Muscarinic inhibitory transmission in mammalian sympathetic ganglia mediated by increased potassium conductance

Noradrenaline-mediated lateral inhibition in the sympathetic ganglion

Intracellular recordings were made from neurons of denervated superior cervical ganglion (SCG) of the rat. Focal electrical stimulation near the recording site evoked a monosynaptic inhibitory postsynaptic potential (IPSP) in about 15% of SCG neurons. The IPSP was reversibly abolished by alpha2-adrenoceptor antagonist, yohimbine. The ionic mechanism of the IPSP appeared to be due to an increase of potassium conductance following the activation of alpha2-adrenoceptor. Electron microscopic observations confirmed that these cells received synapses of intrinsic type. It was concluded that some SCG neurons were under an inhibitory control from neighboring neurons via noradrenergic synapses.

Muscarinic transmission decreases the number of SIF cells demonstrating catecholamine histofluorescence in rat superior cervical ganglia

Preganglionic electrical stimulation of the cervical sympathetic trunk to the rat superior cervical ganglia produced a mean reduction in the number of visible small intensely fluorescent (SIF) cells demonstrating catecholamine histofluorescence to 32% of the unstimulated contralateral control. The reduction in the number of catecholamine-positive SIF cells required the presence of specific blockers of catecholamine uptake and synthesis and was dependent on normal synaptic transmission. No change in the number of catecholamine-positive SIF cells was observed when ganglionic transmission occurred in solutions containing both hexamethonium and atropine or with atropine alone (97% of the unstimulated control). Furthermore, preganglionic stimulation in the presence of high magnesium/low calcium solutions, which effectively blocked synaptic transmission, prevented the stimulation-induced decrease in the number of catecholamine-positive SIF cells. Prolonged antidromic stimulation of the internal carotid nerve only reduced the number of catecholamine-positive SIF cells to 75% of the unstimulated contralateral control. These results suggest that preganglionic synaptic impulses can induce the release of catecholamines from SIF cells via muscarinic receptor activation. Furthermore, the necessity for pharmacological intervention of uptake and synthesis blockers of catecholamines in order to detect the synaptically-induced reduction in the number of catecholamine-positive SIF cells, suggests that synaptic transmission also modulates the synthesis of catecholamines in SIF cells within the rat superior cervical ganglia.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

Adenosine selectively depresses muscarinic compared with non-muscarinic receptor mediated depolarisation of the rat superior cervical ganglion

1. A grease gap d.c. recording technique was used to measure electrophysiological responses of the isolated rat superior cervical ganglion. 2. Adenosine at 100 microM depressed depolarisations to the muscarinic agonists carbachol, muscarine and methylfurmethide. In contrast adenosine (100 microM) did not alter depolarisations to 1,1-dimethyl-4-phenylpiperazinium, 2-methyl-5-hydroxytryptamine and potassium and enhanced depolarisations to 5-hydroxytryptamine and gamma-aminobutyric acid. 3. Adenosine-induced depressions of the depolarisations to carbachol, muscarine, and methylfurmethide tended to be increased in the presence of 0.3 microM methoctramine (a muscarinic receptor antagonist with slight selectivity for M2 receptors). The increase was statistically significant (P < 0.01) for carbachol. 4. Medium containing 0.1 mM Ca2+ and 0.3 microM pirenzepine augmented the hyperpolarising phase of the response to carbachol. Adenosine (10-300 microM) hyperpolarised ganglia and did not significantly alter the hyperpolarisation to 0.3 or 1 microM carbachol but selectively reduced the depolarisation response to 3 microM carbachol. 5. Adenosine-induced hyperpolarisations (100 microM) were enhanced when applied during depolarisations to muscarinic agonists (muscarine, pilocarpine, N-methyl-N-(l-methyl-4-pyrrolidine-2-butynyl)acetamide (BM-5)), and other M-current inhibitors, barium and eledoisin-related-peptide. Adenosine induced hyperpolarisations were not affected by D-Ala6-luteinizing-hormone-releasing-hormone or uridine 5'-triphosphate which produced small depolarisations. 6. It is concluded that adenosine acts selectively in opposing mechanisms of depolarisation of the rat SCG that are due to the action of muscarinic agonists (acting via M1-receptors) and by other M-current inhibitors.

Effects of axotomy, deafferentation, and reinnervation on sympathetic ganglionic synapses: a comparative study

The main physiological and morphological features of the synapses in the superior cervical ganglia of mammals and the last two abdominal ganglia of the frog sympathetic chain are summarized. The effects of axotomy on structure and function of ganglionic synapses are then reviewed, as well as various changes in neuronal metabolism in mammals and in the frog, in which the parallel between electrophysiological and morphological data leads to the conclusion that a certain amount of synaptic transmission occurs at "simple contacts." The effects of deafferentation on synaptic transmission and ultrastructure in the mammalian ganglia are reviewed: most synapses disappear, but a number of postsynaptic thickenings remain unchanged. Moreover, intrinsic synapses persist after total deafferentation and their number is strongly increased if axotomy is added to deafferentation. In the frog ganglia, the physiological and morphological evolution of synaptic areas is comparable to that of mammals, but no intrinsic synapses are observed. The reinnervation of deafferented sympathetic ganglia by foreign nerves, motor or sensory, is reported in mammals, with different degrees of efficiency. In the frog, the reinnervation of sympathetic ganglia with somatic motor nerve fibers is obtained in only 20% of the operated animals. The possible reasons for the high specificity of ganglionic connections in the frog are discussed.

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.

