RESUMO
Neuromuscular transmission is clinically monitored using the train-of-four ratio (TOFratio), which is the quotient between twitch tension produced by the fourth (T4) and by the first (T1) stimulus within a train-of-four stimulation at 2Hz. Neostigmine causes a paradoxical depression of the TOFratio (TOFfade) that is amplified by intra-arterial atropine in cats. This led us to question the usefulness of the TOFratio as a sole testing element to control neostigmine-induced reversal of neuromuscular transmission block caused by non-depolarizing agents. We hypothesized that the inhibition of cholinesterase activity might increase acetylcholine bioavailability and consequently cholinoceptor activation, leading to concomitant adenosine release from nerve endings and skeletal muscle fibers. The involvement of presynaptic muscarinic and adenosine receptors in neostigmine-induced TOFfade in the rat phrenic nerve diaphragm was investigated. Blockade of adenosine A2A receptors with ZM241385 and of muscarinic M2 receptors with methoctramine or atropine amplified neostigmine-induced TOFfade. Notwithstanding TOFfade amplification, the blockade of M2 or A2A receptors increased both T1 and T4 twitch tensions above control during the first 3min of neostigmine application. Beyond that period, the T1 twitch tension returned to baseline, whereas T4 decreased further until the control value with neostigmine alone. Blockade of M1 receptors by pirenzepine did not change neostigmine-induced TOFfade, unless A2A receptors were concurrently blocked with ZM241385. Data indicate that the paradoxical neostigmine-induced fade involves presynaptic mechanisms that regulate transmitter release and synaptic adenosine accumulation, including the activation of adenosine A2A and muscarinic M2 receptors.
Assuntos
Inibidores da Colinesterase/farmacologia , Neostigmina/farmacologia , Nervo Frênico/efeitos dos fármacos , Receptores Colinérgicos/fisiologia , Receptores Purinérgicos P1/fisiologia , Animais , Atropina/farmacologia , Diaminas/farmacologia , Diafragma/efeitos dos fármacos , Diafragma/fisiologia , Técnicas In Vitro , Masculino , Antagonistas Muscarínicos/farmacologia , Nervo Frênico/fisiologia , Pirenzepina/farmacologia , Antagonistas de Receptores Purinérgicos P1/farmacologia , Ratos , Ratos Wistar , Receptores Pré-Sinápticos/fisiologia , Transmissão Sináptica/efeitos dos fármacos , Triazinas/farmacologia , Triazóis/farmacologiaRESUMO
The source size and density determine the extent of nitric oxide (NO) diffusion which critically influences NO signaling. In the brain, NO released from postsynaptic somas following NMDA-mediated activation of neuronal nitric oxide synthase (nNOS) retrogradely affects smaller presynaptic targets. By contrast, in guinea pig trigeminal motor nucleus (TMN), NO is produced presynaptically by tiny and disperse nNOS-containing terminals that innervate large nNOS-negative motoneurons expressing the soluble guanylyl-cyclase (sGC); consequently, it is uncertain whether endogenous NO supports an anterograde signaling between pre-motor terminals and postsynaptic trigeminal motoneurons. In retrogradely labeled motoneurons, we indirectly monitored NO using triazolofluorescein (DAF-2T) fluorescence, and evaluated sGC activity by confocal cGMP immunofluorescence. Multiple fibers stimulation enhanced NO content and cGMP immunofluorescence into numerous nNOS-negative motoneurons; NOS inhibitors prevented depolarization-induced effects, whereas NO donors mimicked them. Enhance of cGMP immunofluorescence required extracellular Ca(2+), a nNOS-physiological activator, and was prevented by inhibiting sGC, silencing neuronal activity or impeding NO diffusion. In conclusion, NO released presynaptically from multiple cooperative tiny fibers attains concentrations sufficient to activate sGC in many motoneurons despite of the low source/target size ratio and source dispersion; thus, endogenous NO is an effective anterograde neuromodulator. By adjusting nNOS activation, presynaptic Ca(2+) might modulate the NO diffusion field in the TMN.
