Muscarinic excitation and inhibition of neurons in the submucous plexus of the guinea-pig caecum

Effects of cholera toxin on the potential difference and motor responses induced by distension in the rat proximal small intestine in vivo

Cholera toxin (CT) may induce uncontrolled firing in recurrent networks of secretomotor neurons in the submucous plexus. This hypothesis was tested in chloralose-anesthetized rats in vivo. The secretory reflex response to graded intestinal distension was measured with or without prior exposure to luminal CT. The transmural potential difference (PD) was used as a marker for electrogenic chloride secretion. In controls, distension increased PD, and this response was reduced by the neural blocker tetrodotoxin given serosally and the vasoactive intestinal peptide (VIP) receptor antagonist [4Cl-d-Phe(6),Leu(17)]VIP (2 mug.min(-1).kg(-1) iv) but unaffected by the serotonin 5-HT(3) receptor antagonist granisetron, by the nicotinic receptor antagonist hexamethonium, by the muscarinic receptor antagonist atropine, or by the cyclooxygenase inhibitor indomethacin. Basal PD increased significantly with time in CT-exposed segments, an effect blocked by granisetron, by indomethacin, and by [4Cl-d-Phe(6),Leu(17)]VIP but not by hexamethonium or atropine. In contrast, once the increased basal PD produced by CT was established, [4Cl-d-Phe(6),Leu(17)]VIP and indomethacin had no significant effect, whereas granisetron and hexamethonium markedly depressed basal PD. CT significantly reduced the increase in PD produced by distension, an effect reversed by granisetron, indomethacin, and atropine. CT also activated a specific motility response to distension, repeated cluster contractions, but only in animals pretreated with granisetron, indomethacin, or atropine. These data are compatible with the hypothesis that CT induces uncontrolled activity in submucous secretory networks. Development of this state depends on 5-HT(3) receptors, VIP receptors, and prostaglandin synthesis, whereas its maintenance depends on 5-HT(3) and nicotinic receptors but not VIP receptors. The motility effects of CT (probably reflecting myenteric activity) are partially suppressed via a mechanism involving 5-HT(3) and muscarinic receptors and prostaglandin synthesis.

Characteristics of mucosally projecting myenteric neurones in the guinea-pig proximal colon

1. Using retrograde tracing with 1,1'-didodecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI) in combination with electrophysiological and immunohistochemical techniques we determined the properties of the putative intrinsic primary afferent myenteric neurones with mucosal projections in the guinea-pig proximal colon. 2. Eighty-four out of eighty-five DiI-labelled myenteric neurones were AH neurones with a late after-hyperpolarization. Thirty-three per cent of them exhibited atropine- and tetrodotoxin-resistant spontaneously occurring hyperpolarizing potentials (SHPs) during which the membrane resistance and excitability decreased. 3. DiI-labelled AH neurones had multipolar Dogiel type II morphology, primarily of the dendritic type. Sixty-one per cent of the neurones were immunoreactive for choline acetyltransferase (ChAT) and calbindin (Calb) and 23 % were ChAT positive but Calb negative. 4. DiI-labelled neurones did not receive fast excitatory postsynaptic potentials but 94 % (34/36) received slow excitatory postsynaptic potentials (sEPSPs). The neurokinin-3 (NK-3) agonist (MePhe7)-NKB but not the NK-1 agonist [(SAR9,Met(O2)11]-SP mimicked this response. The NK-3 receptor antagonist SR 142801 (1 microM) significantly decreased the amplitude and duration of the sEPSPs; the NK-1 receptor antagonist CP-99,994 (1 microM) was ineffective. Atropine (0.5 microM) increased the duration but not the amplitude of the sEPSPs. 5. Microejection of 100 mM sodium butyrate onto the neurones induced in 90 % of the DiI-labelled neurones a transient depolarization associated with an increased excitability. In neurones with SHPs sodium butyrate evoked, additionally, a late onset hyperpolarization. Perfusion of 0.1-10 mM sodium butyrate induced a dose-dependent increase in neuronal excitability. Sodium butyrate was ineffective when applied directly onto the mucosa. 6. Mucosally projecting myenteric neurones of the colon are multipolar AH neurones with NK-3-mediated slow EPSPs and somal butyrate sensitivity.

