Hyperpolarizing 'alpha 2'-adrenoceptors in rat sympathetic ganglia

Mechanisms of electroacupuncture-induced analgesia on neuropathic pain in animal model

Neuropathic pain remains as one of the most difficult clinical pain syndromes to treat. Electroacupuncture (EA), involving endogenous opioids and neurotransmitters in the central nervous system (CNS), is reported to be clinically efficacious in various fields of pain. Although multiple experimental articles were conducted to assess the effect of EA-induced analgesia, no review has been published to assess the efficacy and clarify the mechanism of EA on neuropathic pain. To this aim, this study was firstly designed to evaluate the EA-induced analgesic effect on neuropathic pain and secondly to guide and help future efforts to advance the neuropathic pain treatment. For this purpose, articles referring to the analgesic effect of acupuncture on neuropathic pain and particularly the work performed in our own laboratory were analyzed. Based on the articles reviewed, the role of spinal opioidergic, adrenergic, serotonergic, cholinergic, and GABAergic receptors in the mechanism of EA-induced analgesia was studied. The results of this research demonstrate that μ and δ opioid receptors, α₂-adrenoreceptors, 5-HT_1A and 5-HT₃ serotonergic receptors, M₁ muscarinic receptors, and GABA_A and GABA_B GABAergic receptors are involved in the mechanisms of EA-induced analgesia on neuropathic pain.

Effects of α1- and α2-adrenoreceptor antagonists on cold allodynia in a rat tail model of neuropathic pain

Systemic administrations (0.1, 0.5, and 2 mg/kg) of alpha1-adrenoreceptor (AR) antagonist prazosin dose-dependently attenuated cold allodynia in a rat tail model of neuropathic pain, whereas alpha2-AR antagonist yohimbine exacerbated it. These results suggest that the functions of alpha1- and alpha2-AR in this model are excitatory and inhibitory, respectively, consistent with their general properties. It is also proposed that cold allodynia can be reversed by alpha1-AR antagonist and exacerbated by alpha2-AR antagonist.

Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens

1. Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drugs such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. 2. In chronically saline treated (CST) preparations, clonidine (0.5 microM) completely abolished evoked transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca(2+)](o)) of 2 mM. The inhibitory effect of clonidine was reduced by increasing [Ca(2+)](o) from 2 to 4 mM and the stimulation frequency from 0.1 to 1 Hz. 3. The nerve terminal impulse (NTI) was not affected by concentrations of clonidine that completely abolished evoked transmitter release. 4. Sympathetic varicosities developed a tolerance to clonidine (0.5 microM) following 7-9 days of chronic exposure to clonidine. 5. Acute withdrawal of preparations following chronic clonidine treatment (CCT) resulted in a significant (P < 0.005) enhancement of neurotransmitter release (3.75 times) above control levels observed in CST preparations. 6. The present findings demonstrate an enhancement of neurotransmitter release from sympathetic varicosities following acute withdrawal from chronic clonidine treatment.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

One hundred years of adrenaline: the discovery of autoreceptors

The active principle of suprenal extract that produces its pressor effects was isolated by the joint research of John Abel in 1899 and Jokichi Takamine in 1901. Within three years Elliott, working in Langley's laboratory, suggested that this active principle, referred to by British physiologists as "adrenaline" and named "Adrenalin" by Takamine, was released from sympathetic nerve terminals to act on smooth muscle cells. However, it was not until 1946 that von Euler showed that demythelated adrenaline (noradrenaline) rather than adrenaline is a sympathetic transmitter. The possibility that this sympathetic transmitter could also act on nerve terminals was not developed until 1971. Research on autoreceptors culminated in the identification of adrenergic receptors on nerve terminals different to those on muscle cells. This paper assesses the contributions that established the idea of the adrenergic autoreceptor, 100 years after the discovery of adrenaline.

Receptor subtype mediating the adrenergic sensitivity of pain behavior and ectopic discharges in neuropathic Lewis rats

Receptor subtype mediating the adrenergic sensitivity of pain behavior and ectopic discharges in neuropathic Lewis rats. We attempted to identify the subtype of alpha-adrenergic receptor (alpha-AR) that is responsible for the sympathetic (adrenergic) dependency of neuropathic pain in the segmental spinal injury (SSI) model in the Lewis strain of rat. This model was chosen because our previous study showed that pain behaviors in this condition are particularly sensitive to systemic injection of phentolamine (PTL), a general alpha-AR blocker. We examined the effects of specific alpha1- and alpha2-AR blockers on 1) behavioral signs of mechanical allodynia, 2) ectopic discharges recorded in the in vivo condition, and 3) ectopic discharges recorded in an in vitro setup. One week after tight ligation of the L5 and L6 spinal nerves, mechanical thresholds of the paw for foot withdrawals were drastically lowered; we interpreted this change as a sign of mechanical allodynia. Signs of mechanical allodynia were significantly relieved by a systemic injection of PTL (a mixed alpha1- and alpha2-AR antagonist) or terazosin (TRZ, an alpha1-AR antagonist) but not by various alpha2-AR antagonists (idazoxan, rauwolscine, or yohimbine), suggesting that the alpha1-AR is in part the mediator of the signs of mechanical allodynia. Ongoing ectopic discharges were recorded from injured afferents in fascicles of the L5 dorsal root of the neuropathic rat with an in vivo recording setup. Ongoing discharge rate was significantly reduced after intraperitoneal injection of PTL or TRZ but not by idazoxan. In addition, by using an in vitro recording setup, spontaneous activity was recorded from teased dorsal root fibers in a segment in which the spinal nerve was previously ligated. Application of epinephrine to the perfusion bath enhanced ongoing discharges. This evoked activity was blocked by pretreatment with TRZ but not with idazoxan. This study demonstrated that both behavioral signs of mechanical allodynia and ectopic discharges of injured afferents in the Lewis neuropathic rat are in part mediated by mechanisms involving alpha1-ARs. These results suggest that the sympathetic dependency of neuropathic pain in the Lewis strain of the rat is mediated by the alpha1 subtype of AR.

