Copyright © 2001 by Institute of Pharmacology Polish Academy of Sciences
Polish Journal of Pharmacology
Pol. J. Pharmacol., 2001, 53, 635639 ISSN 1230-6002
SHORT COMMUNICATION
REPEATED IMIPRAMINE ADMINISTRATION ENHANCES THE EFFECTS OF NMDA RECEPTOR LIGANDS ON SYNCHRONOUS ACTIVITY IN RAT FRONTAL CORTEX IN VITRO Bartosz Bobula1,#, Krzysztof Tokarski1, Grzegorz Hess1,2 Department of Physiology, Institute of Pharmacology, Polish Academy of Sciences, Smêtna 12, PL 31-343 Kraków, Poland, Institute of Zoology, Jagiellonian University, Ingardena 6, PL 30-060 Kraków, Poland
Repeated imipramine administration enhances the effects of NMDA receptor ligands on synchronous activity in rat frontal cortex in vitro. B. BOBULA, K. TOKARSKI, G. HESS. Pol. J. Pharmacol., 2001, 53, 635–639. This study assessed the effects of repeated administration (14 days) of imipramine on the function of NMDA receptors by measuring the frequency of spontaneous epileptiform discharges which develop in rat frontal cortical slices incubated in Mg2+-free conditions. Imipramine significantly enhanced both the excitatory effect of NMDA and the inhibitory effect of the competitive NMDA receptor antagonist CGP 37849 on the frequency of discharges. These results are consistent with studies indicating that chronic administration of antidepressant drugs induces adaptive changes in NMDA receptor/ channel complex in the cerebral cortex. Key words: antidepressant drug, neocortical slice, epileptiform activity, rat
correspondence; e-mail:
[email protected]
B. Bobula, K. Tokarski, G. Hess
INTRODUCTION It has generally been accepted that effects of antidepressant drugs on central monoaminergic transmission are involved in the mechanisms of their therapeutic action, since most of these drugs are known to inhibit the uptake or metabolism of amines (for review see [3]). However, the observation that about 3 weeks of antidepressant administration is required to achieve a therapeutic effect, encouraged studies of adaptive changes induced by prolonged treatment with antidepressants. It has been demonstrated that excitatory amino acid transmission may be involved in the neuronal response to such treatment. NMDA receptor antagonists exhibit antidepressant-like actions in animal models, and combined administration of NMDA receptor antagonists and antidepressants exerts synergistic effects [7, 14, 15]. Another line of evidence, obtained using neurochemical, molecular and behavioral approaches, indicated that chronic administration of antidepressants affects the NMDA receptor/channel complex [13, 14]. However, the underlying mechanisms and functional consequences of these effects are still not well understood. The present study was aimed at finding the effects of repeated imipramine administration on synchronous, epileptiform activity of neocortical circuitry. We employed this model because the population activity in an interconnected network is a sensitive measure of pharmacological manipulations. Moreover, neocortical neurons have been hypothesized to be the primary site of action of hallucinogens and they may be critically involved in the action of many atypical antipsychotics and antidepressants [8]. We examined the influence of two NMDA receptor ligands, an agonist and a competitive antagonist, on spontaneous epileptiform discharges, which develop in a Mg2+ free artificial cerebrospinal fluid (ACSF) in slice preparations of rat frontal cortex.
dures were approved by the Animal Care and Use Committee at the Institute of Pharmacology. Animals were divided into two experimental groups: treated and control (10 rats per group). Imipramine (dissolved in water) was applied twice daily (20 mg/kg/day po) for 14 days. Control rats received water (2 ml/kg po). In order to avoid possible acute effects of the drug, brain slices were prepared 48 h after the last treatment. Slice preparation and electrophysiological recording Rats were decapitated, their frontal cortices were dissected and cut into 400–450 mm-thick slices which were stored in a gassed (95% O2 and 5% CO2) ACSF, consisting of (in mM): 127 NaCl, 2 KCl, 2.5 CaCl2, 1.3 MgSO4, 1.25 KH2PO4, 24 NaHCO3 and 10 glucose. A single slice was then transferred to the recording chamber (volume 1 ml) and superfused at a rate of 1.5 ml/min with a modified ACSF devoid of magnesium ions and with added picrotoxin (30 mM) at 32oC. Spontaneously occurring field potentials were recorded using Axoprobe amplifier (Axon Instruments, USA) by glass micropipettes filled with 2 M NaCl (2–5 MW), displayed on a chart recorder (Gould TA 240) and stored on a PC computer (1401 interface, SIGAVG software, CED, UK) for further analysis. After the frequency of discharges stabilized for 30 min, one of the tested NMDA receptor ligands was bath-applied for 10 min. The frequency of discharges was measured before, during the application, and upon the washout of the drug. The effects were expressed as a percentage of baseline frequency ± SEM. Statistical analysis was carried out using Student’s t-test. Drugs N-methyl-D-aspartate (NMDA) was purchased from RBI, DL-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849) was kindly donated by CIBA-GEIGY (Basel, Switzerland).
