Long-term potentiation and depression of synaptic responses in the rat hippocampus: localization and frequency dependency

Intracellular injections of EGTA block induction of hippocampal long-term potentiation

Hippocampal long-term potentiation (LTP) is a remarkably stable facilitation of synaptic responses resulting from very brief trains of high-frequency stimulation. Because of its persistence and modest induction conditions, LTP represents a promising candidate for a substrate of memory. Some progress has been made in localizing the changes responsible for the effect; for example, it has been shown that LTP is not accompanied by changes in the fibre volleys of the test afferents or by generalized alterations of the dendrites of their target cells. However, it is unknown whether the potentiation is due to pre- or postsynaptic changes and there is evidence in favour of each (for example, see refs 5, 6). We now report that intracellular injections of the calcium chelator EGTA block the development of LTP. These results strongly suggest that LTP is caused by a modification of the postsynaptic neurone and that its induction depends on the level of free calcium.

The excitatory amino-acid antagonist gamma-D-glutamylglycine masks rather than prevents long term potentiation of the perforant path

The effect of the glutamate antagonist gamma-D-glutamylglycine on the induction of long-term potentiation in the dentate gyrus has been investigated in vivo. gamma-D-glutamyglycine (10(-3) M) perfused through a push-pull cannula into the dentate gyrus, rapidly reduced perforant path evoked potentials. Application to the perforant path of a high-frequency train (250 Hz, 500 ms), which in control animals reliably produced long term potentiation, had no effect on the evoked potentials when applied during blockade by gamma-D-glutamylglycine. This result was obtained despite the inability of gamma-D-glutamylglycine completely to inhibit the evoked potentials. However, when standard medium was reintroduced, potentiation was revealed in animals that had received the high-frequency train, whereas in animals that had received no high-frequency train during gamma-D-glutamylglycine inhibition the potentials returned only to pre-drug levels. In additional experiments, in which the dentate gyrus was continuously perfused with [3H]glutamine, and the steady state outflow of [3H]glutamate was measured, it was observed that gamma-D-glutamylglycine (10(-3) M) increased the steady state release of [3H]glutamate into the perfusate. From this result it is likely that gamma-D-glutamylglycine does not have any presynaptic inhibitory activity at the perforant path-granule cell synapse. The results indicate that a high frequency train applied to the perforant path during a period of inhibition by gamma-D-glutamylglycine was able to induce long term potentiation, whose expression was, however, masked until the glutamate antagonist was removed.(ABSTRACT TRUNCATED AT 250 WORDS)

Large long-lasting potentiation in the dentate gyrus in vitro during blockade of inhibition

The generation of long-lasting synaptic potentiation was studied in the dentate gyrus of hippocampal slices before and after blockade of inhibition by application of GABA antagonists. Field potential analysis showed that inhibition in the dentate gyrus is to a large extent dendritically located. During blockade of inhibition, induction of long-lasting potentiation was facilitated, together with a large increase in the maximal potentiation that could be produced. It is suggested that the enhanced production of long-lasting potentiation in inhibition-free slices is related to an increased postsynaptic depolarization in granule cell dendrites.

The adenosine agonist 2-chloroadenosine inhibits the induction of long-term potentiation of the perforant path

The effect of 2-chloroadenosine (5 and 10 microM) on long-term potentiation of the perforant path has been investigated. Experiments were performed in vivo using a push-pull cannula implanted in the dentate gyrus. A high intensity, high frequency train applied to the perforant path during perfusion with the adenosine agonist 2-chloroadenosine, was markedly attenuated in its ability to produce long-term potentiation. Inhibitory effects on pre- and post-synaptic calcium calcium influx are discussed.

Long-term potentiation in Schaffer collateral and commissural systems of the hippocampus: in vitro study in rats pretreated with kainic acid

Experiments were carried out on hippocampal slices from rats pretreated with a unilateral intraventricular injection of kainic acid. The kainic acid produced a lesion of the CA3 area in the hippocampus ipsilateral to the site of injection. The resulting neuroanatomically modified hippocampal slices allowed the independent study of the Schaffer collateral and commissural afferents that synapse onto CA1 pyramidal cells. The Schaffer collateral and commissural afferents were both found to independently support long-term potentiation (LTP), and the properties of the LTP were similar to those observed in slices cut from unlesioned hippocampi.

Long-term potentiation in commissural and Schaffer projections to hippocampal CA1 cells: an in vivo study in the rat

Rats were given unilateral injections of kainic acid into the lateral ventricle to produce a unilateral lesion of CA3 pyramidal cells in the hippocampus. This procedure allowed the commissural and associational Schaffer projections of the surviving contralateral CA3 field to be studied separately for their ability to sustain long-term potentiation (l.t.p.) of synaptic efficacy. Both Schaffer and commissural projections showed l.t.p. of the population excitatory post-synaptic potential (e.p.s.p.) and the population spike. No significant difference was seen in the degree or duration of l.t.p. sustained by the two projections.

