Evidence for the presence of CB1 cannabinoid receptors on peripheral neurones and for the existence of neuronal non-CB1 cannabinoid receptors.

The discovery of CB1 and CB2 receptors and of endogenous agonists for these receptors has sparked renewed interest in the therapeutic potential of cannabinoids. This has led to a need for strategies that will provide a better separation of wanted from unwanted effects, particularly for CB1 receptor agonists. Possible strategies are to target CB1 receptors present on neurones outside the central nervous system or novel types or subtypes of neuronal cannabinoid receptor. This paper reviews evidence for the presence of CB1 receptors on peripheral neurones and for the existence of neuronal non-CB1 cannabinoid receptors.     

Processing of vibratory and acoustic signals by ventral cord neurones in the cricket Gryllus campestris

The responses of single vibratory receptors and ascending ventral cord interneurones were studied extracellularly in Gryllus campestris L. The physiology of the vibration receptors resembled those found in tettigoniids and locusts. The frequency responses of the subgenual receptors provide two possible cues for central frequency discrimination: differences in mean tuning between groups of receptors in the different leg pairs and a range of receptors tuned to different frequencies within one subgenual organ.Most of the ascending vibratory interneurones were highly sensitive in either the low or high frequency range. Broadbanded neurones were less sensitive. The characteristic sensitivity peaks of these units are due mainly to receptor inputs from a particular leg pair, although most central neurones receive inputs from all 6 legs. Only one neurone type, TN1 received excitatory inputs from both auditory and vibratory receptors; its responses were greatly enhanced by the simultaneous presentation of both stimulus modes. The responses to sound stimuli of AN2, on the other hand, were inhibited by vibration. No other auditory interneurones investigated were influenced by inputs from vibration receptors. Central processing of vibratory information in the cricket is compared with that of tettigoniids and locusts.     

Distribution of SchistoFLRFamide-like immunoreactivity in the adult ventral nervous system of the locust,Schistocerca gregaria

SchistoFLRFamide (PDVDHVFLRF-NH₂) is one of the major endogenous neuropeptides of the FMRF-amide family found in the nervous system of the locust,Schistocerca gregaria. To gain insights into the potential physiological roles of this neuropeptide we have examined the distribution of SchistoFLRFamide-like immunoreactivity in the ventral nervous system of adult locusts by use of a newly developed N-terminally specific antibody. SchistoFLRFamide-like immunoreactivity in the ventral nerve cord is found in a subgroup of the neurones that are immunoreactive to an antiserum raised against bovine pancreatic polypeptide (BPP). In the suboesophageal ganglion three groups of cells stain, including one pair of large posterior ventral cells. These cells are the same size, in the same location in the ganglion and have the same branching pattern as a pair of BPP immunoreactive cells known to innervate the heart and retrocerebral glandular complex of the locust. In the thoracic and abdominal ganglia two and three sets of cells, respectively, stain with both the SchistoFLRFamide and BPP antisera. In the abdominal ganglia the immunoreactive cells project via the median nerves to the intensely immunoreactive neurohaemal organs.     

Ionotropic receptors in neuronal-astroglial signalling: What is the role of “excitable” molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca²⁺ or Na⁺ signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.     

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.     

The physiological role of peripheral serotoninergic neurones. A review

1. Serotoninergic neurones and neurones which takes up 5-hydroxytryptamine (5-HT) have been observed in sensory ganglia, but their physiological role remains unknown. 2. Serotoninergic neurones participating in the neural control of gut motility are present within the enteric intramural nervous system. 3. 5-HT applied to the serosa inhibits the peristaltic reflex in the small intestine. In contrast, peristalsis is stimulated by 5-HT applied to the serosa. 4. Intracellular microelectrode investigations indicate that some neurones of the enteric nervous system are depolarized, whereas others are hyperpolarized by 5-HT. In addition, 5-HT can also decrease the release of acetylcholine by acting on presynaptic receptors located on cholinergic nerve endings. The release of 5-HT from serotoninergic enteric neurones is very probably under the control of a noradrenergic mechanism. 5. Electromyographic studies on the rabbit colon indicate that a nerve-mediated descending inhibition is modified by drugs interacting with the synthesis of 5-HT or its reuptake.     

