Autoradiographic localization of 3H-histamine accumulation by the visual system of the locust

Summary The uptake of [³H]-histamine into the retina and optic lobe of the locust, Schistocerca americana gregaria was studied by means of autoradiography at the light- and electron-microscopic levels. Light-microscopic autoradiography showed a significant accumulation of [³H]-histamine in several regions of the optic lobe. Dense accumulations of silver grains were concentrated along the medial border of the medullary neuropil and around the entire periphery of the lobula. No significant accumulations of grains were present within the retina or the neuropil zones of the lamina, medulla or lobula.Electron-microscopic autoradiography showed histamine-accumulating cells along the border of the medulla to exhibit electron density and morphology typical of glial cells. Labelled histamine was present within both glial cell bodies and their processes. In the region surrounding the neuropil of the lobula, [³H]-histamine was concentrated within fine glial processes wrapped around neuronal cell bodies and their axons. No neuronal cell bodies or axons showed accumulation of silver grains above background.These results are consistent with previous studies showing the glial uptake of amino acid and biogenic amine putative neurotransmitters. However, the lack of a demonstration of a specific uptake of histamine in neuropil zones makes it difficult to assess the role of histamine uptake in the inactivation of neurally released histamine in the locust visual system.     

Input and output synapses on identified motor neurones of a locust revealed by the intracellular injection of horseradish peroxidase

Summary Physiologically characterised motor neurones in the thoracic ganglia of the locust were injected with horseradish peroxidase in order that the spatial relationship between their input and output synapses could be observed with the electron microscope. A modification in the development procedure for the peroxidase ensured that the internal fine structure of the stained neurones was not obscured by the diaminobenzidine reaction product. Input and output synapses may occur within 1 m of each other on the neuropilar processes of the motor neurones. This supports physiological evidence that motor neurones may be involved in local circuit interactions within the thoracic ganglia.     

Sublinear summation of responses in locust photoreceptors

Summary The response of a locust, Locusta migratoria, photoreceptor to a simultaneous pair of dim flashes is smaller than the sum of the responses to the individual component flashes, even when the photon absorption sites are separated by a distance of 120 m, which should prevent them from interacting biochemically. A similar depression is observed when a flash is paired with a depolarizing current instead of a second flash, irrespecive of whether the flash response precedes or follows the current response. These results indicate that the sublinear summation is caused by the activation of a voltage-activated shunting conductance. This conductance is not blocked by low intracellular concentrations of tetraethylammonium chloride, and is therefore different from the conductance which causes the decrease of the receptor's step response from transient to steady-state.     

Visual pigment processes and prolonged pupillary responses in insect photoreceptor cells

The visual pigment in the peripheral retinular cells of the hoverfly Syrphus balteatus was investigated by absorbance difference measurements. Different visual pigments were found in the dorsal versus the ventral part of the eye in the male, but not in the female. In the male in the dorsal part of the eye the visual pigment has an isosbestic point at 513 nm; in the ventral part this value is 490 nm. The latter value is found in the female in both parts of the eye.Prolonged pupillary responses were studied in the male Syrphus and appeared to be most marked in the ventral part of the eye. In both hoverfly and blowfly prolonged pupillary responses are induced by short wavelength light only; i.e., by light which excessively can convert rhodopsin into metarhodopsin. By contrast, in butterflies red light (and a long dark adaptation time) is necessary to evoke a prolonged pupillary response. It was demonstrated in both hoverfly and blowfly that long wavelength light, which reconverts metarhodopsin into rhodopsin, inhibits a prolonged pupillary response; or, accelerates pupil opening.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Evidence for the possible existence of a second polarization-vision pathway in the locust brain

For spatial orientation and navigation, many insects derive compass information from the polarization pattern of the blue sky. The desert locust Schistocerca gregaria detects polarized light with a specialized dorsal rim area of its compound eye. In the locust brain, polarized-light signals are passed through the anterior optic tract and tubercle to the central complex which most likely serves as an internal sky compass. Here, we suggest that neurons of a second visual pathway, via the accessory medulla and posterior optic tubercle, also provide polarization information to the central complex. Intracellular recordings show that two types of neuron in this posterior pathway are sensitive to polarized light. One cell type connects the dorsal rim area of the medulla with the medulla and accessory medulla, and a second type connects the bilaterally paired posterior optic tubercles. Given the evidence for a role of the accessory medulla as the master clock controlling circadian changes in behavioral activity in flies and cockroaches, our data open the possibility that time-compensated polarized-light signals may reach the central complex via this pathway for time-compensated sky-compass navigation.     