Norepinephrine inhibits neurons of the intermediate subnucleus of the lateral septum via alpha2-adrenoceptors

The physiological and pharmacological actions of norepinephrine (NE) on neurons of the intermediate subnucleus of the lateral septum (LSI) were examined using intracellular recordings in rat brain-slices. Bath-applied NE inhibited 72.5%, excited 5.5% and had no effect on 22% of LSI neurons tested; this study focused on the inhibitory effects of NE. In current clamp recordings, 100 microM NE produced a hyperpolarization of 10.82+/-0.72 mV (n=84) with a decrease in input resistance. In voltage-clamp, NE produced a direct, post-synaptic outward current of 206.8+/-22 pA (n=37) with a 64. 3+/-4.9% increase in input conductance (IC50-17.7+/-4 microM). The NE-induced inhibition was mimicked by the alpha2-agonist, UK14,304, but not by the alpha1- or beta-adrenoceptor agonists. The alpha2-agonist, clonidine, had a weak effect in LSI neurons. Interestingly, the magnitude of the UK14,304-induced response varied between cells (ranging from 29.5 to 320% of the maximal NE inhibition), possibly suggesting the involvement of alpha2A-(high affinity for UK14,304) and non-alpha2A (low affinity for UK14,304) adrenoceptor subtypes. While the alpha2-antagonists, yohimbine, rauwolscine and idazoxan blocked NE-induced inhibition in all neurons tested, the prototypical alpha1-antagonist, prazosin produced a variable degree of block (9-58%), further indicating the possible involvement of alpha2A (prazosin-insensitive) and non-alpha2A (prazosin-sensitive) receptors. However a lack of more selective pharmacological tools precludes definitive classification of the alpha2-receptor-mediated responses into different subtypes. The alpha2-receptor-mediated current in LSI neurons displayed Ba2+-sensitive inward rectification, reversed polarity near EK and was sensitive to external K+. In conclusion, NE inhibits LSI neurons via alpha2-adrenoceptor subtypes.

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.

Exocytotic release from neuronal cell bodies, dendrites and nerve terminals in sympathetic ganglia of the rat, and its differential regulation

Stimulant-induced exocytosis has been demonstrated in sympathetic ganglia of the rat by in vitro incubation of excised ganglia in the presence of tannic acid, which stabilizes vesicle cores after their exocytotic release. Sites of exocytosis were observed along non-synaptic regions of the surfaces of neuron somata and dendrites, including regions of dendrosomatic and dendrodendritic apposition, as well as along the surfaces of nerve terminals About half the exocytoses associated with nerve terminals were parasynaptic or synaptic, and these appeared mostly to arise from the presynaptic terminal, but occasionally from the postsynaptic element. The results demonstrated that the neurons of sympathetic ganglia release materials intraganglionically in response to stimulation, that release from different parts of the neuron is subject to independent regulation, at least via cholinergic receptors, and that release is partly diffuse, potentially mediating autocrine or paracrine effects, and partly targeted toward other neurons, but that the latter mode is not necessarily, and not evidently, synaptic. Specifically, exocytosis from all locations increased significantly during incubation in modified Krebs' solution containing 56 nm potassium. Observation of the effects of cholinergic agonists (nicotine, carbachol, oxotremorine) and antagonists (atropine, AF-DX 116) showed that nicotinic and muscarinic excitation each, independently, increased the incidence of exocytosis from somata and dendrites. Exocytosis from nerve endings was not altered by nicotine, but was enhanced or, at high initial rates of exocytosis, decreased, by muscarinic stimulation. Evidence was obtained for muscarinic auto-inhibition of exocytosis from nerve terminals, occurring under basal incubation conditions, and for a muscarinic excitatory component of somatic exocytosis, elicitable by endogenous acetylcholine. The M2-selective muscarinic antagonist AF-DX 116 was found to modify the exocytotic response of the dendrites to oxotremorine, widening the range of its variation; this effect is consistent with recent evidence for the presence of M2-like muscarinic binding sites, in addition to M1-like binding, upon these dendrites [Ramcharan E. J. and Matthews M. R. (1996) Neuroscience 71, 797-832]. Over all conditions, disproportionately more sites of somatic and dendritic exocytosis were found to be located in regions of dendrosomatic and dendrodendritic apposition than would be expected from the relative extent of the neuronal surface occupied by these relationships. Such mechanisms of intraganglionic release may be expected to contribute to the regulation and integration of the behaviour of the various functionally distinctive populations of neurons in these ganglia, by autocrine, paracrine, and focal, neuroneuronal, routes of action. Similar phenomena of exocytotic soma-dendritic release might prove to subserve integrative neuroneuronal interactions more widely throughout the nervous system.

Ultrastructure of neuronal circuitry in sympathetic ganglia

To elucidate the intraganglionic circuitry in sympathetic ganglia, attempts have been made to define the nature and source of those neuronal elements that establish synaptic connections there. Intracellular labeling of sympathetic cells is of particular value for this purpose. Dendrites of principal neurons in the rat superior cervical ganglion exhibit a varying complexity in their morphology and arborization. Some dendrites show specializations such as a glomerular plexus, where extensively-branched dendritic collaterals form synaptic connections comprising not only axodendritic synapses between preganglionic axons and principal cell dendrites, but also dendrodendritic synapses between principal cell dendrites. A few of these may represent reciprocal synapses. Most presynaptic elements of adrenergic synapses observed by conventional methods appear to represent these specialized dendritic collaterals of principal neurons. These presynaptic dendrites may be an important addition to the conventional scheme of intraganglionic synaptic organization. However, there seem to be extreme species and even strain differences in the number of these adrenergic synapses, and in the sophistication of the specialized local circuits within sympathetic ganglia. Sympathetic ganglia may thus function as more than a simple relay station, with specialized neuronal circuitry that may be involved in the modulation of cholinergic transmission.

Correlation between dendrodendritic synapses of adrenergic type and synaptically evoked hyperpolarization in the sympathetic ganglion of adult rats

Intracellular recording and labeling with biocytin followed by electron microscopic observation were used to examine the nature and the morphological basis of a synaptically evoked hyperpolarization following spikes in the rat superior cervical ganglion neurons. A large hyperpolarization (the amplitude > 8 mV; the duration > 1 s following spikes) was elicited by repetitive stimulation of the preganglionic nerves in 8% of cells examined (n = 50). The alpha 2-adrenoceptor antagonist, yohimbine, reversibly attenuated the hyperpolarization, without affecting spikes. A nicotinic antagonist, hexamethonium, blocked both the hyperpolarization and spikes. Atropine had no effect of these responses. Electron microscopic observation of dendrites of these cells revealed that they received synaptic inputs of adrenergic type besides a cholinergic one from the preganglionic axons. Some dendrites served as presynaptic elements. These results strongly suggest that the hyperpolarization is an inhibitory postsynaptic potential and that this disynaptic response to the preganglionic stimulation is mediated mainly by two transmitters, acetylcholine and noradrenaline that are released from axodendritic and dendrodendritic synapses, respectively. We conclude that there appears to be an adrenergic inhibitory local circuit that modulates cholinergic transmission in the sympathetic ganglia.

