RESUMO
The brain is an excitable media in which excitation waves propagate at several scales of time and space. "One-dimensional" action potentials (millisecond scale) along the axon membrane, and spreading depression waves (seconds to minutes) at the three dimensions of the gray matter neuropil (complex of interacting membranes) are examples of excitation waves. In the retina, excitation waves have a prominent intrinsic optical signal (IOS). This optical signal is created by light scatter and has different components at the red and blue end of the spectrum. We could observe the wave onset in the retina, and measure the optical changes at the critical transition from quiescence to propagating wave. The results demonstrated the presence of fluctuations preceding propagation and suggested a phase transition. We have interpreted these results based on an extrapolation from Tasaki's experiments with action potentials and volume phase transitions of polymers. Thus, the scatter of red light appeared to be a volume phase transition in the extracellular matrix that was caused by the interactions between the cellular membrane cell coat and the extracellular sugar and protein complexes. If this hypothesis were correct, then forcing extracellular current flow should create a similar signal in another tissue, provided that this tissue was also transparent to light and with a similarly narrow extracellular space. This control tissue exists and it is the crystalline lens. We performed the experiments and confirmed the optical changes. Phase transitions in the extracellular polymers could be an important part of the long-range correlations found during wave propagation in central nervous tissue.
Assuntos
Depressão Alastrante da Atividade Elétrica Cortical/fisiologia , Matriz Extracelular/fisiologia , Substância Cinzenta Periaquedutal/fisiologia , Retina/fisiologia , Visão Ocular/fisiologia , Animais , Galinhas , Percepção de Cores/fisiologia , Processamento de Imagem Assistida por Computador , Cristalino/fisiologia , Luz , Potenciais da Membrana , Espalhamento de RadiaçãoRESUMO
This paper presents some results on the correlation between the electrophysiological and intrinsic optical signals (IOS) of spreading depression waves in chicken retinae. We first show that the peak of the time derivative of the electrophysiological wave occurs precisely when the optical signal reaches the electrode tip. Second, by comparing bath applications of propranolol and glycerol it can be shown that the slow potential shift is not directly correlated to the intrinsic optical signal. Propranolol depresses the amplitude of the electrical wave, although the intrinsic optical signal continues being visible. On the other hand, we observe total absence of the IOS under glycerol, while the electrical wave is always present. Correlations of this kind are relevant for a deeper understanding of the underlying mechanisms of the spreading depression phenomenon.
Assuntos
Depressão Alastrante da Atividade Elétrica Cortical/fisiologia , Potenciais da Membrana/fisiologia , Retina/fisiologia , Visão Ocular/fisiologia , Antagonistas Adrenérgicos beta/farmacologia , Animais , Animais Recém-Nascidos , Galinhas/fisiologia , Depressão Alastrante da Atividade Elétrica Cortical/efeitos dos fármacos , Crioprotetores , Glicerol/farmacologia , Potenciais da Membrana/efeitos dos fármacos , Estimulação Luminosa , Propranolol/farmacologia , Retina/efeitos dos fármacos , Visão Ocular/efeitos dos fármacosRESUMO
A mathematical transcription of the intrinsic circuit of the CA1 region of the rat dorsal hippocampus was made and the model parameters adjusted according to experimental data from intracellular recordings and single channel kinetics. This model was able to simulate well the profile of the field potentials recorded extracellularly and the well known phenomenon of the paired-pulse depression. The results suggest that the depression of the second pulse, often interpreted in the literature as resulting from GABA(A) inhibition, can also be due to 'shunting' effects on the CA1 pyramids' membrane. The rhythmic oscillations of the field potential (EEG) was obtained as an emergent property of the network dynamics. The frequency of the field oscillation followed the main synaptic input in the region (Schaffer collaterals).
Assuntos
Hipocampo/fisiologia , Modelos Neurológicos , Animais , Fenômenos Biofísicos , Biofísica , Simulação por Computador , Eletrofisiologia , Hipocampo/anatomia & histologia , Interneurônios/fisiologia , Matemática , Potenciais da Membrana , Rede Nervosa/anatomia & histologia , Rede Nervosa/fisiologia , Neurônios/fisiologia , Oscilometria , Células Piramidais/fisiologia , Ratos , Sinapses/fisiologia , Ritmo TetaRESUMO
Gangliosides are amphiphilic, sialic acid-containing glycosphingolipids which are found preferentially in complex composition in the cellular membranes of the nervous system of vertebrates, including the vertebrate retina as well as in other membranes. They are always exposed to the extracellular side of the membranes. By virtue of the negative charges they carry at their headgroup, they contribute to the surface charge of the membrane and may affect ion distribution, mainly that of protons and calcium ions, at the outer side of the membranes. Using retinal spreading depression (RSD) as a tool, we show in this study that the addition of exogenous gangliosides to the extracellular space can change the state of excitability of the retinal tissue. In RSD experiments it reduces the propagation velocity as well as the intrinsic optical signal of RSD waves. These effects are concentration dependent (IC50 about 20 microM) and increase with the increasing negative charge of the ganglioside headgroup. As a possible mechanistic basis of the changes found, the change of the calcium homeostasis of the extracellular space by the exogenously added gangliosides is discussed. Gangliosides have been reported to be useful in the treatment of some neuropathological syndromes, including migraine, although experimental verification has not been possible up to now. Taking into account that the retina is a true part of the CNS, our data may be interpreted as the requested verification.