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

Involvement of dorsal hippocampal nicotinic receptors in the effect of morphine on memory retrieval in passive avoidance task

The present study evaluated the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus on morphine-induced amnesia and morphine state-dependent memory in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas in the CA1 regions of the dorsal hippocampi, trained in a step-through type passive avoidance task, and tested 24 h after training to measure step-through latency. Results indicate that post-training subcutaneous (s.c.) administration of morphine (2.5-7.5 mg/kg) dose-dependently reduced the step-through latency, showing an amnestic response. Post-training intra-CA1 microinjection of nicotine (0.5-1 microg/rat) decreased significantly the amnesia induced by post-training morphine (7.5 mg/kg). Moreover, co-treatment of mecamylamine (0.5 and 1 microg/rat, intra-CA1) with an ineffective dose of morphine (2.5 mg/kg), immediately after training, caused inhibition of memory retrieval. On the other hand, amnesia produced by post-training morphine (7.5 mg/kg) was reversed by pre-test administration of the opioid that is due to a state-dependent effect. Interestingly, pre-test intra-CA1 microinjection of nicotine (0.25 and 0.5 microg/rat) improved post-training morphine (7.5 mg/kg)-induced retrieval impairment. Moreover, pre-test administration of the same doses of nicotine in combination with a lower dose of morphine (0.5 mg/kg), which had no effects alone, synergistically improved memory performance impaired by post-training morphine. Pre-test injection of mecamylamine (0.5-2 microg/rat) prevented the restoration of memory by pre-test morphine. It is important to note that post-training or pre-test intra-CA1 administration of the same doses of nicotine or mecamylamine, alone did not affect memory retrieval. These results suggest that nicotinic acetylcholine receptors of the hippocampal CA1 regions may play an important role in morphine-induced amnesia and morphine state-dependent memory.

Opioid systems in the dentate gyrus

Opiate drugs alter cognitive performance and influence hippocampal excitability, including long-term potentiation (LTP) and seizure activity. The dentate gyrus (DG) contains two major opioid peptides, enkephalins and dynorphins, which have opposing effects on excitability. Enkephalins preferentially bind to delta- and mu-opioid receptors (DORs and MORs) while dynorphins preferentially bind to kappa-opioid receptors (KORs). Opioid receptors can also be activated by exogenous opiate drugs such as the MOR agonist morphine. Enkephalins are contained in the mossy fiber pathway, in the lateral perforant path (PP) and in scattered GABAergic interneurons. MORs and DORs are predominantly in distinct subpopulations of GABAergic interneurons known to inhibit granule cells, and are present at low levels within granule cells. MOR and DOR agonists increase excitability and facilitate LTP in the molecular layer. Anatomical and physiological evidence is consistent with somatodendritic and axon terminal targeting of both MORs and DORs. Dynorphins are in the granule cells, most abundantly in mossy fibers but also in dendrites. KORs have been localized to granule cell mossy fibers, supramammillary afferents to granule cells, and PP terminals. KOR agonists, including endogenous dynorphins, diminish the induction of LTP. Recent evidence indicates that opiates and opioids also modulate other processes in the hippocampal formation, including adult neurogenesis, the actions of gonadal hormones, and development of neonatal transmitter systems.

Presynaptic opioid receptors on noradrenergic and serotonergic neurons in the human as compared to the rat neocortex

1. Electrically evoked release of [3H]-noradrenaline ([3H]-NA) or [3H]-5-hydroxytryptamine ([3H]-5-HT) in slices of human and the rat neocortex was used to characterize presynaptic opioid receptors. 2. Release of [3H]-NA in rat neocortical slices was reduced only by the mu-receptor agonist DAMGO (pIC50: 7.27, CI95: [7.22, 7.32]; Imax: 77.6+/-1.6%; antagonized by naloxone: pA2: 8.88, CI95: [8.78, 8.98]). 3. Release of [3H]-NA in human neocortical slices was unaffected by DAMGO, but inhibited by the delta-receptor agonist DPDPE (Imax: 25.7+/-2.2%) and the kappa-receptor agonist U-50,488H (19.7+/-2.7% inhibition at 1 microM). Both effects were antagonized by naltrindole (1 microM). 4. Release of [3H]-5-HT in rat neocortical slices, was inhibited by DAMGO (10 microM) and U-50,488H (1 and 10 microM) only in the presence of the 5-HT receptor antagonist methiotepin (1 microM). 5. Release of [3H]-5-HT in human neocortical slices was unaffected by DPDPE, but U-50,488H (Imax: 40.8+/-8.3%; antagonized by 0.1 microM norbinaltorphimine) and DAMGO (16.4+/-3.9% inhibition at 1 microM; antagonized by 0.1 microM naloxone) acted inhibitory. 6. Release of [3H]-5-HT in human neocortical slices was reduced by nociceptin/orphanin (0.1 and 1 microM). These effects were antagonized by the ORL1 antagonist J-113397 (1-[(3R,4R)-1-cyclo-octylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one; 0.1 microM). 7. This study provides evidence for significant species differences in opioid receptor-mediated modulation of NA and 5-HT-release in human vs rat neocortex. In rats, mu-opioid receptors modulate NA release, but 5-HT release is only weakly affected by mu- and kappa-opioids. In contrast, NA release in human neocortex is modulated via delta-opioid receptors, but 5-HT release mainly via kappa-opioid receptors. In addition also the ORL1 receptor seems to be involved in 5-HT release modulation.

Ultrastructural evidence for mu-opioid modulation of cholinergic pathways in rat dentate gyrus

Within the rat hippocampal formation, cholinergic afferents and mu-opioid receptors (MORs) are involved in many crucial learning processes, including those associated with drug reward. Pharmacological data, and the overlapping distributions of cholinergic and mu-opioid systems, particularly in the dentate gyrus, suggest that MOR activation is a potential mechanism for endogenous opioid modulation of cholinergic activity. To date, anatomical evidence supporting this has not been reported. To delineate the relationship between cholinergic afferents and MOR-containing processes in the dentate gyrus, hippocampal sections were dually immunolabeled for vesicular acetylcholine transporter (VAChT) and MOR-1 and examined by electron microscopy. VAChT immunoreactivity was in unmyelinated axons and axon terminals, and was most often associated with small synaptic vesicles. MOR immunoreactivity was found in axons, axon terminals and, to a lesser extent, perikarya, which resembled GABAergic basket cells. Semi-quantitative ultrastructural analysis revealed that from 5% to 13% (depending on laminar location) of VAChT-immunoreactive (ir) presynaptic profiles contained MOR immunoreactivity. Additionally, 7% of VAChT-ir presynaptic profiles directly apposed MOR-ir axons and terminals, and there were almost no appositions to MOR-ir dendrites. These data suggest that opioids may directly and indirectly modulate acetylcholine release and/or reuptake. In the hilus and molecular layer, 4% of VAChT-ir terminals contacted dendritic shafts that were also contacted by MOR-ir terminals. This suggests that cholinergic afferents and MOR-containing afferents can converge on granule cell dendrites (which are restricted to the molecular layer) and on interneuron dendrites in the hilus. The results of this study provide ultrastructural evidence for direct and indirect modulation of cholinergic systems by mu-opioids in the hippocampal formation.

