Developmental Neurobiology BIPN 144
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Date Created: 10/22/15
LECTURE 16 Synaptic Competition and Coincidence Detection 1 Overview of Synaptic Competition Coincidence counting Establishment of initial connections versus refinement of map First order rule Basic map made by activity independent mechanisms Detailed refined features integrating different information require activity Examples Muscles originally multiply innervated gt synapse elimination Activity dependent TTX action potential curare amp ocBTX AChR Two neurons one muscle cell in culture heterosynaptic suppression Both fire both survive stabilize Fire apart the weaker one retracts 2 Synaptic rearrangement in muscle and nervous system Early connections are polysynaptic presynaptic neurons innervate more than one target and targets receive more than one input Mature connections are monosynaptic or less polysynaptic presynaptic neurons have fewer targets and targets receive only one or a small number of inputs Examples Muscle Neonatal rat diaphragm muscle 3 mot neurmuscle gt 1 begins before birth and is finished in 2wks Autonomic NS Rat submandibular gang 5 gt 1 Rabbit ciliary gang 5 gt 2 4O pregang gt 400 cil gang cells Hamster cil gang 11 gt6 CNS Avian cochlear nucleus 4 gt2 Purkinje cell innervated by 3 gt 1 climbing fiber In general cells which ultimately receive only single input either have no dendrites or simple dendrites whereas cells which remain polyinnervated have more complex dendritic processes Loss of ipsilateral projections in bird retinotectal map or contralateral callosal pro in mammals Blocking nerve slows synapse elimination and stimulation speeds it up Experimental evidence Synapse elimination is activity dependent TTX curare ocBTX Two neurons one muscle cell in culture heterosynaptic suppression both fire both survive stabilize fire apart the weaker retracts In cats Eye gt lateral geniculate gt visual cortex Remove one eye gt shrink target region of Ig for enucleated eye expand target of IQ for other eye In flies mutants lacking eyes eg eyeless or sina ocuus have severe loss of medulla and visual brain nuclei Whisker barrels 11 whiskers to barrels in somatosensory cortex Remove whisker 5 early gt loss of barrel 5 3 synapses removed Extra whiskers in some mice gt extra corresponding barrel 3 Plasticity of maps during normal development Shifting connections map in goldfish and frog Goldfish retina grows radially but new cells are added to caudomedial edge of tectum New fibers grow in map to radial positions and must displace some fibers caudally Competition between fibers 4 Sprouting following denervation Partial denervation induces sprouting of remaining axons Partial denervation of mam skel muscle gt sprouting to previous site of innervation Preganglionic innervation of autonomic ganglia Sensory projections within spinal cord CNS projections in brain Also sprouting of sensory innervation of skin following partial denervation Other inducers of sprouting Block of synaptic transmission abungarotoxin block of muscle ACHR TTX cuff on motor neuron axon Dead nerve or thread on or near muscle induces sprouting Degenerating sensory axons central DRG transection gt motor neuron sprouting Sprouting agents and inhibitors of sprouting may determine balance of innervationprocess retraction Blocking axon transport in one cutaneous nerve colchicine induces sprouting of neighboring cutaneous nerve Explanation nerves transport substance that inhibits sprouting which is induced by a factor provided bythe target A lateral inhibitory mechanism Specificity of sprouting agents nhibitors sprouting of sensory axons and cut nerves more pronounced within original dermatome sprouting of different sensory modalities are independent leech Regenerated original nerve displaces sprouts Forces that drive sprouting and retraction normally in delicate balance Nerve may know how large an arbor to make Pruning part of arbor induces expansion of remaining arbor or aberrant proiections Sprouting and retraction occur at normal NMJ Double label of axon silver stain and post synaptic specializations ACHE gt some new sprouts without post synaptic specializations and some post synaptic specializations without motor axon Age dependent changes fewer neurons reduction of synaptic contacts compensatory increased complexity of some synapses NMJ more complex old termini are still less active than young junctions 5 Retraction of presynaptic inputs following postsynaptic axotom Axotomy of postsynaptic cell or blocking axonal transport colchicine results in retrograde reduction in inputs Distal sectioning of cat phrenic nerve causes dramatic reduction in discharge from remaining proximal stump Axotomy of guinea pig sup cerv gang leads to transient reduction of synaptic potential on typical ganglionic cell Recovery of input magnitude parallels time course of axon regeneration 6 Recap of regulation of trophic factor production see Lecture 14 Persistent dependence on target for presynaptic input survival Axotomy reduces or eliminates factors retrogradely transported into ganglion eg NGF Exogenous NGF can prevent presynaptic loss in axotomized symp gang AntiNGF Ab mimics axotomy in intact symp gang Effect of synaptic function on NGF production in postsynaptic cells Blockade of target function eg muscle decreased cell death Excessive target stimulation increased cell death Model Trophic factors produced by noninnervated target presynaptic cells survive gt innervation gt signal to postsynapic target to decrease production of trophic factor gt only some neurons synapsing subset can survive Effect of synapse formation on survival of postsynaptic targets Remove presynaptic cells increased postsynaptic cell death Add addition presynaptic cells decreased cell death Coupled presynaptic control of target and postsynaptic control of input Targets can regulate cell survival from input cells Increases in inputtarget increased cell death Decreases in inputtarget decreased cell death Model Match mass of input to target by trophic factor provided by target Targets also require presence of inputs to survive evidence for trophic factors and electrical activity due to synaptic input 7 Hebbian mechanisms of correlation detection Basic ideas of synaptic modifiability and learnin i Synaptic connections are constantly made and broken in response to experience ii Efficacy of synaptic transmission is modified by experience and usage Hebbian Mechanisms 1 Asynchronoust firing inputs on a given target compete to displace each other 2 Correlated firing of neighboring cells suppresses competition between fibers Recall during muscle innervation that driving one cell more than the other leads to elimination of other synapse Nerve firing may induce muscle to produce an antiinnervation signal to which a recently fired neuron is refractory Classes of models Postsynaptic receptor preservation linked to activity at synapse Level of memb potential AP rev pot 40 mv vs 0 mv for ACHR Protective transient receptor post trans mod at synapse Postsynaptic densities get larger with time in established synapses and harbor coincidence counting CAMII kinase Presynaptic nerve rewarded by localized retrograde signal NO or arachidonic acid as well as inhibited by general activity In both models synchronous activity leads to stable strongly innervated target