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A Constanti

Publications and source records attributed to A Constanti.

At least 37 records · Page 2Linked to original sources
PubMed1993

Persistent muscarinic excitation in guinea-pig olfactory cortex neurons: involvement of a slow post-stimulus afterdepolarizing current.

The persistent excitatory effects of the muscarinic agonist oxotremorine-M were investigated in guinea-pig olfactory cortex neurons in vitro (28-30 degrees C) using a single-microelectrode current-clamp/voltage-clamp technique. In 40% of recorded cells (type 1), bath-application of oxotremorine-M (2-10 microM; 1-2 min) induced a strong membrane depolarization, an increase in input resistance and a sustained neuronal discharge lasting over 30 min following agonist washout. A large depolarizing stimulus applied during the action of oxotremorine-M, evoked a slow post-stimulus afterdepolarization (approximately 10-15 mV) lasting approximately 30 s. Injection of steady negative current at the peak of this response produced a slow repolarization of the membrane potential (half-time approximately 0.6 min) towards a plateau level ("hyperpolarization recovery"); these effects of oxotremorine-M were slowly reversed on washout or by application of atropine (1 microM). In a second population of neurons (type 2; 39% of total), oxotremorine-M produced a large depolarization, a resistance increase and repetitive firing that did not persist after agonist washout; these neurons failed to generate a prominent slow afterdepolarization on stimulation, and showed no hyperpolarization recovery effect. Their resting membrane properties were not significantly different from those of type 1 cells. The remaining proportion of cells (type 3) elicited little or no muscarinic response to oxotremorine-M and no slow afterdepolarization; these cells showed characteristics spike fractionation (pre-potentials) during an evoked train of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine↗
PubMed1992

Trans-ACPD induces a slow post-stimulus inward tail current (IADP) in guinea-pig olfactory cortex neurones in vitro.

The metabotropic glutamate receptor agonist trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) produced a slow, persistent excitation of guinea-pig olfactory cortical neurones in vitro, and the appearance of a prominent post-stimulus after depolarization. The corresponding slow inward tail current (IADP) revealed under voltage clamp was insensitive to tetrodotoxin (or atropine) but was blocked by Cd2+ or tetrabutylammonium. The IADP properties resembled those of the slow inward tail current induced by muscarinic agonists in these neurones, suggesting a common intracellular transduction mechanism.

Animals↗
PubMed1992

Ganglionic nicotinic receptor agonists exhibit anti-muscarinic effects in guinea-pig olfactory cortical brain slices.

The action of some nicotinic acetylcholine receptor agonists was re-examined on the surface field potentials (N-waves) evoked by electrical stimulation of the lateral olfactory tract in guinea-pig olfactory cortical brain slices. Bath superfusion of nicotine or the nicotinic stimulants dimethylphenylpiperazinium (DMPP), lobeline, cytisine, tetramethylammonium or suberyldicholine (up to 100 microM) had little or no effect on the extracellular N-wave amplitude, or the membrane potential, input resistance or excitability of olfactory neurones recorded intracellularly. In contrast, the muscarinic agonists, carbachol or oxotremorine-M consistently depressed the field in a reversible dose-dependent manner. Interestingly, in the presence of the ganglionic stimulants DMPP (n = 6 slices) or lobeline (n = 5 slices) (10-50 microM), the effects of carbachol or oxotremorine-M were antagonized in a weak competitive-type manner (pA2 values = 5.58 and 5.63 respectively, estimated from Schild plots, constrained to unity slope). This anti-muscarinic action was unaffected by d-tubocurarine or hexamethonium. Nicotine, cytisine, tetramethylammonium and suberyldicholine showed much weaker and inconsistent carbachol-blocking effects. Combination of DMPP with atropine produced dose ratio shifts close to those predicted for a common-site interaction of two competitive antagonists. In conclusion, consistent pre- or postsynaptic nicotinic agonist actions could not be detected in olfactory cortex slices; however, some ganglionic nicotinic agonists were shown to exhibit significant anti-muscarinic effects on this preparation. We suggest this action might be due to a direct atropine-like mechanism.

Alkaloids↗
PubMed1991

Muscarinic receptor activation induces a prolonged post-stimulus afterdepolarization with a conductance decrease in guinea-pig olfactory cortex neurones in vitro.

The persistent excitation of guinea-pig olfactory cortical neurones in vitro by the muscarinic agonist oxotremorine-M (OXO-M) was investigated. In OXO-M (10-20 microM), a slowly-decaying afterdepolarization (sADP) accompanied by sustained repetitive firing was induced following a long depolarizing stimulus. The corresponding slow inward current (IADP) revealed under voltage clamp behaved like a K(+)-mediated tail current, but was associated with a decreased membrane conductance. IADP was insensitive to tetrodotoxin (TTX), Ba2+, Cs+, or 4-aminopyridine (4-AP), but was blocked by 500 microM TEA or TBA (tetrabutylammonium). The OXO-M response and IADP were also reduced by Cd2+ or Ca(2+)-free solution, suggesting a dependence on Ca(2+)-entry. We propose that OXO-M induces a novel outward K+ current that can be slowly de-activated by Ca(2+)-entry during a depolarizing stimulus. Summation of IADP tail currents could contribute to the sustained muscarinic excitation of mammalian cortical neurones.