Altered localization of choline transporter sites in the mouse hippocampus after prenatal heroin exposure

Prenatal heroin exposure disrupts hippocampal cholinergic synaptic function and related behaviors. Biochemical studies indicate an increase in the number of presynaptic high-affinity choline transporter (HACT) sites, as assessed by [3H]hemicholinium-3 (HC-3) binding. The present study was designed to assess whether this effect involves global upregulation of the transporter, or whether disruption occurs with a specific tempero-spatial distribution. Pregnant mice were given 10mg/kg per day of heroin subcutaneously on gestational days (GD) 9-18. Autoradiographic distribution of HC-3 binding sites was evaluated in the hippocampus of the offspring at postnatal days 15, 25, and 53. These results, suggestive of hippocampal "miswiring," are likely to explain the net impairment of cholinergic synaptic function after prenatal heroin exposure, despite the simultaneous upregulation of both presynaptic cholinergic activity and postsynaptic receptors. Understanding the subregional selectivity of hippocampal defects can lead to the development of strategies that may potentially enable therapeutic interventions to offset or reverse the neurobehavioral defects.

Prenatal heroin exposure alters cholinergic receptor stimulated activation of the PKCβII and PKCγ isoforms

Prenatal exposure of mice to heroin (SC injection of 10mg/kg to the dams on gestational days 9-18) resulted at adulthood in behavioral deficits related to septohippocampal cholinergic innervation accompanied with both presynaptic and postsynaptic cholinergic hyperactivity; including an increase membrane PKC activity, and a desensitization of PKC to cholinergic input which were highly correlated with the behavioral performance and were reversed by cholinergic grafting. Therefore, we studied the receptor induced activation of the behaviorally relevant PKCgamma and PKCbetaII isoforms and the less behaviorally relevant PKCalpha isoform. Time course studies revealed peak translocation after 40 min incubation with carbachol for PKCgamma (110% increase from basal, i.e. no carbachol level, P < 0.01), 30 min for phosphorylated PKCbetaII (130%, P < 0.05) and 5 min for non-phosphorylated PKCbetaII (64%, P < 0.05) with no peak for alpha. Prenatal heroin abolished the translocation of PKCgamma and PKCbetaII while PKCalpha remained unaffected. A decrease occurred in basal phosphorylated membrane (-45%, P < 0.01) and cytosol-associated (-29%, P < 0.01) PKCbetaII, in membrane-associated non-phosphorylated PKCbetaII (-32%, P < 0.01) and PKCgamma (-25%, P < 0.01) and in cytosolic PKCalpha (-27%, P < 0.01), while membrane-associated PKCalpha was slightly increased (11%, P < 0.05). The results suggest that prenatal heroin disrupts cholinergic receptor induced PKC translocation and activation with the underlying mechanism of neuroteratogenicity potentially lying in the PKCgamma and PKCbetaII, while PKCalpha remains unaffected.

Role of cholinergic and GABAergic neurotransmission in the opioids-mediated GnRH release mechanism of EBP-primed OVX rats

The effect of intracerebroventricular administration of mu-opioid agonist, morphine (a drug of potential abuse), and its antagonist, naloxone, followed by morphine was studied on the metabolism of acetylcholine and gamma amino butyric acid in seven discrete regions of brain from EBP-primed ovariectomized rats. We also assayed serum luteinizing hormone and follicle stimulating hormone after morphine and naloxone + morphine treatments. Cholineacetyltransferase and acetylcholinesterase, gamma-aminobutyric acid transaminase, succinic semialdehyde dehydrogenase and glutamate dehydrogenase activities were found to decrease significantly in hypothalamic as well as other brain regions studied. Naloxone given prior to morphine injection was seen to reverse the effect of morphine on enzymes activities. Our study provides evidence that opioidergic modulation of GnRH release is mediated through cholinergic and GABAergic neurotransmission besides monoaminergic control and the results may further help to elucidate the basis of neuronal dysfunction in opiate addicts.

Opioid receptor regulation of muscarinic acetylcholine receptor-mediated synaptic responses in the hippocampus

A common feature of many synapses is their regulation by neurotransmitters other than those released from the presynaptic terminal. This aspect of synaptic transmission is often mediated by activation of G protein coupled receptors (GPCRs) and has been most extensively studied at amino acid-mediated synapses where ligand gated receptors mediate the postsynaptic signal. Here we have investigated how opioid receptors modulate synaptic transmission mediated by muscarinic acetylcholine receptors (mAChRs) in hippocampal CA1 pyramidal neurones. Using a cocktail of glutamate and gamma-amino-butyric acid (GABA) receptor antagonists a slow pirenzepine-sensitive excitatory postsynaptic potential (EPSP(M)) that was associated with a small increase in cell input resistance could be evoked in isolation. This response was enhanced by the acetylcholine (ACh) esterase inhibitor physostigmine (1 microM) and depressed by the vesicular ACh transport inhibitor vesamicol (50 microM). The mu-opioid receptor agonists DAMGO (1-5 microM) and etonitazene (100 nM), but not the delta- and kappa-opioid receptor selective agonists DTLET (1 microM) and U-50488 (1 microM), potentiated this EPSP(M) (up to 327%) without affecting cell membrane potential or input resistance; an effect that was totally reversed by naloxone (5 microM). In contrast, postsynaptic depolarizations and increases in cell input resistance evoked by carbachol (3 microM) were unaffected by DAMGO (1-5 microM) but were abolished by atropine (1 microM). Taken together these data provide good evidence for a mu-opioid receptor-mediated presynaptic enhancement of mAChR-mediated EPSPs in hippocampal CA1 pyramidal neurones.