Pharmacological characterization of alpha 2-adrenoceptor regulated serotonin release in the rat hippocampus

The purpose of the present study was to confirm the functional regulation by alpha 2-adrenoceptors of the release of serotonin (5-HT) from the rat hippocampus in vivo. Under several pharmacological conditions, extracellular levels of 5-HT were estimated by assaying its concentrations in the perfusate by high performance liquid chromatography with electrochemical detection. Extracellular 5-HT in the hippocampus was reduced by tetrodotoxin (10 microM) co-perfusion, but increased by perfusion of a selective 5-HT re-uptake inhibitor, fluoxetine (10 microM). Addition of potassium (K+, 120 mM) to the perfusion fluid evoked an approximately 3-fold increase in 5-HT release. When the alpha 2-adrenoceptor agonist UK14,304 (0.1-10 microM) was added to the perfusion solution, the K(+)-evoked 5-HT release was significantly inhibited in a concentration-dependent manner. This inhibitory action of UK14,304 was reversed by pretreatment with an alpha 2-adrenoceptor antagonist, idazoxan (5 mg/kg, i.p.). In rats which were catecholaminergically denervated with 6-hydroxydopamine, UK14,304 (10 microM) still inhibited the K(+)-evoked 5-HT release. Treatment with pertussis toxin (PTX) did not alter the K(+)-evoked release of 5-HT but abolished the inhibitory effect of UK14,304. These findings suggest that 5-HT release is functionally modulated via alpha 2-adrenoceptors located on the serotonergic nerve terminals in the rat hippocampus and furthermore, the possibility that the inhibitory of alpha 2-adrenoceptors is linked to G-proteins which are substrates of PTX.

mRNAs encoding muscarinic and substance P receptors in cultured sympathetic neurons are differentially regulated by LIF or CNTF

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) have previously been shown to regulate neuronal choice of neurotransmitter. In this present study, these factors were shown to specifically and differentially regulate levels of both muscarinic (subtypes m1, m2, m3, m4, and m5) and substance P receptor (SPR) mRNAs in sympathetic neurons of the rat superior cervical ganglion (SCG) using solution hybridization/RNase protection analysis. In vivo, neonatal rat SCG expressed predominantly m2 (10.31 +/- 0.43 pg mRNA/micrograms total RNA) and some m1 (1.54 +/- 0.84 pg/microgram) muscarinic receptor mRNA, which increased developmentally to adult levels (m2 mRNA levels being 60% higher than those in neonates). By contrast, m3, m4, and m5 subtype mRNAs were much less abundant at all time points measured. A similar developmental regulation was found in dissociated SCG neurons in vitro. After 16 days in culture, m2 mRNA increased 334% to 15.76 +/- 0.68 pg/microgram, while m1 mRNA changed little (2.03 +/- 1.00 pg/microgram). However, LIF or CNTF treatment (5 ng/ml, 14 days) in sister cultures completely blocked this developmental increase. Further, LIF treatment blocked the normal muscarinic receptor-mediated increase in intracellular calcium (fura-2 imaging), indicating a functional change in receptor phenotype. By contrast, levels of SPR mRNA, which were low in untreated cultures (0.037 +/- 0.025 pg SPR mRNA/microgram total RNA), were elevated by LIF or CNTF treatment, to 0.866 +/- 0.034 pg/microgram and 0.662 +/- 0.148 pg/microgram, respectively. These observations indicate that muscarinic and SPR receptor expression are differentially regulated by the same factors in SCG neurons and that neuronal choice of receptor phenotype may be, at least in part, specifically regulated by cytokines/growth factors in the cellular milieu.

Norepinephrine modulates excitatory amino acid-induced responses in developing human and adult rat cerebral cortex

These experiments were designed to assess the ability of norepinephrine and its beta-receptor agonist, isoproterenol, to modulate responses induced by activation of excitatory amino acid receptors in brain slices obtained from developing human cortex or adult rat cortex. Human cortical slices were obtained from children undergoing surgery for intractable epilepsy (9 months to 10 yr of age). For comparison, slices were also obtained from rats (2-3 months of age). Iontophoretic application of glutamate, N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) produced excitatory responses consisting of membrane depolarizations accompanied by action potentials. Iontophoretic or bath application of norepinephrine or isoproterenol enhanced responses evoked by glutamate or N-methyl-D-aspartate. Depolarizations occurred with shorter latencies and their amplitudes increased. Action potential frequency was also increased and responses were of longer duration. In contrast, norepinephrine or isoproterenol had no effect on responses induced by AMPA. The enhancement of responses induced by N-methyl-D-aspartate or glutamate was antagonized by the beta-adrenergic receptor antagonist propranolol. Similar findings were obtained from neurons in humans or rats. These results suggest that norepinephrine, possibly via beta-receptors, potentiates responses mediated by glutamate and N-methyl-D-aspartate receptors without affecting those mediated by AMPA receptors. These effects were observed at all ages studied, indicating that the ability of norepinephrine to modulate excitatory neuronal transmission is well developed in human cortex by 9 months of age.

Release of sympathetic neurotransmitter evoked by electrical stimulation is increased in the chronically decentralized artery

The present study was performed to confirm our previous proposal that the increase in neurotransmitter release is responsible for the supersensitivity of chronically decentralized artery to transmural nerve stimulation (TNS) in the rabbit. The ear artery was decentralized unilaterally by removing the preganglionic fiber proximal to the superior cervical ganglion (SCG), and the ear arteries and SCG were dissected 8 weeks after the operation. The increase in the tritium overflow induced by TNS from the chronically decentralized artery, which had been incubated with 3H-noradrenaline (NA) for 1 hr, was markedly increased at lower frequencies (0.1 and 0.2 Hz) than that from the control artery, whereas there was no difference at higher frequencies (> 0.5 Hz). No difference was observed in the neuronal uptake of 3H-NA during incubation for 1 hr between the control and decentralized arteries. There was also no change in the contents of catecholamines in both the artery and SCG after chronic decentralization, when assayed by a radioenzymatic procedure. In conclusion, the results obtained indicate that the supersensitivity to TNS after chronic decentralization is not due to the deranged catecholamine uptake and storage mechanisms in adrenergic nerve terminals and augmented transmitter biosynthesis, but due to the increased release of transmitter in response to low frequencies of TNS.

