Biomedical subjects

G Hertting

Publications and source records attributed to G Hertting.

At least 73 records · Page 4Linked to original sources

PubMed1988

Multiple pertussis toxin substrates as candidates for regulatory G proteins of adenylate cyclase coupled to the somatostatin receptor in primary rat astrocytes.

The involvement of G proteins in receptor mediated astroglial cAMP formation was studied. Isoproterenol or prostaglandin E2 stimulated adenylate cyclase of primary astroglial cells was inhibited by somatostatin. Preincubation of cells with increasing concentrations of islet activating protein (IAP) diminished somatostatin inhibition of adenylate cyclase. At an IAP concentration of 50 ng/ml somatostatin inhibition was completely abolished. Studies on IAP catalyzed 32P-ADP-ribosylation of astroglial cell particulate material revealed an incorporation of radiolabel into three polypeptides in the molecular weight range of 41,000-39,000 Dalton. Pretreatment of intact cells with IAP reduced radiolabeling of this molecular species in a concentration dependent manner. No further radiolabeling above background level was detectable after pretreatment of cultures with 10 ng IAP/ml or more. At present, the occurrence of at least three IAP substrates (G proteins) does not permit an identification of the somatostatin receptor coupled G protein. Rather, the finding reveals that astrocytes are endowed with multiple variants of GTP binding proteins likely to be coupled to different receptors.

Adenylate Cyclase Toxin↗

PubMed1988

Characterization of seizure-induced cysteinyl-leukotriene formation in brain tissue of convulsion-prone gerbils.

Tonic-clonic seizures elicited in convulsion-prone gerbils resulted in a large increase in immunoreactive prostaglandin (PG) F2 alpha and in a smaller increase in immunoreactive leukotriene (LT) C4-like material in brain tissue. Brain tissue contents of both eicosanoids were found to reach a maximum at 6 min after the onset of seizures and were still elevated at 54 min after the beginning of convulsions. By reversed phase HPLC the immunoreactive LTC4-like material was identified as LTC4 and LTD4 at 6 min after the onset of convulsions, whereas at 54 min after the onset, transformation of LTD4 to LTE4 could be detected as well. In gerbils showing only weak seizure activity a small increase in PGF2 alpha but no increase in immunoreactive LTC4-like material could be detected at 6 min after the onset of convulsions. Pretreatment with indomethacin abolished the formation of PGF2 alpha but significantly enhanced the biosynthesis of immunoreactive LTC4-like material at 18 min after the beginning of seizures. The results demonstrate formation of cysteinyl-LT following tonic-clonic convulsions in spontaneously convulsing gerbils which could be enhanced by inhibition of the cyclooxygenase pathway of arachidonic acid metabolism. Since cysteinyl-LT have potent biological actions in various organs this finding warrants further investigations on the potential role of cysteinyl-LT in the CNS.

Animals↗

PubMed1988

Protein kinase C and presynaptic modulation of acetylcholine release in rabbit hippocampus.

1. The involvement of protein kinase C in the presynaptic modulation of stimulated acetylcholine release was investigated in rabbit hippocampus. 2. Slices of the rabbit hippocampus, labelled with [3H]-acetylcholine, were superfused with medium and stimulated electrically during superfusion. 3. The protein kinase C activating phorbol ester 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) enhanced the electrically evoked tritium overflow in a concentration-dependent manner. Its biologically inactive 4 alpha-isomer was without any effect on transmitter release. 4. The protein kinase C inhibitor polymyxin B decreased the stimulation-evoked tritium overflow and counteracted the enhancement of release caused by 4 beta-PDB. 5. The stimulation-evoked tritium overflow was facilitated when the muscarine receptor antagonist atropine was present. The effects of both atropine and 4 beta-PDB, given in combination, were additive. 6. The net inhibition of the evoked tritium overflow caused by the muscarine receptor agonists carbachol and oxotremorine was similar, irrespective of whether 4 beta-PDB was present or not. 7. Similar results to those for muscarine autoreceptor-mediated inhibition, were obtained for inhibition of the stimulated tritium overflow caused by the adenosine receptor agonist (-)-N6-(R-phenylisopropyl)-adenosine ((-)-PIA) and the opioid receptor agonist ethylketocyclazocine (EKC). The net inhibition of both agonists was independent of the presence of the phorbol ester. 8. The above results provide further evidence for participation of a presynaptically located protein kinase C in the modulation of acetylcholine release. However, the modulatory mechanisms which are coupled to presynaptic receptors and mediate inhibition of release seem not to be directly affected by protein kinase C.

