Korean J Physiol Pharmacol.  2013 Dec;17(6):553-558. 10.4196/kjpp.2013.17.6.553.

Long-Term Potentiation of Excitatory Synaptic Strength in Spinothalamic Tract Neurons of the Rat Spinal Cord

• Hur SW
• Park JM

• KMID: 2285484
• DOI: http://doi.org/10.4196/kjpp.2013.17.6.553

Abstract

Spinal dorsal horn nociceptive neurons have been shown to undergo long-term synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Here, we focused on the spinothalamic tract (STT) neurons that are the main nociceptive neurons projecting from the spinal cord to the thalamus. Optical technique using fluorescent dye has made it possible to identify the STT neurons in the spinal cord. Evoked fast mono-synaptic, excitatory postsynaptic currents (eEPSCs) were measured in the STT neurons. Time-based tetanic stimulation (TBS) was employed to induce long-term potentiation (LTP) in the STT neurons. Coincident stimulation of both pre- and postsynaptic neurons using TBS showed immediate and persistent increase in AMPA receptor-mediated EPSCs. LTP can also be induced by postsynaptic spiking together with pharmacological stimulation using chemical NMDA. TBS-induced LTP observed in STT neurons was blocked by internal BAPTA, or Ni2+, a T-type VOCC blocker. However, LTP was intact in the presence of L-type VOCC blocker. These results suggest that long-term plastic change of STT neurons requires NMDA receptor activation and postsynaptic calcium but is differentially sensitive to T-type VOCCs.

Keyword

Long-term potentiation; NMDA receptor; Spinothalamic tract neurons; T-type VOCC

MeSH Terms

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid
Animals
Calcium
Depression
Egtazic Acid
Excitatory Postsynaptic Potentials
Horns
Long-Term Potentiation*
N-Methylaspartate
Neurons*
Nociceptors
Plastics
Rats*
Spinal Cord*
Spinothalamic Tracts*
Thalamus
Calcium
Egtazic Acid
N-Methylaspartate
Plastics
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid

Figure

• 

  Fig. 1 Spinothalamic tract neurons (STT) identified in rat spinal cord slices. (Aa, Ac) An example of recorded STT neuron in the bright field (magnitude ×400, ×100, respectively), dashed line-stimulation electrode, solid line-recording electrode. (Ab) Labeling of neurons using fluorescent dye (Δ9 Di-I, Abs.-549 nm, Em.-564 nm). (Ba, Bc) An example of STT neurons stained by lipophilic tracer (Abs.-549 nm, Em.-564 nm). (Bb) STT neuron was highlighted by lucifer yellow (Abs.-428 nm, Em.-536 nm) in the recording pipette. (Bc, Bd) Pseudo color for Di-I, lucifer yellow, respectively.

• 

  Fig. 2 LTP is induced by time-based electrical stimulation (TBS) in STT neurons. (A) Fast monosynaptic, excitatory postsynaptic currents recorded from an identified STT neuron. (Aa) CNQX (10 µM), AMPA receptor antagonist, completely abolished the evoked postsynaptic current, indicating AMPA receptor-mediated excitatory postsynaptic current (EPSC). (Ab) STT neurons showed inward current in response to bath application of substance P (2 µM) in the presence of TTX (0.5 µM), Na channel blocker. The effect of substance P was inhibited by L-703,606, NK1 receptor antagonist. (B) LTP is induced by time-based electrical stimulation (TBS). Averaged time course of the change in evoked EPSC (eEPSC) by low-frequency TBS (2 Hz, n=33). Error bars indicate the standard error of the mean. Note that 2 Hz TBS can induce LTP in most recorded STT neurons (33 out of 36 neurons). Bar graphs represent the normalized current amplitude (mean±sem). ***p<0.0010.001 at time point 2 when compared to the time point 1 (before LTP induction). (C) LTP is also induced by different low-frequency TBS (5 Hz, n=4, different shapes indicate different STT neurons). Note that this 5 Hz TBS can induce LTD in a subset of STT neurons (4 out of 8 neurons, data not shown). Bar graphs represent the normalized current amplitude (mean±sem). ***p<0.0010.001 at time point 2 when compared to the time point 1 (before LTP induction).

• 

  Fig. 3 TBS-induced LTP in STT neurons requires NMDA receptor activation and postsynaptic calcium. (A) Original raw traces before (left arrow at time point 1), during bath application of NMDA (middle arrow at time point 2), and after (right arrow at time point 3). LTP is induced by 2 Hz postsynaptic stimulation in the presence of chemical NMDA (100 nM) without presynaptic stimulation. When NMDA alone was applied to acute slices in the presence of test stimulation, evoked EPSC shows no effect on the responses. Different shapes indicate different STT neurons (n=4). Bar graphs represent the normalized current amplitude (mean±sem). ***p<0.001 at time point 3 when compared to the time point 2 (before LTP induction). (B) Original raw traces before (left arrow at time point 1) and after LTP (right arrow at time point 2). LTP is impaired by internal BAPTA (10 mM), Ca2+ chelator. Different shapes indicate different STT neurons (n=3). Bar graphs represent the normalized current amplitude (mean±sem).

• 

  Fig. 4 TBS-induced LTP in the STT neurons requires T-type voltage-operated calcium channel (VOCC), but not L-type VOCC. (A) TBS-induced LTP in the STT neurons is not affected by L-type VOCC blocker, nimodipine (10 µM). Original raw traces before (left arrow at time point 1) and after LTP (right arrow at time point 2). When nimodipine alone was applied to acute slices in the presence of test stimulation, evoked EPSC shows no effect on the responses. Different shapes indicate different STT neurons (n=4). Right, averaged time course of the changes represent the normalized current amplitude (mean±sem). (B) Original raw traces before (left arrow at time point 1), during bath application of NiCl2 (middle arrow at time point 2), and after (right arrow at time point 3). LTP is impaired by the application of NiCl2 (100 µM), T/R type VOCC blocker. When NiCl2 alone was applied to acute slices in the presence of test stimulation, evoked EPSC shows no effect on the responses. Right, averaged time course of the changes represent the normalized current amplitude (mean±sem). Different shapes indicate different STT neurons (n=4). (C) NMDA receptor and T-type VOCC, but not L-type VOCC in TBS-induced LTP in the STT neurons. Spike-timing dependent TBS-LTP in the STT neurons requires postsynaptic calcium but are differentially sensitive to T-type VOCC blocker, NiCl2, indicating the role of T-type VOCC in LTP of STT neurons.

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