Characterization of the adenosine receptors of the rat superior cervical ganglion

1. Adenosine analogues caused hyperpolarization and inhibition of the depolarizing response to muscarine of the rat isolated superior cervical ganglion (SCG) measured by a 'grease gap' recording technique. The receptors mediating these responses have been characterized by use of a range of selective adenosine analogues and adenosine receptor antagonists. 2. In decreasing order of potency N6-cyclopentyladenosine (CPA), 2-chloroadenosine (2CA), adenosine, 2-phenylaminoadenosine (PAA), caused concentration-dependent hyperpolarizations whilst N6-(9-fluorenylmethyl)adenosine (PD 117,413) was inactive at up to 100 microM. 3. The order of potency of adenosine analogues in depressing depolarization caused by a submaximal concentration of muscarine (100 nM) was: CPA > R-PIA = 2CA > NECA > S-PIA > BZA > adenosine > PAA, where R- and S-PIA = R(-)- and S(+)-N6-(2-phenylisopropyl)adenosine, NECA = 5'N-ethylcarboxamidoadenosine and BZA = N6-benzyladenosine. PD 117,413 was inactive at concentrations up to 100 microM. The maximum inhibitions of the muscarine-induced depolarization by CPA, 2CA, NECA and BZA were similar. R-PIA, S-PIA and PAA produced similar maximal inhibitions which were significantly smaller than those produced by CPA. 4. Hyperpolarizations caused by adenosine were antagonized by the P1-purinoceptor selective antagonist 1,3-dimethyl-8-phenylxanthine (8PT) and by the selective A1-adenosine receptor antagonist, 1,3-dipropyl-8-(4-((2-aminoethyl)amino)carbonylmethyloxyphenyl++ +)xanthine (XAC). Hyperpolarizations caused by CPA, adenosine and PAA were antagonized by the A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2-selective antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX). 5. Inhibition of the muscarinic-induced depolarization by CPA was antagonized by 8PT and DPCPXbut not by DMPX.6. It is concluded that the neurones of the rat SCG possess P1-purinoceptors of the Al-adenosine receptor subtype which mediate hyperpolarization and inhibition of depolarization caused by muscarine.

Studies on the location of catecholamine receptors in canine sympathetic ganglia

Receptors mediating catecholamine-induced inhibition were studied in cardiac ganglia of pentobarbital-anesthetized dogs. Using selective agonists and antagonists the presence of three receptor subtypes was verified: alpha 1- and alpha 2-adrenoceptors and dopamine D2 receptors. Activation of alpha 1-adrenoceptors or dopamine D2 receptors reduced the response to preganglionic nerve stimulation but not to direct stimulation of the nicotinic acetylcholine receptors of the principal ganglion cells: response to both types of stimulation were reduced by activation of ganglionic alpha 2-adrenoceptors. These results suggested that two inhibitory systems were present in canine sympathetic ganglia and mediated the effects of exogenous catecholamines. One system involved alpha 1-adrenoceptors and dopamine D2 receptors located proximal to the synapse of the pre- and postganglionic neurons and the other involved alpha 2-adrenoceptors located distal to the intraganglionic synapse.

The effect of centrally acting myorelaxants on NMDA receptor-mediated synaptic transmission in the immature rat spinal cord in vitro

1. The effect of the myorelaxant drugs baclofen, diazepam and tizanidine have been compared on in vitro preparations of baby rat spinal cord and adult rat superior cervical ganglion. 2. Dorsal root-elicited long duration (time to half decay 9.71 +/- 0.29 s.e. mean, n = 31) ipsilateral ventral root reflexes (DR-VRP), measured as integrated area, of immature rat spinal cord preparations were abolished by RS-2-amino-5-phosphonopentanoate (AP5) (EC50 8.13 +/- 0.92 microM, n = 3). The initial short latency component of DR-VRP was resistant to AP5. 3. Baclofen abolished both components of the DR-VRP. Respective EC50 values for the AP5-insensitive and AP5-sensitive components were 237 +/- 68 nM (n +/- 7) and 57 +/- 10 nM (n = 7). These effects of baclofen were reversed by the GABAB antagonist, CGP35348. The apparent Kd values (16.7 +/- 6.4 microM, n = 3 and 14.3 +/- 3.9 microM, n = 6 respectively) for this reversal were not significantly different. 4. Tizanidine, clonidine and diazepam had no effect on the AP5-insensitive component of the DR-VRP. 5. The AP5-sensitive long duration component of the DR-VRP was depressed to respective maximal levels of 23.2 +/- 1.4% (n = 7), 18.8 +/- 3.8% (n = 4) and 47.6 +/- 1.6% (n = 5) of control (100%) levels by tizanidine (EC50 135 +/- 33 nM), clonidine (EC50 26.0 +/- 2.2 nM) and diazepam (EC25 114 +/- 12 nM, n = 4). The depressant effects of tizanidine and clonidine were reversed by idazoxan (1 microM). Flumazenil (I microM) failed to reverse the depressant effect of tizanidine. The depressant effect of diazepam was reversed by flumazenil (1 microM) but not by idazoxan (1 microM). Naloxone 1 M did not reverse the effects of either tizanidine or diazepam.6. In the presence of tetrodotoxin (0.1 SAM) which abolished synaptic activity, clonidine, tizanidine or diazepam (10, 100 and 101JM respectively) produced no significant antagonism of NMDA-induced depolarizations recorded from ventral roots.7. Control (100%) synaptic responses of rat superior cervical ganglion preparations were depressed respectively to near maximal levels of 60.0 +/- 5.2% (n = 4) and 60.7 +/- 5.6% (n = 5) by clonidine (0.5 JAM,EC25 15.3 +/- 3.0 nM) and tizanidine (1 JAM, EC25 227 +/- 83 nM). These depressant effects were reversed by idazoxan (1 AM). Baclofen (EC25 28.7 +/- 10.0, n = 3) depressed the postganglionic response to a maximum level of 71.8 + 2.4% (n = 4) control at a concentration of 100 microM. The latter depressant action was reversed by the GABAB receptor antagonist, CGP35348 (1 mM). Diazepam (1 microM) had no significant effect on ganglionic transmission.8. It is concluded that the activation of benzodiazepine or M2-noradrenaline receptors can modulate NMDA receptor-mediated excitatory synaptic pathways whereas synaptic excitation from primary afferent terminals, mediated by non-NMDA receptors, appears to lack the propensity for this type of modulation. The results show also that the isolated spinal preparation can be used to identify central myorelaxant actions that are mediated through functional benzodiazepine or X2-noradrenaline receptors.

Norepinephrine activates potassium conductance in neurons of the turtle cerebral cortex

Whole-cell voltage and current clamp recordings were obtained from cortical neurons of the pond turtle, Pseudemys scripta elegans. Norepinephrine (NE) induced an outward current in 50% of pyramidal neurons. This current had a reversal potential of -88.3 +/- 3.2 mV, consistent with a K+ conductance increase, and had a mean amplitude of 18.3 +/- 7.2 pA at -40 mV. The ionic dependence and pharmacological analyses are both consistent with alpha 2 adrenergic receptor stimulation. Inhibition of Na(+)-dependent action potentials with TTX did not diminish the NE-induced K+ conductance, indicating that NE acts directly on the postsynaptic neuron. In addition to effects on postsynaptic conductance, NE dramatically decreased the amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in 55% of pyramidal neurons. The decrease in spontaneous IPSCs was observed both in those neurons which exhibited an increase in K+ conductance in response to NE administration (81%) and in those which did not (33%). Thus, NE modulates neuronal excitability both directly by activating a postsynaptic K+ conductance and indirectly by decreasing spontaneous IPSCs.

