Axonal wiring of guanylate cyclase-D-expressing olfactory neurons is dependent on neuropilin 2 and semaphorin 3F

The olfactory system of the mouse includes several subsystems that project axons from the olfactory epithelium to the olfactory bulb. Among these is a subset of neurons that do not express the canonical pathway of olfactory signal transduction, but express guanylate cyclase-D (GC-D). These GC-D-positive (GC-D+) neurons are not known to express odorant receptors. Axons of GC-D+ neurons project to the necklace glomeruli, which reside between the main and accessory olfactory bulbs. To label the subset of necklace glomeruli that receive axonal input from GC-D+ neurons, we generated two strains of mice with targeted mutations in the GC-D gene (Gucy2d). These mice co-express GC-D with an axonal marker, tau-beta-galactosidase or tauGFP, by virtue of a bicistronic strategy that leaves the coding region of the Gucy2d gene intact. With these strains, the patterns of axonal projections of GC-D+ neurons to necklace glomeruli can be visualized in whole mounts. We show that deficiency of one of the neuropilin 2 ligands of the class III semaphorin family, Sema3f, but not Sema3b, phenocopies the loss of neuropilin 2 (Nrp2) for axonal wiring of GC-D+ neurons. Some glomeruli homogeneously innervated by axons of GC-D+ neurons form ectopically within the glomerular layer, across wide areas of the main olfactory bulb. Similarly, axonal wiring of some vomeronasal sensory neurons is perturbed by a deficiency of Nrp2 or Sema3f, but not Sema3b or Sema3c. Our findings provide genetic evidence for a Nrp2-Sema3f interaction as a determinant of the wiring of axons of GC-D+ neurons into the unusual configuration of necklace glomeruli.     

Ca2+- or Mg2+-Stimulated ATPase Activity in Bullfrog Spinal Nerve: Relation to Ca2+ Requirements for Fast Axonal Transport

Adenosine triphosphatase (ATPase) activity stimulated by Ca2+ or Mg2+ was characterized in spinal nerve and spinal sensory ganglion of bullfrog. Enzyme activity of homogenates from both sources reached a maximum at a 1-2 mM concentration of either cation, although the level of maximal activity in nerve trunks was approximately twice that in ganglia. Enzyme activation was not observed with 2 mM-Sr2+ or Ba2+. Co2+ or Mn2+, at 2 mM, depressed Ca2+ activation of the enzyme by 50-60% in nerve but had no inhibitory effect on ganglia activity. In intact spinal ganglion/spinal nerve preparations, incubated for 20 h in medium containing 0.2 mM-Co2+, no effect was detected on Ca2+/Mg2+ ATPase activity in ganglia or nerve trunks whereas fast axonal transport was inhibited by 80%. Incubation in medium containing 0.02 mM-Hg2+ depressed enzyme activity in ganglia by 64% and in nerve trunks by 44%, whereas fast transport was again inhibited by 80%. When only nerve trunks were exposed to these ions, Hg2+ but not Co2+ was observed to slow the rate of fast axonal transport. The divalent cation specificity of the Ca2+/Mg2+ ATPase activity is distinct from the ion specificities, determined in previous work, of the Ca2+ requirement during initiation of fast axonal transport in the soma, and of the Ca2+ requirement during translocation in the axon. Thus, previous observations of Ca2+-dependent events in fast axonal transport cannot be taken per se to suggest the involvement of Ca2+/Mg+ ATPase in the transport process.     

