Repeated shock stress facilitates basolateral amygdala synaptic plasticity through decreased cAMP-specific phosphodiesterase type IV (PDE4) expression

Abstract

Previous studies have shown that exposure to stressful events can enhance fear memory and anxiety-like behavior as well as increase synaptic plasticity in the rat basolateral amygdala (BLA). We have evidence that repeated unpredictable shock stress (USS) elicits a long-lasting increase in anxiety-like behavior in rats, but the cellular mechanisms mediating this response remain unclear. Evidence from recent morphological studies suggests that alterations in the dendritic arbor or spine density of BLA principal neurons may underlie stress-induced anxiety behavior. Recently, we have shown that the induction of long-term potentiation (LTP) in BLA principal neurons is dependent on activation of postsynaptic D1 dopamine receptors and the subsequent activation of the cyclic adenosine 5'-monophosphate (cAMP)-protein kinase A (PKA) signaling cascade. Here, we have used in vitro whole-cell patch-clamp recording from BLA principal neurons to investigate the long-term consequences of USS on their morphological properties and synaptic plasticity. We provided evidence that the enhanced anxiety-like behavior in response to USS was not associated with any significant change in the morphological properties of BLA principal neurons, but was associated with a changed frequency dependence of synaptic plasticity, lowered LTP induction threshold, and reduced expression of phosphodiesterase type 4 enzymes (PDE4s). Furthermore, pharmacological inhibition of PDE4 activity with rolipram mimics the effects of chronic stress on LTP induction threshold and baseline startle. Our results provide the first evidence that stress both enhances anxiety-like behavior and facilitates synaptic plasticity in the amygdala through a common mechanism of PDE4-mediated disinhibition of cAMP-PKA signaling.

Keywords: Basolateral amygdala; Chronic stress; Compartmentalization; Macromolecular complexes; Morphology; a-Kinase anchoring protein.

Figure 1

4 days of unpredictable shock stress rats leads to heightened anxiety. A) Schematic of unpredictable shock stress paradigm, including acoustic startle pre- and post-tests, and 4 days of USS. B) USS rats show increase in acoustic startle amplitude from pre-test to post-test.

Figure 2

Chronic stress has no effect on dendritic morphology of BL A principal neurons. A) Example reconstructed BLA principal neuron. B–D) No effect of chronic stress on total dendritic length (B), spine density (C), or distribution of dendritic material with respect to the cell body (D).

Figure 3

Chronic stress enhances LTP generated by HFS and abolishes LTD generated by LFS. A) Experimental schematic showing stimulation of the external capsule and recording from BLA. B–E) Synaptic stimulation at different frequencies shows a spectrum of potentiation and depression in non-stress animals which is altered by exposure to chronic stress. A summary of the effects on synaptic plasticity is shown in (F), with asterisks indicating comparisons between stress and non-stress groups after 30 minutes.

Figure 4

Threshold for potentiation is lowered by exposure to chronic stress. In non-stress animals, the 2x100 Hz stimulation protocol is subthreshold for generating LTP (A), but produces substantial potentiation in stressed animals (B). C) Enhanced LTP in stressed animals is still dependent on cAMP-PKA-dependent pathways as has been previously established.

Figure 5

Inhibition of PDE4 activity recapitulates the effects of stress on LTP threshold. Paired, simultaneous recordings of BLA principal neurons (A) in which one patch pipette contains rolipram, a PDE4 inhibitor, reduces the LTP threshold (B), similar to stressed animals. C) Rolipram-induced reduction in LTP threshold is also D1-dependent.

Figure 6

Chronic stress radically alters expression of PDE4 subtypes B and D. Quantitative whole-tissue analysis of transcripts shows significant reduction in expression of PDE4B and D, and a small increase in expression of PDE4A.

Figure 7

Intra-BLA administration of rolipram produces a phenotype of enhanced anxiety. Injection sites identified by post-hoc histology and illustrated in (A). Vehicle and rolipram animals show comparable anxiety in the pretest (B), but 1hr and 24hr after infusion, rolipram animals show significantly enhanced stress at 110 dB.

Figure 8

Diagram illustrating important components of the cAMP cascade and their interactions in BLA synapses. Note the negative feedback loop whereby cAMP activates PKA, which enhances PDE4 activity, which in turn reduces cAMP concentration; this negative feedback loop is an essential component of compartmentalization.