U. Misgeld

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.

Effects of (-)baclofen on inhibitory neurons in the guinea pig hippocampal slice.

Intracellular recordings were made from electrophysiologically identified inhibitory neurons in the dentate hilus. (−)Baclofen (0.1–0.5 μmol/l), applied by the bath, strongly hyperpolarized inhibitory neurons, reduced their input resistance and induced outward currents under voltage clamp at holding potential of −60 mV in cells recorded with KCl-filled electrodes. Increasing the (−)baclofen concentration (up to 1 μmol/l) did not increase the amplitude of the outward current, but increased its duration. (−)Baclofen depressed Cl-dependent IPSPs evoked by perforant path stimulation in inhibitory neurones, granule cells and CA3 neurons. In the case of inhibitory neurons and CA3 neurons, depression of IPSPs, membrane hyperpolarization and increase in membrane conductance concurred. All effects were blocked by BaCl2 (1 mmol/l) in the superfusate. In the case of granule cells, depression of IPSPs by (−)balcofen out-lasted an only small membrane hyperpolarization, conductance increase or outward current. High concentrations (up to 10 μmol/l) of (−)baclofen depressed evoked IPSPs of granule cells for an extended period of time, but the other effects remained small and transient. IPSPs elicited in granule cells by microdrop application of glutamate to the dentate hilus were also blocked by (−)baclofen, but spontaneous IPSPs were only reduced in amplitude. We suggest that the blockade of GABAA receptor-mediated IPSPs of hippocampal neurons by the GABAB receptor agonist (−)baclofen can be explained by a K-dependent hyperpolarization of inhibitory neurons.

Activity dependent alkaline and acid transients in guinea pig hippocampal slices.

Changes of extracellular proton concentration ([H+]o) and K+ activity ([K+]o) were simultaneously measured by ion-sensitive microelectrodes in the CA3 region of guinea pig hippocampal slices. Repetitive electrical stimulation and application of glutamate or GABA were associated with prominent alkaline transients of up to 0.2 pH units lasting 2-10 s followed by smaller acid transients lasting up to 4 min. About 10-fold smaller alkaline transients were induced by spontaneous field discharges in the presence of bicuculline. The time to the maximal amplitude of the alkaline transients and the time to maximal increases of [K+]o were in the same range, concurring with the assumption that alkaline transients are due to a proton influx through cationic channels. However, spontaneous field discharges in low-calcium solution in which synaptic transmission is reduced were associated with acid transients of up to 0.02 pH units lasting 2-20 s. An alkaline transient was superimposed on the acid transient only when increases of [K+]o exceeded 1.5 mM. The effects of changing [H+]o on electrically evoked field potentials and spontaneous field discharges were studied in the range from pH 7.00 to 7.80. Electrically evoked field potentials were markedly depressed from pH 7.15 to 7.00 and enhanced from pH 7.60 to 7.80. The frequency of spontaneous field discharges in the presence of bicuculline significantly decreased by reducing pH from 7.40 to 7.30 and continuously increased from pH 7.40 to 7.80. In the same way, the frequency and the amplitude of spontaneous field discharges in low-calcium solution decreased from pH 7.40 to 7.15 and increased from pH 7.40 to 7.80.(ABSTRACT TRUNCATED AT 250 WORDS)

Central cholinergic synaptic transmission

Cholinergic neurons, pathways and synapses.- Behavioral neuroanatomy of cholinergic innervation in the primate cerebral cortex.- Afferent connections of the forebrain cholinergic projection neurons, with special reference to monoaminergic and peptidergic fibers.- Central cholinergic synapses: The septohippocampal system as a model.- Cholinergic-GABAergic synaptic interconnections in the rat amygdaloid complex: An electron microscopic double immunostaining study.- Topography of ?NGF receptor-positive and AChE-reactive neurons in the central nervous system.- Chol-1: A cholinergic-specific ganglioside of possible significance in central nervous system neurochemistry and neuropathology.- Acetylcholine-induced postsynaptic potentials and excitatory changes.- Pharmacological characterization of muscarinic responses in rat hippocampal pyramidal cells.- Mediation of acetylcholines excitatory actions in central neurons.- Presynaptic cholinergic action in the hippocampus.- Opposing effects of acetylcholine on the two classes of voltage-dependent calcium channels in hippocampal neurons.- Muscarinic slow EPSPs in neostriatal and hippocampal neurons in vitro.- Carbachol and pirenzepine discriminate effects mediated by two muscarinic receptor subtypes on hippocampal neurons in vitro.- Cholinergic activation of medial pontine reticular formation neurons in vitro.- Cholinergic responses in human neocortical neurones.- Cholinergic modulation of hippocampal epileptic activity in vitro.- The cholinergic nucleus basalis: A key structure in neocortical arousal.- Cholinergic mechanisms in the telencephalon of cat and chicken.- Cholinergic receptor types and their modulation.- Central nicotinic acetylcholine receptors in the chicken and Drosophila CNS: Biochemical and molecular biology approaches.- Modulation of the sensitivity of nicotinic receptors in autonomic ganglia.- Muscarinic modulation of acetylcholine release: Receptor subtypes and possible mechanisms.- Characterization of muscarinic receptors modulating acetylcholine release in the rat neostriatum.- Distribution of cholinergic receptors in the rat and human neocortex.- Acetylcholine and plasticity of the CNS.- Effects of chronic in vivo replacement of choline with a false cholinergic precursor.- Development of the septohippocampal projection in vitro.- A role of basic fibroblast growth factor for rat septal neurons.- Survival, growth and function of damaged cholinergic neurons.- Restoration of cholinergic circuitry in the hippocampus by foetal grafts.- Effects of colchicine treatment on the cholinergic septohippocampal system.- Effect of early visual pattern deprivation on development and laminar distribution of cholinergic markers in rat visual cortex.- The role of muscarinic acetylcholine receptors in ocular dominance plasticity.- Acetylcholine-dopamine balance in striatum: Is it still a target for antiparkinsonian therapy?.

