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

Opioid neurotoxicity: fentanyl-induced exacerbation of cerebral ischemia in rats

We tested the hypothesis that fentanyl would worsen ischemia-induced brain damage. In two sequential protocols forty rats were physiologically monitored and controlled. In protocol 1, rats were randomized (n=10/group) to 30 min of control (N2O plus 0.4% halothane), low dose fentanyl (loading dose [LD] 50 micrograms kg-1, maintenance dose [MD] 2 micrograms kg-1 min-1), or high-dose fentanyl (LD 800 micrograms kg-1, MD 32 micrograms kg-1 min-1). After 15 min of fentanyl or sham infusion trimethaphan 0.5 mg was given i.v. and 3 min later bilateral carotid artery occlusion and blood withdrawal-induced hypotension were maintained for 12 min. At 18 h postischemia rats underwent cerebral perfusion fixation. Brain areas were graded from 0 (normal) to 5. In addition to analysis of specific regions, neuropathologic scores were also summated over all brain regions and analyzed to compute a summed neuropathologic score. In protocol 2, five control and five high-dose fentanyl rats were treated identically except that post-ischemic oxygenation was maintained for 6 h and cerebral perfusion-fixation was performed 6 h post-ischemia. Only the caudate/putamen was examined in protocol 2. Fentanyl worsened lesions in both fentanyl groups' summed neuropathologic scores (P=0.002) in protocol 1 and specifically, in the caudate/putamen (P<0.01) in both protocols. Fentanyl in both high and low doses can exacerbate incomplete forebrain ischemia in rats.

Cellular sites for activation of delta-opioid receptors in the rat nucleus accumbens shell: relationship with Met5-enkephalin

The shell compartment of the nucleus accumbens (AcbSh) is prominently involved in the rewarding aspects of delta-opioid receptor (DOR) agonists, including one of its putative endogenous ligands, Met5-enkephalin (Enk). We examined the ultrastructural immunocytochemical localization of an antipeptide DOR antiserum and an antibody against Enk to determine the major cellular sites for DOR activation and the spatial relationship between DOR and Enk in this region. Sixty percent of DOR-immunoreactive profiles were axon terminals and small unmyelinated axons, whereas the remainder were mainly dendrites and dendritic spines. In axons and terminals, DOR labeling was distributed along plasma and vesicular membranes. DOR-containing terminals were mainly large and primarily formed symmetric synapses or occasionally asymmetric synapses. DOR immunoreactivity also was associated with terminals that were small and formed punctate symmetric or nonrecognizable synapses. Dual immunoperoxidase and immunogold labeling showed that 35% of DOR-labeled axons apposed other terminals that contained Enk. In addition, 25% of the DOR-labeled terminals contained Enk. Thirty-five percent of DOR labeling was observed within dendrites and dendritic spines. DOR-labeled spines showed intense immunoreactivity within asymmetric postsynaptic junctions, which were formed by terminals that lacked Enk immunoreactivity. DOR-labeled spines, however, were apposed to Enk-containing terminals in 13% of all associations between dually labeled profiles. These results provide ultrastructural evidence that activation of DOR in the AcbSh is primarily involved in modulating the presynaptic release of mainly inhibitory, but also excitatory, neurotransmitters. These data also suggest that DOR plays a role in determining the postsynaptic response to excitatory afferents.

Reversion of muscarinic autoreceptor agonist-induced acetylcholine decrease and learning impairment by dynorphin A (1-13), an endogenous kappa-opioid receptor agonist

1. We investigated whether carbachol, a muscarinic receptor agonist, induces learning and memory impairment, and if so, dynorphin A (1-13), an endogenous kappa-opioid receptor agonist, ameliorates the impairment of learning and memory induced by carbachol, by use of a step-through type passive avoidance task. 2. Carbachol induced a dose-related dual response. Carbachol (1.66 pmol per rat) administered directly into the hippocampus significantly shortened the step-through latency, while lower (0.166 pmol per rat) and higher (16.6 pmol per rat) doses of carbachol did not induce learning or memory impairment. 3. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) administered 5 min after carbachol injection significantly reversed carbachol-induced impairment of learning and memory. 4. Perfusion with carbachol (3 x 10(-4) M) significantly decreased acetylcholine release in the hippocampus during perfusion as determined by in vivo brain microdialysis. This decrease in acetylcholine release was suppressed by co-perfusion with a low dose of atropine (10(-7) M). 5. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) immediately before carbachol perfusion completely blocked this decrease in extracellular acetylcholine concentration induced by carbachol. 6. These antagonistic effects of dynorphin A (1-13) were abolished by treatment with norbinaltorphimine (5.44 nmol per rat, i.c.v.), a selective kappa-opioid receptor antagonist, 5 min before dynorphin A (1-13) treatment. 7. These results suggest that the neuropeptide dynorphin A (1-13) ameliorates the carbachol-induced impairment of learning and memory, accompanied by attenuation of the reductions in acetylcholine release which may be associated with dysfunction of presynaptic cholinergic neurones via kappa-opioid receptors.

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.

U-50,488H, a selective kappa opioid receptor agonist, ameliorates memory impairments induced by muscarinic autoreceptor agonist, carbachol in mice

We investigated whether carbachol, a muscarinic receptor agonist, induces learning and memory impairments, and U-50,488H, a selective kappa opioid receptor agonist, ameliorates the impairments of learning and memory using a step-down type passive avoidance task in mice. Carbachol induced a dose-related dual response. Carbachol (3 nmol/mouse, i.c.v.) significantly shortened the step-down latency, while lower (1 nmol) and higher (10 nmol) doses of carbachol did not induce learning and memory impairments. U-50,488H (0.64 micromol/kg, s.c.) significantly improved carbachol-induced impairments of learning and memory. These findings suggest that kappa opioid receptor agonists ameliorate learning and memory impairments which may associate with dysfunction of presynaptic cholinergic neurons.