Hyperpolarizing muscarinic responses of freshly dissociated rat hippocampal CA1 neurones

1. Intracellular mechanisms of the muscarinic acetylcholine (ACh) response were investigated in pyramidal neurones freshly dissociated from the rat hippocampal CA1 region. Current recordings were made in the whole-cell mode using the nystatin 'perforated'-patch technique, by which the muscarinic ACh response can be continuously recorded without so-called 'run-down' phenomenon. The amount of intracellular free Ca2+ ([Ca2+]i) was fluorometrically measured using fura-2. 2. In current clamp conditions, ACh induced a transient hyperpolarization accompanied by a decrease in membrane input resistance. 3. Under voltage clamp conditions at a holding potential (Vh) of -40 mV, ACh induced two types of muscarinic currents observed either alone or together: a transient outward current and a slowly activating sustained inward current. 4. The ACh-induced transient outward current reversed the direction at K+ equilibrium potential (EK), and the reversal potential (EACh) shifted 56.7 mV for a tenfold change of extracellular K+ concentration ([K+]o). 5. The ACh-induced transient outward current increased in a sigmoidal fashion with increase in ACh concentration, where the half-maximal concentration (EC50) and the Hill coefficient (n) were 8 x 10(-7) M and 1.9, respectively. Both muscarine and carbamylcholine mimicked the ACh response, but neither McN-A-343 (M1 agonist) nor oxotremorine (cardiac M2 agonist) induced any current. 6. Muscarinic antagonists reversibly blocked the ACh response in a concentration-dependent manner. The inhibitory potency was in the order of atropine > pirenzepine > AF-DX-116. 7. The ACh-induced transient outward current was never recorded when [Ca2+]i was chelated by the acetoxymethyl ester form of 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA AM). On the other hand, in Ca(2+)-free external solution containing 2 mM EGTA and 10 mM Mg2+, the ACh response was elicited by the first application and successive ACh applications did not induce any response. Fura-2 imaging showed that [Ca2+]i was increased when ACh was added to the external medium with or without Ca2+, though in Ca(2+)-free medium only the first application of ACh increased the [Ca2+]i. 8. The ACh response was not affected by pretreatment with pertussis toxin (PTX) but the inhibitory effect of ACh on the high-threshold Ca2+ channel was abolished completely. 9. Pretreatment with Li+ enhanced the amplitude of the transient outward current and the increase in [Ca2+]i induced by ACh. 10. The calmodulin antagonists W-7, chlorpromazine and trifluoperazine reversibly inhibited the ACh response in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential regulation of muscarinic and nicotinic cholinergic receptors and their mRNAs in cultured sympathetic neurons

Mechanisms regulating expression of neuronal muscarinic and nicotinic receptors were examined in cultures of neonatal rat sympathetic neurons. Two factors known to stimulate cholinergic transmitter development in sympathetic neurons were examined for their effects on cholinergic receptor expression. A membrane associated factor (MANS46) and a diffusible factor produced by cultured rat fibroblasts (RFCM) each decreased muscarinic receptor number. By contrast, neither treatment altered levels of nicotinic receptors. Levels of muscarinic (m2) receptor mRNA were decreased by MANS but not by RFCM, indicating that effects of the two treatments were mediated by different mechanisms. Neither MANS nor RFCM altered levels of nicotinic alpha 3 or beta 2 mRNAs, consistent with the lack of change in numbers of nicotinic receptors. These observations indicate that receptor phenotype in developing neurons is subject to regulation by multiple epigenetic factors. Further, the same signals which regulate transmitter development may also regulate receptor expression in sympathetic neurons.

Two dynamic modes of striatal function under dopaminergic-cholinergic control: simulation and analysis of a model

A neural network model based on the anatomy and physiology of the matrix compartment of the striatum is described. The model consists of a network of neurons which are mutually inhibitory within a defined domain. A membrane potassium conductance (GK) under dopaminergic-cholinergic control is included in the model. Computer simulation results show that changes in GmaxK can modulate the behaviour of the network to produce either competition or coactivation among striatal output neurons. An analysis of a two-neuron system based on the model shows that the maximum steepness of the threshold function plays a decisive role in the dynamics, in particular with regard to the competition that exists between the neurons. Competitive interactions predominate at low GmaxK, while coactivation predominates at high GmaxK. We suggest that the former dynamic governs reciprocal inhibition of antagonistic muscles, while the latter governs cocontraction and rigidity. The model offers insights into the control of striatal neurodynamics by GmaxK which establish closer links between dopaminergic actions in the striatum and the mechanism of Parkinsonian rigidity. A prediction of the model is that acetylcholine should increase GKmax in striatal output neurons.

Depression of the cat cortical visual evoked potential by soman

The effects of intravenous administration of the anticholinesterase agent soman (pinacolyl methylphosphonofluoridate, 3-15 micrograms/kg) on the visual evoked potential (VEP) were examined in cats using phase-reversed sine wave grating stimuli of different spatial frequencies and contrasts. Doses of 5-7 micrograms/kg caused a depression of the VEP across all spatial frequencies in an abrupt, non-graded fashion. Studies in which contrast was varied showed that VEP depression resulted primarily from a decrease in the system gain rather than a change in the contrast sensitivity, and that response depression increased with increasing contrast. The dominant changes in gain revealed by these studies are consistent with a modulation of potassium conductance in the cell membrane which previous studies have shown to be dependent on a cholinergic mechanism.

