Alpha 2-adrenoceptor agonist-mediated inhibition of [3H]noradrenaline release from rat hippocampus is reduced by 4-aminopyridine, but that caused by an adenosine analogue or omega-conotoxin is not

Protein kinase C beta regulates the D₂-like dopamine autoreceptor

The focus of this study was the regulation of the D2-like dopamine autoreceptor (D2 autoreceptor) by protein kinase Cβ, a member of the protein kinase C (PKC) family. Together with the dopamine transporter, the D2 autoreceptor regulates the level of extracellular dopamine and thus dopaminergic signaling. PKC regulates neuronal signaling via several mechanisms, including desensitizing autoreceptors to increase the release of several different neurotransmitters. Here, using both PKCβ(-/-) mice and specific PKCβ inhibitors, we demonstrated that a lack of PKCβ activity enhanced the D2 autoreceptor-stimulated decrease in dopamine release following both chemical and electrical stimulations. Inhibition of PKCβ increased surface localization of D2R in mouse striatal synaptosomes, which could underlie the greater sensitivity to quinpirole following inhibition of PKCβ. PKCβ(-/-) mice displayed greater sensitivity to the quinpirole-induced suppression of locomotor activity, demonstrating that the regulation of the D2 autoreceptor by PKCβ is physiologically significant. Overall, we have found that PKCβ downregulates the D2 autoreceptor, providing an additional layer of regulation for dopaminergic signaling. We propose that in the absence of PKCβ activity, surface D2 autoreceptor localization and thus D2 autoreceptor signaling is increased, leading to less dopamine in the extracellular space and attenuated dopaminergic signaling.

The pharmacology of the neurochemical transmission in the midbrain raphe nuclei of the rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain, loaded with [(3)H]serotonin ([(3)H]5-HT), superfused and the release of [(3)H]5-HT was determined at rest and in response to electrical stimulation. Compartmental analysis of [(3)H]5-HT taken up by raphe tissue indicated various pools where the neurotransmitter release may originate from these stores differed both in size and rate constant. 5-HT release originates not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process establishing synaptic and non-synaptic neurochemical transmission in the serotonergic somatodendritic area. Manipulation of 5-HT transporter function modulates extracellular 5-HT concentrations in the raphe nuclei: of the SSRIs, fluoxetine was found 5-HT releaser, whereas citalopram did not exhibit this effect. Serotonergic projection neurons in the raphe nuclei possess inhibitory 5-HT(1A) and 5-HT(1B/1D) receptors and facilitatory 5-HT(3) receptors, which regulate 5-HT release in an opposing fashion. This observation indicates that somatodendritic 5-HT release in the raphe nuclei is under the control of several 5-HT homoreceptors. 5-HT(7) receptors located on glutamatergic axon terminals indirectly inhibit 5-HT release by reducing glutamatergic facilitation of serotonergic projection neurons. An opposite regulation of glutamatergic axon terminals was also found by involvement of the inhibitory 5-HT(7) and the stimulatory 5-HT(2) receptors as these receptors inhibit and stimulate glutamate release in raphe slice preparation, respectively, Furthermore, postsynaptic 5-HT(1B/1D) heteroreceptors interact with release of GABA in inhibitory fashion in raphe GABAergic interneurons. Serotonergic projection neurons also possess glutamate and GABA heteroreceptors; NMDA and AMPA receptors release 5-HT, whereas both GABAA and GABAB receptors inhibit somatodendritic 5-HT release. Evidence was found for reciprocal interactions between serotonergic and glutamatergic as well as serotonergic and GABAergic innervations in the raphe nuclei. Serotonergic neurons in the raphe nuclei also receive noradrenergic innervation arising from the locus coeruleus and alpha-1 and alpha-2 adrenoceptors inhibited [(3)H]5-HT release in our experimental conditions. The close relation between 5-HT transporter and release-mediating 5-HT autoreceptors was also shown by addition of L-deprenyl, a drug possessing inhibition of type B monoamine oxidase and 5-HT reuptake. L-Deprenyl selectively desensitizes 5-HT(1B) but not 5-HT(1A) receptors and these effects are not related to inhibition of 5-HT metabolism but rather to inhibition of 5-HT transporter.

