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     

Positional information determines the anatomy and synaptic specificity of cockroach filiform hair afferents using independent mechanisms

Summary Mutant first instar cockroaches (Periplaneta americana) with supernumerary filiform hair sensilla on their cerci were used to study the effects of cell body position on axonal morphology and synaptic connections. The wild-type cercus has two hairs, one lateral (L) and the other medial (M), each with an underlying sensory neuron. Silver-intensified cobalt fills show that the supernumerary lateral neuron (SIN) in the mutant has the same shape of arborization as L, and electrophysiological recording shows that it forms synaptic connections with the same subset of giant interneurons (GIs) as L in the terminal ganglion: GI3 and GI6. The supernumerary medial neuron (SuM) has the same axonal morphology as M and synapses with the same GIs as does M: ipsilateral GIs 1 and 2 and contralateral GIs 1, 2, 3, 5 and 6. In 0.1% of approximately 8000 animals screened, a supernumerary hair arose on the cereal midline (C hair). The C neuron sends its axon to the CNS in the same branch of the cereal nerve as the L and SIN, and has a similar arborization. However, the C neuron forms synapses with the same GIs as do M and SuM. Electron microscopy of horseradish peroxidase-injected neurons was used to confirm that the C afferent forms a monosynaptic connection to GI2. It was concluded that the position of the sensory neuron cell body does control its axonal morphology and synaptic connectivity, but that these characteristics are produced by independent mechanisms.Abbreviations GI giant interneuron - L lateral - M medial - SI Space Invader - SuM supernumerary medial - C cereal midline     

Ectopic sensory neurons in mutant cockroaches compete with normal cells for central targets.

The cercus of the first instar cockroach, Periplaneta americana, bears two filiform hairs, lateral (L) and medial (M), each of which is innervated by a single sensory neuron. These project into the terminal ganglion of the CNS where they make synaptic connections with a number of ascending interneurons. We have discovered mutant animals that have more hairs on the cercus; the most typical phenotype, called "Space Invader" (SI), has an extra filiform hair in a proximo-lateral position on one of the cerci. The afferent neuron of this supernumerary hair (SIN) "invades the space" occupied by L in the CNS and makes similar synaptic connections to giant interneurons (GIs). SIN and L compete for these synaptic targets: the size of the L EPSP in a target interneuron GI3 is significantly reduced in the presence of SIN. Morphometric analysis of the L afferent in the presence or absence of SIN shows no anatomical concomitant of competition. Ablation of L afferent allows SIN to increase the size of its synaptic input to GI3. Less frequently in the mutant population, we find animals with a supernumerary medical (SuM) sensillum. Its afferent projects to the same neuropilar region as the M afferent, makes the same set of synaptic connections to GIs, and competes with M for these synaptic targets. The study of these competitive interactions between identified afferents and identified target interneurons reveals some of the dynamic processes that go on in normal development to shape the nervous system.     

Postembryonic changes in the response properties of wind-sensitive giant interneurons in cricket

The intensity-response (I-R) relations for four wind-sensitive giant interneurons (GIs 8-1, 9-1, 9-2 and 9-3) in the fourth-, sixth- and last-instar nymphs of the cricket, Gryllus bimaculatus, were investigated using a unidirectional air current stimulus in order to explore the functional changes of GIs during postembryonic development. Contrary to our expectations, the response properties of GIs in nymphs were largely different from those in adults. The response magnitude of GI 8-1 in an intact cricket decreased during development, i.e. the GI in younger insects showed a larger response magnitude. Although the response magnitudes of GIs 9-1 and 9-2 were almost identical during the nymphal period, a significant decrease was observed after the imaginal ecdysis. During the nymphal period, the response magnitude of GI 9-3 increased according to the developmental stage. However, it decreased significantly after the imaginal ecdysis. We also investigated the response magnitudes of the GIs in nymphs after unilateral cercal ablation. From the results of ablation experiments, the changes in excitatory and/or inhibitory connections between filiform hairs and each GI during postembryonic development were revealed.     

