Depolarization of rat isolated superior cervical ganglia mediated by beta 2-adrenoceptors

Functional Significance of Angiotensin Receptor Type 2 in the Neuroplasticity of Autonomic Ganglia in (mRen2)27 Transgenic Hypertensive Rats

ABSTRACT

The over-expression of Ren -2 d gene in (mRen2)27 rats leads to development of hypertension mediated by the renin-angiotensin-system axis and exaggerated sympathetic nerve activity. Exogenously applied angiotensin II (AngII) on the superior cervical ganglion evokes ganglionic compound action potentials (gCAP) and ganglionic long-term potentiation (gLTP). We studied the functional role of angiotensin receptors and expression of reactive oxygen species marker, nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) proteins in AngII-induced postganglionic transmission. Bath-applied AngII revealed that the indices of ganglionic transmission, synaptic strength of gCAP, and decay time for gLTP are remarkably prolonged in (mRen2)27 rats and were abolished by an angiotensin receptor blocker (ARB), suggesting postganglionic AngII Type 1 (AT 1 ) receptor localization and mediation. Receptor density for AT 1 was similar in (mRen2)27 and control animals, and quantitative reverse transcription polymerase chain reaction revealed that it is consistent with the mRNA profile. Furthermore, immunocytochemistry analysis showed similar AT 1 receptor distribution and signals. However, assessment of Type 2 (AT 2 ), Ang-(1-7)-MAS and NOX4-specific proteins showed that AT 2 receptor protein expression was 4-fold lower, consistent with a low mRNA profile. MAS receptor expression was 10-fold lower and NOX4 protein was 2-fold lower. Despite similarity in the densities of AT 1 receptor, the low levels of the components of the protective arm of the renin-angiotensin system at the ganglia may contribute to the differential superior cervical ganglion sensitivity to AngII. The lower NOX4 affects reactive oxygen species balance and possibly results in activation of downstream pathways to promote increased sympathetic nerve activity. We speculate that the significant diminution in AT 2, MAS, and NOX4 protein expressions may play an indirect role in the alteration and efficacy of gCAP and gLTP in hypertension.

Spinal β-adrenergic receptors' activation increases the blood glucose level in mice

We examined the role of spinally located β-adrenergic receptors in the regulation of the blood glucose level. The intrathecal (i.t.) injections with dobutamine (β₁-adrenergic receptor agonist) or terbutaline (β₂-adrenergic receptor agonist) caused an elevation of the blood glucose level, whereas metoprolol (β₁-adrenergic receptor antagonist) or butoxamine (β₂-adrenergic receptor antagonist) did not. In addition, i.t. pretreatment with pertussis toxin (PTX) attenuated the hyperglycemic effect induced by dobutamine or terbutaline. Moreover, plasma insulin level was increased by dobutamine but not by terbutaline, and PTX reduced dobutamine-induced up-regulation of the plasma insulin level. Terbutaline significantly increased plasma corticosterone level, and PTX further enhanced terbutaline-induced corticosterone level. Furthermore, intraperitoneal (i.p.) pretreatment with hexamethonium- (a preganglionic blocker) attenuated dobutamine- and terbutaline-induced hyperglycemic effects. Our results suggest that activation of spinal β₁- and β₂-adrenergic receptors produces hyperglycemic effects in a different manner. Spinally located PTX-sensitive G-proteins appear to be involved in hyperglycemic effect induced by terbutaline. Furthermore, dobutamine- or terbutaline-induced hyperglycemia appears to be mediated through the spinal nerves.

Functional characterization of the beta-adrenergic receptor subtypes expressed by CA1 pyramidal cells in the rat hippocampus

