Disinhibition in the rat septum mediated by M1 muscarinic receptors

A potential cholinergic mechanism of procaine's limbic activation

The local anesthetic procaine, when administered to humans intravenously (i.v.), yields brief intense emotional and sensory experiences, and concomitant increases in anterior paralimbic cerebral blood flow, as measured by positron emission tomography (PET). Procaine's high muscarinic affinity, together with the distribution of muscarinic receptors that overlaps with brain regions activated by procaine, suggests a muscarinic contribution to procaine's emotional and sensory effects. This study evaluates the effects of procaine on cerebral muscarinic cholinergic receptors in the anesthetized rhesus monkey. Whole brain and regional muscarinic receptor binding was measured before and after procaine administration on the same day in three anesthetized rhesus monkeys with PET and the radiotracer 3-(3-(3[18F]fluoropropylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([18F]FP-TZTP), a cholinergic ligand that has preferential binding to muscarinic (M(2)) receptors. On separate days each animal received six different doses of i.v. procaine in a randomized fashion. Procaine blocked up to approximately 90% of [18F]FP-TZTP specific binding globally in a dose-related manner. There were no regional differences in procaine's inhibitory concentration for 50% blockade (IC50) for [18F]FP-TZTP. Tracer delivery, which was highly correlated to cerebral blood flow in previous monkey studies, was significantly increased at all doses of procaine with the greatest increases occurring near procaine's IC50 for average cortex. Furthermore, anterior limbic regions showed greater increases in tracer delivery than nonlimbic regions. Procaine has high affinity to muscarinic M2 receptors in vivo in the rhesus monkey. This, as well as a preferential increase of tracer delivery to paralimbic regions, suggests that action at these receptors could contribute to i.v. procaine's emotional and sensory effects in man. These findings are consistent with other evidence of cholinergic modulation of mood and emotion.

Muscarinic agonist carbachol depresses excitatory synaptic transmission in the rat basolateral amygdala in vitro

Intracellular recordings in slice preparations of the basolateral amygdala were used to test which excitatory amino acid receptors mediate the excitatory postsynaptic potentials due to stimulation of the external capsule. These recordings were also used to examine the action of muscarinic agonists on the evoked excitatory potentials. Intracellular recordings from amygdaloid pyramidal neurons revealed that carbachol (2-20 microM) suppressed, in a dose-dependent manner, excitatory postsynaptic responses evoked by stimulation of the external capsule (EC). This effect was blocked by atropine. The estimated effective concentration to produce half-maximal response (EC(50)) was 6.2 microM. Synaptic suppression was observed with no changes in the input resistance of the recorded cells, suggesting a presynaptic mechanism. In addition, the results obtained using the paired-pulse protocol provided additional support for a presynaptic action of carbachol. To identify which subtype of cholinergic receptors were involved in the suppression of the EPSP, four partially selective muscarinic receptor antagonists were used at different concentrations: pirenzepine, a compound with a similar high affinity for muscarinic M1 and M4 receptors; gallamine, a noncompetitive antagonist for M2; methoctramine, an antagonist for M2 and M4; and 4-diphenylacetoxy-N-methylpiperidine, a compound with similar high affinity for muscarinic receptors M1 and M3. None of them independently antagonized the suppressive effect of carbachol on the evoked EPSP completely, suggesting that more than one muscarinic receptor subtype is involved in the effect. These experiments provide evidence that in the amygdala muscarinic agonists block the excitatory synaptic response, mediated by glutamic acid, by acting on several types of presynaptic receptors.