Receptors for L-glutamate and GABA in the nervous system of an insect (Periplaneta americana)

The nervous system of the cockroach Periplaneta americana is well suited to studies of invertebrate amino acid receptors. Using a combination of radioligand binding and electrophysiological techniques, several distinct receptors have now been identified. These include an l-glutamate-gated chloride channel which has no known counterpart in the vertebrate nervous system, and a putative kainate/quisqualate receptor with pharmacological properties different from those of the existing categories of vertebrate excitatory amino acid receptors. GABA receptors have also been characterized in the cockroach nervous system. Bicuculline, benzodiazepines and steroids have revealed important differences between certain insect GABA-gated chloride channels and vertebrate GABA receptors. Identifiable neurones may facilitate the allocation of specific functions to amino acid receptor subtypes. In view of the existence of subtypes of amino acid receptors in insects, it is of interest to examine how this is reflected at the molecular level in terms of receptor subunit composition and amino acid sequence. Preliminary molecular cloning studies on insect GABA receptors are described.     

Prothrombin overexpressed in post-natal neurones requires blood factors for activation in the mouse brain

Thrombin is thought to mediate, through protease-activated receptors, both protective as well as cytotoxic effects. As thrombin receptors are expressed in the CNS, an important question arises as to whether the intact nervous system is able to generate thrombin by activation of its precursor prothrombin, derived endogenously or only upon extravasation following brain injury. To address this question, transgenic mice that express C-terminally haemagglutinin tagged human prothrombin in post-mitotic neurones were generated. In situ hybridization and immunohistochemical analysis showed abundant and widespread cerebral expression of the transgene. Amidolytic assays of brain homogenates and hippocampal slice cultures demonstrated that activation of transgenic prothrombin required added factors, such as snake venom or blood components. This strongly suggests that any possible action of thrombin in the adult CNS depends on blood-derived factors that activate prothrombin. Furthermore, the results are consistent with the idea that in the non-pathological situation an as yet unidentified ligand activates thrombin receptors in the nervous system.     

The pharynx of Caenorhabditis elegans.

The anatomy of the pharynx of Caenorhabditis elegans has been reconstructed from electron micrographs of serial sections. The pharynx is used for pumping food into the gut, and is composed of 34 muscle cells, 9 marginal cells, 9 epithelial cells, 5 gland cells and 20 neurones. Three regions of specialization in the cuticle lining of the pharyngeal lumen may aid in the accumulation of food particles. A basement membrane isolates the pharynx from the rest of the animal, making the pharyngeal nervous system a nearly self-contained unit which is composed primarily of five classes of motor neurones and six classes of interneurones. Three other classes have also been described, which by their morphology appear to be neurosecretory and motor, motor and interneuronal, and lastly one pair that only innervates three of the marginal cells. Some classes of neurone have free endings just under the cuticle lining the lumen of the pharynx, suggesting that these are mechano- or proprio-receptive endings. The connectivity of these neurones has been described at the level of individual synaptic regions, and after combining this information with video taped observations of the pharynx pumping, some interpretations of how these neurones function have been offered.     

Sensory receptors associated with the stylets and cibarium of the rice brown planthopper,Nilapavarta lugens

Summary The stylets of Nilapavarta lugens consist of two maxillae that interlock to form separate food and salivary ducts partially surrounded by two mandibles. The ultrastructure of the sensory innervation of the stylets is described. Each maxilla possesses five neurones which extend to the tip of the stylet. The mandibles also contain five neurones, four of which are paired. The paired neurones comprise a shorter dendrite extending part of the way along the stylet and a longer one extending to the tip. The possible functions of these neurones are discussed. Gustatory receptors are located in the small passageway leading from the food duct to the cibarium. The receptors are in two distinct groups on the epipharyngeal side and one group on the hypopharyngeal side of the food canal. Two to five neurones innervate each receptor which connects to the food canal via a small pore.     

Neurophysiology of the vibration sense in locusts and bushcrickets: The responses of ventral-cord neurones

In the locustid Locusta migratoria and the tettigoniids Decticus verrucivorus and Tettigonia cantans, comparative aspects of physiological properties of vibratory/auditory ventral-cord neurones were studied by single cell recordings.These neurones all receive inputs from both vibratory and auditory receptors. Nevertheless, they can be classified into “V neurones” responding preferentially to vibration stimuli, “VS neurones” responding to vibration and airborne sound, and “S neurones” responding preferentially to airborne sound. In every group, there are several types with different physiological properties, normally represented by one neurone on each body side.In Locusta and in the tettigoniid species, the same physiological types of vibratory/auditory neurones were found, although there are differences in the synaptic connectivity of the vibration receptors of the different legs. In Locusta, the middle leg receptors have the strongest influence on the generation of suprathreshold responses of the central neurones, whereas in the tettigoniids the receptors of the ipsilateral fore leg are the most influential.Two of the V neurones receive inputs mainly from campaniform sensilla and other low-frequency vibration receptors, the other V and VS neurones are mainly influenced by the subgenual receptors. Central information processing results in preferential responses to different frequency/intensity ranges in different neurones.Most VS neurone types show the same response characteristics (e.g. time pattern of response, habituation) either to vibration or to airborne-sound stimuli. Simultaneous presentation of both stimuli leads to qualitative changes in the response characteristics. Therefore, the co-processing of auditory and vibratory signals seems to be very important in the acoustic behaviour of grasshoppers.     