Calcium permeability changes and neurotransmitter release in cultured rat brain neurons. I. Effects of stimulation on calcium fluxes

The permeability of neuronal membranes to Ca2+ is of great importance for neurotransmitter release. The temporal characteristics of Ca2+ fluxes in intact brain neurons have not been completely defined. In the present study 45Ca2+ was used to examine the kinetics of Ca2+ influx and efflux from unstimulated and depolarized rat brain neurons in culture. Under steady-state conditions three cellular exchangeable Ca2+ pools were identified in unstimulated cells: 1) a rapidly exchanging pool (t1/2 = 7 s) which represented about 10% of the total cellular Ca2+ and was unaffected by the presence of Co2+, verapamil, or tetrodotoxin; 2) a slowly exchanging pool (t1/2 = 360 s) which represented 42% of the total cellular Ca2+ and was inhibited by Co2+, but not by verapamil or tetrodotoxin; 3) a very slowly exchanging pool (t1/2 = 96 min) which represented 48% of the total cell Ca2+ was observed only in the prolonged efflux experiments. The rate of exchange of 45Ca2+ in the unstimulated cells was dependent on the extracellular Ca2+ concentration (half-saturation at 70 microM). Depolarization of the neurons with elevated K+ causes a rapid and sustained 45Ca2+ uptake. The cellular Ca2+ content increased from 56 nmol/mg protein in unstimulated cells to 81 nmol/mg protein during 5 min of depolarization. The kinetics of the net 45Ca2+ uptake by the stimulated neurons was consistent with movement of the ion with a first order rate constant of 0.0096 s-1 (t1/2 = 72 s) into a single additional compartment. The other cellular Ca2+ pools were apparently unaffected by stimulation. The stimulated 45Ca2+ uptake was inhibited by Co2+ and by the Ca2+ channel blocker verapamil but not by the Na+ channel blocker tetrodotoxin. Ca2+ uptake into this compartment was dependent on the extracellular Ca2+ concentration (half-saturation at 0.80 mM Ca2+). Predepolarization of the cells with high K+ for 10-60 s prior to the addition of the radioactive calcium did not alter the rate of 45Ca2+ incorporation into the stimulated cells. It is concluded that the rapidly exchanging, the slowly exchanging, and the depolarization-induced Ca2+ pools observed in intact brain neurons are physically as well as kinetically distinct from each other. In addition, the depolarization-induced component observed in stimulated cells represents movement of the Ca2+ ions through a single class of voltage-sensitive Ca2+ channels. These Ca2+ channels are inhibited by Co2+ ions and by verapamil and are not inactivated during depolarization of the brain neurons.     

Melanocortins and fast axonal transport in intact and regenerating sciatic nerve

The effect of ACTH/MSH peptides on fast axonal transport along intact or regenerating sciatic nerve was examined following injection of tritiated leucine into the rat lumbar spinal cord. The rate of fast axonal transport was not significantly changed by treatment with ACTH/MSH(4-10), the ACTH(4-9) analog ORG 2766, hypophysectomy, or adrenalectomy. Fast axonal transport was unchanged in regenerating nerves and in regenerating, ACTH(4-10)-treated nerves. However, treatment with ORG 2766 in dosages of either 1 or 10 micrograms/kg/day IP for seven days significantly reduced (62% and 64%, respectively) the crest height of the fast axonal transport curve of intact sciatic nerve. The results suggest that the reported peptide-induced enhancement of nerve regeneration is not due to changes in the rate of fast axonal transport.     

Laminin A, B1, and B2 Chain Gene Expression in Transected and Regenerating Nerves: Regulation by Axonal Signals

Abstract: Laminin A, B1, and B2 chain mRNA levels in degenerating and regenerating mouse sciatic nerves were examined using northern blot analysis. In normal intact nerves, B1 and B2 mRNA steady-state levels were high, but when the nerves were crushed, the steady-state levels of B1 and B2 mRNA per milligram wet tissue weight of the distal segments of the nerves increased five- to eightfold over that of control levels as the total RNA and β-actin mRNA levels increased, suggesting that these increases were the consequence of Schwann cell proliferation after axotomy. When the steady-state levels of B1 and B2 mRNA were normalized as the ratio to total RNA or β-actin mRNA levels, however, they drastically decreased to about 20% of the normal nerve levels in the nerve segments distal to both the crush and transaction sites 1 day after injury. In the crushed nerves, B1 and B2 mRNA levels gradually increased as the regenerating nerves arrived at the distal segments and reestablished normal axon–Schwann cell contact, and then returned to normal levels on the 21 st day. In the transected nerves, where Schwann cells continued to be disconnected from axons, both B1 and B2 mRNA levels remained low. Cultured Schwann cells expressed detectable levels of B1 and B2 chain mRNA which significantly increased when the cells were cocultured with sensory neurons. However, mRNA for A chain was not detectable in the normal, axotomized nerves or in cultured Schwann cells. These data indicate that Schwann cells express laminin B1 and B2 chain mRNA that are up-regulated by axonal or neuronal contact, but they do not express A chain mRNA.     