Effects of Tyr-D-Arg-Phe-beta-Ala-NH2, a novel dermorphin analog, on elevated plus-maze learning and spontaneous alternation performance in mice

1. The effects of intracerebroventricular administration of Tyr-D-Arg-Phe-beta-Ala-NH2 (TAPA), a novel dermorphin analog, on plus-maze learning and spontaneous alternation performance were investigated in mice. 2. The pre- or posttraining or preretention administration of TAPA (0.3-3.0 ng) alone failed to affect transfer latency of plus-maze learning, whereas TAPA (3 ng) produced a significant decrease in percent alternation without affecting total arm entries. 3. beta-Funaltrexamine (5 micrograms) almost completely reversed the TAPA (3 ng)-induced decrease in percentage of alternation. 4. These results suggest that stimulation of mu-opioid receptors disrupts spontaneous alternation performance associated with spatial working memory.

mu-Opioid receptor activation decreases N-type Ca2+ current in magnocellular neurons of the rat basal forebrain

Opioid modulation of calcium currents was studied in acutely dissociated rat basal forebrain neurons using the whole cell patch-clamp recording technique. The mu-opioid receptor agonist DAGO reversibly suppressed high-voltage activated calcium currents and slowed their rate of activation, while neither delta- nor kappa-opioid receptor agonists were effective in modifying calcium current in these neurons. The inhibitory effect of DAGO on calcium current was abolished following irreversible blockade of N-type calcium channels by omega-conotoxin GVIA, whereas DAGO-induced inhibitory responses were not affected following blockade of L-type calcium channels by nifedipine. These findings indicate that mu-opioid receptors are negatively coupled to N-type calcium channels on the postsynaptic membrane of basal forebrain neurons. Calcium currents recorded from a significant number of large, mu-opioid sensitive neurons were also suppressed by muscarinic receptor activation, while smaller, mu-opioid sensitive neurons were not sensitive to muscarinic receptor activation. Thus, the present data demonstrate that voltage-activated calcium influx in several subpopulations of basal forebrain neurons can be regulated by mu-opioid receptor activation. These results suggest that mu-opioid regulation of calcium current may be an important functional mechanism in regulating neuronal excitability and synaptic transmission in the basal forebrain.

Evidence for direct and indirect mechanisms in the potent modulatory action of interleukin-2 on the release of acetylcholine in rat hippocampal slices

1. The biphasic nature of the potent modulatory action of interleukin-2 (IL-2) on hippocampal acetylcholine (ACh) release was investigated by use of brain slice superfusion. 2. Both the potentiating (10(-13) M) and inhibitory (10(-9) M) effects of IL-2 on hippocampal ACh release were stimulation-dependent and were blocked by a neutralizing IL-2 receptor antibody, suggesting the activation of typical IL-2 receptors in both cases. 3. Tetrodotoxin (TTX: 10 microM) failed to block the potentiation of ACh release induced by a very low concentration of IL-2 (10(-13) M) suggesting a direct effect on cholinergic nerve terminals. 4. In contrast, the inhibitory effect seen at a higher concentration (10(-9) M) was TTX-sensitive, and hence indicative of an indirect action. 5. To establish the nature of this intermediate mediator, blockers of nitric oxide synthesis, and of opioid and gamma-aminobutyric acid (GABA) receptors were used. Only GABAA and GABAB receptor antagonists altered the inhibitory action of IL-2, suggesting the participation of GABA as mediator. 6. Taken together, these results provide further evidence for the potent role of IL-2 in the modulation of cholinergic function in the rat hippocampus.

Effects of the kappa-opioid dynorphin A(1-13) on learning and memory in mice

The effects of intracerebroventricular administration of dynorphin A(1-13) on scopolamine- and pirenzepine-induced amnesia were investigated in mice by observing the step-down-type passive avoidance response and spontaneous alternation performance. The pre- or post-training, or preretention administration of dynorphin A(1-13) (0.3-10 micrograms) alone failed to affect the passive avoidance response, while scopolamine (1 mg/kg) significantly inhibited it. Dynorphin A(1-13) (1 microgram) given 15 min before training and retention tests, but not immediately after training, significantly improved the scopolamine (1 mg/kg)-induced impairment of passive avoidance response, indicating the anti-amnesic effects of dynorphin A(1-13). A lower dose (1 mg/kg) of the kappa-opioid receptor antagonist (-)-(1R,5R,9R)-5,9-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomorpha n reversed the anti-amnesic effects of dynorphin A(1-13) (1 microgram). In contrast, although dynorphin A(1-13) (1, 3 and 10 micrograms) did not influence spontaneous alternation performance, scopolamine (1 mg/kg) and the muscarinic M1 receptor antagonist pirenzepine (3 micrograms) markedly decreased spontaneous alternation performance. Dynorphin A(1-13) (3, 5.6 and/or 10 micrograms) significantly improved the scopolamine (1 mg/kg)- and pirenzepine (3 micrograms)-induced impairment of spontaneous alternation performance. The improving effects of dynorphin A(1-13) (3 micrograms) were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms), a kappa-selective opioid receptor antagonist. These results suggest that the stimulation of kappa-opioid receptors improves memory dysfunctions resulting from the blockade of muscarinic M1 receptors.

