The role of astrocytes in spinal LTP and hyperalgesia

Chronic pain is a major clinical problem. While acute pain serves as a warning of potential or actual threats to the organism, chronic pain has lost its physiological function. One mechanism thought to contribute to the transition of acute to chronic pain is synaptic long-term potentiation (LTP) at synapses located within the superficial spinal cord dorsal horn. LTP is an amplifier of synaptic strength that may be turned on in an activity-dependent manner by noxious stimuli and is thought to underlie some forms of hyperalgesia. Experimentally, LTP can be induced at spinal C-fibre synapses by various stimuli such as electrical conditioning stimulation, natural noxious stimuli (e.g. capsaicin, formalin), or pharmacologically (e.g. opioids, in slice preparations as well as in deeply anaesthetized animals. There is accumulating evidence suggesting an important role for astrocytes in synaptic plasticity in various brain areas. The role if any of spinal astrocytes for the induction and maintenance of LTP is indirect, at best. Spinal astrocytes react with a strong elevation in intracellular Ca2+ to electrical stimulation of primary afferent fibres in C-fibre strength. Upon activation, astrocytes can release various neuroactive substances such as cytokines, chemokines or D-serine, all of which could potentially contribute to the induction of synaptic plasticity and LTP at spinal synapses. The proposed project aims at investigating the role of spinal cord astrocytes in LTP at C-fibre synapses as well as in hyperalgesia. To selectively activate spinal astrocytes, novel engineered G- protein coupled receptors (GPCRs) known as Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) will be used. These DREADDs are cloned downstream of the glial fibrillary acid protein (GFAP) to enable astrocyte specific expression and will be delivered to the rat spinal cord taking advantage of viral tools. Usingstate-of-the-art electrophysiological, immunohistochemical, behavioraland imaging techniques, I will examine (I.) whether selective activation of spinal astrocyte GPCRs affects basal synaptic transmission in the spinal cord of rats; (II.) whether activation of spinal astrocytes is sufficient to induce LTP at spinal C-fibre synapses; and (III.) whether or not this has any effect on behaviour.

Acute pain serves an important physiological function, as it helps to protect our body from harmful stimuli. When pain persists long after the initial cause has disappeared, it might, however, become a disease on its own right. Chronic pain is a major problem in modern society. It is estimated that about 1.5 million people in Austria are suffering from long-lasting or recurrent pain. One reason for the transition from acute to chronic pain is the formation of a so-called pain memory in the central nervous system. The formation of a pain memory can be triggered by a number of different mechanisms, including a long-lasting enhancement of synaptic strength at the contacts between nerve cells (synapses) in the spinal cord, a phenomenon that is referred to as synaptic long-term potentiation (LTP). Recent findings show that nerve cells are not the only players involved in the generation of a pain memory. Noxious stimuli can lead to activation of another cell type present in the central nervous system called astrocytes. Upon activation, these cells can release neuroactive messengers which might play an important role in the generation of memory traces of pain at spinal cord synapses. Pharmacological blockade of these cells has been shown to prevent or reverse some forms of chronic pain in animal experiments. However, very little is known about the role of astrocytes in the induction of LTP at spinal synapses. The study the role of astrocytes in the induction of spinal LTP, novel pharmacosynthetic tools can be used. Recently developed engineered receptors provide an option to control astrocyte activity. These receptors are known as Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), and are activated solely by a pharmacologically inert ligand. In the present project I developed a method to deliver DREADDs to spinal cord astrocytes using viral vectors. I tested the selectivity and functionality of the DREADDs using biochemical as well as imaging techniques. This lays the foundation to test whether selective activation of spinal astrocyte affects basal synaptic transmission in the spinal cord of rats and whether activation of astrocytes is sufficient to induce LTP at spinal synapses.