Multiple actions of acetylcholine on hippocampal pyramidal cells in organotypic explant cultures

Involvement of TRPC4 and 5 Channels in Persistent Firing in Hippocampal CA1 Pyramidal Cells

Persistent neural activity has been observed in vivo during working memory tasks, and supports short-term (up to tens of seconds) retention of information. While synaptic and intrinsic cellular mechanisms of persistent firing have been proposed, underlying cellular mechanisms are not yet fully understood. In vitro experiments have shown that individual neurons in the hippocampus and other working memory related areas support persistent firing through intrinsic cellular mechanisms that involve the transient receptor potential canonical (TRPC) channels. Recent behavioral studies demonstrating the involvement of TRPC channels on working memory make the hypothesis that TRPC driven persistent firing supports working memory a very attractive one. However, this view has been challenged by recent findings that persistent firing in vitro is unchanged in TRPC knock out (KO) mice. To assess the involvement of TRPC channels further, we tested novel and highly specific TRPC channel blockers in cholinergically induced persistent firing in mice CA1 pyramidal cells for the first time. The application of the TRPC4 blocker ML204, TRPC5 blocker clemizole hydrochloride, and TRPC4 and 5 blocker Pico145, all significantly inhibited persistent firing. In addition, intracellular application of TRPC4 and TRPC5 antibodies significantly reduced persistent firing. Taken together these results indicate that TRPC4 and 5 channels support persistent firing in CA1 pyramidal neurons. Finally, we discuss possible scenarios causing these controversial observations on the role of TRPC channels in persistent firing.

Transition between encoding and consolidation/replay dynamics via cholinergic modulation of CAN current: A modeling study

Hippocampal place cells that are activated sequentially during active waking get reactivated in a temporally compressed (5-20 times) manner during slow-wave-sleep and quiet waking. The two-stage model of the hippocampus suggests that neural activity during awaking supports encoding function while temporally compressed reactivation (replay) supports consolidation. However, the mechanisms supporting different neural activity with different temporal scales during encoding and consolidation remain unclear. Based on the idea that acetylcholine modulates functional transition between encoding and consolidation, we tested whether the cholinergic modulation may adjust intrinsic network dynamics to support different temporal scales for these two modes of operation. Simulations demonstrate that cholinergic modulation of the calcium activated non-specific cationic (CAN) current and the synaptic transmission may be sufficient to switch the network dynamics between encoding and consolidation modes. When the CAN current is active and the synaptic transmission is suppressed, mimicking the high acetylcholine condition during active waking, a slow propagation of multiple spikes is evident. This activity resembles the firing pattern of place cells and time cells during active waking. On the other hand, when CAN current is suppressed and the synaptic transmission is intact, mimicking the low acetylcholine condition during slow-wave-sleep, a time compressed fast (∼10 times) activity propagation of the same set of cells is evident. This activity resembles the time compressed firing pattern of place cells during replay and pre-play, achieving a temporal compression factor in the range observed in vivo (5-20 times). These observations suggest that cholinergic system could adjust intrinsic network dynamics suitable for encoding and consolidation through the modulation of the CAN current and synaptic conductance in the hippocampus.

Cholinergic modulation of the CAN current may adjust neural dynamics for active memory maintenance, spatial navigation and time-compressed replay

Suppression of cholinergic receptors and inactivation of the septum impair short-term memory, and disrupt place cell and grid cell activity in the medial temporal lobe (MTL). Location-dependent hippocampal place cell firing during active waking, when the acetylcholine level is high, switches to time-compressed replay activity during quiet waking and slow-wave-sleep (SWS), when the acetylcholine level is low. However, it remains largely unknown how acetylcholine supports short-term memory, spatial navigation, and the functional switch to replay mode in the MTL. In this paper, we focus on the role of the calcium-activated non-specific cationic (CAN) current which is activated by acetylcholine. The CAN current is known to underlie persistent firing, which could serve as a memory trace in many neurons in the MTL. Here, we review the CAN current and discuss possible roles of the CAN current in short-term memory and spatial navigation. We further propose a novel theoretical model where the CAN current switches the hippocampal place cell activity between real-time and time-compressed sequential activity during encoding and consolidation, respectively.

Pilocarpine-induced seizure-like activity with increased BNDF and neuropeptide Y expression in organotypic hippocampal slice cultures

Organotypic hippocampal slice cultures were treated with the muscarinic agonist pilocarpine to study induced seizure-like activity and changes in neurotrophin and neuropeptide expression. For establishment of a seizure-inducing protocol, 2-week-old cultures derived from 6-8-day-old rats were exposed to 0.1 mM to 5 mM of pilocarpine for 4 h to 7 days. Other cultures were treated with pilocarpine for 7 days and left for 7-14 days in normal medium. Age-matched, non-treated cultures served as controls. Intracellular recordings from CA1 pyramidal cells revealed increased spontaneous activity in 31 of 35 cultures superfused with 0.1 or 5 mM pilocarpine. Epileptiform discharges were recorded in 17 of the 31 cultures, and 19 displayed frequencies specifically in the 6-12-Hz (Theta rhythm) range when superfused with pilocarpine. The pilocarpine effect was blocked by simultaneous superfusion with the muscarinic receptor antagonist atropine (100 microM). Regardless of dose and exposure time, the pilocarpine treatment induced very limited neuronal cell death, recorded as cellular propidium iodide uptake. Cultures exposed to 5 mM pilocarpine for up to 7 days displayed increased BDNF expression when analyzed by Western blot and ELISA. This BDNF increase correlated with increased neuropeptide Y immunoreactivity, known to accompany seizure activity. Addition of BDNF (200 ng/ml) to otherwise untreated cultures also upregulated NPY expression. The pilocarpine-induced seizure-like activity in hippocampal slice cultures, with concomitant increase in BDNF and NPY expression, is compared with in vivo observations and discussed in terms of the potential use of the easily accessible slice cultures in experimental seizure research.

