AMPA-Preferring Receptors Mediate Excitatory Synaptic Inputs to Retinal Ganglion CellsLukasiewicz, Peter D.; Wilson, James A.; Lawrence, Jean E.
doi: N/Apmid: 9120596
Abstract Lukasiewicz, Peter D., James A. Wilson, and Jean E. Lawrence. AMPA-preferring receptors mediate excitatory synaptic inputs to retinal ganglion cells. J. Neurophysiol. 77: 57–64, 1997. Pharmacological studies were performed to determine whether α-amino-3-hydroxy-5-methyl-4-isoazoleprionic acid (AMPA)- and/or kainate (KA)-preferring receptors mediate excitatory synaptic inputs to tiger salamander retinal ganglion cells. Excitatory postsynaptic currents (EPSCs), evoked either by light or by stimulating bipolar cells with puffs of K + , were measured using whole cell recording techniques in the tiger salamander retinal slice. The AMPA/KA component of the EPSCs was isolated by including antagonists of glycine-, γ-aminobutyric acid (GABA)- and NMDA-receptors in the bath. The AMPA-preferring receptor antagonists, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI-52466) and 1-(4-aminophenyl)-3-methylcarbamyl - 4 - methyl - 7,8 - methylenedioxy - 3,4 - dihydro - 5H - 2,3 - benzodiazepine (GYKI-53665), reduced light-evoked EPSCs and K + puff-evoked EPSCs amplitudes in a concentration-dependent manner. The IC 50 values for GYKI-52466 were 3.6 and 4.2 μM for the light- and puff-evoked responses, respectively. The more potent GYKI-53665 had IC 50 values of 0.7 μM for both the light- and puff evoked responses. KA activates both KA- and AMPA-preferring receptors. KA-evoked currents were completely blocked by 10–40 μM GYKI-53665, indicating that little or no excitatory synaptic current was mediated by KA-preferring receptors. Concanavalin A, a compound that preferentially potentiates responses mediated by KA-preferring receptors, did not enhance either EPSCs or glutamate-evoked responses. By contrast, cyclothiazide, which selectively enhances AMPA-preferring receptor mediated responses, was found to enhance both EPSCs and glutamate-evoked currents. Our results indicate that the non-NMDA component of ganglion cell EPSCs is mediated by AMPA-preferring receptors and not significantly by KA-preferring receptors. Footnotes Address for reprint requests: P. D. Lukasiewicz, Dept. of Ophthalmology and Visual Sciences, Campus Box 8096, Washington University School of Medicine, St. Louis, MO 63110-1093.
Comparison of the Neuronal Activity in the SMA and the Ventral Cingulate Cortex During Prehension in the MonkeyCadoret, Geneviève; Smith, Allan M.
doi: N/Apmid: 9120556
Abstract Cadoret, Geneviève and Allan M. Smith. Comparison of the neuronal activity in the SMA and the ventral cingulate cortex during prehension in the monkey. J. Neurophysiol. 77: 153–166, 1997. Two monkeys were trained to use the thumb and forefinger to lift and hold an instrumented apparatus within a narrow position window for 1 s. The device was equipped to measure the position and the grip and lifting forces exerted by the animal. On blocks of trials the weight and surface texture could be varied or a force-pulse perturbation could be systematically delivered 750 ms after the object entered the window. If unopposed, the perturbation would displace the hand from the position window, and in preparation for this perturbation the monkeys either increased their grip force before the perturbation or raised the object higher within the position window. Two clearly separated clusters of cells in the medial wall of the frontal lobe were found to be active in relation to the task. One group of cells ( n = 115) was located in the caudal and medial part of area 6, in the supplementary motor area (SMA), and the other ( n = 92) was located in the ventral bank of the cingulate sulcus (CMAv), in area 23c. In each area, neurons were characterized by their sensorimotor features clearly related to the hand in addition to their modulated activity in the task. In the SMA, 71% (42 of 59) of the neurons tested for receptive fields responded to peripheral and mainly proprioceptive stimulation, and 71% of them (30 of 42) received inputs from the hand. In the CMAv, 77% (48 of 62) of the neurons responded to peripheral proprioceptive