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

Calcium-dependent displacement of haloperidol-sensitive sigma receptor binding in rat hippocampal slices following tissue depolarization

To evaluate the possible existence of an endogenous ligand for the haloperidol-sensitive sigma receptor, we developed an in vitro competition assay to measure endogenous ligand release. Depolarization of in vitro hippocampal slices by either veratridine or potassium reduced [3H]ditolylguanidine binding in a calcium-dependent and transient manner. None of the drugs or iron substitutions directly affected [3H]ditolylguanidine binding to rat brain membranes. Veratridine-induced depolarization also reduced the binding of [3H](+)3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, another sigma radioligand, in a calcium-dependent manner. Radioligand displacement was not associated with alteration in sigma receptor dissociation kinetics or receptor degradation in the hippocampal slice. In contrast, KC1 depolarization had no effect on [3H]ditolyguanidine binding to sigma receptors in liver slices. The results suggest that a calcium-dependent, depolarization-induced reduction in sigma receptor binding may have been caused by the release of an endogenous sigma ligand in rat hippocampal tissue.

Evidence for a chronic loss of inhibition in the hippocampus after kindling: biochemical studies

Brain tissue from kindled animals prepared 1 month after their last seizure was compared to tissue from matched surgical control animals. Quantitative film autoradiography was used to study muscimol binding in the CA1 region and 3 other brain areas (dentate gyrus, cerebral cortex, and thalamus). The Kd values so obtained were constant from region to region and comparable to those published by others. Bmax values varied; of the 4 regions studied CA1 had the lowest value of Bmax. There were no differences in either Kd or Bmax values in any region studied between kindled and surgical control rats. The release of GABA from nerve terminals was assessed with hippocampal tissue maintained in vitro and perfused with different solutions in which the concentrations of K+ and Ca2+ were varied. This allowed the examination of K+-induced depolarization release and the Ca2+ dependence of this process. K+-induced, Ca2+-dependent release of GABA from hippocampus derived from kindled animals was significantly less than that from hippocampus derived from controls. The biochemical studies reported here provide additional support for the hypothesis that there is a chronic decrease in GABA-mediated inhibition in the hippocampus associated with kindling. The data point to a dysfunction at the presynaptic level, within the GABAergic interneuron, but do not exclude changes at a level postsynaptic to the GABAergic interneuron not detected with the methods employed.

Comparison of the actions of phencyclidine and sigma ligands on CA1 hippocampal pyramidal neurons in the rat

To compare the actions of prototypic drugs which are selective for phencyclidine and sigma receptors, the electrophysiological effects of phencyclidine (PCP),3-[3-hydroxyphenyl]-N-(1-propyl)piperidine [+)3-PPP), and 1,3-di(2-tolyl)guanidine (DTG) on CA1 hippocampal pyramidal neurons were examined. A wide range of concentrations of drug was tested to differentiate specific, receptor-mediated effects from nonselective, anesthetic-like actions. At relatively large concentrations (0.1-1 mM), each compound reversibly increased the threshold of action potentials driven by Schaffer collaterals, the duration of action potentials and membrane resistance. The low potencies and rank order of potency suggested that phencyclidine, (+)3-PPP, and DTG were not acting through either high affinity sigma or phencyclidine receptors. These compounds did have receptor-mediated effects at smaller concentrations. Since none of the compounds affected evoked excitatory or inhibitory postsynaptic potentials (EPSP or IPSP) or driven action potentials at subanesthetic concentrations (less than 100 microM), no evidence was found to support the hypothesis that the actions of phencyclidine result from enhanced release of transmitter, caused by the inhibition of a presynaptic potassium conductance. As observed in other neurons, phencyclidine blocked excitations in CA1 pyramidal cells mediated by N-methyl-D-aspartic acid (NMDA) at behaviorally relevant concentrations (1-10 microM). However, (+)3-PPP (1 microM-1 mM) enhanced the pyramidal cell response to NMDA. Alone, DTG did not effect the NMDA-induced response but did inhibit the enhancement induced by (+)3-PPP. The agonist and antagonist actions of the sigma-selective ligands, (+)3-PPP and DTG, suggests that they modify NMDA-induced responses by acting at the sigma receptor.

Transmission of burst responses through slices of rat cerebral cortex

1. Slices of rat's forebrain, 400 micron thick, have been cut and maintained in a bath perfused with warm oxygenated Krebs solution. Records were made with extracellular micropipettes of the neural responses to local stimulation of the cortex itself or the underlying white matter. 2. Single stimuli at either of these sites could produce an all-or-nothing burst response among nearby neurones. This response usually lasted 0.2-0.5 s during which repetitively discharging cortical units could be recorded at all cortical depths. 3. This burst response was transmitted from the stimulated point across the cortex in all directions with a velocity of roughly 0.1 m s-1. 4. Complete recovery of excitability among neurones generating the burst response took about 10 s. 5. Removal of Ca2+ from the perfusate prevented transmission of this response, as did a high concentration of Mg2+ or 160 mg/100 ml of ethanol. 6. Propagation of the burst response was not dependent upon the integrity of the underlying white matter; it required only that any 20% of the cortical thickness was intact and undamaged. 7. In coronal sections of brain the response could be transmitted from one hemisphere to the other provided that the corpus callosum was intact.