The neurochemistry of morphine addiction in the neocortex

Different strategies have been used in an attempt to understand the neurobiology of opioid addiction. Here, Michéle Simonato initially discusses the identification of key anatomical areas involved in the phenomenon and purposes an explanation of opioid addiction based on the theory of complexity. The variable importance of direct and indirect effects in phenotypically different neuronal populations can imply differences in the adaptive changes that occur with chronic morphine exposure. Opioid addiction is therefore proposed as a complex multicellular event, where individual neurones differentially adapt both on the basis of the signals they receive and of their second messengers and genetic programmes.

mu- and kappa-opioids inhibit K+ evoked glutamate release from rat cerebrocortical slices

We have examined the effects of a range of opioid receptor subtype selective agonists on K+ evoked glutamate release from perfused rat cerebrocortical slices. Dual application (S1 and S2) of K+ (46 mM) evoked dual monophasic glutamate release profiles. When areas under the release curves were calculated an S2/S1 ratio for control slices of 1.07 +/- 0.08 (n = 75) was obtained, this was reduced by 80% with EGTA (0.1 mM) treatment confirming the presence of a Ca2+ regulated release process, Morphine produced a dose-dependent inhibition of the S2/S1 ratio. At 1 microM this amounted to 78 +/- 12% (mean +/- SEM; n = 6). (D-Ala2,MePhe4,gly(ol)5)enkephalin (DAMGO; 60 +/- 12%, n = 6 at 1 microM), and spiradoline (53 +/- 14% at 1 and 71 +/- 11% at 100 microM, both n = 6) also inhibited glutamate release in a cyprodime (10 microM) and norbinaltorphimine (10 microM) reversible manner. (D-Pen2.5) enkephalin (DPDPE; 1 microM) was ineffective. All agents tested did not affect basal glutamate release. Collectively these data implicate a role for mu and kappa opioids in the control of evoked glutamate release and their potential for neuroprotective therapy.

Perinatal exposure to methadone affects central cholinergic activity in the weanling rat

Pregnant rats were implanted with osmotic minipumps containing either methadone hydrochloride (initial dose, 9 mg/kg/day) or sterile water. Their offspring were cross-fostered so that they were exposed to methadone prenatally and/or postnatally. Perinatal methadone exposure disrupted cholinergic activity on postnatal day 21 as measured by the turnover rate of acetylcholine (TRACh) in both female and male rats, although there were some sexually-dimorphic responses. The most profoundly affected brain region was the striatum, where prenatal exposure to methadone increased ACh turnover, whether or not the rats continued to be exposed to methadone postnatally. It appears unlikely that neonatal withdrawal contributes to brain regional changes in ACh turnover, as continued postnatal exposure to methadone did not prevent the prenatal methadone induced changes.

Hippocampal acetylcholine release during memory testing in rats: augmentation by glucose

Several lines of evidence indicate that a modest increase in circulating glucose levels enhances memory. One mechanism underlying glucose effects on memory may be an increase in acetylcholine (ACh) release. The present experiment determined whether enhancement of spontaneous alternation performance by systemic glucose treatment is related to an increase in hippocampal ACh output. Samples of extracellular ACh were assessed at 12-min intervals using in vivo microdialysis with HPLC-EC. Twenty-four minutes after an intraperitoneal injection of saline or glucose (100, 250, or 1000 mg/kg), rats were tested in a four-arm cross maze for spontaneous alternation behavior combined with microdialysis collection. Glucose at 250 mg/kg, but not 100 or 1000 mg/kg, produced an increase in spontaneous alternation scores (69.5%) and ACh output (121.5% versus baseline) compared to alternation scores (44.7%) and ACh output (58.9% versus baseline) of saline controls. The glucose-induced increase in alternation scores and ACh output was not secondary to changes in locomotor activity. Saline and glucose (100-1000 mg/kg) treatment had no effect on hippocampal ACh output when rats remained in the holding chamber. These findings suggest that glucose may enhance memory by directly or indirectly increasing the release of ACh. The results also indicate that hippocampal ACh release is increased in rats performing a spatial task. Moreover, because glucose enhanced ACh output only during behavioral testing, circulating glucose may modulate ACh release only under conditions in which cholinergic cells are activated.

Improvement by dynorphin A (1-13) of galanin-induced impairment of memory accompanied by blockade of reductions in acetylcholine release in rats

1. Human galanin (0.32 nmol per rat, i.c.v.), an endogenous neuropeptide, administered 30 min before acquisition or retention trials, significantly impaired the acquisition of learning and recall of memory in a step-through type passive avoidance performance. 2. The role of dynorphin A (1-13) in learning and memory is controversial. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) administered 5 min before galanin injection, completely antagonized these impairments. 3. Galanin significantly decreased acetylcholine release in the hippocampus 40 to 120 min after injection as determined by in vivo brain microdialysis. This peptide also decreased acetylcholine release, albeit to a lesser extent, from the frontal cortex. 4. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) 5 min before galanin injection, completely blocked the decrease in extracellular acetylcholine concentration induced by galanin. 5. These antagonistic effects of dynorphin A (1-13) were abolished by treatment with norbinaltorphimine (5.44 nmol per rat, i.c.v.), a selective kappa-opioid receptor antagonist, 5 min before dynorphin A (1-13). 6. Dynorphin A (1-13) (0.5 nmol) itself had no effect on learning and memory and on the acetylcholine concentration in the hippocampus or the frontal cortex in normal rats. 7. These results suggest that the neuropeptide dynorphin A (1-13) ameliorates the galanin-induced impairment of learning and memory accompanied by abolition of reductions in acetylcholine release via kappa-opioid receptors.