4-Aminopyridine interrupts the modulation of acetylcholine release mediated by muscarinic and opiate receptors

The effect of 4-aminopyridine, a potassium channel blocker on the muscarinic and opiate modulation of acetylcholine release, was investigated. Rat frontal cortical slices were loaded with [3H]choline, superfused continuously, and stimulated electrically. 4-Aminopyridine enhanced the stimulation-evoked release of tritium without affecting basal release. The electrically evoked release of radioactivity was reduced by the muscarinic agonist oxotremorine and the delta selective opiate receptor agonist Metenkephalin, and was enhanced in the presence of the cholinesterase inhibitor physostigmine by the muscarinic antagonist atropine. These effects were completely abolished by 4-aminopyridine. Since 4-aminopyridine blocks potassium permeability of the neuron, it is suggested that the changes in potassium permeability and the consequent alteration of membrane polarization are involved in the presynaptic modulation of acetylcholine release.

A pharmacological study of the responses induced by muscarinic agonists on the isolated superior cervical ganglion of the guinea-pig

We have studied the muscarinic agonist induced responses on the guinea-pig superior cervical ganglion in vitro, as recorded from the internal carotid nerve using a grease-gap. The principal response was a depolarization, but a small hyperpolarizing response could be revealed under certain conditions. We determined the pA2 of a number of muscarinic antagonists against the muscarine induced depolarization. Four selective antagonists and atropine appeared to act competitively. The rank order of their pA2s was 4-DAMP (8.5), atropine (8.4), pirenzepine (8.0), methoctramine (7.2) and AF-DX 116 (6.3). In addition to muscarine, we assessed the potency and relative maximum response of nine other muscarinic compounds to depolarize this preparation: carbachol, 5-methylfurmethide, oxotremorine, oxotremorine-M, pilocarpine, RS 86, AF102B and two novel compounds L-670548 and L-679512. L-670548 was the most potent and AF102B was the least potent agonist tested. Only AF102B evoked a maximum depolarization that was significantly smaller than muscarine. A hyperpolarizing response to carbachol (1 microM) could be recorded when the superfusing medium contained 0.3 microM pirenzepine and only 0.1 mM CaCl2 (cf. usual 2.5 mM). This response was relatively small compared to that evoked on the superior cervical ganglion of the rat. It was blocked by the cardioselective antagonists methoctramine (0.1-0.3 microM) and AF-DX 116 (0.3-1.0 microM). Of the 10 agonists tested, only carbachol, oxotremorine and oxotremorine-M reproducibly evoked a hyperpolarizing response. It was concluded that muscarinic agonists can induce a depolarization of the guinea-pig superior cervical ganglion mediated by M1 receptors. The activation of cardiac-like M2 receptors resulted in a hyperpolarizing response that was relatively small.

Differential effects of the muscarinic M2 antagonists, AF-DX 116 and gallamine, on single neurons of rabbit sympathetic ganglia

Intracellular recording techniques were used to compare the effects of the M2 muscarinic antagonists, AF-DX 116 and gallamine, on membrane potential (Vm), input resistance (Ri), responses induced by methacholine, muscarinic slow postsynaptic potentials and action potentials in the superior cervical ganglion of the rabbit. Gallamine or AF-DX 116 antagonized methacholine-induced or synaptically-evoked muscarinic hyperpolarization, without having significant effect on depolarization induced by methacholine or synaptically. The drug AF-DX 116 reduced evoked muscarinic hyperpolarizing potentials, without significant change in Vm or Ri, recorded in the absence of muscarinic stimulation. In contrast to AF-DX 116, gallamine elicited a concentration-dependent depolarization of the membrane, with a corresponding increase in Ri, when tested in the absence of muscarinic stimulation. These effects of gallamine were accompanied by an increase in duration and decrease in the slope of the descending phase of the action potential. Blockade by gallamine of evoked hyperpolarization was independent of membrane depolarization and readily occurred when gallamine-induced depolarization was prevented by clamping Vm at its pre-gallamine level. The effects of gallamine were maintained during its presence and reversed upon washing with gallamine-free physiological solution. These results indicate that AF-DX 116 and gallamine have a specificity for antagonism of muscarinic responses, mediated by receptors of the M2 type in the superior cervical ganglion. However, gallamine, while an effective antagonist of M2 responses, also has the ability to modify the electrical characteristics of ganglion cells and thus may modify ganglionic transmission by mechanisms other than antagonism of receptors.

Effects of muscarine and adrenaline on neurones from Rana pipiens sympathetic ganglia

1. Neurones dissociated from Rana pipiens paravertebral sympathetic ganglia were studied by means of the whole-cell patch-clamp technique. Responses to agonists were best recorded when cyclic AMP was included in the patch pipette. 2. Two populations of cells were identified on the basis of size (input capacitance, Cin) and the presence or absence of a fast, transient outward current (A-current, IA). This current was usually present in the 'large' cells (Cin = 40.5 +/- 1.5 pF, n = 66) but absent from 'small' cells (Cin = 21.0 +/- 0.8 pF, n = 70). 3. Both cell types exhibited a slowly activating, non-inactivating K+ current (M-current, IM) which was suppressed by luteinizing hormone-releasing hormone (LHRH, 10-100 microM). Threshold for activation of IM was about -75 mV, half-maximal activation was at -50 mV and the M-conductance GM increased e-fold for at 7 mV change in membrane potential. The maximum value for IM studied in large cells by patch-clamp procedures was less than 0.2 nA. More M-channels were available per unit membrane area in the small cells (GM = 1495 microS cm-2) than in the large cells (GM = 1034 microS cm-2). Time constants for IM deactivation at -70 mV were faster in the large cells (37.2 +/- 4.6 ms, n = 16) than in the small cells (66.1 +/- 5.9 ms, n = 9). 4. Muscarine (10 microM) produced inward current in the large cells as a result of IM suppression. In 40% of the large cells, some of the M-channels were also sensitive to adrenaline (10-100 microM). In a few large cells (less than 10%) adrenaline produced outward current by increasing IM. 5. Muscarine failed to effect IM in the small cells and instead produced an inwardly rectifying K+ current which activated within 5 ms at -110 mV. The outward current produced in twenty out of thirty-seven small cells by adrenaline was occluded by that produced by muscarine, suggesting that both agonists affect the same K+ channels. 6. Inclusion of the protein kinase inhibitors, 1-(5-isoquinolinyl-sulphonyl)-2-methyl piperazine (H-7, 50 microM) or gold sodium thiomalate (GST, 50 microM) in the pipette solution failed to antagonize either muscarine-induced current. Both currents were prolonged when the 'internal solution' contained GTP-gamma-S (50 microM). 7. Phorbol-12-myristate-13-acetate (PMA, 2-5 microM) produced an inward current as a result of IM suppression in both small and large cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of acetylcholine on membrane potential in toad dorsal root ganglion neurons and its underlying ionic basis

