Immunocytochemical localization of sodium channels in an insect central nervous system using a site-directed antibody

Putative excitatory and putative inhibitory inputs are localised in different dendritic domains in a Drosophila flight motoneuron

Input-output computations of individual neurons may be affected by the three-dimensional structure of their dendrites and by the location of input synapses on specific parts of their dendrites. However, only a few examples exist of dendritic architecture which can be related to behaviorally relevant computations of a neuron. By combining genetic, immunohistochemical and confocal laser scanning methods this study estimates the location of the spike-initiating zone and the dendritic distribution patterns of putative synaptic inputs on an individually identified Drosophila flight motorneuron, MN5. MN5 is a monopolar neuron with > 4,000 dendritic branches. The site of spike initiation was estimated by mapping sodium channel immunolabel onto geometric reconstructions of MN5. Maps of putative excitatory cholinergic and of putative inhibitory GABAergic inputs on MN5 dendrites were created by charting tagged Dα7 nicotinic acetylcholine receptors and Rdl GABAA receptors onto MN5 dendritic surface reconstructions. Although these methods provide only an estimate of putative input synapse distributions, the data indicate that inhibitory and excitatory synapses were located preferentially on different dendritic domains of MN5 and, thus, computed mostly separately. Most putative inhibitory inputs were close to spike initiation, which was consistent with sharp inhibition, as predicted previously based on recordings of motoneuron firing patterns during flight. By contrast, highest densities of putative excitatory inputs at more distant dendritic regions were consistent with the prediction that, in response to different power demands during flight, tonic excitatory drive to flight motoneuron dendrites must be smoothly translated into different tonic firing frequencies.

Dendritic action potential initiation in hypothalamic gonadotropin-releasing hormone neurons

It is dogma that action potentials are initiated at the soma/axon hillock of neurons. However, dendrites often exhibit conductances necessary for spike generation and represent functionally independent processing compartments within neurons. GnRH neurons provide an interesting neuronal phenotype with simple, relatively unbranched, unipolar or bipolar dendrites of extensive lengths (>1000 microm) covered in spines. These neurons control fertility and must integrate a variety of internal homeostatic and external environmental cues. We used imaging, electrophysiological, and modeling studies to understand how they integrate and process information along dendrites. Simultaneous recordings from distal dendrites and somata of individual GnRH neurons indicate distal dendrites are the primary site of spike initiation in these cells. Compartmental modeling indicates that sites of spike initiation depend upon location of excitatory input and dendrite geometry. Together, these studies demonstrate a novel pattern of spike generation in mammalian neurons and indicate that afferent inputs within distal dendritic microdomains directly initiate action potentials.

A steroid hormone affects sodium channel expression in Manduca central neurons

Neuronal differentiation is characterized by stereotypical sequences of membrane channel and receptor acquisition. This is regulated by the coordinated interactions of a variety of developmental mechanisms, one of which is the control by steroid hormones. We have used the metamorphosis of the holometabolous insect, Manduca sexta, as a model to study effects of 20-hydroxyecdysone on the maturation of thoracic neuron membrane channel expression. To test for direct hormone action, neurons were dissociated into primary cell culture on the first day of pupal life. In situ hybridization demonstrated that the amount of expression of the acetylcholine receptor alpha subunit, MARA1, was not affected by 20-hydroxyecdysone. Immunocytochemistry with an antibody directed against the SP19 segment of voltage-gated sodium channels revealed no effect of 20-hydroxyecdysone treatment during the first 6 days in culture. SP19 sodium channel protein was evenly distributed along all neurites. In contrast, after 8 days in culture, 20-hydroxyecdysone increased the amount of SP19 protein expression and strongly affected its distribution in differentiating neurons. In the presence of 20-hydroxyecdysone, patches of high densities of SP19 sodium channel protein were found in growth cones close to the base of filopodia. This is a further step toward unraveling the blend of membrane proteins under the control of steroids during the development of the central nervous system of postembryonic Manduca. Our results, taken together with previous studies, indicate that 20-hydroxyecdysone does not affect the expression of potassium membrane current or of the nicotinic acetylcholine receptor but instead regulates the amplitude of the calcium membrane current and the amount and distribution of SP19 sodium channel protein.

