Inhibitory interaction of receptor units in the eye of Limulus

The inhibition that is exerted mutually among the receptor units (ommatidia) in the lateral eye of Limulus has been analyzed by recording oscillographically the discharge of nerve impulses in single optic nerve fibers. The discharges from two ommatidia were recorded simultaneously by connecting the bundles containing their optic nerve fibers to separate amplifiers and recording systems. Ommatidia were chosen that were separated by no more than a few millimeters in the eye; they were illuminated independently by separate optical systems. The frequency of the maintained discharge of impulses from each of two ommatidia illuminated steadily is lower when both are illuminated together than when each is illuminated by itself. When only two ommatidia are illuminated, the magnitude of the inhibition of each one depends only on the degree of activity of the other; the activity of each, in turn, is the resultant of the excitation from its respective light stimulus and the inhibition exerted on it by the other. When additional receptors are illuminated in the vicinity of an interacting pair too far from one ommatidium to affect it directly, but near enough to the second to inhibit it, the frequency of discharge of the first increases as it is partially released from the inhibition exerted on it by the second (disinhibition). Disinhibition simulates facilitation; it is an example of indirect effects of interaction taking place over greater distances in the eye than are covered by direct inhibitory interconnections. When only two interacting ommatidia are illuminated, the inhibition exerted on each (decrease of its frequency of discharge) is a linear function of the degree of activity (frequency of discharge) of the other. Below a certain frequency (often different for different receptors) no inhibition is exerted by a receptor. Above this threshold, the rate of increase of inhibition of one receptor with increasing frequency of discharge of the other is constant, and may be at least as high as 0.2 impulse inhibited in one receptor per impulse discharged by the other. For a given pair of interacting receptors, the inhibitory coefficients are not always the same in the two directions of action. The responses to steady illumination of two receptor units that inhibit each other mutually are described quantitatively by two simultaneous linear equations that express concisely all the features discussed above. These equations may be extended and their number supplemented to describe the responses of more than two interacting elements.     

Inhibition in the eye of Limulus

In the compound lateral eye of Limulus each ommatidium functions as a single receptor unit in the discharge of impulses in the optic nerve. Impulses originate in the eccentric cell of each ommatidium and are conducted in its axon, which runs without interruption through an extensive plexus of nerve fibers to become a fiber of the optic nerve. The plexus makes interconnections among the ommatidia, but its exact organization is not understood. The ability of an ommatidium to discharge impulses in the axon of its eccentric cell is reduced by illumination of other ommatidia in its neighborhood: the threshold to light is raised, the number of impulses discharged in response to a suprathreshold flash of light is diminished, and the frequency with which impulses are discharged during steady illumination is decreased. Also, the activity that can be elicited under certain conditions when an ommatidium is in darkness can be inhibited similarly. There is no evidence for the spread of excitatory influences in the eye of Limulus. The inhibitory influence exerted upon an ommatidium that is discharging impulses at a steady rate begins, shortly after the onset of the illumination on neighboring ommatidia, with a sudden deep minimum in the frequency of discharge. After partial recovery, the frequency is maintained at a depressed level until the illumination on the neighboring receptors is turned off, following which there is prompt, though not instantaneous recovery to the original frequency. The inhibition is exerted directly upon the sensitive structure within the ommatidium: it has been observed when the impulses were recorded by a microelectrode thrust into an ommatidium, as well as when they were recorded more proximally in single fibers dissected from the optic nerve. Receptor units of the eye often inhibit one another mutually. This has been observed by recording the activity of two optic nerve fibers simultaneously. The mediation of the inhibitory influence appears to depend upon the integrity of nervous interconnections in the plexus: cutting the lateral connections to an ommatidium abolishes the inhibition exerted upon it. The nature of the influence that is mediated by the plexus and the mechanism whereby it exerts its inhibitory action on the receptor units are not known. The depression of the frequency of the discharge of nerve impulses from an ommatidium increases approximately linearly with the logarithm of the intensity of illumination on receptors in its vicinity. Inhibition of the discharge from an ommatidium is greater the larger the area of the eye illuminated in its vicinity. However, equal increments of area become less effective as the total area is increased. The response of an ommatidium is most effectively inhibited by the illumination of ommatidia that are close to it; the effectiveness diminishes with increasing distance, but may extend for several millimeters. Illumination of a fixed region of the eye at constant intensity produces a depression of the frequency of discharge of impulses from a nearby ommatidium that is approximately constant, irrespective of the level of excitation of the ommatidium. The inhibitory interaction in the eye of Limulus is an integrative process that is important in determining the patterns of nervous activity in the visual system. It is analogous to the inhibitory component of the interaction that takes place in the vertebrate retina. Inhibitory interaction results in the exaggeration of differences in sensory activity from different regions of the eye illuminated at different intensities, thus enhancing visual contrast.     

