Etude topographique des interneurones ocellaires et de quelques uns de leurs prolongements chez Periplaneta americana

The topography of the largest ocellar interneurons in the brain of the cockroach Periplaneta americana was shown with cobalt chloride. The ocellar interneurons coloured from one nerve are confined to the ipsilateral side of the pars intercerebralis; their number and their position vary along the ocellar tract. If two ocellar nerves colour from the ocelli, the interneurons show a bilateral symmetry. Only one interneuron runs through the brain between each ocellus and the contralateral connective to the mesothoracic ganglion. When the injection of cobalt chloride is done without any current from the ocellus, the second-order ocellar neurons only are coloured, but when it is done using a current the higher order interneurons are also coloured.Axonal iontophoresis from a cervical connective back into the brain, has revealed that the cellular body of the contralateral higher-order interneuron is situated in the postero-ventral part of the protocerebrum. This pericaryon with a long cellular process is the largest of the ocellar ones (Ø = 50–60 μm). These results are discussed in relation to the ocellar and visual pathways of Schistocerca.     

The mapping of visual space by identified large second-order neurons in the dragonfly median ocellus

In adult dragonflies, the compound eyes are augmented by three simple eyes known as the dorsal ocelli. The outputs of ocellar photoreceptors converge on relatively few second-order neurons with large axonal diameters (L-neurons). We determine L-neuron morphology by iontophoretic dye injection combined with three-dimensional reconstructions. Using intracellular recording and white noise analysis, we also determine the physiological receptive fields of the L-neurons, in order to identify the extent to which they preserve spatial information. We find a total of 11 median ocellar L-neurons, consisting of five symmetrical pairs and one unpaired neuron. L-neurons are distinguishable by the extent and location of their terminations within the ocellar plexus and brain. In the horizontal dimension, L-neurons project to different regions of the ocellar plexus, in close correlation with their receptive fields. In the vertical dimension, dendritic arborizations overlap widely, paralleled by receptive fields that are narrow and do not differ between different neurons. These results provide the first evidence for the preservation of spatial information by the second-order neurons of any dorsal ocellus. The system essentially forms a one-dimensional image of the equator over a wide azimuthal area, possibly forming an internal representation of the horizon. Potential behavioural roles for the system are discussed.     

Fine structure of the first optic ganglion (lamina) of the cockroach, Periplaneta americana

The structural organization of the first optic ganglion (lamina) of the cockroach (Periplaneta americana) was investigated by the use of light and electron microscopy. Each compound eye of the cockroach is composed of up to 2000 visual units (ommatidia) of the fused rhabdom type. The ommatidia themselves consist of eight receptor cells which terminate as axons in either the first or second optic ganglion. Three different short visual fibre types end in two separate strata in the lamina, and one long fibre type ends in the second optic ganglion. Monopolar second-order neurons with wide field branching patterns in the middle stratum of the first synaptic region have postsynaptic contacts with short visual fibres. Horizontal fibre elements with branching patterns at different levels of the lamina apparently form three horizontal plexuses with presynaptic and/or postsynaptic connections to first- and secondorder neurons. The lack of well-organized fibre cartridges containing a constant number of first and second order neurons in each fascicle and the presence of only unistratified wide field monopolar cells could represent, as compared to other insect orders, a primitive stage in the development of the first optic ganglion.     

Dynamics of cockroach ocellar neurons

The incremental responses from the second-order neurons of the ocellus of the cockroach, Periplaneta americana, have been measured. The stimulus was a white-noise-modulated light with various mean illuminances. The kernels, obtained by cross-correlating the white-noise input against the resulting response, provided a measure of incremental sensitivity as well as of response dynamics. We found that the incremental sensitivity of the second-order neurons was an exact Weber-Fechner function; white-noise-evoked responses from second-order neurons were linear; the dynamics of second-order neurons remain unchanged over a mean illuminance range of 4 log units; the small nonlinearity in the response of the second-order neuron was a simple amplitude compression; and the correlation between the white-noise input and spike discharges of the second-order neurons produced a first-order kernel similar to that of the cell's slow potential. We conclude that signal processing in the cockroach ocellus is simple but different from that in other visual systems, including vertebrate retinas and insect compound eyes, in which the system's dynamics depend on the mean illuminance.     

