Role of dopamine receptors in the modulation of acetylcholine release in the rabbit hippocampus

Characterization of muscarinic autoreceptors in the rabbit hippocampus and caudate nucleus

Oxotremorine-induced inhibition of electrically evoked release of 3H-acetylcholine from brain slices preincubated with 3H-choline was used to characterize muscarinic autoreceptors in rabbit hippocampus and caudate nucleus. From the shifts to the right of the concentration-response curves of oxotremorine in the presence of muscarinic receptor antagonists, the following pKB values [95% C.I.] were determined in the hippocampus: tripinamide: 8.7 [8.5, 8.8]; himbacine: 8.4 [8.3, 8.5]; AQ-RA 741: 8.3 [8.2, 8.5]; 4-DAMP: 8.2 [8.0, 8.3]; hexahydrosiladifenidol: 7.4 [7.2, 7.5]; AF-DX 116: 7.3 [7.1, 7.4]; pirenzepine: 6.8 [6.6, 7.0]; and PD102807: 6.3 [6.0, 6.5]. In the caudate nucleus: tripinamide: 9.1 [8.9, 9.2]; 4-DAMP: 8.3 [8.2, 8.5]; himbacine: 8.1 [8.0, 8.2]; AQ-RA 741: 8.1 [8.0, 8.3]; hexahydrosiladifenidol: 7.3 [7.2, 7.4]; AF-DX 116: 7.1 [7.0, 7.2]; pirenzepine: 6.7 [6.6, 6.8]; and PD102807: 6.5 [6.2, 6.8]. These pKB values fit best to literature values for M2 receptors, suggesting that the muscarinic autoreceptor of the rabbit hippocampus and caudate nucleus is the m2 gene product.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

The dopaminergic mesencephalic projections to the hippocampal formation in the rat

1. The dopaminergic mesencephalic neurons projecting to the hippocampal formation are distributed in three cell groups: A8 region in the retrorubral field, A9 region in the substantia nigra and A10 region in the ventral tegmental area. 2. Anterograde and retrograde tract-tracing techniques combined with immunohistochemical procedures indicate a topographical organization of mesencephalic dopaminergic projections towards the hippocampal formation. 3. Electrophysiological evidence suggest that dopaminergic mesencephalic neurons could have a regulatory role in suppressing hippocampal excitability. 4. The functional significance of the mesohippocampal dopaminergic system is largely unknown, although it was suggested that this projection could have a role in methamphetamine-produced hypermotility and in modulation of memory processes.

Muscarine receptors regulating electrically evoked release of acetylcholine in hippocampus are linked to pertussis toxin-sensitive G proteins but not to adenylate cyclase

[3H]Acetylcholine release elicited with 360 pulses/3 Hz from slices of rabbit hippocampus is facilitated in the presence of the muscarine (M) receptor antagonist atropine (indicating the existence of autoinhibition) and diminished by the M receptor agonists carbachol and oxotremorine. N-Ethylmaleimide (30 microM) and pertussis toxin (8 micrograms/ml) counteracted antagonist-induced facilitation and agonist-induced inhibition of release, suggesting that a pertussis toxin-sensitive GTP-binding protein is involved in the chain of events mediating activation of M receptors to inhibition of release. Neither 8-bromo-cyclic AMP (300 microM), a membrane analogue of cyclic AMP, nor rolipram (10 microM), a phosphodiesterase inhibitor, affected electrically evoked release of [3H]acetylcholine. They also did not influence the oxotremorine-induced inhibition of transmitter release. In conclusion, no evidence was found for the assumption that activation of M autoreceptors is linked to inhibition of adenylate cyclase.

