Overexpression of Bcl-xl protects septal neurons from prolonged hypoglycemia and from acute ischemia-like stress

Overexpression of Bcl-xl, a member of the Bcl-2 protein family, is reported to protect from a variety of stresses involving delayed cell death. We tested the ability of Bcl-xl overexpression to protect primary cultures of embryonic rat septal neurons subjected to one of four different stresses: 6 h of combined oxygen-glucose deprivation, which produces rapid cell death, or a 24 h exposure to hypoglycemia, hyperglycemia, or 1mM 3-nitropropionic acid (an inhibitor of mitochondrial respiration), which results in a more slowly-developing death. Prior to the stress neurons were transiently transfected to overexpress either green fluorescent protein only or green fluorescent protein along with wild-type Bcl-xl. Immediately after oxygen-glucose deprivation, many neurons expressing green fluorescent protein only showed process blebbing and disintegration, with only 49% of the initial cells remaining intact with processes. Neurons expressing both green fluorescent protein and Bcl-xl showed less damage (68% intact post-stress, P<0.05). This result indicates that Bcl-xl's saving effects are not due solely to blocking delayed (apoptotic) death, because death following oxygen-glucose deprivation was rapid and was not accompanied by increased activation of caspase-3. Bcl-xl expression also significantly protected against the hypoglycemic stress (23% intact 24 h post-stress with green fluorescent protein only, compared with 70% with Bcl-xl and green fluorescent protein), but did not protect from hyperglycemia or 3-nitropropionic acid. Thus Bcl-xl does not protect against all forms of delayed death. Bcl-xl's protective effects may include blocking early damaging events, perhaps by increasing mitochondrial function in the face of low levels of energy substrates. Bcl-xl's protective effects may require an intact electron transport chain.

Bone morphogenetic proteins (BMP6 and BMP7) enhance the protective effect of neurotrophins on cultured septal cholinergic neurons during hypoglycemia

The effects of two bone morphogenetic proteins (BMP6, BMP7), alone and in combination with neurotrophins, were tested on cultures of embryonic day 15 rat septum. A week-long exposure to BMP6 or BMP7 in the optimal concentration range of 2-5 n M increased the activity of choline acetyltransferase (ChAT) by 1.6-2-fold, in both septal and combined septal-hippocampal cultures. The increase in ChAT activity reached significance after 4 days and continued to increase over an 11-day exposure. Under control culture conditions neither BMP significantly altered the number of cholinergic neurons, and BMP effects on ChAT activity were less than linearly additive with those of nerve growth factor. The effects of BMPs and BMP + neurotrophin combinations were also assayed under two stress conditions: low-density culture and hypoglycemia. In low-density cultures BMPs and BMP + neurotrophin combinations preserved ChAT activity more effectively than neurotrophins alone. During 24 h hypoglycemic stress, BMPs alone did not preserve ChAT activity, but BMP + neurotrophin combinations preserved ChAT activity much more effectively than neurotrophins alone. These results demonstrate that BMP6 and BMP7 enhance ChAT activity under control and low-density stress conditions, and that during a hypoglycemic stress their trophic effect requires and complements that exerted by neurotrophins.

Brief exposure to neurotrophins produces a calcium-dependent increase in choline acetyltransferase activity in cultured rat septal neurons

We demonstrate that brief (30-min) exposure of cultured embryonic rat septal neurons to neurotrophins (NTs) increases choline acetyltransferase (ChAT) activity by 20-50% for all tested NTs (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4, each at 100 ng/ml). The increase in ChAT activity was first detected 12 h after NT exposure, persisted at least 48 h, and was not mediated by increased neuronal survival or action-potential activity. Under some conditions, the response to brief NT exposure was as great as that produced by continuous exposure. NT stimulation of ChAT activity was inhibited by inhibitors of p75- and Trk kinase-mediated signaling, by removal of extracellular Ca2+ during the period of NT exposure, and by buffering intracellular Ca2+ with BAPTA. Application of nerve growth factor and brain-derived neurotrophic factor transiently increased [Ca2+] within a subpopulation of neurons. NT stimulation of ChAT activity was not affected significantly by cyclic AMP agonists or antagonists. These findings suggest that brief exposure to NTs can have a long-lasting effect on cholinergic transmission, and that this effect requires Ca2+, but not cyclic AMP.

Resealing of transected myelinated mammalian axons in vivo: evidence for involvement of calpain

The mechanisms underlying resealing of transected myelinated rat dorsal root axons were investigated in vivo using an assay based on exclusion of a hydrophilic dye (Lucifer Yellow-biocytin conjugate). Smaller caliber axons (<5 microm outer diameter) resealed faster than larger axons. Resealing was Ca2+ dependent, requiring micromolar levels of extracellular [Ca2+] to proceed, and further accelerated in 1 mM Ca2+. Two hours after transection, 84% of axons had resealed in saline containing 2 mM Ca2+, 28% had resealed in saline containing no added Ca2+ and only 3% had resealed in the Ca2+ buffer BAPTA (3 mM). The enhancing effect of Ca2+ could be overcome by both non-specific cysteine protease inhibitors (e.g., leupeptin) and inhibitors specific for the calpain family of Ca2+ -activated proteases. Resealing in 2 mM Ca2+ was not inhibited by an inhibitor of phospholipase A2. Resealing in low [Ca2+] was not enhanced by agents which disrupt microtubules, but was enhanced by dimethylsulfoxide (0.5-5%). These results suggest that activation of endogenous calpain-like proteases by elevated intra-axonal [Ca2+] contributes importantly to membrane resealing in transected myelinated mammalian axons in vivo.

Purification from bovine serum of a survival-promoting factor for cultured central neurons and its identification as selenoprotein-P

We purified from bovine serum a glycoprotein that promotes the survival of rat embryonic neurons cultured from septum and other brain regions. A 40,000-fold purification was achieved by using a combination of ammonium sulfate precipitation, Zn2+ affinity chromatography, Cibacron blue 3-GA dye affinity chromatography, ABx ion exchange chromatography, and preparative PAGE. The active protein had an apparent molecular weight of 50-60 kDa. The concentration required for half-maximal survival (EC50) was 12 ng/ml ( approximately 200 pM) for the final fraction. Amino acid sequencing after cyanogen bromide cleavage yielded two sequences that are homologous to regions of deduced sequence of the selenoprotein-P (SPP) family in bovine, rat, and human. Antibodies against a synthetic peptide within the bovine SPP sequence immunoprecipitated and inhibited the survival-promoting activity of a partially purified serum fraction. The purified protein supported neuronal survival more effectively than inorganic selenium. These results suggest that SPP or an SPP-like protein contributes to the neuronal survival-promoting activity of serum.

Evidence that mitochondria buffer physiological Ca2+ loads in lizard motor nerve terminals

1. Changes in cytosolic and mitochondrial [Ca2+] produced by brief trains of action potentials were measured in motor nerve terminals using a rapidly scanning confocal microscope. Cytosolic [Ca2+] was measured using ionophoretically injected Oregon Green BAPTA 5N (OG-5N). Mitochondrial [Ca2+] was measured using rhod-2, bath loaded as dihydrorhod-2. 2. In response to 100-250 stimuli at 25-100 Hz the average cytosolic [Ca2+] showed an initial rapid increase followed by a much slower rate of increase. Mitochondrial [Ca2+] showed no detectable increase during the first fifteen to twenty stimuli, but after this initial delay also showed an initially rapid rise followed by a slower rate of increase. The onset of the increase in mitochondrial [Ca2+] coincided with the slowing of the rate of rise of cytosolic [Ca2+]. The peak levels of cytosolic and mitochondrial [Ca2+] both increased with increasing frequencies of stimulation. 3. When stimulation terminated, the initial rate of decay of cytosolic [Ca2+] was much more rapid than that of mitochondrial [Ca2+]. 4. After addition of carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 1-2 microM) to dissipate the proton electrochemical gradient across the mitochondrial membrane, cytosolic [Ca2+] rose rapidly throughout the stimulus train, reaching levels much higher than normal. CCCP inhibited the increase in mitochondrial [Ca2+]. 5. These results suggest that mitochondrial uptake of Ca2+ contributes importantly to buffering presynaptic cytosolic [Ca2+] during normal neuromuscular transmission.