4-Aminopyridine↗
PubMed1991

Muscarinic inhibition of excitatory neurotransmission in guinea-pig olfactory cortex slices: weak antagonism by M3-muscarinic receptor antagonists.

Dose-dependent depression of the electrically evoked surface-negative field potential (N-wave) produced by bath-superfusion of carbachol was measured in guinea-pig olfactory cortex slices maintained in vitro. The possible involvement of M3 (smooth muscle/glandular) type muscarinic receptors in partly mediating this response was investigated by testing the effectiveness of the muscarinic M3 receptor antagonists hexahydro-sila-difenidol (HHSiD) and p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD). Low doses of HHSiD (10-100 nM) or p-F-HHSiD (up to 1 microM), pre-applied for 30 min, produced no obvious antagonism of carbachol responses. However, a clear competitive-type inhibition of carbachol effects was observed in 250 nM-1 microM HHSiD or 10-50 microM p-F-HHSiD respectively. Schild plot analysis (regression slope constrained to unity) of pooled data yielded pA2 values of 6.6 for HHSiD (n = 6 slices) and 5.5 for p-F-HHSiD (n = 6 slices) respectively, suggesting a weak competitive antagonism by both compounds. In addition, combination experiments using either HHSiD or p-F-HHSiD with atropine, produced dose-ratio shifts close to those predicted for two antagonists competing for a common receptor site. By comparison, another suggested M3-receptor antagonist, 4-diphenyl-acetoxy-N-methyl-piperidine methiodide (4-DAMP) was a potent competitive blocker of carbachol responses. Schild analysis for 4-DAMP versus carbachol gave a pA2 of 7.9 (n = 6 slices). It is concluded that the muscarinic receptors involved in the suppression of the olfactory cortical N-wave possess a low affinity for HHSiD and particularly for p-F-HHSiD, but not 4-DAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1990

Muscarinic suppression of the evoked N-wave by oxotremorine-M recorded in the guinea-pig olfactory cortex slice.

The effect of the muscarinic agonist oxotremorine-M has been studied on the surface-negative field potential (N-wave) evoked by orthodromic stimulation of the lateral olfactory tract in slices of guinea-pig olfactory cortex. Bath-application of oxotremorine-M (5-80 microM) or carbachol (10-300 microM) produced a reversible depression of the N-wave amplitude without affecting the lateral olfactory tract compound action potential. Oxotremorine-M was approximately 5 times more potent than carbachol in this respect, and the effects of both agonists were competitively blocked by telenzepine (5-100 nM), a selective M1-receptor antagonist. In contrast, methoctramine or AF-DX 116, two 'cardioselective' M2-receptor antagonists, had little or no blocking effect on the agonist responses. It is suggested that oxotremorine-M (like carbachol) inhibits the evoked field potential by activating presynaptic M1-type muscarinic receptors in the olfactory cortex slice.

Action Potentials↗
PubMed1990

Differential effect of zinc on the vertebrate GABAA-receptor complex.

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

Aging↗
PubMed1988

Quantitative effects of some muscarinic agonists on evoked surface-negative field potentials recorded from the guinea-pig olfactory cortex slice.

1. The effects of muscarinic receptor agonists on the electrically-evoked surface-negative field potential (N-wave) were measured in the guinea-pig olfactory cortex slice maintained in vitro. 2. Bath-superfusion of (+/-)-muscarine, acetylcholine (ACh), carbachol (CCh), or methacholine (MCh) (10-200 microM) produced reversible, dose-dependent depressions of the N-wave (ACh and MCh effects were observed in the presence of 10 microM neostigmine). The order of potencies (based on agonist dose causing 50% field depression: IC50) was: ACh greater than or equal to muscarine greater than CCh greater than MCh. All four agonists depressed the field potential by 100% at doses greater than 500 microM. 3. Pilocarpine and bethanechol were weak agonists and only produced measurable effects at high doses (1-2 mM). Neither agonist evoked a maximum response at doses up to 10 mM. 4. The muscarinic ganglion stimulant, McN-A-343 yielded inconsistent results, depressing the field potential in some slices, but having no effect in others. Pre-application of a conditioning dose (100 microM) of McN-A-343 reduced subsequent responses to CCh, suggesting possible partial agonist properties. 5. Oxotremorine (up to 100 microM) did not depress the field potential, but it reversibly antagonized the effects of CCh. 6. It is concluded that reproducible, quantifiable responses to muscarinic agonists can be evoked in the olfactory cortex slice. We suggest this preparation may be useful for conducting pharmacological studies of 'intact' central muscarinic receptors.