Early differential induction of C-jun in the central nervous system of hens treated with diisopropylphosphorofluoridate (DFP)

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester induced delayed neurotoxicity (OPIDN) in the hen, human and other sensitive species. We studied the effect of a single dose of DFP (1.7 mg/kg/sc) on the expression of c-jun, which is one of the heterodimerizing ITFs (Inducible Transcriptional Factors) of the AP-1 family. The hens were sacrificed at different time points ie 0.25, .0.50, 1 and 2 hrs. Total RNA was extracted from the following brain regions: cerebrum, cerebellum, brainstem, midbrain and as well as spinal cord. Northern blots prepared using standard protocols were hybridized with c-jun as well as b-actin and 18S RNA cDNA (control) probes. The results indicate differential regulation of c-jun levels which may be due to the activation of both cholinergic and non-cholinergic pathways of CNS, besides changing roles of c-jun (as mediator of degeneration or regeneration) depending on heterodimerization with other ITFs. In the highly susceptible tissues like brainstem and spinal cord c-jun transcript levels increased at 15 minutes and continued to increase gradually till it reached the maximum at 2 hrs. Overall spinal cord showed the maximum levels of c-jun induction (207%) at 2 hrs time point of all the CNS tissues. The enhancement of cholinergic transmisson by the inhibition of cholinestrase may be responsible for the gradual increase mediated by neural and vascular factors. In contrast, less susceptible tissue, cerebellum showed almost immediate induction to high level of (179%) at 15 minutes and the levels stayed more or less the same until it peaked to 185% at 2 hrs. Relatively low abundance of cholinergic neurons and high number of sensitized specialized cell types like Bergman glia and Purkinje cells may be responsible for the immediate higher induction. Non-susceptible tissue cerebrum did not show any changes in the c-jun levels. In midbrain the induction pattern was very similar to that of brainstem. This differential induction pattern of c-jun encomposing the differences in the quantity and time course was directly proportionate to the degree of susceptibility and cellular heterogeneity of different regions of CNS. The significant increase in c-jun levels along with our earlier observation on the increased c-fos levels indicate that AP-1 family of genes may be one of the IEGs involved in the long term changes which eventually lead to OPIDN.

Tetanus toxin-enhanced GABA immunoreactivity in living neurons

Analysis of the connectivity between different neuronal cell types is dependent on an appreciation of their dendritic and axonal arborizations. A detailed study of the dendrites and axons of GABAergic neurons has been thwarted by the lack of a suitable technique for enhancing GABA immunoreactivity. This article describes a procedure using tetanus toxin which, when applied to organotypic hippocampal cultures, considerably enhances the immunoreactivity in the dendrites and axons of the GABA- and somatostatin-containing neurons and clearly demonstrates the co-localization of GABA and somatostatin immunoreactivities in the same neuron. Tetanus toxin was applied to the culture medium on Day 14 for a 24-hr period and the cultures were fixed at the end of Day 18. Tetanus toxin-treated cultures (n = 30) or untreated cultures (n = 40) were incubated for either GABA or somatostatin immunoreactivity. Tetanus toxin-treated cultures used for co-localization studies (n = 20) were incubated for both GABA and somatostatin immunoreactivity.

Interaction between 3 alpha-hydroxy-5 alpha-pregnan-20-one and carbachol in the control of neuronal excitability in hippocampal slices of female rats in defined phases of the oestrus

The effects of 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone) and carbachol on CA1 and dentate gyrus action potentials were studied in hippocampus slices in premature, follicular and luteal phase rats. A 0.5 nL droplet of allopregnanolone (12.5 mumol L-1), carbachol (5 mumol L-1) or a mixed solution of 12.5 mumol L-1 allopregnanolone and 5 mumol L-1 carbachol was applied locally onto the stratum oriens-pyramidale or granular layer. The amplitude of CA1 population spike (POPSP) was reduced by allopregnanolone (-38 +/- 3%) and carbachol (-21 +/- 4%) in the luteal phase slices. The mixture of allopregnanolone and carbachol doubled this inhibition (-77 +/- 6%). The inhibition caused by allopregnanolone and the mixture of allopregnanolone and carbachol in CA1 was significantly larger in the luteal phase than in the follicular phase (P = 0.02 and 0.0002). In the granular layer of the dentate gyrus, these inhibitions showed no significant difference between the phases. Neither in CA1 nor in the dentate gyrus did the carbachol inhibition differ between the phases. Perfusion with 5-10 mumol L-1 carbachol caused an increasing inhibition of the POPSP during the first few minutes. Thereafter the inhibition gradually diminished and was replaced by a facilitation. The local allopregnanolone inhibition was enhanced by simultaneous carbachol perfusion. Picrotoxin (100 mumol L-1) substantially reduced the allopregnanolone but not the carbachol inhibition. Atropine (10 mumol L-1) blocked the carbachol response, but not the allopregnanolone inhibition. Perfusion with a mixed solution of picrotoxin and atropine reduced, but did not block, the inhibition caused by local application of allopregnanolone or by the mixture of allopregnanolone and carbachol. Our data suggest that neuroprogestine modulators of the GABAA-receptor-mediated inhibition may play a significant role in the control of the cholinergic excitation in the hippocampus.

Ionic mechanisms of muscarinic depolarization in entorhinal cortex layer II neurons

The mechanisms underlying direct muscarinic depolarizing responses in the stellate cells (SCs) and non-SCs of medial entorhinal cortex layer II were investigated in tissue slices by intracellular recording and pressure-pulse applications of carbachol (CCh). Subthreshold CCh depolarizations were largely potentiated in amplitude and duration when paired with a short DC depolarization that triggered cell firing. During Na+ conductance block, CCh depolarizations were also potentiated by a brief DC depolarization that allowed Ca2+ influx and the potentiation was more robust in non-SCs than in SCs. Also, in non-SCs, CCh depolarizations could be accompanied by spikelike voltage oscillations at a slow frequency. In both SCs and non-SCs, the voltage-current (V-I) relations were similarly affected by CCh, which caused a shift to the left of the steady-state V-I relations over the entire voltage range and an increase in apparent slope input resistance at potentials positive to about -70 mV. CCh responses potentiated by Ca2+ influx demonstrated a selective increase in slope input resistance at potentials positive to about -75 mV in relation to the nonpotentiated responses. K+ conductance block with intracellular injection of Cs+ (3 M) and extracellular Ba2+ (1 mM) neither abolished CCh depolarizations nor resulted in any qualitatively distinct effect of CCh on the V-I relations. CCh depolarizations were also undiminished by block of the time-dependent inward rectifier Ih, with extracellular Cs . However, CCh depolarizations were abolished during Ca2+ conductance block with low-Ca2+ (0.5 mM) solutions containing Cd2+, Co2+, or Mn2+, as well as by intracellular Ca2+ chelation with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. Inhibition of the Na+-K+ ATPase with strophanthidin resulted in larger CCh depolarizations. On the other hand, when NaCl was replaced by N-methyl-D-glucamine, CCh depolarizations were largely diminished. CCh responses were blocked by 0.8 microM pirenzepine, whereas hexahydro-sila-difenidolhydrochloride,p-fluoroanalog (p-F-HHSiD) and himbacine were only effective antagonists at 5- to 10-fold larger concentrations. Our data are consistent with CCh depolarizations being mediated in both SCs and non-SCs by m1 receptor activation of a Ca2+-dependent cationic conductance largely permeable to Na+. Activation of this conductance is potentiated in a voltage-dependent manner by activity triggering Ca2+ influx. This property implements a Hebbian-like mechanism whereby muscarinic receptor activation may only be translated into substantial membrane depolarization if coupled to postsynaptic cell activity. Such a mechanism could be highly significant in light of the role of the entorhinal cortex in learning and memory as well as in pathologies such as temporal lobe epilepsy.

