Acquisition and expression of conditioned taste aversion differentially affects extracellular signal regulated kinase and glutamate receptor phosphorylation in rat prefrontal cortex and nucleus accumbens.

Roberto Marotta,Sandro Fenu,Elio Acquas,Simona Scheggi,Michela Rosas,Stefania Vinci,Carla Gambarana,Andrea Falqui,M Graziella De Montis

doi:10.3389/fnbeh.2014.00153

Abstract

Conditioned taste aversion (CTA) can be applied to study associative learning and its relevant underpinning molecular mechanisms in discrete brain regions. The present study examined, by immunohistochemistry and immunocytochemistry, the effects of acquisition and expression of lithium-induced CTA on activated Extracellular signal Regulated Kinase (p-ERK) in the prefrontal cortex (PFCx) and nucleus accumbens (Acb) of male Sprague-Dawley rats. The study also examined, by immunoblotting, whether acquisition and expression of lithium-induced CTA resulted in modified levels of phosphorylation of glutamate receptor subunits (NR1 and GluR1) and Thr34- and Thr75-Dopamine-and-cAMP-Regulated PhosphoProtein (DARPP-32). CTA acquisition was associated with an increase of p-ERK-positive neurons and phosphorylated NR1 receptor subunit (p-NR1) in the PFCx, whereas p-GluR1, p-Thr34- and p-Thr75-DARPP-32 levels were not changed in this brain region. CTA expression increased the number of p-ERK-positive neurons in the shell (AcbSh) and core (AcbC) but left unmodified p-NR1, p-GluR1, p-Thr34- and p-Thr75-DARPP-32 levels. Furthermore, post-embedding immunogold quantitative analysis in AcbSh revealed that CTA expression significantly increased nuclear p-ERK immunostaining as well as p-ERK-labeled axo-spinous contacts. Overall, these results indicate that ERK and NR1, but not GluR1 and DARPP-32, are differentially phosphorylated as a consequence of acquisition and expression of aversive associative learning. Moreover, these results confirm that CTA represents an useful approach to study the molecular basis of associative learning in rats and suggest the involvement of ERK cascade in learning-associated synaptic plasticity.

Highlights

Conditioned taste aversion (CTA) is a rapid to establish, robust, and long-lasting experimental model of associative learning based on animals’ ability to avoid a taste that has been previously associated with visceral malaise (Garcia et al, 1955; Scott, 2011)
Least significant differences (LSD) post-hoc analysis revealed that CS+-US+ resulted in a significant reduction of sucrose intake compared to CS+-US− group
Acquisition of lithium-induced CTA resulted in increased phosphorylation of extracellular signal regulated kinase (ERK) and NR1 in the prefrontal cortex (PFCx) while, expression of lithium-induced CTA disclosed an increase of ERK phosphorylation in AcbSh and AcbC

Summary

Introduction

Conditioned taste aversion (CTA) is a rapid to establish, robust, and long-lasting experimental model of associative learning based on animals’ ability to avoid a taste (conditioned stimulus, CS) that has been previously associated with visceral malaise (unconditioned stimulus, US) (Garcia et al, 1955; Scott, 2011). CTA has offered, over the years, a valuable tool to investigate, in rodents, the neurotransmitter receptors (Fenu et al, 2001, 2005, 2009; Fenu and Di Chiara, 2003; Akirav, 2007; Barki-Harrington et al, 2009) involved in aversive associative learning, its underpinning molecular mechanisms (Berman et al, 1998; Yasoshima et al, 2006; Bernstein and Koh, 2007; Kwon and Houpt, 2012) as well as its anatomical substrates (Dunn and Everitt, 1988; Yamamoto et al, 1995; Berman et al, 1998; Ferreira et al, 2006; Ramírez-Lugo et al, 2007; Elkobi et al, 2008; Kwon and Houpt, 2012). The role of glutamate transmission in the facilitation of the enduring changes underlying memory trace formation is supported by studies with NMDA receptor blockade (Rosenblum et al, 1997) and with NMDA receptor knock-out mice (Cui et al, 2005)

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