Psychostimulant Response

Addiction represents a complex behavior characterized by uncontrolled intake of a drug despite negative consequences. This behavior involves many aspects including reinforcement, sensitization, tolerance, dependence, withdrawal and ultimately relapse. Sensitization to a drug is defined as enhanced response following repeated administration of the same dose the drug. Psycholocomotor sensitization is a relatively straight forward measurement that is amenable to high throughput screening and even drugs that are traditionally defined as sedatives such as alcohol show this type of sensitization.

Cocaine is a well characterized psychostimulant with a high addictive liability. It blocks the reuptake of dopamine in the nucleus accumbens contributing to its rewarding properties. Sensitization is associated with increasing levels of dopamine in the nucleus accumbens following drug administration however any imbalance in dopamine levels in the nucleus accumbens resulting from perturbed degradation, reuptake and synthesis of the neurotransmission can play potential role in addiction. In addition any perturbation in dopamine signaling mechanism in the presynaptic or postsynaptic cell such as kinases, adapter proteins, and transcription factors can also lead to altered response to the cocaine.

We are conducting a screen to identify mutations that affect psychomotor response to cocaine. Mice, 8-10 weeks old, are tested for acute psychomotor response to cocaine (20mg/kg) on week one (Figure 1 top, green line). One week following this acute response test mice are retested for a sensitized response to cocaine (Figure 1 top, red line). The overall distribution of 30 minute response following injection displays a normal distribution (Figure 1 bottom). Activity is recorded in a 3x3 matrix using BigBrother software from Actimetrics (Figure 2). Mutants that are of interest to us are those that have high acute response (Figure 3), normal first response but high sensitized response (Figure 4), and those that have low or no response on both tests (Figure 5). Since our screen makes few assumptions about the nature of the mutation we can also expect to find mutations that perturb known pathways as well as novel pathways.

Figure 1: Average response. TOP: Average response over time. -60 to 0 minutes consists of pre injection baseline period and 1 to 60 minutes consists of post injection activity after injection. The first acute response to cocaine (green line) and the second sensitized response (red line) are shown. BOTTOM: Distribution of average response after injection. Return to text.

Figure 2: Procedure for measuring the activity. Top: Mice are placed in a 3x3 matrix with a camera directly overhead. Each box in the matrix is approximately 10 inch in length and width. Data is recorded with video based tracking software (Big Brother, Actimetrics, Inc.). Bottom: Sample output from Big Brother. The trace consists on one mouse's activity during baseline (Green period) and injection (Red Period). We can measure up to 51 animals in a single experiment. Return to text.

Figure 3: Example of a high acute response outlier. Both animals are from the same G1 founder (see breeding scheme) and have high +3 and +5SD acute response. Return to text.

Figure 4: Example of a sensitization outlier. The first response (red line) is normal but the second response (green line) is high. Return to text.

Figure 5: No response outlier. This animal failed to respond to cocaine on either the first (Blue) or the second (green) test. Return to text.