stimulation, and 77% (37 of 48) exhibited receptive fields originating from the hand. Intracortical microstimulation applied to 43 sites in the SMA evoked discrete hand movements at 12 loci, whereas in the CMAv hand movements were observed at 8 of 27 sites tested with an average threshold of >15 μA. A strong similarity was observed between the SMA and CMAv neurons in their sensorimotor features as well as the modulation of their activity in relation to the prehension task. In both areas the activity was poorly related to grip force and significant correlation with peak grip force was observed for only 9 and 7% of the CMAv and SMA neurons, respectively. In the SMA only five cells exhibited increased activity before the perturbation and in the CMAv no changes in activity were found despite the presence of clear preparatory increases in grip force in anticipation of the perturbation. The perturbation evoked reflexlike excitation of 38% (25 of 65) of the neurons in the CMAv and 28% (20 of 71) of the cells in the SMA; these cells were similar in magnitude and latency (∼50 ms) in both areas. In both the SMA and CMAv, most of the neurons increased their firing rate <200 ms before the grip force onset and the overlap in the distribution of neuronal response times suggests a parallel activation of the SMA and CMAv neurons during the prehension task. Footnotes Address for reprint requests: A. Smith, Centre de Recherche en Sciences Neurologiques, Département de Physiologie, Université de Montréal, Case Postale 6128, succ Centre Ville, Montreal, Quebec H3C 3T8, Canada.
Differential Effects of 4-Aminopyridine, Serotonin, and Phorbol Esters on Facilitation of Sensorimotor Connections in AplysiaSugita, Shuzo; Baxter, Douglas A.; Byrne, John H.
doi: N/Apmid: 9120559
Abstract Sugita, Shuzo, Douglas A. Baxter, and John H. Byrne. Differential effects of 4-aminopyridine, serotonin, and phorbol esters on facilitation of sensorimotor connections in Aplysia. J. Neurophysiol. 77: 177–185, 1997. Serotonergic modulation of sensory neurons in Aplysia and their synaptic connections with follower cells has been used extensively as a model system with which to study mechanisms underlying neuronal plasticity. Serotonin (5-HT)-induced facilitation of sensorimotor connections is due to at least two processes: a process related to the broadening of presynaptic action potentials and a spike-duration-independent (SDI) process that may involve mobilization of transmitter. We have examined the relationship between spike broadening and synaptic facilitation of relatively nondepressed sensorimotor connections in the intact pleural-pedal ganglia. Previously, 5-HT-induced spike broadening in the sensory neuron was shown to be primarily due to the modulation of a voltage-dependent K + current ( I K,V ). Low concentrations (20–30 μM) of 4-aminopyridine (4-AP) were used to rather selectively block I K,V . 4-AP increased spike duration in the sensory neuron and the excitatory postsynaptic potential (EPSP) in the motor neuron. The temporal development of 4-AP-induced spike broadening closely paralleled that of synaptic facilitation. Thus spike broadening via the reduction of I K,V can directly contribute to synaptic facilitation. The relationship between spike broadening induced by 5-HT (10 μM) and enhancement of the EPSP was also analyzed. We found that components of 5-HT-induced synaptic facilitation preceded the development of 5-HT-induced spike broadening. The comparison between the results of 4-AP and 5-HT revealed that the SDI processes made an important contribution to the rapid development of 5-HT-induced synaptic facilitation and that spike broadening made an important contribution to its maintenance. The SDI process and a slowly developing component of 5-HT-induced spike broadening are mediated, at least in part, by the activation of protein kinase C (PKC). Application of phorbol 12,13-diacetate (PDAc), an activator of PKC, partially mimicked the effects of 5-HT on spike duration and the EPSP.PDAc-induced enhancement of the EPSP preceded the slower development of PDAc-induced spike broadening. Like 5-HT, PDAc enhanced the EPSP via both spike broadening and the SDI processes. In