A stereo-compound hybrid microscope for combined intracellular and optical recording of invertebrate neural network activity

Photodiode-Based Optical Imaging for Recording Network Dynamics with Single-Neuron Resolution in Non-Transgenic Invertebrates

The development of transgenic invertebrate preparations in which the activity of specifiable sets of neurons can be recorded and manipulated with light represents a revolutionary advance for studies of the neural basis of behavior. However, a downside of this development is its tendency to focus investigators on a very small number of "designer" organisms (e.g., C. elegans and Drosophila), potentially negatively impacting the pursuit of comparative studies across many species, which is needed for identifying general principles of network function. The present article illustrates how optical recording with voltage-sensitive dyes in the brains of non-transgenic gastropod species can be used to rapidly (i.e., within the time course of single experiments) reveal features of the functional organization of their neural networks with single-cell resolution. We outline in detail the dissection, staining, and recording methods used by our laboratory to obtain action potential traces from dozens to ~150 neurons during behaviorally relevant motor programs in the CNS of multiple gastropod species, including one new to neuroscience - the nudibranch Berghia stephanieae. Imaging is performed with absorbance voltage-sensitive dyes and a 464-element photodiode array that samples at 1,600 frames/second, fast enough to capture all action potentials generated by the recorded neurons. Multiple several-minute recordings can be obtained per preparation with little to no signal bleaching or phototoxicity. The raw optical data collected through the methods described can subsequently be analyzed through a variety of illustrated methods. Our optical recording approach can be readily used to probe network activity in a variety of non-transgenic species, making it well suited for comparative studies of how brains generate behavior.

Monitoring Spiking Activity of Many Individual Neurons in Invertebrate Ganglia

Optical recording with fast voltage sensitive dyes makes it possible, in suitable preparations, to simultaneously monitor the action potentials of large numbers of individual neurons. Here we describe methods for doing this, including considerations of different dyes and imaging systems, methods for correlating the optical signals with their source neurons, procedures for getting good signals, and the use of Independent Component Analysis for spike-sorting raw optical data into single neuron traces. These combined tools represent a powerful approach for large-scale recording of neural networks with high temporal and spatial resolution.

Recent developments in VSD imaging of small neuronal networks

Voltage-sensitive dye (VSD) imaging is a powerful technique that can provide, in single experiments, a large-scale view of network activity unobtainable with traditional sharp electrode recording methods. Here we review recent work using VSDs to study small networks and highlight several results from this approach. Topics covered include circuit mapping, network multifunctionality, the network basis of decision making, and the presence of variably participating neurons in networks. Analytical tools being developed and applied to large-scale VSD imaging data sets are discussed, and the future prospects for this exciting field are considered.

The spontaneous electrical activity of neurons in leech ganglia

Using the newly developed voltage-sensitive dye VF2.1.Cl, we monitored simultaneously the spontaneous electrical activity of ∼80 neurons in a leech ganglion, representing around 20% of the entire neuronal population. Neurons imaged on the ventral surface of the ganglion either fired spikes regularly at a rate of 1–5 Hz or fired sparse spikes irregularly. In contrast, neurons imaged on the dorsal surface, fired spikes in bursts involving several neurons. The overall degree of correlated electrical activity among leech neurons was limited in control conditions but increased in the presence of the neuromodulator serotonin. The spontaneous electrical activity in a leech ganglion is segregated in three main groups: neurons comprising Retzius cells, Anterior Pagoda, and Annulus Erector motoneurons firing almost periodically, a group of neurons firing sparsely and randomly, and a group of neurons firing bursts of spikes of varying durations. These three groups interact and influence each other only weakly.

Simultaneous measurement of membrane potential changes in multiple pattern generating neurons using voltage sensitive dye imaging

Optical imaging using voltage-sensitive dyes (VSDs) is a promising technique for the simultaneous activity recording of many individual neurons. While such simultaneous recordings are critical for the understanding of the integral functionality of neural systems, functional interpretations on a single neuron level are difficult without knowledge of the connectivity of the underlying circuit. Central pattern generating circuits, such as the pyloric and gastric mill circuits in the stomatogastric ganglion (STG) of crustaceans, allow such investigations due to their well-known connectivities and have already contributed much to our understanding of general neuronal mechanisms. Here we present for the first time simultaneous optical recordings of the pattern generating neurons in the STG of two crustacean species using bulk loading of the VSD di-4-ANEPPS. We demonstrate the recording of firing activities and synaptic interactions of the circuit neurons as well as inter-circuit interactions in their functional context, i.e. without artificial stimulation. Neurons could be uniquely identified using simple event-triggered averaging. We tested this technique in two different species of crustaceans (lobsters and crabs), since several crustacean species are used for studying motor pattern generation. The signal-to-noise ratio of the optical signal was high enough in both species to derive phase-relationship between the network neurons, as well as action potentials and excitatory and inhibitory postsynaptic potentials. We argue that imaging of neural networks with identifiable neurons with well-known connectivity, like in the STG, is crucial for the understanding of emergence of network functionality.

