Spontaneous and evoked electrical activity of neurons in leech Hirudo medicinalis studied by a new generation of voltage sensitive dyes

By using the newly developed voltage sensitive dye VF2.1.Cl invented by Miller and colleagues (Miller et al. 2012), I monitored simultaneously the spontaneous electrical activity of approximately 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 , Leydig 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. I was able to obtain long optical recordings for several minutes. I studied, also, the evoked response of nervous system by stimulating mechanosensory neurons. This work paves the way for further studies of multicellular networks using the new voltage sensitive dye.