The effects of temperature on signalling in ocellar neurons of the desert locust, <Emphasis Type="Italic">Schistocerca gregaria</Emphasis>

In Schistocerca gregaria ocellar pathways, large second-order L-neurons use graded potentials to communicate signals from the ocellar retina to third-order neurons in the protocerebrum. A third-order neuron, DNI, converts graded potentials into axonal spikes that have been shown in experiments at room temperature to be sparse and precisely timed. I investigated effects of temperature changes that a locust normally experiences on these signals. With increased temperature, response latency decreases and frequency responses of the neurons increase. Both the graded potential responses in the two types of neuron and the spikes in DNI report greater detail about a fluctuating light stimulus. Over a rise from 22 to 35°C the power spectrum of the L-neuron response encompasses higher frequencies and its information capacity increases from about 600 to 1,700 bits/s. DNI generates spikes more often during a repeated stimulus but at all temperatures it reports rapid decreases in light rather than providing a continual measure of light intensity. Information rate carried by spike trains increases from about 50 to 185 bits/s. At warmer temperatures, increased performance by ocellar interneurons may contribute to improved aerobatic performance by delivering spikes earlier and in response to smaller, faster light stimuli.     

Dynamic relationship between the slow potential and spikes in cockroach ocellar neurons

The relationship between the slow potential and spikes of second-order ocellar neurons of the cockroach, Periplaneta americana, was studied. The stimulus was a sinusoidally modulated light with various mean illuminances. A solitary spike was generated at the depolarizing phase of the modulation response. Analysis of the relationship between the amplitude/frequency of voltage modulation and the rate of spike generation showed that (a) the spike initiation process was bandpass at approximately 0.5-5 Hz, (b) the process contained a dynamic linearity and a static nonlinearity, and (c) the spike threshold at optimal frequencies (0.5-5 Hz) remained unchanged over a mean illuminance range of 3.6 log units, whereas (d) the spike threshold at frequencies of less than 0.5 Hz was lower at a dimmer mean illuminance. The voltage noise in the response was larger and the mean membrane potential level was more positive at a dimmer mean illuminance. Steady or noise current injection during sinusoidal light stimulation showed that (a) the decrease in the spike threshold at a dimmer mean illuminance was due to the increase in the noise variance: the noise had facilitatory effects on the spike initiation; and (b) the change in the mean potential level had little effect on the spike threshold. We conclude that fundamental signal modifications occur during the spike initiation in the cockroach ocellar neuron, a finding that differs from the spike initiation process in other visual systems, including Limulus eye and vertebrate retina, in which it is presumed that little signal modification occurs at the analog-to-digital conversion process.     

Nonlinear signal transmission between second- and third-order neurons of cockroach ocelli

Transfer characteristics of the synapse made from second- to third-order neurons of cockroach ocelli were studied using simultaneous microelectrode penetrations and the application of tetrodotoxin. Potential changes were evoked in second-order neurons by either an extrinsic current or a sinusoidally modulated light. The synapse had a low-pass filter characteristic with a cutoff frequency of 25-30 Hz, which passed most presynaptic signals. The synapse operated at an exponentially rising part of the overall sigmoidal input/output curve relating pre- and postsynaptic voltages. Although the response of the second-order neuron to sinusoidal light was essentially linear, the response of the third-order neuron contained an accelerating nonlinearity: the response amplitude was a positively accelerated function of the stimulus contrast, reflecting nonlinear synaptic transmission. The response of the third-order neuron exhibited a half-wave rectification: the depolarizing response to light decrement was much larger than the hyperpolarizing response to light increment. Nonlinear synaptic transmission also enhanced the transient response to step-like intensity changes. I conclude that (a) the major function of synaptic transmission between second- and third-order neurons of cockroach ocelli is to convert linear presynaptic signals into nonlinear ones and that (b) signal transmission at the synapse between second- and third-order neurons of cockroach ocelli fundamentally differs from that at the synapse between photoreceptors and second-order neurons of visual systems so far studied, where the synapse operates in the midregion of the characteristic curve and the transmission is essentially linear.     