Biology of mammalian photoperiodism and the critical role of the pineal gland and melatonin

In mammals, photoperiodic information is transformed into a melatonin secretory rhythm in the pineal gland (high levels at night, low levels during the day). Melatonin exerts its effects in discrete hypothalamic areas, most likely through MT1 melatonin receptors. Whether melatonin is brought to the hypothalamus from the cerebrospinal fluid or the blood is still unclear. The final action of this indoleamine at the level of the central nervous system is a modulation of GnRH secretion but it does not act directly on GnRH neurones; rather, its action involves a complex neural circuit of interneurones that includes at least dopaminergic, serotoninergic and aminoacidergic neurones. In addition, this network appears to undergo morphological changes between seasons.     

Integrative mechanisms of the formation of the small intestine motor effects

Contractile responses of the small intestine to serotonine and histamine are mediated by cholinergic neurones, while the inhibitory responses of the substances--by nonadrenergic inhibitory neurones of the enterometasympathetic nervous system. An inhibitory response of the small intestine to met-enkephalin results from its depressing action on the effector cholinergic neurones. Catecholamines activate enteric cholinergic neurones via presynaptic beta-adrenoceptors and inhibit them via pre- and postsynaptic alpha-adrenoceptors. The cholinergic neurones of the enterometasympathetic nervous system seem to be under a double adrenergic control, and a mechanisms seems to exist in the small intestine for adrenergic activation of its contractile apparatus.     

Leech Retzius cells and 5-hydroxytryptamine

A pair of giant Retzius (R) cells in each segmental ganglion of the leech contain 5-hydroxytryptamine (5-HT). They are the only 5-HT-containing neurones in the central nervous system to send branches to the periphery, yet many peripheral tissues (e.g. body wall muscles, heart, reproductive organs, nephridia and gut) possess 5-HT-like immunoreactive nerve fibres. 5-HT and/or R cell stimulation relax basal tension of body wall muscles and reduce their relaxation times following contraction, enhance pharyngeal movements and salivary gland secretion but inhibit muscle movements of the posterior gut regions and of the reproductive tract. It is suggested that R cells are multifunction neurones modulating activity of many tissues so that feeding behaviour of the leech is carried out as efficiently as possible.     

SEROTONIN AND FREE AMINO ACID ANALYSIS OF GANGLIA AND ISOLATED NEURONES OF APLYSIA DACTYLOMELA

—The presence of serotonin and different amino acids was investigated in the ganglia and in isolated giant neurones of Aplysia dactylomela. With a few exceptions the pattern of substances was similar in all the ganglia. Of the many identified neurones studied only one giant neurone located in each cerebral ganglion was found to contain serotonin. GABA was detected in most extracts, including those of the serotonin-containing neurone, known cholinergic, and known neurosecretory neurones. Putrescine, recently detected in extracts of nervous tissue and isolated neurones of Helix, was not detected in Aplysia nervous tissue.     

Fibroblast-like synovial cells from normal and inflamed knee joints differently affect the expression of pain-related receptors in sensory neurones: a co-culture study

Innervation of the joint with thinly myelinated and unmyelinated sensory nerve fibres is crucial for the occurrence of joint pain. During inflammation in the joint, sensory fibres show changes in the expression of receptors that are important for the activation and sensitization of the neurones and the generation of joint pain. We recently reported that both neurokinin 1 receptors and bradykinin 2 receptors are upregulated in dorsal root ganglion (DRG) neurones (the cell bodies of sensory fibres) in the course of acute and chronic antigen-induced arthritis in the rat. In this study, we begin to address mechanisms of the interaction between fibroblast-like synovial (FLS) cells and sensory neurones by establishing a co-culture system of FLS cells and DRG neurones. The proportion of DRG neurones expressing neurokinin 1 receptor-like immunoreactivity was not altered in the co-culture with FLS cells from normal joints but was significantly upregulated using FLS cells from knee joints of rats with antigen-induced arthritis. The proportion of DRG neurones expressing bradykinin 2 receptors was slightly upregulated in the presence of FLS cells from normal joints but upregulation was more pronounced in DRG neurones co-cultured with FLS cells from acutely inflamed joints. In addition, the expression of the transient receptor potential V1 (TRPV1) receptor, which is involved in inflammation-evoked thermal hyperalgesia, was mainly upregulated by co-culturing DRG neurones with FLS cells from chronically inflamed joints. Upregulation of neurokinin 1 receptors but not of bradykinin 2 and TRPV1 receptors was also observed when only the supernatant of FLS cells from acutely inflamed joint was added to DRG neurones. Addition of indomethacin to co-cultures inhibited the effect of FLS cells from acutely inflamed joints on neurokinin 1 receptor expression, suggesting an important role for prostaglandins. Collectively, these data show that FLS cells are able to induce an upregulation of pain-related receptors in sensory neurones and, thus, they could contribute to the generation of joint pain. Importantly, the influence of FLS cells on DRG neurones is dependent on their state of activity, and soluble factors as well as direct cellular contacts are crucial for their interaction with neurones.     

