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. 2015 Dec 14;112(52):16030–16035. doi: 10.1073/pnas.1501731113

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Fig. 5. — ZD7288 reduces HCN current in LFS motor neurons and the NMDA-like component of the LE-LFS EPSP. (A) Evidence for HCN current in LFS siphon motor neurons, reduction by ZD7288, and enhancement by DEA-NO. (A1) Example of hyperpolarization of an LFS neuron by a 6-s constant current injection before (control) and after perfusing the ganglion with ZD7288. ZD7288 reduced the “sag” or depolarization from the beginning to the end of the current injection, which is characteristic of HCN current. (A2) Average results from experiments like the one shown in A1 (n = 4), each with four different amplitudes of current injection. (A3) The nitric oxide donor DEA-NO enhanced the sag, normalized to the value before DEA-NO perfusion (control) in each experiment (n = 6). The average control value was 18.6 mV. (B) ZD7288 did not affect the baseline monosynaptic LE-LFS EPSP, but reduced the NMDA-like component of the EPSP. (B1) Example of the monosynaptic LE-LFS EPSP in artificial sea water (ASW) (control) and CNQX. (B2) Average peak and late (50–75 ms after peak) amplitude of the LE-LFS EPSP in ASW with the abdominal ganglion perfused with ZD7288 after the second test, normalized to the average value on the pretests in each experiment (n = 3). The average pretest values were 12.5 (peak) and 5.4 (late) mV. (B3) Average amplitude of the late component of the EPSP in ASW (control), CNQX, and CNQX + ZD7288, normalized to the average control value in each experiment (n = 3). There was a significant overall effect of drug (F[2,4] = 54.21, P < 0.01). The average control value was 2.7 mV. (B4) Similar experiments with ASW (control), CNQX, CNQX + APV, and CNQX + APV + ZD7288 (n = 3). There was a significant overall effect of drug (F[3,6] = 62.18, P < 0.01). The average control value was 3.6 mV. (C) Hypothetical model of postsynaptic mechanisms by which HCN channels contribute to classical conditioning of the siphon-withdrawal reflex. Nitric oxide (NO) released by L29 interneurons acts directly in the LFS motor neurons to stimulate production of cGMP and activation of HCN channels, which in turn enhances activation of NMDA-like channels and Hebbian potentiation at LE-LFS synapses.