Stimulation-induced changes in [Ca2+] in lizard motor nerve terminals

1. Motor axons were injected ionophoretically with one of five Ca(2+)-sensitive dyes (fluo-3, Calcium Green-2, Calcium Green-5N, fluo-3FF and Oregon Green BAPTA-5N). Changes in fluorescence (delta F/Frest) within motor terminal boutons following a single action potential and brief stimulus trains were monitored with high temporal resolution using a confocal microscope. 2. Stimulation-induced increases in delta F/Frest were confined primarily to boutons, with roughly uniform increases in all the boutons of a terminal. The increase in delta F/Frest began prior to, and decayed more slowly than, the endplate potential (EPP) recorded in the underlying muscle fibre. delta F/Frest was graded with bath [Ca2+]. Both delta F/Frest and the EPP were reduced, but not eliminated, by omega-conotoxin GVIA (5-10 microM). 3. For dyes with lower affinity for Ca2+ (e.g. Oregon Green BAPTA-5N, Kd approximately 60 microM) stimulation-induced increases in delta F/Frest were measured in the presence of the K+ channel blocker 3,4-diaminopyridine (3,4-DAP, 100 microM). During brief stimulus trains (4 at 50 Hz) in 3,4-DAP, the EPP exhibited profound depression, but the fluorescence increase associated with each stimulus showed little decrement, suggesting that depression was not mediated by a reduction in Ca2+ entry. 4. For dyes with a higher affinity for Ca2+ (e.g. fluo-3, Kd approximately 0.5-1 microM) stimulation-induced increases in delta F/Frest could also be measured in normal physiological saline. Increases in delta F/Frest were much greater with 3,4-DAP present, but the amplitude decreased with successive stimuli due to partial dye saturation. 5. Calculations suggested that following a single action potential the average [Ca2+] within a bouton increased by up to 150 nM in normal saline and 940 nM in 3,4-DAP. With low affinity dyes the delta F/Frest measured near the membrane had a higher peak amplitude and a faster early decay than that measured in the centre of the bouton, suggesting that substantial spatial [Ca2+] gradients exist within boutons for at least 15 ms following stimulation.

Spatiotemporal gradients of intra-axonal [Na+] after transection and resealing in lizard peripheral myelinated axons

1. Post-transection changes in intracellular Na+ ([Na+]i) were measured in lizard peripheral axons ionophoretically injected with the Na(+)-sensitive ratiometric dye, sodium-binding benzofuran isophthalate (SBFI). 2. Following axonal transection in physiological saline [Na+]i increased to more than 100 mM in a region that quickly extended hundreds of micrometers from the transection site. This post-transection increase in [Na+]i was similar when the bath contained 5 microM tetrodotoxin, but was absent in Na(+)-free solution. Depolarization of uncut axons in 50 mM K+ produced little or no elevation of [Na+]i until veratridine was added. These results suggest that the post-transection increase in [Na+]i was due mainly to Na+ entry via the cut end, rather than via depolarization-activated Na+ channels. 3. The spatiotemporal profile of the post-transection increase in [Na+]i could be accounted for by movement of Na+ from the cut end with an apparent diffusion coefficient of 1.3 x 10(-5) cm2 s-1. 4. [Na+]i began to decline toward resting levels by 20 +/- 15 min (mean +/- S.D.) post-transection, except in regions of the axon within 160 +/- 85 microns of the transection site, where [Na+]i remained high. The boundary between axonal regions in which [Na+]i did or did not recover probably defines a locus of resealing of the axonal membrane. 5. [Na+]i returned to resting values within about 1 h after resealing, even in axonal regions where the normal transmembrane [Na+] gradient had completely dissipated. The recovery of [Na+]i was faster and reached lower levels than expected by diffusional redistribution of Na+ along the axon. Partial recovery occurred even in an isolated internode, indicating that the internodal axolemma can actively extrude Na+.

Neurotrophin effects on survival and expression of cholinergic properties in cultured rat septal neurons under normal and stress conditions

These studies tested the hypothesis that survival-promoting effects of neurotrophins on basal forebrain cholinergic neurons are enhanced under stress. Septal neurons from embryonic day 14-15 rats exposed for 10-14 d to neurotrophin [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4 (NT-4), each at 100 ng/ml] showed a two- to threefold increase in choline acetyltransferase (ChAT) activity, with little evidence of synergistic interactions. Neurotrophins produced no significant increase in the survival of total or acetylcholinesterase (AChE)-positive neurons at moderate plating density (1200-1600 cells/mm2). However, with very low plating densities (2-28 cells/mm2) BDNF, NT-3, and NT-4 (but not NGF) increased total neuronal survival, and BDNF increased survival of AChE-positive neurons. NGF and BDNF enhanced ChAT activity and survival of cholinergic neurons after a 24 hr hypoglycemic stress, even when added 1 hr after stress onset. All four tested neurotrophins increased total neuronal survival after hypoglycemic stress. These results suggest that neurotrophins are important for preservation of central cholinergic function under stress conditions, with different neurotrophins protecting against different stresses. The stress-associated survival-promoting effects of neurotrophins were not limited to the cholinergic subpopulation.

Electrical and morphological factors influencing the depolarizing after-potential in rat and lizard myelinated axons

1. Intra-axonal recording and electron microscopy were applied to intramuscular myelinated axons in lizards and rats to investigate factors that influence the amplitude and time course of the depolarizing after-potential. 2. Depolarizing after-potentials in lizard axons had larger peak amplitudes and longer half-decay times than those recorded in rat axons (mean values 10 mV, 35 ms in lizard; 3 mV, 11 ms in rat). These differences were not due to differences in temperature, resting potential or action potential amplitude or duration. 3. For a given axon diameter, the myelin sheath in lizard fibres was thinner and had fewer wraps than in rat fibres. There was no significant difference in myelin periodicity. Calculations suggest that the thinner myelin sheath accounts for < 30% of the difference between depolarizing after-potential amplitudes recorded in lizard and rat axons. 4. Consistent with a passive charging model for the depolarizing after-potential, the half-time of the passive voltage transient following intra-axonal injection of current was shorter in rat than in lizard axons. 5. Aminopyridines prolonged the falling phase of the action potential and increased the amplitude of the depolarizing after-potential in both types of axon. 6. During repetitive stimulation the depolarizing after-potentials following successive action potentials exhibited little or no summation. Axonal input conductance in the interspike interval increased during the train. 7. These findings suggest that the amplitude and time course of the depolarizing after-potential are influenced not only by the passive properties of the axon and myelin sheath, but also by persisting activation of axolemmal K+ channels following action potentials.