Effects of botulinum A toxin on presynaptic modulation of evoked transmitter release

A possible influence of botulinum A toxin on the modulation of evoked neurotransmitter release was investigated in hippocampus tissue. Rabbit hippocampal slices prelabelled with [3H]noradrenaline ([3H]NA), [3H]5-hydroxytryptamine ([3H]5-HT) or [3H]choline were superfused with physiological medium and were stimulated electrically during superfusion. The evoked release of [3H]NA, [3H]5-HT and [3H]acetylcholine [( 3H]ACh) was inhibited by botulinum A toxin in a concentration- and time-dependent manner. Neither the inhibition of release of [3H]NA and [3H]5-HT by the alpha 2-adrenoceptor agonist clonidine nor facilitation of release in the presence of alpha 2-antagonists were influenced by pretreatment of the tissue with botulinum toxin. The toxin caused no [32P]ADP ribosylation of synaptosomal proteins of hippocampus. The facilitation of the stimulation-induced [3H]NA and [3H]5-HT release by the specific protein kinase C (PKC) activator 4 beta-phorbol-12,13-dibutyrate (PDB) was significantly diminished by botulinum A toxin. These results show that the evoked transmitter release is inhibited by botulinum A toxin by a mechanism which does not involve ADP ribosylation or an interaction with the alpha 2-adrenoceptor mechanism.

Functional supersensitivity of the hippocampal dopaminergic system after prolonged treatment with haloperidol

The effect of acute and prolonged (21 days) treatment with haloperidol (1 or 5 mg/kg SC) on the dopamine-, apomorphine- and LY 171555-induced changes in the firing rate of CA1 layer neurons was studied in a hippocampal slice preparation. Dopamine and apomorphine administration evoked either an excitatory or inhibitory reaction, while the selective D2 receptor agonist LY 171555 increased the firing rate of hippocampal neurons. The excitatory effects of dopamine and LY 171555 were blocked by sulpiride and haloperidol. Prolonged administration of haloperidol potentiated the excitatory reaction of dopamine and apomorphine; however, even a single dose of the neuroleptic enhanced the dopamine-induced effect. The reaction evoked by LY 171555 was not significantly affected by either acute or chronic treatment with haloperidol. The present findings indicate that long-term administration of haloperidol results in an increased sensitivity of hippocampal neurons to the mixed dopamine agonists, dopamine and apomorphine, but not to selective stimulation of the D2 receptor.

Transmitter release patterns of noradrenergic, dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains, and the operation of presynaptic autoreceptors

Slices of rabbit brain were field-stimulated either by single electrical pulses or by trains of 4 or 8 pulses at 1 or 100 Hz in order to study transmitter release patterns and the autoinhibition of transmitter release. The slices were preincubated with 3H-noradrenaline (cortex), 3H-dopamine (caudate nucleus) or 3H-choline (caudate nucleus). Slices preincubated with 3H-noradrenaline were superfused with medium containing desipramine 1 mumol/l. The overflow of tritium elicited by single pulses amounted to 0.19% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was similar, whereas that elicited by 4 pulses/100 Hz was 5.1-fold higher. Yohimbine 10-1000 nmol/l increased up to 2.5-fold the overflow evoked by 4 pulses/1 Hz but did not change the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-dopamine were superfused with medium containing nomifensine 1 mumol/l. The overflow of tritium elicited by single pulses was 0.39% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was 1.3-fold and the overflow elicited by 4 pulses/100 Hz 1.4-fold higher. Domperidone 1-100 nmol/l and sulpiride 10-1000 nmol/l increased up to 2.4-fold the overflow evoked by 4 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-choline were superfused either with physostigmine-free medium or with medium containing physostigmine 1 mumol/l. In physostigmine-free medium, atropine did not increase the evoked overflow of tritium at any stimulation condition. In physostigmine-containing medium, the overflow elicited by single pulses was 0.18% of the tritium content of the tissue. The overflow elicited by 8 pulses/1 Hz was 2.0-fold and the overflow elicited by 8 pulses/100 Hz 2.2-fold higher. Atropine 2-200 nmol/l increased up to 2.4-fold the overflow evoked by 8 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 8 pulses/100 Hz. In physostigmine-free medium, sulpiride 10-1000 nmol/l did not change the single-pulse-evoked overflow of tritium in the absence but increased it in the presence of nomifensine 1 mumol/l. Single pulses elicit a large release of 3H-noradrenaline, 3H-dopamine and 3H-acetylcholine under the conditions of these experiments. Release elicited by single pulses is not subject to autoinhibition except for a small inhibition by spontaneously released transmitter in the case of dopaminergic and cholinergic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Central interactions between dihydropyridines and cholinergic systems in the control of blood pressure in rat

