Morphological and electrophysiological aspects of dorsal median paired neurons generating plateau action potentials in the terminal abdominal ganglion of the insect central nervous system

Digger wasp versus cricket: mechanisms underlying the total paralysis caused by the predator's venom

The data presented here describe neurophysiological experiments addressing the question of cellular mechanisms underlying the total paralysis of locomotor behavior in crickets occurring after being stung by females of the digger wasp species Liris niger. The Liris venom effects have been studied by both in vivo recordings from identified neurons of the well-described giant fiber pathway and in vitro recordings from cultured neurons isolated from the terminal ganglion of crickets. The total paralysis of the prey is characterized by a general block of action potential generation as well as by a block of synaptic transmission. Intracellular recordings from neurons in intact ganglia under single electrode voltage-clamp conditions, as well as whole-cell patch-clamp recordings from cultured cricket neurons consistently show that the block of action potential generation by the Liris venom is due to a block of voltage-gated sodium inward currents in neurons of the stung ganglia. Furthermore, our data provide evidence that the Liris venom also blocks calcium currents in identified neurosecretory neurons. On the other hand, outward currents are not affected by the Liris venom. The in vitro recordings suggest that the Liris venom contains active venom components, which, at least for the observed block of inward currents, do not require a metabolic modification. Because venom application does not affect the ACh-induced EPSPs in giant interneurons, the Liris venom does not seem to influence the postsynaptic ACh receptors. The possible pre- and postsynaptic sites of venom action and the functional consequences on synaptic transmission within the giant fiber system are discussed.

Na+-Dependent neuritic spikes initiate Ca2+-dependent somatic plateau action potentials in insect dorsal paired median neurons

The origin of plateau action potentials was studied in short-term cultures of dorsal paired median (DPM) neurons dissociated from the terminal abdominal ganglion of the cockroach, Periplaneta americana. Spontaneous plateau action potentials were recorded by intracellular microelectrodes in cell bodies that had neurite stumps. These action potentials featured a fast initial depolarization followed by a plateau. However, only fast spikes of short duration were observed when the cell was hyperpolarized from the resting membrane potential. These two different components of the action potentials could be separated by applying depolarizing current pulses from a hyperpolarized holding potential. Application of 200 nM tetrodotoxin (TTX) abolished both fast and slow phases, but depolarization to the original resting potential by steady current injection triggered slow monophasic action potentials that could be blocked by 3 mM CoCl2. In contrast, DPM neurons without neurites were not spontaneously active. In these cells, calcium-dependent slow monophasic action potentials were only recorded immediately after impalement or with current pulse stimulation. Immunocytochemical observations showed that dorsal unpaired median (DUM) neuron cell bodies, which are known to exhibit spontaneous sodium-dependent action potentials, reacted with an antibody directed against a synthetic peptide corresponding to the SP19 segment of voltage-activated sodium channels. In contrast, the antibody did not stain DPM neuron cell bodies but gave intense, patchy staining only in the neurite. Whole cell patch-clamp experiments performed on isolated DPM neuron cell bodies without a neurite revealed the presence of an inward current that did not inactivate completly within the duration of the test pulse. This current was insensitive to both 100 nM TTX and sodium-free saline. It was defined as a high-voltage-activated calcium current according to its high threshold of activation (-30 mV) and its sensitivity to 1 mM CdCl2 and 100 nM omega-conotoxin GVIA. Our findings demonstrate that spontaneous sodium-dependent spikes arising from the neurite are required to initiate slow somatic calcium-dependent action potentials in DPM neurons.

Proctolin-like immunoreactivity in dorsal paired median neurons generating plateau action potentials in cockroach Periplaneta americana

The proctolinergic nature of dorsal paired median (DPM) neurons generating plateau action potentials (PAPs) of the cockroach CNS has been demonstrated using a double labelling technique (lucifer yellow staining and proctolin-like immunochemistry). Electrophysiological results support morphological evidence that a direct pathway exists from axons passing throughout the proctodeal nerves to electrically active DPM neuron somata. However, the occurrence of spike failure along the neuritic membrane and the existence of a neuritic spike initiating site has been postulated because spontaneous PAPs recorded intracellularly were not associated on a one-to-one basis with peripheral axon spikes.

Activation of picrotoxin-resistant GABA receptors by GABA and related compounds induces modulation of cockroach dorsal paired median (DPM) neuron firing

Activation of gamma-aminobutyric acid (GABA) receptors in insect dorsal paired median (DPM) neurons induced two types of response which appeared to be mediated by two different GABA receptor subtypes. When activated by bath application of GABA, one receptor subtype, insensitive to picrotoxin (PTX), mediated a drastic reduction in the firing frequency, leading to a blockade of the spontaneous electrical activity. These effects were accompanied by decreases in the amplitude and duration of the plateau action potential (AP) and the spike after-hyperpolarization (AHP). In most cases, a slight depolarization of the resting membrane potential occurred. Bath application of the vertebrate GABA(B) receptor agonists 3-aminopropyl(methyl)phosphinic acid (SKF 97541) and 3-aminopropylphosphinic acid (CGA 147823/CGP 27492) induced similar responses. Another GABA receptor subtype, less sensitive to GABA, mediated a chloride dependent hyperpolarization that was suppressed by bath application of PTX. The approximate locations of these two GABA receptor subtypes were determined by local pressure microapplications of GABA and vertebrate GABAergic agonists. The PTX-sensitive receptors were located predominantly on the surface of the ganglion where the apical pole of the soma is situated, while the PTX-resistant receptors appeared to be located deeper within the ganglion.These results reveal the existence of two GABA receptor subtypes on the DPM neurons and provide evidence for a functional role for PTX-resistant GABA receptors in the regulation of spontaneous firing.