Neonatal odor-shock conditioning alters the neural network involved in odor fear learning at adulthood

Abstract

Adult learning and memory functions are strongly dependent on neonatal experiences. We recently showed that neonatal odor-shock learning attenuates later life odor fear conditioning and amygdala activity. In the present work we investigated whether changes observed in adults can also be observed in other structures normally involved, namely olfactory cortical areas. For this, pups were trained daily from postnatal (PN) 8 to 12 in an odor-shock paradigm, and retrained at adulthood in the same task. (14)C 2-DG autoradiographic brain mapping was used to measure training-related activation in amygdala cortical nucleus (CoA), anterior (aPCx), and posterior (pPCx) piriform cortex. In addition, field potentials induced in the three sites in response to paired-pulse stimulation of the olfactory bulb were recorded in order to assess short-term inhibition and facilitation in these structures. Attenuated adult fear learning was accompanied by a deficit in 2-DG activation in CoA and pPCx. Moreover, electrophysiological recordings revealed that, in these sites, the level of inhibition was lower than in control animals. These data indicate that early life odor-shock learning produces changes throughout structures of the adult learning circuit that are independent, at least in part, from those involved in infant learning. Moreover, these enduring effects were influenced by the contingency of the infant experience since paired odor-shock produced greater disruption of adult learning and its supporting neural pathway than unpaired presentations. These results suggest that some enduring effects of early life experience are potentiated by contingency and extend beyond brain areas involved in infant learning.

Figure 1.

Behavioral performances in infant and adult animals during CS retention test carried out the day after odor-shock conditioning. (A) Infant learning: mean number of choices toward CS odor at PN13. (B) Adult learning: mean percentage (±SEM) of adult freezing behavior before and during CS odor presentation. (*) Significant intragroup difference with the pre-odor period (P < 0.05). (#) Significant intergroup difference (P < 0.05).

Figure 2.

Mean 2-DG uptake (±SEM) relative to corpus callosum during adult fear conditioning in the different experimental groups, in three structures: anterior piriform cortex (aPCx), posterior piriform cortex (pPCx), and cortical nucleus of the amygdala (CoA). (*) Significant intergroup differences (P < 0.05).

Figure 3.

Mean paired-pulse ratios (mean test signal amplitude/mean conditioning signal amplitude ± SEM) obtained in the different experimental groups, in four structures: anterior (aPCx) and posterior (pPCx) piriform cortex, cortical nucleus of the amygdala (CoA), and olfactory bulb (OB). Paired-pulse stimulations were delivered using different interpulse interval durations (20–120 msec). A ratio <1 characterizes paired-pulse inhibition, whereas a ratio >1 corresponds to paired-pulse facilitation. (*) Significant difference with all the other groups (P < 0.05). (#) Significant differences with all but Infant Naive/Adult Naive groups (P < 0.05).

Figure 4.

Schematic representation of the implanted electrodes and recorded evoked field potential (EFPs) signals. (A) A bipolar stimulation electrode was inserted in the mitral cell layer of the olfactory bulb (OB) and in the lateral olfactory tract (LOT) for a subgroup of animals. Three monopolar recording electrodes were, respectively, implanted in the anterior piriform cortex (aPCx), posterior piriform cortex (pPCx), and cortical nucleus of the amygdala (CoA). Moreover, the bipolar electrode implanted in the OB also served as a recording electrode during stimulation of the LOT. (B) An example of EFPs induced in the four recording sites in response to paired-pulse stimulation of the OB (aPCx, pPCx, CoA) or the LOT (OB) using a 20-msec interpulse interval. Paired-pulse ratio was defined as the ratio of mean test signal amplitude (A₂, second pulse) over mean conditioning signal (A₁, first pulse). A decrease in the size of the response to the second pulse suggests inhibition and is reflected in a ratio <1.