Activity-dependent accumulation of Ca2+ in axon and dendrites of the leech Leydig neuron

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Complex intrinsic membrane properties and dopamine shape spiking activity in a motor axon

We studied the peripheral motor axons of the two pyloric dilator (PD) neurons of the stomatogastric ganglion in the lobster, Homarus americanus. Intracellular recordings from the motor nerve showed both fast and slow voltage- and activity-dependent dynamics. During rhythmic bursts, the PD axons displayed changes in spike amplitude and duration. Pharmacological experiments and the voltage dependence of these phenomena suggest that inactivation of sodium and A-type potassium channels are responsible. In addition, the "resting" membrane potential was dependent on ongoing spike or burst activity, with more hyperpolarized values when activity was strong. Nerve stimulations, pharmacological block and current clamp experiments suggest that this is due to a functional antagonism between a slow after-hyperpolarization (sAHP) and inward rectification through hyperpolarization-activated current (IH). Dopamine application resulted in modest depolarization and "ectopic" peripheral spike initiation in the absence of centrally generated activity. This effect was blocked by CsCl and ZD7288, consistent with a role of IH. High frequency nerve stimulation inhibited peripheral spike initiation for several seconds, presumably due to the sAHP. Both during normal bursting activity and antidromic nerve stimulation, the conduction delay over the length of the peripheral nerve changed in a complex manner. This suggests that axonal membrane dynamics can have a substantial effect on the temporal fidelity of spike patterns propagated from a spike initiation site to a synaptic target, and that neuromodulators can influence the extent to which spike patterns are modified.

Ca2+ influx into identified leech neurons induced by 5-hydroxytryptamine

The role of 5-hydroxytryptamine (5-HT, serotonin) in the control of leech behavior is well established and has been analyzed extensively on the cellular level; however, hitherto little is known about the effect of 5-HT on the cytosolic free calcium concentration ([Ca(2+)](i)) in leech neurons. As [Ca(2+)](i) plays a pivotal role in numerous cellular processes, we investigated the effect of 5-HT on [Ca(2+)](i) (measured by Fura-2) in identified leech neurons under different experimental conditions, such as changed extracellular ion composition and blockade of excitatory synaptic transmission. In pressure (P), lateral nociceptive (N1), and Leydig neurons, 5-HT induced a [Ca(2+)](i) increase which was predominantly due to Ca(2+) influx since it was abolished in Ca(2+)-free solution. The 5-HT-induced Ca(2+) influx occurred only if the cells depolarized sufficiently, indicating that it was mediated by voltage-dependent Ca(2+) channels. In P and N1 neurons, the membrane depolarization was due to Na(+) influx through cation channels coupled to 5-HT receptors, whereby the dose-dependency suggests an involvement in excitatory synaptic transmission. In Leydig neurons, 5-HT receptor-coupled cation channels seem to be absent. In these cells, the membrane depolarization activating the voltage-dependent Ca(2+) channels was evoked by 5-HT-triggered excitatory glutamatergic input. In Retzius, anterior pagoda (AP), annulus erector (AE), and median nociceptive (N2) neurons, 5-HT had no effect on [Ca(2+)](i).

Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.