Recent studies have demonstrated that activation of the beta-adrenergic receptor (AR) using the selective beta-AR agonist isoproterenol (ISO) facilitates pyramidal cell long-term potentiation in the cornu ammonis 1 (CA1) region of the rat hippocampus. We have previously analyzed beta-AR genomic expression patterns of 17 CA1 pyramidal cells using single cell reverse transcription-polymerase chain reaction, demonstrating that all samples expressed the beta2-AR transcript, with four of the 17 cells additionally expressing mRNA for the beta1-AR subtype. However, it has not been determined which beta-AR subtypes are functionally expressed in CA1 for these same pyramidal neurons. Using cell-attached recordings, we tested the ability of ISO to increase pyramidal cell action potential (AP) frequency in the presence of subtype-selective beta-AR antagonists. ICI-118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol] and butoxamine [alpha-[1-(t-butylamino)ethyl]-2,5-dimethoxybenzyl alcohol) hydrochloride], agents that selectively block the beta2-AR, produced significant parallel rightward shifts in the concentration-response curves for ISO. From these curves, apparent equilibrium dissociation constant (K(b)) values of 0.3 nM for ICI-118,551 and 355 nM for butoxamine were calculated using Schild regression analysis. Conversely, effective concentrations of the selective beta1-AR antagonists CGP 20712A [(+/-)-2-hydroxy-5-[2-([2-hydroxy-3-(4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy)propyl]amino)ethoxy]-benzamide methanesulfonate] and atenolol [4-[2'-hydroxy-3'-(isopropyl-amino)propoxy]phenylacetamide] did not significantly affect the pyramidal cell response to ISO. However, at higher concentrations, atenolol significantly decreased the potency for ISO-mediated AP frequencies. From these curves, an apparent atenolol K(b) value of 3162 nM was calculated. This pharmacological profile for subtype-selective beta-AR antagonists indicates that beta2-AR activation is mediating the increased AP frequency. Knowledge of functional AR expression in CA1 pyramidal neurons will aid future long-term potentiation studies by allowing selective manipulation of specific beta-AR subtypes.

Alterations in Sympathetic Ganglionic Transmission in Response to Angiotensin II in (mRen2)27 Transgenic Rats

Hypertension in (mRen2)27 transgenic rats is partly dependent on activation of the sympathetic nervous system, but the role of ganglionic transmission is unknown. We assessed indices of synaptic plasticity (post-tetanic short-term potentiation [PTP] and long-term potentiation [LTP]) and sympathetic ganglionic transmission without tetany in superior cervical ganglia (SCG) of Hannover Sprague-Dawley rats (HnSD) versus (mRen2)27 rats. There were no differences in decay time constants [PTP=9 minutes; LTP=120 to 150 minutes in both (mRen2)27 and HnSD]. However, angiotensin (Ang) II increased PTP and LTP in SCG isolated from (mRen2)27 rats to a greater extent than HnSD. Candesartan (an AT 1 antagonist) blocked the potentiation in both groups. Without a preceding tetanic pulse, 16-nM Ang II induced similar significant increases in ganglionic transmission of ≈14% in both strains. Assessment of Ang II receptors by 125 I-[Sar 1 Thr 8 ]-Ang II binding showed that the AT 1 -receptor subtype predominates in the ganglia. The density of receptors in the SCG was comparable in (mRen2)27 and HnSD rats, whether measured in tissue from ganglia removed and frozen versus ganglia used in the transmission testing, suggesting that upregulation of receptors in vitro after removal of SCG did not occur. The divergence of effects of Ang II on LTP and PTP [greater in (mRen2)27 than HnSD] and nontetany ganglionic transmission (similar in both strains) may reflect different locations of receptors (pre- versus postsynaptic) or different signaling mechanisms involved in the two responses. We suggest that functional Ang II receptors in SCG mediate physiological actions of Ang II on ganglionic transmission and may play a pivotal role in hypertension.

Beta-adrenergic and muscarinic cholinergic receptor densities in the human sinoatrial node: identification of a high beta 2-adrenergic receptor density

The objective of this study was to measure autonomic receptor densities in the human sinoatrial node and adjacent atrial myocardium to gain further insights into autonomic regulation of sinoatrial node function in the human heart. Sinoatrial nodes (n = 9) were acquired from human donors. Quantitative light microscopic autoradiography of radioligand binding sites in tissue sections was used to compare beta-adrenergic and muscarinic cholinergic receptor densities within specific tissue compartments of the sinoatrial node and adjacent myocardium. Total beta-adrenergic receptors were measured with the nonsubtype selective radioligand [125I]iodocyanopindolol. beta 2-Adrenergic receptors were determined by measuring the amount of radioactivity bound to sections incubated with radioligand in the presence of the highly beta 1-selective antagonist CGP-20712A. Specific autoradiographic grain densities were normalized to myocyte area/unit tissue area. Myocytes in the sinoatrial node occupied 47.7% +/- 0.1% of the total tissue area compared with 92.8% +/- 0.1% in myocardium (P < 0.001). Total specific beta-adrenergic receptor density per unit myocyte area was 3.5 +/- 0.9 times greater in the sinoatrial node than in myocardium (P < 0.001). The relative densities of beta 1-(4.2, P < 0.002), beta 2-(2.6, P < 0.002), and muscarinic (3.3, P < 0.001) receptors were significantly greater in the sinoatrial node than in the atrium. Thus, total beta-adrenergic and muscarinic cholinergic receptor densities are > 3-fold higher in the sinoatrial node than adjacent atrial myocardium, reflecting their specialized roles in regulating cardiac rate and rhythm. The beta 1-subtype is predominant in both regions. The beta 2-subtype, however, is > 2.5-fold more abundant in the sinoatrial node than in atrial myocardium. The relatively high beta 2-receptor density in the human sinoatrial node is consistent with physiologic studies that implicate this receptor in regulating cardiac chronotropism.