Distinct muscarinic receptor subtypes suppress excitatory and inhibitory synaptic responses in cortical neurons

Simultaneous whole cell recordings from monosynaptically connected cortical cells were performed with the use of two patch pipettes to determine the effect of acetylcholine (ACh) on both excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in cultured neurons from rat visual cortex. For 96% of EPSPs and 73% of IPSPs, ACh potently suppressed postsynaptic potentials in a dose-dependent manner. The estimated effective concentrations to produce half maximal response (EC50S) were 30 and 210 nM for EPSPs and IPSPs, respectively. To identify what subtypes of ACh receptors are involved in the suppression of postsynaptic potentials, three different, partially selective muscarinic receptor antagonists were used. According to the comparison of estimated Schild coefficients for each of the three antagonists against the suppression by ACh, EPSPs are most likely mediated by m4 receptors, and IPSPs by m1 receptors. When cells were treated with pertussis toxin, which inactivates m2 and m4 receptors while leaving m1, m3, and m5 receptors intact, 7 of 8 EPSPs were resistant to ACh whereas 8 of 12 IPSPs were still suppressed by ACh. This result supports the interpretation that the suppression of EPSPs was mediated by m4 receptors and that of IPSPs by m1 receptors. To obtain an indication as to whether ACh works presynaptically or postsynaptically, 1/CV2 analysis was carried out. The resultant diagonal alignment of the ratio of 1/CV2 plotted against the ratio of the amplitude of postsynaptic potentials suggests a presynaptic mechanism for the suppression of both EPSPs and IPSPs. In addition, in many cases a large synaptic suppression was observed without an obvious change in the input resistance. Furthermore, in one case where a single inhibitory driver cell was recorded with three different follower cells sequentially, none of the three IPSPs was suppressed by ACh, providing additional support for the presynaptic localization of ACh action. These results suggest that in cerebral cortex ACh has, in addition to its direct facilitatory effect via m3 pharmacology, a suppressive effect on EPSPs and IPSPs via m1 and m4 muscarinic receptors, respectively, probably with a presynaptic site of action. Separation of the actions of ACh into different receptor-second messenger pathways with potential for independent interactions with other neuromodulatory systems may be an important aspect of the mechanism of cholinergic regulation of functional state in cortex. Separation of cholinergic effects at different receptors might also offer a means for selective pharmacological intervention in disorders of sleep or memory.

Hippocampal and cerebellar extracellular amino acids during pilocarpine-induced seizures in freely moving rats

Limbic seizures were provoked in freely moving rats by intrahippocampal administration of the muscarinic receptor agonist pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microliters/min). Changes in extracellular hippocampal and cerebellar glutamate, aspartate and gamma-aminobutyric acid (GABA) levels were monitored during and after pilocarpine administration. Effects of systemic or local administration of anticonvulsants on the seizures and concomitant changes in amino-acid concentrations, were investigated. Pilocarpine-induced seizures were completely abolished after intraperitoneal premedication for 7 days with phenobarbital (15 mg/kg per day) and after intrahippocampal administration of 10 mM phenobarbital and 1 mM carbamazepine (180 min at 2 microliters/min). Rats premedicated with carbamazepine (5 mg/kg per day) still developed seizures. The changes in extracellular hippocampal amino-acid levels suggest that glutamate, aspartate and GABA are not involved in seizure onset, but may play a role in seizure maintenance and/or spread in the pilocarpine animal model of epilepsy. The increases in extracellular amino acids in ipsi- and contralateral cerebellum following limbic seizures provoked in the hippocampus, probably play a role in the 'reversed' diaschisis phenomenon.

A microiontophoretic study of acetylcholine effects in the inferior colliculus of horseshoe bats: implications for a modulatory role

