Cocaine increases the intracellular calcium concentration in brain independently of its cerebrovascular effects

Abstract

Cocaine abuse increases the risk of life-threatening neurological complications such as strokes and seizures. Although the vasoconstricting properties of cocaine underlie its cerebrovascular effects, the mechanisms underlying its neurotoxicity remain incompletely understood. Here, we use optical techniques to measure cerebral blood volume, hemoglobin oxygenation (S(t)O(2)), and intracellular calcium ([Ca(2+)](i)) to test the hypothesis that cocaine increases [Ca(2+)](i) in the brain. The effects of cocaine were compared with those of methylphenidate, which has similar catecholaminergic effects as cocaine (except for serotonin increases) but no local anesthetic properties, and of lidocaine, which has similar local anesthetic effects as cocaine but is devoid of catecholaminergic actions. To control for the hemodynamic effects of cocaine, we assessed the effects of cocaine in animals in which normal blood pressure was maintained by infusion of phenylephrine, and we also measured the effects of transient hypotension (mimicking that induced by cocaine). We show that cocaine induced significant increases ( approximately 10-15%) in [Ca(2+)](i) that were independent of its hemodynamic effects and of the anesthetic used (isofluorance or alpha-chloralose). Lidocaine but not methylphenidate also induced significant [Ca(2+)](i) increases ( approximately 10-13%). This indicates that cocaine at a dose within the range used by drug users significantly increases the [Ca(2+)](i) in the brain and its local anesthetic, but neither its catecholaminergic nor its hemodynamic actions, underlies this effect. Cocaine-induced [Ca(2+)](i) increases are likely to accentuate the neurotoxic effects from cocaine-induced vasoconstriction and to facilitate the occurrence of seizures from the catecholaminergic effects of cocaine. These findings support the use of calcium channel blockers as a strategy to minimize the neurotoxic effects of cocaine.

Figure 1.

A, Schematic illustration of the optical diffusion fluorescence experimental setup used for all studies. B, Example of a real-time physiological monitoring acquired during experiments.

Figure 2.

A, Hemoglobin absorbance spectrum and excitation spectra of calcium fluorescence indicator Rhod2 obtained from the surface of rat brain cortex. The vertical lines illustrate the center wavelengths of the excitation (λex1, λex2, and λex3) and fluorescence emission (λem4). B, Example of calcium-dependent fluorescence recording along with the reflectance of the excitations from the cortex of the brain simultaneously at those wavelengths in response to drug challenges. AU, Arbitrary units.

Figure 3.

MABP, heart rate (electrocardiogram), and body temperature (Temp) as a function of time in response to 1 mg/kg cocaine (isoflurane-anesthetized rats; A), 1 mg/kg cocaine (α-chloralose-anesthetized rats; B), 1 mg/kg methylphenidate (C), and 1 mg/kg lidocaine (D). Data are presented as a mean ± SEM.

Figure 4.

Optical diffusion and fluorescence recordings of CBV, tissue oxygenation (S_tO₂), and intracellular calcium ([Ca²⁺]_i) from rat cortex after 1 mg/kg cocaine in isoflurane-anesthetized rats (A–C), 1 mg/kg cocaine in α-chloralose-anesthetized rats (D–F), 1 mg/kg methylphenidate in isoflurane-anesthetized rats (G–I), and 1 mg/kg lidocaine in isoflurane-anesthetized rats (J–L). The data are presented as relative changes from baseline (100%). The vertical dashed lines in each graph represent the time of intravenous drug administration.

Figure 5.

Optical diffusion and fluorescence recordings of CBV, tissue oxygenation (S_tO₂), and intracellular calcium ([Ca²⁺]_i) from rat cortex after intravenous administration of vehicle/saline (A–C), blood exsanguination (D–F), and 1 mg/kg cocaine (G–I). In the experiment illustrated in G–I, the MABP was maintained normal with phenylephrine. The vertical lines represent the beginning of the injection or hemodynamic challenge.