RESUMO
Electrocytes are muscle-derived cells that generate the electric organ discharge (EOD) in most gymnotiform fish. We used an in vitro preparation to determine if the complex EOD of Gymnotus carapo was related to the membrane properties of electrocytes. We discovered that in addition to the three Na(+)-mediated conductances described in a recent paper [Sierra F, Comas V, Buño W, Macadar O (2005) Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo. J Comp Physiol A 191:1-11] there were four K(+)-dependent conductances. Membrane depolarization activated a delayed rectifier (I(K)) and an A-type (I(A)) current. I(A) displayed fast voltage-dependent activation-inactivation kinetics, was blocked by 4-aminopyridine (1 mM) and played a major role in action potential (AP) repolarization. Its voltage dependence and kinetics shape the brief AP that typifies Gymnotus electrocytes. The I(K) activated by depolarization contributed less to AP repolarization. Membrane hyperpolarization uncovered two inward rectifiers (IR1 and IR2) with voltage dependence and kinetics that correspond to the complex "hyperpolarizing responses" (HRs) described under current-clamp. IR1 shows "instantaneous" activation, is blocked by Ba(2+) and Cs(+) and displays a voltage and time dependent inactivation that matches the hyperpolarizing phase of the HR. The activation of IR2 is slower and at more negative potentials than IR1 and is resistant to Ba(2+) and Cs(+). This current fits the depolarizing phase of the HR. The EOD waveform of Gymnotus carapo is more complex than that of other gymnotiform fish species, the complexity originates in the voltage responses generated through the interactions of three Na(+) and four K(+) voltage- and time-dependent conductances although the innervation pattern also contributes [Trujillo-Cenóz O, Echagüe JA (1989) Waveform generation of the electric organ discharge in Gymnotus carapo. I. Morphology and innervation of the electric organ. J Comp Physiol A 165:343-351].
Assuntos
Membrana Celular/metabolismo , Órgão Elétrico/metabolismo , Gimnotiformes/metabolismo , Músculo Esquelético/metabolismo , Canais de Potássio de Abertura Dependente da Tensão da Membrana/metabolismo , Animais , Bário/farmacologia , Membrana Celular/efeitos dos fármacos , Césio/farmacologia , Órgão Elétrico/citologia , Eletricidade , Gimnotiformes/anatomia & histologia , Ativação do Canal Iônico/efeitos dos fármacos , Ativação do Canal Iônico/fisiologia , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Músculo Esquelético/anatomia & histologia , Músculo Esquelético/efeitos dos fármacos , Técnicas de Cultura de Órgãos , Técnicas de Patch-Clamp , Potássio/metabolismo , Bloqueadores dos Canais de Potássio/farmacologia , Canais de Potássio de Abertura Dependente da Tensão da Membrana/efeitos dos fármacos , Canais de Sódio/efeitos dos fármacos , Canais de Sódio/metabolismo , Especificidade da EspécieRESUMO
Single intrastriatal microinjections of 25, 50 and 100nmol/microl of flunarizine in normal rats produced a dose-dependent turning behavior toward the injected side when they were challenged with apomorphine (1mg/kg, s.c). This effect was seen at 1, 3 and 7 days following administration of the high dose of flunarizine, but had subsided by 24h after administration of the intermediate dose; the low dose was ineffective. However, intrastriatal injection of the high dose of flunarizine resulted in a local lesion and thereafter this dose was not used. A similar dose-response relationship was determined for nifedipine, an L-type calcium channel antagonist. Injection of this antagonist did not result in apomorphine-elicited rotational behavior, reflecting its lack of antidopaminergic action. Intrastriatal injections of haloperidol (5microg/microl), an antagonist of dopamine D(2) receptors, or the sodium channel blocker lidocaine (40microg/microl), were given in order to compare their effects to those observed with flunarizine. Intracerebral injection of haloperidol produced ipsilateral turning in response to systemic administration of apomorphine given 60min after. The same response was obtained with the injection of apomorphine 10min after the injection of intracerebral lidocaine. This effect was no longer apparent 24h after the microinjection of haloperidol and 60min after the injection of lidocaine. In rats rendered hemiparkinsionian by lesioning the nigrostriatal pathway with 6OHDA, intrastriatal microinjection of flunarizine (50nmol/microl) significantly reduced apomorphine (0.2mg/kg, s.c.)-elicited turning behavior towards the non-lesioned side. These results suggest an antidopaminergic effect of flunarizine mediated by antagonistic action of post-synaptic striatal dopamine receptors. However, an action of the drug on sodium channels may not be ruled out. These studies offer additional supporting evidence for the induction or aggravation of extrapyramidal side-effects in patients receiving flunarizine.
