Virtual Cocaine Lab
BOOK 3: The Experiment and Lab Procedures

Paul Garris: Primary Author

Ch. 1: The Hypothesis & Overview

Introduction

This experiment will investigate how cocaine acts on dopamine neurons in the brain. Cocaine is a drug of abuse that increases extracellular dopamine levels in the nucleus accumbens by blocking the dopamine transporter. The nucleus accumbens links the limbic system (the brain network controlling motivation and emotion) to behavior. Addictive behavior is a kind of motivated behavior. Motivated behavior is divided into two components. The appetitive component consists of those behaviors related to "approaching" the goal (e.g., pressing a bar). The consummatory component represents the actual "consuming" of the goal (e.g., cocaine reaches the brain).

An important question in the neurobiology of cocaine concerns the role of dopamine: If cocaine increases brain dopamine when consumed, does dopamine only play a role during the consummatory phase of cocaine use? If so, is dopamine solely related to the reward or pleasurable aspects of cocaine consumption? Or, similar to other motivated behaviors like sex and intracranial self-stimulation, is dopamine involved in the appetitive phase when the rat is seeking cocaine?

Objective, hypothesis, and prediction

The overall objective of this experiment is to investigate the role of dopamine neurons in cocaine use. Three important questions will be asked:

(1)   What happens to brain dopamine levels when cocaine is used?

(2)   When do changes in brain dopamine levels occur when cocaine is used?

(3)   How do changes in brain dopamine levels affect behavior?

With most research, it is often best to focus on one or two questions at a time by simplifying the experiment. Therefore, in this experiment, we shall investigate what cocaine does to dopamine neurons during the appetitive phase of motivated behavior and what this action of dopamine neurons means for behavior. Thus, we will be examining how cocaine affects the behaviors of the animal directed towards obtaining cocaine. These behaviors have collectively been called 'cocaine seeking'. Since we shall be monitoring cocaine levels continuously, we should be able to observe what happens to dopamine levels when cocaine reaches the brain.

The hypothesis to be tested by our experiment is the following:

 Hypothesis: Dopamine is involved in cocaine seeking.

To test our hypothesis, we will make a prediction regarding the outcome of our experiments. If the outcome of the experiment agrees with our prediction, then our hypothesis is supported. The prediction we shall make is this:

Prediction: Dopamine release will increase during cocaine seeking.

If our prediction is verified, then there are two other questions our experiment can answer:

(1)   When during cocaine seeking is dopamine release increased?

(2)   Does the increase in dopamine release cause a subsequent change in behavior?

Technical issues

Determining whether dopamine is involved in cocaine seeking requires us to consider several technical issues. The first is that we need a laboratory animal to engage in cocaine seeking. This issue is not too difficult, because suitably trained rats will self-administer for cocaine.

Perhaps the most challenging technical issue is the actual monitoring of dopamine during cocaine seeking. Several measurement requirements must be considered: temporal resolution (How fast is the measurement?), selectivity (What is being measured?), and spatial resolution (Over what area of the brain is the measurement collected?). As described below, the chemical microsensor technique that will be used in our experiment, fast-scan cyclic voltammetry at a carbon-fiber microelectrode, has suitable measurement characteristics for monitoring dopamine during cocaine seeking. .

First, the monitoring of dopamine must be sufficiently fast to capture changes during the cocaine seeking portion of cocaine self-administration. Rats self-administer cocaine at a rate of about once every 10 min on average. (NOTE: Our virtual rats will lever press more frequently.) At this rate, even slow monitoring techniques could assess dopamine changes before and after a lever press for cocaine. However, the goal of this experiment is to monitor dopamine during the behavior(s) just prior to a lever press for cocaine self-administration. During this time, dopamine neurons may be responding to external and internal cues to direct the rat to move towards the lever and to press it. Thus, a fast measurement technique will be necessary to monitor dopamine levels during the time just before the lever press. Fortunately, fast-scan cyclic voltammetry, which monitors dopamine several times a second, is well suited for these measurements. Therefore, voltammetry has the appropriate temporal resolution to capture changes in brain dopamine levels during cocaine seeking.