addition, a 15-min exposure to PDAc occluded 5-HT-induced enhancement of the EPSP, suggesting that PKC and 5-HT engage similar or overlapping mechanisms. On the basis of these results and others, we propose a time-dependent hypothesis for the 5-HT-induced synaptic facilitation of nondepressed synapses, in which multiple second-messenger/protein kinase systems mediate the actions of 5-HT via both spike-duration-dependent and SDI processes. Footnotes Address for reprint requests: J. H. Byrne, Dept. of Neurobiology and Anatomy, University of Texas Medical School-Houston, P.O. Box 20708, Houston, TX 77225. Present address of S. Sugita: Dept. of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75235-9046. Copyright © 1997 the American Physiological Society
In Vivo Responses of Single Olfactory Receptor Neurons of Channel Catfish to Binary Mixtures of Amino AcidsKang, Jiesheng; Caprio, John
doi: N/Apmid: 9120550
Abstract Kang, Jiesheng and John Caprio. In vivo response of single olfactory receptor neurons of channel catfish to binary mixtures of amino acids. J. Neurophysiol. 77: 1–8, 1997. For the first time in any vertebrate, in vivo responses of single olfactory receptor neurons to odorant mixtures were studied quantitatively. Extracellular electrophysiological response of 54 single olfactory receptor neurons from 23 channel catfish, Ictalurus punctatus, to binary mixtures of amino acids and to their components were recorded simultaneously with the electroolfactogram (EOG). For 57% (73 of 128) of the tests, no significant change (N) from spontaneous activity occurred. Responses to the remaining 55 tests of binary mixtures were excitatory (E; 13%) or suppressive (S; 30%). No response type was associated with any specific mixture across the neurons sampled. Eighty-six percent of the responses of catfish olfactory receptor neurons to binary mixtures were classifed similar to at least one of the component responses, a percentage comparable (i.e., 89%) with that observed for single olfactory bulb neurons in the same species to equivalent binary mixtures. The responses of single olfactory receptor neurons to component-similar binary mixtures (i.e., component responses were both E, both S, and both N, respectively) were generally (80% of 59 tests) classified similar to the responses to the components. For E+N and S+N binary mixtures, the N component often (66% of 58 tests) reduced or concealed (i.e., “masked”) the excitatory and suppressive responses, respectively. For the majority (6 of 11 tests) of E+S binary mixtures, null activity resulted. Responses to the remaining five tests were either excitatory ( n = 3) or suppressive ( n = 2). Footnotes Address reprint requests to J. Caprio. Present address of J. Kang: Monell Chemical Senses Center, 3500 Market Street, Philadelphia, PA 19104.
Characterization of a P-Type Calcium Current in a Crayfish Motoneuron and Its Selective Modulation by Impulse ActivityHong, Sungwon J.; Lnenicka, Gregory A.
doi: N/Apmid: 9120598
Abstract Hong, Sungwon J. and Gregory A. Lnenicka. Characterization of a P-type calcium current in a crayfish motoneuron and its selective modulation by impulse activity. J. Neurophysiol. 77: 76–85, 1997. Previous studies have demonstrated that the voltage-dependent Ca 2+ current recorded from the cell body of the crayfish abdominal motoneuron, F3, undergoes a long-term reduction as a result of increased impulse activity. The properties of the Ca 2+ channels undergoing this long-term change were examined with the use of two-electrode voltage-clamp techniques. The Ca 2+ current was activated at −50 to −40 mV and its amplitude was maximal at 0 mV (−135.0 ± 25.8 nA, mean ± SE, n = 14). The current-voltage relationship and the greater sensitivity of the Ca 2+ channel to Cd 2+ than Ni 2+ indicated that Ca 2+ influx occurs through high-voltage-activated (HVA) Ca 2+ channels. Loose-patch recordings demonstrated that the Ca 2+ current was generated by the membrane of the cell body. When Ba 2+ was substituted for extracellular Ca 2+ , there was a 40% increase in the amplitude of the inward current and a negative shift of ∼10 mV in the I-V relationship. Application of the P-type Ca 2+ channel