Extracellular signal-related kinase activation during natural reward learning: a physiological role for phasic nucleus accumbens dopamine?

Neuroplastic Changes in the Superior Colliculus and Hippocampus in Self-rewarding Paradigm: Importance of Visual Cues

Coincident excitation via different sensory modalities encoding objects of positive salience is known to facilitate learning and memory. With a view to dissect the contribution of visual cues in inducing adaptive neural changes, we monitored the lever press activity of a rat conditioned to self-administer sweet food pellets in the presence/absence of light cues. Application of light cues facilitated learning and consolidation of long-term memory. The superior colliculus (SC) of rats trained on light cue showed increased neuronal activity, dendritic branching, and brain-derived neurotrophic factor (BDNF) protein and mRNA expression. Concomitantly, the hippocampus showed augmented neurogenesis as well as BDNF protein and mRNA expression. While intra-SC administration of U0126 (inhibitor of ERK 1/2 and long-term memory) impaired memory formation, lidocaine (local anaesthetic) hindered memory recall. The light cue-dependent sweet food pellet self-administration was coupled with increased efflux of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens shell (AcbSh). In conditioned rats, pharmacological inhibition of glutamatergic signalling in dentate gyrus (DG) reduced lever press activity, as well as DA and DOPAC secretion in the AcbSh. We suggest that the neuroplastic changes in the SC and hippocampus might represent memory engrams sculpted by visual cues encoding reward information.

Molecular and cellular mechanisms of dopamine-mediated behavioral plasticity in the striatum

The striatum is the input structure of the basal ganglia system. By integrating glutamatergic signals from cortical and subcortical regions and dopaminergic signals from mesolimbic nuclei the striatum functions as an important neural substrate for procedural and motor learning as well as for reward-guided behaviors. In addition, striatal activity is significantly altered in pathological conditions in which either a loss of dopamine innervation (Parkinson's disease) or aberrant dopamine-mediated signaling (drug addiction and L-DOPA induced dyskinesia) occurs. Here we discuss cellular mechanisms of striatal synaptic plasticity and aspects of cell signaling underlying striatum-dependent behavior, with a major focus on the neuromodulatory action of the endocannabinoid system and on the role of the Ras-ERK cascade.

Stimulus-specific and differential distribution of activated extracellular signal-regulated kinase in the nucleus accumbens core and shell during Pavlovian-instrumental transfer

The ability of reward-predictive cues to potentiate reward-seeking behavior--a phenomenon termed Pavlovian--instrumental transfer (PIT)--depends on the activation of extracellular signal-regulated kinase (ERK) in the nucleus accumbens (NAc). Here, we utilized immunohistochemistry to investigate the subregional pattern of ERK activation during PIT, and the contribution of different elements in the PIT condition to the distribution of ERK signaling in the NAc of rats. We found that the occurrence of reward-seeking behavior (lever pressing) did not affect ERK activation in either the core or the shell of the NAc. In contrast, presentation of the reward-predictive cue (auditory conditioned stimulus) caused a significant increase in ERK activation in both subregions of the NAc, with the effect being slightly more robust in the core than the shell. Different from the pattern evoked by the reward-predictive cue, presentation of the reward itself (food pellets) had no effect on ERK activation in the core but caused a pronounced increase in ERK activation in the shell. Taken together, our results demonstrate that ERK signaling in the NAc during PIT involves both the core and the shell and is driven by the conditioned cue irrespective of whether the situation permits engagement in reward-seeking behavior. Furthermore, our results show that the subregional distribution of ERK signaling in the NAc evoked by rewards differs from that evoked by cues that predict them. The stimulus-specific differential pattern of ERK signaling described here may present the molecular complement to stimulus-specific increases in NAc cell firing reported previously.

Epigenetic mechanisms in cognition

Although the critical role for epigenetic mechanisms in development and cell differentiation has long been appreciated, recent evidence reveals that these mechanisms are also employed in postmitotic neurons as a means of consolidating and stabilizing cognitive-behavioral memories. In this review, we discuss evidence for an "epigenetic code" in the central nervous system that mediates synaptic plasticity, learning, and memory. We consider how specific epigenetic changes are regulated and may interact with each other during memory formation and how these changes manifest functionally at the cellular and circuit levels. We also describe a central role for mitogen-activated protein kinases in controlling chromatin signaling in plasticity and memory. Finally, we consider how aberrant epigenetic modifications may lead to cognitive disorders that affect learning and memory, and we review the therapeutic potential of epigenetic treatments for the amelioration of these conditions.