The in vitro brain slice as a useful neurophysiological preparation for intracellular recording

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Opioid pharmacology in the rat hippocampal slice

The ability of several opioids in potentiating the synaptic activation of CA1 pyramidal cells in the rat hippocampal slice were compared. Morphine and the opioid peptides, (D-ala2, D-leu5)-enkephalin (DADL), morphiceptin, beta-endorphin, and Tyr-D-Ser-Gly-Phe-Leu-Thr (DSThr) caused a concentration-dependent, naloxone-reversible shift to the left in the input-output (IO) curve constructed by plotting the population spike as a function of the field EPSP. These opioids then produced an increase in the size of the population spike while leaving the EPSP unaffected. In contrast, the kappa agonist prototype, ethylketazocine, had no effect on the IO curve when perfused in concentrations up to 10 microM. The rank order of potency for the opioids in the CA1 region of the hippocampus was DADL greater than DSThr greater than beta-endorphin greater than morphiceptin greater than morphine much greater than ethylketazocine. Thus, opioids that are more specific for delta opiate receptors were the most potent and mu receptor agonists, the least potent in this action. Taken together with previous studies suggesting that morphine and DADL may interact with a common opiate receptor in the CA1 region, the results are consistent with the notion that these epileptiform effects may be primarily mediated by delta opiate receptors in this area although the potency of morphiceptin indicates that mu receptors play some role in this effect.

Voltage-clamp analysis of muscarinic excitation in hippocampal neurons

Pyramidal cells in the CA1 field of guinea pig hippocampal slices were voltage-clamped using a single microelectrode, at 23-30 degrees C. Small inwardly relaxing currents triggered by step hyperpolarizations from holding potentials of -80 to -40 mV were investigated. Inward relaxations occurring for negative steps between -40 mV and -70 mV resembled M-currents of sympathetic ganglion cells: they were abolished by addition of carbachol, muscarine or bethanechol, as well as by 1 mM barium; the relaxations appeared to invert at around -80 mV; they became faster at more negative potentials; and the inversion potential was shifted positively by raising external K+ concentration. Inward relaxations triggered by steps negative to -80 mV, in contrast, appeared to reflect passage of another current species, which has been labelled IQ. Thus IQ did not invert negative to -80 mV, it was insensitive to muscarinic agonists or to barium, and it was blocked by 0.5-3 mM cesium (which does not block IM). Turn-on of IQ causes the well known droop in the hyperpolarizing electrotonic potential in these cells. The combined effects of IQ and IM make the steady-state current-voltage relation of CA1 cells slightly sigmoidal around rest potential. It is suggested that activation of cholinergic septal inputs to the hippocampus facilitates repetitive firing of pyramidal cells by turning off the M-conductance, without much change in the resting potential of the cell.

PO2-profiles in hippocampal slices of the guinea pig

For a detailed analysis of the oxygen supply of hippocampal slices, tissue PO2 (Pt,O2) was recorded polarographically in the neural layers of thick and thin slice preparations from the guinea pig. The experiments showed that the Pt,O2-gradients were extremely steep in the outer zones of vital slices. In an air equilibrated salt solution the surface PO2 was reduced to less than 50% within ca. 25 micron. Minimum values were measured at a depth of ca. 150 micron. A rise of temperature lowered the oxygen supply in the deeper layers of the excised tissue. An elevation of the surface PO2 hardly improved Pt,O2 in the deep structures, because the O2-consumption of the hippocampal slices increased with rising PO2.

Aspartate-induced changes in extracellular free calcium in 'in vitro' hippocampal slices of rats

Extracellular free calcium ([Ca2+]0) and neuronal activity were measured with double-barreled Ca2+-selective/reference microelectrodes in the CA1 field of rat hippocampal slices during stimulation of Schaffer collaterals and during iontophoretic application of the excitatory amino acid aspartate (Asp). Baseline [Ca2+]0 was 1.5 mM. Repetitive stimulation and aspartate application caused decreases in [Ca2+]0 (delta Ca), which were largest in the pyramidal cell layer. During iontophoretic application of GABA, aspartate often failed to induce significant alterations in [Ca2+]0. Mg2+ (9 mM) and Ni2+ (3mM) when added to the perfusion medium reduced aspartate-dependent delta Ca by 15 and 70% respectively. In tetrodotoxin (10(-6) M) containing Ringer the aspartate-induced delta Ca were about 10% smaller. These findings suggest that the decreases in [Ca2+]0 are mainly due to a postsynaptic Ca2+ entry through selective Ca2+ conductances.

Intradentate colchicine retards the development of amygdala kindling

The mechanisms underlying the kindling model of epilepsy are unknown. Presumably, an altered network of neural circuits underlie amygdala kindling. Biochemical and radiohistochemical studies have pointed to the dentate granule cells (DGC) of the hippocampal formation as a member of this altered circuit. To test the role of these cells, colchicine, a neurotoxin of DGC, was directly injected into the dentate gyrus. Prior destruction of DGC retarded the development of amygdala kindling. Destruction of DGC after kindling was completed did not reverse the kindling effect. We conclude that DGC play a key role in the development, but not the permanence, of amygdala kindling. We propose a model whereby the greater the input to the hippocampal formation, the faster limbic kindling will proceed.

