Fig 1.
Glutamate receptor antagonists do not block TA-induced SC depotentiation.
(A) The graph shows the time course of change in SC EPSPs following a single 100 Hz x 1 sec HFS delivered to the SC pathway (arrow). After establishing SC LTP, 1 Hz LFS of the TA (perforant path) inputs to CA1 (PLFS, hatched bar) reversed LTP. (B) Following stable SC LTP induction 30 μM CNQX (black bar) was administered and inhibited SC EPSPs completely. CNQX did not prevent LTP-D by PLFS (white circles) and had no effect on LTP by itself (black circles). (C) A combination of glutamate receptor antagonists (30 μM CNQX, 100 μM APV and 500 μM MCPG) also failed to prevent PLFS-induced LTP-D. Traces to the right of the graphs show representative field EPSPs at the times indicated (black lines) with baseline EPSPs shown as dashed traces. Calibration: 1 mV, 5 ms.
Fig 2.
GABA-A but not GABA-B receptors contribute to TA-induced SC LTP-D.
(A) In the presence of 1 μM picrotoxin, a GABA-A receptor antagonist (PTX, black bar), PLFS (hatched bar) failed to depotentiate SC LTP. (B) 2-Hydroxysaclofen (200 μM, black bar), a GABA-B receptor antagonist did not alter TA-induced LTP-D. Traces show representative EPSPs at the times indicted in the graphs. Calibration: 1 mV, 5 ms.
Fig 3.
GABAergic neurosteroids contribute to homosynaptic LTP-D in the SC pathway but not to TA-induced LTP-D.
The graph shows the ability of 1 μM finasteride, a 5α reductase inhibitor that blocks synthesis of 5α-reduced neurosteroids, to block LTP-D induced by 1 Hz stimulation of the homosynaptic SC pathway (black circles). In contrast, finasteride had no effect on SC LTP-D induced by PLFS (hatched bar) in a separate set of slices (white circles). HFS was administered to the SC pathway at the arrow. Traces show representative EPSPs at the times indicated. Calibration bar: 1 mV, 5 ms.
Fig 4.
Exogenous administration of an endocannabinoid produces chemical depotentiation of SC LTP.
(A) Perfusion of 20 μM 2AG (white bar), an endocannabinoid agonist, reversed LTP established by SC HFS (arrow). (B) Following LTP-D induced by PLFS, 20 μM 2AG failed to produce further suppression of SC EPSPs. (C) The effects of 2AG (white bar) on SC LTP were blocked by 5 μM AM251 (black bar), an inhibitor of CB1 receptors. (D,E) Similarly, the effects of 2AG on SC LTP were blocked by pretreatment with 1 μM PTX (D) and 200 nM DPCPX, an adenosine A1 receptor antagonist (E). Traces show representative EPSPs at the times indicated in the graphs. Calibration: 1 mV, 5 ms.
Fig 5.
Endocannabinoids contribute to TA-induced LTP-D of SC synapses.
(A) Treatment of slices with 10 μM tetrahydrolipstatin (THS, black bar), an inhibitor of diacylglycerol lipase and 2AG synthesis, blocked the ability of PLFS (hatched bar) to depotentiate SC LTP. SC HFS was administered at the arrow. (B) TA-induced LTP-D was also blocked by 5 μM AM251, a CB1 receptor antagonist (black bar). AM251, however, did not block the ability of 10 nM cyclopentyladenosine (CPA, white bar), an adenosine A1 receptor agonist, to induce chemical depotentiation of SC LTP. (C) PTX (black bar) also failed to alter depotentiation by CPA (white bar) as shown with black squares. White circles show effects of CPA in the absence of PTX. Traces show representative EPSPs. Calibration: 1 mV, 5 ms.
Fig 6.
The MAP kinases ERK 1/2 and p38 contribute to TA-induced LTP-D.
(A) In the presence of 10 μM PD98059 (black bar), a MEK inhibitor that blocks signaling in the ERK pathway, PLFS (hatched bar) failed to induce LTP-D. HFS of the SC pathway was administered at the arrow. (B) Similarly, administration of 10 μM SB20358, a p38 MAPK inhibitor, also blocked TA-induced LTP-D. (C) In contrast, 10 μM SP600125, an inhibitor of the JNK pathway, did not prevent PLFS-induced LTP-D. Traces show representative EPSPs at the times indicated. Calibration: 1 mV, 5 ms.
Fig 7.
An ERK inhibitor does not alter chemical LTP-D by 2AG or CPA.
(A,B) In the presence of 10 μM PD98059, both 2AG (A), the endocannabinoid agonist, and CPA (B), the adenosine A1 receptor agonist, induced chemical depotentiation. Traces show EPSPs at the times indicated. Calibration: 1 mV, 5 ms.
Fig 8.
A p38 MAPK inhibitor blocks chemical LTP-D by CB1 receptor activation but not A1 receptor activation.
(A) In the presence of 10 μM SB20358, 20 μM 2AG failed to induce LTP-D of SC LTP. B. SB20358, however, did not prevent chemical depotentiation by CPA. Traces show EPSPs at the times indicated. Calibration 1 mV, 5 ms.
Fig 9.
Mechanisms contributing to TA-induced depotentiation.
The figure presents a scheme for signaling involved in TA-induced SC LTP-D based on the effects of selective agonists and antagonists. Based on results to date, activation of ERK 1/2 appears to occur early in the cascade, while A1R activation is a late event.