Population Spike Responses Research Articles

Acetylcholine mediates the effects of fenfluramine on dentate granule cell excitability in the rat.

Reactivity of the hippocampal system to stimulation of its main afferent, the perforant path, was studied in the intact, anesthetized rat. Parentral administration of fenfluramine caused a marked elevation of population spike response to perforant path stimulation. An injection of atropine before, but not after fenfluramine, blocked the potentiating effect of fenfluramine. The atropine blockade was dose-dependent and not mimicked by the peripheral muscarinic receptor antagonist methyl atropine. This effect of fenfluramine was also prevented by an injection of the 5-HT receptor antagonist spiperone. The effect of fenfluramine was mimicked by the anticholinesterase physostigmine, which was not affected by spiperone pretreatment. It is proposed that release of 5-HT (5-hydroxytryptamine) by fenfluramine potentiates reactivity to afferent stimulation by interacting with cholinergic terminals in the hippocampus.

Multiple effects of long-term morphine treatment on postsynaptic β-adrenergic receptor function in hippocampus: an intracellular analysis

We previously reported that β-adrenergic receptors are increased in cerebral cortex and hippocampus in rats treated chronically with morphine and subsequently down-regulated after morphine withdrawal [22,23]. The changes in receptor density in hippocampus were accompanied by a corresponding super- and subsensitivity, respectively, in β-adrenergic responsiveness, as assessed electrophysiologically by measuring the ability of isoproterenol to augment population spike responses in the slice. In this study, we compared the ability of isoproterenol to reduce the Ca 2+-activated K + slow afterhyperpolarization (slow AHP) in pyramidal neurons in hippocampal slices from opiate-naive and chronic morphine-treated rats to determine whether such changes in β-adrenergic receptor function are localized postsynaptically. Chronic treatment of rats with morphine produced a 3.5-fold parallel shift to the left in the concentration-response curve for isoproterenol and reduced the EC 50 from 4.8 ± 1.3 to 1.4 ± 0.5 nM. In contrast, sensitivity and maximal responsiveness to isoproterenol was markedly decreased in pyramidal neurons recorded in slices from morphine withdrawn animals. The concentration-response curves for inhibition of the slow AHP by carbachol or forskolin were not affected by chronic morphine treatment. However, blockade of the slow AHP by forskolin was significantly reduced in pyramidal neurons studied after morphine withdrawal. These data suggest that the increase in electrophysiological responsiveness to β-adrenergic receptor stimulation found in hippocampus after chronic morphine treatment most likely resulted from an up-regulation in postsynaptic membrane receptors, whereas alterations occurring beyond the receptor level may be involved in the desentization that is associated with morphine withdrawal.

Inhibition of nitric oxide synthase protects against hypoxic neuronal injury

We investigated the action of nitric oxide in hypoxic neuronal injury, using the hippocampal slice. Inhibition of nitric oxide synthase with the competitive inhibitor, NG-monomethyl-L-arginine, provided significant protection against hypoxia for population spike, EPSP and fiber volley responses, with an EC50 of 30 microM for protection of antidromic population spike amplitude. Confirming a stereo-specific site of action, this protection was reversed by the addition of L-arginine, but not D-arginine. These results indicate that nitric oxide generation may mediate acute CA1 neuronal injury during hypoxia, and that inhibition of nitric oxide synthase may be a useful neuroprotective strategy.

In vivo modulation of N-methyl- d- aspartate receptor-dependent long-term potentiation by the glycine modulatory site