Methylated analogues of methyl (R)-4-(3,4-dichlorophenylacetyl)- 3-(pyrrolidin-1-ylmethyl)piperazine-1-carboxylate (GR-89,696) as highly potent kappa-receptor agonists: stereoselective synthesis, opioid-receptor affinity, receptor selectivity, and functional studies

Analogues of the kappa-receptor agonist methyl (R)-4-(3,4-dichlorophenylacetyl)-3-(pyrrolidin-1-ylmethyl)piperazine-1-carboxylate (GR-89,696, 6) bearing an additional methyl substituent in the side chain are synthesized and evaluated for their kappa-receptor affinity and selectivity. A key step in the synthesis is the stereoselective reductive amination of the ketones 9, 18, and 19 with pyrrolidine and NaBH(3)CN, which succeeds only in the presence of the Lewis acid Ti(OiPr)(4). Whereas the BOC-substituted ketone 9 affords the unlike and like diastereomers of 10 in a ratio of 70:30, the diastereoselectivity during the reductive amination of the butyl and phenyl substituted ketones 18 and 19 is enhanced to 85:15 (butyl derivative) and >95:<5 (phenyl derivative) in favor of the unlike diastereomers. In receptor binding studies using the radioligand [(3)H]U-69,593 the (S,S)-configured methyl carbamate (S,S)-14 reveals the highest kappa-receptor affinity (K(i) = 0.31 nM) within this series, even exceeding the lead kappa-agonist 6 (GR-89,696). A slightly reduced kappa-receptor affinity is observed with the propionamide (S,S)-13 (K(i) = 0.67 nM). The kappa-receptor affinity of piperazines with acyl or alkoxycarbonyl residues at both nitrogen atoms (11, 13, 14) decreases in the order (S,S) > (R,R) > (S,R) > (R,S). The methyl carbamate (S,S)-14 discloses a unique activity profile also binding at mu-receptors in the subnanomolar range (K(i) = 0.36 nM). In a functional assay, i.e., by measuring acetylcholine release in rabbit hippocampus slices, the agonistic effects of the methyl carbamate (S,S)-14 and the propionamide (S,S)-13 are demonstrated. Only weak kappa- and mu-receptor affinities are found with the butyl- and phenyl-substituted piperazines 22 and 23. However, considerable sigma(1)-receptor affinity is determined for the enantiomeric, unlike-configured butyl derivatives (R,S)-22 and (S,R)-22 with K(i)-values of 40.2 nM and 81.0 nM, respectively.

Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7

Recent evidence demonstrates that the fragment of angiotensin II, angiotensin II (3-8) termed angiotensin IV, binds with high affinity to a specific binding site, the AT(4) receptor. Intracerebroventricular injection of AT(4) receptor agonists improves the performance of rats in passive avoidance and spatial learning paradigms. AT(4) receptors and cholinergic neurons are closely associated in regions involved in cognitive processing, such as the hippocampus and neocortex. We therefore postulated that AT(4) receptors affect cognitive processing by modulating cholinergic neurotransmission. To test this, we examined the effect of AT(4) receptor ligands, angiotensin IV and LVV-hemorphin-7, on potassium-evoked [(3)H]acetylcholine ([(3)H]ACh) release from rat hippocampal slices. Hippocampal slices from male Sprague--Dawley rats were incubated with [(3)H]choline chloride, perfused with Krebs--Henseleit solution and [(3)H]ACh release was determined. Angiotensin IV and LVV-hemorphin-7 both potentiated depolarisation-induced [(3)H]ACh release from the rat hippocampus in a concentration-dependent manner with the maximal dose (10(-7)M) of each inducing an increase of 45+/-7.5% (P<0.01) and 95.8+/-19% (P<0.01) above control, respectively. Potentiation of release by both agonists was attenuated by the AT(4) receptor antagonist, divalinal-Ang IV. Angiotensin IV-induced potentiation was not affected by AT(1) and AT(2) receptor antagonists. These results indicate that stimulation of AT(4) receptors can potentiate depolarisation-induced release of ACh from hippocampal slices and suggest that potentiation of cholinergic transmission may be a mechanism by which AT(4) receptor ligands enhance cognition.

Modulation of electrically evoked acetylcholine release in cultured rat septal neurones

The electrically evoked release of acetylcholine and its modulation via auto- and heteroreceptors were studied in primary cell cultures prepared from embryonic rat septum (ED 17). Cultures were grown for 1, 2 or 3 weeks on circular, poly D-lysine-coated glass coverslips. They developed a dense network of non-neuronal and neuronal cells, only some of which were immunopositive for choline acetyltransferase. To measure acetylcholine release, the cells on the coverslips were pre-incubated with [3H]choline (0.1 micromol/L), superfused with modified Krebs-Henseleit buffer at 25 degrees C and electrically stimulated twice for 2 min (S1, S2; 3 Hz, 0.5 ms, 90-100 mA). The electrically evoked overflow of [3H] from the cells consisted of approximately 80% of authentic [3H]Ach, was largely Ca2+-dependent and tetrodotoxin sensitive, and hence represents an action potential-evoked, exocytotic release of acetylcholine. Using pairs of selective agonists and antagonist added before S2, muscarinic autoreceptors, as well as inhibitory adenosine A1- and opioid mu-receptors, could be detected, whereas delta-opioid receptors were not found. Evoked [3H] overflow from cultures grown for 1 week, although Ca2+ dependent and tetrodotoxin sensitive, was insensitive to the muscarinic agonist oxotremorine, whereas the effect of oxotremorine on cells grown for 3 weeks was even more pronounced than that in 2-week-old cultures. In conclusion, similar to observations on rat septal tissue in vivo, acetylcholine release from septal cholinergic neurones grown in vitro is inhibited via muscarinic, adenosine A1 and mu-opioid receptors. This in vitro model may prove useful in the exploration of regulatory mechanisms underlying the expression of release modulating receptors on septal cholinergic neurones.