The responses of Limulus optic nerve fibers to patterns of illumination on the receptor mosaic

The inhibition that is exerted mutually among receptor units (ommatidia) of the compound eye of Limulus is less for units widely separated than for those close together. This diminution of inhibition with distance is the resultant of two factors: (1) the threshold of inhibitory action increases with increasing distance between the units involved; and (2) the coefficient of inhibitory action decreases with increasing distance. The discharge of nerve impulses from ommatidia at various distances from one another may be described quantitatively by a set of simultaneous linear equations which express the excitatory effects of the illumination on each ommatidium and the inhibitory interactions between each ommatidium and its neighbors. The values of the thresholds and coefficients of inhibitory action, which appear as parameters in these equations, must be determined empirically: their dependence on distance is somewhat irregular and cannot yet be expressed in an exact general law. Nevertheless the diminution of inhibitory influences with distance is sufficiently uniform that patterns of neural response generated by various patterns of illumination on the receptor mosaic can be predicted qualitatively. Such predictions have been verified experimentally for two simple patterns of illumination: an abrupt step in intensity, and a simple gradient between two levels of intensity (the so-called Mach pattern). In each case, transitions in the pattern of illumination are accentuated in the corresponding pattern of neural response.     

Inhibitory Fields in the Limulus Lateral Eye

The inhibition that is exerted mutually among receptor units (ommatidia) of the lateral eye of Limulus does not diminish uniformly with increasing distance between units. Instead the response of a receptor unit is most effectively inhibited by other units separated from it by approximately 1 mm (three to five receptor diameters); the effectiveness diminishes with distances both greater and less than this value. The ommatidial inhibitory field as measured by the spatial function of the inhibitory coefficients contains a uniform depression in the central region, a uniformly high annulus at some distance from the center, and a gradual tapering off toward the periphery. The field is large—covering over 30 % of the retina—and is somewhat elliptical in shape with its major axis in the anteroposterior direction on the lateral eye. A number of experiments reveal similar configurations in a sizable part of the eye. Control experiments show that the diminution of the inhibitory effects near the center of the field is not an artifact of the measuring technique and cannot be explained readily by local neural excitatory processes.     

Insulin-like growth factor-I receptors and estrogen receptors interact in the promotion of neuronal survival and neuroprotection

Several in vitro and in vivo studies have shown that estrogen has neuroprotective properties. The neuroprotective effects of estrogen are probably exerted through several mechanisms. It is established that estrogen can provide neuroprotection by actions that are independent of estrogen receptor activation. In addition, in several experimental models, activation of estrogen receptors appears to be indispensable for neuroprotection. This review focuses on neuroprotection mediated by estrogen receptors. The interaction of estrogen with growth factor receptor signaling to induce neuroprotection is discussed. Evidence is presented that estrogen receptors and insulin-like growth factor-I receptors interact in the promotion of neuronal survival and neuroprotection.     

Toll-Like Receptor 11 (TLR11) Interacts with Flagellin and Profilin through Disparate Mechanisms

Toll-like receptors (TLRs) are innate immune receptors that sense a variety of pathogen-associated molecular patterns (PAMPs) by interacting with them and subsequently initiating signal transduction cascades that elicit immune responses. TLR11 has been shown to interact with two known protein PAMPs: Salmonella and E. coli flagellin FliC and Toxoplasma gondii profilin-like protein. Given the highly divergent biology of these pathogens recognized by TLR11, it is unclear whether common mechanisms are used to recognize these distinct protein PAMPs. Here we show that TLR11 interacts with these two PAMPs using different receptor domains. Furthermore, TLR11 binding to flagellin and profilin exhibits differential dependency on pH and receptor ectodomain cleavage.     