Electrophysiological and biochemical sequelae of the destruction of hippocampal noradrenergic afferents by DSP4

The effects of DSP4 lesions were examined 20-53 days postlesion in the rat hippocampus. A single treatment with DSP4 produced decreases of 42-94% in the norepinephrine (NE) content of this brain region. There was, however, no effect of DSP4 treatment on either the number or affinity of beta-adrenergic receptor sites as determined by radioligand binding studies with (-)-[125I]pindolol; furthermore, there was no relationship between the concentrations of NE and the number of receptor sites in individual hippocampi. The DSP4-induced depletion of functionally releasable NE was confirmed by the loss of electrophysiological responsiveness to amphetamine in the in vitro hippocampus following such lesions. In contrast, electrophysiological responses to direct acting beta-adrenergic or alpha-adrenergic agonists were unchanged following DSP4 treatment. This finding again suggests the lack of any change in postsynaptic sensitivity. The results of this study demonstrate that while the potent noradrenergic neurotoxin DSP4 is able to reduce NE concentrations significantly in noradrenergic target regions in brain, these lesions are not necessarily associated with postsynaptic changes in adrenergic systems.

Regulation by calcium ions of glutamate receptor binding in hippocampal slices

Hippocampal slices were incubated in a Krebs-bicarbonate buffer with various concentrations of calcium and [3H]glutamate receptor binding was measured in crude synaptic membranes derived from these slices. Increasing the calcium concentration from 0 to 2.5 mM resulted in a 2.2-fold increase in the maximal number of the Na-independent [3H]glutamate binding sites without changes in their affinity for [3H]glutamate. This effect was totally blocked by the addition of the protease inhibitor leupeptin (50 microM) to the slice incubation medium. No effect was observed on the Na-dependent [3H]glutamate binding nor on the Na-independent [ 3H ]glutamate binding measured in the presence of a concentration of calcium of 250 microM. Increasing the calcium concentration also resulted in an increased proteolytic activity which was inhibited by about 70% by the addition of leupeptin. Finally, increasing the calcium concentration induced the degradation of high-molecular weight proteins, the microtubule-associated proteins (MAPs) and the 220 000 dalton doublet protein corresponding to fodrin. Both effects were partially prevented by the addition of leupeptin in the slice incubation medium. These results indicate that the same calcium-dependent processes which were previously shown to regulate [ 3H ]glutamate receptor binding to hippocampal membranes occur in the hippocampal slice preparation, and they suggest a mechanism by which fluctuations in calcium levels can activate a calcium-dependent proteinase, the degradation of cytoskeletal-associated proteins and the unmasking of additional glutamate receptors. The participation of such processes in various forms of plasticity is discussed.

Differential sensitivity of cerebellar purkinje neurons to ethanol in selectively outbred lines of mice: maintenance in vitro independent of synaptic transmission

The effects of ethanol on spontaneous firing of cerebellar Purkinje neurons were examined in outbred lines of mice (short-sleep, SS; and long-sleep, LS) which exhibit differential behavioral sensitivity to ethanol. In order to determine whether the differences in Purkinje cell ethanol sensitivity which are observed in situ reflect differences in intrinsic properties of Purkinje neurons, we developed an isolated in vitro preparation of mouse cerebellum. Even when synaptic transmission was largely inhibited by elevating Mg2+ and decreasing Ca2+ concentrations, Purkinje cells demonstrated stable long-term firing rates quite similar to those observed in vivo. Purkinje cells responded to superfusion of ethanol with both increases and decreases in firing rate. Inhibition of rate was more commonly observed, and was the only response which was demonstrably dose-dependent. The differential sensitivity to ethanol which we have previously reported in vivo was maintained even under under these conditions, with the LS mice being approximately 5 times more sensitive to the depressant effects of ethanol. In addition, it was shown that ethanol, at the concentrations used in these experiments, decreased the amplitude and increased the duration of single action potentials. Thus, taken together, these results suggest that the differential sensitivity of outbred lines to the soporific effects of ethanol are paralleled by differences in the sensitivity of Purkinje neurons in vitro to superfusion with ethanol. Because these differences can be observed even when synaptic transmission is largely suppressed, it would appear that these differences are intrinsic to the purkinje neurons themselves.

Anticonvulsant and proconvulsant actions of alpha- and beta-noradrenergic agonists on epileptiform activity in rat hippocampus in vitro

The ability of l-norepinephrine to influence epileptiform activity was examined in an in vitro rat hippocampal slice preparation. Exogenously applied norepinephrine (NE) had anticonvulsant properties in that it slowed or stopped spontaneous interictal discharges which had been initiated by superfusion of slices with medium containing penicillin and elevated levels of potassium. This anticonvulsant property of NE was shared by the alpha receptor agonists 6-fluoro-norepinephrine, l-alpha-methyl-norepinephrine, and clonidine, but not by d-alpha-methyl-norepinephrine or l-phenylephrine. The beta receptor agonists 2-fluoro-norepinephrine and l-isoproterenol, on the other hand, were proconvulsant in that they increased interictal discharge rate. The alpha receptor antagonist phentolamine selectively blocked anticonvulsant responses, whereas the beta receptor antagonist timolol selectively blocked proconvulsant activity. These results suggest that activation of alpha or beta receptors has opposing inhibitory or excitatory effects respectively on epileptiform discharges in the rat hippocampus.

