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

The spontaneous electrical activity of neurons in leech ganglia

Using the newly developed voltage-sensitive dye VF2.1.Cl, we monitored simultaneously the spontaneous electrical activity of ∼80 neurons in a leech ganglion, representing around 20% of the entire neuronal population. Neurons imaged on the ventral surface of the ganglion either fired spikes regularly at a rate of 1–5 Hz or fired sparse spikes irregularly. In contrast, neurons imaged on the dorsal surface, fired spikes in bursts involving several neurons. The overall degree of correlated electrical activity among leech neurons was limited in control conditions but increased in the presence of the neuromodulator serotonin. The spontaneous electrical activity in a leech ganglion is segregated in three main groups: neurons comprising Retzius cells, Anterior Pagoda, and Annulus Erector motoneurons firing almost periodically, a group of neurons firing sparsely and randomly, and a group of neurons firing bursts of spikes of varying durations. These three groups interact and influence each other only weakly.

Real-time measurements of synaptic autoinhibition produced by serotonin release in cultured leech neurons

We studied autoinhibition produced immediately after synaptic serotonin (5-HT) release in identified leech Retzius neurons, cultured singly or forming synapses onto pressure-sensitive neurons. Cultured Retzius neurons are isopotential, thus allowing accurate recordings of synaptic events using intracellular microelectrodes. The effects of autoinhibition on distant neuropilar presynaptic endings were predicted from model simulations. Following action potentials (APs), cultured neurons produced a slow hyperpolarization with a rise time of 85.4 +/- 5.2 ms and a half-decay time of 252 +/- 17.4 ms. These inhibitory postpotentials were reproduced by the iontophoretic application of 5-HT and became depolarizing after inverting the transmembranal chloride gradient by using microelectrodes filled with potassium chloride. The inhibitory postpotentials were reversibly abolished in the absence of extracellular calcium and absent in reserpine-treated neurons, suggesting an autoinhibition due to 5-HT acting on autoreceptors coupled to chloride channels. The autoinhibitory responses increased the membrane conductance and decreased subsequent excitability. Increasing 5-HT release by stimulating with trains of ten pulses at 10 or 30 Hz produced 23 +/- 6 and 47 +/- 2% of AP failures, respectively. These failures were reversibly abolished by the serotonergic antagonist methysergide (140 muM). Moreover, reserpine-treated neurons had only 5 +/- 4% of failures during trains at 10 Hz. This percentage was increased to 35 +/- 4% by iontophoretic application of 5-HT. Increases in AP failures correlated with smaller postsynaptic currents. Model simulations predicted that the autoinhibitory chloride conductance reduces the amplitude of APs arriving at neuropilar presynaptic endings. Altogether, our results suggest that 5-HT autoinhibits its subsequent release by decreasing the excitability of presynaptic endings within the same neuron.

Sodium-dependent potassium channels in leech P neurons

In leech P neurons the inhibition of the Na(+)-K(+) pump by ouabain or omission of bath K(+) leaves the membrane potential unaffected for a prolonged period or even induces a marked membrane hyperpolarization, although the concentration gradients for K(+) and Na(+) are attenuated substantially. As shown previously, this stabilization of the membrane potential is caused by an increase in the K(+) conductance of the plasma membrane, which compensates for the reduction of the K(+) gradient. The data presented here strongly suggest that the increased K(+) conductance is due to Na(+)-activated K(+) (K(Na)) channels. Specifically, an increase in the cytosolic Na(+) concentration ([Na(+)](i)) was paralleled by a membrane hyperpolarization, a decrease in the input resistance (R(in)) of the cells, and by the occurrence of an outwardly directed membrane current. The relationship between R(in) and [Na(+)](i) followed a simple model in which the R(in) decrease was attributed to K(+) channels that are activated by the binding of three Na(+) ions, with half-maximal activation at [Na(+)](i) between 45 and 70 mM. At maximum channel activation, R(in) was reduced by more than 90%, suggesting a significant contribution of the K(Na) channels to the physiological functioning of the cells, although evidence for such a contribution is still lacking. Injection experiments showed that the K(Na) channels in leech P neurons are also activated by Li(+).

Beyond the central pattern generator: amine modulation of decision-making neural pathways descending from the brain of the medicinal leech

The biological mechanisms of behavioral selection, as it relates to locomotion, are far from understood, even in relatively simple invertebrate animals. In the medicinal leech, Hirudo medicinalis, the decision to swim is distributed across populations of swim-activating and swim-inactivating neurons descending from the subesophageal ganglion of the compound cephalic ganglion, i.e. the brain. In the present study, we demonstrate that the serotonergic LL and Retzius cells in the brain are excited by swim-initiating stimuli and during spontaneous swim episodes. This activity likely influences or resets the neuromodulatory state of neural circuits involved in the activation or subsequent termination of locomotion. When serotonin (5-HT) was perfused over the brain, multi-unit recordings from descending brain neurons revealed rapid and substantial alterations. Subsequent intracellular recordings from identified command-like brain interneurons demonstrated that 5-HT, especially in combination with octopamine, inhibited swim-triggering neuron Tr1, as well as swim-inactivating neurons Tr2 and SIN1. Although 5-HT inhibited elements of the swim-inactivation pathway, rather than promoting them, the indirect and net effect of the amine was a reliable and sustained reduction in the firing of the segmental swim-gating neuron 204. This modulation caused cell 204 to relinquish its excitatory drive to the swim central pattern generator. The activation pattern of serotonergic brain neurons that we observed during swimming and the 5-HT-immunoreactive staining pattern obtained, suggest that within the head brain 5-HT secretion is massive. Over time, 5-HT secretion may provide a homeostatic feedback mechanism to limit swimming activity at the level of the head brain.