Combined lesions of cholinergic and serotonergic neurons in the rat brain using 192 IgG-saporin and 5,7-dihydroxytryptamine: neurochemical and behavioural characterization

This study assessed behavioural and neurochemical effects of i.c.v. injections of both the cholinergic toxin 192 IgG-saporin (2 microgram) and the serotonergic toxin 5,7-dihydroxytryptamine (5,7-DHT; 150 microgram) in Long-Evans female rats. Dependent behavioural variables were locomotor activity, forced T-maze alternation, beam walking, Morris water-maze (working and reference memory) and radial-maze performances. After killing by microwave irradiation, the concentrations of acetylcholine, monoamines and 5-hydroxyindoleacetic acid (5-HIAA) were measured in the hippocampus, frontoparietal cortex and striatum. 192 IgG-saporin reduced the concentration of acetylcholine by approximately 40% in the frontoparietal cortex and hippocampus, but had no effect in the striatum. 5,7-DHT lesions reduced the concentration of serotonin by 60% in the frontoparietal cortex and 80% in the hippocampus and striatum. Noradrenaline was unchanged in all structures except the ventral hippocampus where it was slightly increased in rats given 192 IgG-saporin. Cholinergic lesions induced severe motor deficits but had no other effect. Serotonergic lesions produced diurnal and nocturnal hyperactivity but had no other effect. Rats with combined lesions were more active than those with only serotonergic lesions, showed motor dysfunctions similar to those found in rats with cholinergic lesions alone, and exhibited impaired performances in the T-maze alternation test, the water-maze working memory test and the radial-maze. Taken together and although cholinergic lesions were not maximal, these data show that 192 IgG-saporin and 5,7-DHT lesions can be combined to selectively damage cholinergic and serotonergic neurons, and confirm that cholinergic-serotonergic interactions play an important role in some aspects of memory, particularly in spatial working memory.