Modulatory effects of neuropsychopharmaca on intracellular pH of hippocampal neurones in vitro

Abstract

Background and purpose: The intracellular pH (pHi) of neurones is tightly regulated by, for example, membrane-bound acid-exchangers and loaders. Nevertheless, excessive bioelectric activity lowers steady-state pHi. In turn, even a moderate acidification can inhibit neuronal activity, a process believed to be part of a negative feedback loop controlling neuronal excitation. As moclobemide, an antidepressant, and also some antiepileptic drugs can reduce neuronal pHi in hippocampus slices in vitro, we screened a panel of currently used neuropsychopharmaca for comparable effects.

Experimental approach: BCECF-AM loaded hippocampal slices were superfused with 16 different neuroleptics, antidepressants and antiepileptics under bicarbonate-buffered conditions. Changes in steady-state pHi of CA3 neurones were measured fluorometrically.

Key results: The antipsychotics haloperidol, clozapine, ziprasidone, and the antidepressants amitriptyline, doxepin, trimipramine, citalopram, mirtazapine, as well as the anticonvulsive drug tiagabine reversibly reduced the steady-state pHi by up to 0.35 pH-units in concentrations of 5-50 microM. In contrast, venlafaxine, the anticonvulsants carbamazepine, clonazepam, gabapentin, lamotrigine, zonisamide, and the mood stabilizer lithium had no effect on neuronal pHi.

Conclusion and implications: These data substantiate the view that clinically relevant concentrations of neuroleptics and antidepressants can mediate changes in neuronal pHi, which may contribute to their pharmacological mode of action. Effects on pHi should be taken into account when therapeutic or even harmful effects of these drugs are evaluated.

Figure 1

Effects of typical and atypical neuroleptics on the steady-state intracellular pH (pHi) of BCECF-laden hippocampal CA3 neurones. Application periods of drugs at indicated concentrations are shown as shaded areas. (A) Ziprasidone, (B) haloperidol, (C) clozapine. All curves represent sliding averages from eight data points measured in single neuronal somata. Open circles in (A) show original data points measured at 20 s intervals. Definition of ΔpHi (see Tables 1–3 for group data) is demonstrated in (A).

Figure 2

Effects of antidepressants on steady state intracellular pH (pHi) of BCECF-laden hippocampal CA3 neurones. Application periods of drugs at indicated concentrations are shown as shaded areas. (A) Amitriptyline, (B) trimipamine, (C) citalopram, (D) mirtazapine. Curves represent sliding averages from four to eight data points measured in one pyramidal soma. In (B), pHi deflections within near-by regions (150–250 µm²) of the stratum radiatum are additionally shown.

Figure 3

Effects of antiepileptics on steady state intracellular pH of BCECF-laden hippocampal CA3 neurones. Application periods of drugs at indicated concentrations are shown as shaded areas. (A) Tiagabine, (B) zonisamide. Curves represent sliding averages from eight data points measured in three pyramidal somata. Note that neurones respond to tiagabine but not to zonisamide.

Figure 4

Schematic overview of major components and systems that are relevant to neuronal intracellular pH regulation. (A) Main acid extruders comprise monocarboxylate transporters (MCT), Na⁺/H⁺ exchangers (NHE), Na⁺-dependent Cl^-/HCO₃^- -exchanger (NCBE), Na⁺/HCO₃^- -exchanger (NBC), and H⁺-ATPases. Anion exchangers (AE), e.g. Cl^-/HCO₃^- exchanger, act as acid loaders. (B) Intracellular protons mainly result from lactic acid production (glycolysis) and H₂CO₃ formation from metabolic CO₂ production (citrate cycles) involving the mitochondria. Carbonic anhydrases are important for the level of cellular bicarbonate and, thus, for intracellular buffering capacity. (C) Membrane potential indirectly drives transmembane fluxes of H⁺ and bicarbonate. (D) Presynaptic mitochondria are preferentially fueled by lactate and pyruvate to provide ATP, for example, for neurotransmitter re-uptake by vesicular and transmembraneous transporters. (E) The pH gradient across the synaptic vesicle membrane is a driving force for transmitter re-uptake mediated by catecholamine-transporters (CAT), which comprise 5-HT transporters (SERT) dopamine transporters, noradrenaline transporters, GABA-transporters and excitatory acid transporters. As an example, the effect of SERT is shown. Arrows numbered 1–9 point to known targets of the following neuropsychopharmaca (see also Discussion): (1) valproate (Rumbach et al., 1986); (2) levetiracetam (Leniger et al., 2004a); (3) topiramate (Leniger et al., 2004b); (4) moclobemide (Bonnet et al., 2000b); (5) fluoxetine, haloperidol, valproate (Rumbach et al., 1986; Wallace and Starkow, 2000); (6) valproate (Benavides et al., 1982); (7) topiramate, sulthiame (Woodbury and Kemp, 1989; Leniger et al., 2002); (8) tiagabine, topiramate, valproate (Kaila, 1994; Lueckermann et al., 1997; Bonnet et al., 2002); (9) 5-HT re-uptake inhibitors, tricyclics, mirtazapine, venlafaxine (Cao et al., 1997).