Intracellular recordings were made to investigate the responses of membrane potential to acetylcholine (ACh) on neurons in isolated toad dorsal root ganglion (DRG). In the 73 neurons examined, 67 were of type A, and the remaining 6 of type C cell. The resting membrane potential of these two types of cells was -67.5 +/- 1.3 mV (means +/- SE). During the application of ACh (4 x 10(-4)-6 x 10(-4) mol/L), the changes in membrane potential were as follows: 1) hyperpolarization, with amplitude of 9.1 +/- 3.0 mV (means +/- SE; n = 23); 2) depolarization, with amplitude of 12.9 +/- 2.2 mV (means +/- SE; n = 20); 3) biphasic response, i.e., hyperpolarization with amplitude of 8.0 +/- 2.4 mV (means +/- SE) followed by depolarization with amplitude of 10.9 +/- 2.1 mV (means +/- SE) (n = 24); no effect (n = 6). The hyperpolarization induced by ACh was blocked by superfusion with atropine (1.3 x 10(-5) mol/L; n = 23), while ACh depolarization was blocked by the mixture of d-tubocurarine (1.4 x 10(-5) mol/L) and hexamethonium (1.4 x 10(-5) mol/L) (n = 18). When ACh caused hyperpolarization, the membrane conductance was increased by 13.8% and the reversal potential was about -96 mV (n = 3). TEA (20 mmol/L) superfusion enhanced ACh depolarization amplitude by 48.2 +/- 3.2% (means +/- SE; n = 6), and depressed ACh hyperpolarization amplitude by 79.4 +/- 4.3% (means +/- SE; n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic localization of muscarinic receptors on cultured, peptide-containing neurones from newborn rat superior cervical ganglion

In order to identify subpopulations of cultured rat superior cervical ganglion (SCG) neurones which express muscarinic receptors, a combination of immunocytochemistry and autoradiography was performed on these cultures. Antibodies to vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) were used to immunostain cultures that had previously been labelled with the irreversible muscarinic antagonist, [3H]propylbenzylylcholine mustard (PrBCM). Binding sites for [3H]PrBCM were observed on a large subpopulation of 65-85% of the ganglionic neuronal cell bodies. Specific labelling was not associated with non-neuronal cells found in these cultures. Approximately 60% of the SCG neurones were NPY-like immunoreactive (NPY-LI), a high proportion of which expressed muscarinic receptors. Five to 10% of the SCG neurones were VIP-LI, a small subpopulation of which displayed [3H]PrBCM binding sites. Receptor distribution on cell bodies was usually uniform, but occasionally, regions of high receptor density were seen. Dense networks of both varicose and non-varicose NPY-LI fibres were seen throughout the culture, a subpopulation of which expressed muscarinic receptors. Occasional VIP-LI fibres were also labelled with silver grains for [3H]PrBCM, but in less abundance than those for NPY-LI fibres. Conversely, neurones expressing muscarinic receptors were often immunonegative for either VIP or NPY: therefore, the identity of some of the neurones which express muscarinic receptors remains to be determined.

Selective antagonism of muscarinic potentials on the superior cervical ganglion of the rat

Selective antagonists have been used to classify the muscarinic receptors involved in the slow excitatory postsynaptic potential and slow inhibitory postsynaptic potential of the superior cervical ganglia of the rat, as recorded in 1 microM neostigmine, using a grease-gap method. Cardioselective M2 antagonists, e.g. AF-DX 116, depressed the slow inhibitory postsynaptic potential and enhanced the slow excitatory postsynaptic potential. The M1 selective antagonist pirenzepine depressed both potentials equally. The high potency of pirenzepine against the slow excitatory postsynaptic potential, however, indicates that it is mediated by M1 receptors. The slow excitatory and inhibitory postsynaptic potentials were found to be pharmacologically similar to the muscarinic agonist-induced depolarisation and hyperpolarisation of this preparation, respectively. The actions of two muscarinic agonists on the postsynaptic potentials were also studied. It is concluded that the slow excitatory postsynaptic potential is mediated by M1 receptors and the slow inhibitory postsynaptic potential by cardiac-like M2 receptors.

