascending axon and parallel fibers does not lead to beam formation but rather to vertical columns of activation. (3) The olivo-cerebellar loop could perform complex timing operations rather than error detection and teaching. (4) Purkinje cell firing dynamics are much more complex than for a linear integrator and include pacemaking, burst-pause discharges, and bistable states in response to mossy and climbing fiber synaptic inputs. (5) Long-term synaptic plasticity is far more complex than traditional parallel fiber LTD and involves also other cerebellar synapses. (6) Oscillation and resonance could set up coherent cycles of activity designing a functional geometry that goes far beyond pre-wired anatomical circuits. These observations clearly show that structure is not sufficient to explain function and that a precise knowledge on dynamics is critical to understand how the cerebellar circuit operate

The cerebellar network: from structure to function and dynamics

Since the discoveries of Camillo Golgi and Ramón y Cajal, the precise cellular organization of the cerebellum has inspired major computational theories, which have then influenced the scientific thought not only on the cerebellar function but also on the brain as a whole. However, six major issues revealing a discrepancy between morphologically inspired hypothesis and function have emerged. (1) The cerebellar granular layer does not simply operate a simple combinatorial decorrelation of the inputs but performs more complex non-linear spatio-temporal transformations and is endowed with synaptic plasticity. (2) Transmission along the ascending axon and parallel fibers does not lead to beam formation but rather to vertical columns of activation. (3) The olivo-cerebellar loop could perform complex timing operations rather than error detection and teaching. (4) Purkinje cell firing dynamics are much more complex than for a linear integrator and include pacemaking, burst-pause discharges, and bistable states in response to mossy and climbing fiber synaptic inputs. (5) Long-term synaptic plasticity is far more complex than traditional parallel fiber LTD and involves also other cerebellar synapses. (6) Oscillation and resonance could set up coherent cycles of activity designing a functional geometry that goes far beyond pre-wired anatomical circuits. These observations clearly show that structure is not sufficient to explain function and that a precise knowledge on dynamics is critical to understand how the cerebellar circuit operate

HDAC6 inhibition extinguishes autophagy in cancer: Recent insights

Autophagy is an essential intracellular catabolic mechanism involved in the degradation and recycling of damaged organelles regulating cellular homeostasis and energy metabolism. Its activation enhances cellular tolerance to various stresses and is known to be involved in drug resistance. In cancer, autophagy has a dual role in either promoting or blocking tumorigenesis, and recent studies indicate that epigenetic regulation is involved in its mechanism of action in this context. Specifically, the ubiquitin-binding histone deacetylase (HDAC) enzyme HDAC6 is known to be an important player in modulating autophagy. Epigenetic modulators, such as HDAC inhibitors, mediate this process in different ways and are already undergoing clinical trials. In this review, we describe current knowledge on the role of epigenetic modifications, particularly HDAC-mediated modifications, in controlling autophagy in cancer. We focus on the controversy surrounding their ability to promote or block tumor progression and explore the impact of HDAC6 inhibitors on autophagy modulation in cancer. In light of the fact that targeted drug therapy for cancer patients is attracting ever increasing interest within the research community and in society at large, we discuss the possibility of using HDAC6 inhibitors as adjuvants and/or in combination with conventional treatments to overcome autophagy-related mechanisms of resistance