Changes in the response of cultured septal cholinergic neurons to nerve growth factor exposure and deprivation during the first postnatal month

The effects of nerve growth factor (NGF) and a blocking anti-NGF antibody were studied in cultures plated from postnatal day 1-28 (P1-P28) rat septum and maintained 3 weeks in vitro. 7S NGF (100 ng/ml = 0.75 nM) increased choline acetyltransferase (ChAT) activity in P7-P21 cultures. The largest increase was measured in P7-P14 cultures, where NGF addition produced ChAT activities 5-12 times higher than those measured in cultures grown in anti-NGF antibody. NGF also increased the number of acetylcholinesterase (AChE)-positive neurons in P7-P14 cultures. To determine whether this increase was due to enhanced survival of cholinergic neurons or simply to enhanced AChE expression, we examined cultures to which NGF was added only after an initial 1-2-week exposure to anti-NGF antibody. This delayed addition of NGF also increased ChAT activity and the number of AChE-positive neurons, indicating that cholinergic neurons survived the initial exposure to anti-NGF antibody. Thus even during a period when ChAT activity was most sensitive to NGF, postnatal septal cholinergic neurons did not require NGF for survival in vitro.

Activation of internodal potassium conductance in rat myelinated axons

1. Voltage changes associated with currents crossing the internodal axolemma were monitored using a microelectrode inserted into the myelin sheath (peri-internodal region) of rat phrenic nerve fibres. This microelectrode was also used to change the potential and the ionic environment in the peri-internodal region. 2. Following stimulation of the proximal nerve trunk, the peri-internodal electrode recorded a positive-going action potential whose amplitude increased (up to 75 mV) with increasing depth of microelectrode penetration into the myelin. The resting potential recorded by the peri-internodal electrode remained within 4 mV of bath ground. 3. Confocal imaging of fibres injected peri-internodally with the fluorescent dye Lucifer Yellow revealed a staining pattern consistent with spread of dye throughout the myelin sheath of the injected internode. 4. After ionophoresis of K+ (but not Na+) into the peri-internodal region, the action potential was followed by a prolonged negative potential (PNP) lasting hundreds of milliseconds to several seconds. The duration of the PNP increased as the frequency of stimulation decreased. PNPs could also be evoked by sub-threshold depolarization of the internodal axolemma with peri-internodally applied current pulses. In the absence of action potentials or applied depolarization PNPs sometimes appeared spontaneously. 5. Peri-internodal application of Rb+ also produced evoked and spontaneous PNPs. These PNPs had longer durations (up to 20 s) than those recorded from K(+)-loaded internodes. 6. Spontaneous action potentials sometimes appeared during the onset of the PNP, suggesting that PNPs are associated with depolarization of the underlying axon. 7. Passage of current pulses during the PNP demonstrated that the PNP is associated with an increased conductance of the pathway linking the peri-internodal recording site to the bath. At least part of this conductance increase occurs across the internodal axolemma, since peri-internodally recorded action potentials evoked during the PNP had larger amplitudes than those evoked before or after the PNP. 8. PNPs were suppressed by tetraethylammonium (TEA, 10-20 mM) and by 4-aminopyridine (1 mM). 9. These results suggest that the PNPs recorded in K(+)- or Rb(+)-loaded myelin sheaths are produced by a regenerative K+ or Rb+ current that enters the internodal axolemma via K+ channels opened by action potentials or subthreshold depolarizations. 10. When normal extracellular [K+] was preserved (by using Na+ rather than K+ salts in the peri-internodal electrode), action potentials recorded within the myelin sheath were instead followed by a brief, positive after-potential that was inhibited by TEA.(ABSTRACT TRUNCATED AT 400 WORDS)

Posttetanic hyperpolarization produced by electrogenic Na(+)-K+ pump in lizard axons impaled near their motor terminals

1. The hyperpolarization that follows tetanic stimulation was recorded intra-axonally from the internodal region of intramuscular myelinated motor axons. 2. The peak amplitude of the posttetanic hyperpolarization (PTH) that followed stimulation at 20-100 Hz for < or = 35 s increased with increasing train duration, reaching a maximum of 22 mV. PTH decayed over a time course that increased from tens to hundreds of seconds with increasing train duration. For a given frequency of stimulation the time integral of PTH was proportional to the number of stimuli in the train, averaging 3-4 mV.s per action potential. 3. Ouabain (0.1-1 mM) and cyanide (1 mM) depolarized the resting potential and abolished PTH. Tetanic stimulation in ouabain was followed by a slowly decaying depolarization (probably due to extra-axonal K+ accumulation) whose magnitude and duration increased as the duration of the train increased. 4. Axonal input resistance showed no consistent change during PTH in normal solution but increased during PTH in the presence of 3 mM Cs+ (which blocks axonal inward rectifier currents). 5. PTH was abolished when bath Na+ was replaced by Li+ or choline. PTH persisted after removal of bath Ca2+ and addition of 2 mM Mn2+. 6. Removal of bath K+ abolished the PTH recorded after brief stimulus trains and greatly reduced the duration of PTH recorded after longer stimulus trains. 7. A brief application of 10 mM K+, which normally depolarizes axons, produced a ouabain-sensitive hyperpolarization in axons bathed in K(+)-free solution. 8. These observations suggest that in these myelinated axons PTH is produced mainly by activation of an electrogenic Na(+)-K(+)-ATPase, rather than by changes in K+ permeability or transmembrane [K+] gradients. This conclusion is supported by calculations showing agreement between estimates of Na+ efflux/impulse based on PTH measurements and estimates of Na+ influx/impulse based on nodal voltage-clamp measurements. Pump activity also appears to contribute to the resting potential. 9. The stimulus intensity required to initiate a propagating action potential increased during PTH but decreased during the posttetanic depolarization recorded in ouabain. Thus changes in axonal excitability after tetanic stimulation correlate with changes in the posttetanic membrane potential. 10. Action potentials that propagated during PTH had a larger peak amplitude and were followed by a larger and longer depolarizing afterpotential than action potentials elicited at the resting potential. This enhancement of the depolarizing afterpotential is consistent with previous reports of an increased superexcitable period after action potentials evoked during PTH.

Reversibility of nerve growth factor's enhancement of choline acetyltransferase activity in cultured embryonic rat septum

Choline acetyltransferase (ChAT) activity and survival of acetylcholinesterase (AChE)-positive neurons were measured in low-density cultures of embryonic (Day 14-15) rat septum exposed to various sequences of nerve growth factor (NGF) exposure and deprivation for up to 7 weeks in vitro. Most septal cultures grown 4-5 weeks with no exogenous NGF (including exposure to monoclonal or polyclonal antibodies against NGF) retained both a basal ChAT activity and the ability to increase ChAT activity in response to subsequently added NGF. When cultures were exposed to NGF (7S, 0.75 nM) for 2-3 weeks and then deprived of NGF for 2 weeks, ChAT activity fell gradually, but the number of AChE-positive neurons remained unchanged, and in many cases ChAT activity could be restored by subsequent re-exposure to NGF. Thus NGF's enhancement of ChAT activity in embryonic septal neurons in vitro is largely reversible and is not mediated by differential survival of cholinergic neurons.

Rat embryonic septal neurons survive and express cholinergic properties in isolation and without nerve growth factor

We studied survival and expression of cholinergic properties in embryonic septal neurons grown in very low density microcultures (1-7 cells per Terasaki well). Even in cultures containing only a single neuron, at least 10% of plated neurons survived for 2 weeks or more in medium containing fetal calf serum or an acid-stable fraction (55,000 Da) of horse serum. Of these surviving neurons, 30-40% stained positively for acetylcholinesterase (AChE) or nerve growth factor (NGF) receptor, even though the culture medium lacked detectable levels of NGF, brain-derived neurotrophic factor, and fibroblast growth factor. Addition of NGF or an antibody against NGF had no effect on either neuronal survival or the percentage of neurons staining positively for AChE or NGF receptor after 18-20 days in vitro. There was no cell division in medium containing the serum fraction, but when 10% fetal calf serum was present cell division occurred in some of the cultures, and in half of these cases at least one of the clonal progeny became AChE-positive. These results demonstrate that some embryonic septal cells can survive at least 2 weeks and develop cholinergic neuronal properties in the absence of other cells or NGF.