Studies on the functional role of alpha-adrenoceptors in the cardiac sympathetic ganglia of the dog

Inhibitory alpha-adrenoceptors were studied in cardiac ganglia of pentobarbital-anesthetized dogs. Blockade of alpha 1- or alpha 2-adrenoceptors augmented preganglionic nerve stimulation induced tachycardia without altering the response to postganglionic nerve stimulation. The effect produced by blockade of ganglionic alpha 1-adrenoceptors with terazosin had different frequency-response characteristics from, was of smaller magnitude than, and was additive with the effect produced by blockade of ganglionic alpha 2-adrenoceptors with rauwolscine. The response to activation of ganglionic nicotinic cholinergic receptors in the absence of electrical stimulation of the preganglionic nerve was not affected by blockade of either alpha 1- or alpha 2-adrenoceptors. The response to nicotinic cholinergic receptor activation during periods of continuous preganglionic nerve stimulation was augmented following blockade of alpha 2-adrenoceptors but unaffected by alpha 1-adrenoceptor blockade. These results suggest that there are two different inhibitory pathways involving alpha-adrenoceptors in mammalian sympathetic ganglia and provide evidence that these inhibitory pathways are operative under the experimental conditions of ganglionic transmission.

Stimulation by acetylcholine and inhibition by norepinephrine of transglutaminase activity in superior cervical ganglia excised from adult rats

Very rapid changes in activity of transglutaminase (TG), a calcium-dependent enzyme contributing to cross-linkage formation of intracellular polypeptide chains, were observed in vitro in rat superior cervical ganglion (SCG) and nodose ganglion (NG) following application of cholinergic or adrenergic agonists and antagonists. In SCG, a tissue rich in synapses, the depolarizing agent acetylcholine (ACh, 0.1 mM) produced an 8.7-fold increase in TG activity within 5 min that lasted for 30 min and returned to control levels by 2 h. In contrast, the ACh-induced increase in TG activity in NG, a tissue containing neuronal cell bodies with few synapses, was more gradual and of smaller magnitude, reaching a peak of approximately 2.4 times control by 30 min that was maintained for at least 2 h. In both tissues the ACh-stimulation was effectively blocked by the nicotinic antagonist, hexamethonium (0.1 mM), whereas the muscarinic antagonist, atropine (0.1 mM), partially blocked the ACh effect in SCG and was without effect in NG. Addition of the hyperpolarizing adrenergic agonists norepinephrine (NE, 50 microM), isoproterenol (0.2 mM) or dopamine (0.1 mM) produced an inhibition of TG activity in SCG but had no effect in NG. The inhibitory effects of the adrenergic agonists in SCG were blocked by the beta-adrenergic antagonist, propranolol (10 microM) and alpha 2-adrenergic antagonist, yohimbine (10 microM). A kinetic study revealed that the ACh-induced stimulation of TG activity in SCG and NG was a result of decrease in apparent Km and an increase in Vmax value, whereas the NE-induced inhibition of SCG enzyme activity was a result of an increased Km and decreased Vmax. 45Ca2+ influx into excised SCG or NG was significantly reduced by the application of either ACh or NE. The ACh inhibition was effectively blocked by either hexamethonium or atropine. The NE inhibition was more effectively blocked by yohimbine than by propranolol. These results suggest that the rapid alterations of TG activity in SCG produced by cholinergic and adrenergic neurotransmitters are attributable to the processes of receptor-mediated depolarization and hyperpolarization, respectively, via modulation of nerve-impulse-induced Ca2+ fluxes during synaptic activity.

Paraganglionic cell response to chronic imipramine and handling stress: an ultrastructural study

The ultrastructure and connectivity of monoamine-storing paraganglionic cells in the rat superior cervical ganglion were investigated following chronic treatment with imipramine (Tofranil, Ciba-Geigy) and compared with uninjected unhandled controls and saline injected animals. The study reveals a significant decrease in the number of dense core vesicles in the drug-treated group (P less than 0.001) which is regarded as a specific effect due to receptor blocking actions of imipramine. A significant reduction in the maximum diameter of the external rim and internal cores of the vesicles (P less than 0.05) in the drug-treated group is mimicked to a certain extent by saline injections, indicating a mixed effect of stress handling and specific alteration. Although the paraganglionic cell morphology is unaltered in the group comparisons, the interrelationship of the paraganglionic cells to surrounding neural processes is significantly altered in both the control versus saline and the control versus drug group comparisons (P less than 0.05). The drug- and saline-induced alterations of neural connectivity may reflect stress-induced general changes demonstrating the plasticity of the paraganglionic cell population.

Postsynaptic long-term enhancement (LTE) by dopamine may be mediated by Ca2+ and calmodulin

Long-term enhancement (LTE), of postsynaptic slow depolarizing responses to a muscarinic agonist (MCh), follows a brief exposure of the rabbit superior cervical ganglion to another transmitter, dopamine (DA). Either reduction of external Ca2+ (to 1.0 mM or 0.2 mM) or presence of a specific calmodulin antagonist (calmidazolium at 5 microM) blocked DA induction of this LTE. However, unlike LTP in hippocampus, induction of LTE is not mediated by depolarization-dependent influx of Ca2+.

Validation of a novel technique allowing assessment of the functional role of cardiac prejunctional receptors in the rat

1. The electrical stimulation of the thoracic portion of the spinal cord (preganglionic nerve fibres) in the pithed rat is a widely used technique to investigate drug effects on the sympathetic nervous system. However, it does not allow discrimination between ganglionic and neuronal ending sites of action of drugs. This report describes and validates an experimental approach in which postganglionic nerve fibres are stimulated in pithed rats, thereby overcoming the above mentioned difficulty. 2. Electrical stimulation of a nerve below the thymus gland produced frequency-related increases in heart rate which were not modified by the ganglion blockers, hexamethonium or mecamylamine, but were reduced by the beta 1-adrenoceptor antagonist, betaxolol. Thus, the studied nerve fibres can be classified as sympathetic postganglionic neurons. 3. The parameters for electrical stimulation of postganglionic cardioaccelerator nerve fibres giving optimal responses were of similar pulse widths as required for the stimulation of the thoracic spinal cord, but of lower strength of current, higher frequencies and shorter stimulation times to achieve steady-state responses. 4. Clonidine reduced the tachycardia evoked by sustained electrical stimulation of postganglionic cardioaccelerator nerve fibres. This effect was antagonized entirely by the selective alpha 2-adrenoceptor antagonist, idazoxan. Therefore, sympathetic cardiac postganglionic nerve fibres are endowed with alpha 2-adrenoceptors.