Acetylcholine↗

PubMed1988

An in vitro model of 1-methyl-4-phenyl-pyridinium (MPP+) toxicity: incubation of rabbit caudate nucleus slices with MPP+ followed by biochemical and functional analysis.

1. Slices of rabbit caudate nucleus were preincubated for up to 24 h in vitro in the presence of the neurotoxic compound 1-methyl-4-phenyl-pyridinium (MPP+). Subsequently the levels of endogenous monoamines in the slices were determined by h.p.l.c. with electrochemical detection. MPP+, in concentrations higher than 32 nM significantly diminished the dopamine levels within the slices in a concentration- and time-dependent manner; at 32 microM the depletion was more than 95%. The concentration of the major metabolite of dopamine, dihydroxyphenyl acetic acid (DOPAC) was decreased at concentrations of MPP+ that did not alter dopamine levels. Thus, MPP+ increased the dopamine/DOPAC ratio. 2. In contrast, both 5-hydroxytryptamine (5-HT) levels and 5-HT/5-hydroxyindolacetic acid (5-HIAA) ratios were increased at nanomolar concentrations of MPP+. 5-HT was significantly reduced only at 32 microM. 3. The dopamine uptake inhibitor nomifensine reduced the depletory effect of MPP+ on dopamine and DOPAC content. 4. Following 24 h pretreatment with MPP+, the uptake of [3H]-dopamine into rabbit caudate nucleus slices was either enhanced (at 0.32 microM, 1 microM and 3.2 microM MPP+) or reduced (at 32 microM MPP+). 5. Preincubation of slices with 10 microM MPP+ for only 1 h increased their 3H-labelling (in contrast to 24 h pretreatment) whereas after 9 h no net increase was detectable. After 1 and 9 h MPP+ pretreatment, much less deaminated metabolites of [3H]-dopamine were found in the incubation medium of MPP+ treated slices than in the medium of control slices. These findings suggest that MPP+ strongly inhibits the enzyme monoamine oxidase (MAO) within dopaminergic (and 5-hydroxytryptaminergic) terminals before destroying them. 6. To validate the proposed in vitro model functionally, the electrically evoked release of [3H]-acetylcholine ([3H]-ACh) was investigated in MPP+ treated slices and controls. MPP+ reduced both the facilitatory effect of the D2-receptor antagonist domperidone and the inhibitory effect of the catecholamine uptake inhibitor nomifensine on [3H]-ACh release; effects compatible with a diminished inhibitory dopaminergic input on cholinergic neurones. 7. These findings also show that the terminal region of dopaminergic neurones, the caudate nucleus, is a site for MPP+ toxicity. The present in vitro model may be useful for investigating the effects of MPP+ and its interaction with other drugs under defined conditions.

1-Methyl-4-phenylpyridinium↗

PubMed1988

The stable prostacyclin-analogue, iloprost, unlike prostanoids and leukotrienes, potently stimulates cyclic adenosine monophosphate synthesis of primary astroglial cell cultures.

The effect of different eicosanoids on adenosine-3', 5'-cyclic-monophosphate (cAMP) accumulation in primary astroglial cell cultures prepared from newborn rat brain was studied. The stable prostacyclin-analogue, iloprost, effectively stimulated cAMP synthesis in a concentration-dependent, saturable manner, the EC50 being about 3 x 10(-8) M. Prostaglandin (PG) E2 was less potent, without reaching plateau even at 10(-5) M. Prostaglandins D2 and F2 alpha, and the stable thromboxane A2-analogue, U 46619, as well as leukotrienes (LT) B4, C4, D4 and E4 were not effective and did not attenuate basal or isoprenaline (10(-8) M)-stimulated astroglial cAMP formation. This is the first indication for the existence of a prostacyclin receptor coupled positively to the adenylate cyclase in astrocytes. Other eicosanoids are unlikely to be involved in receptor-mediated regulation of astroglial cAMP levels.

Animals↗

PubMed1987

Possible involvement of protein kinase C (PKC) in the regulation of electrically evoked serotonin (5-HT) release from rabbit hippocampal slices.