Reduced ATP concentration as a basis for synaptic transmission failure during hypoxia in the in vitro guinea-pig hippocampus

1. Experiments were performed to determine whether a decrease in tissue ATP contributes to the rapid failure of cerebral synaptic transmission during hypoxia. Transmission between the perforant path and the dentate granule cells in the in vitro hippocampus was studied.2. Hippocampal slice ATP is decreased by approximately 15% at the time that the evoked response begins to diminish in standard Krebs bicarbonate buffer. This is about 2 min after the onset of hypoxia.3. When transmission failure is accelerated by increasing extracellular K(+) from 4.4 to 13.4 mM, the evoked response begins to decay about 30 sec after exposure to hypoxia. There is no decrease in hippocampal slice ATP at this time.4. However, ATP in the molecular layer (the synaptic region of the tissue) is decreased by approximately 15% at the time the evoked response begins to decay in the slices exposed to elevated K(+) concentration.5. Exposing the hippocampal slice to 25 mM-creatine for 3 hr elevates molecular layer phosphocreatine fourfold. Synaptic transmission during hypoxia survives three times as long as it does in the absence of creatine.6. In the creatine fortified medium, molecular layer ATP no longer declines within 30 sec of hypoxia. However the molecular layer ATP does decline within 90 sec of hypoxia, the time at which the evoked response begins to decay in this creatine-fortified buffer.7. The results establish that ATP in the region of the active synapses is lowered when the first signs of electrophysiological failure appear during hypoxia. They also show that maintaining ATP for longer than normal during hypoxia is associated with a prolonged maintenance of the evoked response. They thus suggest that a decline in ATP is one factor causing hypoxic block of synaptic transmission.8. It is further suggested that the very rapid failure of the electroencephalogram during anoxia may also result from a decline in ATP.

Dependence of the viability of neurons in hippocampal slices on oxygen supply

Hippocampal slices were tested for their viability by recording monosynaptic responses in granule cell and CA3 pyramidal cell layer. Subsequently, they were studied by light and electron microscopy. Cross sections through the slices contained intact cells clustered in the central part, while the rims consisted of swollen neurons. Electrophysiological and morphological findings indicated that a greater number of granule cells than of CA3 neurons was preserved. This difference was not due to different sensitivity of the neurons to hypoxia, since synaptic responses disappeared within 3-5 min in both cell layers, when the slices were incubated in N2/CO2 instead of O2/CO2. The block of synaptic transmission during hypoxia was caused by hyperpolarization, decrease of EPSP/IPSP amplitudes and increase of membrane conductance as revealed by intracellular recording. Synaptic potentials recorded extracellularly recovered even after 20 min of hypoxia. Morphologically these slices were not different from control slices. After 1 hr of hypoxia no recovery of electrical potentials was seen. In cross sections of these slices, swollen neurons outnumbered intact cells.

The structural integrity of neurons in the hippocampal slice preparation as revealed by intracellular injection of Lucifer Yellow

Hippocampal neurons in slice preparations were injected with the fluorescent dye Lucifer Yellow CH. Pyramidal neurons in the CA1 and CA3 regions and granule cells in the area dentata were identified and their processes traced. The perikarya and dendrites of injected neurons were clearly visible. Axons could be traced in some cases for up to 500 micrometer before they passed out of the slice. In several cases dye was observed in more than one neuron after a single neuron alone was injected.

The actions of cholecystokinin and related peptides on pyramidal neurones of the mammalian hippocampus

The presence of cholecystokinin octapeptide (CCK-8) immunoreactivity in the vicinity of the pyramidal neurones of the mammalian hippocampus has allowed us to investigate the central postsynaptic actions of CCK-8 and a number of related peptides, at a site thought to be innervated by a peptidergic pathway. Intracellular records from pyramidal cells of the CA1 region of the hippocampal slice preparation were used to determine changes in excitability and associated changes in membrane potential and resistance evoked by the pressure application of peptides into the cell body layer, from an independently mounted multibarrelled micropipette. The tetra- and octa-peptide C-terminal fragments of cholecystokinin evoked abrupt and rapidly reversible depolarizations which were accompanied by marked increases in excitability and a decrease in membrane input resistance. A comparison was made of the actions of these peptides with those of glutamate, released by iontophoresis from an adjacent barrel of the same multibarrelled pipette. The rate of onset of the cholecystokinin-evoked response was similar to that of the response evoked by glutamate. C-terminal fragments of gastrin (G-13 and G-14) and bombesin were also found to be excitatory to pyramidal neurones in the CA1 region. However, the nonsulphated form of CCK-8 was inactive, as was substance P, a peptide not present in the hippocampus.