Effects of multiple spikes initiation zones on signal integration properties in leech touch cells.

J. KRETZBERG; F. KRETSCHMER; ANTONIA MARIN BURGIN

Edimburgo

Conferencia; Computational Neuroscience Meeting; 2006

Organization for Computational Neuroscience

Combined experimental and modeling study: Experimental data from Burgin & Szczupak (2003): intracellularely measured responses of leech T-cells to combinations of somatic current injections and synaptic input input on the one hand or mechanical touch stimulation on the other hand. A compartmental model of a leech T-cell was simulated in Neuron (reference) based on the paper of Cataldo et al. (2005). The morphology of the neuron is reproduced in detail, including the soma and processes in the same ganglion, dendritic branches to the major receptive field and processes extending to the adjacent ganglia and the minor receptive fields. Electric activity is modeled by passive membrane and Hodgkin Huxley equations.   Main findings and conclusions of Burgin & Szczupak (2003): Finding 1: P-cell input consisting of only few spikes depolarizes the T-cell and triggers spike responses. In contrast, several P-cell spikes hyperpolarize the T-cell. => Hypothesis 1: The synaptic connection between P- and T-cells is polysynaptic, mediated by at least two interneurons, giving fast excitatory and slower inhibitory input Finding 2: P-cell induced hyperpolarization reduces the T-cell spike responses to somatic electrical stimulation. In contrast, T-cell spike responses to mechanical stimulation of the skin are not altered by synaptic input from P-cells => Hypothesis 2: There are at least two spike-initiation zones in T-cells, one to signal peripheral stimulation of the skin, one to process central synaptic stimulation.   To test these two hypotheses, the following steps will be performed in our model study: 1.      Comparison of a T-cell model with homogeneously distributed voltage dependent channels to a T-cell model with distinct spike initiation zones. The results of these two alternatives models will be compared to electrophysiologically optained spike responses to somatic current injection and to peripheral touch stimulation. 2.      Both models will be combined with two synaptic inputs, one excitatory and one inhibitory which are triggered by the same series of spikes presynaptic to them (representing the P-cell input to the interneurons). By varying the parameters of these synaptic inputs (synaptic conductance, time course, location on the T-cell processes) to fit the experimental data, we hope to be able to draw conclusions about properties of the experimentally unidentified interneurons between P- and T-cell. 3.      Combining steps 1 and 2, we will try to reproduce the experimental finding 2, that synaptic input interferes with somatic current injection but not with stimulation of the skin. Our goal is to determine whether a model with two distinct spike initiation zones is better suitable to fit the experimental data than a model with homogeneous channel distribution. If this turns out to be the case, we will try to make predictions about the location of spike initiation zones and synaptic inputs from interneurons relative to each other.       References: Cataldo E, Brunelli M, Bryne JH, Evyatar AR, Cai Y, Baxter DA (2005) Computational model of touch sensory cells (T cells) of the leech: Role of afterhyperpolarization (AHP) in activity-dependent conduction failure. J Computat Neurosci 18: 5-24   Burgin AM, Szczupak L (2003) Network interactions among sensory neurons in the leech. J Comp Physiol A 189:59-67