Enhancement of passive avoidance learning through small doses of intra‐amygdaloid physostigmine in the young rat. Its relation to the development of acetylcholinesterase

Abstract

Passive avoidance learning was studied in young rats 7-20 days old, in control conditions and after bilateral injections of physostigmine into the lateral amygdaloid nucleus. Acquisition in controls was possible from postnatal Day 8 on, progressed markedly after Day 11, and nearly reached maturity by Day 20. Physostigmine differentially altered acquisition depending on the dose: facilitation with low doses, no effect with moderate doses, and impairment with high doses. Enhanced learning through small doses of physostigmine was observed at all ages from Day 8 on, and was greater with 0.2 microgram than with 0.1 microgram. Maturation of the cholinergic innervation of the amygdaloid region was also studied between Days 9-20 using acetylcholine-esterase histochemistry. The results suggest that passive avoidance learning is dependent on amygdaloid cholinergic mechanisms early in life. In addition, very immature cholinergic systems, which are known to be uninfluenced by anticholinergic agents, react to anticholinesterases.

Cholinergic effects on fear conditioning I: the degraded contingency effect is disrupted by atropine but reinstated by physostigmine

“…Cholinergic drugs have been regularly shown to produce inverted U-shaped response curves during learning processes in normal subjects: low-to-moderate doses of a cholinergic agonist may improve performance, but high doses are ineffective (e.g., Dumery et al 1988;Myers et al 1996). Atropine is a nonspecific muscarinic antagonist (Caulfield and Birdsall 1998) and therefore inhibits at least both postsynaptic muscarinic receptors and inhibitory autoreceptors whose activation decreases acetylcholine release.…”

The Pharmacology of Physostigmine

“…The effects of physostigmine have been examined experimentally in a variety of memory paradigms, passive, active, and those induced by cholinergic deficits and lesions. These effects have been reviewed (81,184).…”

Early inhibitory learning in the rat I. Learning and memory development

“…The task and criteria we chose obviously meet these prerequisites, whereas those of the French laboratories (Blozovski & Cuddenec, 1980;DumCry, Derer, & Blozovski, 1988), which require the severe criterion of 300 sec in the safe part of the apparatus, do not. The latter procedures require that the animal be exposed for up to 5 min to an air flow of 23°C.…”

“…Recently a paper was published from the same laboratory showing a possibility of PA learning at postnatal day 8 and pointing to cholinergic mechanisms as well, but alleging that this is the day in the ontogeny when PA learning first appears (DumCry, Derer, & Blozovski, 1988).…”