antagonist ω-agatoxin IVA (ω-AgTX IVA) produced a significant 33% ( n = 6) reduction in the peak amplitude of the Ba 2+ current, whereas neither the L-type Ca 2+ channel antagonist nifedipine nor the N-type channel antagonist ω-conotoxin GVIA produced a reduction in the Ba 2+ current. The voltage-dependent activation of this P-type (ω-AgTX-IVA-sensitive) Ca 2+ channel was similar to previously identified P-type channels, but different from that of the non-P-type (ω-AgTX-IVA-resistant) Ca 2+ channels. When Ca 2+ currents were measured6–7 h after an increase in impulse activity (5-Hz stimulation for45–60 min), there was a 43% reduction in the amplitude of the P-type current, but no significant changes in the non-P-type current amplitude. These results demonstrate that at least two subtypes of HVA Ca 2+ channels contribute to the macroscopic Ca 2+ current observed in the cell body of this crayfish phasic motoneuron: one belongs to the previously described P-type Ca 2+ channel and the other(s) does not belong to the N-, L-, or P-type Ca 2+ channel. The long-term, Ca 2+ -dependent reduction in Ca 2+ current previously demonstrated in motoneuron F3 is produced by the selective reduction of this P-type Ca 2+ current. This activity-dependent reduction in the P-type Ca 2+ current is likely involved in the long-term depression of transmitter release observed at the neuromuscular synapses of this motoneuron. Footnotes Address for reprint requests: G. A. Lnenicka, Dept. of Biological Sciences, University at Albany, SUNY, 1400 Washington Ave., Albany, NY 12222. Copyright © 1997 the American Physiological Society
Primary Afferent Neurons Innervating Guinea Pig DuraBove, Geoffrey M.; Moskowitz, Michael A.
doi: N/Apmid: 9120572
Abstract Bove, Geoffrey M. and Michael A. Moskowitz. Primary Afferent Neurons Innervating Guinea Pig Dura. J. Neurophysiol. 77: 299–308, 1997. We made recordings from filaments of guinea pig nasociliary nerve to study response properties of afferent axons innervating the anterior superior sagittal sinus and surrounding dura mater. We analyzed 38 units in 14 experiments. Units were initially located with the use of mechanical stimuli, and were then characterized by their conduction velocity and sensitivities to mechanical, thermal, and chemical stimuli. Single-unit recordings revealed innervation of dura and superior sagittal sinus by slowly conducting axons, mostly in the unmyelinated range. The receptive fields were 1–30 mm 2 , and typically had one to three punctate spots of highest sensitivity. All units tested responded to topical application of chemical agents. Ninety-seven percent of units responded to 10 −5 M capsaicin, 79% responded to a mixture of inflammatory mediators, and 37% responded to an acidic buffer (pH 5). These data underline the importance of chemical sensitivity in intracranial sensation. Heat and cold stimuli evoked responses in 56 and 41% of units tested, respectively. Although the response patterns during heating were typical of polymodal nociceptors innervating other tissues, the thresholds were lower than for other tissues (32.3–42°C). Cooling led to a phasic discharge, with thresholds between 25 and 32°C. Although units had different combinations of responses to mechanical, chemical, and thermal stimuli, when grouped by their sensitivities the groups did not differ regarding mechanical thresholds or presence of ongoing activity. This suggests that meningeal primary afferents are relatively homogeneous. Sensitivities of these units are in general consistent with nociceptors, although the thermal thresholds differ. These data provide the first detailed report of response properties of intracranial primary afferent units, likely to be involved in transmission of nociception and possibly mediation of intracranial pain. Footnotes Present address and address for reprint requests: G. M. Bove, Odense Universitet, Institute of Medical Biology (Biomekanik), Campuvej 55, DK-5230 Odense M, Denmark. Copyright © 1997 the American Physiological Society
Cholinergic Activation of Startle Motoneurons by a Pair of Cerebral Interneurons in the Pteropod Mollusk Clione limacinaNorekian, Tigran P.; Satterlie, Richard A.
