Visceral hypersensitivity is common among patients with irritable bowel syndrome (IBS). The anterior cingulate cortex (ACC) is a brain center which mediates affective responses to pain and gut motor function. Imaging of the human brain indicates abnormal processing of visceral sensory signals by the ACC in IBS patients, however the mechanism is unknown. The current proposal is designed to characterize the electrophysiological properties of ACC neurons, and to explore the synaptic plasticity following the induction of visceral hypersensitivity. We hypothesize that persistence of a heightened tonic visceral afferent nociceptive input to the ACC induces ACC neuronal plasticity characterized by an increase in synaptic transmission. The sensitization of ACC neurons may occur as a result of alteration of activity-dependent plasticity (long-term potentiation, LTP and long-term depression, LTD). This heightened synaptic transmission leads to a reduction in pain threshold and an amplification of affective responses to pain. To test this hypothesis, we plan to use two visceral hypersensitive rat models: colonic anaphylaxis (egg albumin) and colorectal irritation (acetic acid). Electrophysiological recording of single ACC neuronal spike firing in response to colorectal distension will be combined with reversal microdialysis to directly infuse drugs to the dendritic area of neurons to demonstrate the enhancement of synaptic glutamatergic transmission in the ACC. We will record the local field potential and characterize the facilitation of LTP and loss of LTD, a key synaptic mechanism of cortical plasticity, following initiation of visceral hypersensitivity. The mechanisms and intracellular signal events underlying the enhanced ACC neuronal excitability and synaptic plasticity will be explored. Finally, we will characterize the role of ACC in pain-related affective processing and elucidate the cellular mechanisms in the induction of learning and memory in ACC neurons. Understanding the processes that lead to ACC neuronal plasticity and its consequences in pain anticipation that precedes avoidance behavior may prove vital to our understanding of the etiology and treatment of CNS abnormalities associated with visceral hypersensitivity. Relevance to public health: Patients with functional Gl disorders commonly demonstrate visceral hypersensitivity. This study seeks to understand the causes and provide clues for the treatment of this condition.
