After-hyperpolarization and receptor potential attenuation following bursts of action potentials in an insect mechanoreceptor

The receptor potential in the sensory neuron of the cockroach femoral tactile spine was recently observed by raising the axon into an oil bath and measuring the decrementally conducted receptor current. Although action potential discharge in this receptor adapts rapidly, there was no evidence of adaptation in the receptor potential. In the present work we report that bursts of action potentials in the neuron produce a prolonged after-hyperpolarization and attenuate the receptor potential. Both of these effects could be important in receptor adaptation and we sought to identify their origin. It was impossible to control ionic concentrations in the fluid surrounding the sensory neuron because of an effective glial barrier, but it was possible to infuse the tissues with chemical agents which are known to block ionic membrane processes. Cobalt and cadmium, which inhibit calcium influx, eliminated the effects of action potentials, and ouabain had similar effects. These results suggest that both a calcium-activated potassium conductance and an electrogenic sodium pump are involved in these phenomena. However, it is argued that the former is probably more important.     

Neuromodulatory effect of creatine on extracellular action potentials in rat hippocampus: role of NMDA receptors

The creatine (Cr) and phosphocreatine (PCr) system is essential for the buffering and transport of high-energy phosphates. Although achievements made over the last years have highlighted the important role of creatine in several neurological diseases, the adaptive processes elicited by this guanidino compound in hippocampus are poorly understood. In the present study, we showed that creatine (0.5-25mM) gradually increases the amplitude of first population spike (PS) and elicits secondary PS in stratum radiatum of the CA1 region, in hippocampal slices. Creatine also decreased the intensity of the stimulus to induce PS, when compared with hippocampal slices perfused with artificial cerebrospinal fluid (ACSF). The competitive NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP5; 100microM) attenuated creatine-induced increase of amplitude of PS and appearance of secondary PS, providing pharmacological evidence of the involvement of NMDA receptors in the electrophysiological effects of creatine. Accordingly, creatine (0.01-1mM) increased [3H]MK-801 binding to hippocampal membranes by 55%, further indicating that this compound modulates NMDA receptor function. These results implicate the NMDA receptor in amplitude and population spike increase elicited by creatine in hippocampus. Furthermore, these data suggest that this guanidino compound may also play a putative role as a neuromodulator in the brain, and that at least some of its effects may be mediated by an increase in glutamatergic function.     

Up regulation of calbindin-D28K mRNA in the rat hippocampus following focal stimulation of the perforant path

Calbindin-D28K is a constitutive Ca2(+)-binding protein expressed in hippocampal neurons that are resistant to various forms of excitotoxic injury. However, the local factors controlling calbindin-D28K expression within the central nervous system are unknown. We report that neuronal excitation via the perforant path leads to an increased expression of calbindin-D28K mRNA within dentate granule cells. This response is related specifically to stimulation that induces prolonged periods of bursting afterdischarges and precedes cellular injury. The up regulation of calbindin-D28K mRNA occurs during the type of neuronal activation associated with elevated cytosolic Ca2+ and suggests that the maintenance of Ca2+ homeostasis includes a system of feedback control at the level of gene expression.     

The incidence of giant spike bursts and repetitive bursts of action potentials in the ovine small bowel before and after cholecystokinin administration

Giant spike bursts (GSBs) or giant contractions (GCs) and repetitive bursts of action potentials (RBAPs) are less common motility patterns as compared to the migrating motor complex (MMC), fed pattern or minute rhythm. They are present in small and large intestines in various animal species. Their occurrence in ruminants has not been satisfactorily evidenced. Thus, the aim of this study was to present the incidence of these patterns in the ovine small bowel before and after different doses of cholecystokinin octapeptide (CCK-OP) and cerulein as well as to demonstrate the motor correlates of RBAPs.Six sheep equipped with electrodes in the antrum and entire small intestine and with duodenal strain gauge force transducer were used. In fasted and non-fasted animals, continuous myoelectrical and motor recordings were performed before and after the slow injection of cholecystokinin octapeptide (20, 200 and 2000 ng/kg i.v.) and cerulein (1, 10 and 100 ng/kg i.v.) during phase 2 MMC. The incidence of GSBs and RBAPs was assessed and these patterns arrived before and after Cholecystokinin (CCK). During the control period RBAPs were most frequently observed in the ileum. GSBs and RBAPs were induced by the highest dose of the hormones. RBAPs exhibited the motor correlates and their tonic component was more pronounced following CCK-OP and cerulein injection.It is concluded that GSBs and RBAPs occur in the small intestine and the administration of CCK peptides further increases their incidence.     