Intracerebroventricular (i.c.v.) injection of the 1,4-dihydropyridine (DHP) calcium channel agonist, Bay K8644 (30 micrograms/kg) increased mean blood pressure and the K+-evoked release of [3H]acetylcholine ([3H]ACh) from hippocampal slices in spontaneously hypertensive rats (SHR). The Bay K8644-induced hypertension was inhibited by a pretreatment with methylatropine (80 micrograms/kg i.c.v.). In SHR, nicardipine, a DHP calcium channel antagonist, reduced mean blood pressure when i.c.v. injected (10 micrograms/kg). The nicardipine-induced hypotension was reduced by a pretreatment with hemicholinium-3 (20 micrograms, i.c.v.). Nicardipine (1 microM) did not modify, in SHR, the K+-evoked release of [3H]ACh, but inhibited the Bay K8644-induced increase in the ACh release. In normotensive rats, neither Bay K8644 nor nicardipine modify blood pressure, when centrally injected, or the stimulated release of [3H]ACh from hippocampal slices. The participation of central DHP sites in the cholinergic transmission in genetic hypertension is discussed.

Pentobarbital antagonizes the A1 adenosine receptor-mediated inhibition of hippocampal neurotransmitter release

Barbiturates have been shown to be competitive antagonists at A1 adenosine receptors in radioligand binding studies. The present study investigates the effects of pentobarbital on the A1 receptor-mediated inhibition of neurotransmitter release from rabbit hippocampal slices. The inhibition of the electrically evoked release of [3H]noradrenaline by the A1 receptor agonist (R)-N6-phenylisopropyladenosine (R-PIA) was antagonized by pentobarbital with an apparent pA2 value of 3.5. Low concentrations of pentobarbital alone altered neither basal nor evoked release of [3H]noradrenaline, whereas 1,000 microM pentobarbital enhanced the basal and reduced the evoked release. In the presence of 8-phenyltheophylline, pentobarbital (200 microM and 1,000 microM) reduced the evoked noradrenaline release. Pentobarbital also antagonized the inhibition of [3H]acetylcholine release by R-PIA. In contrast to the noradrenaline release model, the evoked release of acetylcholine was enhanced by the presence of pentobarbital (50-500 microM), an effect that was lost in the presence of 8-phenyltheophylline. These results indicate that pentobarbital, in addition to a direct inhibitory action at higher concentrations, has a facilitatory effect on neurotransmitter release by blocking presynaptic A1 adenosine receptors. The possible relevance of these findings for the excitatory effects of barbiturates is discussed.

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity

The VTA contains the A10 group of DA containing neurons. These neurons have been grouped into nuclei to be found on the floor of the midbrain tegmentum--Npn, Nif, Npbp and Nln rostralis and caudalis. The VTA is traversed by many blood vessels and nerve fibers. Close to its poorly defined borders are found DA (A8, A9, A11) and 5-HT containing neurons (B8). Efferent projections of the VTA can be divided into 5 subsystems. The mesorhombencephalic projects to other monoaminergic nuclei, the cerebellum and a fine projection descends to other tegmental nuclei as far as the inferior olive. Fibers to the spinal cord have not been demonstrated. The mesodiencephalic path projects to several thalamic and hypothalamic nuclei and possibly the median eminence. Functionally important examples are the anterior hypothalamic-preoptic area, N. medialis dorsalis and reuniens thalami. These two subsystems are largely non-dopaminergic. A minor mesostriatal projection is overshadowed by the large mesolimbic projection to the accumbens, tuberculum olfactorium, septum lateralis and n. interstitialis stria terminalis. There are also mesolimbic connections with several amygdaloid nuclei (especially centralis and basolateralis), the olfactory nuclei and entorhinal cortex. A minor projection to the hippocampus has been detected. The mesocortical pathway projects to sensory (e.g. visual), motor, limbic (e.g. retrosplenial) and polysensory association cortices (e.g. prefrontal). Prefrontal, orbitofrontal (insular) and cingulate cortices receive the most marked innervation from the VTA. A more widespread presence of DA in other cortices of rodents becomes progressively more evident in carnivores and primates. Most but not all projections are unilateral. Some neurons project to more than one area in mesodiencephalic, limbic and cortical systems. The majority of these fibers ascend in the MFB. Most areas receiving a projection from the VTA (DA or non-DA) project back to the VTA. The septohippocampal complex in particular and the limbic system in general provide quantitatively much less feedback than other areas. The role of the VTA as a mediator of dialogue with the frontostriatal and limbic/extrapyramidal system is discussed under the theme of circuit systems. The large convergence of afferents to certain VTA projection areas (prefrontal, entorhinal cortices, lateral septum, central amygdala, habenula and accumbens) is discussed under the theme of convergence systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of dopamine receptors in homogenates of rat hippocampus and other brain areas