Localization of the enhanced input to cockroach giant interneurons after partial deafferentation

The ventral giant interneurons (GIs) in the cockroach have two distinct dendritic fields: a small one ipsilateral to the soma, and a larger, contralateral field from which the axon arises. The major input to these GIs is from the cercus on the axon side; when this cercus is ablated in the last instar before the adult stage, input from the other cercus becomes more effective within 30 days (Vardi and Camhi, 1982b). I wished to determine if the input from the intact, soma-ipsilateral cercus contacted the GIs purely ipsilaterally and if EPSPs at this site were larger in deafferented animals. Consistent with earlier anatomical findings, intracellular recordings from the GI somata showed that the majority of cercal inputs synapse on their own side of the ganglion in normal animals. This was evidenced by differences in the size and shape of the synaptic potentials evoked from the two cerci and by the presence of large EPSPs after a ganglion had been split along the midline. Unitary EPSPs produced by stimulation of single, identified cercal afferents, ipsilateral to the soma, were compared between normal and deafferented animals. Column "h" afferents were chosen because they make a large contribution to the receptive fields of GIs 1 and 2 after ablation of the contralateral cercus. In addition, the arbors of these afferents, when stained with cobalt, did not cross the ganglionic midline in normal animals. Unitary EPSPs recorded in GI 2 were significantly larger in the deafferented animals. There was, however, no significant change in the size of EPSPs in GI 1. Nevertheless, the results from GI 2 suggest that partial deafferentation in the central nervous system can increase the efficacy of synapses distant from the locus of denervation.     

The generation of direction selectivity in the auditory system

Both human speech and animal vocal signals contain frequency-modulated (FM) sounds. Although central auditory neurons that selectively respond to the direction of frequency modulation are known, the synaptic mechanisms underlying the generation of direction selectivity (DS) remain elusive. Here we show the emergence of DS neurons in the inferior colliculus by mapping the three major subcortical auditory nuclei. Cell-attached recordings reveal a highly reliable and precise firing of DS neurons to FM sweeps in a preferred direction. By using in vivo whole-cell current-clamp and voltage-clamp recordings, we found that the synaptic inputs to DS neurons are not direction selective, but temporally reversed excitatory and inhibitory synaptic inputs are evoked in response to opposing directions of FM sweeps. The construction of such temporal asymmetry, resulting DS, and its topography can be attributed to the spectral disparity of the excitatory and the inhibitory synaptic tonal receptive fields.     

Excitatory and inhibitory receptive fields of tectal cells are differentially modified by magnocellular and parvocellular divisions of the pigeon nucleus isthmi

It has been known that magnocellular and parvocellular divisions of the pigeon nucleus isthmi exert excitatory and inhibitory actions on tectal cells, respectively. The present study shows that injection of N-methyl-D-aspartate into the parvocellular division results in an increase in responsive strength and extent of the inhibitory receptive fields, which expand into the excitatory receptive fields of tectal cells. This injection concurrently leads to a decrease in responsiveness and extent of the excitatory fields. On the other hand, injection of acetylcholine into the magnocellular division enhances visual responsiveness, although the excitatory field is not obviously changed in extent. Meanwhile, strength and extent of the inhibitory fields are decreased by acetylcholine. The excitatory and inhibitory fields are reduced in both strength and extent by magnocellular and parvocellular injection of lidocaine, respectively. It suggests that isthmic inputs from both parvocellular and magnocellular divisions converge onto the same tectal cells, and the magnocellular and parvocellular subnuclei can modulate excitatory and inhibitory receptive fields of tectal cells, respectively, with some interactions between both fields. Accepted: 1 March 2000     

Visual cortical unit responses to photic stimulation of different zones of the receptive fields in cats

