Modulation of action potential during the late slow excitatory postsynaptic potential in bullfrog sympathetic ganglia

TRH regulates action potential shape in cerebral cortex pyramidal neurons

Thyrotropin releasing hormone (TRH) is a neuropeptide with a wide neural distribution and a variety of functions. It modulates neuronal electrophysiological properties, including resting membrane potential, as well as excitatory postsynaptic potential and spike frequencies. We explored, with whole-cell patch clamp, TRH effect on action potential shape in pyramidal neurons of the sensorimotor cortex. TRH reduced spike and after hyperpolarization amplitudes, and increased spike half-width. The effect varied with dose, time and cortical layer. In layer V, 0.5µM of TRH induced a small increase in spike half-width, while 1 and 5µM induced a strong but transient change in spike half-width, and amplitude; after hyperpolarization amplitude was modified at 5µM of TRH. Cortical layers III and VI neurons responded intensely to 0.5µM TRH; layer II neurons response was small. The effect of 1µM TRH on action potential shape in layer V neurons was blocked by G-protein inhibition. Inhibition of the activity of the TRH-degrading enzyme pyroglutamyl peptidase II (PPII) reproduced the effect of TRH, with enhanced spike half-width. Many cortical PPII mRNA+ cells were VGLUT1 mRNA+, and some GAD mRNA+. These data show that TRH regulates action potential shape in pyramidal cortical neurons, and are consistent with the hypothesis that PPII controls its action in this region.

Compound 48/80 blocks transmission and increases the excitability of ganglion neurons

Compound 48/80 (5.0-50 micrograms/ml) significantly and reversibly decreased (1) the amplitude, but not the shape of the compound action potential, (2) the amplitude and duration of the acetylcholine potential and (3) the residual fast excitatory postsynaptic potential recorded from neurons of the 9th and 10th paravertebral ganglia of the bullfrog Rana catesbeiana. The excitability of B-type ganglion neurons in the presence of nicotinic and muscarinic receptor antagonists was increased by compound 48/80 without altering the input resistance or membrane potential. In addition, compound 48/80 (10-50 micrograms/ml) significantly decreased the duration of the spike afterhyperpolarization (AHP). The amplitude but not the decay rate of the current underlying the slow component of the spike AHP was decreased by compound 48/80. Compound 48/80 did not, however, alter either the amplitude or the duration of calcium-dependent spikes. Intracellular recordings from dissociated sympathetic neurons also demonstrated a compound 48/80-induced increase in neuronal excitability. These results suggest that compound 48/80 interacts with the nicotinic receptor/channel complex to decrease ganglionic transmission, and also has a direct action to increase neuronal excitability by blocking potassium channels mediating the duration of the spike AHP.

The effects of soman on the electrical properties and excitability of bullfrog sympathetic ganglion neurones

1. The effects of soman (0.1-10 microM), an irreversible inhibitor of acetylcholinesterase (AChE), were examined on the electrical properties of ganglion neurones of the paravertebral sympathetic chain of the bullfrog, Rana catesbeiana. 2. Soman (10 microM) depolarized 29 of 35 (83%) ganglion neurones studied by 6.4 +/- 0.65 mV within 10 min of application and reduced the cell input resistance in 9 of 11 neurones examined (82%) to 55 +/- 5.3% of control. 3. Soman (10 microM) significantly reduced the maximum amplitude and the maximum rate of rise of the action potential and the duration, but not the amplitude, of the after-hyperpolarization (AHP) following the action potential elicited by either direct or antidromic stimulation. The maximum rate of fall and the duration of the action potential were not significantly affected by soman. These actions of soman were independent of the agent-induced depolarization. When examined by a single microelectrode voltage clamp, soman reduced the amplitude and the time constant of the current underlying the slow AHP, IAHs. 4. Soman (1-10 microM) produced an increase in neuronal excitability which was evidenced as either an increase in the number of action potentials or a decrease in the interspike interval in response to constant-current depolarizing pulses. The soman-induced increase in excitability occurred independently of both the agent-induced depolarization and the decrease in input resistance, was reversible with washing, was not caused by an inhibition of the M-current and was also recorded in dissociated sympathetic ganglion neurones.5. The effects of soman on the membrane potential, input resistance and the duration of the AHP but not cell excitability were blocked by pretreatment with atropine (10 microM). Pretreatment with dihydro-/J-erythroidine (DHbetalE) (10 microM) was ineffective in blocking or reversing the effects of soman. These results suggest that the direct actions of soman on the electrical properties of these neurones are mediated by activation of muscarinic receptors.

Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

Galanin-induced hyperpolarization and decreased membrane excitability of neurones in mudpuppy cardiac ganglia

1. Membrane hyperpolarization and decreased excitability produced by galanin were investigated in vitro on parasympathetic postganglionic neurones in the cardiac ganglion of the mudpuppy, Necturus maculosus. 2. Galanin produced a slowly developing hyperpolarization which, in 2.5 mM-KCl, reversed at -105.4 +/- 2.7 mV. The reversal potential was shifted by 38.7 +/- 4.9 mV following a fourfold elevation of the extracellular potassium concentration. 3. Galanin inhibited action potential firing in spontaneously active neurones and decreased the number of spikes in a train produced by long (500-680 ms) depolarizing current pulses. Both effects were independent of the galanin-induced hyperpolarization. 4. Galanin increased the threshold for spike generation, prolonged the spike hyperpolarizing after-potential and decreased the maximum rate of rise, amplitude and maximum rate of fall of the sodium spike. These effects occurred independently of the galanin-induced hyperpolarization. 5. Galanin decreased the amplitude and duration of TTX-insensitive spikes initiated in cells maintained in a solution containing 9 mM-calcium, 20 mM-TEA and 1.5 microM-TTX. 6. These results suggest that a galanin-like peptide may act as an inhibitory transmitter in the mudpuppy cardiac ganglion.

Luteinizing hormone-releasing hormone inhibits nicotinic transmission in bullfrog sympathetic ganglia

Intracellular and voltage-clamp recordings were made from neurons in bullfrog sympathetic ganglia to investigate the effects of luteinizing hormone-releasing hormone (LH-RH) on nicotinic transmission. LH-RH (50 nM-4 microM) decreased the amplitude of the fast excitatory postsynaptic potential (fast EPSP) in a dose-dependent manner. LH-RH (1-4 microM) reduced the quantal content of the fast EPSP by 60-85%. LH-RH did not change the frequency of the miniature (m) EPSP, but it slightly depressed the mEPSP amplitude. LH-RH (1-4 microM) caused a 22-32% decrease in the amplitude of the acetylcholine-induced synaptic responses due to the iontophoretic application of acetylcholine (ACh) to neurons in the presence of atropine (1 microM). These results suggested that LH-RH decreased nicotinic transmission in the bullfrog sympathetic ganglion, primarily by reducing the release of ACh from the preganglionic nerve terminals.