Acetylcholine↗
PubMed1988

A quantitative study of the effects of some muscarinic antagonists on the guinea-pig olfactory cortex slice.

1. Muscarinic depression of the electrically-evoked surface-negative field potential (N-wave) was measured in guinea-pig olfactory cortex slices maintained in vitro. 2. The effects of three muscarinic receptor antagonists, pirenzepine, atropine and gallamine on this muscarinic response were analysed in detail. 3. Pirenzepine was a potent competitive antagonist of carbachol (CCh)-evoked responses. Schild plot analysis yielded a pA2 value of 7.9 (Schild slope constrained to unity). A similar analysis for atropine versus CCh responses gave a pA2 of 8.9. 4. Combination experiments using pirenzepine and atropine produced dose-ratio shifts close to those expected for two antagonists competing for a similar receptor site. 5. Gallamine was only a weak antagonist of responses to CCh. 6. Oxotremorine behaved as a competitive antagonist at this muscarinic receptor (pA2 = 6.1). 7. It is concluded that the presynaptic muscarinic receptor mediating depression of the N-wave in the olfactory cortex slice is of the M1-subtype.

Animals↗
PubMed1987

GABA receptors induced in Xenopus oocytes by chick brain mRNA: evaluation of TBPS as a use-dependent channel-blocker.

Sensitivity to GABA was induced in Xenopus laevis oocytes by injection of poly(A)+ mRNA extracted from 19-day chick embryo brain. The effect of the convulsant drug t-butylbicyclophosphorothionate (TBPS) was studied on the responses to bath-applied GABA using voltage-clamp techniques. TBPS reversibly inhibited the GABA-evoked current (IGABA) in a dose-dependent manner; however, the chloride currents evoked by carbachol or serotonin, or the spontaneous chloride fluctuations, were unaffected. The onset of the block by TBPS was faster in the presence of GABA. The recovery from the block after TBPS wash-out was also agonist-stimulated. At the steady state block, TBPS showed a mixed type of inhibition: increasing the GABA concentration decreased but failed to abolish completely the inhibition by TBPS. The TBPS block was independent of the direction of the chloride flow: both inward and outward IGABA were blocked. The time course of the decay of IGABA was markedly changed in the presence of TBPS: above 40 microM GABA, this time course showed essentially two exponentials and TBPS abolished only the fast component, whereas at a low concentration (less than or equal to 4 microM), IGABA was relatively constant and uniformly reduced by TBPS. It is suggested that TBPS may stabilize a closed form of the liganded receptor-channel complex.

Animals↗
PubMed1987

Muscarinic receptors mediating suppression of the M-current in guinea-pig olfactory cortex neurones may be of the M2-subtype.

Guinea-pig olfactory cortical neurones in vitro were voltage clamped by means of a single intracellular microelectrode technique. Hyperpolarizing voltage commands from holding potentials between -40 to -50 mV produced slow inward current relaxations reflecting deactivation of the M-current (IM). IM was reversibly suppressed by 30 microM muscarine or carbachol; this suppression was insensitive to pirenzepine (up to 300 nM) but was inhibited by gallamine (10-20 microM) or 4-diphenyl-acetoxy-N-methylpiperidine (100, 500 nM), suggesting the involvement of the M2-type muscarinic receptor.

Animals↗
PubMed1987

Calcium-dependent potassium conductance in guinea-pig olfactory cortex neurones in vitro.

1. Guinea-pig olfactory cortex neurones in vitro (23-25 degrees C) were voltage clamped by means of a single-micro-electrode sample-and-hold technique. 2. Under current clamp at the resting potential (approximately -80 mV), brief depolarizing stimuli evoked trains of action potentials with little visible after-potential. However, in 90% of recorded cells held at membrane potentials between -70 and -45 mV, depolarizing current pulses evoked a slow after-hyperpolarization (a.h.p.) (approximately 8 mV) lasting several seconds and accompanied by an increase in input conductance. 3. The outward membrane current underlying the a.h.p. was revealed either by switching rapidly to voltage clamp at the end of a spike train ('hybrid' clamp) or by applying brief depolarizing commands from potentials between -60 to -45 mV. The tail current showed a distinct rising phase (time to peak approximately 1 s) and exponential decay (tau approximately 3 s) and was suppressed by removal of external Ca2+, or adding Co2+ (1-2 mM), Cd2+ (200 microM) or Mg2+ (6 mM). The a.h.p. current reversal potential was -96 mV in 3 mM-K+ medium. 4. Low concentrations (1-2 microM) of muscarine, carbachol, oxotremorine or the muscarinic ganglion stimulant, McN-A-343 (1-10 microM) reduced the a.h.p. current and leak conductance and induced a steady inward current, without affecting M-current (IM) relaxations. IM inhibition generally required higher (greater than 10 microM) agonist concentrations, although oxotremorine remained ineffective at up to 50 microM. 5. The a.h.p. current was reduced by noradrenaline and tetraethylammonium (TEA), but not by apamin or tubocurarine. Apart from TEA, these agents had no effect on IM. 6. Addition of tetrodotoxin (TTX, 1 microM) or removing external Na+ depressed the a.h.p. current amplitude recorded under voltage clamp. The residual tail current could be further reduced by adding Cd2+ or muscarinic agonists. 7. Repolarizing tail currents induced following positive voltage commands consisted mainly of IM and slow a.h.p. current with little evidence of a 'fast' Ca2+-activated K+ current (IC). 8. It is concluded that the slow a.h.p. current that underlies the post-burst after-hyperpolarization of olfactory neurones, is a Ca2+-dependent K+ current distinct from IM. It is suggested that the cholinergic modulation of this current (rather than IM) may provide a more subtle control of cell excitability in cortical neurones.

