This approach can help elucidate the role of specific ventral tegmental area, somatodendritic receptors and phasic dopamine release in the nucleus accumbens. The main advantage of this approach is that questions about receptor function control over phasic dopamine release can be addressed in the intact brain. Optimizing the carbon fiber microelectrode and bipolar stimulator location can be challenging, and some advice for beginners is starting recording in the dorsal striatum.
Due to the use and manipulation of multiple types of electrodes, including recording, reference, and stimulating electrodes, a visual guide of when and where these electrodes are implanted will be useful. After confirming a lack of response to pedal reflex, clean the scalp of an anesthetized rat with three sequential iodopovidone and 70%ethanol scrubs. After the last scrub, use sterilized needle nose tweezers and surgical scissors to cut away enough scalp tissue to make room for the implanted electrodes.
Gently clean the exposed skull surface with sterilized cotton tip applicators before applying two to three drops of 3%hydrogen peroxide to help identify lambda and bregma. Next, use a one millimeter drill bit set to approximately 20, 000 revolutions per minute to make a 1.5 millimeter diameter hole, 2.5 millimeters anterior and 3.5 millimeters lateral to the bregma. Implant a 1.59 millimeter outer diameter, 3.2 millimeter long screw about halfway into the hole until the screw is firmly in place.
Next, drill a one millimeter diameter reference electrode hole in the left hemisphere, 1.5 millimeters anterior, and 3.5 millimeters lateral to the bregma. Insert approximately two millimeters of reference wire into the hole while wrapping the wire around and under the head of the implanted screw. Then fully implant the screw, pinning the reference electrode in place.
In the right hemisphere, drill a 1.5 millimeter diameter hole, 1.2 millimeters anterior, and 1.4 millimeters lateral to the bregma, and use tweezers to carefully remove the dura. For the stimulating electrode, drill a square hole two millimeters anterior-posterior and five millimeters medial-lateral centered at 5.2 millimeters posterior and one millimeter lateral to the bregma. Using stereotactic arm bars, lower the bipolar stimulating electrode guide cannula five millimeters below the dura.
Using the stereotactic arm bars, lower the carbon fiber microelectrode four millimeters below the dura at the most dorsal portion of the striatum and connect the reference wire and carbon fiber to a potentiostat. Then apply a minus 0.4 to 1.4 volt, 400 volt per second triangular waveform for 15 minutes at 60 Hertz, followed by a 10-minute stimulus at 10 Hertz. To optimize the stimulating electrode location, set the stimulator to produce a bipolar electrical waveform with a 60 Hertz frequency, 24 pulses, 300 microamp current, and two millisecond per phase pulse width.
Gently lower the stimulator in 0.2 millimeter increments from 5 to 7.8 millimeters below the dura, stimulating the VTA at each increment. Continue to lower the bipolar-stimulating electrode guide cannula until a stimulation produces phasic dopamine release at the carbon fiber microelectrode within the dorsal striatum. Once a dopamine release has been observed, lower the carbon fiber microelectrode until it is at least six millimeters below the dura into the most dorsal part of the nucleus accumbens core.
When the electrode is in position, stimulate the VTA and record the peak amplitude of the dopamine peak, then lower or raise the carbon fiber microelectrode to the site that produces the greatest dopamine release, making sure that the peak of the dopamine response is represented as a clear oxidation peak at 0.6 volts and a reduction peak at minus 0.2 volts. Once the carbon fiber and stimulating electrode guide cannula location has been optimized, stimulate the animal every three minutes for 20 to 30 minutes. After achieving a stable baseline, gently lower the internal cannula by hand into the guide cannula pre-fitted into the bipolar stimulator and obtain an additional two to three baseline recordings to confirm that the cannula insertion did not cause a change in the evoked signal.
Once a baseline response has been established, use a syringe pump and a microsyringe to infuse 0.5 microliters of the drug solution of interest into the VTA over a two-minute period. After the infusion, leave the internal cannula in place for at least one minute prior to removal, continuing to record the response every three minutes to measure the post-infusion effects. As demonstrated in these representative color plots, saline infusion does not alter the stimulated phasic dopamine release.
The infusion of NMDA produces a robust increase in stimulated phasic dopamine release, while the NMDA receptor competitive antagonist, AP5, produces a robust decrease. Both the non-competitive nicotinic acetylcholine receptor antagonist mecamylamine and the non-selective competitive muscarinic acetylcholine receptor antagonist scopolamine produce a robust decrease in the stimulated phasic dopamine release. In this graph, a summary of the effects of the tested drugs over time can be observed.
The infusion cannula must be carefully inserted into the bipolar stimulating electrode. Otherwise, excessive damage can occur, leading to artifactual changes in phasic dopamine release. This approach has allowed investigators to more finely address the specific receptor mechanisms by which phasic dopamine release is regulated in vivo.