In acute experiments on unanesthetized curarized cats the intensity functions, response thresholds, inhibition thresholds, and differential sensitivity of 96 neurons in the primary visual projection cortex were investigated by extracellular recording of unit activity during central and peripheral stimulation of their receptive fields. In darkness the neurons had wide threshold and above-threshold reliefs (3–30°). The threshold reliefs of the receptive fields of some cells were found to be V-shaped, whereas others were marked by alternation of zones of increased and reduced excitability. Sensitivity of both excitatory and inhibitory inputs of the receptive field as a rule was greatest in the center. Inhibitory inputs of different cortical neurons were much more standard and less sensitive to light, and they were mainly activated within the intermediate (mesoptic) range of brightnesses. During light adaptation the threshold contour of the receptive field narrows sharply, mainly because of the fall in sensitivity of its peripheral inputs. Compared with the lateral geniculate body and retina, the relative number of low-threshold elements, sensitivity in the system of inhibitory elements, and differential brightness sensitivity are greater in the cortex. The mechanisms of formation of receptive fields of cortical neurons and their modification during changes in the level of adaptation, and also the role of excitatory and inhibitory inputs of the cell in these effects are discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 227–235, May–June, 1979.     

A ring model for spatiotemporal properties of simple cells in the visual cortex

 A neural model is proposed for the spatiotemporal properties of simple cells in the visual cortex. In the model, several cortical cells are arranged on a ring, with mutual excitatory or inhibitory connections. The cells also receive excitatory inputs either from lagged and nonlagged cells of the lateral geniculate nucleus in one setting or from nonlagged cells in the other. Computer simulation shows that the cortical cells have spatiotemporally inseparable receptive fields in the former setting and separable fields in the latter; spatial profiles at a given time in the spatiotemporal fields are described with a Gabor function whose phase parameter varies regularly from 0 to 2π with rotation along the ring; the inseparable cells have directional selectivity as observed physiologically. Received: 13 November 1995 / Accepted in revised form: 1 July 1997     

INDeGenIUS,a new method for high-throughput identification of specialized functional islands in completely sequenced organisms

Genomic islands (GIs) are regions in the genome which are believed to have been acquired via horizontal gene transfer events and are thus likely to be compositionally distinct from the rest of the genome. Majority of the genes located in a GI encode a particular function. Depending on the genes they encode, GIs can be classified into various categories, such as ‘metabolic islands’, ‘symbiotic islands’, ‘resistance islands’, ‘pathogenicity islands’, etc. The computational process for GI detection is known and many algorithms for the same are available. We present a new method termed as Improved N-mer based Detection of Genomic Islands Using Sequence-clustering (INDeGenIUS) for the identification of GIs. This method was applied to 400 completely sequenced species belonging to proteobacteria. Based on the genes encoded in the identified GIs, the GIs were grouped into 6 categories: metabolic islands, symbiotic islands, resistance islands, secretion islands, pathogenicity islands and motility islands. Several new islands of interest which had previously been missed out by earlier algorithms were picked up as GIs by INDeGenIUS. The present algorithm has potential application in the identification of functionally relevant GIs in the large number of genomes that are being sequenced. Investigation of the predicted GIs in pathogens may lead to identification of potential drug/vaccine candidates.     

Nicotinic acetylcholine receptors on a cholinergic nerve terminal in the cockroach,Periplaneta americana