MATERIALS and METHODS Animals Male Wistar rats (purchased from a licensed dealer), weighing approximately 100 g at the beginning of the experiment, were housed in the groups of 7 per cage under a controlled light/dark cycle (light on: 7.00–19.00), and had free access to standard food and tap water. The experimental proce-
636
RESULTS and DISCUSSION The removal of Mg2+ block from NMDA receptors and the reduced membrane surface charge screening enhance postsynaptic excitatory potentials and neuronal excitability which results in synchronous, spontaneous discharges of cortical neurons [6]. These discharges represent a network-
Pol. J. Pharmacol., 2001, 53, 635–639
REPEATED IMIPRAMINE AND SYNCHRONOUS ACTIVITY IN NEOCORTEX
Fig. 1. NMDA-induced increase in spontaneous discharge frequency in control artificial cerebrospinal fluid. A. Recording obtained in a representative experiment. Bar above the trace indicates time of NMDA (0.5 mM) application. B. Dose-dependence of the effect. Each point represents a mean (± SEM) change relative to dicharge frequency before application of the drug. Numbers below the points on the chart represent numbers of slices
Fig. 2. CGP 37849-induced decrease in spontaneous discharge frequency in control artificial cerebrospinal fluid. A. Recording obtained in a representative experiment. Bar above the trace indicates time of CGP 37849 (0.25 mM) application. B. Dose-dependence of the effect. Each point represents a mean (± SEM) change relative to control dicharge frequency. Numbers below the points on the chart represent numbers of slices
ISSN 1230-6002
dependent activity, and their frequency is dependent on both the excitable properties of neurons and on synaptic transmission. These events could be recorded throughout the depth of the cortex, however, the most pronounced ones were visible in superficial cortical layers (II/III). The discharges occurred at a frequency ranging from 0.01 to 0.7 Hz (mean 0.041 ± 0.002, n = 56; control rats). Each discharge consisted of a negative-going wave with superimposed afterpotentials (for details see [2]). In the first series of experiments spontaneous epileptiform discharges were induced in frontal cortical slices (n = 17) taken from control rats within 12 ± 1 min of superfusion with a Mg2+-free ACSF and with added picrotoxin. In contrast, in slices prepared from imipramine-treated animals (n = 15), the occurrence of spontaneous activity was significantly delayed (23 ± 2 min, p < 0.01). Additionally, the frequency of discharges was decreased compared to control rats (mean 0.034 ± 0.008, n = 39) In the second set of experiments, bath application of NMDA for 10 min resulted in a reversible increase in the frequency of recorded epileptiform discharges in all tested slices taken from control rats (n = 28, Fig. 1A). The increase in the discharge rate by NMDA (0.25–1.0 mM) was dose-dependent (Fig. 1B). This excitatory action of NMDA demonstrated no desensitization during the time of application. In contrast to the effect of NMDA receptor agonist, the competitive antagonist CGP 37849 (0.1–0.5 mM) dose-dependently and reversibly suppressed spontaneous discharges (n = 23, Fig. 2). Repeated administration of imipramine enhanced both the excitatory effect of NMDA and the inhibitory effect of the competitive NMDA receptor antagonist CGP 37849 on the frequency of spontaneous discharges. While in slices prepared from control animals the application of NMDA (0.5 mM) resulted in the frequency increase by 40 ± 3% (n = 11), the effect on slices taken from rats subjected to imipramine was significantly larger (55 ± 5%, n = 10, Fig. 3A). The suppressive effect of CGP 37849 (0.25 mM) after imipramine (50 ± 2%, n = 10) was significantly stronger than in controls (40 ± 2%, n = 11, Fig. 3B) These results suggest that repeated imipramine administration strengthens the effects of activation as well as of blockade of the glutamate recognition site of NMDA receptor complex in rat frontal cortex. Imipramine and most antidepressant drugs affect noradrenaline and/or serotonin reuptake and
637
B. Bobula, K. Tokarski, G. Hess
% of control
A
NMDA 160
*
140 120 100 CONTROL n = 11 14 × IMIPRAMINE n = 10
B
CGP 37849
% of control
60 40
*
20 0
Fig. 3. Prolonged imipramine treatment enhances the excitatory effect of 0.5 mM NMDA (A) and inhibitory effect of 0.25 mM CGP 37849 (B). * denotes a significant difference from control (p < 0.05)
their prolonged administration induces adaptive changes in noradrenergic and 5-HT receptors [3, 10]. It has been reported that antidepressant treatment also affects other neurotransmitter systems [3], probably via indirect interactions involving noradrenergic and serotonergic transmission. Noradrenaline and 5-HT can influence, both directly and via regulation of g-aminobutric acid (GABA)-mediated synaptic inhibition, the activity of glutamatergic system in various brain structures [1, 12]. Additionally, noradrenaline and 5-HT can regulate glutamate release via presynaptic heteroceptors [4, 11]. Therefore, it is conceivable that adaptive changes in noradrenaline and 5-HT receptors, induced by antidepressants, may modify synaptic release of glutamate and, as a consequence, induce adaptive changes in postsynaptic NMDA receptors in the frontal cortex. It has recently been reported that chronic administration of imipramine significantly decreased potassium-induced outflow of glutamate
638
in rat frontal cortical slices [9], which suggests that the antidepressant administration may result in a decreased release of transmitter in cortical glutamatergic synapses, a finding confirmed also with the use of the selective 5-HT reuptake inhibitor [5]. If this is the case, the increase in the responsiveness of the glutamate recognition site of NMDA receptor complex to selective exogenous ligands, observed in the present study, may be connected with the functional adaptation of NMDA receptors to the decreased level of glutamatergic transmission. It remains to be established whether the observed changes occur pre- or postsynaptically. Acknowledgment. This work was supported by the State Committee for Scientific Research, grant no. 4P05A 06419, Warszawa, Poland.