Neurotoxic components in normal serum

Serum neurotoxicity was studied by adding whole or fractionated serum (adult human, adult horse, or newborn calf) to neuron-rich cultures prepared from various regions of embryonic (Days 14-15) rat brain, including spinal cord, ventral mesencephalon, cerebellum, septum, and striatum. Effects of serum were also tested on several types of embryonic non-neuronal cells (skeletal muscle myotubes, cardiac muscle myocytes, and fibroblasts from skin and lung). Serum concentrations of 50% or more killed more than 95% of all neurons within 3 days. Serum concentrations as low as 10% also killed some neurons, especially those from cerebellum. Septal, cerebellar, and spinal cord neurons were more sensitive than striatal or mesencephalic neurons. All the tested non-neuronal cells survived much better than neurons at serum concentrations of 20% or more. Neurotoxicity was present in both fresh (human) and previously frozen (human and animal) sera, and affected both young (4 days in vitro) and older (42 days in vitro) cultures. Neurotoxicity was greatly diminished by heating the serum to 56 degrees C for 30 min. Experiments indicated that serum toxicity was not due to lipoprotein, complement, or tumor necrosis factor. All serum neurotoxicity was retained by an ultrafilter with a nominal molecular weight cutoff of 10 kDa. The profile of neurotoxicity following gel filtration at neutral pH was variable, with high toxicity most consistently observed in fractions with apparent molecular weights exceeding 100 kDa, and variable degrees of toxicity at lower molecular weights.

Differential effects of insulin on choline acetyltransferase and glutamic acid decarboxylase activities in neuron-rich striatal cultures

We studied the effects of insulin, nerve growth factor (NGF), and tetrodotoxin (TTX) on cellular metabolism and the activity of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) in neuron-rich cultures prepared from embryonic day 15 rat striatum. Insulin (5 micrograms/ml) increased glucose utilization, protein synthesis, and GAD activity in cultures plated over a range of cell densities (2,800-8,400 cells/mm2). TTX reduced GAD activity; NGF had no effect on GAD activity. Insulin treatment reversibly reduced ChAT activity in cultures plated at densities of greater than 4,000 cells/mm2, and the extent of this reduction increased with increasing cell density. The number of acetylcholinesterase-positive neurons was not reduced by insulin, suggesting that insulin acts by down-regulating ChAT rather than by killing cholinergic neurons. Insulin-like growth factor-1 (IGF-1) reduced ChAT activity at concentrations 10-fold lower than insulin, suggesting that insulin's effect on ChAT may involve the IGF-1 receptor. NGF increased ChAT activity; TTX had no effect on ChAT activity. These results suggest that striatal cholinergic and GABAergic neurons are subject to differential trophic control.

Evidence that action potentials activate an internodal potassium conductance in lizard myelinated axons

1. We have studied action potentials and after-potentials evoked in the internodal region of visualized lizard intramuscular nerve fibres by stimulation of the proximal nerve trunk. Voltage recordings were obtained using microelectrodes inserted into the axon (intra-axonal) or into the layers of myelin (peri-internodal), with the goal of studying conditions required to activate internodal K+ currents. 2. Peri-internodal recordings made using K2SO4-, KCl- or NaCl-filled electrodes exhibited a negligible resting potential (less than 2 mV), but showed action potentials with peak amplitudes of up to 78 mV and a duration less than or equal to that of the intra-axonally recorded action potential. 3. Following ionophoretic application of potassium from a peri-internodal microelectrode, the peri-internodal action potential was followed by a prolonged (hundreds of milliseconds) negative plateau. This plateau was not seen following peri-internodal ionophoresis of sodium. The prolonged negative potential (PNP) was confined to the K(+)-injected internode: it could be recorded by a second peri-internodal microelectrode inserted into the same internode, but not into an adjacent internode. 4. The peri-internodally recorded PNP was accompanied by an equally prolonged intra-axonal depolarizing after-potential, and by an increase in the conductance of the internodal axolemma. However, the K+ ionophoresis that produced the PNP had little or no detectable effect on the intra-axonally or peri-internodally recorded resting potential or action potential. These findings suggest that the PNP is generated by an inward current across the axolemma of the K(+)-injected internode, through channels opened following the action potential. 5. Following peri-internodal K+ ionophoresis a PNP could also be evoked by passage of depolarizing current pulses through an intra-axonal electrode or by passage of negative current pulses through an electrode in the K(+)-filled peri-internodal region. The threshold for evoking a PNP was less than the threshold for evoking an action potential, and the PNP persisted in 10 microM-tetrodotoxin. Thus the PNP is evoked by depolarization of the axolemma rather than by Na+ influx. 6. The PNP was reversibly blocked by tetraethylammonium (TEA, 2-10 mM), but was not blocked by 100 microM-3,4-diaminopyridine or 5 mM-4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane resealing in cultured rat septal neurons after neurite transection: evidence for enhancement by Ca(2+)-triggered protease activity and cytoskeletal disassembly

Neurites of cultured septal neurons were transected with a laser under sterile conditions, and the subsequent membrane resealing was assayed using a dye exclusion method. In agreement with findings in other preparations, Ca2+ enhanced resealing: in normal culture medium the percentage of lesioned neurons that resealed within 20-30 min after transection increased with increasing bath [Ca2+] over the range 10(-7) to 2 x 10(-3) M; about 75% of cells resealed in 2 mM Ca2+. Mn2+ and Sr2+ also enhanced resealing, but Mg2+ inhibited it. The percentage of resealing neurons was sensitive to agents known to alter the stability of cytoskeletal components. Agents that tend to disassemble microtubules and/or neurofilaments (e.g., colchicine, low-ionic-strength media) strongly promoted resealing, whereas treatments that tend to stabilize microtubules (taxol, Mg2+) inhibited resealing. Addition of exogenous proteases (papain, trypsin, or dispase) enhanced resealing, whereas inhibitors of cysteine proteases (including a specific inhibitor of calpain, a Ca-activated neutral protease) strongly inhibited resealing. Calmodulin inhibitors inhibited resealing, consistent with reports that calmodulin facilitates calpain-mediated proteolysis of fodrin, a component of the cortical cytoskeleton. Based on these results, we hypothesize that one of the major mechanisms involved in resealing is activation of endogenous proteases by Ca2+ entry into the injured neurite. The resulting changes in the cellular cytoskeleton might promote fusion and resealing of the cut ends of the plasma membrane by enhancing membrane mobility and/or by removing structures that normally prevent membrane-membrane contact.

