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The Francesconi Lab

Research

Altered neural function associated with the acute and chronic effects of drugs of abuse and, alcohol have been the subject of intense investigation and many cellular and molecular mechanisms have been identified. Several electrophysiological studies showed profound effects of drugs of abuse and, alcohol on both Gabaergic and Glutamatergic synaptic transmission. More recently, modification on intrinsic voltage-dependent membrane conductances are beginning to be also recognized using acute brain slices.

In most brain areas including the motivational and reward systems, the primary function of neurons is to produce action potentials (spikes) that they use to communicate with neurons in other areas of the brain. One important and often overlooked aspect of cellular and circuit dynamics is that neurons receive a continuous and irregular synaptic bombardment in the intact brain, unlike in acute brain slices where they are most commonly investigated. Signal integration in the brain is usually associated with high conductance states of neurons, i.e., when they receive a continuous bombardment from presynaptic neurons. In such conditions, spike responses and integrative properties of the postsynaptic neurons differ markedly from those observed in low-conductance states, such as when pharmacologically isolated for in vitro experiments. Indeed, neuronal information processing strongly depends on the conductance state of the interacting neurons. To obtain a better understanding of temporal integrative properties of neurons, we need a type of stimulation that better approximates the natural conditions the neurons experience (in vivo-like conditions) in the functioning circuit. A technique referred to as dynamic clamp is of great value in performing such experiments. The dynamic clamp, sometimes referred to as conductance injection is a form of intracellular stimulation where a simulated synaptic and/or voltage-dependent intrinsic conductance is being injected in real-time while the voltage output of the neuron is continuously monitored by computer. Importantly, the high level of background synaptic input can significantly alter the integrative properties of those neurons with consequences for how neurons process and transmit information. Indeed, changes in the excitability of neurons, such as those under the influence of drugs of abuse or alcohol can alter their firing patterns subsequently resulting in changes in their cooperative dynamics and synchronization. The complex interplay between synaptic and intrinsic biophysical properties results in sophisticated features of neuronal information processing, such as various types of filtering, resonances, and even pattern-selectivity in the signal transduction.

The general dynamic clamp setup using Stdp

Diagram of the dynamic clamp setup using Stdp

The dynamic clamp system forms a closed observation-stimulus loop in which the measured and amplified membrane potential (Vout) is input to the StdpC software (StdpC workstation), which calculates a corresponding trans-membrane current according to a model specified by the user. The software then issues an appropriate current command, which is converted into a physical current injection by the amplifier. A second, independent, analog to digital converter and PC with standard electrophysiology recording software monitors and saves all aspects of the experiment (Electrophysiology workstation). The measurement-injection loop is repeated at 10-20 kHz making the interaction essentially instantaneous for the target neuron (From: Dynamic clamp with StdpC software. Kemenes I et al., 2011 Nat Protoc. 6(3):405-17).

The dynamic clamp system data

jcBNST neurons fire reliably and with high precision when receiving conductance inputs via dynamic clamp. The green trace under the voltage response is the current (Im) that is generated by the dynamic clamp system. The synaptic activation functions for the AMPA-, NMDA- and GABA-type inputs are displayed below (SAMPA, SNMDA and SGABA). A brief section (gray bar) of the voltage response and the input waveforms is zoomed and shown in the right (B). Separate excitatory and inhibitory (Exc and Inh, bottom traces) voltage waveforms are used to elicit barrages of EPSCs and IPSCc in the neuron. A selected 'spike' in the excitatory waveform, the corresponding AMPA- and NMDA-type conductance transients as well as the injected EPSC are indicated by red triangles. Blue triangles indicate the same for the inhibitory input. The peri-stimulus scatter plot (C) demonstrates reliable and precise reproduction of spikes under the action of such input (From: Excitability of jcBNST neurons is reduced in alcohol-dependent animals during protracted alcohol withdrawal. Szücs A et al., 2012)