Sensory gating in a computer model of the CA3 neural network of the hippocampus

We have developed a unique computer model of the CA3 region of the hippocampus that simulates the P50 auditory evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory processing deficit associated with schizophrenia. Our computer model of the CA3 hippocampal network includes recurrent activation from within the CA3 region as well as input from the entorhinal cortex and the medial septal nucleus. We used the model to help us determine if the cortical and septal inputs to the CA3 hippocampus alone are responsible for the gating of auditory evoked activity, or if the strong recurrent activity within the CA3 region contributes to this phenomenon. The model suggests that the medial septal input is critical for normal gating; however, to a large extent the activity of the medial septal input can be replaced by simulated stimulation of the hippocampal neurons by a nicotinic agonist. The model is thus consistent with experimental data that show that nicotine restores gating of the N40 evoked potential in fimbria-fornix lesioned rats and of the P50 evoked potential in schizophrenic patients.

Pre-exposure to subtoxic levels prevents kainic acid lesions in organotypic hippocampal slice cultures: effects of kainic acid on parvalbumin-immunoreactive neurons and expression of heat shock protein 72 following the induction of tolerance

The effects of kainic acid on the survival of principal neurons and parvalbumin-immunoreactive (PARV-IR) neurons, and on the expression of heat shock protein 72 immunoreactivity (HSP72-IR) were investigated in organotypic hippocampal slice cultures. Untreated cultures displayed an organotypic organization and the development and morphology of PARV-IR neurons in the hippocampus paralleled that reported to occur in vivo, with the exception of the hilar region of the dentate gyrus which exhibited a marked lack of PARV-IR neurons. No constitutive expression of HSP72 was found in untreated cultures. The lesion of CA3 neurons and the reduction in numbers of PARV-IR neurons in both CA3 and CA1 after chronic exposure to 5 microM kainic acid were similar to those reported to occur in vivo. Exposure to 1 microM doses of kainic acid resulted in a widespread appearance of HSP72-IR and the induction of tolerance to a previously toxic dose of kainic acid. These results suggest the presence of endogenous neuroprotective mechanisms, activated by a stress response which induces HSP72, and is reminiscent of the induced tolerance reported to occur after a mild ischaemic insult.

Ethanol enhances muscarinic cholinergic neurotransmission in rat hippocampus in vitro

Previous studies from our laboratory showed that ethanol enhances muscarinic excitatory responses in rat hippocampal neurons in vivo and, like muscarinic agonists, reduces the M-current (IM) in these neurons in vitro. Therefore, we used extracellular and intracellular recording techniques in the hippocampal slice preparation to examine the mechanisms underlying this ethanol-muscarinic interaction. Surprisingly, superfusion or local application of low concentrations of acetylcholine (ACh), carbachol (CCh) or muscarine reduced the amplitudes of CA1 field potentials evoked by stratum radiatum (SR) stimulation. This effect was blocked by 1 microM atropine but was independent of the method of agonist application, the site of application or the SR stimulus paradigm. In intracellular and extracellular single unit recordings, cholinergic depressions of field potentials were correlated with: (1) depolarization of pyramidal neurons; (2) spike discharge increases; (3) reduction of amplitudes of postsynaptic potentials and (4) reduction of late afterhyperpolarizations (AHPs). Superfusion of low ethanol concentrations (11-22 mM) alone had little effect on SR-evoked field potentials but enhanced (by 10-90%) both the depressions of evoked field potentials and depolarizations elicited by the muscarinic agonists. Ethanol (22-44 mM) also enhanced both the amplitude and duration of the muscarinic slow excitatory postsynaptic potentials (sEPSPs) recorded intracellularly in CA1 and CA3 neurons. This effect was enhanced by eserine and blocked by atropine, verifying involvement of muscarinic receptors. These results suggest that: (1) caution be used in interpreting results of field potential studies regarding drug-induced excitability changes; and (2) ethanol in just-intoxicating concentrations enhances endogenous muscarinic synaptic transmission as well as responses to exogenous muscarinic agonists.

Enhanced NMDA conductance can account for epileptiform activity induced by low Mg2+ in the rat hippocampal slice

1. Why does lowering extracellular Mg2+ cause synchronous neuronal bursts and after-discharges? To address this question, a computer model of the CA3 region was constructed with 1000 pyramidal neurones and 100 inhibitory neurones. Pyramidal neurones were multicompartmental and contained five ionic conductances, distributed non-uniformly on the membrane. In parallel, experiments were performed on rat hippocampal slices perfused in solutions without added Mg2+. 2. Model neurones were interconnected randomly as follows. Recurrent excitatory connections between pyramidal neurones, and from pyramidal neurones to inhibitory cells, stimulated both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (rapid, voltage and Mg2+ independent) and NMDA receptors (slow conductance decay, voltage and Mg2+ dependent). A time-dependent 'desensitization' process was included whereby the NMDA-mediated conductance declined after the onset of synchronized firing. Half of the inhibitory neurones activated GABAA receptors on pyramidal cells (perisomatic, rapid), and half activated GABAB receptors (dendritic, slow onset and decay). 3. We examined patterns of synchronous firing in the pyramidal cells as parameters defining model features were manipulated. These parameters included the maximum conductance of individual synapses, [Mg2+]o, excitatory connectivity, and parameters that defined the NMDA 'desensitization' process. Comparisons were made with experiment where possible. 4. GABAA blockade in 1 mM [Mg2+]o induces single bursts and bursts with after-discharges. Synchronized bursts and after-discharges also occurred in the model when NMDA conductances were sufficiently enhanced, even with GABAA inhibition present. Both in simulated and experimental after-discharges in low-Mg2+ solutions, the level of GABAA inhibition was important in determining the number of secondary bursts and the number of somatic spikes per wave. 5. The model of low-Mg(2+)-induced synchrony predicts that each somatic wave is induced by a dendritic Ca2+ spike and that the dendritic spikes are superimposed on a tonic dendritic depolarization generated by the enhanced NMDA conductance. We further predict the recurrent activation of interneurones by NMDA receptors, based both on experiments and simulations in which AMPA receptors are blocked. 6. Many of the mechanisms underlying low-Mg(2+)-induced after-discharges appear to resemble those underlying picrotoxin-induced after-discharges. These mechanisms can operate in low-Mg2+ solutions because of the increase in NMDA conductance in the recurrent excitatory connections.