Transplantation of human fetal dopamine cells for Parkinson's disease. Results at 1 year

In an effort to improve the clinical signs of Parkinson's disease, we have implanted mesencephalic dopamine cells from a 7-week human embryo into the caudate and putamen of a 52-year-old man with Parkinson's disease. Fetal tissue was obtained from elective abortion. The woman and the patient with Parkinson's disease were unknown to each other. The woman gave specific consent and was not paid. The patient had a 20-year history of parkinsonism treated with multiple drug therapies including levodopa/carbidopa (Sinemet) every 2 1/2 hours. His symptoms were worse on the left side. For 5 months prior to transplantation, the patient underwent clinical evaluations by both a neurologist and a computer system installed in his home for daily measurement of walking and hand movements. Preoperative positron emission tomographic scanning with 6-L[18F]fluorodopa (fluorodopa) demonstrated severe dopamine depletion bilaterally. Fetal tissue was matched to the patient for ABO blood antigens, and maternal serum was screened for hepatitis B and human immunodeficiency virus type 1 prior to surgery. Fetal tissue was implanted stereotactically throughout the caudate and putamen on the right side of the brain via 10 needle tracks. The patient was not immunosuppressed. Results 12 months after surgery showed 42% improvement in left-hand speed before the first morning dose of drug and 40% greater response to drug therapy. Right-hand speed increased 15% before drug therapy and 23% after drug therapy. Reaction time was unaffected. Walking speed increased 33% after drug administration, although walking speed before the first morning dose of drugs declined 40%. Walking speed on an all-day basis improved 17%.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the altered form of the c-src gene product in neuronal cells

The pp60c-src protein that is expressed at high levels in cultures of neurons from rat embryos displays an altered mobility on SDS-polyacrylamide gels due to a structural difference in the amino-terminal region of the molecule. In this report we show that the expression of this unique form of pp60c-src, designated pp60c-src(+), is not restricted to cultured neuronal cells since the pp60c-src molecules expressed in tissues from avian and rat neural tissues also display a retarded electrophoretic mobility. The amino-terminal region from pp60c-src(+) was found to contain a novel phosphorylated tryptic peptide that contains phosphoserine. However, this phosphorylation does not appear to be responsible for the retarded electrophoretic mobility of pp60c-src(+), since the mobility of this protein is not altered by phosphatase treatment under conditions that remove greater than 95% of the radiolabeled phosphate on pp60c-src(+). The altered electrophoretic form of pp60c-src was also shown to be radiolabeled with [3H]myristate, indicating that pp60c-src is fatty-acylated in neurons, as is pp60c-src in fibroblasts. The pp60c-src molecules synthesized in vitro using rabbit reticulocyte lysates programmed with mRNA from embryonic brain migrated more slowly on SDS-polyacrylamide gels than the pp60c-src protein that was translated in vitro using RNA from embryonic limb tissue. These results suggest the possibility that the c-src mRNA expressed in neurons may undergo a unique form of processing to generate the structurally distinct form of neuronal pp60c-src(+).

1-Methyl-4-phenylpyridinium (MPP+) but not 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) selectively destroys dopaminergic neurons in cultures of dissociated rat mesencephalic neurons

Dopaminergic neurons were studied in cultures of dissociated cells from the ventral mesencephalon of fetal rat embryos (gestational day E15-16). After a week of growth, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP+) was added to the growth medium for 24 h. Dopaminergic neurons were then visualized with tyrosine hydroxylase (TH) immunocytochemistry or catecholamine (CA) cytofluorescence. Concentrations of MPTP in the range of 10 to 100 microM obliterated CA fluorescence without affecting the number of TH-positive neurons. At concentrations greater than 100 microM, MPTP decreased the number of TH-positive neurons as well as the number of all other cell types. MPP+ (0.1-10.0 microM) produced a decrease in the number of TH-positive neurons without decreasing the total number of all cell types. The findings indicate that MPP+ but not MPTP is able to selectively destroy rat dopaminergic neurons in our cultures. The selective toxicity of MPP+ for dopaminergic neurons was partially prevented by pretreatment and co-incubation with mazindol (a selective inhibitor of dopamine uptake) but not by desipramine or deprenil, in confirmation of the notion that MPP+ enters dopaminergic neurons by the specific uptake mechanism for dopamine.

Effects of ischemia-like conditions on cultured neurons: protection by low Na+, low Ca2+ solutions

An in vitro system was used to mimic several aspects of ischemia, including low oxygen pressure, low nutrient levels, and the accumulation of cellular products thought to contribute to damage during ischemia. We replaced normal culture medium from 3-week-old basal ganglia cultures with oxygen-depleted, nutrient-deficient medium. After incubation in an atmosphere of 94% N2, 6% CO2 for 5 hr at 37 degrees C, the cultures were returned to normal medium. After a 24 hr recovery period, cell viability was assessed in terms of cell number, electrophysiological properties, and immunohistochemical markers. When the medium used during the ischemic period was a normal balanced salt solution, more than 70% of the cells were damaged by the low-oxygen, low-glucose stress. Loss of cell processes and cell swelling were the most evident signs of damage. The majority of the cells remaining viable were astrocytes. Neuronal damage was observed only when both glucose and oxygen were deficient. Some damage was evident even at oxygen tensions of 60 mm Hg when glucose was absent from the medium; much more extensive damage was observed at tensions below 1.0 mm Hg. Lowering both extracellular sodium and calcium resulted in more than a 2-fold increase in survival (70 vs 28%). These results indicate that damage to neurons during conditions of extreme energy deprivation such as ischemia may be mediated by the influx of calcium and/or sodium.

Cryopreservation of primary neurons for tissue culture

We have developed new cryopreservation methods which allow storage of fetal rat central nervous system tissues for more than 1 week at 3-8 degrees C or for several months at -70 or -90 degrees C prior to tissue culture. For refrigeration, small brain regions (less than 2 mm thick) were placed intact into 35 mm petri dishes of 'hibernation medium' inside a humidified chamber. Optimal preservation was obtained with hibernation media of pH 6.8-7.4, containing 30-70 mM K+, 10-30 mM Na+, 5-50 mM PO4(2-), 20 mM lactic acid, 5 mM glucose, and less than 0.1 mM Ca2+. The media were made approximately isotonic by addition of sorbitol. For freezing, brain tissues were dissociated by gentle trituration (without enzymes) in the above medium supplemented with 5-10% dimethylsulfoxide. After refrigeration or freezing, neurons were very sensitive to damage from mechanical stress (e.g. centrifugation, harsh rinsing or trituration). Rapid changes in osmotic pressure or excessive polylysine on the tissue culture substratum also reduced neuronal survival after cryopreservation. Pretreatment of tissue culture substrata with media from lung cell cultures, or plating of neurons at higher density (2000 cells/mm2) improved neuronal survival after cryopreservation.

Astrocytes produce interferon that enhances the expression of H-2 antigens on a subpopulation of brain cells

Using primary culture methods, we show that purified astrocytes from embryonic mouse or rat central nervous system (CNS) can be induced to produce interferon (IFN) activity when pretreated with a standard IFN-superinducing regimen of polyribonucleotide, cycloheximide, and actinomycin D, whereas IFN activity was not inducible in neuronal cultures derived from mouse CNS. Astrocyte IFN displays inductive, kinetic, physicochemical, and antigenic properties similar to those of IFN-alpha/beta, but is dissimilar to lymphocyte IFN (IFN-gamma). Treatment of pure astrocytic cultures or astrocytes cultured with neurons with astrocyte IFN or IFN-alpha/beta induced a dramatic increase in the expression of H-2 antigens on a subpopulation of astrocytes. Neither neurons nor oligodendroglia expressed detectable levels of H-2 antigens when exposed to astrocyte IFN, IFN-alpha/beta, or to IFN-beta. Injection of astrocyte IFN or IFN-alpha/beta directly into brains of newborn mice indicated that H-2 antigens were also induced in vivo. None of the IFNs (astrocyte, alpha/beta, or beta) tested induced Ia antigens on CNS cells in vitro or in vivo. Since H-2 antigens have a critical role in immune responses, astrocyte IFN may initiate and participate in immune reactions that contribute to immunoprotective and immunopathological responses in the CNS.