Pertussis toxin sensitivity of drug-induced potentials on the rat superior cervical ganglion

We have investigated the action of pertussis toxin on a range of receptor-mediated responses of the rat superior cervical ganglion in vitro. The ganglia were treated with pertussis toxin for 24 h at 37 degrees C using an in vitro method. Appropriate controls were also carried out. Pertussis toxin (1 microgram/ml) reduced ganglionic hyperpolarisations mediated by adenosine, alpha 2, 5-HT1A, M2 and GABAB receptors. The GABAB-mediated hyperpolarisation of this preparation, evoked by baclofen and GABA in a bicuculline-resistant manner, has not previously been reported. Pertussis toxin did not reduce ganglionic depolarisations evoked by potassium chloride and 5-HT3, GABAA and nicotinic receptors. Depolarisations to muscarine and noradrenaline, probably mediated by M1 and beta-receptors, also appeared to be resistant to pertussis toxin. The similar sensitivity of the various ganglionic hyperpolarisations to pertussis toxin indicates that they may all be mediated by similar G-proteins.

5-Hydroxytryptamine evokes three distinct responses on the rat superior cervical ganglion in vitro

In addition to the 5-HT3-mediated fast depolarisation, 5-hydroxytryptamine (5-HT) evoked two additional responses on the rat superior cervical ganglion: a hyperpolarisation and a slow depolarisation. These responses appeared to be direct actions on 5-HT receptors since they were present in a low calcium medium containing tetrodotoxin and were not abolished by a variety of non-serotonin antagonists. The hyperpolarisation was not antagonised by 5-HT3 or 5-HT2 antagonists. The 5-HT1 ligands 5-carboxamidotryptamine (5-CT) and 8-OH-DPAT also evoked a hyperpolarisation. The hyperpolarisation was antagonised by six 5-HT1A antagonists including WB-4101 and spiroxatrine. It was therefore concluded to be mediated by a 5-HT1A receptor. The slow depolarisation was only evoked by 5-HT. The receptor involved in this response, however, could not be determined. We conclude that in addition to 5-HT3 receptors the rat superior cervical ganglion possesses 5-HT1A receptors and another uncharacterised 5-HT receptor.

Inhibition of Ca-spikes in rat preganglionic cervical sympathetic nerves by sympathomimetic amines

1. Propagated Ca-spikes were recorded from isolated cervical sympathetic nerve trunks of the rat when bathed in a solution containing 5 mM Ca2+, 0.5 or 1 microM tetrodotoxin (to block Na currents) and 1 mM 4-aminopyridine (to reduce K currents). 2. Spikes persisted when external Ca2+ was replaced with Sr2+ or Ba2+, but were blocked by the addition of the following inorganic Ca-channel blockers (in descending order of potency): Cd2+ greater than La3+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+. 3. Ca-spike amplitude was reduced by up to 90% by (-)-noradrenaline (IC50 1.5 microM). The following sympathomimetic amines imitated this effect (in descending order of potency): clonidine greater than or equal to (-)-adrenaline greater than or equal to [(-)-noradrenaline] greater than or equal to dopamine greater than (-)-phenylephrine greater than or equal to (+/-)-amidephrine. 4. Ca-spike inhibition by (-)-noradrenaline was antagonized by phentolamine (pA2 6.5). Yohimbine was about 10 times weaker than phentolamine; (+/-)-propranolol (1 microM) and prazosin (10 microM) had no clear effect. 5. (-)-Noradrenaline reduced the amplitude of the compound action potential recorded from the superior cervical sympathetic ganglion following supramaximal preganglionic trunk stimulation when recorded in normal Krebs solution and hyperpolarized the ganglion with respect to the post-ganglionic trunk. Depression of the transmitted ganglionic action potential was antagonized by phentolamine (5 microM) but not by yohimbine (1 microM); in contrast 1 microM yohimbine completely prevented the ganglionic hyperpolarization. (-)-Noradrenaline did not hyperpolarize the preganglionic cervical sympathetic nerve trunk under these recording conditions. 6. It is suggested that inhibition of transmitter release from sympathetic preganglionic fibres produced by noradrenaline results from a depression of the voltage-gated Ca current in the fibres and/or their terminals, and that this action is mediated by an alpha-adrenoceptor which does not fully conform to either alpha 1 or alpha 2 subtypes.

The effects of estradiol and mestranol on alpha-adrenoceptors in select regions of the rat brain

The role of estrogens in modulating the concentration of CNS alpha-adrenoceptors has not been elucidated nor has it been determined how different estrogenic compounds affect these receptors. In this study brain alpha-1 and alpha-2-adrenoceptor binding was measured in female rats treated with estradiol and/or the synthetic estrogen mestranol. Rats treated biweekly for 12 weeks with mestranol (50 micrograms/100 g b.wt.) had a significant reduction in the apparent number of alpha-2-adrenoceptors in the frontal cortex and nucleus tractus solitarius (NTS), while apparent numbers of both alpha-1 and alpha-2-adrenoceptors were depressed in the locus coeruleus. Estradiol treatment (50 micrograms/100 g b.wt.) caused a significant elevation in apparent alpha-1-adrenoceptor numbers in the NTS relative to control. Alpha-adrenoceptor numbers in the rostral and caudal hypothalamus were not affected by either steroid treatment. These results suggest that regulation of apparent numbers of alpha-1 and alpha-2-adrenoceptors in the CNS depends on the type of estrogen used for treatment.

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.

Inhibition of K+ permeability diminishes alpha 2-adrenoceptor mediated effects on norepinephrine release

The effect of two different potassium channel blockers, 4-aminopyridine (4-AP) and quinine, on the alpha 2-adrenoceptor mediated modulation of norepinephrine (NE) release was investigated. Pairs of mouse vasa deferentia were loaded with 3H-norepinephrine (3H-NE), superfused continuously, and stimulated electrically. 4-AP (5.3 x 10(-4) M), and quinine (10(-5) M) enhanced the stimulation-evoked release of tritium significantly. The electrically induced release of radioactivity was reduced by alpha 2-adrenoceptor agonists (1-NE and xylazine) and enhanced by the alpha 2-adrenoceptor antagonist yohimbine. Both effects were affected markedly by 4-AP or quinine: the depressant action of 1-NA and xylazine was partially antagonized and the facilitatory effect of yohimbine was completely abolished during the blockade of the potassium channels. It is suggested that the blockade of the potassium permeability counteracts negative feedback modulation; therefore, it seems likely that the stimulation of alpha 2-adrenoceptors leads to an enhanced potassium permeability and hyperpolarization of varicose axon terminals.