Characterization of a calcium-dependent current generating a slow afterdepolarization of CA3 pyramidal cells in rat hippocampal slice cultures

A depolarization-induced, slowly decaying inward current was examined in slice-cultured CA3 pyramidal cells by voltage-clamp techniques and microfluorometric measurements of cytosolic free Ca2+ concentration ([Ca2+]i). Action potentials elicited by intracellular injection of short-lasting (50-100 ms) depolarizing current pulses were followed by a slowly decaying afterhyperpolarization (AHP). After switching to voltage-clamp mode, short-lasting (50-100 ms) depolarizing voltage jumps from -60 mV to between -30 and 0 mV induced a slowly decaying outward aftercurrent (IAHP) which was depressed by bath application of muscarine (0.5 microM). In the presence of muscarine, the same depolarizations induced a slowly decaying afterdepolarization (ADP) or inward aftercurrent (IADP) in voltage-clamp mode. This current was also induced in the presence of trans(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 5 microM), an agonist of metabotropic glutamate receptors, but not in the presence of noradrenalin (5 microM), while both of these agonists depressed IAHP. IADP was depressed by reducing the external Ca2+ concentration from 3.8 to 0.5 mM, by external Co2+ (1 mM) and by external Cd2+ (10-100 microM). Combined voltage-clamp recordings and microfluorometric measurements of [Ca2+]i using the Ca2+ indicator fura-2 revealed that the amplitude of IADP was correlated with the amplitude of depolarization-induced Ca2+ influx. IADP was absent at membrane potentials < -90 mV, and reached maximal amplitudes at approximately -55 mV. Raising the extracellular K+ concentration from 2.7 to 13.5 mM increased the amplitude of IADP and resulted in a positively directed shift of the apparent reversal potential of IADP. When the external Na+ concentration was reduced from 157 to 33 or 18 mM the current reversed at more negative potentials and was reduced to 40 and 21%, respectively, of control amplitude. Lowering the external CI- concentration from 159 to 20 mM did not affect IADP. We conclude that IADP most likely represents a Ca(2+)-activated cation current, rather than a Ca2+ tail current, or an electrogenic Ca2+ extrusion current.

NMDA-receptor mediated electrical epileptogenesis in the organotypic culture of rat hippocampus

Extracellular field recordings were made in CA1 in the hippocampal explant cultures in oxygenated artificial cerebrospinal fluid. Schaffer collaterals were stimulated with 1-s trains of 60 Hz pulses every 10 min. Seizures were reliably elicited with progressive lengthening over 1-2 h. D-APV, an N-methyl-D-aspartate (NMDA) antagonist, stereoselectively blocked the development of seizures. Thus we have demonstrated that in vitro epileptogenesis occurs in hippocampal explant cultures through NMDA receptor mediated mechanisms.

Computer simulation of carbachol-driven rhythmic population oscillations in the CA3 region of the in vitro rat hippocampus

1. We used simulations of the in vitro CA3 region of the hippocampus to analyse the 5 Hz population oscillations recorded experimentally in carbachol. 2. A simulation model of the in vitro CA3 region was constructed with 1000 pyramidal neurones and 200 inhibitory neurones (100 producing fast inhibitory postsynaptic potentials (IPSPs) and 100 producing slow IPSPs of delayed onset). Each neurone contained nineteen soma-dendritic compartments. Pyramidal neurones contained six voltage- and/or calcium-dependent ionic currents, whose kinetics were consistent with voltage-clamp data. The connectivity and waveform of unitary synaptic events for excitatory and fast inhibitory synapses were consistent with dual intracellular recordings. This network was shown to generate previously described network oscillations, including synchronized bursts recorded in the presence of GABAA blockers, and synchronized synaptic potentials observed during partial blockade of GABAA inhibition. 3. The model generated 5 Hz oscillations as recorded in carbachol under the following conditions: (a) excitatory synaptic conductance was within a limited range; (b) there was blockade of fast and slow IPSPs (consistent with the experimental lack of effect of bicuculline and phaclofen on carbachol oscillations and the known depression of IPSPs by acetylcholine); (c) the after hyperpolarization (AHP) conductance was reduced (consistent with the known pharmacology of carbachol); (d) the apical dendrites of the pyramidal cells were depolarized, as suggested by the carbachol-induced depolarization of pyramidal neurones. Each oscillation was associated in pyramidal cells with a burst of action potentials riding on a depolarizing wave. The N-methyl-D-aspartate (NMDA) type of excitatory synapse was not necessary for the oscillations to occur. 4. Progressive reduction of excitatory synaptic strength led to an oscillation of the same frequency, with bursts riding on smaller EPSPs (consistent with the experiment). Further reduction of excitatory synaptic strength abolished the population oscillation by uncoupling the neurones. When excitatory synaptic conductance was too large, population oscillations were attenuated as the cells switched from a bursting mode to a repetitively firing mode. 5. Increasing the AHP conductance prolonged the interburst interval as expected. Inclusion of slow IPSPs exerted a similar effect. 6. When fast IPSPs were included, an oscillation with different characteristics emerged: a 10 Hz oscillation that was gated by compound GABAA IPSPs. On any oscillatory wave, few pyramidal neurones fired, and the firing of individual neurones was irregular.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiovascular responses to combined microinjection of substance P and acetylcholine in the intermediolateral nucleus of the rat

As microinjection of either substance P (SP) or acetylcholine (ACh) into the right intermediolateral cell nucleus (IML) at the T2 level elicits increases in heart rate (HR) in the anesthetized rat, we investigated the possibility of a synergistic effect on HR and arterial pressure (AP) of ACh and SP microinjected in this nucleus. Moreover, we studied the effect on HR and AP of microinjection of either ACh or SP into the IML combined with activation of cardiovascular neurons in the ipsilateral rostral ventrolateral medulla (RVLM) by microinjection of glutamate (Glu). Male Wistar rats (n = 16) were anesthetized with urethane (1.4 g/kg i.p.), artificially ventilated, and the dorsal medulla and spinal cord (T1-T3) were exposed. Micropipettes containing SP and ACh were positioned in the right IML at the T2 level. Microinjection of threshold amounts of ACh (5 x 10(-2) M, 2-10 nl) and SP (3 x 10(-6) M, 2-10 nl) that caused small or no changes in HR or AP (less than 10 bpm or mmHg) elicited statistically significant synergistic increases in HR (22.9 +/- 3.3 bpm) but no changes in AP. Threshold microinjections of Glu (0.18 M, 2-10 nl) into the right RVLM combined with microinjections of threshold amounts of SP or ACh into the ipsilateral IML elicited significant synergistic increases in HR of 13.1 +/- 1.9 bpm and 10.6 +/- 1.9 bpm and in AP of 9.7 +/- 1.9 mmHg and 10.8 +/- 1.7 mmHg, respectively. These results indicate that SP and ACh interact to influence cardioacceleratory spinal preganglionic neurons (SPN) and interact with the transmitter released in the IML by RVLM stimulation to elicit increases in HR.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-lasting facilitation of excitatory postsynaptic potentials in the rat hippocampus by acetylcholine