Interaction between brain L-type calcium channels and alpha2-adrenoceptors in the inhibition of sodium appetite

Calcium channels mediate the actions of many drugs. The present work investigated whether diltiazem, an L-type calcium channel blocker, alters the inhibition of sodium appetite induced by noradrenaline and the alpha2-adrenoceptor agonist clonidine. Adult male Holtzman rats (N=4-8) with cannula implanted into the third cerebral ventricle were submitted to sodium depletion (furosemide sc+24-h removal of ambiente sodium). Sodium depleted control animals that received 0.9% NaCl as vehicle injected intracerebroventricularly (i.c.v.) ingested 13.0+/-1.5 ml/120 min of 1.8% NaCl. Intracerebroventricular injection of either noradrenaline (80 nmol) or clonidine (20 nmol) inhibited 1.8% NaCl intake from 70 to 90%. Prior i.c.v. injection of diltiazem (6-48 nmol) inhibited from 50 to 100% the effect of noradrenaline and clonidine in a dose-response manner. Diltiazem alone at 100 nmol inhibited, but at 50 nmol had no effect on, sodium appetite. The results suggest: (1) common ionic mechanisms involving calcium channels for the inhibition that noradrenaline and clonidine exert on sodium appetite and (2) a dual role for the benzothiazepine site of L-type calcium channels in the control of sodium appetite.

Response to prejunctional adenosine receptors is dependent on stimulus frequency

PURPOSE

To characterize the adenosine receptor modulation of norepinephrine (NE) release from sympathetic neurons in the isolated rabbit iris/ciliary body.

METHODS

Iris/ciliary bodies were isolated from New Zealand White rabbits and incubated in the presence of 3H-NE. Norepinephrine release was elicited by field stimulation with varied frequencies from 5 to 30 Hz. The effects of adenosinergic and alpha 2 adrenergic compounds on NE release were determined and compared.

RESULTS

At a stimulation frequency of 5 Hz, the addition of the adenosine A1 agonist CHA did not significantly alter 3H-NE release. However, in the presence of the alpha 2 adrenergic antagonist yohimbine, the addition of CHA produced a dose-related reduction in 3H-NE release. The EC50 for this reduction was 14 nM. At a stimulus frequency of 20 Hz, the addition of CHA alone (10(-6) M) produced a significant reduction in 3H-NE release of 41%. The EC50s for the adenosine A1 agonists CHA- and R-PIA-induced suppression of 3H-NE release at 20 Hz were 32 and 24 nM, respectively. The adenosine A2 agonist CV-1808 did not alter 3H-NE release at stimulation frequencies of 5 or 20 Hz. Pretreatment of tissues with the adenosine A1 antagonist CPT or pertussis toxin reversed the suppression of 3H-NE release induced by CHA. Comparison of the inhibitory responses of CHA to the alpha 2 adrenergic agonist UK-14,304 at stimulus frequencies of 5 to 30 Hz demonstrated that this adenosine A1 agonist was effective only in suppressing NE release at frequencies of 20 Hz or greater. In contrast, the alpha 2 adrenergic agonist UK-14,304 was most effective in reducing NE release at 5 Hz.

CONCLUSIONS

These results provide evidence that adenosine agonists inhibit 3H-NE release in the iris/ciliary body via prejunctional adenosine A1 receptors linked to Gi/o-protein. However, the expression of this response was dependent on the frequency of neuronal stimulation. Hence, prejunctional adenosine A1 receptors may act to selectively limit high-frequency neurotransmission.

3,4-Diaminopyridine masks the inhibition of noradrenaline release from chick sympathetic neurons via presynaptic alpha 2-adrenoceptors: insights into the role of N- and L-type calcium channels

