Christian Carr and Scott Steffensen, Department of Psychology
Introduction
Dopamine is an important brain neurotransmitter regarding brain disorders such as substance abuse, Schizophrenia, Parkinson’s disease, attention deficit hyperactivity disorder (ADHD), and Restless Leg Syndrome (RLS). Dopamine neurons in the ventral tegmental area (VTA) and substantia nigra (SN) are inhibited by DA via DA D2 autoreceptor (D2R) activation (Adell and Artigas, 2005). D2R expression in the brain is a well-known biomarker for brain DA levels, drug abuse, and dependence. Many previous studies have demonstrated that D2R expression is directly proportional to DA levels in the brain. Thus, D2Rs are highly responsive to brain levels. This provides an aid in diagnosing these illnesses. Recently, it has been found that D2R expression is not only detectable in the brain but is also expressed in the peripheral tissues (e.g. blood), where DA appears to play a pivotal role in mediating communication between the nervous and immune systems (Basu and Dasgupta, 2000).
Dopamine can only poorly cross the blood brain barrier (BBB), due to its hydrophilic properties. It is therefore often used as a treatment for many things such as low blood pressure and shock stemming from heart attacks. However, in our previous study, intravenous (IV) DA injection greatly increased extracellular DA levels in the NAc in freely-moving rats (unpublished). In addition, we found that D2Rs are expressed on multiple classes of white blood cells (WBCs) in humans and rodents. This result suggests that peripheral mechanisms will enhance the levels of DA in the brain.
Methodology
An IV catheter was inserted into the jugular vein and a microdialysis probe (CMA 11, 2 mm, Harvard Apparatus) was stereotaxically inserted into the NAc shell (AP 1.7, ML 0.8, DV -8.0 mm) of male Wistar rats under isoflurane anesthesia. The rats were then maintained under isoflurane anesthesia as the probe was perfused with aCSF at a rate of 1.5 uL/min. Samples were collected every twenty minutes for the first two hours to establish baseline DA levels. At two hours the rats were injected with either phentolamine (1 mg/kg, IV) or domperidone (5 mg/kg, IV) followed by DA (1 mg/kg, IV). Following injections brain dialysates were collected at 20 minute intervals for an additional two hours. The brain dialysates were then analyzed using high performance liquid chromatography (HPLC) with electrochemical detection (ECD) for quantification of DA. HPLC was performed using a flow rate of 0.6 ml/min through a HR-80 column (Thermo Fisher). ECD was performed using a Guard Cell (5020, Thermo Fisher) set a +400 mV and an analytic cell (5014B, Thermo Fisher) set at -100 mV/+350mV.
We used IBM SPSS Statistics 20 to analyze the results.
Results
Phentolamine did not alter the effects of IV DA on DA levels in the NAc. Domperidone administration abolished the IV DA mediated increases in NAc DA levels.
Discussion
Our research showed that the effects of IV DA on NAc DA levels are mediated through D2-like receptors located in the periphery, not through the blood pressure effects that result from the action of DA at α- adrenergic receptors located on the periphery. This suggests not only that the effects of some drugs of abuse may be mediated by these peripheral mechanisms but that drugs which target only peripheral D2- like receptors may have potential therapeutic uses.
This research will help contribute to our understanding of the mechanisms through which these substances work, as well as the role of peripheral mechanisms in extracellular DA enhancement in the brain. This has enormous implications for the use of intravenous injections of DA to treat certain disorders and would therefore contribute to creating better treatments for addiction patients, as well as altering treatments for other DA-dependent disorders.
Conclusion
D2Rs are important DA receptors that are found both in the CNS and peripherally. They mediate important interactions in the brain such as movement and are critical in the reward pathway. The peripheries are responsible for many activities including immune function, gastric activity, and functions of the heart as well as others. The results of our experiments suggest a neuroimmune interaction that may be important for testing and treating certain disorders that are involved in these pathways. Many members of my family have suffered from disorders involving DA including Parkinson’s and alcohol and drug dependence. My project has thus been personally significant for me and allowed me to be a part of this important research.
Scholarly Sources
Adell A, Artigas F. The somatodendritic release of dopamine in the ventral tegmental area and its
regulation by afferent transmitter systems. Neurosci Biobehav Rev. 2004 ;28(4):415-31.
Basu S, Dasgupta PS. Role of dopamine in malignant tumor growth. Endocrine 2000;12(3):237-41.