Disruption of Long-Term Depression Potentiates Latent Inhibition: Key Role for Central Nucleus of the Amygdala

Abstract

Background: Latent inhibition (LI) reflects an adaptive form of learning impaired in certain forms of mental illness. Glutamate receptor activity is linked to LI, but the potential role of synaptic plasticity remains unspecified.

Methods: Accordingly, the present study examined the possible role of long-term depression (LTD) in LI induced by prior exposure of rats to an auditory stimulus used subsequently as a conditional stimulus to signal a pending footshock. We employed 2 mechanistically distinct LTD inhibitors, the Tat-GluA23Y peptide that blocks endocytosis of the GluA2-containing glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, or the selective glutamate n-methyl-d-aspartate receptor 2B antagonist, Ro25-6981, administered prior to the acquisition of 2-way conditioned avoidance with or without tone pre-exposure.

Results: Systemic LTD blockade with the Tat-GluA23Y peptide strengthened the LI effect by further impairing acquisition of conditioned avoidance in conditional stimulus-preexposed rats compared with normal conditioning in non-preexposed controls. Systemic Ro25-6981 had no significant effects. Brain region-specific microinjections of the Tat-GluA23Y peptide into the nucleus accumbens, medial prefrontal cortex, or central or basolateral amygdala demonstrated that disruption of glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor endocytosis in the central amygdala also potentiated the LI effect.

Conclusions: These data revealed a previously unknown role for central amygdala LTD in LI as a key mediator of cognitive flexibility required to respond to previously irrelevant stimuli that acquire significance through reinforcement. The findings may have relevance both for our mechanistic understanding of LI and its alteration in disease states such as schizophrenia, while further elucidating the role of LTD in learning and memory.

Keywords: Synaptic plasticity; central amygdala (CeA); cognitive flexibility; latent inhibition (LI); long-term depression (LTD).

Figure 1.

Latent inhibition (LI) of 2-way avoidance. (A) Strong LI was observed in our paradigm, as pre-exposure (PE, filled squares) to the CS significantly impaired acquisition of avoidance responding relative to non-preexposed (NPE, empty circles) rats, as observed in 10 trial bins. Successful avoidance responses involved shuttling to the opposite side of the avoidance chamber during the 10-second tone presentation, which predicted a 2-second footshock. (B) Overall avoidance percentage across 100 trials was significantly lower in PE vs NPE rats, reflecting LI. (C) Escape responses declined progressively over trials, with fewer escape responses in NPE vs PE rats. (D) Overall escape responses percentage across 100 trials was significantly lower in PE vs NPE rats. *P < .05, error bars represent SEM.

Figure 2.

Systemic Tat-GluA23Y potentiates latent inhibition (LI) of 2-way avoidance. (A) The AMPA receptor endocytosis inhibitor Tat-GluA23Y (2.25 nmol/g, i.v., black, solid lines) was administered systemically to rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls. Tat-GluA23Y markedly strengthened the LI effect by further impairing avoidance responding in PE rats (squares) relative to rats administered a scrambled control peptide (red, dashed lines), while having no effect in the NPE group (circles). (B) Overall avoidance percentage indicated a significant reduction of avoidances in Tat-GluA23Y-treated PE rats compared with the NPE group and the corresponding scrambled control, indicating a potentiation of LI. (C) Escape responses progressively declined over trials, with fewer escapes in NPE vs PE rats, although this was not observed in the PE GluA23Y group. No effect of GluA23Y treatment or interaction was observed. (D) Overall escape responses percentage was significantly lower in PE vs NPE rats, and no effect of GluA23Y treatment or interaction was observed. **P < .01, ***P < .001, error bars represent SEM.

Figure 3.

Ro25-6981 does not affect latent inhibition of 2-way avoidance. (A) The GluN2B receptor antagonist Ro25-6981 (6 mg/kg, i.p., black, solid lines) or vehicle (red, dashed lines) was administered systemically to rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls irrespective of the drug treatment, with no effect on LI. (B) Overall avoidance percentage was significantly lower in PE vs NPE rats, with no effect of drug treatment. ***P < .001, error bars represent SEM.

Figure 4.

Nucleus accumbens (NAc) administration of Tat-GluA23Y does not affect latent inhibition (LI) of 2-way avoidance. (A) Tat-GluA23Y (0.5 μL/hemisphere, 22.5 pmol/0.5 μL, IC, black, solid lines) or scrambled control peptide (red, dashed lines) was administered into the NAc of rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls irrespective of the drug treatment, with no effect on LI. (B) Overall avoidance percentage was significantly lower in PE vs NPE rats, with no effect of drug treatment. *P < .05, error bars represent SEM.

Figure 5.

Medial prefrontal cortex (mPFC) administration of Tat-GluA23Y does not affect latent inhibition (LI) of 2-way avoidance. (A) Tat-GluA23Y (1.0 μL/hemisphere, 22.5 pmol/0.5 μL, IC, black, solid lines) or scrambled control peptide (red, dashed lines) was administered into the mPFC of rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls irrespective of the drug treatment, with no effect on LI. (B) Overall avoidance percentage was significantly lower in PE vs NPE rats, with no effect of drug treatment. **P < .01, error bars represent SEM.

Figure 6.

Central nucleus of the amygdala (CeA) administration of Tat-GluA23Y potentiates latent inhibition of 2-way avoidance. (A) Tat-GluA23Y (0.5 μL/hemisphere, 22.5 pmol/0.5 μL, IC, black, solid lines) or scrambled control peptide (red, dashed lines) was administered into the CeA of rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls. Intra-CeA Tat-GluA23Y markedly strengthened the LI effect, by further impairing avoidance responding in PE rats (squares) relative to rats administered a scrambled control peptide (red, dashed lines), while having no effect in the NPE group (circles). (B) Overall avoidance percentage indicated a significant reduction of avoidances in intra-CeA Tat-GluA23Y-treated PE rats compared with the NPE group and the corresponding scrambled control, indicating a potentiation of LI. (C) Escape responses progressively declined over trials, with fewer escapes in NPE vs PE rats, while intra-CeA GluA23Y treatment tended to elevate escapes. (D) Overall escape responses percentage was significantly lower in PE vs NPE rats, while intra-CeA Tat-GluA23Y treatment increased escapes regardless of preexposure status. **P < .01, ***P < .001, error bars represent SEM.

Figure 7.

Basolateral amygdala administration of Tat-GluA23Y did not recapitulate the effect on LI observed following CeA administration. (A) Tat-GluA23Y (0.5 μL/hemisphere, 22.5 pmol/0.5 μL, IC) was administered into the CeA (red, dashed lines) or BLA (black, dashed lines) to rats prior to conditioning. Rats pre-exposed (PE, squares) to the CS avoided significantly less than non-preexposed (NPE, circles) controls. Intra-BLA Tat-GluA23Y administration had no effects on avoidance responding and thus failed to recapitulate the significant effect on LI observed following intra-CeA administration. This confirmed the CeA as the locus of action Tat-GluA23Y, excluding spill over to the BLA. (B) Overall avoidance percentage indicated a potentiation of LI in intra-CeA but not intra-BLA Tat-GluA23Y-treated rats. **P < .01, ***P < .001, error bars represent SEM.