Inhibition of yolk formation in locust oocytes by trypan blue and suramin

Summary Trypan blue and suramin inhibit receptor-mediated endocytosis of vitellogenin in Locusta migratoria. Both drugs bind to cationic side chains of the vitellogenin molecule, which are presumably also the binding sites for the specific vitellogenin receptor. Thus binding of vitellogenin to the receptor is prevented in isolated oocytes and in oocyte membrane preparations. Small amounts of trypan blue may unspecifically enter oocytes by piggy-back endocytosis. Suramin has a high affinity for vitellogenin and in contrast to trypan blue it does not form insoluble complexes with the protein. Therefore, it may be a useful tool for further analysis of the locust vitellogenin receptor.     

Interneurones subserving ocelli in two species of trichopterous insects: morphology and central projections

Summary The central projections of ocellar interneurones in two species of trichopterous insects Agrypnia varia F. and Limnephilus flavicornis F. were analysed by use of cobalt iontophoresis. The interneurones were classified into three groups: large-, medium- and small-caliber neurones based on the diameters of the axons. Seven large-diameter neurones project from each lateral ocellus into the central nervous system. Of these, four neurones terminate in the posterior slope (three ipsilateral and one contralateral). Three neurones possess branches in the contralateral posterior slope and proceed down the cervical connective into the thoracic ganglia. Medium-sized neurones connect the neuropiles of the three ocelli to each other. Small-diameter neurones contact the contralateral lobula and medulla of the optic lobes and connect the three ocellar neuropiles. Large-diameter neurones of the median ocellus were found to terminate bilaterally or ipsilaterally in the posterior slope. In the posterior slope four different subregions can be recognised: (1) the dorso-lateral, (2) the ventro-lateral, (3) the lateral, into which large-diameter interneurones of the lateral ocelli send branches, and (4) the medial, innervated by interneurones of the median ocellus. Interneurones of the median ocellus send branches into the lateral region as well.     

Synaptic specificity in the first instar cockroach: patterns of monosynaptic input from filiform hair afferents to giant interneurons

Summary Direct evidence for monosynaptic connections between filiform hair sensory axons and giant interneurons (GIs) in the first instar cockroach, Periplaneta americana, was obtained using intracellular recording and HRP injection followed by electron microscopy. GIs 1–6 all receive monosynaptic input from at least one filiform afferent axon. GI1, GI2 and GI5 receive input only from the medial (M) axon, while GI3, GI4 and GI6 receive input from both M and lateral (L) axons. The dendrites of GI3 and GI6 which are contralateral to the cell bodies receive input from both axons whereas the smaller ipsilateral dendritic fields have synapses only from the L axon. GI5 has M axon input only onto its contralateral dendrites. In 50% of preparations GI7 receives weak input from the ipsilateral L axon. There is no obvious relationship between the morphology of the giant interneurons and the pattern of input they receive from the filiform afferents.Abbreviations GI giant interneuron - HRP horseradish peroxidase - L lateral axon - M medial axon     

Reflex activation of locust flight motoneurones by proprioceptors responsive to muscle contractions

 This report investigates the reflex activation of locust flight motoneurones following their spiking activity. As shown elsewhere, an electrical stimulus applied to a flight muscle produces multiple waves of delayed excitation in wing elevator and depressor motoneurones. Nerve ablation experiments show that this response is initiated by the mechanical movement of the stimulated muscle, and not the antidromic spike evoked in the motoneurone. The delayed excitation still occurs in the absence of inputs from the wing receptor systems, and also when all other sources of afferent feedback are abolished, excepting thoracic nerve 2. Following complete deafferentation, spikes in flight motoneurones had no influence on other flight motoneurones. Numerous afferents in the purely sensory nerve 2 are excited by flight muscle contractions. The responses are consistent for repeated contractions of the same muscle, but differ when other muscles are stimulated. During tethered flight, changes in the activation of single flight muscles are reflected in changes of the nerve 2 discharge pattern. Electrical stimulation of this nerve causes delayed excitation of flight motoneurones, and can initiate flight activity. It is suggested that internal proprioceptors, such as those associated with nerve 2, will contribute to shaping the final motor output for flight behaviour. Accepted: 24 April 1996     