A Boolean complete neural model of adaptive behavior

A multi-layered neural assembly is developed which has the capability of learning arbitrary Boolean functions. Though the model neuron is more powerful than those previously considered, assemblies of neurons are needed to detect non-linearly separable patterns. Algorithms for learning at the neuron and assembly level are described. The model permits multiple output systems to share a common memory. Learned evaluation allows sequences of actions to be organized. Computer simulations demonstrate the capabilities of the model.

Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus

1. The effects of excitatory amino acids and some antagonists applied by ionophoresis to stratum radiatum in the CA1 region of rat hippocampal slices were examined on the locally recorded field e.p.s.p. evoked by stimulation of the Schaffer collateral-commissural projection. 2. L-glutamate, L-aspartate and the more potent and selective excitatory amino acids quisqualate, kainate and N-methyl-DL-aspartate (NMA) depressed the e.p.s.p., presumably through depolarization and/or a change in membrane conductance. 3. The depression induced by kainate considerably outlasted the period of ejection whereas NMA depressions were rapidly reversible and were often followed by a potentiation of the e.p.s.p. In higher doses NMA also depressed the presynaptic fibre volley. The possible involvement of these effects in neurotoxicity and synaptic plasticity is raised. 4. The selective NMA antagonist, DL-2-amino-5-phosphonovalerate (APV) applied in doses which abolished responses to NMA, had no effect on the e.p.s.p. but prevented long term potentiation (l.t.p.) of synaptic transmission evoked by high frequency stimulation of the Schaffer collateral-commissural pathway. Other antagonists which had little or no effect on normal synaptic transmission included D-alpha-aminoadipate (DAA), the optical isomers of 2-amino-4-phosphonobutyrate (APB) and L-glutamate diethylester (GDEE). 5. In contrast, gamma-D-glutamylglycine (DGG), applied in amounts which affected quisqualate and kainate actions as well as those of NMA, was an effective synaptic antagonist whilst having no effect on the presynaptic fibre volley. 6. These results indicate that the synaptic receptor in the Schaffer collateral-commissural pathway may be of the kainate or quisqualate type. Although NMA receptors do not appear to be involved in normal synaptic transmission in this pathway they may play a role in synaptic plasticity. The interaction of L-glutamate and L-aspartate with these receptors is discussed.

Modulation of long-term potentiation: effects of adrenergic and neuroleptic drugs

A variety of drugs which either mimic or antagonize the effects of norepinephrine and dopamine were tested for their ability to modulate long-term potentiation (LTP) in the rat hippocampus in vitro. Neither administration of norepinephrine, amphetamine or adrenergic antagonists, nor pretreatment with reserpine or DSP4 (which selectively destroys noradrenergic afferents to the hippocampus) had any significant effect on the magnitude of LTP. Isoproterenol, a beta-adrenergic receptor agonist, was able to partially block LTP, but this did not appear to be due to a direct action of isoproterenol on LTP. Neuroleptic drugs such as trifluoperazine were able to block LTP almost completely; however, this action was not stereospecific. Other dopamine antagonists such as sulpiride had no effect on LTP. The ability of neuroleptics to antagonize LTP was more closely related to their ability to block calmodulin than to their relative potencies as dopamine antagonists. It would appear that neither norepinephrine nor adrenergic antagonists influence the amount of LTP elicited by repetitive stimulation; however, drugs which have been shown to interfere with calmodulin-mediated cellular processes do antagonize this phenomenon.

Vincamine: a psychogeriatric agent blocking synaptic potentiation in hippocampus

The action of vincamine on the physiology of the CA1 region of the in vitro hippocampal slice preparation was investigated. At concentrations of 1, 10 and 100 microM, a five-minute perfusion with vincamine did not affect the synaptically-mediated activation of pyramidal neurons evoked by stimulation of the Schaffer-commissural fiber system. The effect of vincamine on the excitability of the pyramidal neurons was investigated by studying its effect on the antidromically-elicited field potential and the input-output relation of Schaffer-commissural fiber input. No effect on either of the two parameters was seen at a concentration of 100 microM of vincamine. Vincamine did, however, attenuate both the post-tetanic (PTP) and long-term potentiation (LTP) evoked by repetitive stimulation of the Schaffer-commissural fiber system. At a concentration of 100 microM of vincamine, PTP was significantly reduced and LTP was almost completely suppressed.