1. The effects of acetylcholine (ACh) on excitatory postsynaptic potentials (EPSPs) evoked by stimulating Schaffer-commissural afferents and on ionophoretically applied L-glutamate ligands, were investigated in CA1 neurones of hippocampal slices using current- and voltage-clamp techniques. 2. ACh produced a transient suppression followed by a long-lasting facilitation of EPSPs. The facilitation was also seen in Cs(+)-filled cells under voltage-clamp conditions. Both suppressing and facilitating effects were blocked by atropine. 3. All components of the EPSP were reduced in the initial phase of ACh action, while only the slow component was enhanced during the later phase. The facilitation was blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonovalerate (2-APV) and by hyperpolarization. 4. ACh also facilitated responses to ionophoretically applied NMDA in voltage-clamped, Cs(+)-filled cells in Ba2(+)-treated slices. ACh facilitated responses to L-glutamate which was blocked by 2-APV. ACh failed to affect responses to kainate or quisqualate. 5. We conclude that ACh, acting on muscarinic receptors, exerts a primary effect in the hippocampus to specifically amplify NMDA receptor-mediated synaptic responses and thereby facilitate EPSPs.

Effects of diisopropyl phosphorofluoridate (DFP) on CA3 and CA1 responses in rat hippocampus

Diisopropyl phosphorofluoridate (DFP), an insecticide, is a potent anticholinesterase that binds essentially irreversibly to acetylcholinesterase, resulting in severe, acute neurologic pathology, and less severe, but longer-lasting, delayed neuropathy. We report here on the short-term effects of bath-applied DFP on extracellularly recorded responses from CA3 and CA1 of rat hippocampus. Exposure to 10 microM DFP evokes low amplitude, spontaneous bursts in CA3 generally within 10 minutes, and the bursting does not reverse with washing. The CA1 neuronal population usually bursts synchronously with CA3, but the population events are of low amplitude and sometimes not detectable, implying a differential sensitivity to DFP. These effects were partially blocked by the muscarinic antagonist atropine, while the cholinergic antagonist gallamine had little effect. Also, the reversible anticholinesterase physostigmine could, within temporal limits, protect slices from DFP's effects, implicating the cholinergic system as the probable mediator in the first stages of DFP-induced epileptogenesis.

Formation of [3H]inositol metabolites in rat hippocampal formation slices prelabelled with [3H]inositol and stimulated with carbachol

Rat hippocampal formation slices were prelabelled with [3H]inositol and stimulated with carbachol for times between 7 s and 3 min. The [3H]inositol metabolites in an acid extract of the slices were resolved with anion-exchange HPLC. Carbachol dramatically increased the concentration of [3H]inositol monophosphate, [3H]inositol bisphosphate (two isomers), [3H]inositol 1,3,4-trisphosphate, [3H]inositol 1,4,5-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate. The levels of [3H]inositol 1,4,5-trisphosphate rose most rapidly; they were maximally elevated after only 7 s and declined toward control levels in 1 min followed by a more sustained elevation in levels for up to 3 min. When [3H]inositol 1,4,5-trisphosphate was incubated with hippocampal formation homogenates in an ATP-containing buffer it was very rapidly metabolised. After 5 min [3H]inositol 1,4-bisphosphate, [3H]inositol 1,3,4-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate could be detected in the homogenates. Under similar experimental conditions [3H]inositol 1,3,4,5-tetrakisphosphate is metabolised to [3H]inositol 1,3,4-trisphosphate and an inositol bisphosphate isomer that is not [3H]inositol 1,4-bisphosphate. We conclude that like other tissues the primary event in the hippocampus following carbachol stimulation is the activation of phosphatidylinositol 4,5-bisphosphate selective phospholipase C.

Cholinergic modulation of epileptiform activity in the developing rat neocortex

The effects of carbachol on picrotoxin-induced epileptiform activity and membrane properties of neurons in the developing rat neocortex were examined in an in vitro slice preparation. Intracellular recordings were obtained in layer II-III neurons of slices prepared from rats 9-21 days of age. Epileptiform activity in 9- to 14-day-olds consisted of a sharply rising, sustained (10-30 s) membrane depolarization with superimposed action potentials. Bath application of carbachol (5-50 microM) raised the threshold for evoking epileptiform activity but, when such responses were evoked, their underlying depolarizations were increased in amplitude. Orthodromic stimulation in slices from 15- to 21-day-old animals evoked a prolonged epileptiform burst response that triggered an episode of spreading depression (SD). Carbachol reduced epileptiform responses and suppressed the occurrence of SD. It did not significantly affect the resting membrane potential or the height of the action potential but decreased the rheobase current needed to evoke an action potential and increased the input resistance. All effects of carbachol were antagonized by atropine (1 microM). These results indicate that carbachol has both pre- and postsynaptic effects in the developing neocortex and can significantly modulate neuronal excitability in the immature nervous system.

Presynaptic cholinergic inhibition in hippocampal cultures

Activity of hippocampal neurons was recorded in a dissociated culture under patch-clamp conditions. Excitatory and inhibitory postsynaptic currents (PSCs) were evoked in response to short pulse application of glutamate to other neurons in the culture dish. These PSCs were suppressed by topical application of acetylcholine (ACh) near the recorded neuron. The dose-dependent effect of the muscarinic antagonist pirenzepine indicates that the effect of ACh is mediated by an M2 receptor. ACh did not affect inward current responses to direct application of glutamate onto postsynaptic neurons. This indicates that ACh may interfere with the release process and not with the postsynaptic response to the neurotransmitter. In some cells, ACh reduced inward Ca currents recorded in the presence of Na and K channel blockers. This effect was atropine sensitive and may underly the reduced PSCs. It is suggested that ACh modulates release of neurotransmitters by reducing presynaptic Ica and thereby reducing evoked PSCs.