Flight-correlated activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria

The study investigates activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria during wind-elicited tethered flight. Neurons with ascending projections from the ventral nerve cord to the lateral accessory lobes showed flight-associated excitations which were modulated in the flight motor rhythm. Descending neurons with ramifications in the lateral accessory lobes were tonically excited corresponding to flight duration. The onset of wind-elicited responses in the descending neurons preceded the onset of flight motor activity by 22–60 milliseconds. Neurons connecting the lateral accessory lobes with the central body, the anterior optic tubercles, or other brain areas showed a variety of responses including activity changes during flight initiation and flight termination. Activity in many of these neurons was less tightly coupled to the flight situation and often returned to background levels before flight was terminated. Most of the recorded neurons responded, in addition, to stationary visual stimuli. The results suggest that the lateral accessory lobes in the locust brain are integrative links between the central body, visual pathways, and the ventral nerve cord. The possible involvement of these brain areas in flight control is discussed.     

Adaptation accentuates responses of fly motion-sensitive visual neurons to sudden stimulus changes

Adaptation in sensory and neuronal systems usually leads to reduced responses to persistent or frequently presented stimuli. In contrast to simple fatigue, adapted neurons often retain their ability to encode changes in stimulus intensity and to respond when novel stimuli appear. We investigated how the level of adaptation of a fly visual motion-sensitive neuron affects its responses to discontinuities in the stimulus, i.e. sudden brief changes in one of the stimulus parameters (velocity, contrast, grating orientation and spatial frequency). Although the neuron''s overall response decreased gradually during ongoing motion stimulation, the response transients elicited by stimulus discontinuities were preserved or even enhanced with adaptation. Moreover, the enhanced sensitivity to velocity changes by adaptation was not restricted to a certain velocity range, but was present regardless of whether the neuron was adapted to a baseline velocity below or above its steady-state velocity optimum. Our results suggest that motion adaptation helps motion-sensitive neurons to preserve their sensitivity to novel stimuli even in the presence of strong tonic stimulation, for example during self-motion.     

Peaking of response in locust visual interneurones to objects of small angular subtense

One compound eye of an immobilised locust viewed a large screen on to which were projected discs of light for periods of 2 sec every 40 sec. The spike response was counted concurrently in the DCMD and the next largest axon in the contralateral nerve cord connective. The average score for 10 trials, after correction for background was plotted for a series of discs subtending a range of angles from 0.05 to 84°.It was found that the response of the DCMD peaked sharply and consistently at a subtense of 0.3°, and fell away to a low plateau or to zero over the range 2° to 84°. The response could exceed background down to subtenses as low as 0.05° (3′ of arc). The response of the next largest axon also showed an early peak, but it was inhibitory and resembled a mirror image of that of the DCMD, although it did not always coincide, ranging from 0.2 to 0.3°. The response, by contrast with the DCMD, rose to a high level at large subtenses, forming a flat peak.No explanation in optical terms could be found for this peaking at small subtenses, and a scheme is proposed by which a peak response could develop by the interaction of excitatory and inhibitory processes in the optic lobe.The peak value of 0.3° corresponds with the resolution limit for moving periodic patterns repeatedly demonstrated by Burtt and Catton (e.g. 1962, 1969). Such peaking behaviour would serve to lift the response curve of the whole visual system at high spatial frequencies, and thus extend the resolution limit.With stepwise reduction in intensity of a small luminous target there was a steep fall in the DCMD response, but a similar reduction for a large target had only a small effect. This could be explained by assuming that excitatory processes were prevalent for small targets, subtending about 0.3°, whereas for larger targets the excitatory and inhibitory processes came into balance over a wide range of intensities, thereby stabilising the response, at a low level.     