Effects of high-frequency synaptic stimulation on glumate receptor binding studied with a modified in vitro hippocampal slice preparation

Slices of the field CA1 minus the stratum moleculare were prepared from the rat hippocampus and maintained in an in vitro recording chamber. The physiological properties of these "minislices" were similar to those reported for non-dissected hippocampal slices. Slices receiving various patterns of electrical stimulation through multiple electrodes were subsequently homogenized and crude membrane fractions prepared; the binding of [3H]glutamate was measured by a rapid filtration assay. Binding in membranes prepared from control slices exhibited kinetic properties and sensitivities to pharmacological and ionic manipulations which were comparable to those found in previous studies using conventional fractionation and assay techniques. Brief bursts of high-frequency stimulation increased [3H]glutamate binding compared to non-stimulated controls in 3 separate experiments. Stimulation at low frequency or at high frequency in low calcium medium did not produce this effect. In addition to suggesting that glutamate binding sites are regulated by patterns of afferent activity, these findings indicate that the minislice preparation should be of general utility in relating synaptic physiology to synaptic chemistry.

Stereoselectivity of opiate antagonists in rat hippocampus and neocortex: responses to (+) and (-) isomers of naloxone

The relative potencies of the (+) and (-) isomers of naloxone in antagonizing electrophysiological responses to D-alanine2-methionine enkephalinamide were compared in rat frontal cortex and hippocampus. In the in vitro hippocampus, the (-) isomer was found to be at least a 100 times more potent than the (+) isomer in antagonizing opiate-induced changes in field potentials. Similar stereoselectivity was observed in vivo in both frontal cortex and hippocampus in terms of the antagonism of enkephalin-induced changes in spontaneous cell firing. The direct effects of (+) and (-)-naloxone were examined as well. In hippocampus both in vivo and in vitro, no differential effect was observed, whereas in the neocortex (-)-naloxone was considerably more potent than the (+) isomer in eliciting depressions of spontaneous activity. These direct effects of naloxone in the cortex do not appear to be due to an antagonism of the effects of endogenously released opioids. These results demonstrate that the stereoselectivity of naloxone isomers in antagonizing electrophysiological responses to opiates in the cortex and hippocampus parallels that previously observed in other brain regions and in other tissues. In addition, they suggest that naloxone may have interactions with other unknown opiate (or possibly non-opiate) receptors which are of physiological significance.

Calcium-induced long-term potentiation in the hippocampus

The effect of a transient increase in extracellular calcium concentration on the Schaffer collateral-commissural evoked excitatory postsynaptic potential and population spike responses of CAI pyramidal neurons was investigated using the rat in vitro hippocampal slice preparation. Brief exposure of slices (5-10 min) to twice the normal concentration of calcium (4 mM) induced a marked potentiation of both the excitatory postsynaptic potential and population spike that could persist for at least 3 h. No long-term changes were observed in either the presynaptic fiber volley of antidromically evoked CAI population spike, indicating that the potentiation could not be attributed to an increase in the number of fibers activated or a generalized increase in cellular excitability. The response of CAI pyramidal neurons to the iontophoretic application of L-glutamate in the apical dendritic zone was also unaffected after exposure to high calcium perfusate, suggesting a lack of alteration in membrane excitability or receptor sensitivity restricted to the region of synaptic input. In addition, total intracellular calcium content of individual slices, measured by atomic absorption spectrophotometry, was significantly increased for at least 1 h following return to the control medium. These data indicate that brief exposure of in vitro hippocampal slices to a high extracellular calcium concentration results in a long-term increase in synaptic efficacy which is similar in many respects to long-term potentiation induced by tetanic stimulation of hippocampal excitatory afferents. The results further suggest that the mechanisms underlying calcium-induced long-term potentiation may reside in presynaptic components and involve an enhanced transmitter release.

Hippocampal noradrenergic responses in vivo and in vitro. Characterization of alpha and beta components

Pressure ejection of l-norepinephrine (NE) in the in vivo rat hippocampus generally produced depression of pyramidal cell spontaneous activity. In addition, both excitation and biphasic responses were observed. NE-induced inhibition of firing rate was effectively antagonized by concurrent administration of the alpha antagonist phentolamine, but was largely unaltered by the beta antagonist timolol. On the other hand, NE-induced elevation in spontaneous firing rate was effectively blocked by timolol, and largely unaffected by phentolamine. Another beta antagonist, sotalol, did not selectively antagonize either NE-induced inhibition or NE-induced excitation. The beta agonist 2-fluoro-NE produced increases in pyramidal cell firing rates in most cells studied, while the alpha agonist 6-fluoro-NE inhibited the majority of cells examined. The effects of sotalol were also examined on alpha and beta receptor-mediated field responses in the in vitro hippocampal slice. Sotalol was shown to be a selective beta antagonist in this system, blocking excitation evoked by the beta agonist isoproterenol while having no effect on inhibition elicited by the alpha agonist clonidine; however, the potency of sotalol (Ki = 3.5 microM) was considerably less than that of timolol (Ki = 50 nM). Taken together, these results suggest that NE-induced depression and elevation in hippocampal pyramidal cell spontaneous discharge in vivo are mediated via alpha and beta adrenoceptors, respectively.

