Drugs of abuse, such as cocaine, produce long-lasting synaptic adaptations that increase the compulsive nature of addiction, undermine self-control, and increase the likelihood of relapse. Identifying and understanding the molecules that regulate these synaptic changes may suggest novel therapies. Recently, we found that acid-sensing ion channels (ASICs) and brain pH play critical roles in the synaptic plasticity thought to underlie addiction. Our findings suggest that ASIC1a is activated during synaptic transmission in medium-spiny neurons (MSNs) of the nucleus accumbens (NAc), a site firmly implicated in addiction-related behavior. Genetically deleting ASIC1a in mice led to a number of synaptic changes paralleling those previously observed following cocaine withdrawal. Consistent with these synaptic effects, disrupting ASIC1a in mice throughout the body or specifically in the NAc increased conditioned place preference (CPP) to cocaine and to morphine, indicating important behavioral consequences that generalize to multiple drugs of abuse. Confirming the NAc as a key site of ASIC1a action in cocaine-dependent behavior, restoring ASIC1a expression to the NAc of ASIC1a-/- mice reversed the synaptic abnormalities and normalized cocaine CPP. We also tested synaptic and behavioral effects of ASIC1a in rats and found results similar to those in mice. In rats, overexpressing ASIC1a in the NAc doubled the ASIC-mediated synaptic current, and significantly reduced cocaine self- administration. Together, these observations indicate that ASIC1a inhibits addiction-related behavior. Furthermore, these results suggest the hypothesis that ASIC1a and brain pH might be targeted to reduce the synaptic changes underlying addiction and relapse. To test this hypothesis, we propose to explore genetic and pharmacological approaches to increase ASIC1a function at synapses and to determine their ability to affect cocaine-related synaptic physiology and behavior in mice and rats. The planned studies capitalize on novel insight into the roles of ASICs and pH in synaptic transmission, and take advantage of state-of-the-art electrophysiological approaches and an innovative collaboration between principal investigators with extensive experience in ASICs, brain pH, and drug-related behavior. Our planned behavioral analyses include models of craving/relapse using long-access cocaine self-administration in rats, widely considered one of the best models of addiction because animals control their own drug intake, thus facilitating assessment of various stages of drug-seeking behavior. Because ASIC1a structure and function in rodents are nearly identical to those in humans, these studies will be highly relevant to the human brain. Moreover, the knowledge gained through these experiments will inform innovative strategies to interrupt addictive behaviors by targeting ASICs and/or brain pH.
Drugs of abuse produce long-lasting changes in synaptic physiology that increase drug-seeking behavior. Understanding and correcting these molecular abnormalities may be critical for interrupting addiction and relapse. Our recent studies suggest a novel mode of synaptic transmission involving synaptic pH and acid-sensitive receptors, and suggest that these receptors oppose the synaptic alterations caused by drugs of abuse. The goal of this work is to determine whether acid-activated receptors might be manipulated to reverse or normalize the detrimental synaptic effects of repeated cocaine exposure. In so doing, this work will lead to a new avenue of therapeutic possibilities for addiction.