Summary Intracellular microelectrode recording and ionophoretic application of carbamylcholine (CCh) were used to compare the cholinergic sensitivity of postsynaptic dendrites of an identified neurone with that of an identified presynaptic cholinergic axon.The axon of the lateral filiform hair sensory neurone (LFHSN) in the first-instar cockroachPeriplaneta americana was found to be as sensitive to CCh as the dendritic regions of giant interneurone 3 (GI 3). The CCh response of both neurones was unaffected by replacing Ca²⁺ with Mg²⁺, confirming that the ACh receptors are present on the neurones under test. The CCh response of both neurones was mimicked by ionophoretic application of nicotine. The responses were blocked by 10^–5 M mecamylamine and 10^–6 M d-tubocurarine and were not affected by muscarinic antagonists, suggesting that the ACh receptors present on GI 3 and LFHSN are predominantly nicotinic.The muscarinic agonist oxotremorine and the antagonists atropine and quinuclidinyl benzilate had no modulatory effect on LFHSN-GI 3 synaptic transmission.The latency of the LFHSN response to CCh was consistent with the hypothesis that ACh receptors are situated on the main axon/terminal within the neuropil of the ganglion. It has previously been shown that this region of the axon does not form output synapses (Blagburn et al. 1985a). This indirect evidence indicates that presynaptic or extrasynaptic ACh receptors are present in the membrane of a cholinergic axon.LFHSN was depolarized by synaptically-released ACh after normal or evoked spike bursts, suggesting that the nicotinic ACh receptors act as autoreceptors. However, it was not possible to obtain direct evidence to support the hypothesis that these receptors modulate ACh release.Abbreviations CCh carbamylcholine - GI giant interneurone - FHSN filiform hair sensory neurone - LFHSN lateral filiform hair sensory neurone - R _in input resistance - V depolarization - V _m resting potential     

Cercal sensory system and giant interneurons in Gryllodes sigillatus

The external morphologies of two cricket species, Gryllodes sigillatus and Gryllus bimaculatus, were investigated. Despite its small body length, G. sigillatus possessed longer cerci and longer cercal filiform hairs than G. bimaculatus. The estimated number of filiform hairs on a cercus was also larger in G. sigillatus than in G. bimaculatus. Wind-sensitive interneurons receiving sensory inputs from cercal filiform hairs and running in the ventral nerve cord (VNC) were investigated in G. sigillatus both morphologically and physiologically. By intracellular staining, these interneurons were proved to be morphologically homologous with previously identified giant interneurons (GIs 8-1, 9-1, 9-2, 9-3, 10-2, and 10-3) in G. bimaculatus and Acheta domesticus. In G. sigillatus, the intensity-response relationship (I-R curve) for each GI was investigated using a unidirectional air current stimulus. The stimulus was applied from 12 different directions, and an I-R curve was obtained for each stimulus direction. Each GI showed a characteristic I-R curve depending on stimulus direction. The directionality curve expressed in terms of threshold velocity showed that each GI had a distinctive directional characteristic. The functional properties of GIs in G. sigillatus, such as I-R curve, threshold velocity, and directional characteristics, were compared with those of homologous GIs in G. bimaculatus in Discussion.     

Balanced Excitatory and Inhibitory Synaptic Currents Promote Efficient Coding and Metabolic Efficiency

A balance between excitatory and inhibitory synaptic currents is thought to be important for several aspects of information processing in cortical neurons in vivo, including gain control, bandwidth and receptive field structure. These factors will affect the firing rate of cortical neurons and their reliability, with consequences for their information coding and energy consumption. Yet how balanced synaptic currents contribute to the coding efficiency and energy efficiency of cortical neurons remains unclear. We used single compartment computational models with stochastic voltage-gated ion channels to determine whether synaptic regimes that produce balanced excitatory and inhibitory currents have specific advantages over other input regimes. Specifically, we compared models with only excitatory synaptic inputs to those with equal excitatory and inhibitory conductances, and stronger inhibitory than excitatory conductances (i.e. approximately balanced synaptic currents). Using these models, we show that balanced synaptic currents evoke fewer spikes per second than excitatory inputs alone or equal excitatory and inhibitory conductances. However, spikes evoked by balanced synaptic inputs are more informative (bits/spike), so that spike trains evoked by all three regimes have similar information rates (bits/s). Consequently, because spikes dominate the energy consumption of our computational models, approximately balanced synaptic currents are also more energy efficient than other synaptic regimes. Thus, by producing fewer, more informative spikes approximately balanced synaptic currents in cortical neurons can promote both coding efficiency and energy efficiency.     