Adrenaline inhibits muscarinic transmission in bullfrog sympathetic ganglia

The effect of adrenaline (Ad) on muscarinic transmission was examined in B neurones of bullfrog sympathetic ganglia by using intracellular and voltage-clamp recording methods. Bath-application of Ad (5-500 microM) caused a depression of the slow excitatory postsynaptic potential (EPSP) elicited by repetitive stimulations of preganglionic nerve fibres in the presence of curare (30 microM). Ad also depressed the 'muscarinic' ACh potential induced by ionophoretic application of ACh directly to curarized sympathetic neurones in a concentration-dependent manner. Isoprenaline mimicked the effect of Ad in producing the inhibition of the 'muscarinic' ACh potential. Propranolol antagonized the inhibitory action of Ad. Dibutyryl adenosine 3',5'-monophosphate had no significant effect on the 'muscarinic' ACh potential. Under voltage-clamp conditions, Ad caused an inward current associated with inhibition of the M-current (Brown and Adams 1980). Ad depressed the amplitude of slow postsynaptic currents produced by applications of ACh and muscarinic. At a concentration of 100 microM, Ad produced a 68 +/- 8% (n = 12) depression of the amplitude of the muscarinic ACh current. The inhibition of muscarinic transmission induced by Ad is due to a direct suppression of the muscarinic current at the postsynaptic membrane in bullfrog sympathetic ganglia.

Long-term enhancement (LTE) of postsynaptic potentials following neural conditioning, in mammalian sympathetic ganglia

Orthodromic, preganglionic conditioning stimulation can consistently induce long-term enhancement (LTE) (greater than 3 h) of the muscarinically mediated slow excitatory postsynaptic potential and the slow inhibitory postsynaptic potential. This was shown for superior cervical ganglia of rabbit and rat. Effective conditioning stimuli are in a physiologically observed range (3/s for 7 min, 5/s for 4 min, 10/s for 2 min, 20/s for 1 min). LTE was producible both homosynaptically and heterosynaptically. LTE can thus be associative, with conditioning synaptic input in one line inducing long-term changes in postsynaptic responses to another (heterosynaptic) input. The dopamine antagonist butaclamol depressed LTE, particularly that following the initial postconditioning period of 30 min. Adrenergic antagonists had no effect. This pharmacological evidence, coupled with the heterosynaptic induction of LTE, supports the view that neurally induced LTE may be at least partly mediated by endogenous dopamine. Another non-cholinergic but non-adrenergic transmitter (possibly a peptide) might contribute to the LTE seen in the initial 30 min postconditioning. The present, orthodromically induced LTE is clearly different from the long-term potentiation widely studied in hippocampus, etc., in the modes of induction and synaptic mediation.

Dual synaptic effects of activating M1-muscarinic receptors, in superior cervical ganglia of rabbits

Postsynaptic potentials elicited by various muscarinic agonists and by preganglionic stimuli in the presence of such agonists were recorded from rabbit superior cervical ganglia using sucrose-gap and air-gap methods. While methacholine and bethanechol (both at 10(-4) M) induced biphasic potential changes, McN-A-343 and a novel synthetic compound AF-102B (10(-7) M-10(-5) M) produced only a depolarizing response which was depressed by the M1-antagonist pirenzepine (10(-7) M), but not by the M2 antagonist AF-DX 116 (same concentration), indicating that these compounds act purely as M1-muscarinic agonists in this system. These agonists selectively depressed the orthodromic slow excitatory postsynaptic potential (EPSP) in a dose-dependent manner without substantially affecting the fast EPSP; this is in accord with the view that their depolarizing action is on the same postsynaptic muscarinic receptor that mediates the slow EPSP. The slow inhibitory post synaptic potential (IPSP), on the other hand, was found potentiated in the presence of these agonists. This potentiation was antagonized not only by pirenzepine but also by yohimbine; the potentiation was itself enlarged by nomifensine (a dopamine-uptake inhibitor). We postulate that M1-muscarinic receptors are present not only on the postganglionic principal cells but also on the interneurons; the former were already known to be responsible for the generation of slow EPSP, but the latter may be on terminals of dopamine-containing small intensely fluorescent cells and regulate the orthodromic release of dopamine and are to be distinguished from the M2-receptors.

Pharmacological studies in frog sympathetic ganglion: support for the cholinergic monosynaptic hypothesis for slow IPSP mediation

The slow inhibitory postsynaptic potential (slow IPSP), the slow excitatory postsynaptic potential (slow EPSP), the late slow excitatory postsynaptic potential (late slow EPSP), and the fast excitatory postsynaptic potential/compound action potential (fast EPSP) were recorded from the 9th or 10th paravertebral sympathetic ganglia of bullfrogs (and some Rana pipiens frogs) by the sucrose-gap technique. The adrenergic antagonists phentolamine, dihydroergotamine and propranolol did not show any antagonistic effect on the slow IPSP when used at concentrations of up to 10, 100 and 10 microM, respectively. U-0521 (3',4'-dihydroxy-2-methylpropriophenone, 50 micrograms/ml), a specific inhibitor of catechol-O-methyltransferase, did not show any potentiating effect on the slow IPSP. The cholinesterase inhibitor neostigmine (0.5-1 microM) induced a large increase in the duration and amplitude of slow IPSP. When phentolamine and propranolol at concentrations greater than 10 microM were used the slow IPSP (and all other synaptic potentials) were non-specifically reduced in amplitude by these drugs. The results reported in this paper do not lend any support to the hypothesis that the slow IPSP in frog sympathetic ganglia is mediated by an adrenergic interneuron. The results are consistent with the proposal that the slow IPSP in this ganglion is mediated by a direct action of acetylcholine released from cholinergic preganglionic fibers.