Intraseptal microinjection of beta-funaltrexamine blocked a microwave-induced decrease of hippocampal cholinergic activity in the rat

Acute (45 min) exposure to pulsed (2 microseconds pulse width, 500 pulses per second) 2450-MHz microwaves at a power density of 1 mW/cm2 (whole body specific absorption rate 0.6 W/kg) microwaves caused a decrease in cholinergic activity in the hippocampus of the rat as measured by the sodium-dependent high-affinity choline uptake. Microinjection of beta-funaltrexamine (1 microgram) into the septum before microwave exposure blocked this effect. These data indicate that mu-opioid receptors in the septum mediate a microwave-induced decrease in cholinergic activity in the hippocampus and support our hypothesis that microwaves at a whole body SAR of 0.6 W/kg can activate endogenous opioids in the brain.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Attenuation of memory with Tyr-D-Arg-Phe-beta-Ala-NH2, a novel dermorphin analog with high affinity for mu-opioid receptors

The involvement of mu-opioid receptors in memory retrieval was examined in mice by using Tyr-D-Arg-Phe-beta-Ala-NH2 (TAPA), a novel dermorphin analog with high affinity for mu-opioid receptors, and passive avoidance learning. TAPA was intracerebroventricularly administered to mice before retention tests of passive avoidance learning. A 0.3-ng dose of TAPA markedly shortened step-down latency of passive avoidance learning, and the shortening of step-down latency was reversed by treatment with beta-funaltrexamine (5 micrograms), a mu-opioid receptor antagonist, indicating that TAPA (0.3 ng) attenuates memory retrieval. Although the attenuating dose (0.3 ng) of TAPA failed to affect horizontal or vertical locomotor activity, a 3-ng dose of TAPA showed a tendency to decrease vertical locomotor activity. A 30-ng dose of TAPA produced a significant increase in horizontal locomotor activity accompanied by a marked reduction of vertical locomotor activity. TAPA (3 ng) produced a significant increase in step-down latency of passive avoidance learning with lower intensity of electroshock or without electroshock during training. These results suggest that the activation of mu-opioid receptors impairs memory retrieval.

Depression of LTP in rat dentate gyrus by naloxone is reversed by GABAA blockade

Long-term potentiation (LTP) of the lateral perforant path (LPP) to dentate granule cell (DGC) synapse is suppressed by the opioid antagonist, naloxone, and thus appears to be dependent upon the release of endogenous opioids from the LPP. It has been suggested that endogenous opioids enhance LTP by depressing GABAA inhibition. As one test of this hypothesis, we determined whether blockade of GABAA inhibition would alleviate the naloxone block of LTP in the LPP. Consistent with the hypothesis that endogenous opioids enable LTP by disinhibition of the DGCs, naloxone no longer blocked LTP in the presence of the GABAA antagonist, bicuculline methiodide. Furthermore, although blockade of mu receptors suppressed LTP of the slope of the population excitatory potential (pEPSP), blockade of both mu and delta opioid receptors was needed to suppress LTP of both the pEPSP and the orthodromic population spike (OPS).

kappa-Opioid receptor agonists improve pirenzepine-induced disturbance of spontaneous alternation performance in the mouse

We investigated the effects of kappa-opioid receptor agonists such as dynorphin A-(1-13) and U-50,488H on the muscarinic M1-selective receptor antagonist pirenzepine (3 micrograms, i.c.v.)-induced impairment of spontaneous alternation performance in the mouse. Although dynorphin A-(1-13)(1-5.6 micrograms, i.c.v.) or U-50,488H ((+/-)trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide, methanesulfonate hydrate) (0.1-1 mg/kg, i.p.) alone did not influence either spontaneous alternation performance or total arm entries, pirenzepine (3 micrograms, i.c.v.) impaired spontaneous alternation performance without producing any significant change in total arm entries. In contrast, dynorphin A-(1-13) (3 and 5.6 micrograms, i.c.v.) and U-50,488H (0.3 and 1 mg/kg, i.p.) ameliorated the pirenzepine (3 micrograms, i.c.v.)-induced impairment of spontaneous alternation performance. The ameliorating effects of dynorphin A-(1-13)(3 micrograms, i.c.v.) and U-50,488H (0.3 mg/kg, i.p.) were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-opioid receptor antagonist. These results suggest that the stimulation of kappa-opioid receptors improves memory dysfunctions resulting from the blockade of muscarinic M1 receptors.

Enhancement of retinal acetylcholine release by DAMGO: possibly a direct opioid receptor-mediated excitatory effect

1. An eye-cup preparation in anaesthetized rabbits was used to examine opioid modulation of acetylcholine (ACh) release from cholinergic neurones in the retina. 2. The mu-opioid receptor agonist, [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAMGO), when applied locally to the retina at concentrations between 1-30 microM significantly increased the light-evoked release of ACh. The effect of DAMGO was completely blocked by the selective mu-receptor antagonist CTOP but the kappa-receptor antagonist nor-binaltorphimine (norBNI) did not affect the action of DAMGO on ACh release indicating that the opioid produced its effect by activation of mu-receptors (the rabbit retina has negligible delta-receptors). 3. Blockade with bicuculline and strychnine of GABAergic and glycinergic inputs to the cholinergic neurones did not affect the action of DAMGO on ACh release. Also DAMGO did not reduce the potassium-evoked release of either GABA or glycine from rat isolated retinas. 4. Exposure of the rabbit retina to a combination of an A1-adenosine receptor antagonist, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), and adenosine deaminase did not affect the enhancing action of DAMGO on the light-evoked release of ACh. 5. When the retina in the rabbit eye-cup was exposed to kainate, the release of ACh was increased by approximately three times the resting release. In the presence of DAMGO the kainate-evoked release of ACh was enhanced by 44%. 6. These experiments show that activation of mu-opioid receptors by DAMGO increases the release of ACh elicited by physiological stimulation (flickering light). Since we could find no evidence thatDAMGO reduces inhibitory inputs to the cholinergic neurones, it seems that the enhancing action ofDAMGO on the light-evoked release of ACh involves a direct excitatory effect rather than disinhibition.This conclusion is supported by the enhancing action of DAMGO on the kainate-evoked release of ACh because kainate is thought to act directly on the cholinergic neurones.