Examination of the role of calcium in the adrenaline-induced hyperpolarization of bullfrog sympathetic neurons

The adrenaline-induced hyperpolarization, which was recorded in neurons of bullfrog paravertebral sympathetic ganglia by means of the sucrose gap technique, was antagonized by 1 mM 4-aminopyridine. The response was unaffected by drugs which influence intracellular Ca2+ movements or Ca2+-sensitive K+ conductances, i.e. 100 or 200 microM Cd2+, 60 microM dantrolene Na+, 10 mM tetraethylammonium bromide, 0.5-2.0 microM apamin or 70 microM (+)-tubocurarine chloride. The spontaneous, rhythmic hyperpolarizations which occur in ganglionic neurons in the presence of 5 mM caffeine and reflect activation of Ca2+-sensitive K+ conductances following mobilization of intracellular Ca2+, were examined by means of intracellular recording. These responses were often biphasic, comprising a transient rapid early phase and a slow late phase. Tetraethylammonium (10 mM) and 0.5-2.0 microM apamin antagonized the rapid early phase and 70 microM (+)-tubocurarine chloride antagonized both phases of the response. Neither phase of these spontaneous, rhythmic, caffeine-induced hyperpolarizations were affected by 1 mM 4-aminopyridine. Although the adrenaline-induced hyperpolarization was antagonized by 50 microM 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate and by 50 microM quinidine, the majority of the results argue against the hypothesis that mobilization of intracellular Ca2+ is required for activation of the K+ conductance thought to underlie the adrenaline-induced hyperpolarization.

Agonist profile of ergometrine (ergonovine) on a population of postsynaptic alpha-adrenoceptors

Ergometrine (0.02-5 microM) produced concentration-related contractions of the mouse anococcygeus muscle, which were unaffected by cocaine (2 microM) or by pretreatment of mice with 6-hydroxydopamine. Contractions were reduced by alpha-adrenoceptor antagonists; the rank order of potency was prazosin greater than phentolamine greater than yohimbine. With phenoxybenzamine as antagonist, the estimated dissociation constant (KD) for ergometrine was 0.41 microM. It is concluded that ergometrine causes direct activation of postsynaptic alpha 1-adrenoceptors, and it is suggested that it acts on the same subtype of the receptor as imidazoline agonists.

The effects of noradrenaline on neurones in the rat dorsal motor nucleus of the vagus, in vitro

1. Intracellular recordings were made from vagal motoneurones identified by antidromic stimulation in the dorsal motor nucleus of the vagus (d.m.v.) in slice preparations of rat medulla oblongata. 2. Noradrenaline (NA) applied by perfusion (0.01 microM to 1 mM) depolarized 55%, hyperpolarized 32% and produced a biphasic response (hyperpolarization followed by depolarization) in 9% of the d.m.v. neurones tested. 3. The NA effects persisted after complete elimination of synaptic inputs during perfusion with Ca2+-free high-Mg2+ solution, and therefore probably resulted from a direct action on the postsynaptic membranes. 4. The NA depolarization was blocked by prazosin and the NA hyperpolarization by yohimbine, but neither was blocked by propranolol or timolol. Phenoxybenzamine blocked both responses. The results indicate that NA depolarization is mediated by alpha 1-adrenoceptors and hyperpolarization by alpha 2-adrenoceptors. 5. The neurones which were depolarized by NA were also hyperpolarized by NA when the alpha 1-adrenoceptors were blocked by prazosin (all of seven neurones tested). This result suggests that most vagal motoneurones in the d.m.v. have both alpha 1-and alpha 2-adrenoceptors. 6. The NA depolarization was accompanied by a decrease in membrane conductance and the hyperpolarization by an increase in membrane conductance, both of which were measured under manual-clamp conditions. 7. The reversal potentials for the NA responses were around -85 mV in normal Ringer solution, and shifted as predicted by the Nernst equation when the extracellular K+ concentration was changed. 8. The inhibitory postsynaptic potentials evoked by focal electrical stimulation on the slice surface of the commissural part of the nucleus of the tractus solitarius (n.t.s.), which contains an A2 catecholaminergic cell group, were abolished by yohimbine. 9. The results suggest that NA modulates vagal output by decreasing or increasing the K+ conductance of d.m.v. neurones through alpha 1- or alpha 2-adrenoceptors. In addition, the A2 noradrenergic cell group within the n.t.s. may send inhibitory inputs to the d.m.v.

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.

Origin of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion and vagus nerve

1 5-Hydroxytryptamine (5-HT)-induced membrane potential changes were recorded extracellularly from rat superior cervical ganglia (SCG) and cervical vagus nerves in vitro. 2 On the SCG, low concentrations of 5-HT (1 X 10(-8)-3 X 10(-7) M) induced concentration-related hyperpolarization responses. Higher concentrations of 5-HT (1 X 10(-6) 1 X 10(-4) M) induced complex responses which typically consisted of an initial hyperpolarization, followed by a depolarization and subsequent after-hyperpolarization. The depolarization, but not the initial hyperpolarization, was blocked by metoclopramide (3 X 10(-5) M), quipazine (1 X 10(-6) M) or MDL 72222 (1 X 10(-5) M). 3 5-HT-induced hyperpolarization of the SCG was potentiated when the amount of calcium chloride added to the superfusion medium was reduced from 2.5 to 0.15 mmol l-1. Hyperpolarization responses recorded from SCG preparations superfused with this low-calcium medium were unaffected by the substitution of lithium chloride for sodium chloride and were potentiated by the omission of potassium ions. Ouabain (1 X 10(-3) M) abolished both the hyperpolarization and the depolarization induced by 5-HT. 4 On the vagus nerve, 5-HT (1 X 10(-7) - 3 X 10(-5)M) did not induce initial hyperpolarization in either normal or low-calcium Krebs-Henseleit medium. However, in the latter solution only, depolarization responses induced by 5-HT at concentrations of 1 X 10(-6)M or greater were followed by hyperpolarization. Both the depolarization and the post-5-HT hyperpolarization were blocked by metoclopramide (3 X 10(-5)M) but were unaffected by spiperone (1 X 10(-7)M). 5 On the vagus nerve, post-5-HT hyperpolarization responses were selectively and reversibly inhibited by ouabain, and by superfusion with Krebs-Henseleit medium that was either potassium-free or contained lithium chloride in place of sodium chloride. 7 These results demonstrate the generation in the rat SCG of a 5-HT-induced hyperpolarization response that is not mediated through 5-HT3 receptors and is unlikely to be a consequence of depolarization. In contrast, on the rat vagus nerve, the post-5-HT hyperpolarization observed in the present study had the characteristics expected of depolarization-dependent activation of a sodium ion pump.