The role of the glycine modulatory site in N-methyl- d-aspartic receptor function was examined by determining the effect of the glycine site antagonist, 7-chlorokynurenic acid, on the induction of long-term potentiation at the commissural-CA1 synapse in anesthetized rats. Robust long-term potentiation of population excitatory postsynaptic potentials and population spike responses recorded extracellularly in the stratum pyramidale and in stratum radiatum of CA1 developed after high frequency stimulation (100 Hz for 1 s) of commissural fibers during continuous intrahippocampal administration of vehicle solution (0.15 M NaCl). In contrast, infusion of either 7-chlorokynurenic acid (400 μM) or of the N-methyl- d-aspartic receptor antagonist, d-2-amino-5-phosphonovaleric acid (100 μM), significantly attenuated or completely blocked the development of long-term potentiation. When 7-chlorokynurenic acid was infused together with the glycine analog, d-serine (1 mM), long-term potentiation developed that was comparable to that observed in control animals. Intrahippocampal administration of d-serine alone was associated with slightly greater magnitude of long-term potentiation than observed in control animals. Collectively, these findings establish that in intact hippocampus, activity at the glycine modulatory site is necessary for activation of the N-methyl- d-aspartic receptor complex. Furthermore, these results suggest that the glycine modulatory site may not be fully saturated in vivo, and thus can serve to regulate N-methyl- d-aspartate receptor function.

Kappa-opioids decrease excitatory transmission in the dentate gyrus of the guinea pig hippocampus

In the guinea pig hippocampus, kappa 1-opioid binding sites were primarily localized in the molecular layer of the dentate gyrus as shown by autoradiography using either the kappa 1-selective radioligand 3H-U69,593 or the nonselective radioligand 3H-diprenorphine in the presence of unlabeled mu- and delta-blocking ligands. In this region, the electrophysiological effects of kappa 1-receptor activation were identified using extracellular and intracellular recordings of dentate granule cell responses. The amplitude of the extracellularly recorded population spike was reduced by U69,593 with an EC50 of 26 nM; this effect was reversible and blocked by the opioid antagonist naloxone. The kappa 1-selective antagonist norbinaltorphimine also blocked the effect of U69,593 with an apparent equilibrium dissociation constant (Ki) of 0.26 nM determined by Schild analysis in the physiologic assay. This value agreed well with the Ki for norbinaltorphimine at kappa 1-binding sites measured by radioligand binding displacement (0.24 nM). These results indicate that the electrophysiologic response observed was likely mediated by kappa 1-receptors. As seen with U69,593, dynorphin B, an endogenous opioid peptide that is present in the dentate gyrus, also inhibited the population spike response. mu- and delta-selective opioid agonists had no effect on the amplitude of the maximally evoked response. Intracellular recordings of dentate granule cells showed no direct effects of U69,593 on the granule cells themselves. However, analysis of synaptic potentials revealed that U69,593 significantly reduced the amplitude of glutaminergic EPSPs evoked by afferent stimulation without affecting IPSP amplitudes. The specific effect of U69,593 application on granule cell EPSPs indicates that presynaptic kappa 1-receptor activation inhibits glutamate release from perforant path terminals in the molecular layer of the dentate gyrus. These results suggest that endogenous dynorphins present in the granule cells may act as feedback inhibitors of the major excitatory input to the dentate gyrus.

Long-term increases in the evoked population spike in the CA1 region of rat hippocampus induced by beta-adrenergic receptor activation

The effects of the selective beta-adrenergic receptor agonist isoproterenol (ISO) were characterized in the CA1 region of the rat hippocampal slice preparation. As has been previously described, 500 nM ISO increased the amplitude of the evoked population spike response without having any effect upon field EPSP (fEPSP) responses. However, the increase in the population spike response was quite persistent and was not reversed by greater than 30 min of washout in the majority of the slices tested; we have termed this prolonged increase beta-adrenergic potentiation (BAP). As with the acute effect of ISO, BAP is confined to an increase in the population spike response and not the fEPSP. In input-output curves, this was clearly observed as a persistent leftward shift in the EPSP-population spike relationship. Similar long-term increases in the population spike could also be elicited by superfusion of the slices for 10 min with 20-25 microM norepinephrine (NE). Although both the acute and the long-term effects of ISO were blocked by pretreatment with timolol, a beta-adrenergic antagonist, the long-term effects were not reversed by superfusion with timolol following ISO treatment, demonstrating that the prolonged effects were not due to slow washout of ISO from the tissue. BAP was not blocked by pretreatment with 50 microM 2-amino-5-phosphonovaleric acid, an NMDA receptor antagonist that blocks hippocampal long-term potentiation and the long-lasting changes in synaptic responses induced in the dentate gyrus by NE and ISO.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of mu- and delta-receptor selective opioid agonists on feedforward and feedback GABAergic inhibition in hippocampal brain slices.