Interactions of a neuroleptic drug (fluphenazine) with catecholamines in hippocampus

The interactions of fluphenazine with the electrophysiological responses to catecholamines were studied in the rat hippocampus and parietal cortex. In the in vitro hippocampal slice, changes in synaptically evoked responses induced by norepinephrine, isoproterenol and dopamine were not altered by superfusion of fluphenazine. Both alpha- and beta- components of adrenergic responses were unaffected by neuroleptic administration in this preparation. Similarly, alterations in the spontaneous firing of single hippocampal pyramidal neurons in situ to adrenergic agonists or dopamine were not affected by local fluphenazine and administration using pressure ejection through multibarreled micropipettes. In contrast, norepinephrine- or isoproterenol-induced inhibitions of parietal cortical neurons in situ were potently antagonized by fluphenazine. A similar interaction was observed from a hippocampal basket neuron. It is concluded that while fluphenazine can antagonize well-defined noradrenergic effects in some brain regions (e. g., cerebellum, cortex), this property is not generalized to all brain regions receiving noradrenergic input.

Long-term potentiation of excitatory synaptic transmission in the rat hippocampus: the role of inhibitory processes

1. The possibility that changes in inhibitory processes are responsible for long-term potentiation (l.t.p.) was examined using the rat hippocampal slice preparation.2. Inhibitory pathways were characterized using both extra- and intracellular recordings from the CA1 pyramidal cell layer. Stimulating electrodes were placed in either stratum radiatum or the alveus to allow orthodromic or antidromic activation of the pyramidal cells.3. Using extracellular recordings, inhibition was studied by applying paired pulses at interstimulus intervals of 20-500 msec through either the same or different stimulating electrodes, and quantifying the reduction in the population spike. An antidromic conditioning pulse was least effective in influencing the test response, while paired stimuli delivered through separate stimulators in stratum radiatum revealed the longest duration effects. Inhibition was either reduced or enhanced, depending upon the stimulation paradigm, with increasing stimulus intensity.4. With l.t.p., alterations in paired-pulse inhibition were observed corresponding to the changes in conditioning pulse amplitude. Reducing stimulus intensity to restore the initial conditioning pulse amplitude eliminated these effects.5. Using intracellular recordings, the effects of l.t.p. on inhibition were studied by examining changes in e.p.s.p.-i.p.s.p. sequences, i.p.s.p.s evoked by antidromic stimulation, and spontaneous depolarizing i.p.s.p.s observed with KCl-filled electrodes.6. Following l.t.p. enhanced e.p.s.p.s and slightly reduced, but prolonged, i.p.s.p.s were observed in response to orthodromic stimulation. Antidromically evoked, as well as spontaneous, i.p.s.p.s were unaffected.7 It is concluded that alterations in inhibitory processes are not responsible for l.t.p. in hippocampal subfield CA1. However, changes in the strength of inhibitory synapses as a consequence of long-term potentiation may modify the functional character of the hippocampal connexions.

Long-term potentiation investigated in a slice preparation of striate cortex of young kittens

In slice preparations from kitten striate cortex, long-term potentiation (up to 15 h) of field potentials was demonstrated after conditioning stimulation of white matter. Current source-density analysis and measurement of response latencies of cortical units indicated that the potentiation represented enhancement of both geniculo-cortical transmission in layer IV and intracortical transmission in layer II--III.

Augmentation of short-term plasticity in CA1 of rat hippocampus after chronic ethanol treatment

The neurotoxic effects of chronic ethanol exposure were investigated in rat hippocampus by electrophysiological analysis of the Schaffer collateral-commissural input to stratum radiatum of CA1. Experimental animals were fed an ethanol-containing liquid diet for 20 weeks but were withdrawn from the special diet at least eight weeks prior to acute electrophysiological recordings. Ethanol treatment had no effect on input-output relationships for either the population EPSP or the population spike (PS). During paired-pulse stimulation, the ethanol group exhibited a greater facilitation of the test pulse PS relative to the control group, although potentiation of the EPSP was unchanged. In addition, the ethanol group showed a trend toward greater facilitation of the PS during 5 and 10 Hz tetani. No differences between groups were observed in the magnitude or duration of the long-term potentiation produced by 5, 10 or 100 Hz stimulus trains. Ethanol treatment did significantly reduce the transient spike depression after low frequency stimulation. This pattern of results is similar to that found for treatments which reduce hippocampal recurrent inhibition. Thus, chronic ethanol treatment may produce a lasting disruption of intrinsic inhibitory neurotransmission in the rat hippocampus.

Calcium uptake and retention during long-term potentiation of neuronal activity in the rat hippocampal slice preparation

Alterations in 45Ca uptake and retention as well as total intracellular calcium content were measured in hippocampal 'in vitro' slice preparations following the induction of long-term potentiation (LTP) in CA1 pyramidal neurones. The results indicate that tetanic stimulation of the Schaffer-commissural input produces a significant increase in the uptake and retention of calcium which parallels LTP of the CA1 population spike response.