Postnatal development of opioid receptors modulating acetylcholine release in hippocampus and septum of the rat

The postnatal development of presynaptic opioid receptors inhibiting the release of acetylcholine (ACh) was studied in rat brain hippocampus, medial septum (MS) and diagonal band of Broca (DB). To this end, the corresponding brain slices (350 microm thick) of rats of various postnatal ages (postnatal day 4 [P4] to P16, and adult) were preincubated with [(3)H]choline and stimulated twice for 2 min (S(1), S(2): at 3 Hz, 2 ms, 60 mA) during superfusion with physiological buffer containing hemicholinium-3. In parallel, the activity of choline acetyltransferase (ChAT) was determined in crude homogenates of the tissues as a marker for the development of cholinergic neurons. At any postnatal age, the electrically evoked overflow of tritium from slices preincubated with [(3)H]choline was highest in the DB, followed by the MS and the hippocampus. The evoked [(3)H]overflow increased with postnatal age, reached about 50% (MS, DB) or 30% (hippocampus) of the corresponding adult levels at P16 and correlated significantly with the corresponding ChAT activities. Presence of the preferential mu-opioid receptor agonist DAMGO during S(2) significantly inhibited the evoked overflow of tritium already at P4 in DB and MS, whereas in the hippocampus significant inhibitory effects were first observed at P8 only. Moreover, adult levels of inhibition due to DAMGO were reached at P16 in the DB and MS but not in the hippocampus. In septal areas, also the effect of the preferential delta-opioid receptor agonist DPDPE on the evoked [(3)H]overflow was studied: in contrast to DAMGO, however, significant inhibitory effects of DPDPE were first observed at P12 only. In conclusion, the postnatal development of presynaptic mu-opioid receptors on cholinergic neurons in the DB and MS starts earlier than in the hippocampus and precedes that of presynaptic delta-opioid receptors.

Neurobehavioral damage to cholinergic systems caused by prenatal exposure to heroin or phenobarbital: cellular mechanisms and the reversal of deficits by neural grafts

Despite the basic differences in their underlying biological targets, prenatal exposure to heroin or phenobarbital produces similar syndromes of neurobehavioral deficits, involving defects in septohippocampal cholinergic innervation-related behaviors. At the cellular level, these deficits are associated with cholinergic hyperactivity, characterized by increased concentrations of muscarinic receptors and enhanced second messenger activity linked to the receptors. In the present study, we determined whether the cellular changes are mechanistically linked to altered behavior, using two different approaches: neural grafting and correlations between behavior and biochemistry within the same individual animals. Mice were exposed transplacentally to phenobarbital or heroin on gestation days 9-18 and, as adults, received fetal cholinergic grafts or were sham-operated. Prenatal drug exposure resulted in deficits in behavioral performance tested in the eight-arm radial maze, accompanied by increases in hippocampal M(1)-muscarinic receptor expression and muscarinic receptor-mediated IP formation. Neural grafting reversed both the behavioral deficits and the muscarinic hyperactivity. In the drug-exposed offspring, there was a significant correlation between maze performance and carbachol-induced inositol phosphate (IP) formation. These studies indicate that deficits of cholinergic function underlie the neurobehavioral deficits seen in the hippocampus of animals exposed prenatally to heroin or phenobarbital, and consequently that the observed cholinergic hyperactivity is an unsuccessful attempt to compensate for the loss of cholinergic function. The fact that the damage can be reversed by neural grafting opens up novel approaches to the restoration of brain function after prenatal insults.

Different modulation of cholinergic neuronal systems by dynorphin A (1-13) in carbon monoxide-exposed mice

The effects of dynorphin A (1-13), a kappa-opioid receptor agonist, on the content of acetylcholine (ACh) and high K+-induced release of endogenous ACh were studied in mice exposed to carbon monoxide (CO). Mice were exposed to CO 3 times at 1-hr intervals and used 7 days after CO exposure. Administration of dynorphin A (1-13) (1.5 and 5.0 nmol/mouse, intracerebroventricularly) 15 min before killing significantly increased the ACh content in the striatum and hippocampus of control mice, but had no effect on the ACh content in CO-exposed mice. Dynorphin A (1-13) did not change the choline acetyltransferase (EC 2.3.1.6) activity in control or CO-exposed mice. The high K+-induced endogenous ACh release from hippocampal slices in CO-exposed mice was significantly lower than that of controls, although exposure to CO did not affect the basal release of endogenous ACh from hippocampal slices compared with controls. Dynorphin A (1-13) caused dose-dependent decreases in high K+-induced release of endogenous ACh from hippocampal slices in control mice. This inhibitory effect of dynorphin A (1-13) was blocked by co-perfusion with nor-binaltorphimine, a selective K-opioid receptor antagonist. On the other hand, dynorphin A (1-13) did not decrease high K+-induced release of endogenous ACh from hippocampal slices in CO-exposed mice. These results suggest that dysfunction of the cholinergic system occurred after exposure to CO, and as a result the inhibitory effects of dynorphin A (1-13) may be blocked in CO-exposed mice.

Gabapentin modifies extracellular opioid peptide content in amygdala: a microdialysis study

Opioid peptide release was monitored in the amygdala and hippocampus of freely moving rats following a single oral administration of gabapentin using microdialysis. Extracellular opioid peptide levels were elevated above basal levels in the amygdala within the first 60 (54%) and 90 min (68%) after gabapentin administration. Levels returned to basal conditions 120 min following the treatment. No significant changes were detected in the hippocampus. The majority of immunoreactive material recovered from the amygdala following gabapentin administration was identified as Leu-enkephalin and Met-enkephalin by high performance liquid chromatography (HPLC) analysis. It is proposed that the enhanced opioid peptide release in the amygdala induced by gabapentin might be involved with the antiepileptic effects as well as with some adverse events produced by this drug.

Modulation of acetylcholine release in human cortical slices: possible implications for Alzheimer's disease

Superfused slices of human neocortex, prepared from surgically removed tissue (to gain access to subcortical tumors) and prelabelled with [3H]choline, were stimulated electrically to evoke action potential-induced, exocytotic [3H]acetylcholine release. For comparison, rat cortex slices were also used. [3H]ACh release decreased with the age of the patients and was modulated by muscarinic autoreceptors and by 5-hydroxytryptamine1F, neurokinin1, and kappa-opioid receptors located on cholinergic terminals. In addition, 5-hydroxytryptamine2 and delta-opioid receptors located on interneurons were also involved in the modulation of [3H]ACh release. The present findings might help to explain pathological conditions in Alzheimer's disease.