Previous studies have suggested that opioid receptor activation in the hippocampus increases pyramidal neuron excitability by reducing GABAergic inhibition. This hypothesis has received support with regard to mu-receptor agonists but has not been adequately tested with selective delta-receptor agonists. In the present investigation we compared the effects of the selective mu-opioid receptor agonist [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO) and the delta-receptor agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE) to those of bicuculline methiodide (BMI) on extracellularly recorded feedforward (FFW) and recurrent (feedback; FB) inhibition. It was discovered that the control population spike response, evoked by Schaffer collateral/commissural axon stimulation, increased in response to DAGO, DPDPE, and BMI, while the secondary or test response increased only in the presence of DAGO and BMI. The resulting hypothesis that delta-opioid receptor activation facilitates synaptically evoked responses independently of a reduction of inhibition was investigated by examining the effect of DPDPE on the field EPSP response recorded in stratum radiatum of CA1, or postsynaptically on a burst response activated through antidromic stimulation of pyramidal neurons in low calcium medium. delta-Opioid receptor activation had no effect on either the field EPSP response or the burst response, suggesting that neither synaptic transmission nor postsynaptic excitability were augmented. Finally, the possibility that DPDPE acts to enhance pyramidal cell excitability independently of GABAergic transmission was further investigated by examining responses to both mu- and delta-opioid agonists following treatment with BMI (30 microM). Responses to DPDPE and DAGO were completely blocked by this treatment, supporting the involvement of a GABAergic circuit in the actions of these enkephalins. These results suggest that the delta-opioid receptor agonist DPDPE may mediate a reduction in GABAergic inhibition which is not detectable using paired stimulation techniques designed to examine FFW and FB inhibition in the hippocampal slice.

Chronic theophylline treatment in vivo increases high affinity adenosine A1 receptor binding and sensitivity to exogenous adenosine in the in vitro hippocampal slice

The present investigation examined the effects of chronic treatment with the adenosine receptor antagonist theophylline in vivo, on in vitro hippocampal electrophysiology and adenosine A1 receptor binding in the same animals. Adult rats were injected once daily (i.p.) with theophylline for 1 week at 75 mg/kg, followed by an additional week at 100 mg/kg, or with saline for the same 2-week period. Two days following the last injection, hippocampal slices were prepared and population spikes recorded from the pyramidal cell layers of area CA1 were elicited by Schaffer collateral-commissural fiber stimulation. The degree of inhibition caused by superfused adenosine was compared between hippocampal slices from theophylline- and saline-treated rats. Tissue from the contralateral hippocampus was used in [ 3H]cyclohexyladenosine ([ 3H]CHA) receptor binding. Hippocampi from theophylline-treated animals showed a significantly greater number of [ 3H]CHA binding sites (apparent B max; 125% of control, P < 0.05), without a significant change in binding affinity, and were more sensitive than controls to the inhibitory effects of adenosine on the population spike response. These results suggest that chronic adenosine receptor antagonism results in the up-regulation of adenosine A1 receptors which are functional and physiologically relevant in the in vitro hippocampus, and further supports the hypothesis that methylxanthine tolerance is mediated, at least in part, by an increase in adenosine receptor density.

Differential effects of isoquinolinesulfonamide protein kinase inhibitors on CA1 responses in hippocampal slices

