Alpha-adrenoceptor and dopamine receptor antagonists do not block the slow inhibitory postsynaptic potential in sympathetic ganglia

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.

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.

Neuropeptide Y mimics a non-adrenergic component of sympathetic vasoconstriction in the bullfrog

The effects of preganglionic sympathetic nerve stimulation and exogenous agents upon vascular tone were observed in hindlimb preparations of pithed adult bullfrogs. Repetitive electrical stimulation of the sympathetic C, but not the B, system elicited arterial vasoconstriction and reduced blood flow in vascular beds supplying the sartorius muscle and the skin. Close-arterial injections of epinephrine and neuropeptide Y each mimicked neurogenic vasoconstriction. After close-arterial injection of phentolamine, an alpha-adrenergic antagonist, the maximal effects of nerve stimulation were delayed in onset and reduced in magnitude, but not eliminated. Pretreatment with phentolamine blocked the vasoconstriction caused by injection of epinephrine, and produced a mild reduction in responses to neuropeptide Y. These observations demonstrate the vasomotor function of the sympathetic C system and they support the hypothesis that neuropeptide Y and epinephrine function as cotransmitters in postganglionic C neurons.

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.

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.

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.

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.

The interaction between metoclopramide and dopaminergic agonists at the level of sympathetic ganglion

The interaction of metoclopramide and two dopaminergic agonists, dopamine and apomorphine, was investigated on ganglionic transmission at the superior cervical ganglion of the anaesthetized cat. Dopamine (100 micrograms, i.a.) and apomorphine (100 micrograms, i.a.) decreased the height of action potential induced by supramaximal preganglionic stimulation. Metoclopramide (3 mg/kg, i.v.) antagonized the inhibition caused by dopamine and apomorphine. But it did not alter the transmission block caused by hexamethonium (100 micrograms, i.a.) or noradrenaline (100 micrograms, i.a.). The results suggest that dopamine-induced inhibition of the ganglionic transmission is related to the activation of dopaminergic receptors of the ganglion cells.

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

Slow muscarinic inhibition may be a powerful influence on membrane properties in the peripheral and central nervous system. But the location of the muscarinic receptors in sympathetic ganglia, either on interneurones or on the postganglionic membrane, and the underlying mechanism of the inhibitory response, remains controversial. In mammalian sympathetic ganglia synaptic activation of muscarinic receptors located on inhibitory interneurones was thought to release catecholamines leading to a membrane hyperpolarization called the slow inhibitory postsynaptic potential, or s.-i.p.s.p.. However, the s.-i.p.s.p. in parasympathetic ganglia and in amphibian sympathetic ganglia is due to direct monosynaptic activation of muscarinic receptors, accompanied by an increased potassium conductance (but see ref. 11), and is not mediated by catecholamines. The situation is less clear in mammalian sympathetic ganglia and monosynaptic s.-i.p.s.ps observed in other ganglia could be exceptions to the hypothesis. We showed earlier that the s.-i.p.s.p. in rabbit superior cervical ganglia is not affected by catecholamine antagonists. We now show that the s.-i.p.s.p. in a mammalian sympathetic ganglion is due to the monosynaptic activation of muscarinic receptors, probably by an increase in potassium conductance.

Effects of histamine agonists and antagonists (H1 and H2) on ganglionic transmission and on accumulation of cyclic nucleotides (cAMP and cGMP) in rat superior cervical ganglion in vitro

Histamine and 4-methylhistamine inhibited ganglionic transmission in the rat superior cervical ganglion in vitro via H2-histaminergic receptors. Under blockade of H2-receptors, 4-methylhistamine sometimes showed slight facilitation of ganglionic transmission, when repetitive stimuli were applied. The H1-receptor agonist, 2-pyridylethylamine, was ineffective. Histamine and 4-methylhistamine increased both cyclic AMP (cAMP) and cyclic GMP (cGMP) levels in concentrations depressing ganglionic transmission, whereas 2-pyridylethylamine increased only cAMP concentrations in the isolated ganglia. Histamine-induced accumulation of cyclic nucleotides was only partially prevented by either histamine--H1- and H2-receptor antagonists, but abolished by their combination. It is concluded that changes in intraganglionic cyclic nucleotides induced by histaminergic receptor agonists did not apparently correlate with their effect on ganglionic transmission in vitro.