Second, there are many chemicals in the brain that, like dopamine, are electroactive (which means they can be measured electrochemically). These electroactive substances could potentially interfere with the measurement of dopamine using voltammetry and the chemical microsensor. Fortunately, fast-scan cyclic voltammetry is a unique type of voltammetry that collects a "chemical signature" (called a voltammogram) to identify the chemical being measured by the microsensor. Thus, the measurement of dopamine in the rat brain by fast-scan cyclic voltammetry is selective. However, in the present experiment, the substance measured during cocaine seeking will be chemically compared to dopamine that is released by the direct activation of dopamine neurons with electrical pulses applied to a stimulating electrode implanted in the ventral tegmental area (see Figure 1). Decades of research have established that this type of electrical stimulation elicits "authentic dopamine" in the rat brain. Moreover, for direct comparison, the chemical signature for this authentic dopamine will be collected by the same microsensor in the same area of the brain as the chemical signature recorded during cocaine seeking behavior. 

Fig. 1

Finally, it is necessary to consider where in the brain dopamine will be monitored and the extent of damage the implanted microsensor will cause. While several regions could potentially be sampled, for three main reasons, the best place to record dopamine is the nucleus accumbens. First, this region is part of one of the major midbrain dopamine systems, the mesolimbic dopamine system, which originates in the ventral tegmental area and terminates in the nucleus accumbens. Second, this region links reward-related information processed in the limbic system and cortex with motor output. Thus, the nucleus accumbens occupies a key role in the brain circuitry supporting motivated behavior. Third, it is well established that cocaine self-administration will increase dopamine release in the nucleus accumbens as measured by microdialysis. However, the probe associated with this technique is rather large compared to the size of the nucleus accumbens. And the microdialysis probe is known to cause damage to the adjacent tissue. Fortunately, the carbon-fiber microelectrode used with fast-scan cyclic voltammetry is considerably smaller. Hence, its tip can be readily implanted within the nucleus accumbens to measure dopamine only in this region. Also, because of smaller size, the carbon-fiber microelectrode causes considerably less tissue damage when implanted.

Experimental design

The experimental design will be divided into three parts:

PART 1: Training a rat to self-administer cocaine.

In this experiment, any animal you will work with has already been trained to self-administer for cocaine.

PART 2: Preparation and surgery for voltammetry measurements of dopamine.

In preparation for brain surgery, a rat must be selected, weighed, and anesthetized properly.  You will complete all the steps leading up to surgery. Professor Neuro will complete the surgery procedure while you observe. When surgery is completed, the rat will have the hardware implanted that will allow the recording of dopamine levels and the administration of cocaine.

PART 3: Monitoring dopamine during cocaine seeking.

Two weeks after surgery, the rat is ready to take part in the experiment. You will perform the experiment on a rat that has already recovered. Just before the rat is allowed to engage in cocaine seeking, a microdrive armed with a microsensor will be loaded into the surgically implanted hub. The microsensor will then be lowered into the nucleus accumbens. The electrically evoked dopamine signal will be transmitted to the computer by a wireless piece of equipment in a backpack using radio waves or telemetry. In this way, dopamine can be monitored in the brain of the animal without being connected or "hardwired" to the equipment. The computer will register a lever press and send a signal to the wireless backpack to administer cocaine. 

 

The wireless telemetry and drug delivery systems used in this "virtual" lab were not used in the actual experiments on which this one was based. However, both systems are being developed and employed now. Hence, this "virtual" lab represents some of the latest technology using voltammetry and chemical microelectrodes to measure dopamine during cocaine self-administration in rats.

 

Completing the experiment will require that you perform specific tasks in each of the lab areas and record your observations or responses in your lab book.