The effects of acetylcholine (ACh) in processing acoustical information in the inferior colliculus (IC) of awake horseshoe bats (Rhinolophus rouxi) were examined with single cell recordings and microiontophoresis. Cholinergic agonists, acetylcholine and carbachol raised the stimulus evoked discharge in 37% and suppressed responses in 16% of the sample. They did not alter the shapes of tuning curves and rate-intensity functions but the latter showed parallel shifting. The nicotinic antagonist, hexamethonium raised neuronal activity in 52% of neurons without affecting discharge patterns. The nonspecific muscarinic antagonist atropine was mostly inhibitory (62% of units) and caused changes in temporal discharge patterns by affecting the tonic response component. The selective muscarinic ml antagonist pirenzepine, also had an inhibitory effect (37% of units) and predominantly influenced the tonic response component. The selective m2 antagonist, gallamine however produced mainly excitatory effects (64% of units) and changed temporal discharge patterns by adding tonic response components. These findings may indicate a differential pre- and postsynaptic synaptic distribution of m1/m2 receptors in the inferior colliculus as reported for other brain structures. The results indicate that ACh plays a neuromodulatory transmitter role in the auditory midbrain by setting the level of neuronal activity. Its exact function in particular behavioral contexts remains to be determined, since the origin of cholinergic innervation of the mammalian IC is still unclear.

Extrinsic and intrinsic substance P innervation of the rat lateral septal area calbindin cells

The electrophysiological observations that substance P administration to the lateral septal area elicits both excitatory and inhibitory responses, together with earlier reports on the multiple sources of substance P innervation of the septum, implies that these axons with distinct origins have different functions. This prompted us to examine the origin and neurochemical character of substance P afferents to the lateral septal area. Chronic surgical isolation of the septum from its ventral afferents and retrograde tracer experiments using wheat germ agglutinin-conjugated horseradish peroxidase, both followed by an immunostaining for substance P, were employed to elucidate the origin of these axon terminals. In order to assess the possible co-existence of substance P with other neurotransmitter substances in the parent cells of the septopetal projections, co-localization studies for substance P and choline acetyltransferase, as well as substance P and GABA, were performed. The comparative distribution of substance P fibers and septal calbindin-containing neurons was also investigated using correlated light and electron microscopic double immunostaining. The results are summarized as follows: (i) the substance P innervation of the lateral septal area derives from several hypothalamic nuclei (including the lateral and lateroanterior hypothalamic area, tuber cinereum and ventromedial hypothalamic nucleus) and tegmental nuclei (the majority of fibers from the laterodorsal and a few from the pedunculopontine tegmental nucleus), as well as intrinsic septal cells; (ii) the septopetal substance P fibers of tegmental origin are cholinergic; intraseptal substance P neurons located in the dorsolateral part of the lateral septum also contain GABA, while substance P neurons seen on the border between the medial and lateral septal area and septopetal hypothalamic substance P cells do not contain GABA or acetylcholine; (iii) substance P fibers from pericellular baskets around calbindin-containing lateral septal neurons with a high degree of selectivity; (iv) approximately 90% of the entire calbindin cell population are postsynaptic targets of substance P axons; (v) their terminals contact the soma and the dendrites of these cells, among them the somatospiny neurons; and (vi) the extrinsic substance P boutons establish asymmetric, while the intrinsic substance P axon terminals form symmetric membrane specializations. Because neurons in the lateral septal area receive hippocampal input and project massively to hypothalamic areas, the different types of substance P input on these neurons can modify the information flow arriving from the hippocampus to diencephalic brain structures at the level of the lateral septal area.

Muscarine increases a voltage-independent potassium conductance through an M4 receptor in rat dorsolateral septal nucleus neurons

The direct effect of muscarine on neurons of the rat dorsolateral septal nucleus (DLSN) was examined by using conventional microelectrode and voltage-clamp techniques. Muscarine (1-50 microM) caused a hyperpolarization accompanied by an increase of a voltage-independent potassium conductance. Pirenzepine competitively antagonized the muscarine-induced hyperpolarization with an apparent dissociation constant (Kd) value of 54 nM. Furthermore, intracellular loading with GTP gamma S, a non-hydrolyzable GTP analog, blocked irreversibly the muscarine-induced hyperpolarization. In addition, pretreatment of neurons with pertussis toxin (PTX) prevented the hyperpolarization produced by muscarine. These results suggest that muscarine hyperpolarizes DLSN neurons via a voltage-independent potassium conductance by acting at M4 subtype receptors which are coupled to a PTX-sensitive G-protein in DLSN neurons.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

