Transient benzodiazepine-GABAA receptor increase after a passive avoidance learning in synaptosomal membranes from chick forebrain.

One-day-old chicks were exposed to a one-time passive avoidance learning task. After chicks peak a bead dipped in a bitter-tasting liquid, they learn to stop pecking the bead. Radioligand binding analysis of [3H]flunitrazepam was performed on crude synaptosomal membranes from forebrains, at 10, 30, and 60 min post-training. Water-trained chicks (control) pecked a bead dipped in water, and they did not learn to stop pecking the bead. The water control was complemented with a methyl anthranilate fed control chick to demonstrate that taste per se does not affect the [3H]flunitrazepam binding. At 30 min in relation to 10 min post-training, the Bmax increased 31% in water-trained chicks and 56% in taste-trained chicks, with Bmax of the taste-trained chicks reaching a value 22% higher than that in water-trained chicks. The difference, attributable to the learning, disappeared at 60 min post-training, and at all times the affinity remained unchanged. The Bmax increase in water-trained chicks might be attributable to psychological stress accompanying the task and the Bmax increase in taste-trained chicks attributable to the learning in addition to the stress accompanying the task. The results suggest that the receptor increase associated with learning is involved in early stages of memory formation.