Differential effects of calcium antagonists on viability of adult rat ventricular myocytes

This study was designed to determine whether the various classes of Ca2+ channel blockers have differential protective effects on isolated adult rat ventricular myocytes exposed to high K+ under anoxic (100% N2) conditions. Calcium-tolerant myocytes were incubated under control (4mM K+) aerobic conditions and then subjected to high K+ (75 mM) and N2. The cells were assessed by morphological criteria (i.e. absence of blebbing, granulation etc.), maintenance of ATP levels, exclusion of trypan blue, and the presence or absence of spontaneous contractile activity. Under control conditions, the cells were quiescent and declined at a rate of approximately 10%/h. In the absence of O2, the rate of cell decline was significantly faster. Verapamil, diltiazem and the dihydropyridines had no significant effects on cell decline under these conditions. Cells exposed to 75 mM K0+ exhibited contractile activity and accelerated rate of decline under anoxic conditions; these effects were independent of lowering Na0+ to 75mM. Cells in high K0+ and N2 were significantly protected (i.e. contractile activity and rate of decline were decreased) by verapamil, less so by diltiazem, and not at all by the dihydropyridines. The uptake of 45Ca2+ into cells in high K0+ was not significantly altered by verapamil or diltiazem. Caffeine induced the immediate cessation of contractile activity of cells incubated in high K0+, but did not affect the accelerated rate of cell declined under anoxic conditions. Verapamil and diltiazem still conferred significant protection in this non-beating cell preparation. Neither verapamil nor diltiazem had any effect on the oscillation frequency of skinned heart cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of tissue-derived macromolecules affecting transmitter synthesis in rat spinal cord neurons

Rat spinal cord cells maintained in neuron-rich cultures were exposed to extracts of skeletal muscle or to medium conditioned by non-neuronal cells. The conditioned media enhanced neuronal acetylcholine (ACh) synthesis, choline acetyltransferase activity, and protein synthesis, and decreased gamma-aminobutyric acid (GABA) synthesis. Muscle extract prepared from newborn rats produced similar enhancements but did not depress GABA synthesis. Muscle extracts prepared from normal and denervated adult rat limbs contained relatively little activity. These results suggest that different molecular factors might mediate the effects on GABA and ACh synthesis. Gel filtration of conditioned media and muscle extracts revealed that all of these activities were confined to a macromolecular fraction with an apparent Mr of 40,000. These tissue-derived factors affecting neuronal protein and transmitter synthesis are in turn distinct from a neuronal survival-promoting factor obtained from serum (Kaufman, L. M., and J. N. Barrett (1983) Science 220: 1394-1396).

Neurite growth cone-substratum adherence increases in vitro

During experiments characterizing the turning response of dorsal root ganglion neurites toward NGF, it was observed that growth cone-substratum adherance increased with time in culture. The experiments reported here indicate that the observed increase in growth cone-substratum adherance is significant and can be detected with both collagen and poly-L-lysine substrates. The increased adherance is apparently due to a substance(s) produced and released by the ganglia which binds to the substrate, increasing adherance. Flow chamber studies indicate that the substrate-bound substance(s) may be necessary for neurite growth onto artificial tissue culture substrata.

An investigation of the growth characteristics of oil-palm (Elaeis guineensis) suspension cultures using 31P NMR

High-resolution 31P nuclear-magnetic-resonance (NMR) spectra are reported for oil-palm (Elaeis guineensis) cells in suspension culture. The spectra are a significant improvement on the results that have appeared for other cultures and they are comparable with the spectra of the meristematic tissue in seedling roots. The NMR technique was used in parallel with other analytical methods to investigate the growth characteristics of the suspension culture, including the effect of 2,4-dichlorophenoxyacetic acid.

Serum factor supporting long-term survival of rat central neurons in culture

Gel filtration of serum at pH 3.6 yielded a fraction that supported long-term (months) survival of dissociated rat central neurons in monolayer culture more reliably than the traditionally used unfractionated serum. The cultures remained neuron-rich, because this fraction did not support the proliferation of glia and fibroblasts that occurs in whole serum. With an apparent molecular weight of 55,000 and an isoelectric point of 5.6, the active factor (or factors) in this fraction is distinct from any well-defined growth factor.

Properties of single calcium-activated potassium channels in cultured rat muscle

1. Properties of the Ca-activated K channel were studied in excised patches of surface membrane from cultured rat muscle cells using single channel recording techniques.2. Increasing the concentration of calcium at the intracellular membrane surface [Ca](i), increased both the frequency and effective duration of channel openings. Ca at the extracellular membrane surface was not sufficient to activate the channels.3. An approximate third power relationship (slope = 2.7) was observed between [Ca](i) and the percentage of time the channels spent in the open state.4. Both the frequency and effective duration of channel openings increased as the intracellular membrane surface was made more positive; the percentage of time spent in the open state increased e-fold for a 15 mV depolarization for low levels of activity.5. The percentage of time spent with 1, 2,...n channels open in membrane patches with n channels was described by the binomial distribution, suggesting that the channels opened and shut independently of one another.6. Single channel conductance (144 mM-K on both sides of the membrane) was essentially independent of membrane potential (-50 to +50 mV) and [Ca](i) (0.1 muM -1 mM), but did increase with temperature, from 100 pS at 1 degrees C to 300 pS at 37 degrees C.7. Channel activity occurred in apparent bursts, with the duration of the apparent bursts increasing with increasing [Ca](i).8. Two exponentials were required to describe the distribution of observed channel open times, suggesting two different open channel states of apparently normal conductance. The observed mean channel open time of these states at +30 mV was 0.34 and 2.2 msec with 0.1 muM-Ca(i) and was 0.47 and 6.9 msec with 0.5 muM-Ca(i).9. The channel occasionally entered an apparent third open channel state with a single channel current amplitude about 40% the amplitude of the normally observed single channel currents. The reduced conductance state was immediately preceded and followed by a normal conducting state.10. While the kinetics of the Ca-activated K channel appear complex, its large conductance and high Ca and voltage sensitivity suggest that it is uniquely suited to resist depolarizations of the cell membrane potential that are accompanied by increases in intracellular Ca.

Characterization of neuritic outgrowth-promoting activity of conditioned medium on spinal cord explants

Conditioned medium (CM) from muscle or fibroblast cultures dramatically increases the outgrowth of neurites from fetal rat spinal cord slices in vitro. The factor(s) in CM responsible for this enhanced outgrowth is chymotrypsin-sensitive, but neuraminidase-insensitive. At neutral pH, the factor(s) binds to a concanavalin A-agarose affinity column, a zinc metal chelate affinity column, a DE-52 anion exchange column, but not to a carboxymethyl-52 cation exchange column. These results suggest that the active CM factor(s) is a glycoprotein that is negatively charged at neutral pH. Following gel chromatography the major peak of outgrowth-promoting activity elutes at a molecular weight of approximately 50,000 daltons.

Two components of conditioned medium increase neuritic outgrowth from rat spinal cord explants

Conditioned medium (CM) from muscle or fibroblast cultures dramatically increases the outgrowth of neurites from fetal rat spinal cord slices in vitro. We report here that there are two separable fractions in conditioned medium that cause this increase in neuritic outgrowth. One fraction, with a molecular weight of approximately 50,000 daltons, enhances neuritic outgrowth only when it is present in the culture medium so that the slices are directly exposed to it. The second component has a much larger molecular weight (above 300,000 daltons), and can enhance neuritic outgrowth by directly binding to the culture substrate on pretreatment. Only when these two fractions are combined by pretreating the substrate with the higher molecular weight fraction and then growing the slices in the lower molecular weight fraction is the full outgrowth promoting activity of whole conditioned medium reconstituted. These two components act synergistically to reproduce nearly the full outgrowth promoting activity of non-fractionated, whole conditioned medium.