The binding of [3H]spiperone (24-26 pM) specifically to the dopaminergic (DAergic) D2 site has been investigated in homogenates of rat hippocampus, pineal gland and cerebellum. It was found that specific binding in hippocampal homogenates was heat-labile and for a part pH-sensitive. Like striatal tissue, binding to the dopaminergic D2 site in hippocampal and cerebellar homogenates was greatly reduced by (pre)incubation with the iron chelator o-phenanthroline. The binding characteristics in hippocampus and cerebellum have been compared to those in the striatum. In all tissues tested, low (74-138 times fewer than in striatum) but significant numbers of DAergic D2 sites were found. The DAergic D2 site in the hippocampus, as far as measured in cerebellum and pineal gland exhibited similar binding characteristics to that in the striatum.

False labelling of dopaminergic terminals in the rabbit caudate nucleus: uptake and release of [3H]-5-hydroxytryptamine

The effect of the catecholamine uptake inhibitor nomifensine and of the 5-hydroxytryptamine (5-HT) uptake blocker 6-nitroquipazine on the accumulation of [3H]-5-HT (0.1 microM, 60 min incubation) and [3H]-dopamine (0.1 microM, 30 min incubation) into slices of hippocampus and caudate nucleus of the rabbit was investigated. In addition, the influence of nomifensine on the electrically evoked [3H]-5-HT release from caudate nucleus slices and of nomifensine and 6-nitroquipazine on [3H]-5-HT released from caudate nucleus slices was analysed. In hippocampal slices, which contain practically no dopaminergic but densely distributed 5-hydroxytryptaminergic and noradrenergic nerve terminals (ratio of dopamine:5-HT:noradrenaline about 1:30:25), nomifensine (1, 10 microM) did not affect the accumulation of [3H]-5-HT; 6-nitroquipazine (1 microM) reduced [3H]-5-HT uptake to about 35% of controls. In the caudate nucleus, however, where dopamine is the predominant monoamine (ratio of dopamine:5-HT:noradrenaline about 400:25:15) nomifensine (1, 10 microM) reduced the tritium accumulation to 65% whereas 6-nitroquipazine (1 microM) was ineffective. The combination of both drugs (1 microM each) led to a further decrease to about 15%. The uptake of [3H]-dopamine into hippocampal slices was blocked by both nomifensine (1 microM) and 6-nitroquipazine (1 microM) whereas in caudate nucleus slices only nomifensine (1, 10 microM) reduced the accumulation of [3H]-dopamine. The combination of both drugs was not more effective than nomifensine alone. The different effects of both uptake inhibitors in the hippocampus and caudate nucleus suggest a neurone specific rather than a substrate specific mode of action. 4 In caudate nucleus slices incubated with [3H]-5-HT and superfused continuously the electrically evoked 5-HT release was diminished by the D2-dopamine receptor agonist LY 171555 and enhanced by the D2-receptor antagonist domperidone. If, however, the labelling of caudate nucleus slices was performed in the presence of I microM or 1O microM nomifensine, the modulation of 5-HT release via D2- receptors was reduced or abolished, respectively. In the hippocampus both LY 171555 and domperidone were completely ineffective in modulating 5-HT release regardless of the absence or presence of nomifensine. 5 The present results indicate that an inverse cross labelling of [3H]-5-HT into dopaminergic and of [3H]-dopamine into 5-hydroxytryptaminergic terminals may occur despite the low concentration (0.1 microM) oftritiated transmitters used. Such cross labelling, as demonstrated with the incubation period of 60 min in the caudate nucleus, may falsely indicate the existence of D2-dopamine receptors modulating [3H]-5-HT release. If both 5-hydroxytryptaminergic and dopaminergic terminals are present within the brain region under investigation false labelling can be corrected using neuronally specific uptake inhibitors.