Inhibition in the Limulus lateral eye in situ

Inhibition in the Limulus lateral eye in situ is qualitatively similar to that in the excised eye. In both preparations ommatidia mutually inhibit one another, and the magnitude of the inhibitory effects are linear functions of the response rate of individual ommatidia. The strength of inhibition exerted between single ommatidia is also about the same for both preparations; however, stronger effects can converge on a single ommatidium in situ. At high levels of illumination of the retina in situ the inhibitory effects are often strong enough to produce sustained oscillations in the discharge of optic nerve fibers. The weaker inhibitory influences at low levels of illumination do not produce oscillations but decrease the variance of the optic nerve discharge. Thresholds for the inhibitory effects appear to be determined by both presynaptic and postsynaptic cellular processes. Our results are consistent with the idea that a single ommatidium can be inhibited by more of its neighbors in an eye in situ than in an excised eye. Leaving intact the blood supply to the eye appears to preserve the functional integrity of the retinal pathways which mediate inhibition.     

Triterpenes from Alisma orientalis act as androgen receptor agonists,progesterone receptor antagonists,and glucocorticoid receptor antagonists

Alisma orientalis, a well-known traditional medicine, exerts numerous pharmacological effects including anti-diabetes, anti-hepatitis, and anti-diuretics but its bioactivity is not fully clear. Androgen receptor (AR), progesterone receptor (PR), and glucocorticoid receptor (GR) are three members of nuclear receptor superfamily that has been widely targeted for developing treatments for essential diseases including prostate cancer and breast cancer. In this study, two triterpenes, alisol M 23-acetate and alisol A 23-acetate from Alisma orientalis were determined whether they may act as androgen receptor (AR), progesterone receptor (PR), or glucocorticoid receptor (GR) modulators. Indeed, in the transient transfection reporter assays, alisol M 23-acetate and alisol A 23-acetate transactivated AR in dose-dependent manner, while they transrepressed the transactivation effects exerted by agonist-activated PR and GR. Through molecular modeling docking studies, they were shown to respectively interact with AR, PR, or GR ligand binding pocket fairly well. All these results indicate that alisol M 23-acetate and alisol A 23-acetate from Alisma orientalis might possess therapeutic effects through their modulation of AR, PR, and GR pathways.     

Afferent signals from pigeon extraocular muscles modify the vestibular responses of units in the abducens nucleus.

Although the extraocular muscles contain stretch receptors it is generally believed that their afferents exert no influence on the control of eye movement. However, we have shown previously that these afferent signals reach various brainstem centres concerned with eye movement, notably the vestibular nuclei, and that the decerebrate pigeon is a favourable preparation in which to study their effects. If the extraocular muscle afferents do influence oculomotor control from moment-to-moment they should exert a demonstrable effect on the oculomotor nuclei. We now present evidence that extraocular muscle afferent signals do, indeed, alter the responses of units in an oculomotor nucleus (the abducens, VI nerve nucleus, which supplies the lateral rectus muscle) to horizontal, vestibular stimulation induced by sinusoidal oscillation of the bird. Such stimuli evoke a vestibulo-ocular reflex in the intact bird. The extraocular stretch receptors were activated by passive eye movement within the pigeon's saccadic range; such movements modified the vestibular responses of all 19 units studied which were all, histologically, in the abducens nucleus. The magnitude of the effects, purely inhibitory in 15 units, depended both on the amplitude and the velocity of the eye movement and most units showed selectivity for particular combinations of plane (e.g. horizontal versus vertical) and direction (e.g. rostral versus caudal) of eye movement. The results show that an afferent signal from the extraocular muscles influences vestibularly driven activity in the abducens nucleus to which it carries information related to amplitude, velocity, plane and direction of eye movement in the saccadic range. They thus strongly support the view that extraocular afferent signals are involved in the control of eye movement.     

