"When we started to play the tone that had been paired with cocaine, the animals began to press the lever at a fairly high rate," said West. "It indicated that the animals had a persistent memory they remembered the significance of the tone. We interpreted the resumption of lever pressing as a behavioral relapse." During this relapse of drug seeking, brain activity recordings showed that accumbens shell neurons responded almost instantaneously when the tone was sounded. In contrast, accumbens neurons had not responded to the tone before conditioning.
Eventually the animals gave up pressing the lever, even in the presence of the tone, since no cocaine was forthcoming. "Even though the lever pressing behavior was extinguished, we were still seeing accumbens neuron activity in response to the tone," said West.
West explained that the brain is slow to forget the stimuli. After a relatively long period of drug abstinence, the persistence of the memories those conditioned associations reflected in the neural activity may partially explain why treating drug addiction is an uphill battle, West observed.
"The neural mechanisms of learning are still not understood. This is what we were investigating here neural mechanisms in one part of a brain circuit that participate in creating memories of these environmental stimuli," said West. "As medical science seeks to develop chemicals that alleviate drug craving, our data may help scientists or clinicians know what part of the brain to target. With cocaine, we haven't yet discovered a magic bullet that can go in and just cure the problem."