A novel muscarinic receptor antagonist AF-DX 116 differentially blocks slow inhibitory and slow excitatory postsynaptic potentials in the rabbit sympathetic ganglia

Muscarinic, slow postsynaptic potentials (s-epsp and s-ipsp) in the rabbit superior cervical ganglia were shown to be differentially depressed by a novel cardioselective M2-type antagonist AF-DX 116: it antagonized the s-ipsp with IC50 value of 1.5 X 10(-7) M, which is 16-fold more potent in depressing the s-ipsp than the s-epsp. A hyperpolarizing component in the biphasic potential changes induced by a muscarinic agonist, methacholine, was selectively eliminated by this antagonist. AF-DX 116 was thus shown to be an useful tool for discriminating the M2-type muscarinic responses from those of M1-type in the nervous system.

Pharmacological differences between two muscarinic responses of the rat superior cervical ganglion in vitro

1 Pharmacological differences have been observed between the muscarinic agonist-induced depolarizing and hyperpolarizing responses of the rat isolated superior cervical ganglion. 2 Pirenzepine (0.3 microM) selectively reduced the depolarizing response and unmasked the hyperpolarizing response. No such selectivity was observed with a concentration of N-methylatropine which was equipotent with pirenzepine in antagonizing the depolarizing response. 3 The neuromuscular blocking agents gallamine (10 microM) and pancuronium (3 microM) exhibited the oppositive selectivity to pirenzepine, both dramatically reduced the hyperpolarization but only slightly antagonized the depolarization. 4 The potencies of a range of agonists in evoking the depolarizing and hyperpolarizing responses, the latter in the presence of 0.3 microM pirenzepine, have been determined. Methylfurmethide failed to hyperpolarize the ganglion at concentrations which evoked maximal depolarizations. 5 The muscarinic hyperpolarization did not appear to be mediated by the secondary release of catecholamines. 6 It was concluded that the two muscarinic responses on the rat superior cervical ganglion, the slow depolarization and faster hyperpolarization, are mediated by different muscarinic receptor subtypes.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

Cholinergic receptors, ion channels, neurotransmitter synthesis, and neurite outgrowth are independently regulated during the in vitro differentiation of a human neuroblastoma cell line

The differentiation of human nerve cells was investigated using a cell model comprising human neuroblastoma (IMR32) cells that were induced to differentiate by the addition of 5-bromo-2'-deoxyuridine (BrdU) or N6-O2-dibutyryl cyclic adenosine 3'-5' monophosphate (Bt2cAMP). As parameters of differentiation, we studied neurite outgrowth, cholinergic receptors, voltage-activated ion channels, tyrosine hydroxylase activity, and neurotransmitter content. BrdU induced marked morphological differentiation, as indicated by the number and length of neurites, as well as an increase in the number of alpha-bungarotoxin binding sites, muscarinic receptors, and voltage-dependent Na channels. In addition, BrdU induced an increase in tyrosine hydroxylase activity as well as in serotonin, dopamine, and noradrenaline content. Bt2cAMP had a less dramatic effect on the morphological appearance of the cells, induced the expression of alpha-bungarotoxin binding sites (but not of muscarinic receptors), and produced a marked increase in the serotonin and noradrenaline content. Not only the number but also the functional properties of nicotinic and muscarinic receptors were differently affected by the two drugs. We conclude that Bt2cAMP and BrdU induce a different pattern of differentiation in the same cells, and that the expression of specific neuronal markers can be modulated to yield functionally different neurons.

Induction of muscarinic acetylcholine, serotonin and substance P receptors in Xenopus oocytes injected with mRNA prepared from the small intestine of rats

Serotonin and muscarinic acetylcholine (ACh) receptors were clearly induced in Xenopus oocyte injected with mRNA prepared from the small intestines of rats. Their response to ACh and serotonin was composed of 4 distinct components: fast and slow depolarization, slow hyperpolarization and large membrane potential fluctuation. About three-quarters of the injected oocytes responded to substance P. The response of the injected oocytes to substance P was transient and decayed even in the presence of substance P, indicating the presence of desensitization. However, the injected oocytes showed no response to 6 other drugs analyzed: adrenaline, noradrenaline, dopamine, gamma-aminobutyric acid, glycine and glutamate.

Evidence for epinephrine-induced depolarization in neurons of bullfrog sympathetic ganglia

The response to epinephrine (EP) was determined for neurons in bullfrog sympathetic ganglia by intracellular and voltage-clamp recording techniques. EP (5 microM-1 mM) produced a concentration-dependent depolarization mediated through beta-adrenoceptors. The EP-induced depolarization (EPD) was associated with a decrease in the membrane conductance. The EP-induced current (EP1) was decreased at hyperpolarizing potential levels and nullified at -70 mV. No reversal of the EPI polarity was seen. It is concluded that the EPD is generated by the suppression of a voltage-dependent gK, probably the M-channel.

Multiple muscarinic responses directly evoked in isolated neurones dissociated from rabbit sympathetic ganglia

Single sympathetic neurones, enzymatically isolated from the superior cervical ganglia of adult rabbits and maintained in culture medium, responded directly to DL-muscarine with a variety of electrical responses. The falling phase of the action potential was markedly accelerated while the late portion of the after-spike hyperpolarization was depressed. These changes occurred before any detectable change in membrane potential. The slow depolarizing changes in membrane potential induced by muscarine were associated with different changes in membrane conductance: a voltage-independent increase, a decrease at the levels of membrane potential positive to about -70 mV and a voltage-independent decrease. Muscarine can produce these multiple membrane changes often within the same cell, suggesting that the different effects are not due to heterogeneous cell types.