Localization, regulation and functions of neurotransmitters and neuromodulators in cervical sympathetic ganglia

Cervical sympathetic ganglia represent a suitable model for studying the establishment and plasticity of neurochemical organization in the nervous system since sympathetic postganglionic neurons: (1) express several neuromediators, i.e., short acting transmitters, neuropeptide modulators and radicals, in different combinations; (2) receive synaptic input from a limited number of morphologically and neurochemically well-defined neuron populations in the central and peripheral nervous systems (anterograde influence on phenotype); (3) can be classified morphologically and neurochemically by the target they innervate (retrograde influence on phenotype); (4) regenerate readily, making it possible to study changes in neuromediator content after axonal lesion and their possible influence on peripheral nerve regeneration; (5) can be maintained in vitro in order to investigate effects of soluble factors as well as of membrane bound molecules on neuromediator expression; and (6) are easily accessible. Acetylcholine and noradrenaline, as well as neuropeptides and the recently discovered radical, nitric oxide, are discussed with respect to their localization and possible functions in the mammalian superior cervical and cervicothoracic (stellate) paravertebral ganglia. Furthermore, mechanisms regulating transmitter synthesis in sympathetic neurons in vivo and in vitro, such as soluble factors, cell contact or electrical activity, are summarized, since modulation of transmitter synthesis, release and metabolism plays a key role in the neuronal response to environmental influences.

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.

Principal neurons as local circuit neurons in the rat superior cervical ganglion: the synaptology of the neuronal processes revealed by intracellular injection of biocytin

To analyze the local circuitry of the sympathetic ganglion, the synaptic relations of the neuronal processes of the principal neurons in the rat superior cervical ganglion were investigated by correlated light and electron microscopy combined with intracellular injection of biocytin. Intracellular iontophoresis of biocytin followed by avidin-biotinylated horseradish peroxidase cytochemistry allowed complete visualization of the neuronal processes of the principal neurons. The stained principal neurons have a single process (axon), which leaves the ganglion, and several intraganglionic processes (dendrites), some of which show specific terminal arborizations. Some terminals of the dendritic collaterals formed pericellular plexuses or intercellular glomerular plexuses. Electron microscopically, the dendrites and their collaterals contain numerous small vesicles. Synaptic membrane specializations were observed between the stained dendritic collaterals and unlabeled neurites. These may be both preganglionic axon terminals and processes of principal neurons. The likely direction of neurotransmission often could not be determined because of the bidirectional synaptic structures. Our findings show that the dendritic collaterals of principal neurons appear to make both post- and presynaptic contacts with both the principal neurons and the preganglionic axons. It is suggested that the principal neurons might participate in local circuits involving not only preganglionic axons but also neighboring principal neurons.

Ganglion-blocking activity of dequalinium in frog and rat sympathetic ganglia in vitro

The effect of dequalinium on ganglionic transmission and responses to exogenous acetylcholine receptor agonists were studied in frog and rat sympathetic ganglia. In frog ganglia, dequalinium reduced ganglionic transmission (measured using gross extracellular recording) with an EC50 of 2 microM. At 1 microM, dequalinium produced non-surmountable antagonism of the ganglion depolarization evoked by nicotinic receptor activation. Dequalinium reduced the amplitude of evoked and spontaneous excitatory postsynaptic potentials recorded intracellularly from frog neurones, but had no effect on the action potential elicited by injection of a depolarizing current pulse. Hyperpolarizing and depolarizing responses to muscarine recorded extracellularly from frog ganglia were antagonized by 3 microM dequalinium. In rat ganglia, synaptic transmission and depolarization by an exogenous nicotinic agonist were only slightly inhibited by dequalinium at a concentration of 100 microM. At 30 microM, dequalinium produced a 17% reduction in the depolarization of this tissue by muscarine (1 microM). These results add to the evidence of the diversity of nicotinic and muscarinic acetylcholine receptors, and indicate that dequalinium may be a valuable tool for the study of these receptors. However its usefulness as a probe for calcium-activated potassium channels may be limited by its actions at acetylcholine receptors.