Axon outgrowth along segmental nerves in the leech. II. Identification of actual guidance cells

Some peripheral neurons, previously identified as candidate guidance cells for axonal outgrowth along the segmental nerves in embryos of the glossiphoniid leech Helobdella triserialis, were photoablated by laser illumination to ascertain whether their presence is necessary for generation of the normal axonal growth pattern. These experiments showed that focal photoablation of peripheral neurons nz3 or pz8 prevents normal axonal outgrowth along the ultraposterior nerve path or along the distal sector of the medial-anterior nerve path, respectively, in conformance with the inference that these two neurons do function as guidance cells. However, ablation of these neurons affects axon outgrowth only if the neurons are illuminated prior to the end of a sensitive period in segmental development. By contrast, photoablation of previously identified candidate guidance cells situated on the anterior-anterior and posterior-posterior nerve paths, among them peripheral neurons nz1, nz2, oz1, oz2, pz6, and LD1, does not prevent normal axonal outgrowth. It is possible that the guidance role, if any, of these neurons is facultative rather than necessary, since each of the several neurons that lies on either of these nerve paths may provide an alternative axon guidance cue.     

Axonal contribution to subthreshold currents inaplysia bursting pacemaker neurons

The contribution of axonal activity to the ionic currents which generate bursting pacemaker activity was studied by using the two-electrode voltage-clamp technique in Aplysia bursting neuron somata in conjunction with intraaxonal voltage recordings. Depolarizing voltage-clamp pulses applied to bursting cell somata triggered axonal action potentials. The voltage-clamp current recording exhibited transient inward current "notches" corresponding to each of the axonal spikes. The addition of 50 microM tetrodotoxin (TTX) to the bathing medium blocked the fast axonal spikes and current notches, revealing a slower axonal spike which was blocked by the replacement of external Ca2+ with Co2+. The inward current evoked by applying a depolarizing voltage-clamp pulse in the soma is distorted by the occurrence of the axonal Ca2+ spike. Elimination of the axonal spike, by injecting hyperpolarizing current into the axon, changes both the time course and the magnitude of the inward current. The axonal Ca2+ spikes are followed by a series of Ca2+-dependent afterpotentials: a rapid postspike hyperpolarization, a depolarizing afterpotential (DAP) and, finally, a long-lasting postburst hyperpolarization. The long-lasting hyperpolarization is not blocked by 50 mM external tetraethyl ammonium, an effective blocker of Ca2+-activated K+ current [IK(Ca)], and does not appear to reverse at EK. Hence, the axonal long-lasting hyperpolarization may not be due to IK(Ca). Somatic voltage-clamp pulses in bursting neurons are followed by a slow inward tail current, which is sometimes coincident with a DAP in the axon. In some cells, the amplitude of the slow inward tail current is greatly reduced if axonal spikes and DAPs are prevented by hyperpolarization of the axon, while, in other cells, elimination of axonal activity has little effect. Therefore, the slow inward tail current is not necessarily an artifact of poor voltage-clamp control over the axonal membrane potential but probably results from the activation of an ionic conductance mechanism located partly in the axon and partly in the soma.     