doi: N/Apmid: 9120570
Abstract Norekian, Tigran P. and Richard A. Satterlie. Cholinergic activation of startle motoneurons by a pair of cerebral interneurons in the pteropod mollusk Clione limacina. J. Neurophysiol. 77: 281–288, 1997. The holoplanktonic pteropod mollusk Clione limacina exhibits an active escape behavior that is characterized by fast swimming away from the source of potentially harmful stimuli. The initial phase of escape behavior is a startle response that is controlled by pedal motoneurons whose activity is independent of the normal swim pattern generator. In this study, a pair of cerebral interneurons is described that produces strong activation of the d-phase startle motoneurons, which control dorsal flexion of the wings. These interneurons were designated cerebral startle (Cr-St) interneurons. Each Cr-St neuron has a small cell body on the dorsal surface of the cerebral ganglia and one large axon that runs into the ipsilateral cerebral-pedal connective and the neuropile of the ipsilateral pedal ganglion. Each spike in a Cr-St neuron produces a fast, high-amplitude (up to 50 mV) excitatory postsynaptic potential (EPSP) in the d-phase startle motoneurons. This 1:1 ratio of spikes to EPSPs and the stable short synaptic latencies (2 ms) persist in high-Mg 2+ , high-Ca 2+ seawater, suggesting monosynaptic connections. Synaptic transmission between Cr-St neurons and startle motoneurons exhibits a very slow synaptic depression, because a number of spikes in Cr-St neurons is required to achieve a noticeable decrease in EPSP amplitude. Synaptic transmission between Cr-St interneurons and startle motoneurons appears to be cholinergic. In startle neurons, 20 μM atropine and 50 μM d -tubocurarine reversibly block EPSPs produced by spike activity in Cr-St interneurons. Hexamethonium only partially blocks EPSPs in startle neurons, and much higher concentrations are required. Exogenous acetylcholine (1 μM) produces a dramatic depolarization of startle motoneurons in high-Mg 2+ seawater, and this depolarization is reversibly blocked by atropine. Nicotine also has a depolarizing effect on startle motoneurons, although higher concentrations are required. Cr-St interneurons and startle motoneurons are also electrically coupled; however, the coupling is weak. Stimuli that are known to initiate escape responses in intact animals, such as tactile stimulation of the tail or wings, produce excitatory inputs to Cr-St interneurons. In addition, tactile stimulation of the lips and buccal cones, which is known to trigger prey capture reactions in Clione, also produces excitatory inputs to Cr-St interneurons and startle motoneurons, suggesting involvement of the startle neuronal system in prey capture behavior of Clione. Footnotes Address reprint requests to T. P. Norekian.
Study of the Inhibitor of the Crayfish Neuromuscular Junction by Presynaptic Voltage ControlVyshedskiy, Andrey; Lin, Jen-Wei
doi: N/Apmid: 9120551
Abstract Vyshedskiy, Andrey and Jen-Wei Lin. Study of the inhibitor ofthe crayfish neuromuscular junction by presynaptic voltage control. J. Neurophysiol. 77: 103–115, 1997. The inhibitor of the crayfish opener muscle was investigated by a presynaptic voltage control method. Two microelectrodes were inserted into the inhibitor and the amplitude and duration of presynaptic depolarization were controlled by a voltage-clamp amplifier. The inhibitory postsynaptic potential (IPSP) was measured from a muscle fiber located near the presynaptic voltage electrode. Nonlinear summation of IPSP amplitudes was corrected after chloride equilibrium potential was measured. With the use of 5-ms presynaptic pulses, the depolarization-release coupling (D-R) curve constructed from IPSP peak amplitudes (IPSP cor ) had a threshold of about −35 mV and reached its maximal level at −5 to −10 mV. Depolarization beyond the maximum led to a suppression of neurotransmitter release. When transmitter release during a presynaptic pulse was completely suppressed, IPSPs activated by tail current could be identified with an average synaptic delay of 2.5 ms. Transmitter secretion triggered by a calcium current activated during the 5-ms pulses (IPSP on ) was also measured on the rising phase of an IPSP, at 2.5 ms after the end of the 5-ms pulses. D-R coupling