Anatoxin-α is a neurotoxic alkaloid produced by the cyanobacterium Anabaena flos-aquae (Carmichael et al. 1994). The effect of anatoxin-α on striatal dopamine release was investigated in conscious and freely moving rats using the microdialysis technique.
Animals were anaesthetized I.P. with choral hydrate (400 mg kg-1) and a microdialysis probe was stereotaxically implanted in the striatum. The day after surgery, a Ringer solution (147 mM NaCl, 4 mM KCl, 3.4 mM CaCl2, pH 7.4) was pumped through the microdialysis probe at a flow rate of 1.5 µl min-1. Dialysates were collected every 20 min, and after three basal samples anatoxin-α (1, 2, 3.5 and 7 mM) was infused over 20 min. The perfusate was then switched back to the unmodified perfusion medium and the infusion was carried out for an additional period of 120 min. Dopamine striatal levels were analysed by HPLC with electrochemical detection (Duran et al. 1998). The results are shown as the mean ± S.E.M. of five experiments expressed as a percentage of basal levels. Statistical analysis of the results was performed by means of repeated measures ANOVA and the Student-Newman-Keuls multiple range test. The experiments were performed according to the guidelines of the European Union Council (86/609/EU) for the use of laboratory animals. All animals were humanely killed.
Anatoxin-α caused a concentration-dependent increase in dopamine output in striatum. The infusion of 1 mM anatoxin-α induced no significant changes in striatal dopamine release The administration of 7 mM and 3.5 mM anatoxin-α produced a maximum increase in striatal dopamine levels, 40 min after the anatoxin-α perfusion (1461 ± 396 and 454 ± 165 % of basal levels, respectively). Basal values were restored 80 min after administration of anatoxin-α. Administration of 2 mM anatoxin-α increased the striatal dopamine levels to 202 ± 25 % 20 min after toxin administration and returned to basal values 20 min later. The increase in striatal dopamine levels was smaller when the flow rate was changed to 2 µl min-1 or when the time of infusion was reduced to 10 or 5 min (254 ± 30 and 132 ± 56, respectively).
Anatoxin-α has been found to act as a potent agonist at the neural nicotinic acetylcholine receptor (Thomas et al. 1993). It is well known that the striatal dopamine release is modulated by such cholinergic receptor agonists as nicotine (Calabresi et al. 2000). Several in vitro reports have indicated a stimulatory effect of anatoxin-α on the striatal dopamine release (Soliakov et al. 1995); however, no data of microdialysis studies on the effects of anatoxin-α on dopamine release have been published. The present results are the first in vivo evidence indicating that the local application of anatoxin-α by microdialysis produced an increase in striatal dopamine levels and they provide another piece of evidence that the striatal dopaminergic system may be under cholinergic modulation.
This study was supported by grants from the MCyT (Spain) in conjunction with FEDER (BQU2002-00083) and the Conselleria de Educacion e Ordenación Universitaria da Xunta a de Galicia (PGIDT02PXIB30101PR).