Anticholinesterase agents affect contrast gain of the cat cortical visual evoked potential

The effects of the anticholinesterase agents physostigmine, pyridostigmine, diisopropylfluorophosphate and soman on the cortical visual evoked potential (VEP) were examined using phase reversed sine wave gratings of varying contrasts. All 4 agents decreased the slope (gain) of the response vs log contrast function while the intercept with the noise level (threshold, an indicator of contrast sensitivity) was unaffected. These results suggest that anticholinesterases modify the VEP by changing a contrast gain control mechanism.

Acetylcholine-activated currents in mouse neuroblastoma cells

The nicotine and muscarinic responses of differentiated mouse neuroblastoma cells from the clonal line N1E 115 to applied cholinergic agents were recorded using single channel and whole cell patch clamp techniques. An inward macroscopic current induced by acetylcholine (ACh) at the resting potential was blocked by curare; cell-attached recordings revealed a single channel conductance of 18 pS and a lifetime of 36 ms at 30 degrees C, with 200 nM ACh. The zero current potential was close to 0 mV. The kinetics of these nicotinic currents were described by multiexponential functions for both the open and closed time distributions. An outward single channel current, present at resting and slightly depolarized potentials, was also observed and has been tentatively described as being dependent on muscarinic receptor activation, as it was usually blocked by atropine. Under our conditions of whole cell clamp, no macroscopic outward current sensitive to ACh was observed.

Three types of muscarinic conductance changes in sympathetic neurons discriminately evoked by the different concentrations of acetylcholine

Three different types of changes in membrane conductance were induced in isolated mammalian sympathetic neurons by the muscarinic actions of acetylcholine at different levels of concentration. These include a voltage-independent increase and two decreases, voltage-dependent and -independent, respectively. Muscarinic receptors in sympathetic neurons are thus shown to be coupled to multiple ionic channels, even within a single cell, and controlled by the different concentrations of a neurotransmitter.

On the mechanism of activation of muscarinic K+ channels by adenosine in isolated atrial cells: involvement of GTP-binding proteins

The molecular mechanisms underlying activation of a K+ channel by adenosine (Ado) and acetylcholine (ACh) were examined in single atrial cells of guinea-pig. Whole cell clamp and patch clamp techniques were used to characterize the K+ channel. In the whole cell clamp conditions, Ado and ACh increased the K+ channel current in a dose-dependent manner. The maximum responses and the apparent dissociation constants were different for Ado and ACh activations of the current. Theophylline blocked activation of the K+ current by Ado, while atropine blocked ACh-activation, indicating that two different membrane receptors were involved. Measurements of the conductance and kinetic properties of both whole cell and single channel currents indicate that Ado and ACh regulate the same K+ channels. In "inside-out" patch conditions, GTP was required in the intracellular side of the membrane for activation of the K+ channel by agonists (present in the patch electrode). The A promoter of pertussis toxin inhibited the channel activation only when NAD was also present. Furthermore, GTP-gamma S, a non-hydrolyzable GTP analogue, gradually caused activation of the K+ channel in the absence of agonists. Therefore, it was concluded that Ado and m-ACh receptors link with the same population of K+ channels via GTP-binding proteins Ni and/or No in the atrial cell membrane.

Hyperpolarizing responses of single mammalian sympathetic neurones to acetylcholine and to dopamine

Acetylcholine (ACh) applied iontophoretically can elicit not only nicotinic fast (f-) and/or muscarinic slow (s-) depolarizations (DP) but also hyperpolarizing responses (HP), all in the same cell. HPs were either 'initial' (sharp onset at the end of ACh delivery) or 'secondary' (variable later onsets). Pretreatment with bretylium eliminated secondary, but not initial, responses to ACh. Pretreatment with both BCh and bretylium eliminated essentially all HP responses to ACh, with good DP responses remaining. Test responses to the more purely muscarinic agent BCh exhibited only a smooth, large s-DP with no secondary HP components, even in ganglia not pretreated with bretylium. The foregoing evidence is explainable by our previous proposals that ACh can elicit two types of HPs, one mediated by muscarinic receptors (putatively located on DA interneurones) and another by non-muscarinic receptors (putatively releasing norepinephrine at dendro-dendritic junctions). Dopamine (DA) applied iontophoretically could also elicit HP responses from the same cells. Iontophoretic DA could elicit an HP at any depth (up to 290 micron tested) in the ganglion. However, bulk application of DA (in the extra-ganglionic medium) was only effective on neurones impaled in the outer 30 micron. This may help explain the variable findings with applied DA.

Autoradiographic localization of peripheral M1 muscarinic receptors using [3H]pirenzepine

A novel antimuscarinic agent, pirenzepine, has been proposed to distinguish at least two subtypes of muscarinic receptor. M1 receptors have been designated as those displaying a high affinity for pirenzepine. Both functional and binding studies have revealed a prevalence of M1 receptors in sympathetic ganglia while autonomic effector tissues have only low densities of M1 receptors. In the present study, in vitro autoradiographic procedures have been used to localize specifically high affinity binding sites for pirenzepine (M1 receptors) in sections of guinea-pig ileum, rat superior cervical ganglion and rat submaxillary gland. The overall localization of muscarinic receptors was also studied using the non-selective antagonist, [3H]N-methylscopolamine. The highest densities of M1 receptors were found in superior cervical ganglion, sympathetic nerve bundles, myenteric ganglia and mucous secreting cells of the submaxillary gland, while lower densities were found in smooth muscle and serous secreting cells of the submaxillary gland. No area found to possess muscarinic receptors was devoid of M1 receptors.