Protein kinase C (PKC)-activating phorbol esters enhanced the electrically evoked 5-HT release from rabbit hippocampal slices preincubated with [3H]5-HT. The release was diminished by polymyxin B, an inhibitor of PKC. These results are compatible with a stimulatory effect of PKC on the 5-HT release induced by action potentials. The mutual effects of PKC affecting drugs on 5-HT release suggest a functional but not a competitive interaction. The attenuation or the enhancement of effects of 5-HT autoreceptor ligands at various 5-HT biophase concentrations found after PKC-affecting drugs are in line with the view that autoreceptor-mediated events are not directly influenced by the enzyme PKC.

Action Potentials↗

PubMed1987

Increased prostaglandin formation in rat brain following systemic application of kainic acid.

The arachidonic acid (AA) metabolites prostaglandin D2 (PGD2), prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha) and thromboxane B2 (TXB2) were measured in the dorsal hippocampus, amygdala/pyriform cortex and parietal cortex of the rat brain following the application of kainic acid (KA, s.c. 10 mg/kg). The first significant increases in the prostanoids were seen 10 min following the KA injection, at this time the first behavioral change 'staring' was observed. In the hippocampus the highest concentrations of the PGs were reached 30 min after the injection of the neurotoxin. At this time frequent wet dog shakes (WDS) and rare focal convulsions effecting head and extremities occurred. In the amygdala/pyriform cortex and parietal cortex the maximal prostanoid formation was seen after 120 min. At this time tonic clonic seizures were registered. In contrast to other seizure models, where PGD2 was the predominant prostanoid formed, PGF2 alpha was the major AA-metabolite found in KA-treated animals. From the time-course of the seizure-related behavior and the increased prostanoid synthesis we conclude that the initiation of the prostanoid synthesis was triggered by the increased neuronal activity rather than by cell damage.

Amygdala↗

PubMed1987

Are cerebral prostanoids of astroglial origin? Studies on the prostanoid forming system in developing rat brain and primary cultures of rat astrocytes.

Prostanoid forming capacity in vitro and convulsion-induced prostanoid formation in vivo were studied in the developing rat brain. For comparison, prostanoid synthesis in homogenates of primary astrocyte cultures of different ages was also examined. There was no significant prostanoid production in homogenates from primary astrocyte cultures prepared one week after cultivation. Two-week-old astrocyte cultures possessed a prostanoid synthesizing system of high specific activity. The relative proportions of the products were similar to those obtained in brain homogenates of adult rats, prostaglandin D2 (PGD2) being the major product. Prostanoid forming capacity of brain homogenates was low at birth, increased during development and nearly reached adult values by day 21. Generalized convulsions could be evoked by pentylenetetrazol (PTZ) irrespective of age, but convulsion-induced prostanoid formation characteristic of adult rodents did not take place before the third week of postnatal life. The close similarities between the characteristic features of prostanoid synthesis in both brain and astroglial homogenates, together with the coincidence during brain development of the expression of cerebral prostanoid synthesis with the appearance of mature astrocytes suggest that astrocytes are an important source of brain prostanoids.

Animals↗

PubMed1987

Inhibition of central neurotransmitter release by omega-conotoxin GVIA, a peptide modulator of the N-type voltage-sensitive calcium channel.

Tritium overflows, evoked by electrical stimulation (2 min; 2 ms, 3 Hz, 5 V/cm, and 24 mA) of [3H]-dopamine-, [3H]-noradrenaline-, [3H]-5-hydroxytryptamine, and [3H]-acetylcholine-labeled slices prepared from discrete regions of the rabbit central nervous system, were inhibited 39-50% by omega-conotoxin GVIA (omega-CT; 5 nmol/l), a peptide modulator of the N-type voltage-sensitive calcium channel (N-VSCC). Additional experiments using omega-CT (5 nmol/l) and [3H]-noradrenaline-labeled hippocampal slices indicated the time dependence of omega-CT-induced inhibition, the competitive antagonism between buffer calcium concentration and omega-CT, and the lack of effect of prolonged electrical stimulation (15 min; 0.4 Hz) on omega-CT-induced inhibition. These various results suggest that 1) omega-CT competes with calcium for the N-VSCC, 2) the inhibitory effects of omega-CT are independent of the gating state of the N-VSCC, and 3) the molecular nature of the N-VSCC may be similar or identical across central neurotransmitter systems. omega-CT appears to be a useful pharmacological tool in studying the involvement of the N-VSCC in neurotransmitter release.

Acetylcholine↗

PubMed1987

Formation of sulphidopeptide-leukotrienes in brain tissue of spontaneously convulsing gerbils.