Intracellular recording from vertebrate myelinated axons: mechanism of the depolarizing afterpotential

1. Electrophysiological techniques are described which allow intracellular recording from peripheral myelinated axons of lizards and frogs for up to several hours. The sciatic and intramuscular axons studied here have resting potentials of -60 to -80 mV and action potentials (evoked by stimulation of the proximal nerve trunk) of 50-90 mV. They show a prominent depolarizing afterpotential (d.a.p.), which is present both in isolated axons and in axons still attached to their peripheral terminals. This d.a.p. has a peak amplitude of 5-20 mV at the resting potential, and decays with a half-time of 20-100 msec.2. The peak amplitude of the d.a.p. is voltage-sensitive, increasing to up to 26 mV with membrane hyperpolarization. The d.a.p. disappears as the axon is depolarized to -60 to -45 mV, and does not appear to reverse with further depolarization.3. The d.a.p. is not reduced when bath Ca is replaced by 2-10 mm divalent Mn or Ni. The d.a.p. is not reversed when axons depleted of Cl (by prolonged exposure to Cl-deficient, SO(4)-enriched solutions) are bathed in Cl-rich solutions. These results suggest that the d.a.p. is not mediated by a conductance change specific for Ca or Cl ions. Partial substitution of tetramethylammonium for bath Na, or addition of 10(-5)m-tetrodotoxin to the normal bathing solution, reduces the amplitude of both the action potential and the d.a.p.4. The amplitude of the d.a.p. is not sensitive to bath [K] over the range 1-7.5 mm, provided that all measurements are made at the same holding potential. This result argues that the d.a.p. is not mediated by accumulation of K outside the active axon.5. Treatments expected to inhibit the Na-K exchange pump (cooling from 25 to 10 degrees C, or 0.15 mm-ouabain) do not enlarge or prolong the d.a.p., although they do abolish a slower hyperpolarizing afterpotential seen following repetitive stimulation.6. The passive voltage response of the axon to small injected pulses of depolarizing or hyperpolarizing current shows a prominent, slowly decaying component with a time course similar to that of the d.a.p. Depolarizing current reduces the input resistance of the axon, and increases the rate of decay of both the passive voltage response and the d.a.p. There is a slight conductance increase during the peak of the d.a.p., but the same conductance increase can be produced by a comparable passive depolarization.7. We conclude that the d.a.p. is due mainly to a passive capacitative current, probably resulting from discharge of the internodal axonal membrane capacitance through a resistive current pathway beneath or through the myelin sheath. We suggest that this slow capacitative discharge becomes evident as soon as most of the nodal ionic channels activated during the action potential have closed. An electrical model of the myelinated axon that incorporates the postulated internodal leakage pathway can account both for the prolonged d.a.p. recorded inside the axon, and for the potential profile recorded extra-axonally in or near the internodal periaxonal space.

Characterization of the turning response of dorsal root neurites toward nerve growth factor

This study reports that chick dorsal root ganglion neurites undergo a rapid (20 min) reorientation of their direction of growth in response to nerve growth factor (NGF) concentration gradients in vitro. Dorsal root ganglia from chick embryos were explanted onto a collagen-poly-L-lysine substrate. After 24-48 h in culture, NGF gradients were applied to individual growth cones via a micropipette containing 50 biological units NGF/ml. The growth cones turned and grew toward these NGF sources. This turning response was not caused by the trophic effects of NGF on neurite initiation, survival, or growth rate. Dorsal root neurites also grew toward sources of mono- and dibutyryl cyclic adenosine monophosphate (dB cAMP), cyclic guanosine monophosphate (cGMP), and elevated calcium in the presence of the calcium ionophore A23187. These results are consistent with the hypothesis that intracellular levels of cAMP and /or cGMP and calcium may play a role in the turning response of dorsal root neurites toward NGF, but do not establish a causal relationship between the mechanisms of action of NGF, cyclic nucleotides and calcium. Total growth cone adherence to the substrate was measured using a timed microjet of perfusion medium. NGF increased the adherence of growth cones to the substrate, but caffeine and dB cAMP which also elicit the positive turning response, decreased growth cone adherence. Calcium, which did not elicit the positive turning response, produced a greater growth cone adherence to the substrate than that observed with NGF. Although these results do not rule out a role of adhesion changes in axonal turning to NGF, they show that a general increase in adherence does not correlate well with the rapid turning response observed in this study.

Voltage clamp of cat motoneurone somata: properties of the fast inward current

1. The soma membrane of cat spinal motoneurones was voltage clamped using separate intracellular voltage and current electrodes directed into the same motoneurone with a new guide system. 2. Antidromic stimulation of the motoneurone's axon or small depolarizing voltage clamp steps (10-20 mV from the resting potential) evoked a small (30-80 nA) all-or-none action potential current, which was shown by occlusion experiments to originate from the initial segment of the axon. Except for this axonal current spike, there was no indication of active (voltage-dependent) conductance changes in membrane regions not under good voltage clamp control. Calculations based on motoneuronal geometry, and electrophysiological recordings from spinal cord neurones in tissue culture, indicate that the proximal portions of dendritic membranes were also under good voltage clamp control. 3. Clamp depolarizations greater than 20 mV activated a fast, transient inward current, which increased in a smoothly graded manner with depolarization between 20 and 40 mV from the resting potential, reaching a peak magnitude of up to 450 nA, and then decreased smoothly for larger depolarizations. Extrapolation of the current-voltage relationship for this current indicated a reversal potential about 80-116 mV positive to the resting potential. 4. This transient inward current is blocked by tetrodotoxin. After a depolarizing voltage clamp step the conductance system controlling this current first activates with fast, non-linear kinetics, and then inactivates with first-order kinetics. These properties are similar to those of the Na conductance system in squid and frog axons. 5. Conditioning-testing experiments showed that the time constant of inactivation ranges from 1.0-1.3 msec at potentials slightly negative to the resting potential to 0.1-0.3 msec for depolarizations 60 mV from the resting potential. The degree of steady-state inactivation also varied with membrane potential, ranging from total inactivation at depolarizations greater than 30 mV from the resting potential, to minimal inactivation at potentials more than 10 mV negative to the resting potential.

Voltage-sensitive outward currents in cat motoneurones

1. The soma membrane of cat motoneurones was voltage-clamped in vivo using intracellular current and voltage electrodes whose tips were separated by at least 5 micrometer. 2. Depolarization activates two separate, non-interacting K conductance systems whose rates of activation and decay differ by a factor of about 10. These conductances have a similar reversal potential, in the range of -6 to -21 mV (these and all subsequent voltages are expressed relative to the resting potential). Both conductances show linear 'instantaneous' current-voltage relationships. The steady-state magnitudes of both conductances increase with increasing depolarization. Neither conductance inactivates substantially during prolonged depolarizations. 3. The faster K conductance is similar to that described for squid axons and frog node. Activation begins at about +30 mV and is more than 90% complete within 5 msec of a depolarizing voltage step to +50 mV. Activation kinetics appear to be nonlinear. This fast K conductance contributes to the fast falling phase of the action potential. Following repolarization, this conductance decays with a time constant of 2-4 msec. 4. The slower K conductance activates during depolarizations of 10 mV or greater. The activation and decay of this conductance can be described by first-order exponential functions with time constants ranging from 20 to 50 msec. The slow K conductance underlies the prolonged hyperpolarization that follows motoneurone action potentials. Evidence from other studies suggests that this slow K conductance is regulated by intracellular Ca ions. 5. In addition to the two K conductance systems activated by depolarization, motoneurones exhibit another distinct conductance system that is activated by hyperpolarization. This third system has a reversal potential near the resting potential. Activation of this conductance during a hyperpolarizing voltage step can be fitted by a single exponential function with a time constant of 50-60 msec over the range -20 to -50 mV. This hyperpolarization-activated conductance accounts for some aspects of the anomalous rectification reported in cat motoneurones. 6. When the clamp circuit was turned off and the motoneurones were stimulated to discharge repetitively by depolarizing current steps, the apparent soma threshold voltage increased as the applied current (and discharge frequency) increased. 7. The basic features of the motoneurone action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties. These simulations assumed that the kinetics of the fast Na and K conductance systems in motoneurones can be described by equations of the same form as the Hodgkin-Huxley equations. These action potential reconstructions indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites...