Effects of noradrenaline on rat paratracheal neurones and localization of an endogenous source of noradrenaline

1. Intracellular recording techniques were used to study the actions of exogenous noradrenaline (NA) on rat paratracheal neurones in situ. The receptor subtypes underlying these actions were investigated by application of selective adrenoceptor antagonists. 2. Application of NA (0.1-10 microM) by superfusion evoked a membrane depolarization in 85% (52 out of 61) of all paratracheal neurones studied. The response consisted of a slow depolarization which was sometimes accompanied by action potential discharge. In 26 out of 31 cells the response was associated with a change in input resistance of the cell membrane. In 22 out of 26 cells there was a 30% increase, whilst in a further 4 cells there was a 15% decrease in input resistance. The amplitude of the NA depolarization was concentration-dependent. 3. The depolarization evoked by NA was reversibly antagonized by prazosin (1 microM) but unaffected by yohimbine (1 microM) or propranolol (1-10 microM). 4. High performance liquid chromatography with electrochemical detection (h.p.l.c.-e.c.d.) was used to assay for NA and dopamine in samples containing mainly paratracheal ganglia and in samples of tracheal smooth muscle with mucosa. NA was present in all samples assayed at a level of 1.6 micrograms NA g-1 and 0.5 microgram NA g-1 wet weight of the two sample types respectively. Dopamine was not detected in any samples of either ganglia or smooth muscle with mucosa. 5. It is concluded that NA-evoked depolarizations of rat paratracheal neurones result from stimulation of alpha 1-adrenoceptors, and that local levels of NA may be sufficiently high to activate these receptors directly.

Actions of noradrenaline on neocortical neurons in vitro

The effects of noradrenaline (NA) and alpha- and beta-adrenergic agonists on neocortical neurons in rat brain slices were studied by intracellular recording. NA added to the perfusion medium decreased the orthodromically evoked excitatory postsynaptic potential (EPSP) and increased the neuronal excitability on current injection. The decrease of the EPSP by NA was mimicked by alpha-adrenoceptor activation. The beta-adrenergic agonist, isoprenaline, increased the neuronal excitability to current injections similarly to NA. In addition, isoprenaline markedly increased the amplitude of a late component of the EPSP (I-EPSP). The increase of the I-EPSP was long-lasting (up to 90 min). Since this late component of the EPSP is blocked by the NMDA (N-methyl-D-aspartate)-antagonist APV (D-2-amino-5-phosphonovalerate), an enhancement of this excitatory synaptic transient via beta-adrenoceptors may be relevant for the facilitatory effect of NA on neuronal plasticity.

Increased beta 2-adrenoceptors in the superior cervical ganglia of genetically hypertensive rats

[125I]Iodocyanopindolol binding sites were characterized by autoradiography in the superior cervical ganglia of Wistar-Kyoto (WKY) rats. A high concentration of (-)-[125I]iodocyanopindolol binding sites, characterized as beta-adrenoceptors by (-)-propranolol displacement, was distributed throughout the ganglia and in the postganglionic (internal carotid) nerve. ICI 118,551, a beta 2-selective antagonist, displaced more than 85% of the binding sites, whereas CGP 20712A, a beta 1-selective antagonist, displaced less than 10% of the binding sites, indicating that the beta-adrenoceptors were primarily of the beta 2-subtype. Emulsion autoradiography demonstrated that at least part of the binding sites were associated with principal ganglion cells. Unilateral deafferentation did not modify the number of binding sites in the superior cervical ganglia of WKY or spontaneously hypertensive rat (SHR). These results suggest that at least part of these receptors may correspond to prejunctional beta 2-adrenoceptors originated in principal ganglion cells. The concentration of beta 2-receptors was increased in the superior cervical ganglia of young and adult SHR when compared to age-matched WKY rats (49% and 39%, respectively). There were no differences in beta 2-adrenoceptor number in the stellate ganglia of young and adult WKY and SHR. These results suggest that beta 2-adrenoceptor stimulation may be selectively enhanced in some peripheral sympathetic ganglia in SHR and this could play a role in the development and maintenance of the increased sympathetic activity in this strain.

