Pain induced synaptic plasticity in the amygdala

Abstract

Pain is an important defense against dangers in our environment, however some clinical conditions produce pain that outlasts this useful role and persist even after the injury has healed. The experience of pain consists of somatosensory elements of intensity and location, negative emotional/aversive feelings and subsequent restrictions on lifestyle due to learning to associate certain activities with pain. The amygdala contributes negative emotional value to nociceptive sensory information and forms the association between an aversive response and the environment in which it occurs. It can form this association because it receives nociceptive information via the spino-parabrachio-amygdaloid pathway and polymodal sensory information via its basolateral nucleus (BLA) and cortical and thalamic inputs. Within the spino-parabrachio-amygdaloid pathway, nociceptive information is sent from the external lateral nucleus of the parabrachial nucleus (PB) to the laterocapsular region of the central nucleus of the amygdala (CeLC). The PB-CeLC synapse and other brain regions undergo synaptic plasticity in chronic pain conditions with ongoing injury. However very little is known about how plasticity occurs in conditions where pain persists even after the injury has healed. In the first study of this thesis, I used immunohistochemistry, electrophysiology and behavioural assays, to show that a brief nociceptive stimulus with no ongoing injury can produce long-lasting synaptic plasticity at the rat parabrachial-amygdala synapse. I show that this plasticity is caused by an increase in number of postsynaptic AMPARs with a transient change in AMPAR subunit, similar to long-term potentiation. Furthermore, repeated stimuli lengthened this plasticity. The potentiation could be representative of the initial changes that occur in the transition from an acute to a chronic pain state. This could mean greater association of a painful experience with the environment and context and could ultimately facilitate the negative association of certain activities and situations with pain, leading to limiting or avoidance of these activities/situations. The next studies of this thesis focused on potential neuromodulators of activity at the PB-CeLC synapse and the polymodal BLA inputs to the CeLC. Opioids and Calcitonin gene-related peptide (CGRP) were chosen because of their presence at synapses in the amygdala and their role in pain particularly in the affective component of pain. Opioids reduce pain intensity and the emotional unpleasantness of pain. Opioids inhibit some synapses in the amygdala, however whether opioids specifically modulate the PB-CeLC and the BLA-CeLC is unknown. I used electrophysiology and optogenetics to show that opioids inhibit two synapses important for pain modulation in the amygdala. Given the evidence of the opioid’s role in reducing pain affect, modulation of these synapses could be, in part, the site of opioid action. CGRP is expressed at all levels of the spino-parabrachio-amygdala pathway and modulates pain, as CGRP receptor antagonists injected into the amygdala inhibit nocifensive behaviours in animals. Additionally, CGRP antagonists reverse arthritis-induced synaptic plasticity at the PB-CeLC synapse. CGRP enhances synaptic transmission in the CeLC, however it is unknown whether it also directly regulates the excitability of CeLC neurons. Using electrophysiology, I show that CGRP directly ‘excites’ CeLC neurons even when fast synaptic transmission is blocked. This suggests that in normal physiology CGRP and opioids have opposing effects in the CeLC and the balance of activity in the CeLC will depend on which peptide has the bigger influence on the CeLC. This thesis addressed the question of whether plasticity can outlast a stimulus and the time course of the plasticity. This plasticity was seen in the amygdala, an area important for associative learning and the affective component of pain. This thesis also addressed how neuropeptides, opioids and CGRP regulate amygdala synapses in normal physiology. Knowledge of how these peptides modulate the amygdala synapses will provide information on how they could operate in a pain state.