A new regulation of non-capacitative calcium entry in insect pacemaker neurosecretory neurons. Involvement of arachidonic acid, no-guanylyl cyclase/cGMP, and cAMP

Efferent dorsal unpaired median neurons are pacemaker neurosecretory cells. A Ca(2+) background current contributing to the pacemaker activity of cockroach dorsal unpaired median neurons is up-regulated by neurohormone D (NHD), an octapeptide belonging to the adipokinetic hormone family. This modulation accelerates spiking and increases [Ca(2+)](i). Using patch clamp, calcium imaging, and immunocytochemistry, we investigated the signaling pathway of NHD-induced current modulation. The membrane depolarization produced by NHD was related to the increase in membrane conductance for Ca(2+), Ba(2+), or Sr(2+). This increase was abolished by LOE 908, an inhibitor of noncapacitive Ca(2+) entry (NCCE), and it was strongly attenuated by the phospholipase C inhibitor U37122 and the diacylglycerol lipase inhibitor RHC80267. Arachidonic acid and ETYA mimicked the NHD effect on background current. This was abolished by l-NAME and ODQ, inhibitors of NO synthase and NO-sensitive guanylyl cyclase, respectively, but mimicked by the NO donor sodium nitroprusside and 8-bromo-cGMP. Immunocytochemistry using cGMP antibodies indicated that NHD and ETYA increase cGMP. Inhibition of protein kinase G with KT5823 and R(p)-8-pCPT-cGMPS had no effect, whereas zaprinast, a cGMP-specific phosphodiesterase 5,6,9 inhibitor, mimicked the NHD effect. Furthermore, inhibition of the cGMP-activated phosphodiesterase 2 by EHNA and trequinsin abolished the effect of NHD. We conclude that the final step of the NHD signal transduction is the phosphodiesterase 2-induced down-regulation of the cAMP level. This removes a depression of NCCE directly attributed to cAMP because inhibition of protein kinase A with KT5720, R(p)-cAMPS, and PKI14-22 amide did not mimic the NHD effect. We also demonstrate that any mechanism increasing the cGMP level can induce NCCE.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

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.

Expression and distribution of voltage-sensitive sodium channels in pyrethroid-susceptible and pyrethroid-resistant Musca domestica

Knockdown resistance (kdr) to pyrethroid insecticides has been found in numerous insect species. kdr causes nerve insensitivity by altering the primary target of these insecticides, the voltage-sensitive sodium channel. In Musca domestica, cloning and sequencing of susceptible, kdr, and super-kdr alleles of the sodium channel gene (Msc) homologous to the Drosophila melanogaster para channel gene has revealed point mutations. The conservation of the nature and of the position of these mutations strongly suggests a role in the kdr mechanism. To determine if these mutations are associated with modifications of channel expression in adult flies, we investigated the localization of the Msc transcripts, and the size and the tissue distribution of the channel protein in pyrethroid-susceptible and super-kdr strains. Msc channels were mainly found in the cortical regions of the central nervous system with additional labeling in some neuronal processes and in the eyes. No qualitative or quantitative difference was observed between the strains. In immunoblotting experiments, anti-Msc antibodies bound to only one polypeptide of 260 kDa in adult brain. No differences were found in antibody staining between susceptible and pyrethroid-resistant strains. These results were correlated with those on Drosophila melanogaster, for which two sodium channel genes have been identified.

Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons

Membrane excitability in different tissues is due, in large part, to the selective expression of distinct genes encoding the voltage-dependent sodium channel. Although the predominant sodium channels in brain, skeletal muscle, and cardiac muscle have been identified, the major sodium channel types responsible for excitability within the peripheral nervous system have remained elusive. We now describe the deduced primary structure of a sodium channel, peripheral nerve type 1 (PN1), which is expressed at high levels throughout the peripheral nervous system and is targeted to nerve terminals of cultured dorsal root ganglion neurons. Studies using cultured PC12 cells indicate that both expression and targeting of PN1 is induced by treatment of the cells with nerve growth factor. The preferential localization suggests that the PN1 sodium channel plays a specific role in nerve excitability.

Sodium channel distribution in a spider mechanosensory organ

A site-directed antibody was used immunocytochemically to measure the distribution of sodium channels in the tissues of a spider mechanoreceptor organ. The VS-3 slit sense organ contains 7-8 pairs of bipolar sensory neurons; these neurons are representative of a wide range of arthropod mechanoreceptors. Sensory transduction is thought to occur at the tips of the dendrites and to cause action potentials that are regeneratively conducted to the cell bodies, although it has not been possible to confirm this by direct intracellular recordings from the dendrites. Wholemount preparations were labelled by immunofluorescence and thin sections were immunogold labelled, using an antibody to the highly conserved SP19 sequence of the voltage-activated sodium channel. Labelling for sodium channels was found in the neurons and in their surrounding glial cells. Both cytoplasm and membranes were labelled, but immunogold particles were clearly aligned along cell membranes, indicating that the majority of labelling represented membrane-bound sodium channels. Channel density in the dendrites was similar to the axons and higher than in the cell bodies, supporting the idea of active conduction in the sensory dendrites. Labelling in glial cell membranes was indistinguishable from the neighboring neurons, suggesting a significant role for sodium channels in the functions of these supporting cells.