Gut-derived proteolysis during insulin-induced hypoglycemia: the pain that breaks down the gut

The metabolic events associated with early response to injury have received little attention because of the confounding effects of the hemodynamic alterations that normally occur during this early phase. We have used a well established and reproducible model of insulin-induced hypoglycemia in the conscious dog to define the glucose and amino acid kinetic alterations as well as the hormonal and interorgan amino acid and gluconeogenic precursor flux characteristics of the "ebb" phase of postinjury metabolism. The results from our whole-body response have demonstrated on enhanced rate of whole body proteolysis and amino acid oxidation. The site of the majority of the proteolytic response has been demonstrated to be the extra-hepatic splanchnic tissues or gut. These findings have been supported by studies focusing on the specific organ changes, which have demonstrated alterations compatible with impaired proliferation at the level of the gut mucosa. Furthermore, the regulation of this gut-derived proteolysis has been demonstrated to be mediated by the glucopenia at the level of the central nervous system. The specific site of this response is still elusive; however, the mediators seem to involve not only the traditional hormonal and neurotransmitter pathways but also the release of endogenous opioids and opiates. Although a cause-effect relationship has not yet been demonstrated for the control of gut-derived proteolysis by opioids and opiates, we present evidence that leads us to hypothesize that relationship as a possible regulatory mechanism.

Alterations in striatal acetylcholine overflow by cocaine, morphine, and MK-801: relationship to locomotor output

The activity of cholinergic interneurons in the striatum appears to be modulated by a variety of different systems including dopamine, opiate, and glutamate. The purpose of this study was to characterize the effects of drugs known to act on these three systems (i.e., cocaine, morphine, and MK-801) on striatal ACh overflow with microdialysis procedures, and to determine if alterations in ACh function induced by these agents are related to changes in locomotor activity. Cocaine was found to increase striatal ACh following intraperitoneal injections of 20 and 40 mg/kg, but not 10 mg/kg. The increases in locomotor activity induced by cocaine appeared to be dose dependent, while the effects on striatal ACh were not. Injections of 0.1 mg/kg MK-801 (a non-competitive NMDA receptor antagonist) produced dramatic increases in locomotor activity while decreasing striatal ACh overflow. A lower dose (0.03 mg/kg) of MK-801 failed to alter locomotor activity or striatal ACh. Morphine produced an apparent dose-dependent elevation in striatal ACh while only the lowest dose (5 mg/kg) increased locomotor activity. There appears to be no relationship between alterations in striatal ACh and locomotor output following systemic administration of these psychoactive agents.

Functional expression of Ca(2+)-mobilizing opioid receptors in Xenopus oocytes injected with rat brain mRNA

Functional expression of opioid receptors was detected in the Xenopus oocyte translation system by a voltage-clamp recording. After injection of poly(A)+ RNA isolated from 3-week-old rat striatum or whole brain, the oocytes often demonstrated intracellular Ca(2+)-mediated oscillatory responsiveness to [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMGO), [D-Pen2, D-Pen5]enkephalin (DPDPE) and U50488H at a concentration of 1 microM. These responses were very transiently expressed after injection of the mRNA, however, water-injected oocytes never responded to any of these opioid agonists. After fractionation by a sucrose-density gradient, an RNA size of about 3-4 kb encoded these opioid receptors. In the oocytes injected with size-selected striatal mRNA, DPDPE evoked the fluctuating current with higher probability and larger amplitude than other agonists, whereas oocytes injected with size-selected whole brain mRNA produced DAMGO and U50488H responses predominantly. The DPDPE response of striatal mRNA-injected oocytes was antagonized by naloxone as well as the delta-specific antagonist ICI 174864. The DAMGO and U50488H responses have not been characterized yet because of a strong desensitizing property making repeated recordings impossible. These observations suggest that putative mu, delta and kappa subtypes of opioid receptors mobilizing intracellular Ca2+ are expressed in Xenopus oocytes by rat brain mRNA.

Different responses to opioids measured in terminals and somas of Edinger-Westphal neurons

Neurotransmitter receptors on the axon terminals of a neuron can be located a considerable distance away from comparable receptors on the cell body or dendrites of the same neuron. We examined the effects of activating either nerve terminal receptors or those located on or near the somas of chick Edinger-Westphal neurons. Cell body responses were measured via intracellular recording in a brain slice preparation. To measure nerve terminal responses, intracellular recordings were obtained from the large, calyciform nerve endings in intact ciliary ganglia, which emanate from neurons of the lateral Edinger-Westphal nucleus. Cell bodies of Edinger-Westphal neurons responded to leucine-enkephalin with a dose-dependent hyperpolarization that was associated with a decrease in input resistance. In spontaneously active Edinger-Westphal somas, leucine-enkephalin caused marked inhibition of suprathreshold and subthreshold activity, indicating that, as with a number of other central neurons, the major effect of opioids was to reduce excitability. The response to opioids was sensitive to naloxone (1 microM) and was a direct effect, since it was not blocked by either 0.5 microM tetrodotoxin or 100 microM cadmium. More selective mu ([D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin) and delta ([D-Ser2]-leucine-enkephalin-Thr and [D-Pen2,5]-enkephalin) opioid agonists produced effects similar to those of leucine-enkephalin. Opioids produced strikingly different effects in the nerve terminals of Edinger-Westphal neurons, where the major effect was a depolarization associated with a decrease in input resistance. The effects of opioids in the terminals were reduced in a low sodium buffer, indicating that they were dependent on the presence of extracellular sodium.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of morphine and its metabolites on opiate receptor binding, cAMP formation and [3H]noradrenaline release from SH-SY5Y cells