Presynaptic opioid receptors modulating acetylcholine release in the hippocampus of the rabbit

Slices of the rabbit hippocampus were preincubated with 3H-choline, rinsed and superfused continuously. The release of 3H-acetylcholine in these slices, evoked by electrical field stimulation, was strongly reduced by the preferential kappa-agonists ethylketocyclazocine, dynorphin A (1-13) and dynorphin A (1-17). Dynorphin A (1-9) and (-)MR 2034 [(-)5,9-dimethyl-2'-OH-2-tetrahydrofurfuryl-6, 7-benzomorphan] were less potent, the (+)enantiomer of (-)MR 2034 was ineffective. Whereas the mu-agonist DAGO (D-Ala2-Gly-ol5-enkephalin) showed significant depressant effects, two other mu-agonists morphine and morphiceptine, as well as the delta-agonists DADLE (D-Ala2-D-Leu5-enkephalin) and Leu-enkephalin were much less inhibitory. The preferential mu-antagonist (-)naloxone as well as (-)MR 2266 [(-)N-(3-furylmethyl)-alpha-noretazocine], a preferential kappa-antagonist, did not increase acetylcholine release when given alone, but antagonized the effect of ethylketocyclazocine; (-)MR 2266 (Ke: 1.6 nmol/l) was about 4 times more potent than (-)naloxone (Ke: 6.3 nmol/l). The inhibitory effects of DAGO and DADLE were abolished by (-)MR 2266 (0.1 mumol/l) but not by the delta-antagonist ICI 174864 (N,N-diallyl-Tyr-Aib-Phe-Leu-OH, 0.3 mumol/l). It is concluded that the release of acetylcholine in the hippocampus of the rabbit is inhibited at the level of the axon terminals via kappa-receptors; in addition, mu-receptors may be present. An inhibitory tone of endogenous opioid peptides on hippocampal acetylcholine release could not be demonstrated. Experiments on rat hippocampal slices showed that in this species mu- rather than kappa-receptors may modulate acetylcholine release.

Islet-activating protein (pertussis toxin) diminishes alpha 2-adrenoceptor mediated effects on noradrenaline release

The effect of islet-activating protein (IAP) on alpha 2-adrenoceptor mediated modulation of noradrenaline release in the rabbit hippocampus was studied. Slices of the hippocampus were incubated for 6 h with IAP, subsequently loaded with 3H-noradrenaline and superfused continuously. IAP-pretreatment significantly enhanced the electrically evoked transmitter release and diminished the facilitatory effect of the alpha 2-adrenoceptor antagonist yohimbine. In addition, the inhibitory effect of the alpha 2-adrenoceptor agonist clonidine was reduced. These results provide circumstantial evidence that an inhibitory guanine-nucleotide-binding protein, most probably Ni of a presynaptically located adenylate cyclase, is involved in the alpha 2-autoreceptor mediated modulation of noradrenaline release.

Dopaminergic modulation of hippocampal noradrenaline release. Evidence for alpha 2-antagonistic effects of some dopamine receptor agonists and antagonists

3H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with 3H-noradrenaline, continuously superfused in the presence of cocaine (30 mumol/l) and subjected to electrical field stimulation. The electrically evoked tritium overflow from the slices was reduced by 0.1 and 1 mumol/l dopamine and apomorphine, but significantly enhanced by 10 mumol/l apomorphine or by 0.1 and 1 mumol/l bromocriptine. If the alpha 2-adrenoceptor antagonist yohimbine (0.1 mumol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mumol/l apomorphine and 1 mumol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another alpha 2-antagonist, idazoxane (0.1 mumol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mumol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (greater than 1 mumol/l) and bromocriptine (greater than 0.01 mumol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mumol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mumol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mumol/l, each). At 1 mumol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine: an endogenous modulator of hippocampal noradrenaline release