Inositol lipid metabolism in rat hippocampal formation slices: basal metabolism and effects of cholinergic agonists

Incubation of rat hippocampal formation slices under steady-state conditions with [3H]inositol leads to only three phospholipids becoming labelled: phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate. All three lipids incorporate [32P]Pi into their phosphodiester phosphate group with the polyphosphoinositides also incorporating this tracer into their monoester phosphate groups. As the concentrations of these lipids remain constant during these labelling processes we conclude that the phosphodiester phosphate, the inositol moiety, and the monoester phosphate groups undergo metabolic turnover in hippocampal formation slices incubated in vitro. The rate of incorporation of [3H]inositol into all three inositol phospholipids was stimulated by the addition of methacholine to the medium. Moreover, following steady-state labelling of the inositol lipids with [3H]inositol, methacholine in the presence of 10 mM LiCl caused a transient fall of 13% in the radiochemical concentration of phosphatidylinositol 4,5-bisphosphate after only 30 s stimulation and a fall of 15% in the radiochemical concentration of phosphatidylinositol after 30 min. Concomitantly, there was an approximately stoichiometric rise in the radiochemical concentration of inositol phosphates. Thus, we suggest that methacholine stimulates an inositol phospholipid phosphoinositidase C in rat hippocampal formation slices.

Presynaptic cholinergic action in the hippocampus

The hippocampus is among the regions in the brain richest in M1 cholinergic receptors. Topical application of acetylcholine (ACh) onto hippocampal slices produces a characteristic complex response consisting of a depolarization, an increase in input resistance especially upon depolarization and a blockade of a slow afterhyperpolarization (AHP). The first two of these responses can be recorded also in non-cholinergic septal neurons in an area which contains about 8% of the M1 muscarinic receptors found in the hippocampus. The responses of hippocampal but not septal neurons to ACh involve an increase in the spontaneous synaptic activity and a decrease in evoked responses to afferent stimulation. The dissociated hippocampal culture was used to study these presynaptic effects. The neurons in culture possess muscarinic receptors which develop gradually over a period of several weeks after plating. ACh rarely depolarizes hippocampal neurons in culture. Instead, it causes an increase in spontaneous discharge of small postsynaptic currents (PSC's) and a marked decrease of large, evoked PSC's. In some cultured hippocampal cells ACh reduced ICa without affecting any of several outward K currents studied. It is suggested that ACh reduces evoked activity by reducing Ca currents at presynaptic terminals.

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.

Calcium-dependent pirenzepine-sensitive muscarinic response in the rat hippocampal slice

Using intracellular recording techniques in the rat hippocampal slice, we observed that muscarinic agonists produce a transient Ca2+-dependent depolarization that may be related to the phosphatidylinositol cycle. First, it was more readily produced by muscarinic group A agonists, which strongly enhance the breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) than by group B agonists, which are less efficacious. Second, the Ca2+-dependent response was blocked by pirenzepine (PRZ), a selective muscarinic antagonist that blocks PIP2 breakdown in forebrain. Both group A and group B muscarinic agonists caused equivalent maintained levels of depolarization that were relatively insensitive to PRZ. The data suggest that the Ca2+-dependent response is fundamentally unlike other muscarinic responses that have been described in hippocampus.

Synaptic activation of a cholinergic receptor in rat hippocampus

Tetanic stimulation of the basal dendritic field (stratum oriens) of Ca1 area of rat hippocampal slice can produce a slow depolarization associated with an increase in input resistance, decrease in accommodating properties of the cell to a depolarizing drive and an increase in synaptic activity. These effects are enhanced by an acetylcholinesterase inhibitor, eserine, and blocked by the muscarinic antagonists, pirenzepine and atropine. The accommodating property of the cell is more sensitive to the stimulation than the membrane potential or input resistance. The effects of oriens stimulation can be obtained also in fornix-fimbria transected hippocampal slice indicating that it may activate a local cholinergic or non-cholinergic pathway. The stimulation causes a heterosynaptic enhancement of reactivity of neurons to afferent stimulation indicating that acetylcholine may cause an enhanced excitability of hippocampal neurons.

Stimulation of muscarinic receptor in hippocampal neuron induces characteristic increase in cytosolic free Ca2+ concentration

Changes in cytosolic free Ca2+ concentration ([Ca2+]i) in response to acetylcholine (ACh) were examined by fura-2 fluorometry in cultured rat hippocampal neurons. ACh (greater than or equal to 10(-5) M) induced an increase in [Ca2+]i composed of fast transient and slow long-lasting phases. Atropine (10(-8) M) abolished the fast component and greatly reduced the slow component. The slow component was selectively blocked by pirenzepine (10(-6) M). The effect of ACh remained partially in a Ca2+-deficient medium where effects of L-glutamate and KCl (50 mM) were abolished. Present results suggest that ACh elevates [Ca2+]i by activation of muscarinic receptor subtypes, one of which is coupled with ion channels and the other of which transduces the ACh binding to mobilization of intracellularly stored Ca2+.

Selective kainic acid lesions in cultured explants of rat hippocampus

The influence of the excitotoxin kainic acid (KA) on cultivated explants of rat hippocampus was investigated. Addition of 3 microM KA to the culture medium over 24-48 h induced a destruction of the pyramidal cells in the CA3 region, whereas the CA1 pyramidal cells and the granule cells were left undamaged. Higher concentrations (10-100 microM) of KA destroyed also the latter cell groups. The selectivity of the KA lesion at 3 microM was further indicated by the fact that the acetylcholinesterase-positive neurons in the hippocampus were not destroyed through KA administration and that the stereoisomer dihydrokainic acid was ineffective in inducing lesions. Application of tetrodotoxin did not protect the CA3 pyramidal cells from KA lesion, whereas gamma-glutamylaminomethylsulphonic acid (GAMS) only offered a very small, statistically not significant, protection. Baclofen protected the cultures slightly from KA lesions but not when added together with GAMS. Possible mechanisms responsible for the KA lesions in these cultures are discussed.

Slow cholinergic excitation of guinea pig hippocampal neurons is mediated by two muscarinic receptor subtypes

Stimulation of cholinergic fibers or bath application of carbachol (0.1-10 microM) induced a slow excitability increase in CA3 neurons and dentate granule cells of hippocampal slices. This effect which was antagonized by atropine (1 microM) was mediated by two receptor subtypes: a pirenzepine (10 microM)-insensitive receptor, 'M2', and a pirenzepine (1 microM)-sensitive receptor, 'M1'. The M2-receptor activation led to a blockade of slow afterhyperpolarizations following trains of action potentials and to the occurrence of threshold-activated plateau-depolarizations associated with a conductance increase. The M1-receptor mediated a membrane depolarization sometimes associated with a conductance decrease which reversed its polarity at membrane potentials negative to -80 mV. The 'slow excitatory postsynaptic potential' which results from activation of cholinergic fibers is thus caused by the activation of two receptor subtypes.