大鼠下丘神经元对重复刺激适应性的时频表征

自由声场下,通过给大鼠不同刺激呈现率（presentation rate,PR）的重复声刺激,用钨丝单电极记录神经元的放电信号,系统分析了下丘神经元对重复刺激的表征特性。作为发放率表征的刺激后脉冲发放数（spike count,SC）随着刺激重复不断减少,作为时间表征的首次发放潜伏期（first spike latency,FSL）逐渐延长,时间过程均呈指数形式变化。起始型神经元FSL 的时间常数大于SC,在FSL上呈现慢适应;持续性神经元FSL 的时间常数小于SC,在SC 上呈现慢适应。随着刺激呈现率PR 的增加,过渡过程的时间常数缩短,稳态SC减少,
稳态FSL延长。稳态SC 和FSL与PR呈对数线性关系,SC 的线性度更高。下丘神经元的适应性能够提高对新奇刺激的响应能力,为皮层下检测异常信息提供了可能。     

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.     

Responses and locations of neurones in the locust metathoracic femoral chordotonal organ

Summary Insect legs possess chordotonal organs which monitor leg angle, and the direction, velocity and acceleration of leg movements. The locust metathoracic femoral chordotonal organ (mtFCO) has previously been studied morphologically and physiologically, but no detailed analysis of the responses of individual neurones, and their location in the organ has so far been produced. By recording from, and staining mtFCO neurones I have been able to compile for the first time such a map. The distribution of neurone somata in the locust mtFCO is more complex than previously thought: receptors sensitive to both stretch and relaxation of the apodeme are distributed throughout the organ. Seventeen response types were encountered. Neurones with a particular response type have somata in comparable locations within the mtFCO. Comparisons are made between the response types found in the stick insect and those in the locust. The possible functions of some of the responses are discussed.Abbreviation (mt)FCO (metathoracic) femoral chordotonal organ - F-T femur-tibia     

Characterisation of columnar neurons and visual signal processing in the medulla of the locust optic lobe by system identification techniques

We describe visual responses of seventeen physiological classes of columnar neuron from the retina, lamina and medulla of the locust (Locusta migratoria) optic lobe. Many of these neurons were anatomically identified by neurobiotin injection. Characterisation of neuronal responses was made by moving and flash stimuli, and by two system identification techniques: 1. The first-order spatiotemporal kernel was estimated from response to a spatiotemporal white-noise stimulus; 2. A set of kernels to second order was derived by the maximal-length shift register (M-sequence) technique, describing the system response to a two-channel centre-surround stimulus. Most cells have small receptive fields, usually with a centre diameter of about 1.5°, which is similar to that of a single receptor in the compound eye. Linear response components show varying spatial and temporal tuning, although lateral inhibition is generally fairly weak. Second-order nonlinearities often have a simple form consistent with a static nonlinear transformation of the input from the large monopolar cells of the lamina followed by further linear filtering.Abbreviations LMC large monopolar cell - LVF long visual fibre - RF receptive field - SMC small monopolar cell - SVF short visual fibre     

DUM neurons in locust flight: a model system for amine-mediated peripheral adjustments to the requirements of a central motor program

In both vertebrates and invertebrates, multiple effects of biogenic amines on neuromuscular transmission, muscle contraction kinetics and metabolism have been described. Nevertheless, it is not yet known whether and how these different effects work in concert during the performance of a specific behavior. In the locust flight system, the biogenic amine octopamine is released as a neurohormone into the haemolymph, and also delivered directly onto specific target muscles by individually identified dorsal unpaired median neurons. Determining the connectivity of these neurons and their activation during behavior, we show for the first time that different types of dorsal unpaired median neurons are differentially connected to certain components of the flight circuitry. During flight, all types of pterothoracic dorsal unpaired median neurons innervating flight muscles receive inhibitory inputs from tegula proprioceptive afferents and from the central flight circuitry, whereas all other types of dorsal unpaired median neurons are excited by wind-sensitive pathways and by the central pattern generator. Considering the results of other studies which investigated metabolic effects of octopamine, we propose a model in which the differential activation of dorsal unpaired median neurons during flight may lead to an adequately controlled release or removal of octopamine to adjust metabolic processes to the requirements of a specific motor program. Accepted: 24 February 1999