Opiate receptor occupation leads to a variety of intracellular events including inhibition of adenylyl cyclase and cAMP formation. We have examined the opiate binding characteristics, effects on cAMP formation and [3H]noradrenaline release of morphine, morphine-6 (M6G) and -3 (M3G)-glucuronides, and fentanyl in SH-SY5Y human neuroblastoma cells. M6G and M3G are the major metabolites of morphine formed in vivo whose cellular action remains to be fully elucidated. In binding experiments morphine (affinity, K50 = 96 nM) and fentanyl (K50 = 99 nM) were more potent than M6G (K50 = 393 nM), while M3G was inactive. However, for cAMP inhibition morphine (half maximum inhibition, IC50 = 193 nM) and M6G (IC50 = 113 nM) were roughly equipotent, with fentanyl (IC50 = 27 nM) being more potent and producing a greater maximum inhibition (56%). M3G was inactive. These in vitro data are in general agreement with the in vivo effects of these glucuronides. Moreover, all of the opiates tested failed to inhibit K(+)-evoked release of [3H]noradrenaline. Whilst these data do not support a role for cAMP in neurotransmitter release, alterations in cAMP formation may still have a role to play in the mechanism of analgesia.

Calcium channel modulators modify K opioid-induced inhibition of C-fiber-evoked spinal reflexes in rat

The role of L-type Ca2+ channels on the kappa opioid-induced depression of spinal afferent transmission was assessed in spinalized rats, through recording of the C-fiber-evoked spinal flexor reflex. Six successive i.t. doses of the K agonist U-50,488H produced a dose-dependent decrease of the C-reflex duration (ID50: 25.7 nmol), the log dose-response relationship being shifted to left by pretreatment with 5 mg/kg i.v. of the calcium channel blocker verapamil, or to right by pretreatment with .25 mg/kg i.v. of the calcium channel agonist Bay K8644. Verapamil and Bay K8644, administered i.v. after U-50,488H i.t., respectively potentiated or antagonized the depressor effect of the K ligand on the reflex. The results point to a role for Ca2+ availability as a factor involved in depression of afferent nociceptive transmission by K opioids at the spinal cord.

Differential alterations in adenosine A1 and kappa 1 opioid receptors in the striatum in Alzheimer's disease

The alterations in Alzheimer's disease (AD) of two binding sites in the striatum suggested to have a presynaptic localisation have been investigated by quantitative ligand binding autoradiography. Adenosine A1 binding sites labelled with [3H]cyclohexyladenosine (CHA) and kappa 1 opioid binding sites labelled with [3H]U-69593 were studied in adjacent sections of the striatum obtained postmortem from 10 patients with AD and 9 matched controls. In AD, there was a significant reduction of [3H]CHA binding sites in the caudate nucleus (control = 88 +/- 4; AD = 56 +/- 6 pmol/g tissue; mean +/- S.E.M.) and putamen (control = 83 +/- 4; AD = 58 +/- 7 pmol/g). In control subjects, highest levels of [3H]U-69593 binding were localised to patches within the caudate nucleus (9.66 +/- 0.58 pmol/g) with lower levels in the matrix (5.54 +/- 0.48 pmol/g). There was no alteration in [3H]U-69593 binding sites in either the caudate nucleus (patches and matrix) or putamen of AD patients. The activity of choline acetyltransferase (ChAT), determined in the same tissue samples used for autoradiographic analysis, was significantly reduced in AD (control = 124 +/- 11; AD = 64 +/- 14 nmol/h/mg protein). There was a positive correlation between ChAT activity and [3H]CHA binding (r = 0.769), but not [3H]U-69593 binding (r = 0.197). The results indicate that a marked loss of adenosine A1 receptors occurs in the striatum of AD with no loss of kappa 1 opioid receptors, and that the loss of A1 receptors parallels the loss of choline acetyltransferase activity.

The actions of the kappa 1 opioid agonist U-50,488 on presynaptic nerve terminals of the chick ciliary ganglion

The actions of the kappa 1 opioid receptor agonist U-50,488 (trans-(+-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benz ene - acetamide methane sulfonate) on the membrane properties of presynaptic calyciform nerve terminals of the chick ciliary ganglion were examined using intracellular recordings obtained from intact ganglion preparations maintained in vitro. U-50,488 produced a concentration-dependent (30-1000 microM) hyperpolarization with an apparent increase in input resistance. This hyperpolarization resulted from inhibition of the Na(+)-K+ inward rectifier, since it was blocked by 3 mM Cs+ and was not observed when terminals were depolarized beyond resting potential where inward rectification was voltage inactivated. A depolarizing effect on membrane potential with a further rise in input resistance was commonly observed at the highest perfused U-50,488 concentration (1 mM). The depolarizing event appears to result from a decrease in membrane potassium conductance, as the reversal potential for the response was estimated to be between -70 and -90 mV and the potassium channel blocker Ba2+ (1 mM) abolished the response. The kappa 1 opioid receptor agonist also blocked spontaneously occurring miniature hyperpolarizations in the terminals, which are considered to be due to a Ca(2+)-dependent K+ conductance. Most of the responses to U-50,488 were abolished in the presence of the kappa 1 receptor antagonist norbinaltorphimine. In conclusion, the excitability of presynaptic nerve terminals in the chick ciliary ganglion can be modulated by the inhibition of at least three separate ion conductances following activation of kappa 1 opioid receptor sites in the nerve terminal region.