The effects of the isoquinolinesulfonamide protein kinase inhibitors l-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) andN-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA1004) on CA1 responses in hippocampal slices of the rat were examined to clarify their mode of action, and also to further define the role of Ca 2+-dependent kinases in long-term potentiation. Initially, the inhibitory potencies of H-7 and HA1004 against both protein kinase C and type II Ca 2+/cahmodulin-dependent kinase were examined in standard in vitro phosphorylation assays. The apparentK i values of H-7 and HA1004 for protein kinase C were 9 and 57μ M, respectively. In contrast, theK i values of H-7 and HA1004 for type II calcium/ calmodulin-dependent protein kinase were 156 and 13μ M, respectively. These results indicate that H-7 is a more effective inhibitor of protein kinase C, whereas HA1004 is a more effective inhibitor of type II calcium/calmodulin-dependent protein kinase. Following the induction of long-term potentiation, addition of 50μ M H-7 or HA1004 substantially increased the amplitude of the population spike in a control pathway, while producing no change or a slight increase in the spike amplitude in a previously potentiated long-term potentiation pathway. Moreover, H-7 (50 μ M), but not HA1004, produced multiple population spikes in both pathways. Addition of a higher concentration of H-7 (300 μ M) reduced the amplitude of the initial population spike but still produced multiple spikes. HA1004 (300μ M) typically produced effects similar to those observed with 50 μ M H-7, increasing the amplitude of the control population spike and producing multiple spike activity in both pathways. In contrast to the differential concentration-dependent effects of H-7 on the population spike responses, qualitatively similar effects were observed at both low (50 μ M) and high (300μ M) concentrations with regard to synaptic field responses. The initial slope of the population excitatory postsynaptic potential was significantly reduced by H-7, to a similar degree in both pathways. HA1004 produced a modest, but insignificant reduction in both pathways. These results, in conjunction with other reports, suggest that H-7 and HA1004 exert complex concentration-dependent effects which synchronously affect both excitatory and inhibitory synaptic transmission. We hypothesize that reduction of the population excitatory postsynaptic potential and spike (300 μ M H-7) is due to reduction of excitatory inputs, whereas enhancement of the population spike amplitude (50 μ M H-7) and the production of multiple spikes are due to the reduction of GABA-mediated inhibitory inputs. Moreover, the actions of these drugs on synaptic transmission were not affected by prior induction of long-term potentiation, except for the effects of 50μ M H-7 on the population spike amplitude. However, the lack of further potentiation of the population spike amplitude with 50μ M H-7 is consistent with recent reports suggesting that reduction of inhibitory mechanisms contributes to spike long-term potentiation. The results of the phosphorylation assays suggest that the electrophysiological alterations observed following addition of H-7 and HA1004 can be attributed to inhibition of protein kinase C. The biochemical studies also indicate the potential usefulness of HA1004 as a potent inhibitor of type II calcium/calmodulin-dependent protein kinase.

Apical dendritic depolarizations and field interactions evoked by stimulation of afferent inputs to rat hippocampal ca1 pyramidal cells

The relationship between orthodromic extracellular field potentials and intradendritic depolarizations in apical dendrites of CA1 pyramidal neurons was investigated using the in vitro slice preparation of rat hippocampus. Orthodromic synaptic field potentials evoked by stimulation of afferent inputs in stratum radiatum or stratum oriens were used to measure extracellular voltage gradients generated over the pyramidal cell axis. Extracellular gradients were of opposite polarity over the region of pyramidal cell apical dendrites in stratum radiatum. The stratum radiatum-evoked gradient was negative towards the apical dendrites and the stratum oriens-evoked gradient negative towards the cell body layer, with gradients reaching values of up to 50 mV/mm over the apical dendritic axis. Intradendritic recordings obtained >150 μm from stratum pyramidale directly measured the subthreshold apical dendritic excitatory postsynaptic potentials evoked by stratum radiatum or stratum oriens stimulation. These ground-referenced recordings were then compared to the transmembrane potential calculated by subtraction of the corresponding extradendritic field potential. Both stratum radiatum and stratum oriens stimulation evoked graded excitatory postsynaptic potentials that could be recorded in apical dendritic impalements up to 265 μm from stratum pyramidale. The calculated transmembrane potential of the stratum radiatum-evoked excitatory postsynaptic potential had a significantly greater rate of rise, peak amplitude, and rate of decay than that of the ground-referenced excitatory postsynaptic potential. In contrast, the rates of rise and decay of the transmembrane potential of the stratum oriens-evoked excitatory postsynaptic potential were reduced with respect to the ground-referenced recording. The peak amplitude of the stratum oriens-evoked transmembrane potential, however, varied according to the polarity of the corresponding extradendritic population spike response recorded in stratum radiatum. These data reveal that synaptic activation of either basal or apical dendrites of CA1 pyramidal cells evokes a depolarization that can be recorded over a substantial region of the apical dendritic arbor. Furthermore, extradendritic field potentials evoked by stimulation of these inputs produce opposite effects on the transmembrane potential of apical dendrites. The magnitude of the accompanying extracellular voltage gradients suggest that these shifts in transmembrane potential reflect ephaptic interactions at the apical dendritic level of pyramidal cells.