Phototaxis in Drosophila: R1–6 input and interaction among ocellar and compound eye receptors

All 3 photoreceptor types in the compound eye of Drosophila can evoke positive phototaxis. Here we describe input from R1–6 receptors which are very sensitive. Previous reports in this series of studies described input from R7 and R8, the other less sensitive receptors. Here we studied fast-walking phototaxis using extremely dim stimuli. We also studied input from the simple ocellar eyes. Thus, we can now summarize a complete synthesis of inputs and interactions among all compound eye and ocellar receptor types. Receptor-deficient mutants were used to establish receptor-specific input. Two other findings are presented: (1) eye colour pigments affect the spectral sensitivity for phototaxis; and (2) the ocelli interact to facilitate input from the compound eye receptor types. Possible mechanisms of receptor interaction are discussed in the light of these findings of positive input from all photoreceptor types in Drosophila.     

Interrelationship of the mediator receptors in the terminals of the afferent nerves of the cornea of the eye]

In local application of substances with a mediator action (adrenaline, acetylcholine, histamine) and blockers of specific receptors (dihydroergotoxin, inderal, atropine and of subcutaneous dimedrol) together with dicaine it was possible to reveal a distinct presence in the cornea of the eye of the mediator receptors capable of influencing one another. Pharmacological analysis permitted a supposition to be made that there were two receptor systems in the sensory nerve endings of the cornea; these systems were in reciprocal inhibitory relations; due to this their regular influence on the processes of excitation and inhibition is effected in the sensory nerve ending.     

Fast-acting excitatory amino acids are involved in the enhancement of the aversiveness of the electrical stimulation of the inferior colliculus by systemic injections of muscimol

Gradual increases in the electrical stimulation of the inferior colliculus produces progressive aversive responses from vigilance, through freezing, until escape. These responses are probably mediated by excitatory amino acids (EAA) mechanisms as microinjection of glutamate into the inferior colliculus can trigger freezing responses while microinjections of NMDA cause a mixture of immobility and escape responses. Moreover, it has been shown that the neural substrates for defensive behavior in this structure are regulated by GABA-benzodiazepine mechanisms. Indeed, these responses are depressed by muscimol and midazolam locally injected into the inferior colliculus. In this work we were interested in knowing how GABAergic mechanisms interact with the EAA-mediated neural substrates of aversion generated at the inferior colliculus level. We found that while intraperitoneal injections of muscimol caused the expected antiaversive effects, unexpectedly systemic injections of muscimol enhanced the aversive reactions induced by electrical stimulation of the inferior colliculus of rats. Local injections into the central nucleus of the inferior colliculus of GDEE-an AMPA/kainate receptor antagonist-inhibited whereas AP7-a NMDA receptor antagonist-did not influence these responses. It is suggested that systemic injections of muscimol inhibit GABAergic inputs to the inferior colliculus. The removal of these inhibitory influences reduce the well-known tonic inhibitory control exerted by GABAergic mechanisms on the neural substrates of aversion of the inferior colliculus. Activation of these neural substrates by fast-acting AMPA/kainate receptors trigger the initial steps of the defense reaction in the central nucleus of the inferior colliculus.     

Identification of an interaction between the angiotensin II receptor sub-type AT2 and the ErbB3 receptor, a member of the epidermal growth factor receptor family

To identify the proteins that interact and mediate angiotensin II receptor AT2-specific signaling, a random peptide library was screened by yeast-based Two-Hybrid protein-protein interaction assay technique. A peptide that shared significant homology with the amino acids located between the residues Gly-Xaa-Gly-Xaa-Xaa-Gly721 and Lys742, the residues predicted to be important for ATP binding of the ErbB3 and ErbB2 receptors, was identified to be interacting with the AT2 receptor. The interaction between the human ErbB3 receptor and the AT2 receptor was further confirmed using the cytoplasmic domain (amino acids 671-782) of the human ErbB3 receptor. Moreover, an AT2 receptor peptide that spans the amino acids 226-363, (spans the third ICL and carboxy terminal domain) could also interact with the AT2 receptor in a yeast Two-Hybrid protein-protein interaction assay. Studies using mutated and chimeric AT2 receptors showed that replacing the third intracellular loop (ICL) of the AT2 receptor with that of the AT1 abolishes the interaction between the ErbB3 and the AT2 in yeast Two-Hybrid protein-protein interaction assay. Thus the interaction between the AT2 receptor and the ErbB3 receptor seems to require the region spanning the third ICL and carboxy terminus of the AT2 receptor. Since the third ICL of the AT2 receptor is essential for exerting its inhibitory effects on cell growth, possible involvement of this region in the interaction with the cytoplasmic domain of the ErbB3 receptor suggests a novel signaling mechanism for the AT2 receptor mediated inhibition of cell growth. Furthermore, since both the AT2 and the ErbB3 receptors are expressed during fetal development, we propose that the existence of direct interaction between these two receptors may play a role in the regulation of growth during the initial stages of development.     