Five minutes after the onset of seizures high amounts of immunoreactive prostaglandin (PG) F2 alpha and smaller amounts of sulphidopeptide (SP)-leukotriene (LT)-like immunoreactivity could be detected in gerbil brain tissue. Bilateral carotid artery ligation followed by 15 min of reperfusion even more enhanced brain tissue contents of PGF2 alpha and SP-LT-like material. Analysis of the immunoreactive SP-LT-like material by reversed phase high pressure liquid chromatography (h.p.l.c.) revealed immunoreactivity co-eluting with authentic LTC4 and LTD4.

Animals↗

PubMed1987

The serotonin (5-HT) autoreceptor in the hippocampus of the rabbit: role of 5-HT biophase concentration.

Slices of hippocampus from the rabbit were preincubated with [3H]5-HT), then superfused continuously and twice stimulated electrically. The stimulation-evoked overflow of tritium was inhibited by the 5-HT autoreceptor ligands 5-carboxamido-tryptamine (5-COHT), 5-HT, 5-methoxy-N,N-dimethyl-tryptamine (5-MeOMT), (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), methysergide and (+/-)-cyanopindolol in a concentration-dependent manner. These effects were competitively inhibited by the 5-HT autoreceptor antagonists, metitepin and metergoline. (+/-)-Cyanopindolol also reduced the evoked release of 5-HT from slices of cortex from the rat. The inhibitor of the uptake of 5-HT, 6-nitroquipazine diminished the autoreceptor-mediated depression of release of 5-HT. In cortex tissue from the rat, 6-nitroquipazine reversed the decreased release of 5-HT, due to (+/-)-cyanopindolol, to a facilitation. The disinhibition of the release of 5-HT by autoreceptor antagonists was further enhanced by 6-nitroquipazine. Non-linear regression analysis of concentration-response curves for 5-COHT yielded the following pKd of endogenous 5-HT at the autoreceptor: 7.753 +/- 0.116. This value corresponds to the pKd of 5-HT at the 5-HT1B binding site. The 5-HT biophase concentration at the autoreceptor of 10(-8.220 +/- 0.132)M was markedly enhanced by 6-nitroquipazine (10(-6)M) to 10(-7.476 +/- 0.132)M. It is concluded that the 5-HT autoreceptor belongs to the 5-HT1B subtype of receptor; the corresponding 5-HT biophase concentration can be estimated quantitatively; 8-OH-DPAT decreased the evoked release of 5-HT through both 5-HT autoreceptors and alpha 2-heteroreceptors and (+/-)-cyanopindolol acts as partial agonist at the 5-HT autoreceptor.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1987

The adenosine receptor-mediated inhibition of noradrenaline release possibly involves an N-protein and is increased by alpha 2-autoreceptor blockade.

The stimulation-evoked overflow of [3H]-noradrenaline from slices of the rabbit hippocampus is inhibited by alpha 2-autoreceptors as well as by adenosine (A1)-receptors. Slices of rabbit hippocampus were labelled with [3H]-noradrenaline, superfused continuously and stimulated twice electrically (rectangular pulses; 2 ms, 3 Hz, 24 mA, 5 V cm-1). Treatment of hippocampal slices with N-ethylmaleimide (NEM, 30 microM; 30 min), which functionally disturbs certain N-proteins, decreased the inhibitory action of adenosine receptor agonists like (-)-N6-(R-phenylisopropyl)-adenosine ((-)-PIA) and adenosine on noradrenaline release. Release inhibition caused by (-)-PIA (0.03-1 microM) was antagonized by NEM in a non-competitive manner in the absence and in the presence of the alpha 2-adrenoceptor antagonist yohimbine. The adenosine receptor antagonist 8-phenyltheophylline significantly increased the evoked noradrenaline release by about 15% in control slices by diminishing the inhibitory action of endogenous adenosine. In NEM-treated slices this effect of 8-phenyltheophylline was not seen. In the presence of (-)-PIA (0.1 microM), i.e. under conditions of an increased inhibitory tone, release facilitation by 8-phenyltheophylline was decreased by NEM compared to that in the respective controls. Occupation of the A1-receptor with (-)-PIA prior to and during the NEM treatment did not protect the A1-receptor-coupled signal transduction system from being affected by NEM. In the presence of the alpha 2-adrenoceptor antagonist yohimbine, the inhibitory action of (-)-PIA was strongly increased. The above results suggest the involvement of a regulatory N-protein in the A1-receptor-mediated inhibition of noradrenaline release and an interaction between the alpha 2-autoreceptor and the A1-receptor-coupled signal transduction system, possibly at the level of a N-protein.