Time discrete model of recurrent inhibition in hippocampal dentate gyrus

A time discrete model of recurrent inhibition in the hippocampal dentate gyrus is made and analyzed. This model assumes that (1) each granule cell can generate only one action potential in response to a single stimulation of the perforant path, (2) each interneuron receives synaptic inputs from many granule cells, and (3) an output of the interneuron is inhibitory for granule cells. Although each granule cell generates an action potential in the all-or-none fashion, the population spike is shown to be approximated by a piecewise linear function of the population excitatory post-synaptic potential (EPSP). From this model six patterns in the population spike responses to the paired-pulse stimulation are deduced. Each pattern is composed of some broken lines whose slopes and intercepts are explicitly expressed by the average and variance of the microscopic parameters and the population size of the cells. This model clarifies the relation between the measured quantity in the field potential experiment and the microscopic quantities peculiar to the granule cell and to the interneuron.

Release of endogenous adenosine does not mediate electrophysiological responses to morphine in the hippocampus in vitro

Previous studies have shown that adenosine receptor antagonists can block some of the effects of morphine and that morphine can enhance the release of adenosine. The possibility that some of the effects of morphine in the hippocampus are exerted through the release of adenosine was investigated using electrophysiological responses in vitro. Although both morphine and the adenosine receptor antagonist theophylline increase population spike responses, they appear to do so by independent mechanisms. Also, while adenosine significantly decreases the amplitude of the field excitatory postsynaptic potential (fEPSP) and theophylline increases this response (by blocking the effects of endogenous adenosine), morphine had no significant effect on the fEPSP. No significant interactions between morphine and theophylline were observed using either fEPSPs or the population spike response. These results suggest that, either morphine does not stimulate the release of adenosine in slices of hippocampus under normal electrophysio-logical recording conditions, or that the concentrations of adenosine, released by morphine, are not physiologically relevant in the hippocampus in vitro.

Feedforward excitation of the hippocampus by afferents from the entorhinal cortex: redefinition of the role of the trisynaptic pathway.

For the past 3 decades, functional characterizations of the hippocampus have emphasized its intrinsic trisynaptic circuitry, which consists of successive excitatory projections from the entorhinal cortex to the dentate gyrus, from granule cells of the dentate to the CA3/4 pyramidal cell region, and from CA3/4 to the CA1/2 pyramidal cell region. Despite unequivocal anatomical evidence for a monosynaptic projection from entorhinal to CA3 and CA1/2, few in vivo electrophysiological studies of the direct pathway have been reported. In the experiments presented here, we stimulated axons of entorhinal cortical neurons in vivo and recorded evoked single unit and population spike responses in the dentate, CA3, and CA1 of hippocampus, to determine if pyramidal cells are driven primarily via the monosynaptic or trisynaptic pathways. Our results show that neurons within the three subfields of the hippocampus discharge simultaneously in response to input from a given subpopulation of entorhinal cortical neurons and that the initial monosynaptic excitation of pyramidal cells then is followed by weaker excitatory volleys transmitted through the trisynaptic pathway. In addition, we found that responses of CA3 pyramidal cells often precede those of dentate granule cells and that excitation of CA3 and CA1 pyramidal cells can occur in the absence of granule cell excitation. In total, these results argue for a different conceptualization of the functional organization of the hippocampus with respect to the propagation of activity through its intrinsic pathways: input from the entorhinal cortex initiates a two-phase feedforward excitation of pyramidal cells, with the dentate gyrus providing feedforward excitation of CA3, and with both the dentate and CA3 providing feedforward excitation of CA1.

Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In vivo electrochemical measurements of chronoamperometric recordings from Nafion-coated electrodes were used to investigate monoamine overflow from selected regions of the rat hippocampal slice. Concurrent electrophysiological measurements of evoked CA1 pyramidal cell population spike responses were used to characterize changes in the electrical activity in the slices that occur during potassium-induced neurotransmitter overflow. Superfusion with elevated K+ (10–50 mM, 5 min) elicited consistent concentration-dependent increases in the electrochemical responses recorded from the dentate gyrus. At the onset of K+ perfusion, there was an initial increase in the population spike response, followed by electrical silence, which usually lasted 5–10 min following the return to normal medium, and required 20–30 min for complete recovery of the response. The potassium-induced electrochemical signal always increasedfollowing the decline in the electrophysiological response. Although the electrochemical signal usually returned to baseline much before the electrophysiological response (usually within 5 min), both signals remained refractory for some time. Cocaine pretreatment (10–50 μM) caused a dose-dependent augmentation of the electrochemical responses. Local pressure ejection of K+ via a micropipette elicited dose-dependent increases in the electrochemical signals that were of relativity brief duration as compared to superfusion with K+. Such potassium-evoked responses were highly localized, and were attenuated in amplitude in animals that had been previously treated with the selective noradrenergic neurotoxin, DSP-4. In addition to K+, local applications of methyl-amphetamine, tyramine and veratridine also elicited electrochemical signals, and the time courses of these responses were specific to the releasing agent that was used. Preliminary data obtained using high-speed electrochemical 3ecordings of both oxidation and reduction current suggested that tyramine ejections evoked pimarily norepinephrine overflow, while K+ evoked the overflow of both norepinephrine and serotonin. The present experiments demonstrate that simultaneous electrophysiological and electrochemical experiments can be used in an isolated preparation of brain such as the hippocampal slice to characterize the electrophysiological events that occur during stimulated transmitter release.

Pharmacology of calcium-induced long-term potentiation in rat hippocampal slices

A transient increase (10 min) in extracellular calcium concentration (4 mM) causes a long-lasting (> 2 hr) enhancement of population spike responses evoked by radiatum fibers to CA1 pyramidal neurons in rat hippocampal slices. The phenomenon is similar to tetanic long-term potentiation (LTP), and is also related to memory processes. The influence of various drugs was investigated on calcium-induced LTP. The NMDA antagonist 2 amino-5-phosphonopentanoic acid (AP5; 100 μM) was able to prevent the calcium-induced LTP, while atropine sulphate (10 μM), propranolol hydrochloride (10 μM) and verapamil hydrochloride (100 μM) were ineffective. The results suggest an involvement of the NMDA receptor in the development of calcium-induced LTP.

GENERAL ANAESTHETIC MODIFICATION OF SYNAPTIC FACILITATION AND LONG-TERM POTENTIATION IN HIPPOCAMPUS

The synaptic integrative properties of facilitation and potentiation are important determinants of cortical neurone excitability. The present study measured the effects of halothane and methoxyflurane on synaptic facilitation, paired-pulse potentiation, and long-term potentiation (LTP) in CA1 pyramidal neurones of rat hippocampal slices. Methoxyflurane 0.16vol% and halothane 1.2 vol% depressed population spike amplitudes by approximately 100%, but halothane did so with relatively little (less than 10%) depressant effect on the field excitatory postsynaptic potential (EPSP). Both agents enhanced EPSP facilitation, halothane more than methoxyflurane. Halothane consistently enhanced paired-pulse population spike potentiation; methoxyflurane sometimes diminished it. Halothane reduced the probability of LTP induction to an extent correlated with block of population spike responses in the inducing stimulus train (r = 0.92); methoxyflurane did not block LTP. The results suggest that these agents affect cortical excitability by multiple actions, the distribution of actions being specific to each agent.

