Neurotransmitter systems involved in learning and memory in the rat: a meta-analysis based on studies of four behavioral tasks.

From previous literature, it appears that most classical neurotransmitter systems can in some way influence learning and memory in the rat. A matter of crucial interest is, however, whether the chemical systems contribute in a similar manner or whether they have different abilities to support cognitive processes. The purpose of the present study was to investigate this issue. The investigation was carried out by reviewing relevant studies of neurochemistry and cognition. Inclusion criteria were set for selection of behavioral tasks to be elucidated and for studies employing acceptable tasks. Morris water maze, radial maze, passive avoidance, and spontaneous alternation met the criteria for inclusion, and a table for each of these tests summarizes the neurochemical results of the studies accepted for inclusion. In this way, a reliable comparability of results from relevant studies was obtained. The comparisons revealed that for both systemic and targeted infusions of agents the neurochemical systems possess different abilities to influence learning and memory. Calculation of impact factors (percentage of significant effects of chemical agents like agonists, antagonists, neurotoxins) showed that glutamate was ranking highest (93), followed by GABA (81), dopamine (81), acetylcholine (81), serotonin (55), and norepinephrine (48). No task specific roles were observed for the transmitter systems. The highest sensitivity (percentage of significant effects) to interference with neurochemical systems was found for the spontaneous alternation task (86), followed by water maze (76), passive avoidance (72), and radial maze (58). The multiple memory systems in the rat brain can hardly be related to specific transmitter systems, because of the great extent of interactions between the systems.