State-dependent and state-independent effects of thyrotropin-releasing hormone on medial septum neuronal activity in the brain slices of waking and hibernating ground squirrels

Effects of thyrotropin-releasing hormone on spontaneous activity and responses to medial forebrain bundle stimulation were tested in the units of the medial septum-diagonal band complex in slices taken from the brain of hibernating and waking ground squirrels. Administration of thyrotropin-releasing hormone (0.1 microM) into the flow of incubating medium increased the frequency of spontaneous activity of all the medial septum-diagonal band complex neurons in hibernating ground squirrels and of the majority of neurons in the waking ground squirrels. However, in the septal slices of hibernating ground squirrels this increase was significantly more pronounced. In addition, the neuropeptide slightly increased the frequency of bursts in the majority of cells with rhythmic burst activity. The excitatory influence of thyrotropin-releasing hormone on the units was preserved in conditions of synaptic blockade. In neurons from other structures (lateral septum, medial preoptic area, hippocampus) in the brain slices of both hibernating and waking ground squirrels, thyrotropin-releasing hormone did not usually affect the level of spontaneous discharges. When studying the responses of the medial septum-diagonal band complex neurons to electrical stimulation of medial forebrain bundle it was found that application of thyrotropin-releasing hormone (0.1 microM) led to the disappearance of responses in 50 and 44% of units in the hibernating and waking ground squirrels, respectively; in the rest of the neurons a disturbance of stability and probability of responses was observed. The existence of a modulatory thyrotropin-releasing hormone system which participates post-, and, probably, presynaptically in the regulation of the medial septum-diagonal band complex neuronal activity is suggested. The role of thyrotropin-releasing hormone and of medial septum-diagonal band complex in the neural control of hibernation/euthermic waking cycle is discussed.

Muscarinic responses of rat basolateral amygdaloid neurons recorded in vitro

1. Intracellular recordings were obtained from pyramidal-type neurons in the basolateral amygdaloid nucleus (BLA) in slices of rat ventral forebrain and used to compare the actions of exogenously applied cholinomimetics to the effects produced by electrical stimulation of amygdalopetal cholinergic afferents from basal forebrain. 2. Bath application of carbachol depolarized pyramidal cells with an associated increase in input resistance (Ri), reduced the slow after-hyperpolarization (AHP) that followed a series of current-evoked action potentials and blocked spike frequency accommodation. All of these effects were reversed by the muscarinic antagonist atropine but not by the nicotinic antagonist hexamethonium. 3. Electrical stimulation of amygdaloid afferents within the external capsule evoked a series of synaptic potentials consisting of a non-cholinergic fast excitatory postsynaptic potential (EPSP), followed by early and late inhibitory postsynaptic potentials (IPSPs). Each of these synaptic potentials was reduced by carbachol in an atropine-sensitive manner. 4. Local application of carbachol to pyramidal cells produced a short-latency hyperpolarization followed by a prolonged depolarization. The hyperpolarization and depolarization to carbachol were blocked by atropine but not hexamethonium. 5. The carbachol-induced hyperpolarization was associated with a decrease in Ri and had a reversal potential nearly identical to that of the early IPSP. The inhibitory response was blocked by perfusion of medium containing tetrodotoxin (TTX), bicuculline or picrotoxin, while the subsequent depolarization was unaffected. On the basis of these data, it is concluded that the muscarinic hyperpolarization is mediated through the rapid excitation of presynaptic GABAergic interneurons in the slice. 6. The findings that the carbachol-induced depolarization was associated with an increase in Ri, often had a reversal potential below -80 mV, was sensitive to changes in extracellular potassium concentration and was blocked by intracellular ionophoresis of the potassium channel blocker caesium suggest that it resulted from a muscarinic blockade of one or more potassium conductances. 7. Repetitive stimulation of sites within the slice containing cholinergic afferents evoked a series of fast EPSPs followed by IPSPs. These non-cholinergic potentials were followed by a slow EPSP that lasted from 10 s-4 min. The slow EPSP was enhanced by eserine and blocked by atropine. It was also blocked by TTX or cadmium, indicating that it was dependent on spike propagation and calcium-dependent release of acetylcholine (ACh). 8. Stimulation of cholinergic afferents in the slice mimicked other effects produced by carbachol including blockade of the slow AHP and accommodation of action potential discharge and these actions were potentiated by eserine and blocked by atropine.(ABSTRACT TRUNCATED AT 400 WORDS)