Peripheral nerve lesion-induced uptake and transport of choleragenoid by capsaicin-sensitive c-fibre spinal ganglion neurons

In the present experiments the effect of systemic capsaicin treatment on the retrograde labelling of sensory ganglion cells was studied following the injection of choleratoxin B subunit-horseradish peroxidase conjugate (CTX-HRP) into intact and chronically transected peripheral nerves. In the control rats CTX-HRP injected into intact sciatic nerves labelled medium and large neurons with a mean cross-sectional area of 1,041 +/- 39 gm2. However, after injection of the conjugate into chronically transected sciatic nerves of the control rats, many small cells were also labelled, shifting the mean cross-sectional area of the labelled cells to 632 +/- 118 microm2. Capsaicin pretreatment per se induced a moderate but significant decrease in the mean cross-sectional area of the labelled neurons (879 +/- 79 microm2). More importantly, systemic pretreatment with capsaicin prevented the peripheral nerve lesion-induced labelling of small cells. Thus, the mean cross-sectional areas of labelled neurons relating to the intact and transected sciatic nerves, respectively, did not differ significantly. These findings provide direct evidence for a phenotypic switch of capsaicin-sensitive nociceptive neurons after peripheral nerve injury, and suggest that lesion-induced morphological changes in the spinal cord may be related to specific alterations in the chemistry of C-fibre afferent neurons rather than to a sprouting response of A-fibre afferents.     

The role of the cerebral ganglia in the organization of feeding behavior in the pteropod mollusk Clione limacina]

The role of cerebral ganglia in switching from free swimming over to hunting behaviour in Clione limacina (Phipps) was studied. The cell bodies of central neurones giving processes to the sensory and motor nerves were mapped by retrograde axonal transport of Co2+ ions, and behavioural reactions under the extracellular electric stimulation of different ganglia areas were investigated. Comparison of morphological and behavioural data suggests main role of cerebral ganglia neurones in switching of behaviour from free swimming over to any of the three possible states: hunting (including food uptake), active or passive defensive behaviour.     

DIVALENT CATION SPECIFICITY OF THE CALCIUM REQUIREMENT FOR FAST TRANSPORT OF PROTEINS IN AXONS OF DESHEATHED NERVES

The presence of a requirement for calcium during the fast transport of [³H]protein in axons was assessed in desheathed spinal nerves of bullfrog. The nerves were desheathed locally along 4 mm of their length, and desheathing was judged effective on the basis of an enhanced uptake of [³H]leucine into that region of nerve trunk. Desheathing per se had a slight inhibitory effect on transport. Incubation of desheathed nerve trunks in calcium-free medium reduced transport by 60-80% relative to that in desheathed nerves incubated in normal medium. Addition of Mg²⁺ or Sr²⁺ to the calcium-free medium allowed transport to proceed normally. Addition of Co²⁺ or Mn²⁺ to normal medium did not affect transport in desheathed isolated nerve trunks. When ganglia and nerve trunks were both incubated in medium containing 0.18 mM-CoCl₂, transport was depressed to a similar extent proximal and distal to the desheathed region. This again indicates that Co²⁺ does not inhibit transport in desheathed nerves, whereas it does inhibit transport in the ganglia. Additive inhibitory effects were observed when ganglia were incubated in medium containing 0.018 mM-CoCl₂, and desheathed nerves were incubated in calcium-free medium. Differences in the divalent cation specificities of the axonal and ganglionic calcium requirements suggest that calcium supports transport in nerves in a manner distinct from its role in maintaining transport in spinal ganglia. It is concluded that the ganglionic calcium requirement involves initiation of axonal transport in the soma rather than translocation in the intraganglionic region of axon.     

Impaired fast axonal transport in neurons of the sciatic nerves from dystonia musculorum mice

Dystonia musculorum (dt) mice suffer from a severe sensory neuropathy caused by mutations in the gene encoding the cytoskeletal cross-linker protein dystonin/bullous pemphigoid antigen 1 (Bpag1). Loss of function of dystonin/Bpag1 within neurons leads to a loss in the maintenance of cytoskeletal organization and to the development of focal axonal swellings prior to death of the neuron. In the present study, we demonstrate that neurons within the sciatic nerves of dt27J mice undergo axonal degeneration as has been previously reported for the dorsal roots. Furthermore, ultrastructural studies reveal a perturbed organization of the neurofilament and microtubule networks within the axons of sciatic nerves in dt27J mice. The disrupted cytoskeletal organization suggested that axonal transport is affected in dt mice. To address this, we assessed fast axonal transport by measuring the rate of accumulation of acetylcholinesterase (AChE) proximal and distal to a surgically introduced ligature on the sciatic nerves of normal and dt27J mice. Our findings demonstrate that axonal transport of AChE in both orthograde and retrograde directions is markedly affected, and allow us to conclude that axonal transport defects do exist in the sciatic nerves of dt27J mice.     