REFERENCES 1. Bijak M., Misgeld U.: Effects of serotonin through serotonin ) and serotonin" receptors on inhibition in the guinea-pig dentate gyrus in vitro. Neuroscience, 1997, 78, 1017–1026. 2. Bobula B., Zahorodna A., Bijak M.: Different receptor subtypes are involved in the serotonin-induced modulation of epileptiform activity in rat frontal cortex in vitro. J. Physiol. Pharmacol., 2001, 52, 265–274. 3. Caldecott-Hazard S., Morgan D.G., DeLeon-Jones F., Overstreet D.H., Janowsky D.: Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories. Synapse, 1991, 9, 251–301. 4. Gereau R.W., Conn P.J.: Presynaptic enhancement of excitatory synaptic transmission by beta-adrenergic receptor activation. J. Neurophysiol., 1994, 72, 1438– –1442. 5. Go³embiowska K., Dziubina A.: Effect of acute and chronic administration of citalopram on glutamate and aspartate release in the rat prefrontal cortex. Pol. J. Pharmacol., 2000, 52, 441–448. 6. Jones R.S., Heinemann U.: Synaptic and intrinsic responses of medial entorhinal cortical cells in normal and magnesium-free medium in vitro. J. Neurophysiol., 1988, 59, 1476–1496. 7. Maj J., Rogó¿ Z., Skuza G., Sowiñska H.: Effects of MK-801 and antidepressant drugs in the forced swimming. Eur. Neuropsychopharmacol., 1992, 2, 37–41. 8. Marek G.J., Aghajanian G.K.: The electrophysiology of prefrontal serotonin systems: therapeutic implications for mood and psychosis. Biol. Psychiat., 1998, 44, 1118–1127. 9. Michael-Titus A.T., Bains S., Jeetle J., Whelpton R.: Imipramine and phenelzine decrease glutamate overflow in the prefrontal cortex – a possible mechanism of neuroprotection in major depression? Neuroscience, 2000, 100, 681–684.
Pol. J. Pharmacol., 2001, 53, 635–639
REPEATED IMIPRAMINE AND SYNCHRONOUS ACTIVITY IN NEOCORTEX 10. Mongeau R., Blier P., de Montigny C.: The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments. Brain Res. Rev., 1997, 23, 145–195. 11. Muramatsu M., Lapiz M.D., Tanaka E., Grenhoff J.: Serotonin inhibits synaptic glutamate currents in rat nucleus accumbens neurons via presynaptic 5-HT * receptors. Eur. J. Neurosci., 1998, 10, 2371–2379. 12. Nicoll R.A., Malenka R.C., Kauer J.A.: Functional comparison of neurotransmitter receptor subtypes in mammalian central nervous system. Physiol. Rev., 1990, 70, 513–565. 13. Popik P., Wróbel M., Nowak G.: Chronic treatment with antidepressants affects glycine/NMDA receptor
ISSN 1230-6002
function: behavioral evidence. Neuropharmacology, 2000, 39, 2278–2287. 14. Skolnick P., Layer R.T., Popik P., Nowak G., Paul I.A., Trullas R.: Adaptation of N-methyl-D-aspartate (NMDA) receptors following antidepressant treatment: implications for the pharmacotherapy of depression. Pharmacopsychiatry, 1996, 29, 23–26. 15. Trullas R., Skolnick P.: Functional antagonists at the NMDA receptor complex exhibit antidepressant actions. Eur. J. Pharmacol., 1990, 185, 1–10.
Received: October 8, 2001; in revised form: October 16, 2001.
639