Post- and presynaptic effects of norepinephrine in guinea-pig colonic submucous plexus

Intracellular recording techniques were used to investigate the effects of norepinephrine on submucous neurones in the guinea-pig distal colon. In 81% of the neurones, pressure microejection of norepinephrine produced a membrane hyperpolarization associated with a decrease in excitability and input resistance. Microejection of clonidine (1 microM) mimicked the norepinephrine-induced hyperpolarization, whereas both phentolamine (1 microM) and yohimbine (1 microM) reversibly suppressed it. Superfusion of norepinephrine (1 nM - 10 microM) hyperpolarized the cells in a concentration-dependent manner. Norepinephrine and clonidine (1 nM - 10 microM) caused a concentration-dependent presynaptic inhibition of stimulus-evoked cholinergic fast excitatory postsynaptic potential. Slow inhibitory post-synaptic potentials (sISPSs) were induced by focal electrical stimulation of the interganglionic fibre tracts in 43% of the neurones tested. Superfusion of both phentolamine (1 microM) and yohimbine (1 microM) reduced the sIPSPs while prazosin (1 microM) had no significant effect. We concluded that norepinephrine acted post- and presynaptically via alpha 2-adrenoreceptors to have an inhibitory effect on the guinea-pig colonic submucous. In addition, our study strongly supported the role of norepinephrine as a mediator of the sIPSPs. As a result, norepinephrine would primarily suppress information transfer within the neuronal circuits in guinea-pig colonic submucosal plexus.

Synthesis of models for excitable membranes, synaptic transmission and neuromodulation using a common kinetic formalism

Markov kinetic models were used to synthesize a complete description of synaptic transmission, including opening of voltage-dependent channels in the presynaptic terminal, release of neurotransmitter, gating of postsynaptic receptors, and activation of second-messenger systems. These kinetic schemes provide a more general framework for modeling ion channels than the Hodgkin-Huxley formalism, supporting a continuous spectrum of descriptions ranging from the very simple and computationally efficient to the highly complex and biophysically precise. Examples are given of simple kinetic schemes based on fits to experimental data that capture the essential properties of voltage-gated, synaptic and neuromodulatory currents. The Markov formalism allows the dynamics of ionic currents to be considered naturally in the larger context of biochemical signal transduction. This framework can facilitate the integration of a wide range of experimental data and promote consistent theoretical analysis of neural mechanisms from molecular interactions to network computations.

Somatostatin-mediated inhibitory postsynaptic potential in sympathetically denervated guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones in guinea-pig ileum. In some animals, the extrinsic (sympathetic) nerves to the submucosal plexus were severed 5-7 days previously. The actions of somatostatin and somatostatin analogues on membrane potential, membrane current and inhibitory postsynaptic potentials (IPSPs) were examined. 2. Somatostatin, somatostatin(1-28), [D-Trp8]somatostatin and the somatostatin analogue CGP 23996 all produced equivalent maximum hyperpolarizations or outward currents; half-maximal concentrations (EC50 values) were 9-11 nM. The somatostatin analogue MK 678 had an EC50 of 0.9 nM. Extrinsic sympathectomy did not alter concentration-response relations for somatostatin or its analogues. 3. Somatostatin (> 100 nM) produced hyperpolarization or outward current that declined almost completely during superfusion for 2-4 min; decline of the somatostatin current was exponential with a time constant of 30 s in the presence of 2 microM somatostatin. Desensitization was not altered by extrinsic denervation. 4. Recovery from desensitization was rapid and followed the time course of agonist wash-out. Forskolin, phorbol esters, dithiothreitol, hydrogen peroxide, concanavalin A, or reducing temperature from 35 to 29 degrees C did not alter the time course, degree of, or recovery from desensitization. 5. The somatostatin-induced desensitization was of the homologous type; no cross-desensitization to opiate or alpha 2-adrenoceptor agonists (which activate the same potassium conductance) occurred. 6. Somatostatin desensitization did not alter the adrenergic IPSP seen in sympathetically innervated preparations but abolished the non-adrenergic IPSP recorded from normal preparations and from preparations in which the extrinsic sympathetic nerve supply had been surgically removed. 7. The selective blockade of the non-adrenergic IPSP by the homologous-type somatostatin desensitization characterized in the present study provides strong support for the hypothesis that somatostatin is the neurotransmitter underlying the non-adrenergic IPSP in both normal and extrinsically denervated submucosal neurones.

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.