Influence of extracellular matrix proteins on the expression of neuronal polarity

The influence of laminin (LN) and fibronectin (FN) on the differentiation of individual neurones from the embryonic rat central nervous system was studied in vitro. In control cultures or in the presence of soluble FN, most neurones had several dendrite-like and one axon-like processes. On substratum-bound LN, multipolar and unipolar cells were present. Soluble LN and bound FN induced a very simple neuronal morphology, most neurones having only one axon-like neurite as defined by morphological and immunocytochemical characteristics. The significant reduction of neuronal adhesion and spreading in conditions leading to the growth inhibition of dendrite-like processes suggests that, contrary to that of axons, dendrite growth strongly depends on neuronal adhesion. We propose a model in which the different dependency of axonal and dendritic outgrowth towards adhesion and spreading is explained by the respective physical properties of the two types of neurites.     

Distribution of gaba-receptors and gaba-carriers in the mammalian nervous system.

1. Extrasynaptic GABA-receptors occur on both neurone somata and unmyelinated axons in the mammalian peripheral nervous system. Activation of these receptors leads to depolarization, reduced spike amplitude and slowed conduction, probably mediated through increased Cl- conductance. 2. GABA also depolarizeds preganglionic nerve terminals in the rat superior cervical ganglion and reduces the release of acetylcholine by preganglionic nerve impulses. 3. The Schwann and satellite neuroglial cells surrounding peripheral unmyelinated axons and neurones possess a GABA-carrier promoting net uptake of GABA at external concentrations greater than or equal to 1 microM. 4. The possible significance of extrasynaptic receptors and carriers for GABA is discussed.     

Insect neuronal cultures: an experimental vehicle for studies of physiology, pharmacology and cell interactions

The current status of insect neuronal cultures is discussed and their contribution to our understanding of the insect nervous system is explored. Neuronal cultures have been developed from a wide range of insect species and from all developmental stages. These have been used to study the morphological development of insect neurones and some of the extrinsic factors that affect this process. In addition, they have been used to investigate the physiology of sodium, potassium and calcium channels and the pharmacology of acetylcholine and GABA receptors. Insect neurones have also been grown in culture with muscle and glial cells to study cell interactions.     

Depression of presynaptic excitation by the activation of vanilloid receptor 1 in the rat spinal dorsal horn revealed by optical imaging

Hypoxia alters neuronal function and can lead to neuronal injury or death especially in the central nervous system. But little is known about the effects of hypoxia in neurones of the peripheral nervous system (PNS), which survive longer hypoxic periods. Additionally, people have experienced unpleasant sensations during ischemia which are dedicated to changes in conduction properties or changes in excitability in the PNS. However, the underlying ionic conductances in dorsal root ganglion (DRG) neurones have not been investigated in detail. Therefore we investigated the influence of moderate hypoxia (27.0 ± 1.5 mmHg) on action potentials, excitability and ionic conductances of small neurones in a slice preparation of DRGs of young rats. The neurones responded within a few minutes non-uniformly to moderate hypoxia: changes of excitability could be assigned to decreased outward currents in most of the neurones (77%) whereas a smaller group (23%) displayed increased outward currents in Ringer solution. We were able to attribute most of the reduction in outward-current to a voltage-gated K⁺ current which activated at potentials positive to -50 mV and was sensitive to 50 nM α-dendrotoxin (DTX). Other toxins that inhibit subtypes of voltage gated K⁺ channels, such as margatoxin (MgTX), dendrotoxin-K (DTX-K), r-tityustoxin Kα (TsTX-K) and r-agitoxin (AgTX-2) failed to prevent the hypoxia induced reduction. Therefore we could not assign the hypoxia sensitive K⁺ current to one homomeric K_V channel type in sensory neurones. Functionally this K⁺ current blockade might underlie the increased action potential (AP) duration in these neurones. Altogether these results, might explain the functional impairment of peripheral neurones under moderate hypoxia.