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.

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 potentiation at nicotinic synapses in the rat superior cervical ganglion

1. Nicotinic fast excitatory postsynaptic potentials (fast EPSPs) were recorded intracellularly from postganglionic neurones in the isolated rat superior cervical ganglion. 2. An hours-long potentiation of the fast EPSP could be induced by brief tetanic stimulation of the preganglionic nerve (5 Hz for 5 s to 20 Hz for 20 s). While long-term potentiation (LTP) can be detected in every ganglion by extracellular techniques, LTP was induced in only two-thirds of the nicotinic synaptic responses. 3. Muscarinic blockade with atropine did not prevent LTP of the fast EPSP. 4. LTP of the fast EPSP did not correlate with changes in input resistance nor cell potential, as recorded in the soma. 5. The formation of nicotinic LTP appeared to depend upon stimulation of the nerve terminals. Non-synaptic tetanic depolarization of the postganglionic neurone, effected by injecting depolarizing current pulses through the intracellular microelectrode, was not sufficient. LTP could be induced by synaptic tetani in two-thirds of the same neurones. 6. The response to exogenous 1,1-dimethyl-4-phenylpiperazinium (DMPP), a selective nicotinic agonist, was not increased during nicotinic synaptic LTP. This was true whether DMPP was applied by pressure-ejection from an extracellular micropipette during intracellular recording, or by brief superfusion during sucrose-gap recording of postganglionic responses. 7. Responses to exogenous acetylcholine and carbachol were increased during nicotinic LTP when these non-selective cholinergic agonists were applied by pressure-ejection during intracellular recording. However, the potentiation of the fast EPSP was always at least twofold greater than the potentiation of the response to these exogenous agonists. 8. Potentiation of the responses to acetylcholine and carbachol may have been due to long-term enhancement of muscarinic responses. Thus, no postsynaptic basis for nicotinic LTP was uncovered in these studies.

Long-term regulation of synaptic acetylcholine release and nicotinic transmission: the role of cyclic AMP

1. Using the rat superior cervical ganglion in vitro, the relative efficacy of nicotinic synaptic transmission was estimated by recording the postganglionic compound action potential and the amount of endogenous acetylcholine (ACh) released. These two parameters were correlated in individual ganglia by sampling the bathing medium for the assay of ACh while simultaneously recording the postganglionic response. 2. The beta-adrenoceptor agonist isoprenaline potentiated both the evoked release of ACh and the postganglionic response by about 20% during preganglionic stimulation at 0.2 Hz. 3. The adenosine receptor agonist 2-chloroadenosine inhibited ACh release and the postganglionic response by about 35%. 4. Tetanic preganglionic stimulation for a few seconds induced a long-term potentiation of nicotinic responses and of ACh release. Both of these potentiations were dependent upon extracellular Ca2+ during the tetani. 5. Forskolin and analogues of cyclic AMP also caused a long-lasting potentiation of both the evoked release of ACh and the postganglionic response, indicating that cyclic AMP may regulate transmission by a presynaptic mechanism. The specificity of the cyclic AMP analogues was tested using various butyryl- and bromo-purine nucleotides. 6. The effects of forskolin and 8-bromo-cyclic AMP did not appear to be dependent upon extracellular Ca2+. 7. The potentiation caused by forskolin was consistently augmented by three phosphodiesterase inhibitors--AH 21-132, papaverine and SQ 20-006. However, the effect of forskolin was not consistently enhanced by theophylline, nor was it reduced by the adenylate cyclase inhibitor SQ 22-536. 8. The neurogenic long-term potentiation was augmented by two of the phosphodiesterase inhibitors that also augmented the forskolin-induced potentiation--papaverine and SQ 20-006. 9. It was concluded that cyclic AMP can enhance nicotinic transmission, and can do so by increasing the evoked release of ACh. However, it was not possible to prove that cyclic AMP mediates the long-term potentiation induced by tetanic preganglionic stimulation.

Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 microM-1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2-20 mV). Under voltage-clamp conditions, Ad (100 microM) produced fast, slow and mixed types of inward currents (AdIs) with amplitude of 2.9 +/- 1.3 nA. beta-Adrenoceptors may be responsible for the generation of these AdDs. The slow AdI which lasted for 1-5 min was associated with a decreased membrane conductance. The slow AdI decreased at hyperpolarized potential level and eventually nullified at -70 mV. No reversal of the slow AdI polarity was observed in the Ringer solution. Injection of Cs2+ into the ganglion cells produced a marked depression of the amplitude of the slow AdI. The slow AdI was blocked by bath-applied Ba2+ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range -35 to -55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast AdI decreased and reversed its polarity at -36 +/- 8.3 mV. Removal of Cl ion from superfusing solution depressed the fast AdI, suggesting that activation of Cl- conductances may be involved in the generation of the fast AdI. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.

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 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.

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.

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.

Amino acid neurotoxicity: relationship to neuronal depolarization in rat cerebellar slices

It has long been proposed that the excitatory and toxic properties of acidic amino acid receptor agonists are linked. To test this hypothesis, the depolarizing effects of quisqualate, kainate and N-methyl-D-aspartate in adult and immature rat cerebellar slices have been studied in relation to their neurotoxic effects in the same tissues (reported separately). A "grease-gap" method was used to measure the depolarizing responses of Purkinje cells and granule cells in lobule VI to the agonists. The depolarizing potencies of kainate and quisqualate were apparently similar on both cell types and at both ages studied although maximal responses to kainate were always larger. N-Methyl-D-aspartate was a very weak agonist in the adult slices but was much more effective in the immature tissues, apparently on both Purkinje cells and granule cells. Comparison of the depolarizing effects of the agonists with their neurotoxic effects on Purkinje cells and granule cells suggested that: (a) the ability to depolarize is a required condition for an agonist to be neurotoxic, (b) the magnitude of depolarization, rather than depolarizing potency, is the more pertinent determinant of neurotoxic potency and (c) resistance to the neurotoxicity of an agonist is not necessarily associated with resistance to its depolarizing actions. Histological studies indicated that the neurotoxicity of N-methyl-D-aspartate and kainate in immature cerebellar slices could largely not be replicated by veratridine (50 microM) or high extracellular K+ (124 mM) indicating that receptor-mediated ionic fluxes may be needed in addition to those caused by depolarization. Exposure of the slices to anoxia in the absence of glucose partially reproduced the toxicity of the receptor agonists. Application of ouabain for 30 min caused necrosis of all the cells which are vulnerable to the agonists but spared the cells which are not vulnerable. Profound ionic imbalance thus appears to be a sufficient explanation for amino acid neurotoxicity.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro

CA1 pyramidal neurones were studied in rat in vitro hippocampal slices using standard intracellular and single-electrode voltage-clamp recording techniques to examine the actions of noradrenaline (NA). NA had two different effects on the resting membrane potential of pyramidal neurones; either a hyperpolarization accompanied by a decrease in membrane input resistance, or less commonly, a depolarization accompanied by an increase in input resistance. In many cells, both effects, a hyperpolarization followed by a depolarization were observed. The depolarization was mediated by a noradrenergic beta-receptor. The hyperpolarization was more difficult to characterize, but may result from alpha-receptor activation. NA reduced the amplitude and duration of the slow calcium-activated potassium after-hyperpolarization (a.h.p.) that follows depolarization-induced action potentials. This action of NA was mediated by beta 1-noradrenergic receptors. NA, in the presence of tetrodotoxin and tetraethylammonium, reduced the a.h.p. without reducing the size of the calcium action potential which preceded it. This was unlike the action of the calcium channel blocker, cadmium, which reduced the calcium action potential and the a.h.p. in parallel. Furthermore, NA did not reduce the amplitude of calcium or barium currents recorded under voltage clamp after blockade of potassium currents. A functional consequence of this blockade of the calcium-activated a.h.p. was a reduction of the accommodation of action potential discharge such that the excitatory responses of the neurone to depolarizing stimuli, such as glutamate application or current passed through the recording electrode, were enhanced. We conclude that the effects of NA on calcium-activated potassium conductance and on resting membrane potential can interact to increase the signal-to-noise ratio of hippocampal pyramidal neurone responsiveness.

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.