Acetylcholine receptors in rat cardiac neurones

Membrane potential changes produced by acetylcholine (ACh), and their underlying mechanisms, were studied in neurones of isolated cardiac ganglia of the rat by means of intracellular microelectrodes. Five components of membrane potential change could be detected in cardiac neurones following 1-5 s micro-application of ACh: (i) fast depolarization resulting from an activation of nonselective cationic conductance; (ii) slow depolarization associated with a decreased membrane conductance, presumably for potassium ions; slow hyperpolarization which consisted of (iii) early and (iv) late parts resulting from an activation of calcium-sensitive potassium current and from inhibition of steady-state inward current, respectively; and (v) delayed slow hyperpolarization associated with an increased conductance, most likely for potassium ions. Components (i), (iii) and (iv) persisted in the presence of atropine and were inhibited by nicotinic antagonists. Thus they were due to activation of nicotinic ACh receptors. However, the sensitivity of component (i) to ganglion-blocking agents appeared to be rather low: IC50s for inhibiting (i) were 226 +/- 34.2 microM, 31.2 +/- 4.31 microM and 15.3 +/- 3.27 microM for hexamethonium, d-tubocurarine, and trimetaphan, respectively. Components (ii) and (v) were abolished by atropine (1 microM) and mimicked by muscarine (component (ii) also persisted in d-tubocurarine), hence they resulted from activation of muscarinic ACh receptors. It is concluded that cardiac neurones are endowed with both nicotinic and muscarinic ACh receptors. Their activation leads to membrane depolarization and discharges followed by hyperpolarization and inhibition of discharges.

The effects of muscarine and adrenaline on patch-clamped frog cardiac parasympathetic neurones

1. The whole-cell patch-clamp technique was used to record membrane currents from neurones which were acutely dissociated from the intra-atrial parasympathetic ganglia of Rana pipiens. The effects of muscarine and adrenaline were observed at a holding potential of -30 mV. Extracellular potassium concentration ([K+]o) was 2, 6 or 20 mM. 2. Muscarine (10 microM) produced inward current in thirteen cells, outward current in eighteen cells and seven cells were unaffected. Inward currents were observed in six out of ten neurones in which the intracellular solution contained adenosine triphosphate (ATP; 100 microM) and outward currents were seen in eleven out of fourteen neurones which contained adenosine 3',5'-cyclic monophosphate (cyclic AMP; 100 microM). 3. In five out of nine cells tested, the inward current produced by muscarine was attributable to a 30% depression of a voltage-dependent current which resembled the M-current (IM). Muscarine-induced inward current in the other four cells involved a steady-state conductance increase that reached a null potential at -10 mV. Modest IM suppression also contributed to the response in three of these four cells. 4. Adrenaline (10 or 100 microM) produced inward currents in twelve cells, outward current in ten cells and three cells were unaffected. Outward currents were only seen in cells which contained ATP or cyclic AMP (ten out of sixteen cells) whereas inward currents were seen in eight out of nine cells which did not contain adenosine nucleotides. These inward currents were always attributable to IM suppression. 5. The outward currents induced by muscarine and adrenaline resulted from an increase in a potassium conductance (GK) that exhibited inward rectification.

Mechanisms underlying intracellular signal transduction of the slow IPSP in submucous neurones of the guinea-pig caecum

1. Intracellular recordings were obtained from submucous plexus neurones of the guinea-pig caecum. 2. The resting membrane conductance displayed two types of inward rectification: one which developed at potentials more negative than -70 mV, and another that occurred at potentials more negative than the potassium equilibrium potential. The former inward rectification was blocked by extracellular caesium (Cs+; 1-2 mM) and the latter was blocked by Cs+ (1-2 mM) or barium (Ba2+; 30-100 microM). 3. The noradrenaline-induced current measured by subtraction of the current-voltage (I-V) relation before and after adding the agonist also showed an inward rectification around the resting potential. Ba2+ (30-100 microM) blocked both the outward and inward current induced by noradrenaline. The noradrenaline current was not affected by Cs+ (1-2 mM). Both the slow IPSP and the slow IPSC (inhibitory postsynaptic current) were reduced by Ba2+, but not by Cs+. 4. During the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), multiple repetitive stimulation or repeated applications of noradrenaline produced irreversible membrane hyperpolarizations with a decreased membrane input resistance, until the membrane had approached the potassium equilibrium potential. 5. Pertussis toxin (1-40 micrograms/ml) abolished both the slow IPSP and the noradrenaline hyperpolarization without affecting the nicotinic fast EPSP or the slow EPSP. 6. Superfusion with a Ca(2+)-free, high-Mg2+ (12 mM) solution caused a membrane depolarization associated with an increased input resistance. It eliminated the Ca2+ spikes, the slow after-hyperpolarizations following the spikes, and the synaptic potentials within 3 min. Prolonged exposure (longer than 20 min) to this solution resulted in a progressive decline of the noradrenaline hyperpolarization. 7. Intracellular injection of ethylene glycol-bis(beta-aminoethylether)N,N,N',N'-tetraacetic acid (EGTA) reduced the slow IPSP and the noradrenaline hyperpolarization. Superfusion with a membrane-permeable Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM; 10-200 microM) reduced the noradrenaline hyperpolarization. 8. Procaine reversibly reduced the slow IPSP and noradrenaline hyperpolarization without affecting the fast EPSP or slow EPSP at concentrations up to 300 microM.(ABSTRACT TRUNCATED AT 400 WORDS)