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

Morphine and the Endogenous Opioid Dynorphin in the Brain Attenuate Digoxin-Induced Arrhythmias in Guinea Pigs

The effects of the opioid receptor agonists morphine and dynorphin on digoxin-induced arrhythmias were examined in guinea pigs that had received intravenous digoxin (50 mu/kg bolus plus 500 mu/kg/hr intravenously). Animals received either morphine (50 or 100 micrograms/kg) or dynorphin A(1-13) (50 or 100 micrograms/kg) or saline (the diluent) into the lateral cerebral ventricle (intracerebroventricularly) prior to digoxin. Morphine and dynorphin produced significant (P < 0.05) dose-dependent increases in the threshold of digoxin-induced arrhythmias. The mean digoxin dose at the development of fatal arrhythmias was 775 +/- 42 micrograms/kg in the control group but was significantly higher namely 958 +/- 45 micrograms/kg after 100 micrograms/kg of morphine ICV, and 984 +/- 47 micrograms/kg after 100 micrograms/kg of dynorphin A (1-13) intracerebroventricularly. In the absence of digoxin, the highest doses of each of these opioids did not produce arrhythmias. Changes in blood pressure and heart rate were unlikely explanations for the observed actions of these opioids as morphine accentuated the increase in blood pressure that accompanied digoxin while dynorphin was associated with a lower blood pressure with digoxin, despite similar effects on arrhythmias. In the control group, fatal digoxin-induced arrhythmias were ventricular tachyarrhythmias in two-thirds of cases and complete heart block in the rest. Morphine and dynorphin reduced the development of ventricular tachyarrhythmias. The role of the cholinergic system was explored, with morphine, utilizing atropine sulfate which crosses the blood brain barrier and atropine methylnitrate which does not enter the CNS. Atropine sulfate but not atropine methylnitrate reversed the effects of morphine on digoxin-induced arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of selective opiate antagonists on striatal acetylcholine and dopamine release

We investigated the effect of selective opiate antagonists on striatal acetylcholine (ACh) and dopamine (DA) release. The mu-receptor antagonist beta-funaltrexamine (beta-FNA), the delta-antagonist naltrindole (NTI), and the kappa-antagonist norbinaltorphimine (nor-BNI) were used to selectively block different subtypes of opiate receptors. The experiments were carried out on isolated superfused striatal slices of rats, loaded with [3H]choline or [3H]dopamine. beta-FNA and NTI significantly enhanced the electrical field stimulation-evoked release of ACh but only if the dopaminergic input had been impaired either by chemical denervation or D2 dopamine receptor blockade. By contrast, neither the selective nor nonselective antagonists had any modulatory effect on the release of dopamine. It is concluded, therefore, that the release of ACh is tonically controlled by endogenous opioid peptide(s) through the stimulation of mu- and delta-opiate receptors located on cholinergic axon terminals, in addition to the tonic control by DA.

Opioid receptor subtypes mediating the noise-induced decreases in high-affinity choline uptake in the rat brain

Acute (20 min) exposure to 100-dB white noise elicits a naltrexone-sensitive decrease in sodium-dependent high-affinity choline uptake in the frontal cortex and hippocampus of the rat. In the present study, the subtypes of opioid receptors involved were investigated by pretreating rats with microinjection of specific opioid-receptor antagonists into the lateral cerebroventricle before noise exposure. We found that the noise-induced decrease in high-affinity choline uptake in the hippocampus was blocked by pretreatment with either mu-, delta-, or kappa-opioid-receptor antagonists, whereas the effect of noise on frontal cortical high-affinity choline uptake was blocked by a mu- and delta- but not by a kappa-antagonist. These data further confirm the role of endogenous opioids in mediating the effects of noise on central cholinergic activity and indicate that different neural mechanisms are involved in the effects of noise on the frontal cortical and hippocampal cholinergic systems.

Lesioning of the nucleus basalis of Meynert has differential effects on mu, delta and kappa opioid receptor binding in rat brain: a quantitative autoradiographic study

Opioid receptor binding was investigated in rat brain following lesioning of the nucleus basalis of Meynert (nbM). The nbM, which provides cholinergic input to the cortex, was lesioned unilaterally using ibotenic acid. The efficacy of lesioning was confirmed by the observation of a significant decrease in choline acetyltransferase (ChAT) activity in the ipsilateral prefrontal cortex. The specific laminar and regional distribution of mu, delta and kappa opioid receptor binding was quantitated in various cortical and limbic structures in the rat using autoradiography. Distinct medial to lateral gradients of mu and kappa opioid binding were observed in regions of the cerebral cortex. In the lesioned hemisphere the levels of mu, delta and kappa opioid binding were altered in localized areas of the cerebral cortex and the hippocampus. The direction of these binding changes varied with the opioid receptor type being assessed. Delta opioid binding was increased in the lateral portions of the frontal, occipital, perirhinal and retrosplenial granular cortices. Kappa opioid binding was increased in the lateral portion of the occipital cortex and in the CA3 region of the hippocampus. In contrast, mu opioid binding was decreased in the lateral portions of the frontal, entorhinal and forelimb cortices. These opioid receptor changes are discussed with respect to the interactions between the cholinergic and opioid systems, and relevance of the nbM lesion model to Alzheimer's disease.