Golgi-like staining of neocortical neurons using retrogradely transported horseradish peroxidase.

Neocortical pyramidal neurons stained solidly with horseradish peroxidase (HRP) two days after striatal, thalamic or cortical HRP injections are described in the rabbit, rat and marmoset monkey. The dendritic detail seen in these solid HRP-labeled neurons approaches that produced by the Golgi method.

Differences in baclofen-sensitivity between CA3 neurons and granule cells of the guinea pig hippocampus in vitro

In the slice preparation of the guinea pig hippocampus, the effects of (+/-) baclofen added to the Krebs-Ringer solution on dentate granule cells and CA3 pyramidal cells were investigated by means of intracellular recording techniques. In a 10-25 microM concentration, baclofen reduces the inhibitory postsynaptic potentials of the granule cells evoked by electrical stimulation of the perforant path and hyperpolarizes the granule cell membrane slightly. The reduction of both, the excitatory and inhibitory postsynaptic potentials of CA3 pyramidal cells evoked by mossy fiber stimulation, however, is accompanied by a strong hyperpolarization and conductance increase. Further, repetitive discharges of granule cells elicited in the presence of the convulsant bicuculline (25 microM) are hardly affected by baclofen (50 microM), whereas those of CA3 neurons are blocked.

γ-Aminobutyric acid-induced ion movements in the guinea pig hippocampal slice

gamma-Aminobutyric acid (GABA)-induced regional changes of extracellular Cl, K and Na concentration ([Cl]o, [K]o, [Na]o), as well as of the extracellular space were measured with ion-sensitive microelectrodes in guinea pig hippocampal slices. Microdrop application of GABA to the pyramidal cell layer of CA3 or CA1 induced a decrease of [Cl]o, while application to the dendritic layer of CA3 or CA1 induced an increase of [Cl]o in addition. All changes of [Cl]o persisted in the presence of TTX and were blocked by bath-applied bicuculline. The GABA-induced decrease of [Cl]o was reduced by bicuculline application to the pyramidal cell layer. The increase of [Cl]o was blocked by bicuculline application to the dendritic layer. Additionally, GABA induced an increase of [K]o and decreases/increases of [Na]o. Changes of [Cl]o, [K]o and [Na]o together were approximately electroneutral. [Cl]o increases were exaggerated and [Cl]o decreases partly masked by shrinkage of the extracellular space after GABA application. Changing [K] in the superfusate transiently changed GABA-induced [Cl]o movements in a way predicted from a change in driving force due to the effect of [K] on membrane potential. Then a partial recovery followed towards the original [Cl]o change. We conclude that inward and outward Cl transports maintain [Cl]i below equilibrium in CA3 and CA1 pyramidal somata and above equilibrium in CA3 and CA1 dendrites. The significance of this Cl-distribution for hippocampal inhibition is discussed.

Neurons sensitive to pH in slices of the rat ventral medulla oblongata.