Theoretical simulation of the calcium action potential in squid giant synapse: Repetitive stimulation

A biophysical model of the experimentally observed calcium action potential (CAP) in squid giant synapse is proposed. Whereas the inclusion of the inward calcium current in the Hodgkin-Huxley model can generate the rising phase of CAP, to account for the observed termination of the action potential, a repolarizing process needs to be introduced. Adding a term representing Ca-activated K current, the observed features of CAP can be reproduced. However, one feature of CAP, namely the gradual shortening of the plateau duration on repetitive stimulation, cannot be simulated by this model. In this paper, it is proved that both the termination of the action potential and the gradual shortening of the plateau cannot be accounted for by inclusion of a single repolarizing process. One more repolarizing process, namely a slow voltage-dependent Ca-inactivation, is therefore proposed to account for all the observed features of CAP.     

Hyperuricemia in the rat following hypothalamic stimulation

Ventromedial hypothalamic electrical stimulation elicited a marked elevation of plasma uric acid with a rise in plasma allantoin in the rat. The magnitude of this hyperuricemia was greater than that of the hyperglycemia which was also produced by the ventromedial stimulation. On the other hand, lateral hypothalamic stimulation did not significantly affect the plasma levels of either of the purine metabolites. These results strongly indicate that the ventromedial hypothalamus is specifically very active in producing hyperuricemia in the rat.     

Prolonged action potentials from single nodes of Ranvier

The duration of action potentials from single nodes of Ranvier can be increased by several methods. Extraction of water from the node (e.g. by 2 to 3 M glycerin) causes increased durations up to 1000 msec. 1 to 5 min. after application of the glycerin the duration of the action potential again decreases to the normal value. Another type of prolonged action potential can be observed in solutions which contain K or Rb ions at concentrations between 50 mM and 2 M. The nodes respond only if the resting potential is restored by anodal current. The kinetics of these action potentials is slightly different. Their maximal durations are longer (up to 10 sec.). Like the normal action potential, they are initiated by cathodal make or anodal break. They also occur in external solutions which contain no sodium. The same type of action potentials as in KCl is found when the node is depolarized for some time (15 to 90 sec., 100 to 200 mv.) and is then stimulated by cathodal current. These action potentials require no K or Na ions in the external medium. Their maximal duration increases with the strength and duration of the preceding depolarization. The possible origin of the action potentials in KCl and after depolarization, and their relation to the normal action potentials and the negative after-potential are discussed.     

Repetitive transcranial magnetic stimulation induces kf-1 expression in the rat brain

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive approach used for stimulating the brain, and has proven effective in the treatment of depression, however the mechanism of its antidepressant action is unknown. Recently, we have reported the induction of kf-1 in rat frontal cortex and hippocampus after chronic antidepressant treatment and repeated electroconvulsive treatment (ECT). In this study, we demonstrated the induction of kf-1 after rTMS in the rat frontal cortex and hippocampus, but not in hypothalamus. Our data suggest that kf-1 may be a common functional molecule that is increased after antidepressant treatment, ECT and rTMS. In conclusion, it is proposed that induction of kf-1 may be associated with the treatment induced adaptive neural plasticity in the brain, which is a long-term target for their antidepressant action.     