A method for intracellular injection of horseradish peroxidase by pressure

A technique for intracellular injection of horseradish peroxidase and other tracers into neurones is described. The method utilises gas pressure to force the tracer solution out of glass micropipettes and allows electrophysiological recordings to be made simultaneously with the injection process. Constructional details of the simple and inexpensive equipment are given. The method has the advantages of being equally suitable for charged and uncharged tracer molecules, and of providing reliable indication of the success of the injection at the time of the injection attempt.

Physiological and morphological characterization of striatal neurons transplanted into the striatum of adult rats

Physiological and morphological properties of transplanted striatal neurons (TSNs) were examined in an in vitro slice preparation. Fetal striatal tissue (E13-14) was implanted as a dissociated cellular suspension into the striatum of adult rats. Intracellular records were obtained from TSNs 2-6 weeks after transplantation. TSNs exhibited biophysical, morphological, and synaptic properties characteristic of normal striatal neurons, despite the disruption involved in processing of the fetal tissue. Differences were observed, however, between the TSNs and host striatal neurons. TSNs consistently had higher input resistance values than host striatal neurons as determined by neuronal responses to intracellular current injection. Stimulation of adjacent host striatum elicited both excitatory and inhibitory postsynaptic potentials in TSNs. By contrast, the same stimuli elicited only excitatory responses in host striatal neurons. Morphologically, TSNs resembled host medium-size spiny neurons as demonstrated by intracellular injection of lucifer yellow. However, the complexity of dendritic branching and the density of spines on the dendrites were less than that observed for host striatal neurons. It was concluded that during the posttransplantation period studied, TSNs possess neuronal properties expected of developmentally immature striatal neurons.

Detection of dopamine overflow and diffusion with voltammetry in slices of rat brain

Voltammetric electrodes have been used to monitor extracellular dopamine in rat brain slices. The electrode tips are small enough to be immersed inside the slice. Specificity for dopamine is increased through the use of voltammetry and a cation exchange membrane at the electrode tip. Dopamine overflow is observed in the caudate nucleus following electrical stimulation (60 Hz, 1 s, 3 V) with an adjacent bipolar electrode. The amount of overflow observed is increased when the tissue is perfused with 10 microM cocaine or nomifensine, both recognized inhibitors of dopamine uptake. The ability of dopamine in the perfusion buffer to permeate the slice was monitored with two voltammetric electrodes, one in the cerebral cortex and the other in the caudate nucleus. At a high concentration (100 microM), dopamine rapidly appeared (2.7 +/- 0.4 min) in the interior of the cortex, but dopamine was not observed in the caudate until a significantly later time (8.9 +/- 1.0 min). To examine whether this difference is a reflection of the presence of different uptake systems in the two regions, pressure ejection was employed. In this experiment a double-barrelled pipette was used to eject dopamine or DOPAC at a fixed distance (approximately 70 micron) from the voltammetric electrode. Ejection of small amounts of both substances could be detected in the cortex. When the ejector-detector assembly was moved to the caudate, dopamine could only be observed following pressure ejection after perfusion of the slice with 10 microM nomifensine. Detection of DOPAC was unaffected. All of these experiments indicate that uptake systems in the caudate keep dopamine concentrations very low in the extracellular fluid of the slice.

The N-methyl-D-aspartate receptor and burst firing of CA1 hippocampal pyramidal neurons

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of polyhydroxyl alcohols on protein synthesis in brain slices

Stressors such as tissue slicing, toxic chemicals, and heat shock applied to cultured cells, organ tissues, or whole animals in vivo induce the synthesis of a 71,000-kilodalton stress protein (SP71) that is not normally present in most organ tissues. In the present experiment, an attempt was made to inhibit selectively the synthesis of SP71 in rat brain tissue slices. Of several manipulations to the brain slice incubation medium that were examined, only addition of very high concentrations of certain polyhydroxyl alcohols, i.e., 1.0 M glycerol, selectively inhibited SP71 synthesis. Glycerol also selectively inhibited SP71 synthesis in heat-shocked cerebral microvascular cells in culture but failed to inhibit SP71 synthesis in anesthetized rats in vivo in response to heat shock. The effects of glycerol on SP71 synthesis are discussed in relationship to current hypotheses concerning the function of SP71.

Rat lateral septum in slice preparation with viable transmission

A procedure is described which makes it possible to prepare slices from the rat brain that comprise nearly the entire lateral septum together with the major part of the fimbria-fornix afferent fibers to the lateral septum. The field potentials and monosynaptic excitation of lateral septal neurons elicited by electrical stimulation of the fimbria-fornix afferent fibers were employed to demonstrate that the neurophysiological features of the lateral septal neurons in vitro are similar to those found previously in vivo, and the synaptic transmission between fimbria-fornix afferent fibers and neurons of the lateral septum could be maintained in vitro without significant alterations for at least a 6-h period of incubation.