In slices of hippocampus from the rabbit, preincubated with [3H]noradrenaline and then continuously superfused, the modulation of the release of noradrenaline by adenosine receptors was studied. Electrical field stimulation of the slices elicited a release of [3H]noradrenaline which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. From the order of potency: cyclohexyladenosine greater than (-)phenylisopropyladenosine [(-)PIA] greater than 5'-N-ethylcarboxamide-adenosine (NECA) greater than 2-chloro-adenosine greater than adenosine (+)phenylisopropyladenosine greater than ATP, the inhibitory adenosine receptor was classified as A1- (Ri-) receptor. The effect of the agonist was strongly reduced by adenosine receptor antagonists, the methylxanthines. A role for endogenous adenosine in the modulation of hippocampal noradrenaline release is supported by these findings: (1) that blockade of adenosine receptors by methylxanthines, especially by 8-phenyltheophylline, increased, whereas (2) inhibition of the uptake of adenosine decreased the evoked release of noradrenaline and (3) that deamination of endogenous extracellular adenosine by addition of adenosine deaminase to the medium enhanced the evoked transmitter release. Inhibitors of endogenous adenosine deaminase and 5'-nucleotidase were without effect. It is concluded that release of noradrenaline in the hippocampus is inhibited at the level of the noradrenergic nerve terminals by endogenous adenosine via A1 (or Ri) receptors.

Identification of mechanisms involved in the modulation of release of noradrenaline in the hippocampus of the rabbit in vitro

The modulation of the electrically-evoked release of noradrenaline by various possible neurotransmitters or neuromodulators in the hippocampus was studied in the dorsal part of the hippocampus of the rabbit. Slices of this tissue were preincubated with [3H]noradrenaline and superfused with a medium containing 30 microM cocaine. The evoked overflow of tritium was calcium-dependent, tetrodotoxin-sensitive and subject to modulation by presynaptic alpha 2-autoreceptors. Drugs with affinity for beta-adrenoceptors (up to 1 microM), muscarinic (up to 10 microM), nicotinic (up to 100 microM), GABA- (up to 1000 microM), glutamate- (up to 100 microM) and prostaglandin-receptors (up to 1 microM) did not show any modulatory influence on the evoked release of noradrenaline. In contrast, morphine (1 microM) and fentanyl (1 microM) significantly reduced the evoked overflow; this effect was antagonized by naloxone (10 microM), which, given alone, was ineffective. Apomorphine (1 microM) reduced the release of noradrenaline in the absence, and increased it in the presence, of 0.1 microM haloperidol; haloperidol (0.1 microM), given alone was ineffective. From these results it is concluded that, in addition to the well-known alpha 2-autoreceptor mechanism, presynaptic opiate-, D2- and probably D1-receptors might modulate the release of noradrenaline in the hippocampus.

Endogenous adenosine as a modulator of hippocampal acetylcholine release

Modulation of acetylcholine release via adenosine receptors was studied in rabbit hippocampal slices, which were preincubated with 3H-choline and then continuously superfused. Electrical field stimulation of the slices elicited a release of acetylcholine, which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. The effects of the agonists were antagonized by the methylxanthines. From the order of potency: cyclohexyladenosine greater than (-)phenylisopropyladenosine [-)PIA) greater than 5'-N-ethylcarboxamideadenosine (NECA) greater than 2-chloradenosine greater than (+)phenylisopropyladenosine greater than adenosine, the inhibitory adenosine receptor may be classified as A1-(R1-)receptor. In experiments on rabbit caudate nucleus slices, adenosine receptor agonists only slightly decreased the evoked acetylcholine release. The presence of an inhibitory tone of endogenous adenosine on hippocampal acetylcholine release is supported by the following findings: 1) the methylxanthines theophylline, 8-phenyltheophylline and 3-isobutylmethylxanthine (IBMX) increased the evoked acetylcholine release in concentrations below those required for phosphodiesterase inhibition. 2) Adenosine uptake inhibitors, in contrast, decreased the evoked transmitter release. 3) Deamination of endogenous adenosine by addition of adenosine deaminase to the medium enhanced the acetylcholine release. In conclusion, acetylcholine release in the hippocampus is depressed at the level of the cholinergic nerve terminals by endogenous adenosine via A1-(R1-)receptors.