Assessment of a cholinergic contribution to chlordiazepoxide-induced deficits of place learning in the Morris water maze

This investigation sought to characterize the interaction between benzodiazepine and cholinergic systems in place learning in the Morris water maze. In the first experiment, rats were treated with scopolamine (1 mg/kg) alone or concomitantly with one of two doses of flumazenil (15 and 30 mg/kg) or with chlordiazepoxide (5 mg/kg) alone or concomitantly with flumazenil (15 mg/kg). Chlordiazepoxide and scopolamine severely impaired place learning but not cue learning. The low dose of flumazenil completely reversed the impairment produced by chlordiazepoxide and both high and low doses of flumazenil attenuated the place learning deficit produced by scopolamine. Neither dose of flumazenil affected place learning when administered alone. In the second experiment, rats were administered chlordiazepoxide (5 mg/kg) or scopolamine (1 mg/kg) alone or concomitantly with one of four doses of physostigmine (0.05, 0.10, 0.25, and 0.5 mg/kg). Once again, both chlordiazepoxide and scopolamine impaired place but not cue learning. Physostigmine reversed the impairment produced by scopolamine in a dose-dependent manner but failed at every dose to attenuate the impairment produced by chlordiazepoxide. The higher doses of physostigmine impaired place learning when administered alone. None of the drug treatments impaired cue learning. Together, these results suggest that the scopolamine-induced impairment of place learning is due to an increase in benzodiazepine/GABA activity, and contradict the notion that benzodiazepines impair memory by cholinergic mechanisms.

Muscarinic autoreceptors of Torpedo electric organ are of the M1 subtype: evidence by radioligand binding using selective antagonists

The presynaptic muscarinic autoreceptor of Torpedo marmorata electric organ has been characterised by radioligand binding studies using the subtype-selective antagonists pirenzepine, (+)-telenzepine, methoctramine, and AF-DX 116. The presynaptic receptor had relatively high affinity for the M1 antagonists pirenzepine and (+)-telenzepine (Ki = 35 and 7 nM, respectively) and lower affinities for the M2 antagonists AF-DX 116 and methoctramine (Ki = 311 and 277 nM, respectively). Comparison of these binding data with those from an M2 receptor (rat heart membranes) assayed under identical conditions and with data in the recent literature suggests that the Torpedo muscarinic autoreceptor has a pharmacology most similar to the M1 pharmacological subtype of muscarinic acetylcholine receptor.

Spontaneous bursting and non-bursting activity in morphologically identified neurons of the rat dorsolateral septal nucleus, in vitro

