Differential dynamics of amino acid release in the amygdala and olfactory cortex during odor fear acquisition as revealed with simultaneous high temporal resolution microdialysis

Abstract

Although the amygdala seems to be essential to the formation and storage of fear memories, it might store only some aspects of the aversive event and facilitate the storage of more specific sensory aspects in cortical areas. We addressed the time course of amygdala and cortical activation in the context of odor fear conditioning in rats. Using high temporal resolution (1-min sampling) intracerebral microdialysis, we investigated the dynamics of glutamate and GABA fluctuations simultaneously in basolateral amygdala (BLA) and posterior piriform cortex (pPCx) during the course of the acquisition session, which consisted of six odor (conditioned stimulus)-footshock (unconditioned stimulus) pairings. In BLA, we observed a transient increase in amino acid concentrations following the first odor-shock pairing, after which concentrations returned to baseline levels or slightly below. In pPCx, transient increases were seen after each pairing and were also observed after the last odor-shock pairing, corresponding to the predicted times of anticipated trials. Furthermore, we observed that for the first pairing, the increase in BLA occurred earlier than the increase in pPCx. These data suggest that the amygdala is engaged early during acquisition and precedes the activation of the olfactory cortex, which is maintained until the end of the session. In addition, our data raise the challenging idea that the olfactory cortex might store certain aspects of fear conditioning related to the timing of the associations.

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

“…Here we show that the animals' performances were lower with one pairing than with six and were unaffected by AP5 treatment. Taken together these data suggest that after the first pairing, NMDA receptors are no more involved in the processing of the next pairings or in consolidation processes, comforting our previous data showing an increase in glutamate release in BLA for the first pairing but not for the next ones (Hegoburu et al 2009). This observation is in line with previous studies showing that intra amygdala injections of NMDA antagonists carried out post-training (Maren et al 1996;Walker and Davis 2002) or before testing (Rodrigues et al 2001;Walker et al 2005 in odor fear conditioning) have no effect on consolidation and expression of auditory fear conditioning.…”

“…In a recent study investigating hippocampal -cortical interactions, Lesburguères et al (2011) showed that neurons in the rat cortex undergo a "tagging process" upon encoding to ensure the progressive rewiring of cortical networks that support remote memory storage. We reasoned that the same phenomenon might occur in amygdala-dependent memories and that the amygdala and cortical networks might be involved together at the time of encoding in order for fear memories to be stored in the long term.In a previous study using odor fear conditioning, we compared the dynamics of glutamate fluctuations between basolateral amygdala (BLA) and olfactory cortex (posterior piriform cortex: pPC) during the acquisition session (Hegoburu et al 2009). In the BLA, we observed a transient increase in glutamate in response to the first odor-shock pairing only, whereas in the pPC increases in glutamate were seen after each pairing of the acquisition session.…”

“…In the other six, the injection cannula was implanted in the BLA, while the microdialysis probe was implanted in the ipsilateral pPC (AP 21.8 mm; ML +5.5 mm; DV 28.2 mm from dura). The microdialysis sampling procedure has been described previously (Bert et al 2002;Hegoburu et al 2009). Briefly, sampling (1 mL/ min) started 3 h after the probe/injection cannula implantation.…”

“…Injection needles (30 gauge, extending 2 mm from the tip of the guide cannula) connected to a microinfusion pump (Harvard Apparatus) allowed the infusion of AP5 (Sigma-Aldrich; 2.5 mg/0.5 mL, 0.5 mL per hemisphere). After 1 wk of recovery from the surgery, the animals were trained in an olfactory fear conditioning paradigm (Sevelinges et al 2004;Hegoburu et al 2009). Briefly, the animal received six pairings of 20 sec peppermint odor coterminating with a 1-sec footshock (0.4 mA) using a 4-min intertrial interval (total session duration: 28 min).…”

“…Then the first infusion of the glutamate uptake inhibitor PDC (L-trans-pyrrolidine-2,4-dicarboxylic acid, 0.3 mL at 2 mL/min; Tocris Bioscience) was delivered followed by a second infusion 8 min later. The samples were then analyzed for glutamate content using capillary electrophoresis coupled to a laser-induced fluorescence detector as previously described (Hegoburu et al 2009). Figure 2A illustrates the placement of the microdialysis probes in the BLA and pPC, as well as the PDC infusion sites in BLA.…”