ROLE OF GABA DURING THE MULTIPLE CONSOLIDATION OF MEMORY

Alzheimer's disease (AD) is the most common form of senile dementia and it is a neurodegenerative disorder associated with a progressive deterioration of memory and cognitive capacity. Although one of the most characteristic abnormalities in AD patients is the reduced cholinergic input to the cortex, AD is a disorder that affects different neuronal populations in the brain, including the GABAergic neurons. The data regarding the participation of the central GABAergic system on memory indicate that: (1) drugs that facilitate GABA-A and GABA-B neurotransmission impair memory in experimental animals and in humans; (2) drugs that reduce GABA-A neurotransmission facilitate memory in rodents; (3) the facilitatory effect of these drugs has not been corroborated at the clinical level, as they show a small therapeutic window, but new drugs are presently being evaluated in clinical studies; (4) the cognitive effects of the GABAergic agents are dose- and time-related, and cannot be explained by state-dependency; (5) the effects of the GABAergic antagonists are centrally mediated, as peripherally acting drugs are ineffective in memory tests; (6) GABA and endogenous benzodiazepines are released in different brain areas during learning of different tasks and after the induction of long-term potentiation (LTP); (7) GABA-A antagonists facilitate LTP while diazepam blocks LTP in hippocampal slices; and (8) the amygdala, the basal forebrain, the septo-hippocampal pathway, the trisynaptic circuit and the entorhinal cortex are likely candidate regions for the central actions of GABAergic drugs. The consolidation process of memory storage can be presently envisioned as a multiple consolidation process that takes place in different brain circuits and at different times after the learning experience. The anatomical evidence on the presence of GABAergic neurons in brain areas relevant to memory (and affected in Alzheimer's patients) like the cortex, amygdala, septum, hippocampus and NBM, together with the electrophysiological and biochemical changes induced by the learning experience, suggest that the GABAergic neurons can critically modulate the electrical activity of these brain areas during the ''multiple consolidation'' process of memory storage.