Nimodipine Reverses the Hypoxic Elevation of Serotonin and Tryptophan in the Striatum of Adult Rats.
Michael M. Haile, M.D., Patricia A. Broderick, Ph.D., Yong-Sheng Li, M.D., David Quartermain, Ph.D., Thomas J.J. Blanck, M.D., Ph.D., Alex Y. Bekker, M.D., Ph. D.,
Department of Anesthesiology, NYU Langone Medical Center, Department of Physiology & Pharmacology, CUNY Medical School at CCNY, New York, NY
- Moderate hypoxia has been implicated in the development of postoperative delirium. This phenomenon may be related to a hypoxia-induced dysregulation of the neurotransmitter (NT) Serotonin (5-HT) and of the amino acid Tryptophan (L-TP).
- Hypoxia causes a reduction in cerebral oxidative metabolism, relative membrane depolarization, an influx of calcium (Ca2+), and the release of Ca2+ from internal stores.
- Ca2+ is a potent regulator of neuronal excitability and cellular second messenger that can activate the enzyme tryptophan hydroxylase (TryH), the rate limiting step in the synthesis of 5-HT from L-TP.
- Hypoxia increases levels of 5-HT in the CNS1. The dysregulation of 5-HT2 neurotransmission and L-TP3 metabolism may be part of a “final common neural pathway”4 to delirium.
- Our previous study indicated that Nimodipine (NIMO) when administered immediately after a hypoxic episode, preserves short term memory5. NIMO is an L-type calcium channel blocker that crosses the blood-brain barrier.
- A decrease in intracellular concentrations of Ca2+ during hypoxic episodes may preserve neural homeostasis and normal synaptic transmission to improve cognitive outcomes.
To test the hypothesis that Nimodipine may stabilize neuronal membranes during hypoxia thereby reducing hypoxia induced NT elevations.
- The study was approved by the NYUMC & the CCNY IACUC.
- Six Adult Sprague-Dawley rats, 300-350g, (Charles River Laboratories, Raleigh, NC) were anesthetized with 50 mg/kg Na Pentobarbital intraperitoneal injection (i.p.).
- BRODERICK PROBE® Neuromolecular Imaging micro-voltammetry sensors were implanted in the dorsal striatum at coordinates AP+2.5, ML+2.6, DV-7.3 (Stereotactic Atlas of the Rat Brain, Pellegrino, Pellegrino, and Cushman, 1979).
- NIMO (Sigma-Aldrich, St. Louis, MO) was dissolved in a vehicle of 60% Polyethylene Glycol / 40% methanol by mass in saline and administered by i.p. injection.
- The BRODERICK PROBE® senses specific neurochemicals in the living brain.
- Neurotransmitters (NT), metabolites and precursors, such as dopamine, serotonin, somatostatin, norepinephrine, homovanillic acid, tryptophan, and dynorphin are detected.
- After a period of charging and reduction of neurochemicals in the surrounding tissues, the sensors detect returning oxidation currents. Each neurochemical has a specific signature determined by its characteristic oxidative potential (voltage).
- The scan above is from a human epilepsy patient. The concentration of a neurochemical is proportional to current, according to the Cottrell Equation: (i = nFAcoO(DO)1/2/π1/2t1/2).
- Neurotransmitters are detected within seconds and scans can be repeated within minutes or hours. In subjects where the sensors have been permanently implanted scans can be made days, weeks, or months later.
Recording & Analysis:
In this study extracellular NT levels were recorded serially in four phases:
- Fifteen minutes during the establishment of baseline values in room air.
- Thirty minutes under hypoxia (10% O2) alone.
- Thirty minutes under ongoing hypoxia after i.p. injection of NIMO (0.1mg/kg).
- Thirty minutes under ongoing hypoxia after i.p. injection of NIMO (1.0mg/kg).
Measurements were analyzed with ANOVA with post hoc Tukey’s test. P-values less than 0.05 were considered significant.
- The scans to the right are a series of recordings from one subject.
- NT levels below are expressed as percentages of baseline averages obtained over 15 mins (assigned a value of 100%).
- Treatment values were averaged over each sequential 30 min period following each intervention.
- Moderate hypoxia resulted in the increase of 5-HT to 68% (SEM=4) and LTP to 27% (SEM=3.5) above baseline.
- Under continuing hypoxia, NIMO (0.1mg/kg) caused 5-HT levels to fall to 13% (SEM=5) and LTP to 10% (SEM=1.5) above baseline.
- Under continuing hypoxia, NIMO (1.0mg/kg) caused 5-HT levels to fall to 20% (SEM=2) above baseline and LTP to fall to baseline (SEM=0.5).
Why does hypoxia lead to increased levels of 5-HT and LTP?
- Moderate hypoxia causes elevated intracellular Ca2+ levels.
- Calmodulin (CaM) and in turn Calcium-Calmodulin Kinase II (CaMKII) are activated by Ca2+.
- The phosphorylation of enzymes such as synapsin by CaMKII is a mechanism for controlling neurotransmitter release. CaMKII also phosphorylates tryptophan hydroxylase, the rate limiting enzyme in the synthesis of 5-HT6.
Why may administration of Nimodipine affect cognitive function?
- In neurons, L-type Ca2+ channels are the principal source for Ca2+ entry and membrane depolarization following energy failure7. NIMO may stabilize neuronal membranes preventing Ca2+ leakage during hypoxia.
- CaMKII and synapsin regulation is important for both the control of NT release and for neuronal plasticity.
- The derangement of cerebral Ca2+ homeostasis has been linked to the pathophysiology of aging, Alzheimer’s Disease, psychosis and delirium 8 & 9.
- Delirium may be related to the dysfunction of multiple NT systems4 including Dopamine10 and 5-HT that are associated with Ca2+ homeostasis. Changes in the levels of amino acid precursors of NT may also contribute to the development of delirium2.
Moderate hypoxia increased levels of Serotonin and Tryptophan in the striatum of adult rats. Nimodipine administration during ongoing hypoxia caused the levels of both to fall towards baseline. The results may have implications for understanding and treating cognitive decline in the immediate postoperative period.