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The present experiments determined the consequences of blocking muscarinic cholinergic receptors of the prelimbic (PL) cortex in the acquisition and retention of an odor-reward associative task. Rats underwent a training test (five trials) and a 24-h retention test (two retention trials and two relearning trials). In the first experiment, rats were bilaterally infused with scopolamine (20 or 5 microg/site) prior to training. Although scopolamine rats showed acquisition equivalent to PBS-injected controls, they exhibited weakened performance in the 24-h retention test measured by number of errors. In the second experiment, rats were injected with scopolamine (20 microg/site) immediately or 1 h after training and tested 24 h later. Scopolamine rats injected immediately showed severe amnesia detected in two performance measures (errors and latencies), demonstrating deficits in retention and relearning, whereas those injected 1 h later showed good 24-h test performance, similar to controls. These results suggest that muscarinic transmission in the PL cortex is essential for early memory formation, but not for acquisition, of a rapidly learned odor discrimination task. Findings corroborate the role of acetylcholine in consolidation processes and the participation of muscarinic receptors in olfactory associative tasks.

A rapidly learned odor discrimination task based on spontaneous foraging behavior of the rat was used to evaluate the role of N-methyl-D-aspartate (NMDA) receptors (NMDARs) in ongoing memory consolidation. Rats were trained in a single session to discriminate among three odors, one of which was associated with palatable food reward. Previous experiments showed that the NMDAR antagonist DL-APV induced amnesia for this task when injected immediately after training. In the present study, memory was reactivated 24 h after training by exposure to the rewarded odor within the experimental context after which rats received an intracerebroventricular injection of APV. Combined reactivation-drug treatment induced profound amnesia when tested 48 h later. Animals receiving drug alone, in absence of reactivation, showed perfect retention. It is concluded that NMDARs support a consolidation process taking place after memory reactivation.

Abstract: Rats received bilateral stria terminalis (ST) lesions or were sham-operated. Five days later, the animals were trained in a two-way active avoidance task (one session, 30 trials) and, immediately after the training session, received 0.01 mg/kg i.p. epinephrine or distilled water. Retention was tested 20 days after the acquisition session. In sham-operated groups, epinephrine improved retention in rats that were poor learners and impaired it in rats that were good learners. In poor learners with posttraining epinephrine, lesions of the ST not only blocked the facilitatory effect of epinephrine but also disrupted performance throughout the retention session. In good learners, ST lesions attenuated the disruptive effect of epinephrine. Lesions per se did not affect either acquisition or retention. We conclude that ST is involved in the modulatory effect of posttraining epinephrine on memory consolidation. In addition and considering the results observed in rats that were poor learners, we suggest that emotional factors and/or other amygdaloid pathways different from the ST could participate in the effects of posttraining epinephrine, along with the ST.

A 28-year-old otherwise healthy man presented twice with a peroneal nerve palsy, first on the left and 4 months later on the right side. Both palsies occurred after sitting on a chair for a few hours. Personal and family history were unremarkable. Electrophysiologic examinations revealed conduction blocks of the peroneal nerves slightly above the fibular head and signs of acute denervation in the anterior tibial muscles. Bilateral fabellas (sesamoid bones) within the tendon of the lateral head of gastrocnemius muscles detected by ultrasound and MRI (figure) caused these unusual pressure palsies.1Surgical excision of the fabellae resulted in full recovery.