2-Amino-5-phosphonovalerate and Co2+ selectively block depolarization and burst firing of rat hippocampal CA1 pyramidal neurones by N-methyl-D-aspartate

Intracellular recordings from pyramidal neurones during microiontophoretic ejection of N-methyl-D-aspartate and quisqualate into the pyramidal cell layer of the CA1 region of the rat hippocampal slice showed that both amino acids caused depolarization and evoked spike activity. Whereas quisqualate evoked tetrodotoxin-sensitive spikes, those produced by N-methyl-D-aspartate consisted of bursts of tetrodotoxin-sensitive action potentials superimposed on an underlying depolarizing shift of membrane potential. Both membrane depolarization and the superimposed depolarizing shifts associated with N-methyl-D-aspartate excitation were selectively and reversibly antagonized by the D(-) isomer of 2-amino-5-phosphonovalerate and Co2+. Both amino acids caused an increase in membrane conductance when small ejection currents were used, and the depolarizing response to these compounds was prevented by current injection. However, only the increase by N-methyl-D-aspartate was blocked by 2-amino-5-phosphonovalerate and Co2+. These results provide evidence to support the suggestion that different mechanisms underlie the excitatory response to N-methyl-D-aspartate and quisqualate in CA1 pyramidal neurones.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of 3H-corticosterone binding capacity in hippocampal slices maintained in vitro

Nuclear uptake of 3H-corticosterone was studied in hippocampal slices subjected to varying periods of preincubation before application of steroid. Uptake capacity declined rapidly toward nonspecific levels over the course of 3 hours from the time of cutting. Glucocorticoid receptors in the cytosol fraction of hippocampal slices also were found to decline, and Scatchard analysis indicated a loss of corticosterone binding sites rather than a change in binding affinity. These results are discussed in terms of the reported effects of corticosterone in vitro.

The action of norepinephrine in the dentate gyrus: beta-mediated facilitation of evoked potentials in vitro

The effects of superfusion of norepinephrine (NE) on perforant path (PP) evoked potentials in the dentate gyrus were evaluated in the rat hippocampal slice preparation. Superfusion of NE (10 microM) produced a facilitation of the PP evoked responses. Facilitation of the synaptically-evoked responses was expressed in the field potential as an increase in extracellular excitatory postsynaptic potential (EPSP) (117% of control), a decrease in population spike onset latency (94% of control) and an increase in population spike amplitude (131% of control). In 24% of the slices the facilitation of the population spike amplitude lasted longer than 30 min. Isoproterenol, a beta-agonist, mimicked NE effects while timolol, a beta-antagonist, blocked them. Facilitation of the population spike amplitude by NE could not be accounted for solely by the increase in EPSP slope also produced by NE. Superfusion of NE did not produce facilitation of the antidromically evoked field potentials, but in 4 of 8 slices produced a small decrease. NE effects were activity-independent, since the subsequently evoked PP responses were facilitated even when the PP was not concurrently stimulated during superfusion with NE.

An intracellular study of the interactions of N-methyl-DL-aspartate with ketamine in the mouse hippocampal slice

Intracellular recordings were made from dentate and CA1 pyramidal cells of the mouse hippocampal slice preparation. N-methyl-DL-aspartate (NMDLA), quisqualate and kainate and the anaesthetic agent, ketamine, were applied by microelectrophoresis. Excitation by NMDLA but not by the other amino acids, was associated with increased outward rectification. Ketamine had no effect on the resting potential or current/voltage relation of the cells, but selectively antagonised the responses to NMDLA. Action potentials evoked by NMDLA were characteristically broader than those evoked by the other amino acids or by the passage of depolarising current through the electrode.

Frequency-dependent block of field potentials in the rat hippocampal slice caused by tricyclic antidepressants

The effect of the tricyclic antidepressants imipramine and desipramine were studied on field potentials in the rat hippocampal slice. The electrically evoked stratum radiatum nerve volley, excitatory postsynaptic potential (e.p.s.p.) and pyramidal cell layer population spike (PS) were recorded in the CA1 region. At concentrations of 10(-6)M to 10(-5)M, impramine did not affect the amplitude of the nerve volley, e.p.s.p. or PS at low frequencies of stimulation (0.01 Hz). At higher frequencies of stimulation (1-100 Hz), imipramine caused a frequency-dependent block of the nerve volley, e.p.s.p. and PS. The time course of onset of the frequency-dependent block in the presence of imipramine was very slow. Maximum inhibition was reached after 3-4 h treatment with imipramine. Desipramine (10(-6)-10(-5)M) also caused a frequency-dependent block of the hippocampal field potentials. Only slight frequency-dependent block was observed in slices from rats injected in vivo with desipramine (10 mg kg-1) for 14 days.

A portable in vitro brain slice chamber

A portable chamber is described which is suitable for maintaining live slices of brain tissue while they are being transported by automobile. The chamber supplies nutrient medium, oxygen, temperature control, and shock isolation.

A new method of monitoring membrane potential in rat hippocampal slices using cyanine voltage-sensitive dyes

A novel application of voltage-sensitive dyes is described. Hippocampal slices in vitro accumulated voltage-sensitive cyanine dyes under conditions presumed to cause depolarization and hyperpolarization. Increasing extracellular potassium caused a depression of dye uptake that correlated linearly with the membrane potential calculated from the Goldman equation. Veratrine depressed dye uptake, and this effect was blocked by addition of tetrodotoxin or removal of extracellular sodium. Ouabain also depressed dye uptake. Conversely, hyperpolarizing conditions using reduced extracellular sodium caused increased dye uptake. These results support a voltage-dependent mechanism for the uptake of cyanine dyes in hippocampal slices. Application of this phenomenon as an alternative to 2-deoxyglucose autoradiography for mapping neuronal activity will be presented.

