NIDA-supported researchers Dr. Cheryl Kirstein and Ms. Kirstie Stansfield at the University of South Florida have found that higher scores on tests of impulsivity and some behavioral responses to novelty correlate with a heightened biological response to cocaine in adolescent, but not adult, rats. The findings accord well with scientists' widely shared view that developmental differences in brain systems that use the neurotransmitter dopamine underlie age differences in susceptibility to drug abuse.
Dr. Kirstein and Ms. Stansfield conducted a series of behavioral assays to rate rats' relative responsiveness to novelty, then compared these results with measures of dopamine release in the reward pathway after an injection of cocaine. First, they put adolescent rats (34 days old, which is roughly equivalent to adolescence in people) and fully mature rats (59 days old, equivalent to human young adulthood) through four behavioral protocols. The tests measured activity in a new environment (how much the rat moved around when put into a new cage); impulsivity (how quickly it approached a new object placed into its cage); exploratory drive in response to a new object (how many times it approached the object in a given period of time); and attraction to new objects (what percentage of a given time interval was spent close to the object).
The researchers then injected the animals with saline and then, 2 hours later, with cocaine 20 mg/kg. Every 10 minutes, starting immediately after the saline injection and continuing until 2 hours after administering the cocaine, they measured the concentrations of the neurotransmitter dopamine and its major metabolite in the rats' nucleus accumbens (NAc). The measurements were made using the technique of in vivo microdialysis. By the time of the last measurement, the drug had cleared the animal's system.
On Most Tests, Age Matters
In their analysis, the researchers compared cocaine-induced dopamine release in animals that had responded above the mean level on each test (high responders, HR) to those who had scored below the mean (low responders, LR). The results revealed that among both the adult and adolescent rats, those that exhibited greater activity in a new environment also demonstrated enhanced dopamine release following a cocaine injection. This was the only test, however, in which age did not influence cocaine-induced dopamine release. The other behavioral assays revealed interactions between age and the response to novelty on cocaine-induced dopamine release in the NAc:
- Impulsivity—Adolescent rats with above-the-mean impulsivity scores released more dopamine in response to cocaine than their age mates who were LR. Mature rats exhibited no clear relationship between impulsivity and cocaine-induced dopamine response.
- Exploration of a new object—Adolescent rats with above-the-mean scores on this measure released more dopamine in response to cocaine than their age mates who were LR. Adult rats showed the opposite pattern: Animals with above-the-mean scores showed attenuated cocaine-induced dopamine release compared with age mates who were LR.
- Attraction to a new object—Adolescent rats exhibited no clear relationship between reactivity on this assay and cocaine-induced dopamine release. Mature rats with above-the-mean scores released less dopamine in response to cocaine compared with their age mates who were LR.
Dr. Kirstein's finding that for all the animals, greater activity in a new environment corresponded with increased sensitivity to stimulants is consistent with earlier research. Her team's mixed findings on the impulsivity and other novelty response tests indicates, she says, that those behaviors arise from different physiological mechanisms than does locomotor activity. "My colleagues and I think locomotor activity may reflect primarily dopamine activity in a brain circuit involved with generating and controlling movement. Novelty may instead differentially stimulate mesolimbic dopamine—a pathway implicated in attention as well as reward and motivation," says Dr. Kirstein.
In Vivo Microdialysis
The investigators used In Vivo microdialysis to measure dopamine each animal released from its nucleus accumbens (NAc) in response to cocaine. They implanted a probe into the shell area of the NAc. The probe is a fine tube, about the size of a sewing needle, connected to a mini-pump that continuously perfuses it with artificial cerebrospinal fluid. The membrane tip of the probe captures dopamine and its metabolites. The samples collected by the needle are then analyzed using techniques, such as chromatography, that are able to isolate dopamine and its metabolites from other molecules.
Inhibition Develops Later
The findings on the three tests where age affected the relationship between behavior and cocaine-induced dopamine release may reflect maturation of the brain's reward circuit. When rats are adolescents, dopamine-producing and releasing cells in this circuit may be particularly sensitive both to novelty and to pharmacological stimulation. As part of normal neurological development, areas of the brain that dampen the activity of this circuit come "online" later, explaining the age-related differences observed in Dr. Kirstein's study. "The mesolimbic pathway and the cortical areas that inhibit it to regulate dopamine release are not yet fully matured in the adolescent, and this may explain why the adolescent brain responds to drugs differently than the adult brain," says Dr. Kirstein.
"The results of Dr. Kirstein's study, along with other animal research on the interaction of drugs and developmental stage, indicate that the adolescent brain is more responsive to drugs than the adult brain—both neurochemically and behaviorally," says Dr. Nancy Pilotte of NIDA's Division of Basic Neuroscience and Behavioral Research. Studies that identify the physiological and behavioral processes underlying age-related susceptibility to addiction complement epidemiological work on the individual and social factors contributing to adolescent vulnerability to substance abuse.
Stansfield, K.H., and Kirstein, C.L. Neurochemical effects of cocaine in adolescence compared to adulthood. Developmental Brain Research 159(2):119-125, 2005. [Abstract]