Central projections of first-order ocellar interneurons in two orthopteroid insects Acheta domesticus and Periplaneta americana

Summary The central projections of ocellar first-order interneurons in the cricket, Acheta domesticus, and the cockroach, Periplaneta americana, were examined in silver-intensified cobalt preparations. Ten morphologically different types of ocellar interneurons among a total of 44 are recognized in the cricket, and five different types among a total of 26 in the cockroach, indicating that these species have simpler ocellar systems than those described previously in locusts. Ocellar interneurons arborize in the following regions of neuropil in both the cricket and cockroach: the ocellar foci of the posterior protocerebrum, the posterior deutocerebrum, the protocerebral bridge, the ocellar synaptic plexus, ocellar nerves and tracts, and the lobula and medulla of the optic lobes. Ocellar first-order interneurons thus project predominantly to sites where they are likely to synapse with other ocellar and optic-lobe interneurons.     

Intracellular stainings of the large ocellar second order neurons in the cockroach

Summary The large ocellar second order neurons (L-neurons) in the cockroach,Periplaneta americana have been studied physiologically by intracellular recordings and morphologically by intracellular and whole nerve cobalt stainings. All the recorded L-neurons showed similar light responses, i.e., light on-hyperpolarization and a small number of off-spikes. All the stained L-neurons had an ocellar arborization covering the whole region of the ocellar neuropile and an central arborization in the region posterior to the protocerebral bridge.     

Componental analysis of the ocellar electroretinogram of the locust, Schistocerca gregaria

Ruck's componental analysis of the ocellar electroretinogram (ERG) has been reappraised using techniques of signal averaging and waveform subtraction. Components (1), (3), and (4) can readily be isolated in the locust ocellus but component (2) as recorded in the locust ocellus is probably an artefact. Component (1), produced by the receptor cells, only contributes significantly to the total ERG at higher light intensities and it is this contribution which changes most with the degree of light and dark adaptation employed in these experiments. Component (3), the response of the second-order neurones, indicates that the majority of second-order neurones hyperpolarize on illumination of the ocellus. Component (4), the afferent activity of the second-order cells, indicates that more than one afferent axon is involved in the production of off spikes in the locust ocellus.     

Lobula plate and ocellar interneurons converge onto a cluster of descending neurons leading to neck and leg motor neuropil in Calliphora erythrocephala

Summary In the fly, Calliphora erythrocephala, a cluster of three Y-shaped descending neurons (DNOVS 1–3) receives ocellar interneuron and vertical cell (VS4–9) terminals. Synaptic connections to one of them (DNOVS 1) are described. In addition, three types of small lobula plate vertical cell (sVS) and one type of contralateral horizontal neuron (Hc) terminate at DNOVS 1, as do two forms of ascending neurons derived from thoracic ganglia. A contralateral neuron, with terminals in the opposite lobula plate, arises at the DNOVS cluster and is thought to provide heterolateral interaction between the VS4–9 output of one side to the VS4–9 dendrites of the other. DNOVS 2 and 3 extend through pro-, meso-, and metathoracic ganglia, branching ipsilaterally within their tract and into the inner margin of leg motor neuropil of each ganglion. DNOVS 1 terminates as a stubby ending in the dorsal prothoracic ganglion onto the main dendritic trunks of neck muscle motor neurons. Convergence of VS and ocellar interneurons to DNOVS 1 comprises a second pathway from the visual system to the neck motor, the other being carried by motor neurons arising in the brain. Their significance for saccadic head movement and the stabilization of the retinal image is discussed.     

The Drosophila gap gene giant regulates ecdysone production through specification of the PTTH-producing neurons

