Post-training intra-amygdala amphetamine injections given during acquisition of a stimulus–response (S–R) habit task enhance the expression of stimulus–reward learning: Further evidence for incidental amygdala learning

Peripherally-administered amphetamine induces plasticity in medial prefrontal cortex and nucleus accumbens in rats with amygdala lesions: implications for neural models of memory modulation

The amygdala has been implicated in a variety of functions linked to emotions. One popular view is that the amygdala modulates consolidation in other brain systems thought to be mainly involved in learning and memory processes. This series of experiments represents a further exploration into the role of the amygdala in memory modulation and consolidation. One interesting line of research has shown that drugs of abuse, like amphetamine, produce dendritic changes in select brain regions and these changes are thought to be equivalent to a usurping of normal plasticity processes. We were interested in the possibility that this modulation of plasticity processes would be dependent on interactions with the amygdala. According to the modulation view of amygdala function, amphetamine would activate modulation mechanisms in the amygdala that would alter plasticity processes in other brain regions. If the amygdala was rendered dysfunctional, these effects should not occur. Accordingly, this series of experiments evaluated the effects of extensive neurotoxic amygdala damage on amphetamine-induced dendritic changes in the nucleus accumbens and prefrontal cortex. The results showed that rats with large lesions of the amygdala showed the normal pattern of dendritic changes in these brain regions. This pattern of results suggests that the action of not all memory modulators, activated during emotional events, require the amygdala to impact memory.

Exploring time-dependent changes in conditioned place preference for food reward and associated changes in the nucleus accumbens

The conditioned place preference (CPP) procedure has been used to study the incubation of craving phenomenon with rewarding drugs such as cocaine and methamphetamine. The present study examined whether rats trained in a CPP behavioral design would display an incubation of craving response for chocolate-flavored pellets or milk chocolate chips at the behavioral and neural levels. Rats were conditioned using an unbiased CPP design then underwent abstinence from food reward for 24 hs, 7, 14, or 28 days at which point they were tested for CPP. Brains underwent immunohistochemical staining for c-Fos and FosB as well as Golgi staining to assess dendritic spine density in the nucleus accumbens (NAc). A time-dependent increase in CPP and entries into the previously paired compartment was observed in the chocolate-flavored pellet group but not the milk chocolate group. Time-dependent neural changes were not directly associated with behavioral outcomes but c-Fos labelling was higher in the chocolate pellet group than controls at the 7-day abstinence period. The behavioral results show that chocolate pellets are rewarding and are associated with long-term behavioral changes but, as evidenced by limited neural changes, these food rewards do not have the same effects on the NAc as drugs of abuse.

Part III: Principal component analysis: bridging the gap between strain, sex and drug effects

Previous work has identified the adolescent period as particularly sensitive to the short- and long-term effects of marijuana and its main psychoactive component Δ9-tetrahydrocannabinol (THC). However, other studies have identified certain backgrounds as more sensitive than others, including the sex of the individual or the strain of the rat used. Further, the effects of THC may be specific to certain behavioural tasks (e.g. measures of anxiety), and the consequences of THC are not seen equally across all behavioural measures. Here, data obtained from adolescent male and female Long-Evans and Wistar rats exposed to THC and tested as adults, which, using standard ANOVA testing, showed strain- and sex-specific effects of THC, was analyzed using principal component analysis (PCA). PCA allowed for the examination of the relative contribution of our variables of interest to the variance in the data obtained from multiple behavioural tasks, including the skilled reaching task, the Morris water task, the discriminative fear-conditioning to context task, the elevated plus maze task and the conditioned place preference task to a low dose of amphetamine, as well as volumetric estimates of brain volumes and cfos activation. We observed that early life experience accounted for a large proportion of variance across data sets, although its relative contribution varied across tasks. Additionally, THC accounted for a very small proportion of the variance across all behavioural tasks. We demonstrate here that by using PCA, we were able to describe the main variables of interest and demonstrate that THC exposure had a negligible effect on the variance in the data set.

A complex associative structure formed in the mammalian brain during acquisition of a simple visual discrimination task: dorsolateral striatum, amygdala, and hippocampus

A review of empirical evidence supporting the multiple memory systems view of the organization of learning and memory in the mammalian brain is presented as a powerful component of a broader foundation of basic scientific information necessary for understanding human behavior. However, it is argued that there are significant gaps in our knowledge about these different learning and memory systems, how they interact with one another, and how they interact with the rest of the brain. To demonstrate how little we know about these complex processes, this article reviews recent evidence showing the complexity of associative structure formed during the acquisition of a simple visual discrimination task. The results show that the dorsolateral striatum is necessary for the acquisition of this task but that both the amygdala and hippocampus incidentally acquire and store information during this training period. A new experiment is also presented showing that rats with complete or partial (dorsal vs. ventral) hippocampal lesions show a retrograde amnesic effect on the simple visual discrimination task despite the fact that these same lesions produce no impairment in the anterograde direction. Evidence is presented in support of one interpretation of this effect suggesting that the retrograde amnesia occurs, at least in part, because the hippocampus acquires a context-specific inhibitory association during original training. Although this representation is not required for acquisition of the task in the anterograde direction, removal of this representation has a disruptive effect on expression of the task.

Empirical tests of the functional significance of amygdala-based modulation of hippocampal representations: evidence for multiple memory consolidation pathways

This series of experiments evaluated the effects of amygdala damage on the acquisition and long-term retention of variants of the water task, and tested the hypothesis that the amygdala is an essential neural system for consolidation of hippocampal memories. In Experiment 1, rats with large, neurotoxic lesions of the amygdala (AMYG) showed normal acquisition on the standard spatial version of the water task, as well as normal retention and decay rate profiles on the 24-h and 30-day retention probes. In Experiment 2, AMYG rats showed normal one-trial place learning abilities and could retain this one-trial information over a 24 h delay. Experiment 3 showed that the amygdala lesions used in this study were functionally significant because AMYG rats, from Experiment 2, showed impairments in a discriminative fear conditioning to context paradigm. Experiment 4 was a critical test of the idea that the amygdala is a decisive locus for consolidation of hippocampal memories. AMYG rats were trained to sub-asymptotic levels of performance on the standard version of the water task. Following each training session, the subjects were given a post-training peripheral injection of D-amphetamine. A probe test revealed that normal subjects and AMYG rats showed similar post-training memory improvement effects. Taken together, the results show that hippocampal memory consolidation processes do not require amygdala modulation. Arguments for an alternative view are presented suggesting that there are multiple memory consolidation pathways, one of which may depend on amygdala neural circuitry.