Regeneration of cercal filiform hair sensory neurons in the first-instar cockroach restores escape behavior

Neural regeneration in the escape circuit of the first-instar cockroach is described using behavioral analysis, electrophysiology, intracellular staining, and electron microscopy. Each of the two filiform hairs on each of the animal's cerci is innervated by a single sensory neuron, which specifically synapses with a set of giant interneurons (GIs) in the terminal ganglion. These trigger a directed escape run. Severing the sensory axons causes them to degenerate and perturbs escape behavior, which is restored to near normal after 4–6 days. Within this time, afferents regenerate and reestablish arborizations in the terminal ganglion. In most cases, regenerating afferents enter the cercal glomerulus and re-form most of the specific monosynaptic connections they acquired during embryogenesis, although their morphology deviates markedly from normal; these animals reestablish near normal escape behavior. In a few cases, regenerating afferents remain within the cercus or bypass the cercal glomerulus, and thereby fail to re-form synapses with GIs; these animals continue to exhibit perturbed escape behavior. We conclude that in most cases, specific synapses are reestablished and appropriate escape behavior is restored. This regeneration system therefore provides a tractable model for the establishment of synaptic specificity in a simple neuronal circuit. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 439–458, 1997     

Synaptic mechanisms underlying auditory processing

In vivo voltage clamp recordings have provided new insights into the synaptic mechanisms that underlie processing in the primary auditory cortex. Of particular importance are the discoveries that excitatory and inhibitory inputs have similar frequency and intensity tuning, that excitation is followed by inhibition with a short delay, and that the duration of inhibition is briefer than expected. These findings challenge existing models of auditory processing in which broadly tuned lateral inhibition is used to limit excitatory receptive fields and suggest new mechanisms by which inhibition and short term plasticity shape neural responses.     

Connections between wide-field monocular and binocular movement detectors in the brain of a hawk moth

Summary Recordings were made in the brain of Sphinx ligustri of pairs of directionally selective movement detectors, and the spike trains analysed with a computer for possible synaptic connections between two classes of movement detector. (1) Neurones with large binocular fields which arise in the medial protocerebrum and project to the medulla or lobula of one optic lobe, or to the ventral nerve cord. (2) Movement detectors which project from the lobula complex of one optic lobe to the opposite medial protocerebrum. The majority of the second group had back-to-front preferred directions over the ipsilateral eye, and of these many were weakly sensitive to stimuli to the opposite eye. The ipsilateral receptive field covered most of the eye.Optic lobe output cells with the appropriate preferred direction provide a powerful excitatory input to the binocular movement detectors centrifugal to the medulla. Each centrifugal movement detector probably receives excitatory inputs from no more than two optic lobe output cells with back-to-front preferred direction. The same set of optic lobe output neurones probably feeds several cells projecting to the medulla and lobula of both optic lobes, and, possibly, to the ventral nerve cord.Evidence was obtained that the optic lobe output cells themselves receive few excitatory inputs, and that therefore the receptive fields of their input cells are large.Two moving stimuli were presented in different areas of the receptive field. Movement through the null direction in one area inhibited the response to movement in the preferred direction in another area. This suppression was stronger in optic lobe output cells with front-to-back preferred direction than in units with back-to-front preferred direction. Thus the optic lobe output cells, or wide-field units feeding them, receive inhibitory inputs from wide-field units with the opposite preferred direction.Similar tests in which moving stimuli were presented to both eyes gave results indicating that the binocular centrifugal movement detectors may receive inhibitory inputs from movement detectors with back-to-front preferred direction. The possible functional significance of these inhibitory inputs is discussed.I am very greatful to F. A. Miles for helpful discussion and criticism. Financial support came from the U. K. Science Research Council.     