Single-sweep voltage-sensitive dye imaging of interacting identified neurons

The simultaneous recording of many individual neurons is fundamental to understanding the integral functionality of neural systems. Imaging with voltage-sensitive dyes (VSDs) is a key approach to achieve this goal and a promising technique to supplement electrophysiological recordings. However, the lack of connectivity maps between imaged neurons and the requirement of averaging over repeated trials impede functional interpretations. Here we demonstrate fast, high resolution and single-sweep VSD imaging of identified and synaptically interacting neurons. We show for the first time the optical recording of individual action potentials and mutual inhibitory synaptic input of two key players in the well-characterized pyloric central pattern generator in the crab stomatogastric ganglion (STG). We also demonstrate the presence of individual synaptic potentials from other identified circuit neurons. We argue that imaging of neural networks with identifiable neurons with well-known connectivity, like in the STG, is crucial for the understanding of emergence of network functionality.

Light-induced effects of a fluorescent voltage-sensitive dye on neuronal activity in the crab stomatogastric ganglion

Optical imaging being one of the cutting-edge methods for the investigation of neural activity, it is very important to understand the mechanisms of how dye molecules work and the range of side effects that they may induce. In particular, it is very important to reveal potential toxic effects and effects impairing the functioning of the investigated neural system. Here, we investigate the effects of illumination in the presence of the commonly used di-4-ANEPPS voltage-sensitive dye on the rhythmic motor pattern generated by the pyloric central pattern generator in the crab stomatogastric nervous system, a model system for motor pattern generation. We report that the dye allows long recording sessions with little bleaching and no obvious damage to the pyloric rhythm. Yet, exciting illumination induced a temporary and reversible change in the phase relationship of the pyloric motor neurons and a concomitant speed-up of the rhythm. The effect was specific to the excitation wavelength of di-4-ANEPPS and only obtained when the neuropile and cell bodies were illuminated. Thus, di-4-ANEPPS acts as a photo-switch that causes a quick and reversible change in the phase relationship of the motor neurons, but no permanent impairment of neuronal function. It may thus also be used as a means to study the maintenance of phase relationships in rhythmic motor patterns.

Sensory driven multi-neuronal activity and associative learning monitored in an intact CNS on a multielectrode array

The neuronal network controlling feeding behavior in the CNS of the mollusc Lymnaea stagnalis has been extensively investigated using intracellular microelectrodes. Using microelectrodes however it has not been possible to record from large numbers of neurons simultaneously and therefore little is known about the population coding properties of the feeding network. Neither can the relationships between feeding and neuronal networks controlling other behaviors be easily analyzed with microelectrodes. Here we describe a multielectrode array (MEA) technique for recording action potentials simultaneously from up to 60 electrodes on the intact CNS. The preparation consists of the whole CNS connected by sensory nerves to the chemosensory epithelia of the lip and esophagus. From the buccal ganglia, the region of the CNS containing the feeding central pattern generator (CPG), a rhythmic pattern of activity characteristic of feeding was readily induced either by depolarizing an identified feeding-command neuron (the CV1a) or by perfusing the chemosensory epithelia with sucrose, a gustatory stimulus known to activate feeding. Activity induced by sucrose is not restricted to the buccal ganglia but is distributed widely throughout the CNS, notably in ganglia controlling locomotion, a behavior that must be coordinated with feeding. The MEA also enabled us to record electrophysiological consequences of the associative conditioning of feeding behavior. The results suggest that MEA recording from an intact CNS enables distributed, multiple-source neural activity to be analyzed in the context of biologically relevant behavior, behavioral coordination and behavioral plasticity.

Parameter space analysis suggests multi-site plasticity contributes to motor pattern initiation in Tritonia

This research examines the mechanisms that initiate rhythmic activity in the episodic central pattern generator (CPG) underlying escape swimming in the gastropod mollusk Tritonia diomedea. Activation of the network is triggered by extrinsic excitatory input but also accompanied by intrinsic neuromodulation and the recruitment of additional excitation into the circuit. To examine how these factors influence circuit activation, a detailed simulation of the unmodulated CPG network was constructed from an extensive set of physiological measurements. In this model, extrinsic input alone is insufficient to initiate rhythmic activity, confirming that additional processes are involved in circuit activation. However, incorporating known neuromodulatory and polysynaptic effects into the model still failed to enable rhythmic activity, suggesting that additional circuit features are also required. To delineate the additional activation requirements, a large-scale parameter-space analysis was conducted (~2 x 10(6) configurations). The results suggest that initiation of the swim motor pattern requires substantial reconfiguration at multiple sites within the network, especially to recruit ventral swim interneuron-B (VSI) activity and increase coupling between the dorsal swim interneurons (DSIs) and cerebral neuron 2 (C2) coupling. Within the parameter space examined, we observed a tendency for rhythmic activity to be spontaneous and self-sustaining. This suggests that initiation of episodic rhythmic activity may involve temporarily restructuring a nonrhythmic network into a persistent oscillator. In particular, the time course of neuromodulatory effects may control both activation and termination of rhythmic bursting.