Synergistic and antagonistic effects of NMDA-receptor antagonists and D,L amphetamine: A combined behavioural and neurochemical study

Ulrich O. Kronthaler, Beate D. Kretschmer

Zoological Institute, Dept. Neuropharmacology, University of Tⁿbingen, F.R.G.

Introduction

The striatum and the nucleus accumbens represent the input structures of the basal ganglia, mediating motor and cognitive behaviour. Both structures receive converging glutamatergic afferences from cortical areas and dopaminergic afferences from the midbrain. Numerous experiments addressed the functional role of dopamine- and glutamate-receptor ligands, especially of those selective for the N-methyl-D-aspartate subtype (NMDA) of glutamate-receptors. Nevertheless the interaction of glutamate and dopamine in the basal ganglia is not completely understood. There is accumulating evidence that antagonists acting competitively or non- competitively at the NMDA-receptor-complex can induce different neurochemical (i.e. activation of dopaminergic neurons) and behavioural (i.e. motor stimulatory) effects. For this reason we examined whether this distinction is not only limited to the single application of competitive or non-competitive NMDA-receptor antagonists but may be expanded to the interaction with an indirect dopamine-receptor agonist, D,L-amphetamine. The aim of the experiment was to broaden behavioural by microdialysis studies. For this we monitored, in addition to the behavioural effects, the consequences of NMDA receptor blockade on the dopamine release induced by the dopamine- receptor agonist D,L amphetamine.

Materials and methods

Male Sprague Dawley rats were kept under standard laboratory conditions. Guide cannulae (Carnegie Medicine) were implanted under chloralhydrate anaesthesia into the striatum or nucleus accumbens. Coordinates relative to bregma were: AP + 1.7 mm, L + 2.5 mm, V - 4.5 mm and AP + 3.4 mm, L + 1.5 mm, V - 6.0 mm respectively. Following a minimum recovery period of three days CMA/12 dialysis probes were lowered into the striatum or the nucleus accumbens via the guide cannula, active membrane length was 3 mm or 2 mm. Probes were perfused by a CMA/100 with artificial CSF containing 147 mM NaCl, 2.5 mM KCl, 1.3 mM CaCl2 and 0.9 mM MgCl2 (pH 7.4) at a rate of 2.5 ╡l/min. Dialysate was collected and injected online by a CMA/160 into an HPLC-EC analysis system. Dopamine, DOPAC, HVA and 5-HIAA were separated by a Nucleosil 5-C18-column and detected by an ESA 5011 high sensitivity cell. Drugs were applicated 4-5 hours after probe insertion, when dopamine and its metabolites had reached a steady state level. For the behavioural analysis, animals were gently placed in an open-field for 5 min after the respective delay following drug or vehicle injections. The open-field (69 x 69 cm) was illuminated with four red bulbs (20 W) providing non-aversive conditions. Immediately following testing in the open-field, animals were gently placed in an experimental chamber. The behaviour was recorded on video tape and sniffing-stereotypy (number of snout-wall contacts in the experimental chamber) and locomotion (number of line crossings in the open-field) were manually analysed afterwards supported by a PC.

Results and discussion

The systemic application of the indirect dopamine-receptor agonist D,L-amphetamine (2.0 mg/kg i.p., n=6) induced a pronounced increase in the dopamine release and a pronounced decrease in the concentration of DOPAC and HVA in the striatum and nucleus accumbens. The effects of D,L-amphetamine (increase of dopamine- and decrease of DOPAC-concentration) were augmented by the joint application with the competitive NMDA-receptor antagonist CGP 37849 (8.0 mg/kg i.p., n=6) in the striatum (Fig. 1a; Data shown are MEAN + and - S.E.M.), but were reduced in the nucleus accumbens. The results from the striatal and accumbal microdialysis studies were only partially paralleled by the behavioural studies. The identical dosage of D,L-amphetamine (2.0 mg/kg, i.p.) induced an increased level of locomotor activity and sniffing stereotypy. These effects however were not changed by the additional application of a lower dosage CGP 37849 (4.0 mg/kg, i.p.) and the identical dosage as used in the dialysis experiment (Fig. 1b; C1 represents 4.0 mg/kg, C2 8.0 mg/kg; Data shown are MEAN + S.E.M.; n=10-12). Several additional behavioural studies were conducted combining motor behaviour stimulating dosages of two non-competitive NMDA- receptor antagonists with D,L amphetamine and other competitive NMDA-receptor antagonists. Furthermore the effects of a motor behaviour stimulating dosage of a competitive NMDA receptor antagonist in combination with D,L amphetamine were analysed.

Several studies show that a functionanatomical separation exists between the striatum, mediating primarily sniffing stereotypy, and the nucleus accumbens, mediating primarily locomotor behaviour1. From the results of the microdialysis studies, one may have expected this way an increased sniffing stereotypy and decreased locomotion by the joint application of D,L amphetamine and CGP 37849, which however was not observed. This expands results from Hutson et al.2 who reported the block of amphetamine-induced activation of dopaminergic neurons ba an antagonist acting at the glycine binding site of the NMDA-receptor complex. Further behavioural studies showed that single motor stimulating dosages of several non- competitive NMDA-receptor antagonists, at least tend to, augment effects of D,L amphetamine. Even a motor stimulating dosage of CPPene (competitive NMDA-receptor antagonist) however reduces locomotor activity induced by D,L-amphetamine, supporting the complexity of even simple motor behaviour. Nevertheless the behavioural and neurochemical data reported, were not derived from the identical animals due to methodological limitations. However a recent study3 deriving both data from the identical animals provides further evidence for failing to completely parallel these parameters. In conclusion, these results stress the necessity of carefully conducted behavioural studies before transfering the results of microdialysis studies to the behavioural level. References 1. Kretschmer B.D. and Schmidt W.J. (1996) Behavioral effects mediated by the modulatory glycine site of the NMDA receptor in the anterodorsal striatum and nucleus accumbens, J. Neurosci. 16(4), 1561-1569. 2. Hutson, P.H., Bristow, L.J., Thorn, L., and Tricklebank, M.D. (1994) R-(+)-HA-966, a glycine/NMDA receptor antagonist, selectively blocks the activation of mesolimbic dopamine system by amphetamine, Br. J. Pharmacol. 103(4), 2037-2044. 3. Bubser, M., Zadow, B., Kronthaler, U.O., Rⁿckert, N.G.H., and Schmidt, W.J. (1996) Behavioural pharmacology of the non-competitive NMDA antagonists dextrorphan and [*]-5-aminocarbonyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (ADCI): Relations between psychomotor stimulation, anticataleptic potential and forebrain dopamine metabolism. in prep.

Document created 24. July 1996

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