Receptor activation is required for the induction of both early and late phases of long-term potentiation in rat hippocampal slices

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in mechanisms enabling the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the specific NMDA receptor antagonists (-)-2-amino-7-phosphonoheptanoic acid (D-APH) and 2-amino-5-phosphonovaleric acid (DL-APV) as well as of the inactive isomer L-APH was tested on orthodromic population excitatory postsynaptic potential (EPSP) and population spike (PS) responses recorded extracellularly from CA1 pyramidal cells. If the active D-isomer of APH (10 microM) or DL-APV (50 microM), but not if L-APH was present during tetanization, both EPSP and spike potentiation were markedly reduced or even blocked for the whole recording period (8 h after tetanization). It is concluded that the NMDA receptor component expressed during tetanization is a necessary step not only for initiation but also for subsequent mechanisms enabling late phases of synaptic LTP. Some remaining potentiation of the population spike may be related to a second, NMDA-independent mechanism.

Pertussis toxin pretreatment antagonizes the actions of μ- and δ-opiate agonists in hippocampal slices

The effect of pertussis toxin pretreatment on electrophysiological responses to selective δ- and μ-opioid receptor agonists was examined in rat hippocampal brain slices. The evoked population spike response in the CA1 region following activation of the Schaffer collateral and commissural afferents was increased following perfusion with the δ-selective agonist [ d-Pen 2, d-Pen 5enkephalin (DPDPE), and with the μ-selective agonist [ d-Ala 2,NMe-Phe 4,Gly(O) 5ol]enkephalin (DAGO). Both effects were significantly reduced or abolished in brain slices obtained from animals that had been pretreated with pertussis toxin 2–3 days earlier. These findings suggest that the excitatory responses to opioid agonists in hippocampus are the result of interactions with receptors that are coupled via pertussis toxin sensitive GTP-binding proteins to their respective effector mechanisms.

Local anaesthetic actions of cocaine: effects on excitatory and inhibitory synaptic responses in the hippocampus in vitro.

1. The basis for the proconvulsant action of cocaine was investigated in the CA1 region of the rat hippocampal slice in vitro. 2. Superfusion with 100 microM cocaine depressed inhibitory and excitatory postsynaptic potentials recorded intracellularly from CA1 pyramidal neurones; both types of potentials were inhibited to an equal extent. When inhibition was assessed using extracellular recording of population spike responses before and after conditioning impulses, there did not appear to be any selective effect upon either recurrent or feed-forward gamma-aminobutyric acid (GABA)ergic inhibition. 3. Not all responses showed equivalent sensitivity to the local anaesthetic actions of cocaine. In particular, the antidromic population spike evoked by stimulation of the alveus was significantly more sensitive than the presynaptic fibre spike elicited by stimulation of stratum radiatum. 4. The rate of interictal spiking in hippocampus, induced by penicillin and increased potassium in the perfusion medium, was depressed by superfusion with cocaine in the range 5-100 microM. 5. These results suggest that cocaine does not have a selective depressant effect upon inhibitory pathways in the CA1 region of the hippocampus. Although the hippocampus shows epileptiform activity following systemic administration of local anaesthetics such as cocaine in the intact rat, this effect may not reflect a direct hippocampal site of drug action.

Serotonin releasers modulate reactivity of the rat hippocampus to afferent stimulation

Electrical stimulation of the perforant path produces a characteristic population EPSP and population spike in the dentate gyrus of the anesthetized rat. Parenteral administration of a serotonin releasing drug D-fenfluramine (FFA) caused a marked (30-100%) and highly significant increase in dentate gyrus population spike response to perforant path stimulation without affecting the slope of the population excitatory postsynaptic potential (EPSP). This indicates that FFA modifies granular cell excitability to afferent stimulation. The facilitatory effect of FFA was not present in rats depleted of serotonin following treatment with the synthesis inhibitor p-chlorophenylalanine (PCPA) but was restored after restoration of serotonin synthesis with the precursor 5-hydroxytryptophan indicating that presence of serotonin in terminals is required for the action of FFA.