Astrocyte

combra_loihi supports an astrocyte module simulating the functions of tripartite synapses. We can easily convert an Spiking Neural Network into a Spiking Neural-Astrocytic Network by using this module.

import combra_loihi.api as combra
import nxsdk.api.n2a as nx

net = nx.NxNet()

# Define Spiking Neural Network on Loihi
...

# Add Astrocyte module onto the network
astro = combra.Astrocyte(net, other parameters)
astro.connectInputNeurons(presyn_compartments, number, connectionMask, weight)
astro.connectOutputNeurons(postsyn_compartments, number, connectionMask, weight)
probe_list = astro.probe([combra.ASTRO_SPIKE_RECEIVER_PROBE.COMPARTMENT_VOLTAGE,
                          combra.ASTRO_IP3_INTEGRATOR_PROBE.COMPARTMENT_VOLTAGE,
                          combra.ASTRO_SIC_GENERATOR_PROBE.COMPARTMENT_VOLTAGE,
                          combra.ASTRO_SPIKE_GENERATOR_PROBE.SPIKE,
                          combra.ASTRO_IP3_INTEGRATOR_PROBE.SPIKE])

# Run network
net.run(simulation time)
net.disconnect()

More information about Astrocyte module api and parameters can be found here.

Astrocyte and Tripartite Synapse

Tripartite synapse is a key element of information processing in astrocytic neuromodulation. Astrocytes in tripartite synapses modulate synaptic activity by sensing neurotransmitters' spillover from the presynaptic neuron and translating it to gliotransmitter release.

1. Presynaptic neurotransmitters affect astrocytic receptors and cause the production of an intracellular astrocytic second messenger molecule, Inositol Triphosphate (IP₃).

2. IP₃ concentration reaches a threshold and generates an intracellular Ca²⁺ wave.

3. Ca²⁺ wave triggers the release of gliotransmitters, which in turn induce a Slow Inward Current (I_SIC) affecting the postsynaptic neuron.

By controlling multiple tripartite synapses, astrocytes provide a spatially and temporally alternative neuromodulatory system supplementing the well-studied neural network.

Astrocyte module in Loihi

Our astrocyte module in Loihi simulates the behavior and function of tripartite synapses. This astrocyte module uses 4 Loihi compartments with two major components – (1) input integration, which simulates astrocytic receptors and IP3 production, and (2) output generation, which simulates ISIC projecting to postsynaptic neurons.

1. Input Spike Receiver (SR) integrates synaptic activity into IP3 integrator (IP3).

2. IP3 compartment spikes when its voltage threshold is reached.

3. Whenever IP3 spikes, SIC generator (SIC) produces a voltage signal similar to the observed biological I_SIC.

4. Burst Spike Generator (SG) receive SIC voltage from dendritic tree, transforms the voltage signal into bursting spikes and sends them to postsynaptic neurons.