In Drosophila melanogaster, hypomorphic mutations in the gap gene giant (gt) have long been known to affect ecdysone titers resulting in developmental delay and the production of large (giant) larvae, pupae and adults. However, the mechanism by which gt regulates ecdysone production has remained elusive. Here we show that hypomorphic gt mutations lead to ecdysone deficiency and developmental delay by affecting the specification of the PG neurons that produce prothoracicotropic hormone (PTTH). The gt¹ hypomorphic mutation leads to random loss of PTTH production in one or more of the 4 PG neurons in the larval brain. In cases where PTTH production is lost in all four PG neurons, delayed development and giant larvae are produced. Since immunostaining shows no evidence for Gt expression in the PG neurons once PTTH production is detectable, it is unlikely that Gt directly regulates PTTH expression. Instead, we find that innervation of the prothoracic gland by the PG neurons is absent in gt hypomorphic larvae that do not express PTTH. In addition, PG neuron axon fasciculation is abnormal in many gt hypomorphic larvae. Since several other anteriorly expressed gap genes such as tailless and orthodenticle have previously been found to affect the fate of the cerebral labrum, a region of the brain that gives rise to the neuroendocrine cells that innervate the ring gland, we conclude that gt likely controls ecdysone production indirectly by contributing the peptidergic phenotype of the PTTH-producing neurons in the embryo.     

Ribeiroia marini: Irradiated miracidia and induction of acquired resistance in Biomphalaria glabrata

Biomphalaria glabrata snails sensitized by exposure to X-irradiated miracidia of the trematode, Ribeiroia marini, acquired resistance to challenge with nonirradiated R. marini miracidia. Resistance was acquired within 1 day of sensitization; was strongest at 1 week, when infection rates of sensitized snails were 15% of the controls (i.e., $\text{S}\text{C}\text{= 0.15}$); and persisted for at least 3 weeks. By 30 days the difference between the infection rates of sensitized and control snails was no longer statistically significant. As in previous studies with echinostomes, acquired resistance to R. marini was characterized histologically by the destruction of irradiated sporocysts by host amoebocytes. Following destruction of all irradiated sporocysts, snails became resistant and encapsulated and destroyed nonirradiated challenge sporocysts within 1 day postchallenge. Associated with sporocyst destruction was an enlargement of the amoebocyte-producing organ, which showed intense mitotic activity. A proportion of the nonirradiated challenge sporocysts were also destroyed in most nonsensitized control snails, which consequently had a temporarily enlarged amoebocyte-producing organ. In contrast to acquired resistance reported to echinotomes, which is quite specific, acquired resistance to R. marini was associated with nonsusceptibility to both Echinostoma paraensei ($\text{S}\text{C}\text{= 0.19}$) and Schistosoma mansoni ($\text{S}\text{C}\text{= 0.81}$).     

The projection of neuroendocrine fibers (NCC I and II) in the brains of three orthopteroid insects

Neuronal projections from neuroendocrine tracts (nervi corpori cordiaci I and II) in the brains of the locust (Schistocerca vaga), cricket (Acheta domesticus), and cockroach (Periplaneta americana) were studied using reconstructions of silver-intensified cobalt chloride preparations. Collaterals from the NCC I in these species branch extensively in the dorsal protocerebral neuropile, anterior to the stalk of the corpora pedunculata and ventral to its calyces. Other fibers project from the NCC I bilaterally into the medial protocerebral neuropile, anterior to the central body, and posterior to the beta lobes. NCC II collaterals arborize in the medial, dorsal, and lateral protocerebral neuropile, their region of projection partially overlapping with that of the NCC I. Several NCC II fibers terminate in the superior arch of the central body in Acheta but not in the other two species. Tritocerebral cells filled through the NCC I branch in the medial tritocerebral neuropile in all three species, but most extensively in Schistocerca. No NCC fibers were seen to penetrate any part of the corpora pedunculata, protocerebral bridge, olfactory glomeruli, ocellar tracts, or optic lobes. These neuronal projections from the NCC I and II lie anterior to regions of branching of second-order ocellar fibers and thus provide no anatomical basis for direct ocellar input to neurosecretory cells, contrary to previous reports for orthopteroid species (Brousse-Gaury, '71a, b). However, interneurons filled from the optic lobes were found to terminate in the same region of dorsal protocerebral neuropile as NCC I and II fibers in Acheta, thus providing a possible pathway for optic input to the cerebral neuroendocrine system.     

