Morphofunctional effects of applications of glutamate and dopamine on the goldfish Mauthner neurons

Abstract

In the goldfish, we studied the effects of intramedullar applications of glutamate (Glu), dopamine (DA), and of long-lasting rotational stimulation on the functional activity, dimensional characteristics, and ultrastructure of Mauthner neurons (MNs). Applications of Glu, especially when combined with rotational stimulation, were found to result in suppression of the function of MNs, in a decrease in their dimensions and lengths of desmosome-like contacts (DLCs, whose structure is determined by filamentous actin) in afferent mixed and chemical synapses, and in destruction of actin microfilaments in the cytoskeleton of MNs. Applications of DA, vice versa, induced an increase in the resistance to the effects of long-lasting stimulation and stabilized the dimensions of MNs; the length of DLCs increased in afferent synapses of both the above types, and the number of fibrillar actin bridges in the DLC cleft of mixed synapses also increased. Bundles of the actin filaments, which were preserved after stimulation, appeared in the cytoskeleton of MNs. Testing of the action of neurotransmitters on actin preparations in vitro showed that Glu entirely depolymerizes filamentous actin, while DA, vice versa, polymerizes monomeric actin. Thus, the Glu-and DA-induced reactions are similar in their types and are of a reciprocal nature both in the actin cytoskeleton of MNs in situ and in purified actin in vitro; these effects correlate with suppression of the functional state of MNs under the influence of Glu and with stabilization of this state under the influence of DA. These results agree with the concept on the roles of depolymerization and polymerization of actin in changes of the morphofunctional state of MNs and show that actin of the cytoskeleton of MNs is a cellular target for the actions of Glu and DA. The similarity between the effects of tested neurotransmitters on actin in MNs in situ and in cell-free preparations in vitro allows us to hypothesize that these transmitters can penetrate into the neuron.

Effect of Dopamine on Viability of BHK-21 Cells

“…According to the results of model studies, globular actin serves as the target and substrate for DA-induced direct formation of ¿ laments [1,4,5]. Hence, the effects of DA on BHK-21 cell viability and morphology, observed in our experiments, can be explained by the direct polymerizing effect of DA on cytoplasmic globular actin.…”

“…Hence, the effects of DA on BHK-21 cell viability and morphology, observed in our experiments, can be explained by the direct polymerizing effect of DA on cytoplasmic globular actin. Previously detected ɫhanges in cell shape and hypertrophy of the cell cortical layer in lactotropes and desmosome-like contacts of Mauthner neurons [1,13] can be interpreted from this standpoint. Ultrastructural data on DAinduced hypertrophy of ¿ lamentous actin network in BHK-21 cell cytoplasm (more detailed data in [6]) and recent cytochemical data on DA penetration into BHK-21 cells and its integration in the actin ¿ laments [7] con¿ rm the possibility of non-ligand (trophic) DA interactions with living cells, as was assumed previously.…”

“…Model experiments con¿ rmed this hypothesis, at least for biogenic amine dopamine (DA). It was shown in vitro that DA interacts with isolated monomeric actin (the most prevalent protein of any eukaryotic cell) and poly merizes it [1,4]. Moreover, DA penetrating through the membrane causes polymerization of monomeric actin in phospholipid vesicles (liposomes) [5].…”

Effects induced by an actin-polymerizing peptide in goldfish mauthner neurons

“…A technique of application of physiologically active substances to the MNs, which was developed earlier, allows us to realize such an examination [3,4]. Using this technique, experimenters have recently studied the effect of application of dopamine to the MNs [5]; this agent, as is known, is capable of inducing LTP at mixed MN synapses [6]. The data obtained demonstrated that ultrastructural manifestations of potentiation mediated by the action of dopamine or tetanic electrical stimulation of the corresponding afferent inputs in vitro are practically identical [5,7,8].…”

“…Using this technique, experimenters have recently studied the effect of application of dopamine to the MNs [5]; this agent, as is known, is capable of inducing LTP at mixed MN synapses [6]. The data obtained demonstrated that ultrastructural manifestations of potentiation mediated by the action of dopamine or tetanic electrical stimulation of the corresponding afferent inputs in vitro are practically identical [5,7,8]. It was found that the level of activity of MNs (where dopamine induced LTP) observed in the behavior of the fishes increased nearly two times, as compared with that of MNs of intact and control (application of the same solution but with no dopamine) fishes.…”