Differential effect of posttraining naloxone, β-endorphin, leu-enkephalin and electroconvulsive shock administration upon memory of an open-field habituation and of a water-finding task

Abstract

Rats were trained and tested in an open field for habituation of rearing responses, for a waterfinding task, or for both tasks simultaneously. Training-test interval was 24 hr. The water-finding task consisted of locating a metal tube in one of the walls of the box, which was attached to a water bottle on the outside; animals were water deprived between training and testing. Retention was estimated by measuring the latency to lick from the tube on the test session. Rats learned this task either with or without water deprivation, also prior to training. Habituation learning (reduction of the number of rearings between the training and test session) occurred either simultaneously with the water-finding task or in animals trained without the water tube, so that they could not learn the water-finding task. As happens with many other tasks, training in the open field was followed by a large decrease of hypothalamic β-endorphin immunoreactivity, attributable to a release of this substance. Posttraining IP naloxone (1.6 mg/kg) administration facilitated, and posttraining β-endorphin (2.0 μg/kg), leu-enkephalin (5.0 μg/kg), or electroconvulsive shock (15 mA, 60 Hz, 2 sec) depressed the retention of habituation; this occurred regardless of whether the animals were trained and/or tested with or without water deprivation, and whether the task was acquired alone or simultaneously with the water-finding task. By contrast, none of these treatments had any effect on retention of the water finding task, acquired either with or without prior water deprivation. Thus, habituation was, and water-finding was not, sensitive to posttraining treatments known to affect endogenous opioids: the opioids themselves, their antagonist, naloxone, and electroconvulsive shock which releases brain opioids and causes naloxone-reversible retrograde amnesia. Learning of the water-finding task was merely incidental to exploration of the open field; it took place even when the animals were trained without the water tube. This suggests that the posttraining treatments that affect endogenous opioid function affect memory only of the task(s) that actually cause the release of brain β-endorphin (in this case, probably habituation), and not of others that may occur simultaneously but are merely incidental (water-finding). A feature apparently common to the former is that they must directly involve either the recognition of novelty, or the initiation of an interaction with a new environment, or perhaps the habituation of such interaction.