Involvement of cAMP-dependent protein kinase in mu-opioid modulation of NMDA-mediated synaptic currents

We have previously reported dual effects of mu-opioids on N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic events in the hippocampal dentate gyrus: an indirect facilitating effect via suppression of GABAergic interneurons (disinhibition) and a direct inhibitory effect in the presence of gamma-aminobutyric acid-A (GABA(A)) antagonists. The cellular mechanism underlying the inhibitory effect of mu-opioids remains to be determined. In the present study we examine the role of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) in mu-opioid-induced inhibition of NMDA currents in rat hippocampal slices. NMDA-receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) were evoked by stimulating the lateral perforant path and were recorded from dentate granule cells with the use of whole cell voltage-clamp techniques in the presence of the GABA(A) antagonist and a non-NMDA type of glutamate receptor antagonist. Two selective mu-agonists, [N-MePhe3, D-Pro4]-morphiceptin and [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin, induced dose-dependent inhibition of NMDA EPSCs in a concentration range of 0.3-10 microM. This inhibitory effect could be completely reversed by the opioid antagonists naloxone or prevented by a selective mu-antagonist cyprodime, but was not affected by removal of Mg2+ from the external perfusion medium. Intracellular application of pertussis toxin (PTX) into the granule cell via whole cell recording pipettes completely prevented mu-opioid-induced reduction in NMDA currents, suggesting that a postsynaptic mechanism involving PTX-sensitive G proteins might be responsible for the inhibitory action of mu-opioids. Further studies were conducted to identify the intracellular messengers that coupled with G proteins and transduced the effect of mu-opioids in granule cells. The adenylate cyclase activator forskolin was found to enhance NMDA-receptor-mediated synaptic responses and to reverse the inhibitory effect of mu-opioids. Sp-cAMPS, a specific PKA activator, also enhanced NMDA EPSCs, whereas the PKA inhibitor Rp-cAMPS reduced NMDA EPSCs and occluded further inhibition of the current by mu-opioids. These findings strongly suggest that NMDA receptor function is subject to the modulation by PKA, and that mu-opioids can inhibit NMDA currents through suppression of the cAMP cascade in the postsynaptic neuron. Combined with our previous findings, the present results also indicate that mu-opioids can modulate NMDA-receptor-mediated synaptic activity in a complex manner. The net effect of mu-opioids in the dentate gyrus may depend on the interplay between its disinhibitory action, which facilitates NMDA-receptor-mediated responses, and its inhibitory action on the cAMP cascade.

Regional brain activity changes associated with fentanyl analgesia elucidated by positron emission tomography

Recent positron emission tomography (PET) studies have demonstrated areas of pain processing in the human brain. Given the inhibitory effects of opioids on neuronal activity, we predicted that fentanyl's analgesic effects would be associated with suppression of pain-evoked responses in these distinct brain areas. To test this, PET was used to measure cerebral blood flow responses, as reflections of regional neuronal activity, to painful and nonpainful thermal stimuli both in the absence and presence of fentanyl in humans. During each PET scan in nine healthy volunteers a tonic heat source was placed against the subject's left forearm, delivering a preset temperature of either 40 degrees C (nonpainful) or 47-48 degrees C (painful). Subjects underwent eight blood flow studies, each consisting of 50 mCi [15O]water injection and a PET scan. The first four studies were performed during placebo administration in the stimulus sequence: nonpainful, painful, painful, nonpainful. This sequence was then repeated during intravenous (i.v.) administration of fentanyl 1.5 micrograms/kg [corrected]. Significant differences in regional cerebral blood flow (rCBF) between the placebo and the fentanyl conditions during nonpainful and painful stimuli were identified using statistical parametric mapping. It was found that pain increased rCBF in the anterior cingulate, ipsilateral thalamus, prefrontal cortex, and contralateral supplementary motor area. Fentanyl increased rCBF in the anterior cingulate and contralateral motor cortices, and decreased rCBF in the thalamus (bilaterally) and posterior cingulate during both stimuli. During combined pain stimulation and fentanyl administration, fentanyl significantly augmented pain-related rCBF increases in the supplementary motor area and prefrontal cortex. This activation pattern was associated with decreased pain perception, as measured on a visual analog scale. In contrast to our hypothesis, these data indicate that fentanyl analgesia involves augmentation of pain-evoked cerebral responses in certain areas, as well as both activation and inhibition in other brain regions unresponsive to pain stimulation alone.

Inositol phosphate formation in mice prenatally exposed to drugs: relation to muscarinic receptors and postreceptor effects

Mice were exposed to phenobarbital or heroin [diacetylmorphine (DAM)] prenatally by feeding the mother phenobarbital on gestation day 9-18; DAM was injected into the mother on gestation days 9-18. At the age of 50 days, mice exposed to phenobarbital or DAM prenatally were examined for long-term biochemical changes in the postsynaptic septohippocampal system as measured by alterations in formation of the second messenger inositol phosphate (i.p.). A significant increase in i.p. formation in response to carbachol was found after prenatal exposure to DAM. An increase in i.p. formation in response to 20 mM KCl alone or in the additional presence of 10 mM carbachol or 1mM physostigmine was found after prenatal exposure to phenobarbital or DAM. In addition, a significant increase in IP formation in response to sodium fluoride was found after prenatal exposure to phenobarbital or DAM. It is suggested that an increase in G-protein activation and in the second messenger formation accompanies the early drug-induced upregulation of the muscarinic receptors found in our previous studies.

The effect of haloperidol on met-enkephalin, beta-endorphin, cholecystokinin and substance P in the pituitary, the hypothalamus and the striatum of rats during aging

1. Haloperidol increased the Met-enk level in the striatum at all age groups. However, the Met-enk level was decreased in AL of young and middle-aged rats by the drug. 2. Haloperidol elevated the beta-end level in AL and CCK level in NIL in young rats only. 3. The SP content in NIL was decreased by haloperidol in all age groups. 4. With regard to the effect of aging, Met-enk level in AL of middle-aged rats was higher than that in young rats. The beta-end level in AL also increased in old rats. 5. Aging modified the haloperidol effect on beta-end level in AL and CCK level in NIL as the effect was only observed in young rats. 6. In addition, aging caused a blunted response of Met-enk level to haloperidol in the striatum but an increased response of SP content to haloperidol in the NIL.

mu-Opioid receptors modulate noradrenaline release from the rat hippocampus as measured by brain microdialysis