Syntaxin13 Expression Is Regulated by Mammalian Target of Rapamycin (mTOR) in Injured Neurons to Promote Axon Regeneration

Injured peripheral neurons successfully activate intrinsic signaling pathways to enable axon regeneration. We have previously shown that dorsal root ganglia (DRG) neurons activate the mammalian target of rapamycin (mTOR) pathway following injury and that this activity enhances their axon growth capacity. mTOR plays a critical role in protein synthesis, but the mTOR-dependent proteins enhancing the regenerative capacity of DRG neurons remain unknown. To identify proteins whose expression is regulated by injury in an mTOR-dependent manner, we analyzed the protein composition of DRGs from mice in which we genetically activated mTOR and from mice with or without a prior nerve injury. Quantitative label-free mass spectrometry analyses revealed that the injury effects were correlated with mTOR activation. We identified a member of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family of proteins, syntaxin13, whose expression was increased by injury in an mTOR-dependent manner. Increased syntaxin13 levels in injured nerves resulted from local protein synthesis and not axonal transport. Finally, knockdown of syntaxin13 in cultured DRG neurons prevented axon growth and regeneration. Together, these data suggest that syntaxin13 translation is regulated by mTOR in injured neurons to promote axon regeneration.     

Limited Trafficking of a Neurotropic Virus Through Inefficient Retrograde Axonal Transport and the Type I Interferon Response

Poliovirus is an enteric virus that rarely invades the human central nervous system (CNS). To identify barriers limiting poliovirus spread from the periphery to CNS, we monitored trafficking of 10 marked viruses. After oral inoculation of susceptible mice, poliovirus was present in peripheral neurons, including vagus and sciatic nerves. To model viral trafficking in peripheral neurons, we intramuscularly injected mice with poliovirus, which follows a muscle–sciatic nerve–spinal cord–brain route. Only 20% of the poliovirus population successfully moved from muscle to brain, and three barriers limiting viral trafficking were identified. First, using light-sensitive viruses, we found limited viral replication in peripheral neurons. Second, retrograde axonal transport of poliovirus in peripheral neurons was inefficient; however, the efficiency was increased upon muscle damage, which also increased the transport efficiency of a non-viral neural tracer, wheat germ agglutinin. Third, using susceptible interferon (IFN) α/β receptor knockout mice, we demonstrated that the IFN response limited viral movement from the periphery to the brain. Surprisingly, the retrograde axonal transport barrier was equivalent in strength to the IFN barrier. Illustrating the importance of barriers created by the IFN response and inefficient axonal transport, IFN α/β receptor knockout mice with muscle damage permitted 80% of the viral population to access the brain, and succumbed to disease three times faster than mice with intact barriers. These results suggest that multiple separate barriers limit poliovirus trafficking from peripheral neurons to the CNS, possibly explaining the rare incidence of paralytic poliomyelitis. This study identifies inefficient axonal transport as a substantial barrier to poliovirus trafficking in peripheral neurons, which may limit CNS access for other viruses.     

