Study links reelin to cocaine-induced brain and behavior changes

Cocaine, a drug of abuse, activates just a portion - 10 to 20 percent - of the neurons in the brain's nucleus accumbens, a critical region linked to motivation and addiction. Though small in numbers, this activated neuronal population strongly controls drug-related behavior through downstream changes in gene expression, nerve synapses, neural circuitry and neural function that lead to behavioral change, including addiction.

In a study published in Science Advances, University of Alabama at Birmingham researchers led by Kasey Brida and Jeremy Day, Ph.D., report that the secreted glycoprotein reelin is a marker for those nucleus accumbens neurons that have been activated by cocaine.

In order to identify whether reelin contributes to drug-related cellular and behavioral changes, the team developed a CRISPR interference strategy to target and reduce the expression of the reelin gene in nucleus accumbens neurons. Knockdown of reelin in rats decreased gene expression associated with activation by cocaine, altered expression of ion channels related to neuronal excitability and impaired excitability of nucleus accumbens neurons. The knockdown also abolished cocaine-induced behavioral changes in movement and place preference, and it dampened cocaine self-administration by the rats.

"Together, these results identify reelin as a stable marker of cocaine-sensitive neurons and reveal a key role for reelin in the transcriptional, electrophysiological and behavioral properties of cocaine-induced striatal plasticity," said Brida, a graduate student in Day's UAB Department of Neurobiology lab. "These findings highlight an opportunity for high-precision manipulation of reward circuitry using reelin-based tools, reveal the necessity of reelin in the cellular response to cocaine and implicate the reelin signaling pathway as a potential therapeutic target for cocaine use disorder."

Prior studies have demonstrated that cocaine's effects in the nucleus accumbens occur at medium spiny neurons, which are the principal neuronal type in this brain region and express receptors for the neurotransmitter dopamine. The researchers identified reelin as a marker of cocaine-activated medium spiny neurons by leveraging unbiased single-nucleus RNA sequencing datasets previously collected after cocaine exposure. More than 80 percent of activated medium spiny neurons in the nucleus accumbens expressed reelin mRNA, with average expression about 10 times greater compared to neurons that were not activated by cocaine. They also showed that reelin is enriched in a subpopulation of medium spiny neurons in both the rat and human brain.

Researchers then used the CRISPR interference knockdown of reelin expression to identify its influences on medium spiny neuron physiology and drug-related behaviors. The CRISPR interference system was carried by lentiviruses injected directly into the nucleus accumbens in the rat brain. The knockdown experiments together suggested that reelin expression allows neurons to be excitable and initiate signal transduction cascades that cocaine uses to shape neuronal function over longer timescales, Brida says.

Reelin has long been known to be crucial for development of mammalian brains during embryogenesis, and it is also known to play a role in synaptic plasticity and function in the adult brain. Reelin also has links to diverse neuropsychiatric disorders. Still, the finding that mRNA for the protein reelin was enriched in medium spiny neurons that were activated by cocaine was unexpected, Brida and Day say.

Co-authors with Brida and Day in the study, "Reelin marks cocaine-activated striatal neurons, promotes neuronal excitability, and regulates cocaine reward," are Emily T. Jorgensen, Robert A. Phillips III, Catherine E. Newman, Jennifer J. Tuscher, Emily K. Morring, Morgan E. Zipperly and Lara Ianov, UAB Department of Neurobiology; and Kelsey D. Montgomery, Madhavi Tippani, Thomas M. Hyde, Kristen R. Maynard and Keri Martinowich, Johns Hopkins Medical Campus, Baltimore, Maryland.