Mechanisms of action of acetylcholine in the guinea-pig cerebral cortex in vitro

The mechanisms of action of acetylcholine (ACh) in the guinea-pig neocortex were investigated using intracellular recordings from layer V pyramidal cells of the anterior cingulate cortical slice. At resting membrane potential (Vm = -80 to -70 mV), ACh application resulted in a barrage of excitatory and inhibitory post-synaptic potentials (p.s.p.s) associated with a decrease in apparent input resistance (Ri). ACh, applied to pyramidal neurones depolarized to just below firing threshold (Vm = -65 to -55 mV), produced a short-latency hyperpolarization concomitant with p.s.p.s and a decrease in Ri, followed by a long-lasting (10 to greater than 60 s) depolarization and action potential generation. Both of these responses were also found in presumed pyramidal neurones of other cortical regions (sensorimotor and visual) and were blocked by muscarinic, but not nicotinic, antagonists. The ACh-induced hyperpolarization possessed an average reversal potential of -75.8 mV, similar to that for the hyperpolarizing response to gamma-aminobutyric acid (GABA; -72.4 mV) and for the i.p.s.p. generated by orthodromic stimulation (-69.6 mV). This cholinergic inhibitory response could be elicited by ACh applications at significantly greater distance from the cell than the slow depolarizing response. Blockade of GABAergic synaptic transmission with solution containing Mn2+ and low Ca2+, or by local application of tetrodotoxin (TTX), bicuculline or picrotoxin, abolished the ACh-induced inhibitory response but not the slow excitatory response. In TTX (or Mn2+, low Ca2+) the slow excitatory response possessed a minimum onset latency of 250 ms and was associated with a voltage-dependent increase in Ri. Application of ACh caused short-latency excitation associated with a decrease in Ri in eight neurones. The time course of this excitation was similar to that of the inhibition seen in pyramidal neurones. Seven of these neurones had action potentials with unusually brief durations, indicating that they were probably non-pyramidal cells. ACh blocked the slow after-hyperpolarization (a.h.p.) following a train of action potentials, occasionally reduced orthodromically evoked p.s.p.s, and had no effect on the width or maximum rate of rise or fall of the action potential. It is concluded that cholinergic inhibition of pyramidal neurones is mediated through a rapid muscarinic excitation of non-pyramidal cells, resulting in the release of GABA. In pyramidal cells ACh causes a relatively slow blockade of both a voltage-dependent hyperpolarizing conductance (M-current) which is most active at depolarized membrane potentials, and the Ca2+-activated K+ conductance underlying the a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscarinic receptors in slice cultures of rat brain

Muscarinic acetylcholine receptors in organotypic slice cultures of hippocampus of the rat, have been examined using the tritiated muscarinic antagonist quinuclidinylbenzilate [( 3H]QNB) as a as a marker. Maximum specific binding of [3H]QNB in mature explants of hippocampus amounted to 316 fmol/mg protein and a dissociation constant (KD) of 185 pM was determined. Scatchard analysis suggested binding to one single binding site. In younger cultures smaller KDs were registered. This decrease in ligand affinity in maturer cultures possibly reflects a decrease in the turnover of acetylcholine. Muscarinic antagonists inhibited the total binding of [3H]QNB significantly, whereas muscarinic agonist, nicotinic antagonists and cholinesterase inhibitors had no influence whatsoever on the total binding of [3H]QNB. The content of muscarinic acetylcholine receptors varied between cultures with explants from different brain areas: hippocampus greater than striatum greater than septum greater than spinal cord greater than cerebellum. These in vitro results are generally in good agreement with results obtained in situ by other investigators and suggest that the binding of [3H]QNB observed in these cultures is indeed correlated to specific muscarinic receptor sites.

Transplanted septal neurons make viable cholinergic synapses with a host hippocampus

Cell suspensions from the fetal septal region were injected stereotaxically into the hippocampus of fornix-fimbria-transected adult rats. The host rats were sacrificed up to 3 months after the operation and the hippocampus sliced into 350 microns transverse slices. Intracellular recording was made from CA1 neurons adjacent to the graft. Electrical stimulation of the graft produced a voltage-dependent depolarization in some recorded neurons. This was associated with an increase in spontaneous and anodal break action potential discharges. In addition, a slow after-hyperpolarization (AHP) which typically follows a burst discharge was blocked during the depolarization indicating that the stimulation may block a Ca2+-dependent K+ current. The effects of the stimulation were antagonized by atropine. A response to the stimulation was seen 2 weeks but not 1 week after grafting. Over time, cells that were located away from the graft became activated by the stimulation. This was correlated with the extent of proliferation of acetylcholinesterase-containing fibers around the graft. These results suggest that grafted septal neurons make viable cholinergic connections with a host hippocampus.

Cellular and connective organization of slice cultures of the rat hippocampus and fascia dentata

This study examined the cellular and connective organization of hippocampal tissue taken from 6-8-day-old rats and cultured by the roller tube technique for 3-6 weeks. In the cultures containing the fascia dentata and the hippocampus proper (CA1, CA3, CA4) the main cell and neuropil layers were organotypically organized when observed in ordinary cell stains. The normal distribution of smaller cell populations of AChE-positive neurons and somatostatin-reactive neurons was demonstrated by histochemical and immunohistochemical methods. Both cell types were mainly confined to str. oriens of CA3 and CA1 and the dentate hilus (CA4). Individual dentate granule cells and hippocampal pyramidal cells were injected with lucifer yellow and HRP, revealing great stability of the dendritic patterns of these cells in the culture condition. The same was found for the axonal branching and termination of HRP-filled mossy fibers arising from an HRP-injected granule cell. The preservation of organotypic afferent patterns in the cultures was also shown by Timm staining of the terminal distribution of the mossy fiber system. Mossy fiber terminals, with characteristic ultrastructural features verified in the electron microscope, were thus found in the hilus (CA4) and along the CA3 pyramidal cell layer onto the CA3-CA1 transition. Depending on the amount of dentate tissue relative to CA3 the terminals could stop before reaching CA1 (small fascia dentata) or take up additional intra and infrapyramidal locations along CA3 (small CA3). In cultures with a gap in the CA3 pyramidal cell layer some mossy fiber terminals were found in contact with the CA3 pyramidal cells beyond the gap. In all cultures there was an aberrant projection of supragranular mossy fibers. This projection is analogous to the one known from lesion and transplant studies to form in the absence of the entorhinal perforant path input to the dentate molecular layer. Also, in accordance with these studies the Timm staining pattern of the outer parts of the dentate molecular layer and the entire molecular layer of the hippocampus was altered corresponding to the spread of afferents normally confined to the inner zone of the dentate and str. radiatum of CA3 and CA1. Possibly as a consequence of the lack of normal targets for projections from CA1, this subfield contained an unusually dense Timm staining suggestive of autoinnervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Slice cultures of cerebellar, hippocampal and hypothalamic tissue