Postganglionic muscarinic inhibitory receptors in pulmonary parasympathetic nerves in the guinea-pig

The location of neuronal inhibitory muscarinic receptors in pulmonary parasympathetic nerves was investigated in vivo and in vitro. The effects of an agonist for neuronal muscarinic receptors (pilocarpine) and an antagonist (gallamine) were tested on contractions of airway smooth muscle induced by pre- and postganglionic cholinergic nerve stimulation. In anaesthetized guinea-pigs, gallamine potentiated constriction of the tracheal tube and smaller airways induced by preganglionic stimulation. Gallamine also potentiated postganglionic stimulation induced by transmural stimulation of the tracheal tube and by 1-1-dimethyl-4-phenylpiperazinium iodide in the rest of the lung. Bronchoconstriction and contraction of the tracheal tube induced by intravenous acetylcholine were not potentiated by gallamine, indicating that postjunctional muscarinic receptors in airway smooth muscle were not involved in the potentiation. The muscarinic agonist, pilocarpine, had the opposite effect to gallamine and inhibited contractions of the tracheal tube induced by both types of nerve stimulation. In vitro, the inhibitory effect of pilocarpine was demonstrated in the tracheal tube preparation stimulated both pre- and postganglionically. The effect of pilocarpine was antagonized by gallamine. Because gallamine and pilocarpine affected the responses to postganglionic stimulation, the inhibitory muscarinic receptors must be located on the postganglionic neurones of the parasympathetic nerves innervating the trachea and the smaller airways. However, these experiments cannot exclude the possibility that muscarinic receptors may also be located on the soma of the ganglion cells.

Evidence for a catecholamine-mediated slow hyperpolarizing synaptic response in parasympathetic ganglia

Stimulation of preganglionic nerve trunks in the presence of muscarinic and nicotinic cholinoceptor and purinoceptor antagonists produced a slow-hyperpolarizing synaptic potential that was mimicked by exogenously applied norepinephrine. Both responses were blocked by yohimbine, an alpha-adrenoceptor antagonist, and enhanced by imipramine and cocaine, inhibitors of norepinephrine reuptake. These findings fulfill pharmacological criteria suggesting that norepinephrine is a neurotransmitter in cat bladder parasympathetic ganglia.

Alpha 2-adrenergic hyperpolarization is not involved in slow synaptic inhibition in amphibian sympathetic ganglia

The adrenaline-induced hyperpolarization (AdH), slow inhibitory postsynaptic potential (slow i.p.s.p.) and hyperpolarizing phase of the response to methacholine (MChH) in Rana pipiens sympathetic ganglia were studied by means of the sucrose-gap technique. Desmethylimipramine (DMI, 0.5 microM) lowered the EC50 for adrenaline from 1.65 microM (1.23-2.21 microM, n = 10) to 0.30 microM (0.21-0.41 microM, n = 8). DMI did not potentiate the slow i.p.s.p. or the MChH. Propranolol, sotalol or prazosin (1 microM) did not antagonize the AdH. The response was antagonised by phentolamine (IC50 = 0.53 microM), yohimbine (IC50 = 6.2 nM) and idazoxan (IC50 = 0.59 microM). Yohimbine (0.1 microM) did not reduce the amplitude of the slow i.p.s.p. or the MChH. The slow i.p.s.p. was eliminated in Ringer solution containing Cd2+ (100 microM). This concentration of Cd2+ did not reduce the amplitude of the MChH. Alpha-Methylnoradrenaline produced a concentration-dependent hyperpolarization with an EC50 of 0.31 microM (0.13-0.73 microM, n = 5), in the presence of DMI (0.5 microM). These results are consistent with the hypothesis that the AdH may be generated by activation of a receptor similar to the mammalian alpha 2-adrenoceptor. No evidence was found in support of the hypothesis that an adrenergic interneurone is involved in the synaptic pathway for the slow i.p.s.p.

Acetylcholine hyperpolarizes central neurones by acting on an M2 muscarinic receptor

Acetylcholine (ACh) is considered to act as a neurotransmitter in the mammalian brain by binding to membrane receptors and bringing about a change in neurone excitability. In the case of muscarinic receptors, cell excitability is usually increased; this effect results from a closure of membrane potassium channels in cortical cells. However, some central neurones are inhibited by ACh, and we hypothesized that these two opposite effects of ACh resulted from interactions with different subtypes of muscarinic receptor. We made intracellular recordings from neurones in the rat nucleus parabrachialis, a group of neurones in the upper pons some of which themselves synthesize ACh. ACh and muscarine caused a membrane hyperpolarization which resulted from an increase in the membrane conductance to potassium ions. The muscarinic receptor subtype was characterized by determining the dissociation equilibrium constant (KD) for pirenzepine during the intracellular recording; the value of approximately 600 nM indicates a receptor in the M2 class. This muscarinic receptor is quite different from that which brings about a decrease in potassium conductance in other neurones, which has a pirenzepine KD of approximately 10 nM (M1 receptors). It is possible that antagonists selective for this kind of M2 receptor would be useful in the management of conditions, such as Alzheimer's disease, which are associated with a reduced effectiveness of cholinergic neurones.

Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Non-nicotinic slow synaptic currents were recorded from voltage-clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d-tubocurarine and (usually) 1 microM-neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage-dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.

Innervation of small intensely fluorescent cells in frog sympathetic ganglia

Autonomic ganglia have a population of small intensely fluorescent (SIF) cells with unknown function. We have investigated the afferent innervation of SIF cells in bullfrog sympathetic ganglia using intracellular recordings and light microscopy of stained preganglionic axon terminals. Contrary to previous knowledge, bullfrog SIF cells do indeed receive functional synaptic input and this innervation is provided solely by the C-type preganglionic nerve fibers.