Hippocampal glutamate receptors

For years, the hippocampus has been the privileged domain of anatomists and electrophysiologists for investigating various neurobiological processes. The present review deals with recent work which shows that this structure is also well suited to study the role of glutamate as a neurotransmitter and more particularly the characteristics of glutamate receptors and their possible involvement in hippocampal function. After a brief description of the main anatomical features of the hippocampus, we attempt a critical evaluation of the electrophysiological studies of hippocampal glutamate receptors. We then describe the properties of Na-independent 3H-glutamate binding sites in hippocampal membranes, and discuss the possibility that these binding sites are related to postsynaptic glutamate receptors. Finally we show that these binding sites are extremely labile and that hippocampal membranes possess various mechanisms which regulate their number. In particular we develop the idea that the calcium-stimulation of 3H-glutamate binding in hippocampal membranes may be the mechanism by which electrical activity regulates the number of glutamate receptors at hippocampal synapses and thus induces long-lasting changes in synaptic transmission.

Alkylxanthines elevate hippocampal excitability. Evidence for a role of endogenous adenosine

The effects of four xanthines (theophylline, 8-phenyltheophylline, isobutylmethylxanthine (IBMX), and 7-benzyl-IBMX) were studied in the hippocampal slice in vitro. These agents increased the excitability of this preparation with 8-phenyltheophylline being the most potent, 7-benzyl-IBMX the least potent, and theophylline and IBMX having intermediate potencies. A similar rank order was observed in terms of the potencies of these xanthines in antagonizing a) electrophysiological responses to adenosine, and b) adenosine-stimulated cyclic AMP formation. These results indicate that the excitatory actions of xanthines in the vivo hippocampus can be most easily explained on the basis of their ability to block adenosine's actions; the CNS excitatory actions of these drugs in vivo may depend upon a similar mechanism of action.

Noradrenergic responses in rat hippocampus: evidence for medication by alpha and beta receptors in the in vitro slice

The effect of perfused norepinephrine (NE) on evoked potentials in CA1 of the in vitro rat hippocampus was examined. Weak and variable effects on population spike amplitude were observed, with lower doses of NE generally producing excitations and higher doses more often producing inhibitions. Clonidine, an alpha-receptor agonist, produced a dose-dependent inhibition of population spike amplitude; this inhibition was effectively antagonized by the alpha-antagonist, phentolamine. Isoproterenol (ISO), a beta-agonist, produced marked increases in population spike amplitude which could be antagonized by timolol, a beta-receptor antagonist. Phentolamine did not antagonize the excitations produced by ISO, and timolol had no effect on the inhibitions seen with clonidine. After pretreatment with either phentolamine or timolol, NE perfusion elicited robust and consistent elevations or reductions in the population spike, respectively. A potent cyclic AMP derivative, 8-p-chlorophenylthio cyclic AMP, produced large increases in population spike amplitude which appeared similar to the responses seen with beta-agonists. No changes in field EPSP amplitudes were observed with any of the drugs tested. Taken together, these results suggest that NE may interact with alpha-adrenergic receptors to decrease pyramidal cell excitability, and the beta-adrenergic receptors to increase pyramidal cell excitability; the beta-effect may involve cAMP.

Ethanol-induced depressions in cerebellar and hippocampal neurons of mice selectively bred for differences in ethanol sensitivity: an electrophysiological study

The recently discovered profound differential sensitivity of cerebellar Purkinje (P) cells in long-sleep (LS) verus short-sleep (SS) mice to the depressant effects of locally applied ethanol was extended in this study. First, the sensitivity of Purkinje neurons from HS mice (an outbred stock of mice from which the LS and SS lines were derived), was found to be almost exactly intermediate between the values for the long-sleep and short-sleep animals. Second, no differential sensitivity in long-sleep versus short-sleep hippocampal pyramidal neurons was observed. This was true using both spontaneous and evoked activity. Third, no differential sensitivity of P cells was seen in long- versus short-sleep mice with local application of halothane. Taken together with previous reports, these data strongly suggest that whatever genetically determined central nervous alterations result in the differential soporific effects of ethanol in the two (LS and SS) mouse lines, such alterations are brain region- and depressant drug-specific rather than generalized.

Development of glutamate binding sites and their regulation by calcium in rat hippocampus

The postnasal development of the Na-independent [3H]glutamate binding sites, which exhibit some characteristics of postsynaptic glutamate receptors, has been studied in rat hippocampal membranes. The amount of binding sites (expressed in pmol/hippocampus) represents 4% of the adult level at postnatal day (PND) 4, increases very rapidly until PND 9, and then increases at a slower rate reaching 80% of the adult value at PND 23. In contrast, the density of binding sites (expressed in pmol/mg protein) exhibits a maximum at PND 9 and slowly decreases to reach the adult value at PND 23. These changes seen to be only quantitative since the affinity (about 450nM) and Hill coefficient (about 1.0) of these binding sites remain constant throughout development. Calcium ions have been shown to markedly stimulate [3H]glutamate binding in adult hippocampal membranes. This effect appears on PND 9--10 and increases rapidly until PND 16 when it is similar to that seen in the adult rat. We also determined the minimum age at which long-term potentiation (LTP) of synaptic transmission could be detected in the CA1 field of hippocampal slice preparations following repetitive electrical stimulation of the Schaffer-commissural pathways. LTP was only rarely detected at PND 8 whereas it could be reliably obtained after PND 9. These results indicate that the postnatal development of Na-independent glutamate binding sites closely parallels synapse formation in the hippocampus, further supporting the idea that the binding sites are associated with a physiological receptor. They also show that the appearance of the stimulatory effect of calcium on glutamate binding occurs at a time when several forms of synaptic plasticity appear in the hippocampus. In particular the correlation of the development of LTP with the calcium-stimulation of glutamate binding suggests that these phenomena have similar cellular mechanisms.