plots measured from IPSP on exhibited a more pronounced suppression than that obtained from IPSP cor . The effect of presynaptic pulse duration on the level of transmitter release was analyzed. Transmitter release increased with increasing duration and was nearly saturated by 20-ms pulses depolarized to 0 mV. The following conditions were identified as necessary to obtain a consistent D-R curve with a clear suppression: 1 ) small animals, 3.8 cm head to tail, 2 ) 15°C, 3 ) 40 mM tetraethylammonium and 1 mM 4-aminopyridine, 4 ) an extracellular calcium concentration of ≤10 mM. In addition, a consistent correlation was found among the branching pattern of the inhibitor, the placement of the presynaptic electrode, and the characteristics of the D-R curves. An ideal presynaptic electrode configuration involved placing the voltage electrode in a secondary branch, ∼100 μm from the main branch point, and placing the current electrode at the branch point. Postsynaptically, optimal recordings were obtained from muscle fibers innervated by a single branch of the inhibitor that originated from a point near the presynaptic voltage electrode. A cable-release model was constructed to evaluate the relationship between the shape of the D-R coupling curves and the space constants of the presynaptic terminals. A comparison between the model and the D-R coupling curves suggested that the space constant of an inhibitor branch on a muscle fiber is ≥8 times longer than its actual length. Therefore the upper limit estimate of the space constant of a typical preparation is ∼3 mm. Results reported here outline morphological and physiological conditions needed to achieve optimal control of the presynaptic branch of the crayfish inhibitor. The cable-release model quantitatively defines the extent of presynaptic voltage control. Footnotes Address for reprint requests: J.-W. Lin, Dept. of Biology, Boston University, 5 Cummington St., Boston, MA 02215.
Emergence of Radial Nerve Dominance in Median Nerve Cortex After Median Nerve Transection in an Adult Squirrel MonkeySchroeder, C. E.; Seto, S.; Garraghty, P. E.
doi: N/Apmid: 9120595
Abstract Schroeder, C. E., S. Seto, and P. E. Garraghty. Emergence of radial nerve dominance in median nerve cortex after median nerve transection in an adult squirrel monkey. J. Neurophysiol. 77: 522–526, 1997. Throughout the glabrous representation in Area 3b, electrical stimulation of the dominant (median or ulnar) input produces robust, short-latency excitation, evident as a net extracellular “sink” in the Lamina 4 current source density (CSD) accompanied by action potentials. Stimulation of the collocated nondominant (radial nerve) input produces a subtle short-latency response in the Lamina 4 CSD unaccompanied by action potentials and followed by a clear excitatory response 12–15 ms later. Laminar response profiles for both inputs have a “feedforward” pattern, with initial activation in Lamina 4, followed by extragranular laminae. Such corepresentation of nondominant radial nerve inputs with the dominant (median or ulnar nerve) inputs in the glabrous hand surface representation provides a likely mechanism for reorganization after median nerve section in adult primates. To investigate this, we conducted repeated recordings using an implanted linear multi-electrode array straddling the cortical laminae at a site in “median nerve cortex” (i.e., at a site with a cutaneous receptive field on the volar surface of D2 and thus with its dominant afferent input conveyed by the median nerve) in an adult squirrel monkey. We characterized the baseline responses to median, radial, and ulnar nerve stimulation. We then cut the median nerve and semi-chronically monitored radial nerve, ulnar nerve and median nerve (proximal stump) evoked responses. The radial nerve response in median nerve cortex changed progressively during the weeks after median nerve transection, ultimately assuming the characteristics of the dominant nerve profile. During this time, median, and ulnar nerve profiles displayed little or no change. Footnotes Present address and address for reprint requests: C. Schroeder, Program in Cognitive Neuroscience and Schizophrenia, Nathan Kline Institute for Psychiatric Research, 140 Old Orangeburg Rd., Bldg. 37, Orangeburg, NY 10962.