Faster nerve regeneration after sciatic nerve injury in mice over-expressing basic fibroblast growth factor

Basic fibroblast growth factor (FGF-2) is expressed in the peripheral nervous system and is up-regulated after nerve lesion. It has been demonstrated that administration of FGF-2 protects neurons from injury-induced cell death and promotes axonal regrowth. Using transgenic mice over-expressing FGF-2 (TgFGF-2), we addressed the importance of endogenously generated FGF-2 on sensory neuron loss and sciatic nerve regeneration. After sciatic nerve transection, wild-type and transgenic mice showed the same degree of cell death in L5 spinal ganglia. Also, the number of chromatolytic, eccentric, and pyknotic sensory neurons was not changed under elevated levels of FGF-2. Morphometric evaluation of intact nerves from TgFGF-2 mice revealed no difference in number and size of myelinated fibers compared to wild-type mice. One week after crush injury, the number of regenerated axons was doubled and the myelin thickness was significantly smaller in transgenic mice. After 2 and 4 weeks, morphometric analysis and functional tests revealed no differences in recovery of sensory and motor nerve fibers. To study the role of FGF-2 over-expression on Schwann cell proliferation during the early regeneration process, we used BrdU-labeling to mark dividing cells. In transgenic mice, the number of proliferating cells was significantly increased distal to the crush site compared to wild-types. We propose that endogenously synthesized FGF-2 influences early peripheral nerve regeneration by regulating Schwann cell proliferation, axonal regrowth, and remyelination.     

Nitric oxide nerves in the uterus are parasympathetic,sensory, and contain neuropeptides

Nitric oxide (NO) is synthesized in neurons and is a potent relaxor of vascular and nonvascular smooth muscle. The uterus contains abundant NO-synthesizing nerves which could be autonomic and/or sensory. This study was undertaken to determine: 1) the source(s) of NO-synthesizing nerves in the rat uterus and 2) what other neuropeptides or transmitter markers might coexist with NO in these nerves. Retrograde axonal tracing, utilizing Fluorogold injected into the uterine cervix, was employed for identifying sources of uterine-projecting neurons. NO-synthesizing nerves were visualized by staining for nicotinamide adenine dinucleotide phosphate (reduced)-diaphorase (NADPH-d) and immunostaining with an antibody against neuronal/type I NO synthase (NOS). NADPH-d-positive perikarya and terminal fibers were NOS-immunoreactive (-I). Some NOS-I/NADPH-d-positive nerves in the uterus are parasympathetic and originate from neurons in the pelvic paracervical ganglia (PG) and some are sensory and originate from neurons in thoracic, lumbar, and sacral dorsal root ganglia. No evidence for NOS-I/NADPH-d-positive sympathetic nerves in the uterus was obtained. Furthermore, double immunostaining revealed that in parasympathetic neurons, NO-I/NADPH-d-reactivity coexists with vasoactive intestinal polypeptide, neuropeptide Y, and acetylcholinesterase and in sensory nerves, NOS-I/NADPH-d-reactivity coexists with calcitonin generelated peptide and substance P. In addition, tyrosine hydroxylase(TH)-I neurons of the PG do not contain NOS-I/NADPH-d-reactivity, but some TH-I neurons are apposed by NOS-I varicosities. These results suggest NO-synthesizing nerves in the uterus are autonomic and sensory, and could play significant roles, possibly in conjunction with other putative transmitter agents, in the control of uterine myometrium and vasculature.     

Interaction of phospholipase A with an axon plasma membrane preparation.

The rates at which the phosphalipase A catalyzed inactivation of the axonal cholinergic binding macromolecule (ACBM), activation of acetylcholinesterase, and hydrolysis of fatty acid acyl esters were measured in an axon plasma membrane preparation from lobster nerves. The inactivation of ACBM was shown not to be caused by products of the reaction. The solubilized ACBM was also sensitive to phospholipase A inactivation. These results indicated a direct role for the phospholipid in the binding of cholinergic ligands to ACBM. It is suggested that ACBM may be a postsynaptic cholinergic receptor that is in a different lipid environment in the axon plasma membrane. A comparison with the effect of phospholipase A on axonal conduction in intact nerves suggests that the ACBM is directly involved in this process.     