A rapid-scanning spectrophotometer designed for biological tissues in vitro or in vivo

Rapid and sensitive detection of optical signals from biological materials has been very useful in studies of cell physiology and biochemistry. A rapid-scanning spectrophotometer is described here that has the following advantages: (i) it can be used in transmission or reflection modes, (ii) it can rapidly accumulate spectra and simultaneous kinetic data, (iii) it has high accuracy and sensitivity, and (iv) it can analyze and store large amounts of spectral information. Evaluations described here are aimed toward the measurement of reduction/oxidation shifts of mitochondrial cytochromes in tissues, but the flexibility of the optical components makes this spectrophotometer adaptable to the study of light absorption changes of intrinsic or extrinsic optically active molecules in a variety of light scattering preparations, tissues, and organs in vitro or in vivo.

Intrasomatic and intradendritic recordings of plateau potentials in slices of the dentate gyrus maintained in vitro

Intrasomatic and intradendritic recordings were performed in slices of the dentate gyrus maintained in vitro. When barium ions (2.4 mM) were substituted for calcium ions in the perfusing medium, plateau potentials appeared with an amplitude of 20-40 mV which lasted from 40 ms to more than one min; during these plateau potentials, the input membrane resistance was decreased. In the soma, plateau potentials were also observed in a medium containing barium ions + tetrodotoxin (0.3 or 0.6 microM); whereas, in the dendrites, the barium-induced plateau potentials were abolished after addition of tetrodotoxin to the barium containing perfusion. The somatic plateau potentials had a duration which appeared to be dependent on the stimulus frequency. After being in contact with the barium-tetrodotoxin solution for a long period, the soma membrane potential was observed to jump between two relatively stable levels: a resting state and a depolarized state. In conclusion, calcium conductances appear to be present both at the soma and the dendrites of dentate granule cells; however, at the dendritic level, it appears that, when sodium channel permeability is blocked by tetrodotoxin, there is insufficient inward current to support the generation of action potentials.

Release of purines, noradrenaline, and GABA from rat hippocampal slices by field stimulation

Labelled adenine, noradrenaline (NA), and gamma-aminobutyric acid (GABA) were taken up by the transversely cut hippocampal slice. [3H]NA and [14C]GABA were retained as such, [3H]- (or [14C]-) adenine mainly as adenine nucleotides. There was a spontaneous overflow of all three types of compounds ranging from 0.1 (GABA) to 0.21 (NA) %/min. The rate of [3H]NA overflow increased rapidly during electrical field stimulation. The release rate was well maintained over a 15-min period. The rate of [14C]GABA release also increased rapidly but it was not maintained over a 15-min period even if uptake and/or metabolism was inhibited by nipecotic acid (1 mM) and aminooxyacetic acid (AOAA, 0.1 mM). The bulk of the purines was released after the stimulation period. For all compounds the amounts released were frequency- and calcium-dependent. At a frequency of 3 Hz a 10 V stimulation was sufficient to cause a maximal [3H]NA release and 20 V to cause maximal [14C]GABA release, but 14C-purine release was increased further by increasing the voltage to 40 V. The evoked purine release was inhibited by a nucleoside uptake inhibitor (dipyridamole). On stimulation of [3H]NA-labelled slices the released radioactivity was composed of greater than 95% unchanged NA. The specific activities of NA in the slice and in the superfusate were practically identical. In [3H]adenine-labelled slices the released radioactivity was composed of adenosine, inosine, and hypoxanthine, but the activity in the slice of ATP, ADP, and AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ethanol on Purkinje cells recorded from cerebellar slices

The effects of ethanol on rat cerebellar Purkinje cells were studied using brain slice preparations. The spontaneous activity of Purkinje cells exhibited either steady or cyclic firing patterns. The steady units were generally inhibited by ethanol, but some units were excited by low doses. The cycling units showed a dose-related decrease in firing rate and the duration of cyclic firing. The data indicate that ethanol in concentrations which produce behavioral effects is primarily inhibitory to cerebellar Purkinje cells.

Fluorescent latex microspheres as a retrograde neuronal marker for in vivo and in vitro studies of visual cortex

The use of retrograde axonal transport of various substances (for example, enzymes, lectins, synthetic fluorescent compounds) has yielded much information on the organization of neuronal pathways. Each type of retrograde tracer has its own set of attributes which define the scope of problems it can address. We describe here a new class of retrograde tracer, rhodamine-labelled fluorescent latex microspheres (0.02-0.2 micron diameter), which have distinct advantages over other available tracers for in vivo and in vitro applications. When injected into brain tissue, these microspheres show little diffusion and consequently produce small, sharply defined injection sites. Once transported back to neuronal somata, the label persists for at least 10 weeks in vivo and 1 yr after fixation. Microspheres have no obvious cytotoxicity or phototoxicity as assessed by intracellular recording and staining of retrogradely labelled cells in a cortical brain slice preparation. This approach was further used to visualize and compare, in cat visual cortex slices, neurones with different projection patterns, and revealed significant differences in patterns of intrinsic axons and dendrites. These properties of microspheres open new avenues for anatomical and physiological studies of identified projection neurones in slices as well as in dissociated cell cultures.