Control of cardiac rhythm by stimulation of the vagus nerve in bursts in the rat

Burst stimulation of the vagus nerve in rats made it possible to obtain the ranges of the heart rate regulation. The increase of the number of impulses in a burst from 1 to 16 leads to a consistent decrease in the range of regulation from respectively 72.1 and 66.7% to 30.4 and 25% of the initial rate. The maximal range (10.1% of the initial rate) is reached with 4 impulses in a burst. The phenomenon of the regulation is reversed by atropine.     

Electroconvulsive shock increases the phosphorylation of Pyk2 in the rat hippocampus

Recently we reported the activation MAPKs, MEK, and Rafs by electroconvulsive shock (ECS) in the rat hippocampus. However, the upstream pathways for the activation of Raf-MEK-MAPK cascade after ECS have not been studied yet. Since the proline-rich tyrosine kinase 2 (Pyk2) and Src were reported to be involved in the activation of the MAPKs in neuronal cells, we examined tyrosine phosphorylation and activation of Pyk2 in the rat hippocampus after ECS. ECS transiently increased the phosphorylation of Pyk2 at multiple tyrosine residues (Tyr-402, Tyr-580, and Tyr-881). The phosphorylations reached the peak at 1 min and returned to basal level by 10 min after ECS. At 1 min after ECS, the binding of Pyk2 to Src and Grb2, and of Grb2 to Ras increased. These results suggested that ECS activates Pyk2, which then transmits the signal to MAPK cascade via Src, Grb2, and Ras in the rat hippocampus.     

Collision of two action potentials in a single excitable cell

BackgroundIt is a common incident in nature, that two waves or pulses run into each other head-on. The outcome of such an event is of special interest, because it allows conclusions about the underlying physical nature of the pulses. The present experimental study dealt with the head-on meeting of two action potentials (AP) in a single excitable plant cell (Chara braunii internode).MethodsThe membrane potential was monitored with multiple sensors along a single excitable cell. In control experiments, an AP was excited electrically at either end of the cell cylinder. Subsequently, stimuli were applied simultaneously at both ends of the cell in order to generate two APs that met each other head-on.ResultsWhen two action potentials propagated into each other, the pulses did not penetrate but annihilated (N = 26 experiments in n = 10 cells).ConclusionsAPs in excitable plant cells did not penetrate upon meeting head-on. In the classical electrical model, this behavior is specifically attributed to relaxation of ion channel proteins. From an acoustic point of view, annihilation can be viewed as a result of nonlinear material properties (e.g. a phase change).General significanceThe present results suggest that APs in excitable animal and plant cells belong to a similar class of nonlinear phenomena. Intriguingly, other excitation waves in biology (intracellular waves, cortical spreading depression, etc.) also annihilate upon collision and are thus expected to follow the same underlying principles as the observed action potentials.     

Reduced neurogenesis in the rat hippocampus following high fructose consumption

In this study, we investigated how prolonged consumption of sugar solution affects hippocampal neurogenesis. We gave rats sucrose or fructose solution for four weeks and observed a 40% reduction in BrdU/NeuN-immunoreactive cells in the hippocampal dentate gyrus. This reduction in hippocampal neurogenesis was accompanied by increased apoptosis in the hippocampus and increased circulating levels of TNF-α. Therefore, we hypothesize that the reduction in hippocampal neurogenesis may be due to the increased apoptosis induced by TNF-α. Our results suggest that chronic ingestion of fructose is detrimental to the survival of newborn hippocampal neurones. The results presented in the present study add to the list of harmful effects associated with prolonged and excessive consumption of sugary beverages and soft drinks.     

Oscillations in the amplitude of human peripheral nerve action potentials during repetitive stimulation

Summary Repetitive stimulation of human peripheral nerves in situ produces an amplitude oscillation of the evoked action potentials. The purpose of this experiment was to describe the dynamics of the response as a function of the characteristics of the electrical stimuli. Two attempts to define precisely the origin of the response have proved ineffective.These studies were supported in part by Grant MH 8786-01 from the National Institute of Health and in part by Grant GR 2000 from the National Science Foundation. The author gratefully acknowledges the assistance of Mrs. Madelon Krissoff in the analysis of the data.