Assuntos
Sistema Nervoso Central/fisiologia , Óxido Nítrico/fisiologia , Receptores Pré-Sinápticos/fisiologia , Transdução de Sinais/fisiologia , Animais , Tronco Encefálico/fisiologia , Sinalização do Cálcio/fisiologia , Sistema Nervoso Central/citologia , GMP Cíclico/fisiologia , Fenômenos Eletrofisiológicos , Ativação Enzimática/fisiologia , Fluoresceína , Corantes Fluorescentes , Guanilato Ciclase/metabolismo , Cobaias , Processamento de Imagem Assistida por Computador , Imuno-Histoquímica , Técnicas In Vitro , Microscopia Confocal , Neurônios Motores/fisiologia , Fibras Nervosas/fisiologia , Óxido Nítrico Sintase Tipo I/metabolismo , Recrutamento Neurofisiológico/fisiologia , Transmissão Sináptica/fisiologia , Nervo Trigêmeo/fisiologiaRESUMO
A number of studies have implicated cholinergic activity in the mediation of learning and memory processes. However, the specific role of muscarinic receptors in memory formation mechanisms is less known. The aim of the present study is to evaluate the effects of muscarinic antagonist M2 presynaptic receptor, AFDX-116 (0.5mM) and M1 and M3 post-synaptic receptor pirenzepine (100mM), as well as a non-selective muscarinic antagonist, scopolamine (136mM), in the insular cortex (IC) during acquisition and retrieval of conditioned taste aversion (CTA). In addition, we evaluate the effects of those antagonists in cortical ACh release by in vivo microdialysis and the effects on the induction of in vivo LTP in the BLA-IC projection. The results showed that the cortical microinjections of scopolamine and pirenzepine, but not AFDX-116, produced significant disruption in the acquisition of CTA, without effects during retrieval. Microinjections of scopolamine and AFDX-116 produced significant cortical ACh release, while infusions of pirenzepine did not produce any release. Application of scopolamine and pirenzepine diminished induction of LTP in the BLA-IC projection, but not AFDX-116, as compared with vehicle. The induction of BLA-CI LTP seems to be modulated by post-synaptic muscarinic acetylcholine receptors and not by pre-synaptic muscarinic receptors. These results suggest a differential involvement of cholinergic receptors during acquisition and retrieval of aversive memory formation, as well as a differential role of muscarinic receptors in the biochemical and electrophysiological processes that may underlay aversive memory.
Assuntos
Córtex Cerebral/fisiologia , Memória/fisiologia , Antagonistas Muscarínicos/farmacologia , Pirenzepina/análogos & derivados , Receptores Muscarínicos/fisiologia , Paladar/fisiologia , Animais , Aprendizagem da Esquiva/efeitos dos fármacos , Aprendizagem da Esquiva/fisiologia , Córtex Cerebral/efeitos dos fármacos , Condicionamento Operante/efeitos dos fármacos , Condicionamento Operante/fisiologia , Potenciação de Longa Duração/efeitos dos fármacos , Masculino , Memória/efeitos dos fármacos , Microdiálise , Microinjeções , Pirenzepina/farmacologia , Ratos , Ratos Wistar , Receptores Muscarínicos/efeitos dos fármacos , Receptores Pré-Sinápticos/efeitos dos fármacos , Receptores Pré-Sinápticos/fisiologia , Escopolamina/farmacologia , Paladar/efeitos dos fármacosRESUMO
The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA(A) receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.
Assuntos
Neurônios Aferentes/fisiologia , Receptores Pré-Sinápticos/fisiologia , Medula Espinal/fisiologia , Animais , Humanos , Condução Nervosa/fisiologia , Vias Neurais/citologia , Vias Neurais/fisiologia , Medula Espinal/citologiaRESUMO
The in vitro capacity of sympathetic superior cervical ganglia (SCG) to take up [3H]choline from the extracellular medium, to synthesize acetylcholine from [3H]choline, and to release [3H]acetylcholine in response to a high K+ concentration, were examined in rats throughout a 24-h cycle. Both the release of [3H]acetylcholine and the synthesis of [3H]acetylcholine from [3H]choline exhibited significant diurnal variations, showing maxima during the first half of the night. After these maxima, nocturnal acetylcholine release and synthesis decayed to daytime levels and remained low until the end of the night. [3H]Choline uptake by rat SCG did not vary significantly throughout a 24-h period. A 1.5-h exposure of rats to darkness at the 5th hour of light phase of the daily photoperiod did not change significantly any parameter studied. A 20-min, 5-Hz, electrical stimulation of the preganglionic trunk of SCG excised from rats at noon increased significantly subsequent K(+)-induced [3H]acetylcholine release but did not change [3H]acetylcholine synthesis. In decentralized SCG of rats subjected to a unilateral SCG decentralization and a contralateral sham-operation 7 days earlier, [3H]acetylcholine release and synthesis were highly reduced or abolished at the decentralized side, while [3H]choline uptake remained unaltered. The present results suggest that an activation of preganglionic rat SCG neurons takes place during the first half of the scotophase.