Denervation-induced formation of adrenergic synapses in the superior cervical sympathetic ganglion of the rat and the enhancement of this effect by postganglionic axotomy

A study has been made at the ultrastructural level of the effects of denervation and axotomy on the synapse population of the rat superior cervical ganglion. Superior cervical ganglia were subjected unilaterally to acute (survival, 48 h) or chronic preganglionic denervation (survival, 41-189 days) by cutting the cervical sympathetic trunk; in chronic denervation experiments regeneration of preganglionic nerve fibres into the ganglion was prevented by suturing the proximal (caudal) stump of the trunk into the sternomastoid muscle. In some chronic experiments the preganglionic denervation was combined with simultaneous crush axotomy of the major postganglionic branches of the ganglion, the internal and external carotid nerves (axotomized-denervated ganglia). Control observations were made in contralateral ganglia and in ganglia from normal rats. After excision and before fixation, ganglia were incubated briefly in the presence of 5-hydroxydopamine to label adrenergic vesicles. Chronic denervation caused a statistically significant 12% decrease from control values in the cytoplasmic minor axes of the principal ganglionic neurones; axotomy combined with chronic denervation led to a 6% increase in this dimension, which was not statistically significant. The minor axes of the neuronal nuclei did not differ significantly from control values in either type of experiment. Axotomy combined with denervation led however to a 36% decrease in the incidence of nucleated neuronal profiles per unit area of ganglion. Counts of synapses were made in the various classes of ganglia and their incidence was expressed per nucleated neuronal profile, to permit comparison within and between experiments. Normal and control ganglia showed a high incidence of synapses of preganglionic cholinergic type. Nerve terminal profiles and synapses containing small dense-cored vesicles, as distinct from the efferent synapses of small granule-containing cells, were not found to be present on the principal neurones or their dendrites in these ganglia, despite strong 5-hydroxydopamine labelling of small dense-cored vesicles within cell bodies and dendrites. After acute denervation extremely few residual synapses were found in the ganglion, in areas remote from small granule-containing cells, and these residual synapses were of the cholinergic type. Acute denervation led to the appearance of vacated or isolated postsynaptic densities; such densities were also found, but were fewer in number, in chronically denervated and axotomized-denervated ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenaline hyperpolarization and depolarization in cat vesical parasympathetic neurones

Responses to noradrenaline (NA) applied by superfusion, ionophoresis or pressure pulse were analysed using conventional intracellular recording and voltage-clamp methods in cat vesical parasympathetic ganglia. NA (1 microM) hyperpolarized 60% of the neurones, depolarized 25%, and produced a biphasic potential, which comprised a membrane hyperpolarization followed by a membrane depolarization, in 10%. About 5% of the neurones did not respond to NA. The NA hyperpolarization was blocked by yohimbine (1 microM), an alpha 2-adrenoceptor antagonist, whereas the NA depolarization was blocked by prazosin (0.1-1 microM), an alpha 1-adrenoceptor antagonist. These data indicated that the NA hyperpolarization was mediated through alpha 2-adrenoceptors and the NA depolarization through alpha 1-adrenoceptors. The NA hyperpolarization was accompanied by an increase in conductance, while the NA depolarization was associated with a decrease in conductance measured under manual-clamp conditions. Similar conductance changes were observed under voltage clamp. NA hyperpolarizations became smaller as the membrane was hyperpolarized and reversed polarity beyond -100 mV. NA depolarizations also became smaller at hyperpolarized membrane potentials and reversed polarity around -90 mV. The NA responses were enhanced in low-K media and depressed in high-K Krebs solution. The NA hyperpolarization was blocked by the Ca antagonists, Cd, Mn and Co. Intracellular injection of EGTA caused a slowly developing, progressive block of the NA hyperpolarization. The NA depolarization was not affected by low Ca concentrations, Ca antagonists or intracellular injection of EGTA. In some neurones the NA depolarization was unmasked in solutions containing Ca antagonists and after intracellular EGTA injection. The NA hyperpolarization was depressed by intracellular injection and extracellular superfusion of Cs but not by TEA. Ba (10-100 microM) depressed the NA hyperpolarization by 30%. The NA depolarization persisted in the presence of muscarine (10 microM) and was not blocked by Cs or TEA but was depressed 70% by Ba (10 microM). These data are consistent with the hypotheses that alpha 2-adrenoceptor activation produces a membrane hyperpolarization that is mediated through a Ca-dependent K conductance, and that alpha 1-adrenoceptor activation produces a membrane depolarization through closure of a voltage-insensitive K channel.