We have investigated the participation of the N-type (omega-conotoxin GVIA-sensitive) and L-type (nifedipine-sensitive) calcium channels in the alpha 2-adrenoceptor mediated autoinhibition of the release of [3H]noradrenaline from chick sympathetic neurons in culture. Blockade of 3,4-diaminopyridine-sensitive potassium channels resulted in tetrodotoxin-sensitive and calcium-dependent increase of the release of [3H]noradrenaline evoked by electrical stimulation. Nifedipine attenuated the evoked release under control conditions by 20%, but in the presence of 3,4-diaminopyridine by 51%, while omega-conotoxin decreased the release under control conditions by 87% and in the presence of 3,4-diaminopyridine by only 43%. The L-type calcium channel activator Bay k 8644 increased the evoked release of the transmitter both in the absence and in the presence of 3,4-diaminopyridine. Under control conditions, the alpha 2-adrenoceptor agonist UK 14304 decreased the evoked release by 57% and the alpha 2-adrenoceptor antagonist rauwolscine increased it by 14%. Nifedipine did not prevent this modulation. In the presence of 3,4-diaminopyridine, UK 14304 lost its effect on the release of noradrenaline, but its inhibitory action was restored when nifedipine, but not omega-conotoxin, was added. Changes in the increase of intracellular calcium concentration ([Ca2+]i) evoked by electrical stimulation, measured in the cell processes by microfluorimetry, paralleled the changes in the release of [3H]noradrenaline. Under control conditions, nifedipine attenuated the rise of intracellular calcium by only 16%, while omega-conotoxin did so by 66%. 3,4-Diaminopyridine enhanced the evoked rise of [Ca2+]i; in its presence the rise of intracellular calcium was about equally reduced by nifedipine and omega-conotoxin (by 46 and 36%, respectively). These effects were additive. UK 14304 diminished the peak concentration of [Ca2+]i elicited by the standard electrical stimulation by 31% and rauwolscine antagonised this effect. UK 14304 did not measurably inhibit the stimulation-evoked rise of intraterminal [Ca2+]i in the presence of 3,4-diaminopyridine but it produced an inhibition by 26% if nifedipine had been applied together with 3,4-diaminopyridine. Our observations show that, under control conditions, the stimulated release of [3H]noradrenaline is mainly associated with the opening of N-type channels, while in the presence of 3,4-diaminopyridine the contribution of L-type channels becomes more important. The alpha 2-adrenoceptor stimulation by UK 14304 inhibits the release of [3H]noradrenaline but, in the presence of 3,4-diaminopyridine, the inhibition of release can only be observed if the massive influx through L-type calcium channels is prevented. These data suggest that presynaptic alpha 2-adrenoceptors of chick sympathetic neurons preferentially influence the N-type calcium channels.

The role of various calcium and potassium channels in the regulation of somatodendritic serotonin release

We prepared slices from midbrain containing the raphe nuclei and from hippocampus of rats. The brain slices were loaded with [3H]serotonin and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. No difference was observed in the resting and stimulated fractional release of tritium in the somatodendritic and axon terminal parts of serotonergic neurons. The selective 5-HT1A receptor agonist 8-OH-DPAT decreased the electrically induced tritium efflux from raphe nuclei slices preloaded with [3H]serotonin, and this inhibition was reversed by the 5-HT1A receptor antagonist (+)WAY-100135. The 5-HT1B receptor agonist CGS-12066B but not 8-OH-DPAT, inhibited the stimulation-evoked tritium efflux from hippocampal slices after labeling with [3H]serotonin. The electrical stimulation-evoked tritium efflux in raphe nuclei slices incubated with [3H]serotonin was completely external Ca(2+)-dependent, and omega-conotoxin GVIA and Cd2+, but not diltiazem, inhibited the tritium overflow. In raphe nuclei slices 4-aminopyridine enhanced the electrical stimulation-induced tritium release in a concentration-dependent manner. The inhibition of tritium efflux by 8-OH-DPAT was abolished with 4-aminopyridine. Glibenclamide or tolbutamide proved to be ineffective. These data indicate that (1) different 5-HT receptor subtypes (5-HT1A and 5-HT1B) regulate dendritic and axon terminal 5-HT release; (2) serotonin release from the dendrites may be regulated by the voltage-sensitive N-type Ca2+ channels; (3) the 5-HT1A receptor-mediated inhibition of serotonin release may be due to opening of voltage-sensitive K+ channels.

