Hypoxia-Associated Changes in Striatal Tonic Dopamine Release: Real-Time in vivo Measurements With a Novel Voltammetry Technique

Abstract

Introduction: Striatal tonic dopamine increases rapidly during global cerebral hypoxia. This phenomenon has previously been studied using microdialysis techniques which have relatively poor spatio-temporal resolution. In this study, we measured changes in tonic dopamine during hypoxia (death) in real time with high spatio-temporal resolution using novel multiple cyclic square wave voltammetry (MCSWV) and conventional fast scan cyclic voltammetry (FSCV) techniques.

Methods: MCSWV and FSCV were used to measure dopamine release at baseline and during hypoxia induced by euthanasia, with and without prior alpha-methyl-p-tyrosine (AMPT) treatment, in urethane anesthetized male Sprague-Dawley rats.

Results: Baseline tonic dopamine levels were found to be 274.1 ± 49.4 nM (n = 5; mean ± SEM). Following intracardiac urethane injection, the tonic levels increased to a peak concentration of 1753.8 ± 95.7 nM within 3.6 ± 0.6 min (n = 5), followed by a decline to 50.7 ± 21.5 nM (n = 4) at 20 min. AMPT pre-treatment significantly reduced this dopamine peak to 677.9 ± 185.7 nM (n = 3). FSCV showed a significantly higher (p = 0.0079) peak dopamine release of 6430.4 ± 1805.7 nM (n = 5) during euthanasia-induced cerebral hypoxia.

Conclusion: MCSWV is a novel tool to study rapid changes in tonic dopamine release in vivo during hypoxia. We found a 6-fold increase in peak dopamine levels during hypoxia which was attenuated with AMPT pre-treatment. These changes are much lower compared to those found with microdialysis. This could be due to improved estimation of baseline tonic dopamine with MCSWV. Higher dopamine response measured with FSCV could be due to an increased oxidation current from electroactive interferents.

Keywords: hypoxia; rodent; striatum; tonic dopamine; voltammetry.

FIGURE 1

Striatal dopamine release in response to euthanasia-induced hypoxia measured with FSCV. (A) Pseudo-color plot shows time trend of change in oxidation and reduction current after intracardiac urethane injection (black arrowhead) in a representative animal. A shift in background current can be seen in later part of pseudo-color plot. The 2-D voltammogram inset shows dopamine oxidation and reduction at the time point indicated by dashed white line on the pseudo-color plot. (B) Concentration vs. time plot show phasic dopamine changes after intracardiac urethane injection (bold line represents mean concentration of dopamine over time; thin black lines represent SEM). The point of highest observed dopamine peak response was used to align the time trends.

FIGURE 2

Striatal dopamine (DA) release in response to euthanasia-induced hypoxia measured using multiple cyclic square wave voltammetry (MCSWV). (A) Tonic dopamine levels rise rapidly following intracardiac (IC) urethane injection [black arrows in (A) and (B)] reaching a peak concentration of about 6 times the basal values within 3–4 min, followed by a decline to less than 20% of the baseline concentration by 20 min (n = 5 rats for time point 0–17 min, n = 4 rats from 17–20 min). In most animals, levels gradually declined over a 20 min period following the peak. However, in animal 4, the levels became undetectable at around 8 min following IC injection. (B) Changes in tonic dopamine levels observed in animal groups with and without alpha-methyl-p-tyrosine (AMPT) pretreatment. Bold line represents mean concentration of dopamine over time; red and blue shaded areas represent the SEM for the respective groups. (C) Pretreatment of animals with AMPT lead to a significant decrease in dopamine release (p = 0.035) observed post-euthanasia. Snapshots of pseudo-color plots during (D) baseline tonic recording, (E) peak oxidation signal response, and (F) 20 min after urethane injection in a representative animal from (A).

FIGURE 3

(A) Changes in tonic dopamine release accompanying spontaneous drop of breath rate and labored respiration followed by recovery in a single animal. (B) Record of breath rate (running averaged over 30 s) during the same period as indicated in (A) shows a decrease in breath rate starting 7–8 min before the observed increase in the dopamine oxidation signal signal marked by green arrowheads in both (A) and (B). Snapshots of pseudo-color plots from different time points indicated in (A). (C) Baseline tonic recording, (D) peak of dopamine oxidation signal, (E) post-episode undetectable dopamine, and (F) recovery of dopamine response.