Pharmacological characterization of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion

1 A study has been made of the pharmacology of 5-hydroxytryptamine (5-HT)-induced hyperpolarization responses recorded extracellularly from the rat isolated superior cervical ganglion (SCG). 2 Hyperpolarization responses induced by 5-HT (1 X 10(-8)-1 X 10(-4) M) in the presence of MDL 72222 (1 X 10(-5) M) were not antagonized by phentolamine (1 X 10(-6) M), prazosin (1 X 10(-7)-3 X 10(-7) M), haloperidol (1 X 10(-6) M) or ketanserin (1 X 10(-7)-1 X 10(-6) M). However, the latter two compounds both potentiated and increased the persistence of the hyperpolarization induced by moderate to high concentrations of 5-HT. Spiperone (1 X 10(-7) M) caused similar effects. All further experiments were performed in the presence of ketanserin (1 X 10(-6) M) as well as MDL 72222. 3 8-Hydroxy-2(di-n-propylamino)-tetralin (8-OH-DPAT; 1 X 10(-7)-1 X 10(-4) M) and ipsapirone (3 X 10(-5)-3 X 10(-4) M) behaved as weak hyperpolarizing agonists on the SCG. However, at concentrations below those required to produce hyperpolarization, both compounds acted as unsurmountable antagonists of 5-HT-induced hyperpolarization. 4 5-Carboxamidotryptamine (5-CT; 1 X 10(-9)-1 X 10(-5) M) mimicked the hyperpolarizing activity of 5-HT on the SCG. The EC50 for 5-CT was approximately 9 fold lower than that for 5-HT. 5 Spiperone (1 X 10(-7) - 1 X 10(-5) M) behaved as a reversible competitive antagonist of hyperpolarization responses induced by 5-HT with a pKB value of 7.40 +/- 0.09. Spiperone (1 X 10(-7)-1 X 10(-6) M) also caused concentration-dependent rightward displacement of the 5-CT concentration-hyperpolarization response curve. In this case, the pKB was 7.80 +/- 0.05. 6 (+/-)-Cyanopindolol (3 X 10(-7)-3 X 10(-6) M) caused non-parallel rightward displacements of the 5-HT concentration-response curve. Against 5-CT, (+/-)-cyanopindolol (3 X 10(-7)-3 X 10(-6) M) caused a concentration-independent rightward displacement of the concentration-response curve, accompanied by a large increase in the maximum response. 5-CT-induced hyperpolarization recorded in the presence of (+/-)-cyanopindolol (3 X 10(-7) M) was not significantly antagonized by methiothepin (1 X 10(-6) M) or methysergide (1 X 10(-6) M). 7. It is concluded that 5-HT-induced hyperpolarization of the rat SCG is mediated via a 5-HT1-like receptor which resembles the 5-HT1A binding site. However, a lack of selective drugs precludes more definitive characterization of this receptor.

Do presynaptic opiate receptors and alpha-adrenoceptors alter acetylcholine release from a sympathetic ganglion by a similar mechanism?

The present experiments tested the possible involvement of a calcium-sensitive mechanism in the alpha-adrenoceptor- and opiate receptor-mediated inhibition of acetylcholine release from the cat superior cervical ganglion. First, the calcium-dependence of evoked acetylcholine release was measured in the presence and absence of the alpha-adrenoceptor agonist noradrenaline or of the opiate receptor agonist [Met5]enkephalin-Arg6-Phe7. When ganglia were perfused with Krebs medium containing [Ca2+] = 2.4, 1.2, 0.6, 0.2 mM, evoked release of acetylcholine was depressed by both agonists and the inhibition increased with reduced levels of extracellular Ca2+; this was especially evident when calcium in the medium was reduced to 0.2 mM. Second, the effects of both noradrenaline and [Met5]enkephalin-Arg6-Phe7 on calcium influx into presynaptic nerve endings was determined by measuring the accumulation of 45Ca into ganglia in the presence and absence of either drug. Both agonists reduced the stimulation-induced increase in 45Ca accumulation. The effect of noradrenaline to reduce calcium influx was blocked by yohimbine or by phentolamine; the effect of [Met5]enkephalin-Arg6-Phe7 to decrease 45Ca accumulation by ganglia was blocked by naloxone. It is concluded that activation of presynaptic opiate receptors and alpha-adrenoceptors in the cat superior cervical ganglion can alter acetylcholine release by a similar mechanism, i.e. to reduce Ca2+ influx during preganglionic nerve stimulation.

Evidence that dopamine regulates norepinephrine synthesis in the rat superior cervical ganglion during hypoxic stress

Electrical stimulation of preganglionic nerves is known to increase norepinephrine synthesis in the rat superior cervical ganglion in vitro, an effect which appears to be partially regulated by a non-cholinergic transmitter. In the present study, we sought to determine whether sympathetic stimulation also increases norepinephrine synthesis in the rat ganglion in vivo, and whether dopamine released from ganglionic interneurons might regulate this response. To tackle these questions, rats were pretreated with spiroperidol, a selective dopamine-receptor blocker, and then were sympathetically stimulated by exposure to severe hypoxic stress. Other rats were pretreated with vehicle alone before the hypoxic exposure. Norepinephrine synthesis in ganglia was assessed by measuring endogenous tyrosine hydroxylase activity and norepinephrine turnover. We found that hypoxic stress increased both of these indices of norepinephrine synthesis, but only in rats pretreated with spiroperidol. No such response was detected in rats pretreated with vehicle. These results indicate that sympathetic stimulation increases norepinephrine synthesis in the rat superior cervical ganglion in vivo, and that dopamine released from interneurons might regulate this response.

Quantitative evaluation of alpha- and beta-adrenoceptor modulation of [3H]choline release in guinea pig superior cervical ganglia

[3H] Overflow evoked by 5 min supramaximal preganglionic stimulation at 1 pps has been studied in isolated guinea pig superior cervical ganglion preparations preincubated with [3H]choline. At 15 microM norepinephrine (NE) reduced both the [3H]choline overflow and endogenous acetylcholine release by 59.4 and 54.1% respectively; the dose-response curve for NE inhibitory action is described. Evidence is given that endogenous catecholamines effectively reduce ACh release from the ganglia. After blocking the inhibitory action of endogenous NE, a significant beta-adrenoceptor-mediated facilitatory effect on ACh release could be observed. Preincubation of the ganglia with different combinations of alpha 1 and alpha 2 agonists (phenylephrine, 10 microM and clonidine, 1 microM respectively) and antagonists (prazosin, 10 microM and yohimbine, 3 microM) showed that the adrenoceptors involved in alpha-mediated NE inhibition of ACh output are exclusively of the alpha 2-type.