Protein release from hippocampus in vitro

Physiologically viable slices of rat hippocampus in vitro continuously release protein into the superfusion medium at a rate of about 2 micrograms/mg tissue/h. Assays of a cytoplasmic marker enzyme (lactate dehydrogenase) indicate that this material is not the result of cell lysis. Pulse-chase experiments using [3H]valine indicate that a substantial fraction of the newly synthesized proteins eventually appear in the incubation medium (18.7% +/- 3% of the total TCA precipitable radioactivity during a 6-h superfusion) and that the releasable protein pool has an apparent half-life of about 4 h. Simultaneous labeling of newly synthetized proteins with [3H]fucose and [14C]valine showed a 3-fold higher ratio of [3H]fucose to [14C]valine in the released protein fraction compared to the soluble cytoplasmic protein and to the crude membrane protein fraction, suggesting that the soluble released proteins are more highly glycosylated than the proteins retained in the tissue. Electrophoretic migration patterns on SDS-polyacrylamide gels with both labeled and unlabeled proteins show differences between the released proteins and the soluble cytoplasmic proteins of the tissue. Several molecular weights between 14 kdalton and 86 kdalton appear to be characteristic of the released protein fraction. These results suggest that a distinct group of proteins and glycoproteins exists in hippocampal tissue which is destined to be selectively released into the extracellular space.

Decreased neuronal inhibition in vitro after long-term administration of ethanol

The pathophysiology of brain dysfunction was studied with an animal model of chronic alcoholism. Rats were fed a liquid diet with or without ethanol for 20 weeks and then the diet without ethanol for three more weeks. Hippocampal slices were prepared and intracellular recordings were obtained from dentate granule and CA1 cells. Significant depression of orthodromically elicited inhibitory postsynaptic potentials and postspike afterhyperpolarizations was observed in neurons from ethanol-exposed animals. No differences were observed in other active or passive membrane characteristics. These results suggest that a loss of neuronal inhibition could contribute to brain dysfunction in chronic alcoholism.

Cells in the anteroventral cochlear nucleus are insensitive to L-glutamate and L-aspartate; excitatory synaptic responses are not blocked by D-alpha-aminoadipate

Auditory nerve fibers transmit signals from the cochlea to the 3 regions of the cochlear nuclear complex, the anteroventral (AVCN), posteroventral, and dorsal cochlear nucleus in the brainstem. It has been suggested that the amino acids L-aspartate and L-glutamate might serve as a neurotransmitter in auditory nerve fibers. The sensitivity of postsynaptic cells in the cochlear nuclei to these amino acids has been tested by iontophoretic techniques. One difficulty with these experiments is that responses were recorded only extracellularly. A second difficulty is that the concentrations needed to affect cells could not be determined. To avoid these difficulties a brain slice preparation was used to test the sensitivity of cells in the AVCN to bath applied L-glutamate and L-aspartate at concentrations ranging from 10(-5) to 10(-2) M. All cells that were tested in the cochlear nuclear complex were insensitive at all concentrations used; the resting potentials and the input resistances remained unchanged and the synaptic responses to electrical stimulation of the auditory nerve were not desensitized. All cells that were tested in the hippocampus, however, depolarized in the presence of 10(-4) M L-glutamate and L-aspartate. The synaptic responses to electrical stimulation of the auditory nerve were not blocked by D-alpha-aminoadipate, an amino acid which has been shown to block excitation of cells in the cochlear nuclei by auditory nerve fibers. The results are not consistent with L-glutamate and L-aspartate serving as neurotransmitters in the AVCN.

Factors affecting the phosphorylation of a 41,000 dalton protein of hippocampal subcellular fractions

The phosphorylation of the alpha-subunit of mitochondrial pyruvate dehydrogenase may be involved in the development of long-term potentiation in the hippocampus. Study of this hypothesis is hampered by variability in the incorporation of 32P into pyruvate dehydrogenase of hippocampal subcellular preparations, in vitro. 32P from [gamma-32P]ATP was incorporated into pyruvate dehydrogenase present in mitochondria and in a membrane-enriched synaptic particulate fraction from hippocampus. However, the presence of the synaptic fraction decreased isotopic labeling of the mitochondrial protein. This effect was not due to inhibition of the protein kinase or activation of a protein phosphatase, but the rate of ATP hydrolysis was found to be higher in the synaptic fraction than in the mitochondria (34 nmol/mg protein/min vs 14 nmol/mg protein/min). These data raise a variety of questions about the interpretation of the in vitro phosphorylation assay. It is concluded that variability in in vitro labeling can be minimized if the effect of ATP hydrolysis is diminished by use of a higher concentration of ATP. In addition, these data indicate that quantitative comparisons of the in vitro phosphorylation of diverse subcellular preparations must take into account differential rates of ATP hydrolysis.

