Actions of barium on rapidly inactivating potassium current in bullfrog sympathetic neurons

Opposing inward and outward conductances regulate rebound discharges in olfactory mitral cells

The olfactory bulb, a second-order sensory brain region, relays afferent input from olfactory receptor neurons to piriform cortex and other higher brain centers. Although large inhibitory postsynaptic potentials (IPSPs) are evident in in vivo intracellular recordings from mitral cells, the functional significance of these synaptic responses has not been defined. In many brain regions, IPSPs can function to either inhibit spiking by transiently suppressing activity or can evoke spiking directly by triggering rebound discharges. We used whole cell patch-clamp recordings from mitral cells in olfactory bulb slices to investigate the mechanisms by which IPSPs regulate mitral cell spike discharges. Mitral cells have unusual intrinsic membrane properties that support rebound spike generation in response to small-amplitude (3-5 mV) but not large-amplitude hyperpolarizing current injections or IPSPs. Rebound spiking occurring in mitral cells was dependent on recovery of subthreshold Na currents, and could be blocked by tetrodotoxin (TTX, 1 microM) or the subthreshold Na channel blocker riluzole (10 microM). Surprisingly, larger-amplitude hyperpolarizing stimuli impeded spike generation by recruiting a transient outward I(A)-like current that was sensitive to high concentrations of 4-aminopyridine and Ba. The interplay of voltage-gated subthreshold Na channels and transient outward current produces a narrow range of IPSP amplitudes that generates rebound spikes. We also found that subthreshold Na channels boost subthreshold excitatory stimuli to produce membrane voltages where granule-cell-mediated IPSPs can produce rebound spikes. These results demonstrate how the intrinsic membrane properties of mitral cells enable inhibitory inputs to bidirectionally control spike output from the olfactory bulb.

Effects of lanthanides on voltage-dependent potassium currents in bullfrog sympathetic neurons

The effects of lanthanides (La(3+), Gd(3+), Lu(3+) and Sm(3+)) on voltage-dependent potassium currents were studied in dissociated bullfrog sympathetic neurons. A-type current (I(A)) and M-type current (I(M)) were blocked by lanthanides (0.1-30 microM) with I(M) being much less sensitive to these ions than I(A). The order of potency was Gd(3+)>/=Lu(3+) approximately La(3+) approximately Sm(3+) for I(A) and Gd(3+)&z.Gt;Lu(3+) approximately La(3+)>Sm(3+) for I(M). The I(M) block occurred independently of its activation kinetics while the I(A) block was associated with a positive shift of the activation and inactivation curves. Gd(3+) (100 microM) blocked the delayed rectifier-type current (I(K)) by less than 20%; Lu(3+), La(3+) and Sm(3+) (100 microM for each) were without effect on I(K). It is concluded that I(A) was the most sensitive to lanthanides, and Gd(3+) was the most potent for all the currents in amphibian autonomic neurons.

Mechanisms underlying the M-current block by barium in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the channel blocking actions of barium (3-2000 microM) on an M-type potassium current (I(M)). Barium (IC(50) approximately 105 microM) blocked I(M) without affecting the 50%-activation voltage ( approximately -35 mV) and the slope factor ( approximately 11 mV) of the activation curve. The results indicate that the barium block is independent of the kinetics of I(M).

Effects of quinine on three different types of potassium currents in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the sensitivity of potassium currents to a potassium channel blocker quinine (1-500 microM). Among three currents tested, a rapidly inactivating A-type current (I(A)) was the most sensitive to the block by quinine (IC50 approximately 22 microM). A non-inactivating M-type current (I(M)) was the least sensitive (IC50 approximately 445 microM), and the sensitivity of a slowly inactivating delayed rectifier-type current (I(K)) was in between (IC50 approximately 115 microM). Results suggest that the ability of quinine to block different types of potassium currents such as I(A) and I(M) with significantly different IC50 values would be of help for the potassium channel pharmacology in amphibian autonomic ganglion cells.

Actions of zinc on rapidly inactivating A-type and non-inactivating M-type potassium currents in bullfrog sympathetic neurons

The actions of zinc on A-type potassium current (I(A)) were studied in dissociated bullfrog sympathetic neurons. Zinc (1-300 microM) caused a parallel shift in the activation and inactivation curves to a depolarizing direction, thereby enhancing I(A) around physiological resting potential. An EC50 value was 70-100 microM for these actions. The zinc actions were non-selective in a sense that zinc inhibited M-type potassium current (I(M)) with an IC50 value of 300 microM. Zinc was without effect on the maximum conductance for I(A) and the kinetic behavior for I(M). The ability of low concentrations of zinc to modulate separate set of potassium currents such as I(A) and I(M) in conceptually distinct manner may therefore assume pathophysiological importance for autonomic neurons.