4-Aminopyridine-induced increase in basal and stimulation-evoked [3H]-NA release in slices from rat hippocampus: Ca2+ sensitivity and presynaptic control

1. We have examined the mechanisms by which the K(+)-channel blocker 4-aminopyridine (4-AP) can dose-dependently increase both basal [3H]-noradrenaline ([3H]-NA) release and the [3H]-NA release evoked by electrical stimulation, but not the release of [3H]-acetylcholine ([3H]-ACh), from slices of rat hippocampus. 2. Both the electrically evoked and the 4-AP-induced release were blocked by tetrodotoxin (TTX) (3 microM). The Ca(2+)-dependence of the 4-AP-induced release (EC50 0.15 mM) was, however, different from that of the electrically evoked [3H]-NA release (EC50 0.76 mM). 3. The 4-AP-induced release could be inhibited by CdCl2(10 microM) and omega-conotoxin (30 nM), but not by nifedipine (1 microM). 4. Transmitter release evoked by 100 microM 4-AP could be blocked by the alpha 2-adrenoceptor agonist, UK 14,304 (0.1 microM) and by the A1-receptor agonist R-N6-phenylisopropyl adenosine (R-PIA, 1 microM) and increased by the alpha 2-adrenoceptor antagonist, yohimbine (1 microM), both in 0.25 and 1.3 mM Ca(2+)-containing medium. By contrast, the effect of alpha 2-adrenoceptor agonist and antagonists on transmitter release evoked by electrical stimulation was markedly reduced in the presence of 4-AP (100 microM). 5. The results suggest that 4-AP can depolarize some nerve endings in the central nervous system, leading to transmitter release that is dependent on nerve impulses and Ca2+. Furthermore, the fact that alpha 2-receptors and adenosine A1 receptor agonists can influence the release of NA evoked by 4-AP suggests that these drugs may have actions that are independent of blockade of aminopyridine-sensitive K(+)-channels.

Presynaptic inhibitory action of a metabotropic glutamate receptor agonist on excitatory transmission in visual cortical neurons

A family of metabotropic glutamate receptors (mGluRs) has been elucidated by molecular cloning. To study the possible modulatory role of mGluRs in synaptic transmission, we tested the effect of a mGluR agonist, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), on the excitatory post-synaptic currents (EPSCS) recorded from neurons in thin slices of rat visual cortex, by using the whole-cell patch-clamp method. We found that trans-ACPD markedly suppressed the evoked EPSCS without affecting the mean amplitude of spontaneous miniature EPSCS. This effect on the evoked EPSCS was blocked by a potassium channel blocker, 4-aminopyridine (4-AP) in a dose-dependent manner. We suggest that trans-ACPD presynaptically inhibits EPSCS by a mechanism involving the 4-AP-sensitive channels.

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.

N-type calcium channels are involved in the dopamine releasing effect of nicotine

Mouse striatum was incubated with [3H]dopamine ([3H]DA) and superfused with and the tritium efflux induced by nicotine, electrical stimulation, or simultaneous nicotine and electrical stimulation was measured, to characterize the role of different Ca2+ channels in the transmitter release. Nicotine stimulation and electrical stimulation exerted additive effects on tritium efflux. Separation of the released radioactivity on alumina columns indicated that nicotine or electrical stimulation increases the release of [3H]DA and that the outflow of 3H-labeled metabolites was similar with the two different stimulation procedures. Removal of Ca2+ from the superfusate resulted in a marked reduction in the tritium release evoked by nicotine, whereas the electrical stimulation-evoked tritium release was completely dependent on external Ca2+. The L- and N-type calcium channel blockers omega-conotoxin GVIA and Cd2+ inhibited the tritium release from the striatum evoked by either nicotine or electrical stimulation, whereas the L-type and T-type channel blockers diltiazem and Ni2+ did not alter release of [3H]DA. We conclude that N-type voltage-sensitive Ca2+ channels participate in striatal dopamine release, and we speculate that nicotinic receptor-operated ion channels permeable to cations such as Ca2+ and N-type voltage-sensitive calcium channels may simultaneously open up, and they additively increase free intracellular Ca2+ concentration.