The modulation of noradrenaline (NA) release via presynaptic opioid receptors in the hippocampus of freely moving rats was studied by the use of brain microdialysis. Extracellular levels of NA were estimated by assaying its concentrations in the perfusion fluid using high-performance liquid chromatography (HPLC) with electrochemical detection (ECD). Spontaneous NA levels were reduced by tetrodotoxin (1 microM) co-perfusion and were increased by peripheral administration of desipramine (5 and 10 mg/kg, i.p.). Addition of potassium (K+, 60 and 120 mM) to the perfusion fluid evoked a concentration-dependent release of NA. K+ (120 mM)-evoked NA release was markedly reduced by removal of calcium (Ca2+) from the perfusion fluid. These results indicate that both the spontaneous and the K(+)-evoked NA release measured by the use of brain microdialysis coupled with HPLC-ECD can be used as indices of neuronal release from the noradrenergic nerve terminals. A mu-opioid receptor agonist, morphine (0.01-10 microM), when co-perfused with K+ (120 mM), produced a reduction of K(+)-evoked NA release in a concentration-dependent manner. Neither co-perfusion with a high concentration of [D-Pen2, D-Pen5]-enkephalin (DPDPE) (10 microM), an agonist selective for delta-opioid receptors, nor with U-69593 (10 microM), an agonist selective for kappa-opioid receptors, modified the K+ (120 mM)-evoked release of NA. Morphine-induced (1 microM) inhibition of NA release was blocked by a mu-opioid receptor antagonist, naltrexone (3 and 9 mg/kg, i.p). Naltrexone by itself did not alter the spontaneous NA levels or the K(+)-evoked NA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in the contents of neuropeptides in the rat brain and pituitary

beta-Endorphin, Leu-enkephalin, Met-enkephalin, substance P, somatostatin, and cholecystokinin were measured in the brain and the pituitary of male Sprague-Dawley rats aged 3 months, 12 months, and 22 months. beta-Endorphin, Met-enkephalin and Leu-enkephalin contents in the neurointermediate lobe, and the enkephalin levels in the anterior lobe of the pituitary increased with age. The increases in contents were both in the day and at night for beta-endorphin and Met-enkephalin. However, the increase for Leu-enkephalin content was in the day only. Hypothalamic beta-endorphin content decreased with age only in the day. beta-Endorphin and Leu-enkephalin contents in the brain stem, and Leu-enkephalin levels contents in the cortex decreased with age at night. Leu-enkephalin in the striatum decreased with age in the day. There was also an age-related decrease for somatostatin and substance P contents in the striatum and the hypothalamus in the day, and in cholecystokinin levels in the hippocampus, and the hypothalamus at night. It is concluded that there are age differences in neuropeptide levels, and that these changes may differ according to diurnal rhythms.

No evidence for presynaptic opioid receptors on cholinergic, but presence of kappa-receptors on dopaminergic neurons in the rabbit caudate nucleus: involvement of endogenous opioids

The effects of various opioid receptor agonists and antagonists were studied in rabbit caudate nucleus slices preincubated with either [3H]dopamine or [3H]choline, superfused with medium (containing in most experiments the D2 receptor antagonist domperidone) and subjected to electrical field stimulation. The stimulation-evoked [3H]overflow from slices prelabeled with [3H]dopamine (evoked [3H]dopamine release) was significantly reduced by preferential kappa-opioid receptor agonists, like U-50,488 H, but not by mu- or delta-opioid receptor selective drugs. Opioid receptor antagonists shifted the concentration/response curve of U-50,488 H to the right (apparent pA2-value of the kappa-selective antagonist nor-binaltorphimine: 10.1) and enhanced the evoked dopamine release in the presence of a mixture of peptidase inhibitors. On the other hand, the [3H]overflow from rabbit caudate nucleus slices prelabeled with [3H]choline (evoked acetylcholine release) remained almost unaffected by any opioid receptor agonist, as long as the presynaptic D2 heteroreceptor was blocked with domperidone: in the absence of domperidone, U-50,488 H exhibited facilitatory effects. For comparison, the effects of the preferential delta-opioid receptor agonist DPDPE was also studied in slices of the rat striatum, where it clearly inhibited the evoked acetylcholine release. From our data we conclude that in the rabbit caudate nucleus the evoked dopamine release is inhibited by both exogenous and endogenous opioids via presynaptic kappa-opioid receptors, whereas the evoked release of acetylcholine is not, or only indirectly (via released dopamine) affected by opioids.

Nerve growth factor and its receptor are differentially modified by chronic naltrexone treatment during rat brain development

In order to examine the relationship between the action of opioid neurotransmitters and growth factors in the regulation of brain development, we have studied the long-term effect of the opiate antagonist naltrexone (NTX) on the content of nerve growth factor (NGF) in cortex, hippocampus, septum and neostriatum, and on NGF receptor (NGFRs) levels in cortical membranes. 50 mg/kg NTX treatment induced a decrease in the number of 125I-NGF high-affinity binding sites, without detectable changes in NGF levels. However, low doses of NTX (1 mg/kg) produced no differences in 125I-NGF binding sites, but induced a decrease in NGF levels in hippocampus, septum and neostriatum. These results suggest that NGF and NGFRs could be involved in the trophic effects of opioids during brain development.

Effect of dermorphin on behavior and hippocampal electrical activity in rabbits

Dermorphin, a naturally occurring heptapeptide that selectively binds to mu-opioid receptors, was injected intravenously 0.4 mg/kg in male rabbits. Eight days before injection the spontaneous behavior of the animals was observed in a neutral environment in the absence of external stimuli. At the same time, hippocampal EEG was recorded by telemetry. After dermorphin injection, the same experimental procedure was repeated. Behavior showed a strong increase in quiet immobility and a concomitant reduction in voluntary activities as compared to control periods. Treatment did not affect either the pattern or the frequency of hippocampal electrical activity.

Beta-adrenoceptor-sensitive adenylate cyclase is inhibited by activation of mu-opioid receptors in rat striatal neurons

The beta-adrenoceptor-sensitive adenylate cyclase in primary cultures of rat striatal neurons was inhibited by opioids, unlike that in rat striatal slices. Isoprenaline (1 microM)-stimulated cyclic AMP production was dose dependently inhibited by the mu-opioid receptor agonist. [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO, EC50 = 0.02 microM, 36% inhibition), and only slightly reduced by relatively high concentrations of the delta-opioid receptor agonist, [D-penicillamine2, D-penicillamine5]enkephalin (DPDPE, 1 microM). The highly selective and potent delta-opioid receptor agonist. [D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6 (DSTBULET), and the kappa-opioid receptor agonist, U50-488, were ineffective in concentrations up to 3 microM. Naloxone reversed equally well the inhibitory effects of DPDPE and of DAGO, indicating the involvement of functional mu-opioid receptors. The isoprenaline (1 microM)-stimulated adenylate cyclase activity in cultured glial cells, which exceeded that in neurons about 10-fold, was not affected by opioids. Therefore, opioids were ineffective in rat brain slices probably due to the fact that cyclic AMP production induced by beta-adrenoceptor activation occurs primarily in the glial cells, where it is not subject to inhibition by opioids. These data indicate for the first time the existence of an interaction between functional mu-opioid receptors and beta-adrenoceptors on striatal neurons of the rat.

