Species-specific differences in TBS-induced synaptic plasticity and synaptic tagging and capture (STC) in hippocampal CA1.

(A) (i) Isolated NHP brain. (ii) Representative image of NHP hippocampal slice preparation with electrodes positioned in the CA1 region. (iii) Schematic of a hippocampal slice illustrating two independent synaptic inputs (S1 and S2) onto CA1 pyramidal neurons. Field excitatory postsynaptic potentials (fEPSPs) were recorded from apical dendrites in CA1 using a recording electrode (rec). (B-C) TBS-induced late-phase LTP (late-LTP). Following a stable 30 min baseline, theta-burst stimulation (TBS; 5 Hz) delivered to input S1 induced robust and persistent potentiation in both rat (B) and NHP (C) hippocampal slices. Potentiation was significant from 5 min post-TBS and persisted up to 6 h, whereas the control pathway S2 remained stable. (D-E) Early-LTP induced by weak tetanic stimulation (WTET). WTET applied to S1 induced transient potentiation in both rat (D) and NHP (E) hippocampal slices. Early-LTP decayed to baseline within ∼155-200 min, with no sustained potentiation observed in either species. (F-G) In rat hippocampal slices (F), TBS-induced late-LTP in S1 failed to support STC, as WTET-induced potentiation in S2 decayed to baseline, indicating lack of plasticity-related protein (PRP) capture. In contrast, in NHP hippocampal slices (G), WTET delivered to S2 30 min after TBS in S1 resulted in the conversion of early-LTP into persistent late-LTP, demonstrating robust STC expression. Both S1 and S2 pathways remained significantly potentiated throughout the 6 h recording period. In all panels, figures shown in blue represent experiments conducted in rats, whereas those in dark red represent experiments in nonhuman primates (NHPs). Solid and dotted arrows indicate the time points of application of TBS and WTET, respectively. Analog traces depict representative S1 and S2 fEPSPs: 30 min before any experimental manipulation (dotted line), 30 min (hatched line) and 360 min (solid line) after TBS or WTET. Vertical scale bar: vertical: 3 mV, horizontal scale bar: 5 ms.

Divergent gene expression and proteomic signatures in rat and nonhuman primate (NHP) hippocampal CA1 following TBS-induced LTP.

(A-B) Relative mRNA expression levels of plasticity-related genes in hippocampal CA1 under control conditions (black) and following TBS-induced LTP (red). (A) In NHP hippocampal slices, TBS significantly increased the expression of BDNF and CaMKIV, while CaMKII, PKMζ, and CREB1 remained unchanged. (B) In rat hippocampal slices, TBS did not induce significant changes in the expression of any of the examined genes. (C) Representative Western blot images showing protein levels of PKMζ and BDNF in NHP and rat hippocampal slices under control conditions and following TBS. Tubulin was used as a loading control. (D-E) Quantification of protein expression levels. (D) BDNF and (E) PKMζ protein levels were significantly increased following TBS in NHP hippocampal slices compared to controls, whereas no significant changes were observed in rat hippocampal slices.

Differential roles of BDNF and PKMζ in the maintenance of TBS-induced LTP in rat and nonhuman primate (NHP) hippocampal CA1

(A-B) Rat hippocampal slices. (A) Blockade of BDNF signaling using TrkB/Fc applied 60 min after TBS impaired the late phase of LTP, resulting in a gradual decay of potentiation in S1, while the control pathway S2 remained stable. (B) In contrast, inhibition of PKMζ (PKMZi), applied prior to TBS and maintained throughout the recording, did not affect LTP maintenance, indicating that PKMζ is not required for TBS-induced LTP in rat CA1 and that it can be maintained exclusively by BDNF. (C-D) NHP hippocampal slices. (C) Inhibition of BDNF signaling with TrkB/Fc did not disrupt TBS-induced LTP, with S1 responses remaining persistently potentiated throughout the recording period. (D) Similarly, inhibition of PKMζ alone did not affect LTP maintenance, indicating that TBS-induced LTP in NHP hippocampus is resistant to disruption of either pathway individually. Asterisks denote statistically significant differences between groups (*P < 0.05). (E) Combined inhibition of BDNF and PKMζ in NHP hippocampal slices. Simultaneous blockade of both pathways prevented the maintenance of TBS-induced LTP, resulting in a decay of potentiation in S1 toward baseline levels, while S2 remained stable. This indicates cooperative and partially redundant roles of BDNF and PKMζ in sustaining long-term synaptic plasticity in NHP CA1. In all panels, figures shown in blue represent experiments conducted in rats, whereas those in dark red represent experiments in nonhuman primates (NHPs). Solid arrow indicate the time points of application of TBS. A hatched or empty rectangle represents the timing and duration of application of specific inhibitors. Analog traces depict representative S1 and S2 fEPSPs: 30 min before any experimental manipulation (dotted line), 30 min (hatched line) and 360 min (solid line) after TBS or WTET. Vertical scale bar: vertical: 3 mV, horizontal scale bar: 5 ms.