Membrane potential-dependent changes in the repetitive firing properties of morphologically identified rat dorsolateral septal nucleus neurons were investigated in a submerged slice preparation using intracellular recording techniques and lithium acetate-Lucifer Yellow-filled microelectrodes. The results indicate that the majority of dorsolateral septal nucleus neurons are capable of burst firing and suggest, moreover, the existence of neuronal subtypes with distinct differences in spike waveform and the pattern of spontaneous activity. In the largest proportion of neurons, single spike activity predominated at membrane potentials near rest while burst-like discharges prevailed at more hyperpolarized membrane potentials. Less frequently observed were neurons exhibiting different burst waveforms at various membrane potentials. In a few neurons, hyperpolarization slowed neuronal firing but did not elicit burst-like discharges. Characteristics such as the presence of burst or single spike discharges, spike afterpotentials, and the membrane potential dependence of repetitive firing patterns did not appear to be closely associated with membrane time constant, membrane resistance, or resting membrane potential. A detailed examination of the somatodendritic and axonal morphology of the Lucifer Yellow-filled cells revealed that these electrophysiologically identified neurons in the dorsolateral septal nucleus are morphologically heterogeneous. However, there did not appear to be any correlation between a particular somatodendritic morphology and the expression of a distinct spontaneous firing pattern. The present findings demonstrate that neurons in the rat dorsolateral septal nucleus are morphologically diverse and capable of intrinsically generating rhythmic neuronal activity. Similar patterns of rhythmic neuronal firing in vivo may provide a substrate for the integration of afferent neuronal activity and have a central role in intraseptal circuitry necessary for generation of hippocampal theta rhythm.

Cholinergic disinhibition in area CA1 of the rat hippocampus is not mediated by receptors located on inhibitory neurons

We studied the effects of carbamylcholine (carbachol; CCh) on monosynaptic inhibitory postsynaptic potentials (IPSPs) evoked in the presence of the excitatory amino acid receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D,L-2-amino-5-phosphonovalerate (APV). CCh (30 microM) blocked late afterhyperpolarizations but did not depress GABAA receptor-mediated fast monosynaptic IPSPs or GABAB receptor-mediated late monosynaptic IPSPs. In the presence of CCh the GABAB receptor agonist (+/- )-baclofen (2 microM) reversibly hyperpolarized pyramidal neurons and depressed monosynaptic IPSPs as under control conditions. Phorbol-12,13-diacetate (PDAc; 10 microM) increased fast and depressed late monosynaptic IPSPs, and prevented depression of IPSPs by baclofen. These results suggest that cholinergic disinhibition in area CA1 of the hippocampus results from decreased synaptic excitation of inhibitory neurons.

Pharmacology of the putative M4 muscarinic receptor mediating Ca-current inhibition in neuroblastoma x glioma hybrid (NG 108-15) cells

1. We have assessed the potency of a range of agonists and antagonists on the muscarinic receptor responsible for inhibiting the Ca-current (ICa) in NG 108-15 hybrid cells. 2. Acetylcholine (ACh), oxotremorine-M and carbachol were potent 'full' agonists (EC50 values were 0.11 microM, 0.14 microM and 2 microM, respectively). Maximum inhibition of peak high-threshold ICa by these agonists was 39.5%. (+/-)-Muscarine, methylfurmethide and arecaidine propargyl ester (APE) were 'partial' agonists, with EC50 values of 0.54 microM, 0.84 microM and 0.1 microM, respectively. 3. Atropine, pirenzepine and himbacine were potent antagonists of muscarinic inhibition of ICa, with apparent pKB values of 9.8, 7.74 and 8.83, respectively. Methoctramine was relatively weak (pKB = 7.63). Atropine and pirenzepine depressed maximum responses to agonists, probably because these antagonists have relatively slow dissociation rates. 4. The characteristic pharmacological profile found for the M4 receptors in these functional experiments (himbacine high affinity, pirenzepine moderate to high affinity, methoctramine low affinity) corresponds well with data from earlier binding experiments (Lazareno et al., 1990). Since mRNA hybridising to probes for the m4 receptor genotype can be detected in these cells, it is suggested that these pharmacological characteristics identify the equivalent expressed receptor subtype M4.