Cerebellar, hippocampal and hypothalamic slices prepared from newborn and 7-day-old rats were cultured by means of the roller-tube technique. Identification of cells was made easier by the fact that at least part of the characteristic cytoarchitecture of the tissue was preserved in vitro. Cerebellar Purkinje cells and neurones of the deep cerebellar nuclei were recognized on the basis of their size, their location within the culture and their dendritic arborization. Pyramidal cells of all hippocampal subfields displayed their characteristic basal and apical dendritic trees with numerous spinous processes. Hippocampal granule cells gave rise to a monopolar dendritic arbor; their axons terminated in the dentate hilus and CA3 region. Golgi-like immuniperoxidase staining allowed localization of groups of neurophysin-positive neurones in slices prepared from the anterior hypothalamus. These neurones, bilaterally bordering the third ventricle, usually displayed a simple dendritic arborization and fine beaded axons. - Cultivation of brain slices prepared from young rats offers particular advantages in that the cultured cells are organized in an organotypic monolayer and individual living neurones may be directly visualized.

Organotypic development of neonate rabbit hippocampus in roller tube culture

Maintenance of organotypic cultures of hippocampus derived from neonate rabbit has not been previously reported. The study described here was undertaken to define the conditions most suitable for organotypic development, in vitro, of this structure. Slices of hippocampus, on flying cover-slips, were maintained on plasma clots in roller tubes, for periods of up to 6 weeks. The results showed that the explanted, immature hippocampus developed in a manner which parallels the in vivo development, previously described. Specifically, pronounced neuronal differentiation was noted as the cultures matured. There is evidence that the hippocampus of rabbit, in vivo, at 3 weeks of age has assumed the mature pattern of neuronal and synaptic differentiation. Such differentiation similarly occurred in the cultured hippocampus described in this study. This system would serve as an ideal tool in applications in experimental neuropathology, where the use of a model of a phylogenetically advanced central nervous system is preferred.

Facilitation by acetylcholine of tetrodotoxin-resistant spikes in rat hippocampal pyramidal cells

The electrical activity of hippocampal pyramidal cells was studied in slice cultures during blockade of the regenerative Na currents. In the presence of tetrodotoxin, these neurones had a mean resting potential of -68 mV, a membrane input resistance of 87 M omega and displayed marked non-linearities in their current voltage relationship. In response to depolarizing stimuli, pyramidal cells generated action potentials of small amplitude, slow rise and long duration. These tetrodotoxin-resistant spikes were abolished by calcium conductance blockers such as cobalt and cadmium ions. Acetylcholine applied to the bath or by iontophoresis depolarized pyramidal cells, elicited spontaneous tetrodotoxin-resistant spikes and facilitated spiking evoked by depolarizing rectangular current pulses or a current ramp. The effects of acetylcholine were not only slow in onset, but also prolonged; they were completely reversible and sensitive to atropine and calcium-antagonists such as cadmium and cobalt ions which, respectively, reduced and abolished these effects. After hyperpolarizations following injection of depolarizing current pulses were suppressed by acetylcholine and often transformed into depolarizing afterpotentials. Acetylcholine had no effect on voltage-independent conductances as determined by application of hyperpolarizing current pulses. These results could be explained by inhibition of the voltage-dependent K+-current, i.e. the M current (blockade of the calcium current could remove any depolarizing influence resulting from M current inhibition) or by a direct activation of a voltage-dependent calcium current by muscarinic agonists.

Modulation of low calcium induced field bursts in the hippocampus by monoamines and cholinomimetics

The influence of monoamine transmitter candidates, acetylcholine and related substances on rhythmic depolarization shifts (field bursts) in the CA 1 area of hippocampal slices from rats in low calcium (0.2 mmol X 1(-1) ) high magnesium (4 mmol X 1(-1) ) was investigated. Acetylcholine (ACh), histamine (HA) and H2-agonists, noradrenaline (NA) and beta-agonists at nano- to micromolar concentrations as well as dopamine (DA) and 8-bromo-cyclic AMP at 100 mumol X 1(-1) accelerated the field bursts. H2-antagonists blocked HA actions, beta-antagonists blocked NA actions selectively; muscarinic antagonists blocked ACh, HA and NA actions. H1-agonists, serotonin, dopamine and adenosine slowed the field bursts at micromolar concentrations. These effects parallel the action of the tested substances on afterhyperpolarizations in CA 1 pyramidal cells. High sensitivity and specificity make this response of the field bursts an excellent model to study postsynaptic transmitter actions in the central nervous system.

Chemical transmission in the rat interpeduncular nucleus in vitro

We have used a rat brain-slice preparation to study the effects of some cholinomimetic and amino acid agonists and antagonists on the discharge frequency of neurones in the interpeduncular nucleus (i.p.n.), and on the response of these neurones to electrical stimulation of their main excitatory input, the fasciculus retroflexus of Meynert (f.r.m.). A high proportion of i.p.n. neurones were excited by carbachol, acetylcholine (ACh) and muscarine, but methacholine was less effective. The amino acids L-glutamate and L-aspartate were highly effective stimulants of i.p.n. neurones. The responses to ACh or carbachol were greatly reduced by the nicotinic blocking agents hexamethonium, d-tubocurarine and mecamylamine but only slightly reduced by atropine. The response to muscarine was abolished by low doses of atropine. Alpha-Bungarotoxin did not block the response of i.p.n. neurones to f.r.m. stimulation or to cholinomimetic agonists. The response of i.p.n. neurones to f.r.m. stimulation was not appreciably affected by high doses of nicotinic antagonists or atropine nor was there any enhancement of the response by physostigmine. The amino acid antagonists gamma-D-glutamylglycine (gamma DGG) and 2-amino phosphonovalerate (2-APV) were effective blockers of the response to f.r.m. stimulation and preferentially reduced responses to aspartate while having little effect on responses to glutamate or cholinomimetic agonists. It is concluded that ACh is an unlikely candidate for transmitter in this pathway despite abundant neurochemical evidence in its favour. It is more likely that the transmitter is an excitatory amino acid, probably an aspartate-like substance.