Changes in synaptic membrane phosphorylation after tetanic stimulation in the dentate area of the rat hippocampal slice

Slices of rat brain hippocampus were stimulated electrically in the perforant pathway. After electrical stimulation, known to produce long-lasting post-tetanic potentiation, endogenous phosphorylation of membrane proteins was measured in a crude mitochondrial fraction, prepared from stimulated and unstimulated slices. Tetanic stimulation specifically enhanced the incorporation of [32P]phosphate into a protein band with apparent molecular weight of 50K. When the same number of pulses were given, but at a much slower rate (1 pulse per 4 sec instead of 15 pulses per sec) no posttetanic stimulation and concomitantly, no enhanced incorporation of [32P]phosphate were observed into the 50K band. When the stimulation of the slices was performed in Ca2+-free medium, again no potentiation and no enhanced incorporation into the 50K protein band were observed. It is suggested that electrical stimulation enhances the activity of the protein kinase that phosphorylates the 50K protein.

Endogenously released adenosine regulates excitability in the in vitro hippocampus

The role of adenosine in regulating epileptiform discharge was studied in the in vitro hippocampal slice preparation. Exogenously applied adenosine appeared to have anticonvulsant properties (EC50 approximately 10 microM), in that it slowed or suppressed spontaneous interictal discharges. In addition, the release of adenosine (or related adenine nucleotides) from the slice preparation appeared to exert a tonic inhibitory influence on these types of discharges. This provides further evidence supporting a role for adenosine as an endogenous regulator of the excitability of the central nervous system, particularly in convulsive states.

Trifluoperazine inhibits hippocampal long-term potentiation and the phosphorylation of a 40,000 dalton protein

Brief high frequency stimulation induces long-term potentiation (LTP) and changes in the endogenous phosphorylation of a 40,000 dalton protein in the hippocampus in a calcium-dependent manner. In the present paper we report that 40 microM trifluoperazine (TFP), a phenothiazine that binds calmodulin and blocks its activity, inhibits LTP in the hippocampal slice. In addition, calmodulin stimulates and TFP inhibits the phosphorylation of the 40,000 dalton protein (as well as that of several other proteins) in a dose-dependent fashion.

Strontium supports synaptic transmission and long-lasting potentiation in the hippocampus

(1) Synaptic transmission was studied in isolated transverse hippocampal slices from guinea pigs. Extracellular evoked potentials were recorded in the region CA1. (2) Changing the normal perfusion solution (containing 2 mM Ca2+) to calcium-free Ringer abolished synaptic transmission which was again restored by adding strontium. A synaptic efficacy of 25--50% ofn normal was obtained for 10 mM Sr2+. (3) Two different synaptic inputs to CA1 pyramidal cells were tested with respect to their ability to produce long-lasting synaptic potentiation after tetanization in strontium Ringer. Following a brief tetanus the field EPSP and, especially, the population spike were greatly enhanced. (4) The potentiation so produced was similar to the long-lasting potentiation seen in the normal slice, because it (i) had a very long duration (hours), (ii) was specific for the tetanized pathway, (iii) showed potentiation of both 'volley-EPSP' and 'EPSP-spike' relations, and (iv) was accompanied by short-lasting (less than 5 min) generalized depression.

Potentiating hippocampal stimulation facilitates acquisition of lever-pressing for stimulation but not food

The effect of a prior program of daily hippocampal stimulation on acquisition of lever-pressing for food, and subsequently for stimulation, was studied in 3 groups of rats--stimulated over 30 days with a single daily train (60 Hz) of potentiating stimulation (kindling), stimulated with an equal number of pulses at a non-potentiating frequency (1 Hz), and unstimulated controls. Rate of acquisition of lever-pressing for food pellets was unaffected by either stimulation pretreatment. When reinforcement was switched to hippocampal stimulation, acquisition was very slow for animals which had received no previous potentiating stimulation (1 Hz and unstimulated groups), but significantly faster for the 60 Hz animals. The results suggest that the faster acquisition of lever-pressing in kindled animals is specific to hippocampal reinforcement and that the effect is dependent on the potentiating consequences of the stimulation pretreatment.