Opioid receptor-mediated inhibition of dopamine and acetylcholine release from slices of rat nucleus accumbens, olfactory tubercle and frontal cortex

The modulation of the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) by opioid receptor activation was examined in superfused slices from rat nucleus accumbens, olfactory tubercle, and frontal cortex. In all brain areas examined, [3H]DA release was inhibited by the kappa agonist, U 50,488 (1-100 nM), and this inhibition was fully antagonized by the selective kappa antagonist, norbinaltorphimine (nor-BNI). In the frontal cortex, the mu agonist, [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO, 0.01-1 microM), also inhibited the evoked release of tritium. However, further experiments (including the use of the D2-receptor agonist, LY 171555, and the alpha 2-adrenoceptor agonist, oxymetazoline) suggest strongly that in the frontal cortex DAGO only inhibits the release of [3H]catecholamine from noradrenergic nerve terminals, despite the use of desimipramine to prevent the uptake of [3H]DA into these terminals. [14C]ACh release from both the nucleus accumbens and olfactory tubercle, but not from the frontal cortex, was inhibited by DAGO (0.01-1 microM) and the delta agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE, 0.01-1 microM). These inhibitory effects were antagonized by 0.1 microM naloxone but not by 3 nM nor-BNI. The irreversible delta ligand, fentanyl isothiocyanate (FIT, 1 microM), only antagonized the inhibition caused by DPDPE. The results indicate that the inhibitory effects of opioids on the in vitro release of DA from dopaminergic nerve fibres arising from the substantia nigra and the ventral tegmental area are mediated by presynaptic kappa receptors only. In those regions where ACh release is modulated by opioids, the type of opioid receptor involved may depend on the type of neuron, i.e. interneuron or afferent neuron.

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.

Adrenoreceptor antagonist treatment influences recovery of learning following medial septal lesions and hippocampal sympathetic ingrowth

Previous studies have demonstrated that in male rats hippocampal sympathetic ingrowth (HSI), which is induced by medial septal lesions (MS), is detrimental to recovery of spatial learning. The present study was performed in an attempt to determine if this effect was mediated through adrenergic receptor activity. Adult male Sprague-Dawley rats underwent training on a modified version (i.e., 4 arms baited) of a radial-8-arm maze task. Following attainment of learning criterion animals underwent one of three surgical procedures: CON (sham surgeries); MSGx (MS + superior cervical ganglionectomy--to prevent HSI); MS (MS + sham ganglionectomy). Reacquisition trials were performed in the same manner as initial acquisition except animals were treated with vehicle, propranolol (20 mg/kg), or phentolamine (20 mg/kg) 30 minutes prior to testing. As expected, vehicle-treated MS animals took longer to reacquire the task than MSGx animals, who were in turn more impaired than CON animals. Propranolol (beta-adrenergic antagonist) treatment impaired performance of both the MS and MSGx group, but did not alter the CON group. Phentolamine (alpha-adrenergic antagonist) increased the number of trials to reattain criterion in the CON group, had no effect in the MSGx group, and markedly improved performance in the MS group. The results suggest that HSI mediates its detrimental effects through alpha-receptors, while beta-blockade, in the setting of brain injury, is detrimental to performance regardless of the presence or absence of HSI.

Regulation of endogenous acetylcholine release from mammalian brain slices by opiate receptors: hippocampus, striatum and cerebral cortex of guinea-pig and rat

The effects of opiate agonists on acetylcholine release from hippocampal, striatal and cerebral cortical slices were tested; tissue from rat was compared to that from guinea-pig. The results show that opiate receptors in each of these areas can alter the evoked release of acetylcholine from nerve terminals; however, there are species and tissue differences with respect to the apparent subtype of opiate receptor effective. In the hippocampus and striatum of the two species studied, opiates caused a dose-dependent decrease in evoked acetylcholine release from tissue slices but in the guinea-pig kappa-selective agonists were effective, and mu or delta agonists were not, whereas in the rat, mu-, but not delta- or kappa-selective drugs were effective. Opiates also altered acetylcholine release from the frontal, parietal and occipital cortex of both of these species. In all three regions of the guinea-pig cortex, kappa and delta agonists were active and in the parietal cortex mu agonists were also active; rat cortical slices showed similar results except that delta agonists were not effective. The inhibitory effects of the opiate agonists were effectively antagonized by the non-selective opiate antagonist naloxone and by the calcium channel agonists, BAY K 8644 or YC-170. In addition, the effects of the opiate drugs tested in this study on acetylcholine release were confined to evoked release, that is, spontaneous acetylcholine release was not affected. The results suggest that in guinea-pig and rat brain, opiate receptors regulate acetylcholine release, and that, although the subtypes of opiate receptors involved in this effect are different in the two species and in different tissues from the same species, the effect results from a common mechanism that involves alterations of calcium influx into the nerve terminals during depolarization.

Mu-, delta- and kappa-opioid receptor-mediated inhibition of neurotransmitter release and adenylate cyclase activity in rat brain slices: studies with fentanyl isothiocyanate

We investigated the effects of [D-Ala2,D-Leu5]enkephalin (DADLE). [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.01-1 microM) and bremazocine (0.001-0.3 microM) on the electrically evoked release of radiolabelled neurotransmitters and on the dopamine (DA)-stimulated cyclic AMP efflux from superfused rat brain slices. The differential inhibitory effects of these agonists on the evoked neurotransmitter release indicate that the opioid receptors mediating presynaptic inhibition of [3H]noradrenaline (NA, cortex), [14C]acetylcholine (ACh, striatum) and [3H]DA (striatum) release represent mu, delta and kappa receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the delta-opioid receptor-selective irreversible ligand, fentanyl isothiocyanate (FIT, 0.01-1 microM), antagonized the inhibitory effects of DADLE and DPDPE on striatal [14C]ACh release only. On the other hand, the D-1 DA receptor-stimulated cyclic AMP efflux from striatal slices appeared to be inhibited by activation of mu as well as of delta receptors. In this case, the reversible mu antagonist, naloxone (0.1 microM), fully antagonized the inhibitory effect of the mu agonist, DAGO, without changing the effect of the delta agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 microM). The inhibitory effect of DAGO on the electrically evoked [3H]NA release was antagonized by naloxone whether the receptors were irreversibly blocked by FIT or not. These data not only further support the existence of independent presynaptic mu-, delta- and kappa-opioid receptors in rat brain but also evidence strongly that mu and delta receptors mediating the inhibition of DA-sensitive adenylate cyclase could share a common binding site (for naloxone and FIT) and, therefore, may represent constituents of a functional opioid receptor complex.