Comparison of the effects of four dendrotoxin peptides, 4-aminopyridine and tetraethylammonium on the electrically evoked [3H]noradrenaline release from rat hippocampus

We have examined the effects of four dendrotoxin (DaTX) peptides, alpha-, beta-, gamma- and delta-DaTX, separated from the venom of the green mamba (Dendroaspis angusticeps), on field stimulation-evoked [3H]noradrenaline (NA) release from rat hippocampus and compared their effects with those of two other inhibitors of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP (10-300 microM) and TEA (0.1-5 mM) facilitated the evoked [3H]NA release in a concentration-dependent manner. The evoked [3H]NA release was reduced to about half by alpha 2-adrenoceptor stimulation (UK 14,304; 100 nM) and this reduction was antagonized by 4-AP (10-100 microM), whereas TEA even at 5 mM was a poor inhibitor of alpha 2-effects. alpha-DaTX (10-200 nM) mimicked 4-AP in increasing the electrically evoked [3H]NA release and diminishing the inhibitory effects of UK 14,304 in a concentration-dependent manner. delta-DaTX did not itself alter the electrically evoked [3H]NA release, but at 200 nM, it reduced the effects of alpha 2-receptor stimulation. beta- and gamma-DaTX (up to 200 nM) had no significant effects. 4-AP, 3,4-diaminopyridine (3,4-DAP), TEA and the four dendrotoxins displaced the binding of [3H]p-aminoclonidine ([3H]PAC) from alpha 2-receptors. The IC50 values were 6.6 x 10(-4), 1.42 x 10(-3), 5.6 x 10(-2) for 4-AP, 3,4-DAP and TEA, respectively, and 3.19 x 10(-6) M for alpha-DaTX. Thus, their potency as inhibitors of alpha 2-receptors is apparently too low to account alone for the antagonism by K+ channel inhibitors of alpha 2-effects on NA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenaline receptors participate in the regulation of GABAergic inhibition in area CA1 of the rat hippocampus

1. Standard intracellular recordings from CA1 pyramidal neurones in in vitro hippocampal slices have been used to investigate the effects of excitatory amino acid antagonists and adrenergic agents on evoked synaptic potentials. 2. Ortho- and antidromic stimulation were conducted with remotely placed electrodes in order to minimize the possibility of stimulating the interneurones directly. In addition to the excitatory postsynaptic potential (EPSP), orthodromic stimulation evoked an inhibitory sequence consisting of a fast and slow inhibitory postsynaptic potential (IPSP). The slow-IPSP was blocked by intracellular injection of QX 314. Antidromic stimulation evoked a relatively pure fast-IPSP. 3. In seven neurones the differential effects of glutamatergic receptor blockers on the fast-IPSP were investigated. The N-methyl-D-aspartate (NMDA) receptor blocker, DL-2-amino-5-phosphonovaleric acid (APV) was added after the full effect of the non-NMDA receptor blocker, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) had been achieved. In three neurones, APV had no additional blocking effect, while in the remaining four neurones, both the ortho- and antidromically evoked IPSPs were reduced by 20-50%. This suggests that NMDA receptors participate in the activation of some GABAergic interneurones, which was further confirmed by showing that the IPSP was enhanced by Mg(2+)-free medium. 4. In the presence of CNQX (10 microM) and APV (50 microM) together, the ortho- and antidromically evoked fast-IPSPs were greatly reduced. A small 'residual' IPSP remained which was best studied by depolarizing the neurone to around -50 mV. With maximum stimulation, this amounted to 26.3 +/- 15.4% (mean +/- S.E.M., n = 15) of the control IPSP evoked by orthodromic stimulation and 41 +/- 14.6% of the control IPSP evoked by antidromic stimulation. The following statements apply equally to the ortho- and antidromically activated residual IPSPs. 5. The residual IPSP was completely blocked by low concentrations of bicuculline, indicating that it is mediated by GABAA receptors. When compared with a control IPSP of similar amplitude, the residual IPSP was found to have a faster rise time and time-to-peak, but a similar decay time. 6. Neither the muscarinic cholinergic antagonist, atropine nor the presynaptic glutamate agonist, L-2-amino-4-phosphonobutyric acid (L-APB) had any effect on the residual IPSP. 7. The residual IPSP was completely blocked by the adrenergic beta-receptor antagonist, L-propranolol (50-100 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium channel blockers inhibit D2 dopamine, but not A1 adenosine, receptor-mediated inhibition of striatal dopamine release