Muscarinic agonists block a late-afterhyperpolarization in medial septum/diagonal band neurons in vitro

Intracellular recordings were made from neurons located in the medial septum (MS), and nucleus of the diagonal band (nDB) from slices of guinea pig brain. These forebrain nuclei contain both cholinergic and noncholinergic neurons that project to the cortex and hippocampus and are involved in many cortical functions. Muscarinic agonists (bethanechol, 2-30 microM) had the specific action to reduce a long-duration afterhyperpolarization (long-AHP) while leaving other shorter duration AHPs intact. Since the long-AHP was observed in both cholinergic and non-cholinergic neurons, muscarinic agonists were not selective for any one cell type. Block of a long-AHP was not associated with a consistent increase in cell excitability and therefore can not fully explain the excitatory actions of acetylcholine (ACh) observed in vivo within the MS/nDB.

Pharmacology of nicotinic receptor-mediated inhibition in rat dorsolateral septal neurones

1. Intracellular electrophysiological techniques were employed to investigate the effects of nicotinic receptor stimulation on rat dorsolateral septal nucleus (DLSN) neurones in a submerged rat brain slice preparation. 2. Acetylcholine (in the presence of the muscarinic antagonist, atropine), nicotine or dimethylphenylpiperazinium (DMPP), applied either by pressure ejection or superfusion, produced predominantly a membrane potential hyperpolarization. 3. Following concentration-response comparisons, DMPP appeared to exhibit fewer desensitizing properties and greater efficacy than nicotine with half-maximal hyperpolarizing responses attainable at 3 and 10 microM, respectively. 4. Pharmacological analyses revealed that the agonist-induced membrane hyperpolarization was sensitive to antagonism by mecamylamine (50-100 microM) and neuronal bungarotoxin (0.2-0.3 microM), but not alpha-bungarotoxin (0.5-1.0 microM), curare (10-50 microM) or dihydro-beta-erythroidine (50-100 microM). 5. Hyperpolarizing responses to DMPP were found to reverse near the equilibrium potential for potassium and were sensitive to changes in extracellular potassium concentration as predicted by the Nernst equation. Under single-electrode voltage clamp, application of DMPP produced an outward current (75-100 pA) which approached reversal at around -88 mV. These findings indicated that the hyperpolarizing response to nicotinic receptor stimulation was mediated by changes in membrane permeability to potassium. 6. DMPP-induced membrane hyperpolarization resulted from a direct action on postsynaptic DLSN neurones since the response persisted under conditions of superfusion with calcium-free/high-magnesium media or tetrodotoxin; both conditions blocked orthodromically induced neurotransmission. The hyperpolarizing response remained unaltered in TTX but was diminished in calcium-free/high-magnesium media. Further studies revealed blockade of the DMPP response following intracellular injection of EGTA. This response was also sensitive to antagonism by various calcium-dependent potassium channel blockers including apamin, barium and tetraethylammonium. 7. Our studies reveal a novel class of CNS nicotinic receptor whose action upon stimulation by an agonist results in a membrane hyperpolarization via a calcium-dependent increase in potassium ion conductance.

Distinct muscarinic receptors inhibit release of gamma-aminobutyric acid and excitatory amino acids in mammalian brain

Intracellular recordings were made from neurons of rat lateral amygdala, nucleus accumbens, and striatum in vitro. Synaptic potentials mediated by gamma-aminobutyric acid and by excitatory amino acids were isolated pharmacologically by using receptor antagonists, and their amplitudes were used as a measure of transmitter release. Muscarine and acetylcholine inhibited the release of both gamma-aminobutyric acid and excitatory amino acids, but measurements of the dissociation equilibrium constants for the antagonists pirenzepine, 11-(2-[(diethylamino)methyl]-1-piperidinyl)acetyl-5,11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepine-6-one, methoctramine, and hexahydrosiladifenidol indicated clearly that different muscarinic receptors were involved (M1 and probably M3, respectively). The differential localization of distinct muscarinic receptor subtypes on terminals releasing the major inhibitory and excitatory transmitters of the brain could be exploited therapeutically in some movement disorders and Alzheimer disease.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Catecholaminergic, GABAergic, and hippocamposeptal innervation of GABAergic "somatospiny" neurons in the rat lateral septal area

