3,4-Diaminopyridine (3,4-DAP) increases transmitter release from neuromuscular junctions (NMJs), and low doses of 3,4-DAP (estimated to reach ∼1 μM in serum) are the Food and Drug Administration (FDA)-Approved treatment for neuro muscular weakness caused by Lambert-Eaton myasthenic syn drome. Canonically, 3,4-DAP is thought to block voltage-gated potassium (Kv) channels, resulting in prolongation of the pre synaptic action potential (AP). However, recent reports have shown that low millimolar concentrations of 3,4-DAP have an off-Target agonist effect on the Cav1 subtype ("L-Type") of voltage-gated calcium (Cav) channels and have speculated that this agonist effect might contribute to 3,4-DAP effects on transmitter release at the NMJ. To address 3,4-DAPs mecha nism(s) of action, we first used the patch-clamp electrophysi ology to characterize the concentration-dependent block of 3,4-DAP on the predominant presynaptic Kv channel subtypes found at the mammalian NMJ (Kv3.3 and Kv3.4). We identified a previously unreported high-Affinity (1-10 μM) partial antag onist effect of 3,4-DAP in addition to the well-known low-Af finity (0.1-1 mM) antagonist activity. We also showed that 1.5-μM DAP had no effects on Cav1.2 or Cav2.1 current. Next, we used voltage imaging to show that 1.5-or 100-μM 3,4-DAP broadened the AP waveform in a dose-dependent manner, in dependent of Cav1 calcium channels. Finally, we demonstrated that 1.5-or 100-μM 3,4-DAP augmented transmitter release in a dose-dependent manner and this effect was also independent of Cav1 channels. From these results, we conclude that low micromolar concentrations of 3,4-DAP act solely on Kv chan nels to mediate AP broadening and enhance transmitter release at the NMJ.
Ojala, Kristine S; Ginebaugh, Scott P; Wu, Man; Miller, Evan W; Ortiz, Gloria; Covarrubias, Manuel; and Meriney, Stephen D, "A high affinity, partial antagonist effect of 3,4-diaminopyridine mediates action potential broadening and enhancement of transmitter release at NMJs" (2021). Department of Neuroscience Faculty Papers. Paper 55.
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