Levels of adenosine and adenine nucleotides in slices of rat hippocampus

ATP, ADP, AMP, IMP, adenosine, inosine and cyclic AMP were measured in slices of the rat hippocampus maintained in vitro. Immediately following cutting ATP was low (3.5 +/- 0.6 nmol/mg protein) and AMP high (8.6 +/- 0.9 nmol/mg), giving an energy charge of only 0.34 +/- 0.02. Over the next 90 min the energy charge gradually normalized (to 0.92 +/- 0.01), partly due to conversion of AMP to ATP, but mainly to breakdown to adenosine and other purines which were recovered in the incubation medium. Total purine content decreased from approximately 18 to 10 nmol/mg protein in the first hour following cutting. In slices from old rats the energy charge was lower 60 min following preparation than in younger rats, while cyclic AMP and adenosine levels were higher. The adenosine antagonist 8-phenyl-theophylline tended to enhance the recovery of responsiveness after preparation of the slices. Stimulation of excitatory afferent fibers at a frequency of 10 Hz for 5 min did not significantly alter the purine levels in brain slices, while hypoxia decreased the energy charge significantly and tended to increase adenosine levels. These changes occurred somewhat later than the fall in electrophysiological responsiveness. 8-Phenyl-theophylline was able to delay somewhat the decline in the amplitude of synaptic responses under hypoxic conditions. It is concluded that the viable part of the hippocampal slice, which accounts for about half of the tissue, has levels of adenine nucleotides and adenosine which are similar to those found in the intact rat brain. The return of electrophysiological function following slice preparation is paralleled by a normalization of the energy charge, the adenosine level and the concentration of cyclic AMP. The absence of electrophysiological activity following cutting, and the decreases in such responses following either prolonged afferent stimulation or hypoxia may be related to changes in purine concentration in the slice. Although adenosine accumulating in the slice may contribute to the depression of electrophysiological responses it is probably not the major factor responsible for the reduction in synaptic responsiveness.

Ergopeptine-sensitive calcium-dependent protein phosphorylation system in the brain

We studied a protein phosphorylation system that is regulated by the dopamine-mimetic ergot bromocriptine. Bromocriptine was found to inhibit selectively the endogenous phosphorylation of a threonine residue(s) in 50,000- and 60,000-dalton proteins in a synaptosome fraction. The bromocriptine-sensitive phosphorylation is stimulated by calcium and by calmodulin, and occurs predominantly in the brain. The inhibitory effect of bromocriptine was not mimicked by 3,4-dihydroxyphenylethylamine or by any of the neurotransmitters and related agents tested, but was mimicked, although less effectively, by other ergots that contain peptide moieties. In the hippocampus, the brain region with the highest content of the 50,000- and 60,000-dalton proteins, the ergopeptine-sensitive protein phosphorylation appears to be localized to interneurons or cell bodies whose axons synapse outside the hippocampus. The results raise the possibility that some of the bromocriptine- and ergopeptine-induced pharmacological effects in the CNS may be mediated by the inhibition of the calcium/calmodulin-dependent phosphorylation of these specific proteins.

Effects of folic and kainic acids on synaptic responses of hippocampal neurones

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.

Effects of calcium, strontium, and barium ions on phosphorylation of hippocampal proteins in vitro

Calcium ion alone or in the presence of added calmodulin stimulated in vitro transfer of 32P from [gamma 32P]ATP into several proteins of mitochondrial and synaptosomal particulate fractions from rat brain. Strontium ion was capable of substituting for calcium ion in this stimulation, but barium ion lacked this capacity. These results bring into question the hypothesis that calcium-dependent protein phosphorylation of synaptic proteins is intrinsic to neurotransmitter release during neurotransmission, but they do not rule out that possibility.

Naloxone and long term potentiation of hippocampal CA3 field potentials in vitro

The effects of naloxone on potentiation of CA3 pyramidal cell field potentials induced by tetanization of the mossy fiber pathway was studied in the in vitro guinea pig hippocampal slice preparation. Naloxone in nanomolar concentrations prevented the development of long term potentiation and it is concluded that an opioid peptide is probably involved in the generation of the potentiation.

Oxygen consumption of hypothalamic tissue slices after varying incubation periods

In order to quantitate possible time-related changes in the viability of rat hypothalamic tissue slices, tissue oxygen consumption was measured after incubation periods ranging from 0-4 hours. There were no significant differences in mean tissue oxygen consumption between the various incubation periods; nor was there any trend indicating that oxygen consumption gradually decreases over time. Moreover, no regional differences were observed among the various rostral hypothalamic slices. One obvious trend, however, was that during the first two hours of each experiment, tissue oxygen consumption decreased briefly and then returned to normal higher levels. The exact occurrence of this transitory decrease varied from experiment to experiment; but the subsequent recovery in oxygen consumption was always complete by two hours of incubation. This initial transient decrease in tissue oxygen consumption may reflect the initial period of electrophysiological inactivity reported in several in vitro studies.

An air pressure system for the injection of tracer substances into the brain

A system for injecting traces and other substances into the brain through glass micropipettes using mechanically or electromechanically regulated air pressure as the driving force is described. The system has a number of advantages over currently used devices including: accurate ejection of a wide range of volumes (2 nl to several microliters); the capability to inject all currently utilized tracers; and, the direct visualization of the volume of solution injected. This system has proven reliable for a number of tract tracing experiments ranging from the injection of tracers into deep structures of the monkey brain to injecting small quantities of tracers into the brains of fetal rats.