The effects of extracellular pH changes on neurons in slices of the rat ventral medulla oblongata were investigated by extracellular recording. Changes in discharge rate were correlated with pH changes in the tissue next to the recorded cell, as measured by H+-selective microelectrodes. pH was altered by varying the bicarbonate concentration ([HCO3−]) in the superfusion solution. In 136 out of 316 neurons, the number of spontaneous or electrically evoked discharges per unit time increased with decreasing pH and decreased with increasing pH. Changes of only 0.01–0.04 pH unit were effective in these pH-sensitive neurons. The response was transient; the discharge rate returned to the control value within a few minutes. The pH sensitivity persisted in the presence of 0.5 μM atropine, 20 μM bicuculline and after replacing Ca2+ by Mg2+ in the superfusion solution to reduce synaptic transmission. The response to the same pH decrease was stronger when increasing PCO2 than when reducing [HCO3−]0. The pH-induced response significantly increased during hypoxia. The results show that in the ventral medulla oblongata neurons exist that transiently respond to small decreases and increases of pH. The pH sensitivity is an intrinsic property of these neurons; it is not due to a synaptic mechanism but is modulated by PCO2 and PO2.

Presynaptic muscarinic modulation of nicotinic excitation in the rat neostriatum

In rat neostriatal slices, cholinergic agents were tested for their effects on endogenous ACh release and on electrical activity. ACh release was evoked by 25 mM K+ during two 5-min periods between which a slice was allowed to rest for 20 min; drugs were present during the second stimulation period. In the absence of a cholinesterase inhibitor, only Ch outflow was monitored. For the recording of electrical activity, intrastriatal stimulation evoked field potentials which were monitored in the absence and presence of drugs in the perfusate. Atropine (1-100 microM) increased endogenous ACh release by 32-91% and effective doses were 10-fold lower in the presence of a cholinesterase inhibitor. Atropine also increased the amplitudes of synaptic population spikes in the field potentials. The muscarinic agonists muscarine (100 microM) and oxotremorine (25 and 100 microM) decreased endogenous ACh release. Atropine (10 microM) blocked the depressant effect of muscarine (100 microM). Muscarine (100 microM-1 mM) and oxotremorine (10-100 microM) decreased the electrically evoked excitation in the rat neostriatal slices, and their effects were reversed by atropine. Only higher concentrations of nicotine (1 and 5 mM) decreased the synaptic population spikes, but potassium-stimulated Ch outflow was not affected. It is concluded that in the neostriatum presynaptic muscarinic receptors modulate nicotinic excitation since potassium-stimulated ACh release and intrinsically evoked synaptic excitation are influenced by muscarinic drugs in the same way.

Neurotransmission in the hippocampus

1 Introduction.- 2 Neuronal Elements in the Hippocampus and Their Synaptic Connections.- 2.1 Hippocampal Neurons.- 2.1.1 Pyramidal Cells in CA1.- 2.1.2 Pyramidal Cells in CA3.- 2.1.3 Dentate Granule Cells.- 2.1.4 Nonpyramidal Neurons.- 2.2 Synaptic Connections of Identified Neurons in the Hippocampus.- 2.2.1 Principles of Hippocampal Innervation.- 2.2.2 Extrinsic Afferents to the Hippocampus and Fascia Dentata.- 2.2.2.1 Entorhinal Afferents.- 2.2.2.2 Cholinergic Fibers.- 2.2.2.3 Commissural Afferents.- 2.2.3 Intrinsic Connections.- 2.2.3.1 Mossy Fibers.- 2.2.3.2 GABAergic Terminals.- 2.3 Functional Implications.- 3 Membrane Properties and Postsynaptic Responses of Hippocampal Neurons.- 3.1 Various Cell Types in the Slice Preparation.- 3.2 Short-Distance Excitatory Connections.- 3.3 GABAergic Inhibition.- 3.4 Cholinergic Excitation.- 3.5 Conclusions.- 4 The Enzyme Histochemistry of Neurotransmitter Metabolism.- 4.1 Enzymes Related to the Cholinergic Transmitter System.- 4.1.1 Acetylcholinesterase.- 4.2 Enzymes Related to the Amino Acid Transmitter System.- 4.2.1 Enzymes in Glutamate Metabolism.- 4.2.1.1 Aspartate Aminotransferase.- 4.2.1.2 Glutamate Dehydrogenase.- 4.2.1.3 Glutaminases.- 4.2.2 Enzymes in GABA Metabolism.- 4.2.2.1 GABA Transaminase.- 4.3 Conclusions.- 5 Receptor Autoradiography in the Hippocampus of Man and Rat.- 5.1 Quantitative Receptor Autoradiography as a Functional Approach.- 5.2 Distribution of Receptors in the Hippocampus of the Rat.- 5.2.1 5-HT System.- 5.2.2 Cholinergic System.- 5.2.3 Glutamate System.- 5.2.4 Colocalization of Different Receptors on the Level of Hippocampal Layers.- 5.3 Distribution of Binding Sites in the Human Hippocampus.- 5.3.1 5-HT1 System.- 5.3.2 Glutamate System.- 5.4 Comparison of Receptor Distribution in the Hippocampus of Rat and Man.- 5.4.1 5-HT System.- 5.4.2 Glutamate System.- Concluding Remarks.- Summary.- References.