Mu- and delta-opioid receptor binding peaks and kappa-homogeneity in the molecular layers of rat hippocampal formation

The sublaminar binding profiles of (D-Ala-NMe-Phe-Gly-ol)-enkephalin (DAGO), (2-D-penicillamine), 5-D-penicillamine)-enkephalin (DPDPE), and dynorphin A(1-8) (DYN) were studied in the CA1 subfield and dentate gyrus of rat hippocampal formation. Binding was assayed on cryomicrotome sections using coverslip autoradiographic and single grain counting techniques. DAGO, an agonist for mu-sites, had peak binding in the stratum pyramidale with a secondary peak in the distal part of the stratum radiatum. Binding of DAGO in the dentate gyrus was homogeneous. DPDPE, a delta-site agonist, also had peak binding in the stratum pyramidale, but there was no secondary peak in the molecular layer of the hippocampus. In the dentate gyrus, DPDPE binding was highest in the inner one-third of the molecular layer immediately adjacent to the granular cell layer. The endogenous opioid DYN had a laminar binding profile that mimicked that of DAGO. However, when tritiated DYN was coincubated with unlabeled DAGO and DPDPE, as much as 90% of DYN binding was blocked and remaining binding was homogenous though a small peak remained in the stratum pyramidale. The secondary peak of DAGO binding in the stratum radiatum corresponds to an area previously determined to contain processes immunoreactive for enkephalin and gamma aminobutyric acid. This correspondence suggests that opioid compounds may mediate disinhibition of the distal dendrites of hippocampal pyramidal neurons. In addition, DYN binding patterns indicate that its action in rat hippocampus is likely by both mu- and delta-receptors.

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.

Effect of ethylketocyclazocine on acetylcholine release in guinea-pig brain slices

The effect of ethylketocyclazocine (EKC) on 3H-Choline (Ch) efflux and on endogenous acetylcholine (ACh) release from guinea-pig brain slices was studied. The drug inhibited the 3H-Ch efflux at a low concentration (0.1 mumol/l) in thalamus slices, while only at high concentrations (30-100 mumol/l) did EKC induce deep inhibition in the caudate nucleus and slight reduction in the cerebral cortex. Dynorphin (1-13) and morphine (Mo) inhibited the ACh release from thalamus slices as well. Naloxone (Nx) was more effective in antagonizing Mo than EKC. The experiments carried out with the endogenous ACh bioassay technique confirmed the above results. Mr 2266 and Mr 1452, both proposed as preferential k antagonists, per se enhanced 3H-Ch efflux from thalamus slices, while the dextrorotatory isomer Mr 1453, devoid of opioid properties, did not share such action. The role of k-opioid receptors in cholinergic system modulation in the guinea-pig brain is discussed.

Evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus

Acetylcholinesterase staining and studies on the uptake of [3H]choline into the subsequent efflux of tritium from collicular slices were carried out in order to provide evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus. Acetylcholinesterase staining was dense and homogeneous in superficial layers whereas the staining was arranged in patches with slightly higher density caudally than rostrally in the intermediate layers. The accumulation of tritium in slices incubated with [3H]choline depended on time, temperature and concentration, and was inhibited by hemicholinium-3. Accumulation was slightly higher in caudal than in rostral slices. Electrical stimulation enhanced tritium outflow from slices preincubated with [3H]choline. Tetrodotoxin and a low calcium medium inhibited the evoked overflow whereas hemicholinium-3 caused an enhancement. Oxotremorine decreased the evoked overflow; atropine prevented this effect. The opioids [D-Ala2, MePhe4, Glycol5]enkephalin, [D-Ala2, D-Leu5]enkephalin and ethylketocyclazocine caused an inhibition. The effects of the latter two agonists were antagonized by naloxone. The GABAB-receptor-agonist (-)-baclofen decreased the evoked overflow at lower concentrations than GABA, whereas the GABAA-receptor-agonist muscimol was ineffective. Serotonin produced an inhibition which was prevented by metitepin, alpha- and beta-adrenoceptor as well as dopamine-receptor ligands caused no change. It is concluded that in the rabbit superior colliculus the pattern of acetylcholinesterase staining is comparable, but not identical to the distribution in other species. The accumulation of [3H]choline, as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (reflecting presumably release of [3H]acetylcholine) indicate that acetylcholine has a neurotransmitter function in this tissue. The release of [3H]acetylcholine was modulated by various transmitter substances and related compounds. The pattern of modulation of release differed from the pattern in other cholinergically innervated tissues.

Presynaptic opioid receptors in the portal vein of the rabbit

The effects of opioids on sympathetic neuroeffector transmission were studied in superfused strips of the portal vein of the rabbit. In the first series of experiments, the tissue was stimulated electrically every 10 min with 10 pulses at 8 Hz and a current strength of 200 mA. The contractions evoked were concentration dependently inhibited by the slightly kappa-selective agonist ethylketocyclazocine and by the delta-selective agonists [D-Ala2,D-Leu5]enkephalin (DADLE) and [D-Pen2,D-Pen5]enkephalin (DPDPLE). The mu-selective agonist [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) had no effect. The estimated EC50 values for ethylketocyclazocine and DADLE were 5.5 X 10(-8) and 2.9 X 10(-8) mol/l, respectively. Naloxone shifted the concentration-response curves of ethylketocyclazocine and DADLE to the right; the estimated Kb values were (1.7 +/- 0.4) X 10(-8) mol/l and (2.9 +/- 0.8) X 10(-8) mol/l, respectively. The delta-selective antagonist ICI 174864 antagonized only the inhibitory effect of DADLE but not that of ethylketocyclazocine. In the second series of experiments, the tissue was preincubated with [3H]noradrenaline then superfused and stimulated electrically three times for 3 min at 1 Hz and a current strength of 100 mA. Ethylketocyclazocine and DADLE inhibited the evoked overflow of tritium. The inhibition produced by ethylketocyclazocine was antagonized by naloxone. Our results show for the first time that opioid agonists inhibit noradrenaline release and hence, neurogenic vasoconstriction in a vein. As in other cardiovascular tissues of the rabbit, the presynaptic opioid receptors are of the kappa- and delta- but not the mu-type.