Assuntos
Bloqueadores dos Canais de Cálcio/farmacologia , Dopamina/fisiologia , Flunarizina/farmacologia , Neostriado/fisiologia , Transmissão Sináptica/efeitos dos fármacos , Ácido 3,4-Di-Hidroxifenilacético/metabolismo , Animais , Apomorfina/farmacologia , Canais de Cálcio Tipo L/efeitos dos fármacos , Dopamina/metabolismo , Agonistas de Dopamina/farmacologia , Antagonistas de Dopamina/farmacologia , Antagonistas dos Receptores de Dopamina D2 , Relação Dose-Resposta a Droga , Haloperidol/farmacologia , Masculino , Microinjeções , Neostriado/efeitos dos fármacos , Neostriado/metabolismo , Nifedipino/farmacologia , Doença de Parkinson Secundária/psicologia , Ratos , Ratos Sprague-Dawley , Comportamento Estereotipado/efeitos dos fármacos , Sinapses/efeitos dos fármacosRESUMO
The effects of the membrane permeant Ca2+ chelator BAPTA-AM on voltage-gated Na+, Ca2+, K+ (I(Na), I(Ca) I(K), respectively) and Ca2+-activated K+ (I(KCa)) currents in cultured bovine chromaffin cells were investigated using the whole-cell patch-clamp technique. Superfusion with BAPTA-AM (50 microM) induced a rapid (< 60 s) and reversible block of both I(KCa) and I(K) (approximately 50%), without affecting either I(Ca) or I(Na). Preincubation with BAPTA-AM (50 microM, 30 min) or cell loading with the nonpermeable active form of BAPTA (10 mM in the pipette solution) permanently blocked I(KCa). BAPTA-AM superfusion (50 microM) also blocked I(K) (approximately 53%) after BAPTA-loading or BAPTA-AM preincubation. In conclusion, we show a fast and reversible block of I(KCa) and I(K) by BAPTA-AM, acting directly on K+ channels before it operates as a Ca2+ chelator, in cultured bovine chromaffin cells.
Assuntos
Cálcio/fisiologia , Quelantes/farmacologia , Células Cromafins/fisiologia , Ácido Egtázico/análogos & derivados , Ativação do Canal Iônico/efeitos dos fármacos , Canais de Potássio/fisiologia , Animais , Canais de Cálcio/fisiologia , Bovinos , Células Cultivadas , Células Cromafins/química , Células Cromafins/efeitos dos fármacos , Ácido Egtázico/farmacologia , Estimulação Elétrica , Ativação do Canal Iônico/fisiologia , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Bloqueadores dos Canais de Potássio , Canais de Sódio/fisiologiaRESUMO
The effects of omega-conotoxin-GVIA (omega-CgTX) on synaptic transmission were studied in the electromotoneuron-electrocyte synapses of the electric organ (EO) of the weakly electric fish Gymnotus carapo. omega-CgTX selectively and irreversibly blocked excitatory postsynaptic potentials (EPSPs) in a dose dependent-manner. The toxin had no effect on: (a) resting postsynaptic membrane potential and conductance; (b) postsynaptic action potentials elicited by depolarizing transmembrane current pulses; (c) the action potential conduction in the presynaptic fiber; (d) acetylcholine (ACh)-induced postsynaptic responses. Nifedipine - a selective dihydropyridine antagonist of the L-type voltage-dependent Ca2+ channels (VDCCs) - did not affect synaptic transmission. Transmission was also undisturbed by the peptide omega-Agatoxin (omega-Aga-IVA), the low molecular weight polyamine, funnel-web toxin (FTX) - both included in the venom of the spider Agelenopsis aperta - and its synthetic analog sFTX, all selective blockers of P-type VDCCs. Since omega-CgTX irreversibly blocks the N-type VDCCs, we conclude that presynaptic N-type VDCCs mediate transmitter release at electromotoneuron terminals. The VDCCs involved in fish peripheral electromotoneuron-electrocyte presynaptic transmitter release are therefore similar to those in amphibian, reptilian and avian peripheral synapses, but differ from mammalian and invertebrate motoneuron terminals.
Assuntos
Canais de Cálcio/fisiologia , Peixe Elétrico/fisiologia , Neurônios Motores/ultraestrutura , Neurotransmissores/metabolismo , Sinapses/fisiologia , Animais , Bloqueadores dos Canais de Cálcio/farmacologia , Órgão Elétrico/metabolismo , Órgão Elétrico/ultraestrutura , Eletrofisiologia , Técnicas In Vitro , Nifedipino/farmacologia , Peptídeos/farmacologia , Poliaminas/farmacologia , Venenos de Aranha/farmacologia , ômega-Agatoxina IVA , ômega-Conotoxina GVIARESUMO
Unit pairs were recorded from dorsal hippocampus in curarized rats injected with physostigmine, during spontaneous activity and stimulation of hippocampal afferents. Interactions between neurons and with the theta (theta) rhythm were investigated with statistical procedures which included: autocorrelations, crossocorrelations, peristimulus-time-histograms (PSTHs) and joint-peristimulus-scatter-diagrams (JPSs). Three types of pairs were found: (1) theta pairs, with rhythmical bursting units phaselocked with theta; (2) non-theta pairs, having non-rhythmical cells; and (3) mixed pairs. Theta pairs showed periodical crossrelation between units which fired in phase or out of phase. Non-theta pairs could interact or not. Many interacting non-theta pairs showed a degree of phaselocking with theta. Mixed pairs were rare, but when interacting they always revealed positive periodical crosscorrelations. Stimulus-elicited phase relationships (reset) and rhythmical interactions within the theta pairs suggest a common source for their rhythmicity and reset. In related non-theta pairs the stimulus usually modified their interactions suggesting loose functional connections.