Muscarinic inhibition of sympathetic C neurones in the bullfrog

1. The muscarinic inhibitory post-synaptic potential (i.p.s.p.) in sympathetic C neurones has been characterized in an isolated preparation of bullfrog paravertebral chain ganglia. Interactions between the i.p.s.p. and two other synaptic potentials have also been examined. 2. A single presynaptic stimulus to a C cell produces a nicotinic excitatory post-synaptic potential (e.p.s.p.) followed by a muscarine i.p.s.p. The latency of the i.p.s.p. is 50 msec or longer and the response lasts for seconds. C cells receive multiple cholinergic innervation but the thresholds for activation of the e.p.s.p. and i.p.s.p. are inseparable. Trains of 50 or more presynaptic stimuli produce a non-cholinergic e.p.s.p. which follows the nicotinic e.p.s.p. and i.p.s.p. and which lasts for tens of seconds. 3. The i.p.s.p. produced by a single presynaptic stimulus can be 30 mV in amplitude. However, in most cells, a short train of stimuli applied at an optimal frequency of 10 Hz is required to produce a large i.p.s.p. 4. The i.p.s.p. is blocked by atropine but is not affected by catecholamine antagonists. 5. Ionophoretically applied acetylcholine (ACh) mimics the i.p.s.p. in its latency, time course and amplitude. In addition, the i.p.s.p. and the muscarinic response to ACh reverse polarity at the same membrane potential: -102 mV in normal Ringer solution. The i.p.s.p. reversal potential shifts by 55 mV/decade change in extracellular K+ concentration and is insensitive to the Cl- gradient. 300 microM-Ba2+ totally blocks the muscarinically activated conductance in a reversible manner. 6. Action potentials, when initiated by a supramaximal nicotinic e.p.s.p. or by an antidromic impulse, are not blocked by the i.p.s.p. 7. Near resting potential (-50 to -60 mV), C cells can fire repetitively. The non-cholinergic slow e.p.s.p. is often accompanied by oscillations in membrane potential and firing of action potentials. This repetitive firing of C cells, which appears to be enhanced by the non-cholinergic e.p.s.p., is strongly inhibited by the i.p.s.p. The inhibition can be mimicked by injection of very small hyperpolarizing currents (e.g. 25 pA). Interactions between the i.p.s.p. and the non-cholinergic e.p.s.p. can generate phasic bursting patterns in C cells. 8. The mechanism underlying the i.p.s.p. and the consequences of these findings for ganglionic integration are discussed.

Cholinergic transmission in cat parasympathetic ganglia

1. Intracellular electrical recording techniques were used to study the ionic mechanisms of cholinergic synaptic transmission in cat vesical pelvic ganglia (v.p.g.). 2. Orthodromic nerve stimulation as well as ionophoretic application of acetylcholine (ACh) resulted in, first, a fast excitatory post-synaptic potential (f.e.p.s.p.) and secondly, a slow inhibitory post-synaptic potential (s.i.p.s.p). These distinct post-synaptic responses were direct actions of ACh and not mediated through an interneurone. In addition, a slow excitatory post-synaptic potential (s.e.p.s.p.) was observed in 44% of the cells. 3. The f.e.p.s.p., mediated via nicotinic receptors, had a reversal potential of -10 mV and resembled the conventional rapid depolarization in other ganglia. The s.i.p.s.p., mediated by muscarinic receptors, had a reversal potential of about -100 mV and resulted from an increase in potassium conductance. 4. The slow muscarinic hyperpolarization could be observed in the absence of antagonists and it was elicited at stimulus frequencies in the physiological range (2-10 Hz). the s.i.p.s.p. induced orthodromically or ionophoretically inhibited firing in spontaneously active neurones. These observations suggest that the muscarinic hyperpolarization may occur under physiological conditions and has sufficient magnitude to be inhibitory to neuronal activity.

Control of calcium current in rat sympathetic neurons by norepinephrine

Inward voltage-dependent calcium currents were recorded from clamped rat sympathetic ganglion cells using either one or two microelectrodes. Suppression of potassium current was achieved by applying tetraethylammonium (TEA) externally and TEA plus cesium internally. Peak ICa was observed at 0 mV. ICa was abolished by perfusing cadmium or low calcium medium. ICa was reduced by adding norepinephrine (1-50 micrometers). This effect was not accompanied by any major change in the voltage sensitivity or time course of the residual calcium current. It is suggested that norepinephrine acts by reducing the number of available calcium channels.

Pharmacological properties and monoaminergic mediation of the slow IPSP, in mammalian sympathetic ganglion

Phenoxybenzamine can selectively eliminate the s-IPSP, in the presence of anti-cholinesterases that enhance s-IPSP and s-EPSP; and the alpha 2-antagonist, yohimbine, can partially but consistently depress s-IPSP selectively. The results provide positive pharmacological support for the monoaminergic nature of the transmitter for s-IPSP in mammalian sympathetic ganglia and argue against suggestions that the s-IPSP is a direct hyperpolarizing response to acetylcholine.

Sucrose-gap recordings of nerve-evoked potentials in mammalian parasympathetic ganglia

The sucrose-gap recording technique was used to study mammalian parasympathetic ganglionic transmission. Both a fast nicotinic depolarization potential and a slow muscarinic hyperpolarizing potential were recorded in vesical pelvic ganglia (VPG). A long afterhyperpolarization caused by an electrical response of through-fibers was also recorded. However, a slow excitatory postsynaptic potential (S-EPSP) was not readily observed and may be masked by the long afterhyperpolarization. In addition, the slow inhibitory postsynaptic potential (S-IPSP) of the VPG was due to a direct effect of acetylcholine (ACh). Thus, sucrose-gap recordings of VPG potentials are similar to those obtained in sympathetic ganglia, but the mechanism for transmission of the S-IPSP may be different in the respective ganglia.