Depression of spike adaptation and afterhyperpolarization by 4-aminopyridine in hippocampal neurons

SK channels and the varieties of slow after-hyperpolarizations in neurons

Action potentials and associated Ca2+ influx can be followed by slow after-hyperpolarizations (sAHPs) caused by a voltage-insensitive, Ca2+-dependent K+ current. Slow AHPs are a widespread phenomenon in mammalian (including human) neurons and are present in both peripheral and central nervous systems. Although, the molecular identity of ion channels responsible for common membrane potential mechanisms has been largely determined, the nature of the channels that underlie the sAHPs in neurons, both in the brain and in the periphery, remains unresolved. This short review discusses why there is no clear molecular candidate for sAHPs.

Kindling induces a transient suppression of afterhyperpolarization in rat subicular neurons

To determine whether chronic epilepsy induces persistent cellular changes in subicular neurons intracellular recordings were used to compare membrane properties of control and kindled rats. In both, control and kindled preparations the subiculum contained regular firing cells and an extensive sub-population of bursting cells expressing amplifying membrane characteristics. Subicular cells showed a transient depression of the fast and slow AHP in the course of kindling that may contribute to the induction but not permanence of the kindled state.

Electrophysiological function of the delayed rectifier (IK) in bullfrog sympathetic ganglion neurones

Neurones of the bullfrog sympathetic ganglion exhibit at least 5 distinct K+ currents. Two calcium-dependent K+ currents IC and IAHP, the delayed rectifier IK, the muscarinic-sensitive IM and the transient outward current IA. Each current plays a unique role in controlling the shapes and firing patterns of action potentials observed in these neurones. We have found that 3,4-diaminopyridine (DAP) (greater than 0.1 mM) will selectively block IK and IA. Concentrations as high as 2 mM have no effect on IC, IAHP or IM. Since IA is mostly inactivated at resting potentials in these cells, DAP can be used to explore the electrophysiological function of IK. Under normal conditions DAP has no effect on action potential duration or no patterns of repetitive activity. This indicates that IK is normally not involved in modulating these parameters. When IC and IAHP are blocked by removing extracellular calcium, however, inhibition of IK prolongs action potential duration, reduces a fast afterhypolarization and enhances spike frequency adaptation. When IM and IAHP are reduced by barium (1 mM), inhibition of IK by DAP has smaller effects on action potential duration and afterhyperpolarization amplitude, but still enhances spike frequency adaptation. We conclude that electrophysiological effects of blocking IK are critically dependent on the levels of other K+ currents found in these cells.

Changes in cytosolic free calcium with 1,2,3,4-tetrahydro-5-aminoacridine, 4-aminopyridine and 3,4-diaminopyridine

The effects of 1,2,3,4-tetrahydro-5-aminoacridine (THA), 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP) on cytosolic free calcium (Ca2+i) were determined. Both 4-AP and THA have been used to treat Alzheimer's disease. THA is a structural analog of the aminopyridines, which alter calcium homeostasis in nerve terminals. The structural similarities between these compounds suggest a common mechanism of action. The aminopyridines raised Ca2+i concentrations in non-depolarized synaptosomes, whereas THA had no effect. Neither the aminopyridines nor THA had any effect on Ca2+i concentrations in potassium-depolarized synaptosomes. These results suggest that the beneficial effects of THA may be mediated by other mechanisms (i.e. neurotransmitter degradative enzyme inhibition), whereas those of 4-AP and 3,4-DAP may be due, at least in part, to their elevation of Ca2+i, which may enhance neurotransmitter release or other calcium-dependent processes.

Verapamil blocks the afterhyperpolarization but not the spike frequency accommodation of rat CA1 pyramidal cells in vitro

The effects of prolonged periods (up to 4 h) of perfusion with verapamil (100 microM) or D600 (100 microM) on synaptically and directly evoked responses of rat CA1 pyramidal cells were determined in vitro. The slow depolarization underlying burst generation and the slow afterhyperpolarization following directly evoked repetitive firing were blocked, but spike frequency accommodation was not. There was an increase in threshold for evoking synaptic responses and the amplitude of evoked inhibitory postsynaptic potentials (IPSPs) was decreased slightly. The results suggest that verapamil can partially block voltage dependent Ca influx into CA1 cells and that currents underlying accommodation and the slow afterhyperpolarization (AHP) may differ.

Optical recording of electrical activity from parallel fibres and other cell types in skate cerebellar slices in vitro

1. A reliable and simple fish brain slice preparation was obtained from the cerebellum of the skate, and its properties were described. 2. A potentiometric oxonol dye, RH-482, and multiple site optical recording of transmembrane voltage (MSORTV) were used to reveal the electrophysiological properties of the parallel fibre action potential and to measure its conduction (0.13 m/s). The parallel fibre action potential was blocked in the presence of tetrodotoxin (TTX) and prolonged by tetraethylammonium (TEA), suggesting that the upstroke depends upon sodium entry and the repolarization upon potassium efflux. An after-hyperpolarization results from a calcium-dependent potassium conductance. 3. A second potentiometric dye, RH-155, differing only slightly from RH-482, exhibited a high affinity for glial cell membrane, and could be used to monitor changes in extracellular potassium concentration by detecting changes in glial membrane potential. 4. Calcium channel blockers such as cadmium ions blocked the optical signal that reflected the extracellular accumulation of potassium. 5. Interventions that modified the extracellular volume, and thereby affected the accumulation of potassium, produced large changes in the optical signal that monitored glial depolarization. Hypertonic and hypotonic bathing solutions resulted in decreases and increases, respectively, in the magnitude of the extrinsic absorption change that tracked potassium accumulation. 6. Blocking sodium-potassium pump activity by means of ouabain prolonged the time course of the optical signal that was related to potassium accumulation in the extracellular space. 7. Extracellular potassium accumulation was revealed to be critically dependent upon intracellular calcium ions.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.