Adenosine↗

PubMed1987

Protein kinase C activation and alpha 2-autoreceptor-modulated release of noradrenaline.

1 Effects of phorbol esters on the evoked noradrenaline release were studied in slices of the rabbit hippocampus, labelled with [3H]-noradrenaline, superfused continuously with a medium containing the reuptake inhibitor cocaine and stimulated electrically for 2 min (stimulation parameters: 2 ms, 24 mA, 5 V cm-1, 3 or 0.3 Hz). 2 The electrically-evoked overflow of [3H]-noradrenaline in the slices was increased in a concentration-dependent manner by the protein kinase C (PKC) activators 12-O-tetradecanoylphorbol 13-acetate (TPA) and 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB). Phorbol esters, which do not activate PKC, 4-O-methyl-TPA and 4 alpha-PDB, showed no effect on neurotransmitter release. The effect of 4 beta-PDB was abolished in the presence of tetrodotoxin and in the absence of calcium. The PKC inhibitor polymyxin B inhibited the evoked noradrenaline release. 3 In the presence of 4 beta-PDB the inhibitory effects of the alpha 2-adrenoceptor agonist clonidine or the facilitatory effects of the alpha 2-adrenoceptor antagonist yohimbine seemed to be modified only by changes in the concentration of noradrenaline in the synaptic region. At a stimulation frequency of 3 Hz the inhibitory action of clonidine was reduced whereas the facilitatory effect of the yohimbine was even slightly enhanced by the phorbol ester. At 0.3 Hz and in the presence of 4 beta-PDB the effect of clonidine remained and that of yohimbine was strongly enhanced. 4 Pretreatment of the slices with islet-activating protein or N-ethylmaleimide significantly reduced the enhancement of noradrenaline release caused by 4 beta-PDB. It is possible that a regulatory N-protein is involved in steps following PKC activation. 5 These results suggest that PKC participates in the mechanism of action-potential-induced noradrenaline release from noradrenergic nerve terminals of the rabbit hippocampus and that effects on the autoinhibitory feedback system were not responsible for the 4 beta-PDB-induced increase of neurotransmitter release.

Animals↗

PubMed1987

Effects of urapidil on neurotransmitter release in CNS tissue in vitro.

The effects of the antihypertensive drug urapidil on the electrically evoked release of [3H]-noradrenaline (NA) in rabbit cortex slices, of [3H]-acetylcholine (ACh) and [3H]-5-hydroxytryptamine (5-HT) in rabbit hippocampus slices, and of [3H]-ACh and [3H]-dopamine (DA) in rabbit caudate nucleus slices were investigated. Urapidil significantly increased basal tritium outflow in slices preincubated with [3H]-NA and showed weak alpha 2-adrenoceptor antagonism on the evoked NA-release. The pA2-value of urapidil against the inhibitory effect of the alpha 2-agonist clonidine was about 6.3. In contrast to phentolamine, urapidil exhibited no partial agonistic properties at alpha 2-adrenoceptors in this model when stimulation was performed at low frequency and in the absence of cocaine. Neither the evoked ACh- nor 5-HT-release in hippocampal slices were affected by urapidil in concentrations up to 10 microM, but basal tritium outflow was significantly enhanced from slices preincubated with [3H]-5-HT. In caudate nucleus slices urapidil (greater than 1 microM) significantly facilitated both the evoked ACh- and DA-release, effects which were enhanced by the DA-reuptake inhibitor nomifensine. The effects of the D2-dopamine receptor selective agonist LY 171555 and the D2-receptor antagonist domperidone on the evoked DA-release were reduced by 1 microM urapidil. In addition, urapidil (10 microM) increased basal tritium outflow from slices preincubated with [3H]-DA. It is concluded that the antihypertensive effect of urapidil is not mediated by presynaptic actions affecting NA- or 5-HT-release within the CNS. Whether an increased dopaminergic or cholinergic transmission following the observed enhancement of DA- or ACh-release (due to a weak D2-receptor antagonism) is involved, remains to be further elucidated.

Acetylcholine↗

PubMed1986

Enhancement of noradrenaline release by 12-O-tetradecanoyl phorbol-13-acetate, an activator of protein kinase C.

