Paradoxical facilitatory effect of fornix lesions on acquisition of contextual fear conditioning in mice

Dorsal striatum and the temporal expectancy of an aversive event in Pavlovian odor fear learning

Interval timing, the ability to encode and retrieve the memory of intervals from seconds to minutes, guides fundamental animal behaviors across the phylogenetic tree. In Pavlovian fear conditioning, an initially neutral stimulus (conditioned stimulus, CS) predicts the arrival of an aversive unconditioned stimulus (US, generally a mild foot-shock) at a fixed time interval. Although some studies showed that temporal relations between CS and US events are learned from the outset of conditioning, the question of the memory of time and its underlying neural network in fear conditioning is still poorly understood. The aim of the present study was to investigate the role of the dorsal striatum in timing intervals in odor fear conditioning in male rats. To assess the animal's interval timing ability in this paradigm, we used the respiratory frequency. This enabled us to detect the emergence of temporal patterns related to the odor-shock time interval from the early stage of learning, confirming that rats are able to encode the odor-shock time interval after few training trials. We carried out reversible inactivation of the dorsal striatum before the acquisition session and before a shift in the learned time interval, and measured the effects of this treatment on the temporal pattern of the respiratory rate. In addition, using intracerebral microdialysis, we monitored extracellular dopamine level in the dorsal striatum throughout odor-shock conditioning and in response to a shift of the odor-shock time interval. Contrary to our initial predictions based on the existing literature on interval timing, we found evidence suggesting that transient inactivation of the dorsal striatum may favor a more precocious buildup of the respiratory frequency's temporal pattern during the odor-shock interval in a manner that reflected the duration of the interval. Our data further suggest that the conditioning and the learning of a novel time interval were associated with a decrease in dopamine level in the dorsal striatum, but not in the nucleus accumbens. These findings prompt a reassessment of the role of the striatum and striatal dopamine in interval timing, at least when considering Pavlovian aversive conditioning.

Differential involvement of amygdala and cortical NMDA receptors activation upon encoding in odor fear memory

Although the basolateral amygdala (BLA) plays a crucial role for the acquisition of fear memories, sensory cortices are involved in their long-term storage in rats. However, the time course of their respective involvement has received little investigation. Here we assessed the role of the glutamatergic N-methyl-d-aspartate (NMDA) receptors in the BLA and olfactory cortex at discrete moments of an odor fear conditioning session. We showed that NMDA receptors in BLA are critically involved in odor fear acquisition during the first association but not during the next ones. In the cortex, NMDA receptor activation at encoding is not necessary for recent odor fear memory while its role in remote memory storage needs further investigation.

Long-term consequences of soman poisoning in mice: part 2. Emotional behavior

The organophosphorus compound soman produces long-lasting epileptic seizure activity which is associated to brain damage, more particularly in the hippocampus and the amygdala. The companion paper (see part 1 in the same journal issue) describes the neuropathology in the amygdala of soman-poisoned mice. The present paper examines the long-term effects of soman poisoning on emotional reactivity in mice, 30 or 90 days after intoxication using behavioral tasks involving amygdala function. The emotional behavior was estimated in animal tests of unconditioned fear (light/dark boxes, elevated plus-maze) and conditioned fear (auditory and contextual response). In the light/dark boxes and elevated plus-maze, mice intoxicated with soman (110 microg/kg, 1.2 LD(50)) showed an anxiety-like behavior profile at post-poisoning days 30 and 90. In conditioned fear, results showed that both auditory and contextual conditioned responses are increased on post-soman day 30 but no longer on post-soman day 90, evidencing behavioral recovery overtime. This latter behavioral result is in accordance with the delayed neuronal regeneration patterns described in the companion paper (part 1).

Automated analysis of foot-shock sensitivity and concurrent freezing behavior in mice

Foot-shock is used in a variety of behavioral tasks either as a motivational stimulus, a way to characterize different rodents, or to test various pharmacological agents for their antinociceptive or analgesic effects. All these procedures need to assess foot-shock sensitivity either to rule out possible differences (when the shock is used as a motivational stimulus) or to use the differences to compare animals or treatments. In addition, many of the procedures that utilize foot-shock as a motivational stimulus evaluate freezing as an index of anxiety or fear. In the present study, data obtained by an automated computer system was compared with data obtained by human observers to validate the automated system for examining foot-shock sensitivity in mice. The different computer measures obtained for foot-shock sensitivity exhibited high correlations with human scoring at shock levels as low as 0.2 mA. The computer controlled analysis provided a non-subjective, quantifiable measurement of the foot-shock-induced response as well as freezing behavior. Automated data collection is an improvement over the methods of human visual observation in that the data collection is more standardized, efficient and consistent.