The Caenorhabditis elegans D2-like dopamine receptor DOP-2 physically interacts with GPA-14, a Gαi subunit

Dopaminergic inputs are sensed on the cell surface by the seven-transmembrane dopamine receptors that belong to a superfamily of G-protein-coupled receptors (GPCRs). Dopamine receptors are classified as D1-like or D2-like receptors based on their homology and pharmacological profiles. In addition to well established G-protein coupled mechanism of dopamine receptors in mammalian system they can also interact with other signaling pathways. In C. elegans four dopamine receptors (dop-1, dop-2, dop-3 and dop-4) have been reported and they have been implicated in a wide array of behavioral and physiological processes. We performed this study to assign the signaling pathway for DOP-2, a D2-like dopamine receptor using a split-ubiquitin based yeast two-hybrid screening of a C. elegans cDNA library with a novel dop-2 variant (DOP-2XL) as bait. Our yeast two-hybrid screening resulted in identification of gpa-14, as one of the positively interacting partners. gpa-14 is a G?? coding sequence and shows expression overlap with dop-2 in C. elegans ADE deirid neurons. In-vitro pull down assays demonstrated physical coupling between dopamine receptor DOP-2XL and GPA-14. Further, we sought to determine the DOP-2 region necessary for GPA-14 coupling. We generated truncated DOP-2XL constructs and performed pair-wise yeast two-hybrid assay with GPA-14 followed by in-vitro interaction studies and here we report that the third intracellular loop is the key domain responsible for DOP-2 and GPA-14 coupling. Our results show that the extra-long C. elegans D2-like receptor is coupled to gpa-14 that has no mammalian homolog but shows close similarity to inhibitory G-proteins. Supplementing earlier investigations, our results demonstrate the importance of an invertebrate D2-like receptor's third intracellular loop in its G-protein interaction.     

Electrophysiological basis for the spatial dependence of the inhibitory coupling in the Limulus retina

A technique for measuring, with total optical isolation, the inhibition between two individual receptor units in the Limulus lateral eye is described. The extracellular responses of pairs of units were recorded, using light piping microelectrodes. The inhibitory coupling between two units was found to be nonlinear and describable by a simple hyperbolic equation written in terms of saturation rate (S), half saturation (H), and threshold (ft). By plotting reciprocal frequencies, the data could be linearized and compared for different pairs of units. The magnitude of inhibition (in terms of S and H) was found to decrease monotonically as the anatomical distance between receptors increased. An electrical model of the inhibitory system was developed which accounts for many of the properties of the observed inhibitory interactions. Using the equations from the model and the experimental data, it is shown that the "electrical distances" (which are computed in terms of space constants lambda) of the inhibitory synapses from the impulse-generating region of the test unit are directly related to the anatomical distance between receptors. It is also shown that "synaptic strength" is relatively constant with separation. The electrical distances of the inhibitory synapses range from about 0.1lambda to 0.25lambda for adjacent units to greater than 0.5lambda for units seven to nine receptors away. It is concluded that the nonlinear character of the inhibitory coupling is attributable to synaptic effects, and that the decrease of inhibition with distance between receptors is caused primarily by an increase in the electrical distance of the inhibitory synapses from the test unit.     

The Additivity of Contrast in the Human Eye

Using the method of binocular brightness matching, simultaneous brightness contrast effects were measured on two observers. The effects of a given pattern were invariably smaller than the summation of the effects of the pattern's components. This failure of additivity was valid both for patterns with isolated components as well as for those with components exactly contiguous with one another. This failure was more pronounced the farther the inducing patterns were from the test patch. These findings are interpreted as indicating that in the human (just as in the Limulus) eye, the amount of inhibition exerted by a given region on its neighbors depends upon the inhibition exerted against it as well as its excitation state.     