Fasciculation and guidance of spinal motor axons in the absence of FGFR2 signaling

During development, fibroblast growth factors (FGF) are essential for early patterning events along the anterior-posterior axis, conferring positional identity to spinal motor neurons by activation of different Hox codes. In the periphery, signaling through one of four fibroblast growth factor receptors supports the development of the skeleton, as well as induction and maintenance of extremities. In previous studies, FGF receptor 2 (FGFR2) was found to interact with axon bound molecules involved in axon fasciculation and extension, thus rendering this receptor an interesting candidate for the promotion of proper peripheral innervation. However, while the involvement of FGFR2 in limb bud induction has been extensively studied, its role during axon elongation and formation of distinct nervous projections has not been addressed so far. We show here that motor neurons in the spinal cord express FGFR2 and other family members during the establishment of motor connections to the forelimb and axial musculature. Employing a conditional genetic approach to selectively ablate FGFR2 from motor neurons we found that the patterning of motor columns and the expression patterns of other FGF receptors and Sema3A in the motor columns of mutant embryos are not altered. In the absence of FGFR2 signaling, pathfinding of motor axons is intact, and also fasciculation, distal advancement of motor nerves and gross morphology and positioning of axonal projections are not altered. Our findings therefore show that FGFR2 is not required cell-autonomously in motor neurons during the formation of initial motor projections towards limb and axial musculature.     

Axonal pathways and synaptic inputs of three identified neurons in the buccal ganglion of Helix pomatia.

The axonal pathways and the synaptic inputs of the identified neurons B1 through B3 in the buccal ganglia of Helix pomatia were studied. The axons of neurons B1, B2 and B3 were found to run invariably within the ipsilateral posterior oesophageal nerve, ipsi- and contralateral salivary gland nerves, and ipsilateral cerebrobuccal connective, respectively. Synaptic responses could be elicited by stimulation of most of the nerves of the buccal ganglia. These consisted of an early depolarization which was most frequently followed by a longlasting de- or hyperpolarization. The shape of the synaptic response proved to be related to the different neurons.     

Intrinsic properties guide proximal abducens and oculomotor nerve outgrowth in avian embryos

Proper movement of the vertebrate eye requires the formation of precisely patterned axonal connections linking cranial somatic motoneurons, located at defined positions in the ventral midbrain and hindbrain, with extraocular muscles. The aim of this research was to assess the relative contributions of intrinsic, population-specific properties and extrinsic, outgrowth site-specific cues during the early stages of abducens and oculomotor nerve development in avian embryos. This was accomplished by surgically transposing midbrain and caudal hindbrain segments, which had been pre-labeled by electroporation with an EGFP construct. Graft-derived EGFP+ oculomotor axons entering a hindbrain microenvironment often mimicked an abducens initial pathway and coursed cranially. Similarly, some EGFP+ abducens axons entering a midbrain microenvironment mimicked an oculomotor initial pathway and coursed ventrally. Many but not all of these axons subsequently projected to extraocular muscles that they would not normally innervate. Strikingly, EGFP+ axons also took initial paths atypical for their new location. Upon exiting from a hindbrain position, most EGFP+ oculomotor axons actually coursed ventrally and joined host branchiomotor nerves, whose neurons share molecular features with oculomotor neurons. Similarly, upon exiting from a midbrain position, some EGFP+ abducens axons turned caudally, elongated parallel to the brainstem, and contacted the lateral rectus muscle, their originally correct target. These data reveal an interplay between intrinsic properties that are unique to oculomotor and abducens populations and shared ability to recognize and respond to extrinsic directional cues. The former play a prominent role in initial pathway choices, whereas the latter appear more instructive during subsequent directional choices.     

Relationships between a neuronal buccal population and the peribuccal regions in Aplysia: an electrophysiological study

1. The relationships between Aplysia buccal neurons projecting the cerebral ganglion (L cells) and peribuccal regions were studied by electrophysiological techniques. 2. Stimulation of the cerebral upper labial (UL) and anterior tentacular (AT) nerves produced excitatory postsynaptic potentials in L cells. 3. Sixteen cells out of 24 were found possess an axonal branch in the labial branch of the AT nerve, 1 out of 8 in the UL nerve. 4. These axonal branches did not show any direct motor or sensory function in "reduced" preparations. 5. A modulatory function for the axonal projections and a sensory role for the synaptic relationships are hypothesized.