Blockade of amino acid-induced depolarizations and inhibition of excitatory post-synaptic potentials in rat dentate gyrus

Excitatory post-synaptic potentials (e.p.s.p.s) evoked by stimulation of the medial perforant path and depolarizations induced by excitatory amino acids were recorded from granule cells in the preparation of the hippocampal slice from the rat. The effects of (+/-)-2-amino-5-phosphonovalerate (APV), gamma-D-glutamylglycine (gamma DGG) and cis-2,3-piperidinedicarboxylate (PDA), antagonists of excitatory amino acids on these phenomena were compared. gamma DGG was the most effective antagonist of the e.p.s.p. Its action was reversible and not associated with any change in the passive membrane properties of the granule cells or in the apparent reversal potential of the e.p.s.p. Quantal analysis showed that the reduction in the e.p.s.p. paralleled the decrease in quantal size rather than quantal content, confirming a post-synaptic site of the action of gamma DGG. The potency of gamma DGG against the exogenous agonists was N-methyl-D-aspartate greater than kainate greater than or equal to quisqualate. APV had very little effect on the e.p.s.p. but was a selective antagonist of N-methyl-D-aspartate-induced depolarizations. PDA depolarized granule cells and increased their membrane input resistance. Although gamma DGG was a potent antagonist of both glutamate- and aspartate-induced depolarizations, no clear pattern of specificity could be found. The action of glutamate was unaffected by APV. These results indicate that the receptor for the transmitter at the synapses formed by the fibres of the perforant path with the granule cells is of the quisqualate and/or kainate type. The present data are consistent with the biochemical evidence that glutamate may be the endogenous transmitter at his synapse.

Low-dose benzodiazepine neuronal inhibition: enhanced Ca2+-mediated K+-conductance

The water-soluble inhibitory benzodiazepine, midazolam, was applied in low nanomolar concentrations to CA1 hippocampal neurons in vitro, recorded intracellularly. The drug caused a long-lasting hyperpolarization and moderate conductance increase, which persisted with TTX-induced synaptic blockade or with intracellular injection of Cl- ions, but not in zero Ca2+ perfusate. Calcium spikes elicited in the presence of TTX were enhanced by midazolam. It was concluded that these low nanomolar concentrations, which did not enhance GABA actions, inhibited by augmenting Ca2+ mediated K+-conductance.

The excitatory effects of the specific benzodiazepine antagonist Ro14-7437, measured intracellularly in hippocampal CA1 cells

The specific benzodiazepine antagonist, Ro14-7437, in nanomolar concentrations, caused depolarization, increased spontaneous spiking, and conductance decrease when applied to CA1 cells in vitro. These effects were resistant to intracellularly injected Cl- ions or synaptic blockade by TTX, were prevented in Ca2+-free medium, and occurred with or without prior application of midazolam, an inhibitory benzodiazepine. Ca2+-mediated AHPs and Ca2+ spikes in TTX medium were diminished by the blocker, suggesting that Ro14-7437 acted by inhibiting Ca2+-mediated K+ conductance.

Transmission of cerebrospinal fluid pressure via the cochlear aqueduct and endolymphatic sac

The concept of perilymphatic and endolymphatic pressure balance is generally linked to the theory that the endolymphatic sac transmits cerebrospinal fluid (CSF) pressure changes to the endolymph to equalize CSF pressure changes transmitted to the perilymph via the cochlear aqueduct. This theory, and the significance of other mechanisms of CSF pressure influence on the labyrinth, were evaluated experimentally. Continuous measurements of perilymphatic, CSF, venous, and arterial pressures were performed on cats with the cochlear aqueduct patent or obstructed and the inferior cochlear vein intact or occluded. Intracranial pressure changes were induced by subarachnoid infusion of artificial CSF in live and dead animals. With the cochlear aqueduct patent, CSF pressure changes were transmitted to the perilymph without any significant dampening or time lag. With the cochlear aqueduct obstructed, CSF pressure changes induced significantly lower and delayed changes in perilymphatic pressure. Similar results were obtained whether the animals were alive or dead and the cochlear vein intact or blocked. This indicated a passive mechanism not induced by changes in labyrinthine fluid production or blood flow. Long-standing, stable elevation of CSF pressure with the cochlear aqueduct blocked induced a slowly increasing perilymphatic pressure, always stabilizing at a pressure rise significantly less than that of CSF. The results do not suggest any major pressure transfer via perineural or perivascular routes. The endolymphatic sac is postulated to mediate a reduced and delayed transfer of increased intracranial pressure to the labyrinth.

Supraoptic neurons in the rat hypothalamo-neurohypophysial explant: double-labeling with Lucifer Yellow injection and immunocytochemical identification of vasopressin- and neurophysin-containing neuroendocrine cells

By combining intracellular electrophysiology with double-labeled intracellular dye-marking and immunocytochemical identification of the same magnocellular neuroendocrine cell, we studied supraoptic neurons in the rat hypothalamo-neurohypophysial explant in vitro. This report examines neurophysiological and light microscopical features of vasopressin- and neurophysin-immunoreactive, pituitary-projecting supraoptic neurons.

A slice chamber for intracellular and extracellular recording during continuous perfusion

The design of a tissue slice perfusion system is described, and examples are given showing the stability of this system for intracellular and extracellular recordings during changes in perfusion media. The stability of this system is attributed to several features. Mini-drips serve to cushion transient changes in flow rate when switching from one medium to another. Solenoid valves are used to quickly switch perfusion media with minimal mechanical movement. A finely-controlled adjustable flow valve provides a uniform flow rate for all media. Constant tissue temperature is maintained by media perfusion through a thermoelectric Peltier assembly. In addition, a filter paper wick insures that the perfusate is constantly removed without movement in the tissue slices. With this design, the slices are supported on a net at the interface between the perfusion medium and a humidified, oxygenated atmosphere. This arrangement appears to be conducive to tissue viability and facilitates the placement of microelectrodes in the slices.