High-affinity TrkA and p75 neurotrophin receptor complexes: A twisted affair

Neurotrophin signaling is essential for normal nervous system development and adult function. Neurotrophins are secreted proteins that signal via interacting with two neurotrophin receptor types: the multifaceted p75 neurotrophin receptor and the tropomyosin receptor kinase receptors. In vivo, neurons compete for the limited quantities of neurotrophins, a process that underpins neural plasticity, axonal targeting, and ultimately survival of the neuron. Thirty years ago, it was discovered that p75 neurotrophin receptor and tropomyosin receptor kinase A form a complex and mediate high-affinity ligand binding and survival signaling; however, despite decades of functional and structural research, the mechanism of modulation that yields this high-affinity complex remains unclear. Understanding the structure and mechanism of high-affinity receptor generation will allow development of pharmaceuticals to modulate this function for treatment of the many nervous system disorders in which altered neurotrophin expression or signaling plays a causative or contributory role. Here we re-examine the key older literature and integrate it with more recent studies on the topic of how these two receptors interact. We also identify key outstanding questions and propose a model of inside-out allosteric modulation to assist in resolving the elusive high-affinity mechanism and complex.     

Functional Analyses of Bitter Taste Receptors in Domestic Cats (Felis catus)

Cats are obligate carnivores and under most circumstances eat only animal products. Owing to the pseudogenization of one of two subunits of the sweet receptor gene, they are indifferent to sweeteners, presumably having no need to detect plant-based sugars in their diet. Following this reasoning and a recent report of a positive correlation between the proportion of dietary plants and the number of Tas2r (bitter receptor) genes in vertebrate species, we tested the hypothesis that if bitter perception exists primarily to protect animals from poisonous plant compounds, the genome of the domestic cat (Felis catus) should have lost functional bitter receptors and they should also have reduced bitter receptor function. To test functionality of cat bitter receptors, we expressed cat Tas2R receptors in cell-based assays. We found that they have at least 7 functional receptors with distinct receptive ranges, showing many similarities, along with some differences, with human bitter receptors. To provide a comparative perspective, we compared the cat repertoire of intact receptors with those of a restricted number of members of the order Carnivora, with a range of dietary habits as reported in the literature. The numbers of functional bitter receptors in the terrestrial Carnivora we examined, including omnivorous and herbivorous species, were roughly comparable to that of cats thereby providing no strong support for the hypothesis that a strict meat diet influences bitter receptor number or function. Maintenance of bitter receptor function in terrestrial obligate carnivores may be due to the presence of bitter compounds in vertebrate and invertebrate prey, to the necessary role these receptors play in non-oral perception, or to other unknown factors. We also found that the two aquatic Carnivora species examined had fewer intact bitter receptors. Further comparative studies of factors driving numbers and functions of bitter taste receptors will aid in understanding the forces shaping their repertoire.     

Membrane effects of toxins isolated from a cyanobacterium, Cylindrospermopsis raciborskii, on identified molluscan neurones

The effect of anatoxin (ANTX), the crude extract (AlgTX) and purified fraction (F1) isolated from cyanobacterium C. raciborskii was studied on the neurones of two snail species. ANTX and AlgTX exerted excitatory, inhibitory and biphasic effects on the spontaneous activity of identified neurones. Both ANTX and AlgTX elicited an inward current, which could be decreased by curare or amiloride. On the contrary, F1 had no direct effect on the spontaneous activity; it was not able to induce conductance changes of the neuronal membrane, but it did antagonise the acetylcholine (ACh)-induced inward current. We concluded that ANTX affects the neuronal membrane of neurones acting on ACh receptors. The AlgTX had similar effects, and therefore the extract of C. raciborskii may contain an ANTX-like component. The purified fraction prolonged and decreased the ACh-elicited response, but had no direct membrane effect. We suggest, therefore, that both AlgTX and the purified fraction F1 interact with the ACh receptor, but they have different binding sites on the neuronal ACh receptor-ion channel complex. The possible neurotoxic effects of the C. raciborskii extract and F1 are demonstrated for the first time; the molecular mechanism of their action, however, remains to be elucidated.