12-O-Tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C (PKC), enhanced the electrically evoked overflow of [3H]noradrenaline in a concentration-dependent manner in rabbit hippocampal slices. 4-O-Methyl-TPA, which lacks the ability to activate PKC had no effect on the evoked tritium overflow. The enhancement of noradrenaline release by TPA was affected by neither the alpha 2-adrenoceptor antagonist yohimbine nor the alpha 2-adrenoceptor agonist clonidine. It is concluded from these results that PKC is involved in the mechanism of stimulus-secretion coupling in noradrenergic nerve terminals of the rabbit hippocampus.

Animals↗

PubMed1986

Functional compartmentalization of arginine-vasopressin-activated cyclic AMP in anterior pituitary gland: the presence of a compartment activated by prostaglandin E2.

AVP(10(-8)-10(-6)M) increased ACTH as well as PGE2 release from rat anterior pituitary quarters in vitro in a concentration dependent manner. IBMX (0.1 mM), a phosphodiesterase inhibitor, increased the ACTH response to AVP. The cAMP content in pituitary tissue was increased by AVP. Cyclooxygenase inhibition by indomethacin(1.4 X 10(-5) M) or diclofenac (1.8 X 10(-5)M) led to a potentiation of AVP-evoked ACTH secretion and to a decrease in AVP-stimulated cAMP formation. PGE2(10(-6)M) significantly increased pituitary cAMP content and indomethacin did not affect cAMP levels activated by PGE2. PGE2 attenuated the AVP-induced ACTH release. These results indicate that at least two functional compartments of AVP-activated cAMP responses are involved in the AVP-induced ACTH release. One compartment is directly activated by AVP and participates in the propagation of AVP-induced ACTH release. The second compartment is activated by PGE2. The contribution of the second compartment to the regulation of ACTH secretion is not well understood since PGE2 shows an inhibitory effect on AVP-induced ACTH secretion.

Adrenocorticotropic Hormone↗

PubMed1986

Polymyxin B, a selective inhibitor of protein kinase C, diminishes the release of noradrenaline and the enhancement of release caused by phorbol 12,13-butyrate.

Slices of the rabbit hippocampus were labelled with 3H-noradrenaline, superfused continuously with a modified Krebs-Henseleit medium containing the uptake inhibitor cocaine and stimulated electrically (2 ms, 3 Hz, 24 mA, 5 V/cm). Phorbol 12,13-dibutyrate (PDB), a potent activator of protein kinase C (PKC), strongly enhanced the electrically-evoked overflow of tritium. In contrast, polymyxin B, a relatively selective inhibitor of PKC, diminished the evoked tritium overflow in a time- and concentration-dependent manner. The enhancement of the evoked overflow of tritium caused by PDB was strongly reduced in the presence of polymyxin B (100 mumol/l). These results suggest 1. that PKC may be involved in the physiological mechanism of action-potential-induced noradrenaline release from noradrenergic nerve terminals and 2. that the PDB-induced enhancement of noradrenaline release may be due to a direct activation of PKC.

Animals↗

PubMed1986

N-ethylmaleimide (NEM) diminishes alpha 2-adrenoceptor mediated effects on noradrenaline release.

The effect of N-ethylmaleimide (NEM), which has been shown to abolish rather selectively inhibition of adenylate cyclase, on the alpha 2-adrenoceptor-mediated modulation of noradrenaline release was studied. Slices of the rabbit hippocampus were loaded with 3H-noradrenaline, superfused continuously and stimulated twice electrically. NEM (30 mumol/l) applied for 30 min enhanced both basal and stimulation-evoked tritium overflow significantly. Occupation of the receptor by the alpha 2-adrenoceptor agonist clonidine prior to and during NEM treatment did not protect the alpha 2-adrenoceptor-mediated autoinhibitory feedback system from being affected by NEM. Preincubation of the hippocampal slices with NEM was without any influence on 3H-noradrenaline uptake. The inhibitory effect of clonidine on 3H-noradrenaline release was attenuated in a non-competitive manner. In addition, the facilitatory effect of the alpha 2-adrenoceptor antagonist yohimbine on the stimulus-evoked tritium overflow was reduced. The facilitation of the evoked noradrenaline release by yohimbine or yohimbine or yohimbine and NEM converged with increasing concentrations of yohimbine, suggesting that yohimbine and NEM were acting at the same signal-transduction system. These results are compatible with the idea that NEM, by alkylating the Ni-unit of a presynaptically located adenylate cyclase, prevents the alpha 2-adrenoceptor-mediated modulation of noradrenaline release.

Animals↗