Action Potentials↗
PubMed1986

Calcium-dependent action potentials and associated inward currents in guinea-pig neocortical neurons in vitro.

Calcium-dependent potential changes and inward currents were studied in guinea-pig neocortical neurons maintained in vitro. Under conditions of reduced outward potassium current, induced by external application of tetraethylammonium ions or internal application of caesium ions, regenerative Ca2+-dependent action potentials could be elicited. Strontium and barium ions could substitute for calcium as the charge carrier but not magnesium; cadmium blocked the calcium spikes. In caesium-loaded neurons, in the presence of tetrodotoxin and tetraethylammonium, inward currents were recorded when the membrane potential was step-depolarized to potentials more positive than -50 mV. These currents were blocked by cadmium. It is concluded that guinea-pig neocortical neurons are capable of generating a calcium action potential via the activation of a slow inward current.

Action Potentials↗
PubMed1985

Spontaneous inhibitory postsynaptic potentials in guinea pig neocortex and olfactory cortex neurones.

The membrane potential of olfactory cortex and neocortex neurones in vitro was recorded using conventional microelectrode techniques. During recordings with KCl- or CsCl-filled microelectrodes, spontaneous, subthreshold, transient membrane depolarizations were observed. These were abolished by the GABAA-receptor antagonist, bicuculline methiodide, and were prolonged by the barbiturate pentobarbitone. In most cells they were abolished by tetrodotoxin. It is concluded that these spontaneous depolarizations are inhibitory postsynaptic potentials arising from spontaneous activity in inhibitory interneurones.

Animals↗
PubMed1985

Muscarinic depression of evoked surface-negative field potentials recorded from guinea-pig olfactory cortex in vitro.

The action of some cholinergic drugs has been studied on the field potentials evoked by orthodromic stimulation of the lateral olfactory tract (LOT) in guinea-pig olfactory cortex slices maintained in vitro. A reversible depression of the electrically evoked surface-negative field potential (N-wave) was seen following superfusion of muscarine (10-200 microM) or mixed-agonist choline esters but not nicotinic agonists. This depression was blocked by atropine and pirenzepine, but not d-tubocurarine or by antagonists active at gamma-aminobutyric acid or adenosine receptors. Little effect of muscarinic agonists was observed on the compound action potential recorded from the LOT, or on pial surface DC potential. A possible presynaptic site of action of muscarinic agonists in the olfactory cortex is discussed.

Acetylcholine↗
PubMed1985

Calcium-dependent action potentials in guinea-pig olfactory cortex neurones.

Ca2+-dependent action potentials were recorded in guinea pig olfactory neurones in vitro (23 degrees--25 degrees C). In most cells (in the presence of tetrodotoxin: TTX) the current-voltage relationship displayed 'anomalous' rectification (apparent high slope resistance) at potentials approximately 20 mV depolarized to the resting membrane potential (approximately -80 mV) and strong outward rectification at more positive potentials. Intracellular Cs+-loading blocked outward rectification and increased action potential duration. Such spikes were TTX-insensitive and were further prolonged by external addition of tetraethylammonium (TEA) or Ba2+. Spikes recorded from Cs+-loaded, TTX/TEA-treated neurones displayed a prolonged plateau and an after-depolarization. They persisted when Ba2+ or Sr2+ were substituted for external Ca2+, but not when Mg2+ was the sole extracellular divalent cation. The spikes were blocked in the presence of Cd2+ but persisted when 82% of the extracellular Na+ was substituted by choline. A TTX-insensitive, slowly inactivating inward current at depolarized potentials is believed to account for the subthreshold 'anomalous' rectification and prolonged spike plateau.

Acetates↗