Low-frequency stimulation induces a pathway-specific late phase of LTP in the amygdala that is mediated by PKA and dependent on protein synthesis

Abstract

Activity-dependent changes in synaptic efficacy are thought to be the key cellular mechanism for the formation and storage of both explicit and implicit memory. Different patterns of stimulation can elicit different changes in the efficiency on excitatory synaptic transmission. Here, we examined the synaptic changes in the amygdala of adult mice produced by low-frequency stimulation (1 Hz, 15 min, LFS). We first compared the synaptic changes induced by LFS in three different synaptic pathways of amygdala: cortical-lateral amygdala, thalamic-lateral amygdala, and lateral-basolateral amygdala pathways. We find that the plastic changes induced by LFS are different between synaptic pathways. Low-frequency stimulation selectively elicits a slow onset and protein synthesis-dependent late-phase LTP in the cortical-lateral amygdala pathway, but not in the thalamic-lateral or lateral-basolateral pathways. We next analyzed LTP induced by LFS in the cortical-lateral amygdala pathway and found that three PKA-coupling neurotransmitter receptors are involved: 5-HT4, Dopamine D1, and beta-adrenergic receptors. Antagonists of these receptors block the LFS L-LTP, but the effects of agonists of these receptors are clearly different. These results indicate that the threshold for the induction of LFS L-LTP is different among these pathways and that the maintenance of LFS L-LTP requires a cross-talk among multiple neurotransmitters.

Figure 1.

Plastic changes induced by LFS in different synaptic pathways of the amygdala. (A1) Schematic illustration of stimulating and recording site of EC–LA pathway in a coronal brain slice. The inset shows the representative field potential recorded in this pathway. (A2) LFS (1 Hz, 15 min) induced L-LTP in the EC–LA pathway. Calibration: 3 msec, 1 mV. (B1) Schematic illustration of stimulating and recording site of the TH–BL pathway. The inset shows the representative field potential in this pathway. (B2) LFS induced a transient synaptic depression, followed by a weak potentiation in this pathway. (C1) Schematic illustration of stimulating and recording site of the LA–BL pathway. The inset shows the representative field potential in this pathway. (C2) LFS did not induce L-LTP in this pathway.

Figure 2.

A comparison of plastic changes induced by LFS in three different pathways of amygdala. (A) Schematic model shows the major inputs to the lateral amygdala (LA) and the output from LA to the basolateral amygdala (BL). (B) Histograms show the comparisons of plastic changes induced by LFS in three different synaptic pathways above at different time points after LFS. (Black bars) LA–BL pathway; (red bars) EC–LA pathway; (green bars) TH–LA pathway. There are significant differences in the plastic changes induced by LFS between these pathways. *P < 0.01, ANOVA.

Figure 3.

LFS L-LTP in the amygdala is dependent on PKA and new protein synthesis. (A) NMDA receptors antagonist D-APV (50 μM) did not block LFS L-LTP. (●) Control; (○) D-APV + LFS. (B) Inhibitor of PKA KT5720 (1 μM) completely blocked the LFS L-LTP. (●) Control; (○) KT5720 + LFS; (▴) KT5720 alone; (n = 5). (C) LFS induced a synaptic depression in the presence of anisomycin. (●) Control; (○) Anisomycin + LFS; (▴) Anisomycin alone; (n = 5).

Figure 4.

The effects of 5-HT4, D1, and β-adrenergic receptor antagonists. (A) 5-HT4 receptor antagonist RS39604 (25 μM) blocked LFS L-LTP. (●) LFS; (○) LFS + RS39604; RS39604 alone did not alter the baseline synaptic response (▴, n = 5). (B) D1 receptor antagonist SCH23390 (2 μM) blocked LFS L-LTP. (●) LFS. (○) LFS + SCH; (▴, n = 5) SCH alone did not alter the baseline synaptic response. (C) β-Adrenegic receptor antagonist propranolol (1 μM) blocked LFS L-LTP. (●) LFS; (○) LFS + Propranolol; (▴, n = 5) Propranolol alone did not alter the baseline synaptic response.

Figure 5.

The effects of 5-HT4, D1, and β-adrenergic receptor agonists. (A) 5-HT4 receptor agonist RS67333 (50 μM, perfused for 25 min, in the presence of Zimelidine, 100 μM) induced L-LTP. Co-application RS67333 and LFS induced a profound early synaptic depression. (○) RS67333 + Zimelidine; (●) RS67333 + Zimelidine + LFS. (▴) Zimelidine + LFS. (B) β-Adrenergic receptor agonist Isoproterenol (ISO, 15 μM, perfused for 25 min) induced L-LTP. Co-application ISO and LFS (1 Hz, 15 min) did not alter the LFS L-LTP. (○) ISO; (●) ISO + LFS. (C) D1 receptor agonist 6-APB (50 μM, perfused for 25 min) did not induce L-LTP. Co-application 6-APB and LFS induced a weak synaptic depression. (●) 6-APB; (○) 6-APB + LFS.

Figure 6.

A comparison of the effects of agonists and antagonists. (A) The effects of different antagonists. (1) Early effects (30 min after LFS). (Black bar) β-Adrenergic receptor antagonist (proprenolol); (red bar) 5-HT4 receptor antagonist (RS39604); (green bar) D1 receptor antagonist (SCH23390). (2) Late effects (3.5 h after LFS). The column colors are the same as in 1. (B) The effects of different agonists. (1) The early effects of co-application LFS and agonists (30 min after LFS). (2) The late effects of co-application of LFS and agonists (3.5 h after LFS). (3) The effects of agonists without pairing of LFS. The colors of columns are the same as in A. There are significant differences between the effects of three agonists. *P < 0.01, ANOVA.