Neurophysiology and pharmacology of long-term potentiation in the rat sympathetic ganglion

Brief tetanic stimulation of the preganglionic nerve induced a persistent potentiation of nicotinic synaptic transmission in the rat superior cervical sympathetic ganglion. Quantitative measurements of the post-tetanic increase in synaptic efficacy revealed two distinct time courses. The early, rapidly decaying component, termed post-tetanic potentiation (p.t.p.), had a decay time constant of 2-3 min, as reported elsewhere. The duration of the more persistent component, called long-term potentiation (l.t.p.), was extremely temperature dependent, lasting much longer at 32 degrees C than at 22 degrees C. In half of the experiments performed at 32 degrees C, l.t.p. showed no detectable decay over the course of 1 h or more after a brief tetanic stimulation. Other experiments were conducted at 22 degrees C. The induction of l.t.p. was dependent on the extracellular [Ca2+]. Transient elevation of the extracellular [K+] also produced a long-term enhancement of synaptic efficacy, and this effect was also Ca2+ dependent. The tetani that were effective in inducing l.t.p. (5-20 Hz for 5-20 s) were well within the physiological range of preganglionic activity. The magnitude and time course were related to frequency and duration of stimulation. The occurrence of l.t.p. was restricted to those preganglionic fibres that were tetanically stimulated. This lack of heterosynaptic or generalized effects was demonstrated by splitting the preganglionic nerve into two branches that could be independently tested and conditioned. Physiological activation of muscarinic or nicotinic receptors apparently does not play an essential role in causing ganglionic l.t.p., which is expressed as an enhancement of nicotinic transmission. A muscarinic antagonist (2 microM-atropine) did not block l.t.p. Preganglionic stimulation induced l.t.p. even when a high concentration of a nicotinic antagonist (3 mM-hexamethonium) was present during the tetanic stimulation. Furthermore, bath application of a cholinergic agonist (100-1000 microM-carbachol) could not substitute for tetanic stimulation in provoking l.t.p. Activation of adrenergic receptors also appeared not to play an essential role. Neither a beta-adrenergic antagonist (10 microM-sotolol or 1 microM-propranolol) nor an alpha-adrenergic antagonist (1 microM-phentolamine) had any significant effect on the magnitude or duration of l.t.p. The results indicate that ganglionic l.t.p. is a Ca2+- and temperature-dependent process that can be created independently of the activation of nicotinic, muscarinic or adrenergic receptors.

alpha 2 and alpha 1-Adrenoceptors mediate opposing actions on parasympathetic neurons

We used intracellular recording methods to analyze the membrane responses to norepinephrine in cat vesical parasympathetic ganglia. In parasympathetic neurons, norepinephrine (NE) produces a membrane hyperpolarization, a membrane depolarization often accompanied by cell firing and a biphasic potential, a hyperpolarization followed by a depolarization. We found that the NE hyperpolarization is mediated through alpha 2-adrenoceptors while the NE depolarization is mediated through alpha 1-adrenoceptors. This situation is different than in sympathetic neurons where beta-adrenoceptors mediate a NE depolarization.

Cyclic adenosine 3',5'-monophosphate and beta-effects in rat isolated superior cervical ganglia

Isoprenaline (0.01-1 microM) increased the amount of cyclic adenosine 3',5'-monophosphate (cyclic AMP) in rat isolated superior cervical ganglia by up to 10 times after 10 min application. Cyclic AMP levels returned to control values after 20 min washing. Salbutamol, in concentrations (1-100 microM) that depolarized the ganglion and facilitated submaximal transmission, did not significantly raise ganglionic cyclic AMP levels. The action of isoprenaline was antagonized by butoxamine (apparent KI approximately equal to 0.14 microM) and weakly by practolol (apparent KI approximately equal to 9.1 microM). The effect of 0.1 microM isoprenaline was also inhibited 94% by 100 microM of the adenylate cyclase inhibitor, 9-(tetrahydro-2-furyl)adenine (SQ 22,536). Exogenous dibutyryl cyclic AMP did not replicate the effects of isoprenaline on ganglionic d.c. potentials or submaximal transmission. The phosphodiesterase inhibitors theophylline, isobutylmethylxanthine or 4-(3,4-dibutoxybenzyl)-2-imidazolidinone (Ro 20-1724) did not potentiate these electrical responses to isoprenaline. The adenylate cyclase inhibitor, SQ 22,536, did not inhibit the electrical responses to isoprenaline. It is concluded that available evidence does not support the view that the ganglion depolarization or facilitation of submaximal transmission in rat isolated ganglia produced by isoprenaline are likely to be mediated by cyclic AMP.