D2 dopamine autoreceptors and A1 adenosine heteroreceptors inhibit the evoked release of dopamine from rat striatum. We examined the role of potassium channels in this modulation by determining the effects of two potassium channel blockers, 4-aminopyridine and tetraethylammonium, on the modulation of electrically stimulated release of endogenous dopamine from rat striatal slices. Maximally effective concentrations of the D2 dopamine receptor agonist N-0437 (10 nM) and of adenosine (50 microM) caused a 30% inhibition of evoked dopamine overflow, and their effects were additive. When coperfused with N-0437, both 4-aminopyridine and tetraethylammonium blocked the inhibition caused by N-0437 in a dose-dependent manner. 4-Aminopyridine was approximately three orders of magnitude more potent than tetraethylammonium, with complete blockade occurring at 3 microM and 1 mM, respectively. Binding experiments confirmed that neither 4-aminopyridine nor tetraethylammonium was a direct-acting D2 dopamine receptor antagonist at the concentration necessary to block the release-modulatory effect of D2 receptor activation. In contrast, the inhibitory modulation produced by adenosine was not affected by 4-aminopyridine (30 microM) or tetraethylammonium (1 mM). These results suggest that D2 dopamine and A1 adenosine receptors inhibit dopamine release in the striatum by different mechanisms. D2 dopamine autoreceptor action appears to involve potassium channels, whereas A1 adenosine receptor action does not.

Adenosine A1-receptor-mediated inhibition of evoked acetylcholine release in the rat hippocampus does not depend on protein kinase C

The possible involvement of protein kinase C and/or a lipoxygenase product in the mechanism by which adenosine inhibits release of [3H]acetylcholine evoked by electrical pulses from [3H]choline-labelled hippocampal slices was examined. For comparison, the muscarinic autoreceptors were examined using carbachol. The order of potency of adenosine analogues (CHA = R-PIA greater than NECA much greater than CGS 21680, CV 1808) indicates that the adenosine receptor responsible is of the A1 subtype. Adenosine (10 microM) and R-PIA (0.1 microM) were virtually equiactive as inhibitors and were antagonized to an equal extent by 8-CPT with a potency (IC50 approximately 25 nM) which is also compatible with A1-receptor mediation. The effects of carbachol and of R-PIA were not antagonized by the lipoxygenase inhibitor NDGA (10 or 50 microM). Stimulation of protein kinase C by the phorbol ester 4 beta-phorbol 12,13-dibutyrate caused a concentration-dependent increase in stimulation-evoked 3H overflow, but did not antagonize the presynaptic inhibitory effect of R-PIA or carbachol (0.01-1 microM). Staurosporine (0.1 microM), which inhibited the stimulating effect of phorbol dibutyrate, did not alter the effects of carbachol or R-PIA. The presynaptic effects of phorbol dibutyrate, R-PIA and adenosine were reduced by pretreatment with N-ethylmaleimide (100 microM for 10 min), which inactivates G-proteins. The evoked transmitter release was unaffected by nifedipine (1 microM) in the presence and in the absence of phorbol dibutyrate. These results indicate that adenosine, by acting at presynaptic A1-receptors, reduces transmitter release by a mechanism that involves neither an NDGA-sensitive lipoxygenase nor protein kinase C. The results also indicate that the enhancement of transmitter release by phorbol esters is due to protein kinase C activation and that a G-protein may be involved in the effect but a dihydropyridine-sensitive L-type Ca2+ channel probably is not.

Differential sensitivity to blockade by 4-aminopyridine of presynaptic receptors regulating [3H]acetylcholine release from rat hippocampus

The inhibitory effect of an adenosine analogue, R-N6-phenylisopropyl adenosine (R-PIA), of the cholinergic agonist carbachol, and of morphine on 3H efflux from [3H]choline-labeled field-stimulated rat hippocampal slices was compared with that produced by two inhibitors of N- and L-type Ca2+ channels, omega-conotoxin (CgTx; conotoxin GVIA) and cadmium chloride. 4-Aminopyridine (4-AP) caused a dose-dependent increase in evoked transmitter release, with a maximal effect (an almost threefold increase) at 100 microM. 4-AP (100 microM) did not affect the actions of CgTx, cadmium chloride, and R-PIA but almost abolished the effect of carbachol and morphine. The present results indicate that presynaptic muscarinic and opiate receptors reduce acetylcholine release by a mechanism that is somewhat different from that used by adenosine A1 receptors. Furthermore, the results indicate that presynaptic A1 receptors on hippocampal cholinergic neurons do not primarily regulate 4-AP-dependent potassium channels, but that they might act directly on a Ca2+ conductance.