Ch. 2: Prep area lab procedures

1)     Put on non-sterile surgical gloves.

2)     Select a rat.

3)     Weigh rat and then place the rat in the prep tray.

4)     Determine how much anesthesia to draw.

a)     per weight dose, e.g., for a pre-mixed cocktail of ketamine and xylazine, a common anesthetic of laboratory rodents, 1 ml/kg ( inject 0.4 ml into a 400 g rat)

b)     too much anesthesia, rat dies

c)     too little anesthesia, need to inject again, but secondary doses are tricky and rats may also die even with normal amount

5)     Grab a vile of anesthesia and a syringe.

a)     anesthesia is premixed

b)     place both objects in the prep tray

6)     Inject the needle into the vile and draw the correct amount of anesthesia.

7)     Properly inject the anesthesia.

a)     inject into peritoneal cavity

 

Fig. 2: Inject the needle at the center of the 'X'.

 

(1)   if too high (in thoracic cavity), causes pneumothorax, deflating lungs and killing animal

(2)   if too low (in bladder), causes complications and perhaps death

b)     properly dispose of needle in sharps container

8)     Place the rat in the prep tray.

a)     rat needs to be on a heating pad as anesthesia prevents thermoregulation

(1)   rat can die from being too hot or too cold

 

Ch. 3: Experiment area lab procedures

If this experiment were to be completed in real time instead of through the virtual world, one would have to perform certain tasks before beginning the actual experiment. As the experimenter, you would be responsible for verifying that the hardware is attached properly, secure, and that the animal is healthy. If this is not done, hardware failure will prevent dopamine measurement or cocaine injection. If a rat is unhealthy, it could die from cocaine injection, and the data would be skewed.

A recovered rat will be placed in the experiment area and the telemetry unit will be attached to the animal. The jugular injection line is flushed with saline to verify that the line is not clogged. The microdrive is then loaded with the dopamine microsensor to verify the microsensor functions correctly. The protective cap covering the brain is removed from the cemented hub and the microdrive is then attached to the hub, which was implanted during surgery into the rat's brain. The connections are then attached between the telemetry unit and the reference electrode (which supports the electrochemical measurements of dopamine), the stimulating electrode, the microsensor (carbon-fiber microelectrode), and the jugular cannula. So as not to break the microsensor, the microsensor is lowered at the appropriate speed to the correct depth in the nucleus accumbens – the depth that provides measurement of dopamine. 

The rat must now be allowed to habituate to the experiment area in order to feel comfortable with its surroundings and not be stressed. A stressed animal may behave erratically and have an adverse reaction to cocaine. The microsensors must then be allowed to come to equilibrium in the brain so the microsensors will not perform erratically. A test stimulation will be performed to check the functionality of the microsensor and have it replaced if necessary, and verify the microsensor has been placed in a dopamine-rich area so that appropriate readings will take place.

Next, the stimulating electrodes must be checked to make sure there is not a bad connection, a bad stimulation unit, or the stimulating electrode has been moved. Wireless communication must also be checked to verify there is no problem with the battery, the telemetry pack, or the computer.

The next step is to turn on the video camera in order to record the rat's behavior. It is now time to begin the experiment and to monitor dopamine during cocaine self-stimulation. The following steps will take place during the lab experiment.

a)     start session by pressing the on button

b)     session starts with extension of lever and display of data on monitor

c)     every bar press results in

i)       six second injection of cocaine

ii)      illuminating the light above lever for 20 seconds

iii)     auditory tone for 20 seconds

iv)    during this 20 seconds, additional lever presses will not result in additional injections of cocaine

d)      session ends

i)       lever is retracted

ii)      typical session duration is 120 min (NOTE: Our's will be much, much shorter.)

e)     repeat test stimulation

i)    check for functioning microsensor

Once you have completed the experiment, it is time for you to write up your analysis in your lab report.