This study deals with the neurochemical characterization of the rat lateral septal area (LSA) somatospiny neurons and their innervation by hippocamposeptal, catecholaminergic, and GABAergic fibers. Electron microscopic single and double immunostaining methods were used to label catecholaminergic fibers and GABAergic cells and boutons. Axon terminals originating in the hippocampus were labeled by acute anterograde axon degeneration induced by fimbria-fornix transection 36 hours before sacrifice. Three types of experiments were performed. The convergent catecholaminergic and hippocamposeptal innervation of LSA somatospiny neurons was studied by combining immunostaining for tyrosine hydroxylase (TH) with fimbria-fornix transection. GABAergic neurons and their hippocamposeptal afferents were identified and characterized in colchicine pretreated animals immunostained for glutamic acid decarboxylase (GAD) combined with fimbria-fornix transection. The third experiment aimed at simultaneously visualizing the relationships between catecholaminergic boutons, hippocamposeptal excitatory amino acid containing axon terminals and GABAergic profiles by double immunostaining for TH (the PAP technique) and GAD (the immunogold method) combined with fimbria-fornix transection. The results are summarized as follows: 1) The same LSA somatospiny neurons receive synaptic inputs from the hippocampus and TH immunoreactive fibers which form pericellular baskets around these cells. 2) LSA somatospiny neurons are GABAergic and are postsynaptic targets of GABAergic boutons with unknown origin and hippocamposeptal axon terminals. 3) The double immunostaining experiment, finally, provided direct evidence that the same GABAergic somatospiny neurons are postsynaptic targets of both catecholaminergic and hippocamposeptal afferents. The synaptic interconnections described in this study provide anatomical basis for a better understanding of the action of catecholamines, excitatory amino acids, and GABA on the activity of LSA neurons.

Facilitatory action of muscarine on the slow afterdepolarization of rat dorsolateral septal nucleus neurons in vitro

Intracellular recordings were made from an isolated rat brain slice preparation containing the dorsolateral septal nucleus (DLSN). Calcium-dependent slow afterdepolarizations (slow-ADPs) were observed in a population of neurons, which exhibited bursts of action potentials. Bath application of muscarine (10-20 microM) augmented the slow-ADP and triggered burst firing on top of the slow-ADP. This effect of muscarine was blocked by atropine (1 microM). Muscarine augmented the slowly decaying inward current (slow-ADC) recorded by voltage clamping the afterpotential of cathodally-evoked spikes. Our data suggest that some of the burst-like activity induced by muscarine in a select population of rat DLSN neurons is mediated partly by augmentation of the current which underlies the slow-ADP.

A direct nicotinic receptor-mediated inhibition recorded intracellularly in vitro

Acetylcholine activates both nicotinic and muscarinic receptors in the central nervous system. Although the action of acetylcholine at muscarinic receptor has been well characterized, relatively little is known at the cellular level concerning nicotinic receptor stimulation in brain. Central nicotinic receptors have been implicated in Alzheimer's disease, seizure activity, the generation of slow-wave theta rhythm in the hippocampus and the potential abuse liability of nicotine. At the neuronal level, nicotinic agonists have been most often associated with postsynaptically mediated excitation and membrane depolarization at various sites, including Renshaw spinal motoneurons, locus coeruleus and the medial habenular nucleus. Nicotine acting presynaptically can produce either excitation or inhibition indirectly through the release of endogeneous transmitters or modulators. Whereas a direct inhibitory effect of nicotine has been suggested by one in vivo extracellular recording study in rat cerebellar Purkinje neurons, the mechanism(s) underlying this action is not yet known. We now report our findings obtained using in vitro intracellular methods in a submerged brain slice preparation in which application of nicotinic agonists to rat dorsolateral septal neurons reveal a direct membrane hyperpolarization mediated by an increase in potassium conductance.