Possible mechanisms for long-lasting potentiation of synaptic transmission in hippocampal slices from guinea-pigs

1. Long-lasting potentiation of synaptic transmission was studied in the CA1 region of guinea-pig hippocampal slices maintained in vitro. 2. Stimulating pulses were delivered alternately to two independent afferent pathways, stratum radiatum and stratum oriens. The presynaptic volleys and field e.p.s.p.s. were recorded from the same two layers, while an electrode in the pyramidal cell body layer recorded the population spike or in other experiments the extra- or intracellular potentials from a single pyramidal cell. 3. A short tetanus to either of the two input pathways produced a long-lasting enhancement of the field e.p.s.p. as well as an increased size and a reduced latency of the population spike. This long-lasting potentiation was observed for up to 110 min after tetanization. Extracellular unit recordings showed that this potentiation is accompanied by an increased probability of firing and a reduced firing latency. Intracellular recordings showed an increased e.p.s.p., through the increase was smaller and less regular than for the extracellular field e.p.s.p. 4. No corresponding changes were seen in the field potential responses to stimulation of the untetanized input path, or in the intracellularly measured soma membrane potential, resistance, or excitability. The latter two properties were measured by intracellular injection of current pulses. It is concluded that long-lasting potentiation is specific to the pathway which has received the tetanization. 5. Following tetanization there was also a short-lasting (usually 2-4 min) depression, most often seen for the control pathway but sometimes visible on the tetanized side as well, superimposed on the potentiation. It is concluded that the short-lasting depression is not confined to any particular pathway but is a generalized (unspecific) phenomenon.

Adenine nucleotides and synaptic transmission in the in vitro rat hippocampus

1 The effects of adenosine and various derivatives were examined in the in vitro hippocampal slice preparation from rat.2 The amplitudes of extracellularly recorded field potentials from the CA1 region were depressed by adenosine, and this effect could be antagonized by methylxanthines. Because presynaptic field potentials were unaffected by adenosine, while the field e.p.s.p. was depressed, adenosine would appear to act at a synaptic site to depress transmission.3 Adenosine deaminase, which breaks down adenosine to inosine, increased the amplitude of synaptic responses, while hexobendine, which blocks reuptake of adenosine, had a depressant effect. This strongly suggests that the endogenous release of adenosine from the hippocampal slice preparation is sufficient to exert a tonic inhibitory influence on the amplitude of synaptic responses.4 Cyclic adenosine 3',5'-monophosphate (cyclic AMP) and its dibutyryl derivative had depressant effects on the amplitude of field responses which were blocked by theophylline, suggesting that they are able to act at the extracellular adenosine receptor. (-)-Isoprenaline (which raises tissue cyclic AMP levels), and the 8-p-chlorophenylthio derivative of cyclic AMP both increased the amplitude of population spike responses, and these effects were not blocked by theophylline, suggesting that the physiological effects of adenosine are not mediated via a cyclic AMP-dependent mechanism.5 Since adenosine is not the transmitter at this CA1 pyramidal cell synapse, but is apparently present in the extracellular compartment in sufficient concentrations to affect the synaptic physiology of this region, this provides strong evidence in favour of the concept of a neuromodulatory role for adenosine in the central nervous system.

Electrophysiological interactions of enkephalins with neuronal circuitry in the rat hippocampus. I. Effects on pyramidal cell activity

Effects of enkephalins on hippocampal pyramidal cell activity were studied in situ and in the in vitro hippocampal slice. Active enkephalin derivatives produced a dose-dependent naloxone-reversible excitation in both preparations whereas inactive enkephalin derivatives had no effect. Several different types of experiments, carried out in the slice, strongly suggest that this excitation is due to blockade of inhibitory pathways. First, when the pyramidal cell population spike is increased during enkephalin administration, no change is seen in the simultaneously recorded EPSP. Second, the magnitude of the enkephalin effect is highly correlated with the amount of inhibition, as judged by paired-pulse stimulation, initially present in the slice. Third, if inhibitory pathways are depressed by a brief period of hypoxia, enkephalin has little effect. Finally, enkephalin responses are mimicked by picrotoxin, which selectively antagonizes inhibitory input to the pyramidal neuron. Since enkephalins do not block the effects of GABA, the putative inhibitory transmitter, these data suggest that opioid peptides depress the inhibitory interneurons and disinhibit the pyramidal cells.

Electrophysiological interactions of enkephalins with neuronal circuitry in the rat hippocampus. II. Effects on interneuron excitability

The effects of active and inactive enkephalin derivatives and naloxone on putative interneurons were studied in the in vitro hippocampal slice. Inhibitory interneurons were recorded from extracellularly, and identified electrophysiologically on the basis of their characteristic action potential shape and pattern of evoked firing in response to single and multiple electrical stimuli. Active enkephalin derivatives elicited a dose-dependent depression in excitability whereas inactive derivatives had no effect. Naloxone reliably and reproducibly antagonized the depressant action of active enkephalins. These data confirm the hypothesis outlined in the preceding communication, that the direct effect of enkephalins in the hippocampus is a depression of firing of inhibitory neurons, and support the hypothesis that enkephalin-induced excitations of pyramidal cells are brought about by disinhibition.