Neuronal chemotaxis: chick dorsal-root axons turn toward high concentrations of nerve growth factor

Micropipettes containing 2 to 50 biological units of beta growth factor (NGF) were placed near growing axons of chick dorsal-root ganglion neurons in tissue culture. The axons turned and grew toward the NGF source within 21 minutes. This turning response to elevated concentrations of NGF appears to represent chemotactic guidance rather than a general enhancement of growth rate.

Temperature-sensitive aspects of evoked and spontaneous transmitter release at the frog neuromuscular junction

1. The temperature dependence of presynaptic processes involved in neuromuscular transmission was studied by rapidly increasing the temperature of cooled frog neuromuscular junctions by 4--10 degrees C using pulses from a neodymium laser. The temperature elevation was complete within 0.5 msec, and decayed back to control levels with a time constant of about 7--8 sec. 2. Temperature jumps completed before nerve stimulation increased the quantal content and decreased the latency of the end-plate potential (e.p.p.). The Q10 for e.p.p. quantal content in low [Ca2+] Ringer averaged about 3.9 over the range 1--18 degrees C. 3. Temperature jumps occurring during the synaptic delay (the interval between the presynaptic action potential and the onset of the e.p.p.) also increased the quantal content and decreased the latency of the e.p.p. These effects diminished as the onset of the temperature jump was moved closer to the expected onset of the e.p.p. Temperature jumps applied after the onset of the e.p.p. immediately accelerated the time course of the e.p.p. but did not significantly alter quantal content. These results demonstrate that the magnitude and timing of evoked release are influenced by temperature-sensitive processes that operate both during and shortly after the presynaptic nerve action potential, but are largely complete before the onset of release. 4. Temperature jumps were applied at various times during the interval between two nerve stimuli. The amplitude of the second e.p.p. decreased as the temperature jump was moved earlier in the interstimulus interval, suggesting that the rise in temperature following the first nerve stimulus accelerates the decay of facilitation. When the temperature jump was moved from 10 msec after to 10 msec before the onset of the first e.p.p., the amplitude of the second e.p.p. either decreased or showed no change. The fact that the second e.p.p. did not increase suggests that the temperature-sensitive processes that increase the quantal content of the conditioning e.p.p. do not greatly increase the facilitation following that e.p.p. 5. Temperature jumps immediately accelerated the time course of spontaneous miniature end-plate potentials (m.e.p.p.s) and increased their frequency. Experiments using slow temperature changes revealed that the Q10 for m.e.p.p. frequency in normal Ringer is about 10 over the range 10--20 degrees C. M.e.p.p. frequency was much less sensitive to temperature changes below about 10 degrees C. When the nerve terminal was depolarized by 20 mM-K+ in the presence of Ca2+, the Q10 for the rate of spontaneous release over the range 10--20 degrees C decreased to about 4, similar to the Q10 for e.p.p. quantal content. In the absence of extracellular Ca2+ the Q10 for m.e.p.p. frequency in 20 mM-K+ remained near 10. 6. The marked difference in Q10S for spontaneous transmitter release under different experimental conditions suggests that not all transmitter release uses identical mechanisms...

Epiphysitis in beef cattle fattened on slatted floors

A syndrome which causes severe lameness and affects the fetlock area of the hind limbs was recently encountered in beef cattle housed on slatted floors. This condition does not appear to have been reported to others. On radiological and pathological examination of the limb, a lesion of epiphysitis was found in the distal epiphysis of the metatarsus. The clinical, radiological and pathological features are reported and discussed.

Motoneuron dendrites: role in synaptic integration

Dendrites constitute over 80 per cent of the receptive surface area in cat motoneurons. Calculations based on matched electrical and gemoetrical measurements in these neurons indicate that the specific resistance of dendritic membranes in resting motoneurons is at least 2,000 ohm-cm2. When the specific membrane resistance is this high, even the most distal dendritic synapses can contribute significantly to the depolarization of the soma, and hence influence the rate of action potential generation. However, dendritic membrane resistance depends strongly on the level of background synaptic activity. The conductance changes associated with excitatory synaptic activity on a dendrite can be great enough to reduce significantly both the excitatory synaptic driving potential and the effective membrane resistance on that dendrite, and thus greatly reduce the effectiveness of synapses on the dendrite. Inhibitory synaptic activity produces an even greater reduction in dendritic membrane resistance. Thus the relative effectiveness of dendritic synapses depends on the type, distribution, and intensity of background synaptic activity, as well as on dendritic geometry and resting membrane properties.

Influence of dendritic location and membrane properties on the effectiveness of synapses on cat motoneurones

1. Measurements of the specific membrane properties and neuronal geometry of cat motoneurones were used to calculate the excitatory postsynaptic potentials (e.p.s.p.s) produced by unit (quantal) conductance changes occurring at various locations on the dendritic tree.2. Calculations demonstrate that conductance changes of 80-190 x 10(-10) mho are required to produce e.p.s.p.s having the same rise time and peak amplitude as the quantal e.p.s.p.s recorded in motoneurones by Kuno & Miyahara (1969b). Because quantal conductance changes are so large, synaptic activity can significantly reduce the effective specific resistance of the motoneuronal membrane.3. A quantal conductance change occurring at a high-impedance distal dendritic site is calculated to produce an e.p.s.p. of 15-20 mV peak amplitude at that site. Significant non-linear summation will occur between the e.p.s.p.s produced by conductance changes occurring simultaneously on the same dendritic branch.4. Calculations which take into account both non-linear summation and the loss of synaptic charge through dendritic membranes predict that for these motoneurones the time integral of soma-recorded quantal e.p.s.p.s originating on distal dendrites should be at least 20% as great as the time integral of a quantal e.p.s.p. originating directly on the soma. Quantal conductance changes occurring on 76% of the dendritic tree should produce soma e.p.s.p. time integrals at least 50% as great as those produced by somatic synapses.

Specific membrane properties of cat motoneurones

1. Electrophysiological properties of cat motoneurones were measured using intracellular electrodes, after which Procion dye was injected iontophoretically into the neurone through the recording pipette.2. Histological procedures were chosen to minimize changes in neuronal morphology. Reconstructed motoneurones had more dendritic branches and larger surface areas than the Golgi-stained motoneurones of earlier reports.3. The sum of the 3/2 power of the dendritic diameters (the dendritic trunk parameter; Rall, 1959) of the reconstructed motoneurones was found to decrease with distance from the soma. Thus, the dendritic tree is not satisfactorily approximated by a non-tapering membrane cylinder.4. A computational technique was developed to allow calculation of the specific resistance (R(m)) of the membrane using the measured value of the input resistance of the motoneurone and a more detailed approximation of the dendritic tree. These calculations indicate that the average resting value of dendritic R(m) is at least 1800 Omega cm(2). The specific membrane capacity, calculated assuming uniform R(m), ranged between 2-3 muF/cm(2).

A note on the interaction of spontaneous and evoked release at the frog neuromuscular junction

1. The interaction between spontaneous miniature end-plate potentials and evoked end-plate potentials was investigated at the frog neuromuscular junction using focal extracellular recording techniques.2. End-plate potentials evoked immediately after a spontaneous miniature potential were facilitated by up to 20%. The percentage facilitation was negatively correlated with the average quantal content of the end-plate potential.