On the mechanism of action of GABA in pelvic vesical ganglia: biphasic responses evoked by two opposing actions on membrane conductance

Intracellular recording techniques were used to study the response of cat vesical pelvic ganglion neurones loaded with permeable anions to the application of GABA in vitro. In 106/127 neurones GABA evoked a biphasic response, the initial phase of which was depolarizing and associated with a conductance increase; the latter phase was hyperpolarizing and associated with a conductance decrease. The GABA evoked hyperpolarization and conductance decrease were related and behaved as though generated by closure of ion channels open in the resting membrane. The hyperpolarization had a strong inhibitory action on both spontaneous activity, and excitation evoked by depolarizing current injection and pre-synaptic nerve stimulation. Ion substitution experiments suggest that the conductance decrease is primarily to chloride ions, although other ionic species may contribute. Short iontophoretic applications of GABA-evoked monophasic depolarizing excitatory responses, even during the hyperpolarizing response evoked by perfusion of GABA, suggesting no cross-desensitization between the mechanisms generating the initial and late phases of the biphasic response.

Electrical properties of neurons recorded from the rat supraoptic nucleus in vitro

The electrical properties of neurons in the supraoptic nucleus (so.n.) have been studied in the hypothalamic slice preparation by intracellular and extracellular recording techniques, with Lucifer Yellow CH dye injection to mark the recording site as being the so.n. Intracellular recordings from so.n. neurons revealed them to have an average membrane potential of -67 +/- 0.8 mV (mean +/- s.e.m.), membrane resistance of 145 +/- 9 M omega with linear current-voltage relations from 40 mV in the hyperpolarizing direction to the level of spike threshold in the depolarizing direction. Average cell time constant was 14 +/- 2.2 ms. So.n. action potentials ranged in amplitude from 55 to 95 mV, with a mean of 76 +/- 2 mV, and a spike width of 2.6 +/- 0.5 ms at 30% of maximal spike height. Both single spikes and trains of spikes were followed by a strong, long-lasting hyperpolarization with a decay fitted by a single exponential having a time constant of 8.6 +/- 1.8 ms. Action potentials could be blocked by 10(-6) M tetrodotoxin. Spontaneously active so.n. neurons were characterized by synaptic input in the form of excitatory and inhibitory postsynaptic potentials, the latter being apparently blocked when 4 M KCl electrodes were used. Both forms of synaptic activity were blocked by application of divalent cations such as Mg2+, Mn2+ or Co2+. 74% of so.n. neurons fired spontaneously at rates exceeding 0.1 spikes per second, with a mean for all cells of 2.9 +/- 0.2 s-1. Of these cells, 21% fired slowly and continuously at 0.1 - 1.0 s-1, 45% fired continuously at greater than 1 Hz, and the remaining 34% fired phasically in bursts of activity followed by silence or low frequency firing. Spontaneously firing phasic cells showed a mean burst length of 16.7 +/- 4.5 s and a silent period of 28.2 +/- 4.2 s. Intracellular recordings revealed the presence of slow variations in membrane potential which modified the neuron's proximity to spike threshold, and controlled phasic firing. Variations in synaptic input were not observed to influence firing in phasic cells.

The antagonism of amino acid-induced excitations of rat hippocampal CA1 neurones in vitro

1. The effects of the ionophoretic application of a number of excitatory amino acids and antagonists to the dendrites of CA1 neurones of rat hippocampal slices maintained in vitro were examined. Cells were excited by N-methyl-DL-aspartate (NMA), kainate, quisqualate, L-aspartate and L-glutamate; NMA was unique in causing cells to fire in bursts of repetitive discharges in contrast to the sustained firing seen with the other compounds. 2. D-(-)-alpha-aminoadipate (DAA) and (+/-)-2-amino-5-phosphonovalerate (APV) were selective NMA antagonists, the latter appearing to be the more potent; in addition both compounds potentiated the responses to kainate and quisqualate. L-glutamate excitations were affected less by APV than were those of L-aspartate. The antagonist properties of APV appeared to reside with the D-(-)-isomer. 3. gamma-D-glutamylglycine (DGG) in low ionophoretic doses inhibited NMA-, kainate- and aspartate-induced cell firing but at higher doses the quisqualate and glutamate responses were also decreased. 4. Kainate and NMA responses were blocked by D-(-)-2-amino-4-phosphonobutyrate (D-APB) which also had some action against the excitatory effects of L-aspartate. L-APB had no antagonistic effects, but often produced potentiation of amino acid excitations or was itself an excitant. 5. The effects of NMA and those of kainate and quisqualate were blocked by (+/-)-cis-2,3-piperidine dicarboxylate (PDA), but this compound itself had a direct excitatory effect. L-glutamate diethylester (GDEE) did not show specific antagonism of any amino acid excitations. 6. DGG and APV did not affect ACh excitations and these selective antagonists should be of value in studying the involvement of the excitatory amino acids in synaptic transmission in the hippocampus. Because they are less potent and/or have complicating direct effects PDA, GDEE, D- and L-APB may be less useful in this regard.