Electrophysiology of the Insect Dorsal Ocellus : I. Origin of the components of the electroretinogram

Dorsal ocelli are small cup-like organs containing a layer of photoreceptor cells, the short axons of which synapse at the base of the cup with dendritic terminals of ocellar nerve fibers. The ocellar ERG of dragonflies, recorded from the surface of the receptor cell layer and from the long lateral ocellar nerve, contains four components. Component 1 is a depolarizing sensory generator potential which originates in the distal ends of the receptor cells and evokes component 2. Component 2 is believed to be a depolarizing response of the receptor axons. It evokes a hyperpolarizing postsynaptic potential, component 3, which originates in the dendritic terminals of the ocellar nerve fibers. Ocellar nerve fibers in dragonflies are spontaneously active, discharging afferent nerve impulses (component 4) in the dark-adapted state. Component 3 inhibits this discharge. The ERG of the cockroach ocellus is similar. The main differences are that component 3 is not as conspicuous as in the dragonflies and that in most cases ocellar nerve impulses appear only as a brief burst at "off." In one preparation a spontaneous discharge of nerve impulses was observed. As in the dragonflies, this was inhibited by illumination.     

Direct interactions between immunocytes and neurons after axotomy in Aplysia

Axon growth during development and after injury has processes in common, but also differs in that regeneration requires the participation of cells of the immune system. To investigate how neuron-immunocyte interactions might influence regeneration, we developed an in vitro model whereby neurons and hemocytes from Aplysia californica were cocultured. The hemocytes, which behave like vertebrate macrophages, migrated randomly throughout the dish. When a neuron was encountered, some hemocytes exhibited an avoidance response, whereas others formed stable contacts. Hemocytes did not distinguish between neurons from different animals. Stable contacts occurred on neurites and growth cones, but not the cell soma, and were benign in that the hemocytes did not impede neurite growth. When hemocytes attached to the cell body, it presaged the destruction of the neuron. Destruction was a dynamic process that was initiated when groups of one to three hemocytes adhered to various regions of the cell soma. Each group was then joined by other hemocytes. They did not contact the neuron, but interconnected the initial groups, forming a network around the neuron. The network then contracted to dismember the cell. Once a neuron was destroyed, hemocytes removed the debris by phagocytosis. Both damaged neurons and those without apparent damage were targets for destruction. Severing neurites with a needle resulted in the destruction of only one of six cells. Our studies suggest that hemocytes, and by extrapolation, vertebrate macrophages, exhibit highly complex interactions with neurons that can exert a variety of influences on the course of nerve regeneration.     

Fine structure of ocelli of an anthomedusan,Nemopsis dofleini,with special reference to synaptic organization

Summary An ocellus of an anthomedusan, Nemopsis dofleini, is composed of sensory and pigment cells and underlain by a nerve plexus and a muscle sheet. A sensory cell is divided into three parts: an apical part from which a single cilium arises, a slender middle part with numerous microtubules and an enlarged basal part that contains an oval nucleus but does not send out an axon. The ocellar cup is occupied by variously remodelled ciliary sheaths that are covered by a few lysosomal projections from the pigment cells. Three modes of synaptic connections — centripetal, centrifugal and two-way — are found between sensory cells and either dendrites or somata of second order neurons. Synaptic vesicles in sensory cells are larger in number, smaller in size and more uniform in shape than those of second order neurons. The soma of a second order neuron lies below the surface layer of an ocellar cup and gives rise to a single cilium that lacks rootlets and the second centriole. The possibility of multimodal sensory perception in and around the ocellar region is discussed.The work was supported by research grants from the Ministry of EducationFormerly Tamano Marine Laboratory     

The specificity of the serum agglutinins of Periplaneta americana and Schistocerca gregaria and its relationship to the insects' immune response

The agglutinating activity of insect serum against vertebrate erythrocytes has been examined for two insect species, the cockroach Periplaneta americana and the locust Schistocerca gregaria. Differences were found between the two insect species, in that cockroach serum agglutinated a wider range of erythrocyte types than did locust serum and the titre of the agglutinating activity of cockroach serum was higher in all cases. The results of attempts to inhibit the agglutinating activity using a variety of sugars and glycoproteins revealed that the combining specificities of the agglutinating molecules of the two species differed. Agglutination of rat erythrocytes by cockroach serum was not inhibited by any of the sugars or glycoproteins tested, whereas several of these compounds, in particular sucrose, partially inhibited the agglutination of rat erythrocytes by locust serum.The significance of these results is discussed in relation to the observation that haemocytes of the cockroach respond to a wider range of transplanted tissues in vivo than do those of the locust.