RESUMEN
Spike transmission at the electrical synapse between the giant fibres (GFs) and motor giant neurone (MoG) in the crayfish can be blocked by depolarising postsynaptic chemical inhibition, which has previously been shown to be mediated in part by gamma-aminobutyric acid (GABA). The authors show that glutamate applied to the synaptic region of the MoG mimics the depolarisation of the chemical input and can also block spike transmission from the GFs. The glutamate induces an inward current mediated by a conductance increase that is 30-40% of that induced by GABA and that is blocked substantially by picrotoxin. Glutamate has no effect on the presynaptic GF, and the effects in the MoG are maintained in the presence of cadmium, indicating that the glutamate is acting directly on the MoG. Both GABA and glutamate have similar effects on the cell body, where the response reverses 10-20 mV positive to resting potential, is dependent on chloride concentration, and is inhibited by picrotoxin. Joint application of glutamate and GABA induces a nonadditive current under voltage clamp, suggesting that the transmitters can activate the same postsynaptic receptors. Immunocytochemical staining shows that, whereas some synaptic profiles impinging on the MoG contain pleomorphic agranular vesicles and are immunoreactive to GABA and not glutamate (as previously reported), there are at least as many other profiles that contain round, agranular vesicles and that are immunoreactive to glutamate and not to GABA. Thus, the authors conclude that some of the interneurones mediating inhibition of the electrical synapse use glutamate as their neurotransmitter.
Asunto(s)
Astacoidea/fisiología , Ácido Glutámico/fisiología , Inhibición Neural/fisiología , Neurotransmisores/fisiología , Sinapsis/fisiología , Animales , Electrofisiología , Femenino , Inmunohistoquímica , Masculino , Ácido gamma-Aminobutírico/metabolismoRESUMEN
The endogenous neuropeptides FMRFamide and FLRFamide (tetrapeptides) reversibly reduced a voltage-activated calcium current in the C1 neuron of Helix aspersa by an average of 20%. Two structurally related heptapeptides, pQDPFLRFamide and pQDPFLRIamide, both derived from another precursor protein in this species, did not reduce the current at all.
Asunto(s)
Canales de Calcio/efectos de los fármacos , FMRFamida/análogos & derivados , Neuronas/efectos de los fármacos , Animales , Canales de Calcio/metabolismo , Encefalina Metionina/análogos & derivados , Encefalina Metionina/farmacología , Potenciales Evocados/efectos de los fármacos , FMRFamida/farmacología , Caracoles Helix , Neuronas/metabolismo , Neuropéptidos/farmacología , Oligopéptidos/farmacología , Ácido Pirrolidona Carboxílico/análogos & derivadosRESUMEN
FMRFamide (i.e. Phe-Met-Arg-Phe-NH2) application to the C2 neurone of Helix caused a depolarizing response which consisted of a large, rapidly developing, and rapidly desensitizing inward current, underlain by a smaller, slower inward current which did not desensitize. Both currents were carried through sodium-selective channels which were insensitive to D-tubocurarine, and the to the fast sodium channel blockers tetrodotoxin (TTX) and lignocaine. Only the faster, desensitizing current could be blocked by amiloride. FMRFamide also activated two types of unitary inward currents with slightly differing amplitudes in outside-out patches taken from the C2 neurone, both through sodium-selective ion channels. Only the smaller unitary currents readily desensitized and were susceptible to block by amiloride, and they also activated more rapidly. Unitary currents of both types were recorded in outside-out patches in the absence of freely diffusible intracellular mediators, and were also activated when guanosine 5'-O-(2-thiodiphosphate) (GDP [beta-S]) was included in the recording pipette solution. This supports a tight receptor/channel coupling for both responses, with no involvement of GTP-binding proteins. Further, the very fast rate of activation of the smaller channels, which generally carry the major part of the FMRFamide-induced current, strongly indicates that these channels are ligand gated.
Asunto(s)
Caracoles Helix/fisiología , Activación del Canal Iónico , Canales Iónicos/efectos de los fármacos , Neuronas/fisiología , Neuropéptidos/farmacología , Animales , Electrofisiología , FMRFamida , Hormonas de Invertebrados/farmacologíaRESUMEN
The C3 neurone, which acts as a motoneurone for the tentacle retractor muscle in Helix aspersa, contains both Phe-Met-Arg-Phe-NH2 (FMRFamide) and acetylcholine (ACh). Each of these transmitter substances evokes contraction of the isolated muscle. FMRFamide induces a delayed rise in tension followed by phasic contractions. Unlike the response to ACh, this response is not associated with a depolarization of the muscle cells. Here we show that FMRFamide stimulates the inositol phosphate second messenger system in the muscle and causes a significant increase in total inositol trisphosphate (InsP3) levels. The isomer which releases intracellular Ca2+ stores, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), is increased in a similar proportion to the total InsP3. The production of Ins(1,4,5)P3 is therefore likely to be involved in the response of the muscle to FMRFamide and may account for the oscillatory nature of the mechanical response. The N-terminally extended heptapeptide pGlu-Asp-Pro-Phe-Leu-Arg-Phe-NH2 (pQDPFLRFamide), which relaxes the muscle, had no acute effect on InsP3 levels. Indirect evidence also indicates that intracellular Ca2+ stores are required for the generation of the FMRFamide response.