H4 - Brain Stimulation: Reward

Objectives:

To observe a model of addiction in the rat
To examine the role of dopamine in this reward model

Introduction:

Rewarding behaviors are thought to be rewarding because they activate the release of dopamine into nucleus accumbens (Wise, 1996). This is true of everyday pleasurable activities (eating yummy food, laughing at a funny joke, having sex), but is also true for drugs of abuse. No matter what neurotransmitter system is directly targeted by a drug of abuse, they all seem to increase activity of dopaminergic cells that project to the nucleus accumbens through the medial forebrain bundle. This has lead to the Positive Reward Model of addiction. One animal model that has been particularly useful in studying addiction is the intracranial self stimulation paradigm (Koob, 2000).

Procedure:

Preparation: The male rats will need to have an electrode implanted into their medial forebrain bundle. See Appendices B+C for the procedure. Implant a bipolar electrode (Plastics One, Roanoke, VA, catalogue # MS303/1-B) at the following coordinates with the skull leveled between bregma and lambda: 2.8 mm posterior to bregma, 1.7 mm lateral to midline, 7.8 mm below dura. Prepare the electrode by cleaning the insulation off of the flat bottom of the wires using a scalpel. Do this with a stereomicroscope.

Test: Set up the test cage equipped with a bar. Connect the bar to one trigger on the stimulation unit. A good protocol is available from Carlezon and Chartoff (2007). Connect the rat to the stimulator through a counterbalanced electrical commutator (i.e., electrical swivel). Set the stimulator (Grass SD88 stimulator coupled to a photic stimulus isolation unit (PSIU6); Grass Instrument Company, Quincy, MA, USA) to the following pulse parameters: 100Hz (1 pulse every 10 ms), 100µs pulse duration, pulse train duration at 500ms. Current should be increased slowly from around 10µA. Depending on your placement, threshold for responding is often around 50 µA, while maximal current that will not elicit aversive responses are usually around 400µA. You want a good responses rate and no aversive responses. Too much current can result in tissue damage, so be conservative while increasing the current, being aware of the upper limit. A current that yields about 40 presses a minute is ideal. With a manual trigger, shape the behavior using successive approximation. First reward the animal when it is in the correct half of the cage, then when it is in that half and facing the bar, then when it is near the bar, then when it is touching the bar, then as presses the bar. Once this is established, allow it to self stimulate for 30 minutes, counting the bar presses. Remove the rat from the box and give it an injection of either saline or the D2 receptor antagonist raclopride (Sigma-Aldrich, catalogue # R121, 0.16mg/kg; Panagis and Spyraki, 1996). Place it back in the box 15 minutes later, and allow it to self stimulate. Count the number of bar presses it makes in 30 minutes. Report your numbers and doses to your TA who will tabulate the results for analysis. Variation:

If time permits, proper thresholding could be used (Carlezon and Chartoff, 2007). This requires more days for training.
A variety of drugs may be examined to see how they alter responses. Some include lithium chloride, dopamine agonists, and drugs of abuse. See Carlezon and Chartoff, (2007) for a review of some substances examined and their effects on self stimulation.

Follow-up: In any surgical intervention, histological confirmation of the effectiveness to the surgery is important. Discuss with your TA opportunities for learning about histological procedures (perfusion, tissue slicing and staining, microscopy) that may be available. For this study, perfusion with formalin, tissue sectioning and a nissl stain such as cresyl violet would be appropriate histological procedures.