Patterns of monosynaptic input to the giant interneurons 1–3 in the cereal system of the adult cockroach

1. The cerci of the cockroach Periplaneta americana bear filiform hair mechanoreceptors that are arranged in segmentally repeated rows and longitudinal columns. The monosynaptic connections between receptors of the same column or row and the 3 largest giant interneurons (GIs) were compared using the oil-gap single fibre technique.
2. For many columns, the synaptic efficacy of the afferents decreased gradually from the base to the tip of the cercus, but columns with an inverted gradient or without any gradient were also observed. On the ipsilateral side (relative to the GI axon), the inverted gradients were exclusively found for columns with short proximal hairs. For one column (d) and GI3, the ipsilateral and contralateral gradients were opposite.
3. Monosynaptic EPSPs evoked by stimulating different receptors of the same segment (segment 3) were of very different amplitudes, which partially account for the directional sensitivity of the GIs. Differences in the location, shape and size of the afferent terminals were not sufficient to explain these differences in connection strength.
4. No correlation was found between the size of the EPSPs produced by a sensory neuron and the length of its associated hair.

     

Presynaptic effects of biogenic amines modulating synaptic transmission between identified sensory neurons and giant interneurons in the first instar cockroach

Intracellular recording was used to investigate the modulatory effects of serotonin and octopamine on the identified synapses between filiform hair sensory afferents and giant interneurons in the first instar cockroach, Periplaneta americana. Serotonin at 10(-4) mol l(-1) to 10(-3) mol l(-1) reduced the amplitude of the lateral axon-to-ipsilateral giant interneuron 3 excitatory postsynaptic potentials. and octopamine at 10(-4) mol l(-1) increased their amplitude. Similar effects were seen on excitatory postsynaptic potentials in dorsal giant interneuron 6. Several lines of evidence suggest that both substances modulate the amplitude of excitatory postsynaptic potentials by acting presynaptically, rather than on the postsynaptic neuron. The fitting of simple binomial distributions to the postsynaptic potential amplitude histograms suggested that, for both serotonin and octopamine, the number of synaptic release sites was being modulated. Secondly, the amplitudes of miniature excitatory postsynaptic potentials recorded in the presence of tetrodotoxin were unaffected by either modulator. Finally, recordings from contralateral giant interneuron 3, which has two identifiable populations of synaptic inputs, showed that each modulator had a more pronounced effect on excitatory postsynaptic potentials evoked by the lateral axon than on those evoked by the medial axon. Immunocytochemistry confirmed that neuropilar processes containing serotonin are present in close proximity to these synapses.     

Activity-dependent matching of excitatory and inhibitory inputs during refinement of visual receptive fields

The receptive field (RF) of single visual neurons undergoes progressive refinement during development. It remains largely unknown how the excitatory and inhibitory inputs on single developing neurons are refined in a coordinated manner to allow the formation of functionally correct circuits. Using whole-cell voltage-clamp recording from Xenopus tectal neurons, we found that RFs determined by excitatory and inhibitory inputs in more mature tectal neurons are spatially matched, with each spot stimulus evoking balanced synaptic excitation and inhibition. This emerges during development through a gradual reduction in the RF size and a transition from disparate to matched topography of excitatory and inhibitory inputs to the tectal neurons. Altering normal spiking activity of tectal neurons by either blocking or elevating GABA(A) receptor activity significantly impeded the developmental reduction and topographic matching of RFs. Thus, appropriate inhibitory activity is essential for the coordinated refinement of excitatory and inhibitory connections.     

Inhibitory zones of receptive fields of the lateral geniculate body and visual cortex of the cat

Unit responses to moving strips were investigated. The organization of the inhibitory zones in the receptive fields of the lateral geniculate body and visual cortex of the cat was compared. The response in the receptive field of the lateral geniculate body was inhibited only during simultaneous stimulation of the excitatory and inhibitory zones of the field. Stimulation of the inhibitory zone in the receptive field of the visual cortex was effective for a long time (several hundreds of milliseconds) after stimulation of the excitatory zone. The inhibitory zones of the simple and complex receptive fields of the visual cortex differed significantly. An increase in the width of the strip above the optimal size reduced the inhibitory effect in the complex fields. This was not observed in the simple receptive fields. The functional and structural models of the receptive field of the visual cortex are discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 201–209, March–April, 1973.