Synaptic localization of kappa opioid receptors in guinea pig neostriatum

Distribution of kappa opioid receptors was examined by EM radioautography in sections of guinea pig neostriatum with the selective 125I-labeled dynorphin analog [D-Pro10]dynorphin-(1-11). Most specifically labeled binding sites were found by probability circle analysis to be associated with neuronal membrane appositions. Because of limitations in resolution of the method, the radioactive sources could not be ascribed directly to either one of the apposed plasma membranes. Nevertheless, three lines of evidence favored a predominant association of ligand with dendrites of intrinsic striatal neurons: (i) the high frequency with which labeled interfaces implicated a dendrite, (ii) the enrichment of dendro-dendritic interfaces, and (iii) the occurrence of dendritic profiles labeled at several contact points along their plasma membranes. A small proportion of labeled sites was associated with axo-axonic interfaces, which may subserve the kappa opioid-induced regulation of presynaptic dopamine and acetylcholine release documented in guinea pig neostriatum. Although most membrane-associated kappa sites were found at extrasynaptic locations, approximately 23% were associated with synaptic specializations. This proportion is markedly higher than that previously reported for either mu or delta sites in rat neostriatum. Whether labeled synapses represent preferential sites of action for kappa ligands remains to be established. In any event, these results support the hypothesis that in mammalian brain kappa opioid receptors are conformationally and functionally distinct from mu and delta types.

Opioid receptor subtypes that mediate a microwave-induced decrease in central cholinergic activity in the rat

We performed experiments to investigate subtypes of opioid receptors in the brain involved in the effect of acute (45 min) pulsed microwave exposure (2,450-MHz, 2-microseconds pulses, 500 pps, average power density 1 mW/cm2, peak-power density, 1 W/cm2, average whole body SAR 0.6 W/kg) on cholinergic activity in the rat brain. Rats were pretreated by microinjection of specific antagonists of mu, delta, and kappa opioid-receptors into the lateral cerebroventricle before exposure to microwaves. The data showed that all three subtypes of opioid receptors are involved in the microwave-induced decrease in cholinergic activity in the hippocampus. However, the microwave-induced decrease in cholinergic activity in the frontal cortex was not significantly affected by any of the drug treatments, confirming our previous conclusion that the effect of microwaves on the frontal cortex is not mediated by endogenous opioids.

Microdialysis evidence that acetylcholine in the nucleus accumbens is involved in morphine withdrawal and its treatment with clonidine

This study used microdialysis to measure changes in extracellular acetylcholine (ACh) content in the nucleus accumbens (NAC) of freely moving rats during acute and chronic morphine treatment, and following naloxone-precipitated withdrawal. Morphine injection (20 mg/kg, i.p.) caused a significant decrease in extracellular ACh which was not apparent after repeated exposure to the opiate for 7 days. Basal recovery of ACh was not altered by chronic morphine treatment. On day 8, after morphine dependence had been established, naloxone caused a large increase in ACh levels accompanied by withdrawal symptoms such as wet dog shakes, diarrhea and teeth-chattering. Pretreatment with clonidine (200 micrograms/kg, i.p.) reduced these withdrawal symptoms and eliminated the ACh response. These results suggest that accumbens ACh is involved in some of the aversive aspects of opiate withdrawal.

Systemic morphine simultaneously decreases extracellular acetylcholine and increases dopamine in the nucleus accumbens of freely moving rats

Microdialysis was used to measure extracellular levels of acetylcholine (ACh) and dopamine (DA) simultaneously in the nucleus accumbens (NAC) of freely moving rats. Systemic injection of morphine (20 mg/kg) significantly decreased ACh (30%, p less than .01) while it increased DA (55%, p less than .01). The effects of morphine were eliminated by naloxone. The results confirm that morphine increases DA and in addition, demonstrate an inhibitory influence of this opiate on extracellular levels of ACh in the NAC.

Naloxone enhances the release of acetylcholine from cholinergic interneurons of the striatum if the dopaminergic input is impaired

Naloxone significantly enhanced the release of radioactive acetylcholine ([3H]ACh) from rat striatal slices loaded with [3H]choline either when the nigrostriatal pathway had been destroyed by 6-hydroxydopamine or when the D2 dopamine receptors had been inhibited by sulpiride. This in vitro study supplies the first neurochemical evidence, that, in addition to D2-receptor-mediated dopaminergic tonic control, there is opiate-receptor mediated presynaptic modulation of striatal ACh release, possibly by endogenous enkephalin released from local neurons. Such modulation occurs under conditions in which the dopaminergic input is impaired.

Inhibition of carbachol-stimulated phosphoinositide turnover by U-50,488H in rat hippocampus--involvement of GTP-binding protein

The effect of U-50,488H, a selective kappa-opioid agonist, on carbachol-stimulated phosphoinositide (PI) turnover response in rat hippocampal slices was examined. U-50,488H which stimulates PI turnover response in this preparation (Periyasamy and Hoss, 1990, Life Sci. 47, 219), inhibited carbachol-stimulated PI turnover in a concentration-dependent manner with an IC50 value of 33 +/- 9.0 microM. The inhibitory effect of U-50,488H was not blocked by the kappa-selective antagonists, e.g., nor-binaltorphimine (10 microM), and MR2266 (10 microM), or tetrodotoxin (1 microM) suggesting that the effect of U-50,488H was mediated neither through the kappa-receptors nor through the release of an endogenous neurotransmitter(s). A Lineweaver-Burke plot of the stimulation of PI turnover by carbachol in the presence and absence of U-50,488H showed that the Km was not changed (11.4 +/- 3.4 and 11.5 +/- 2.6 microM) whereas the Vmax was reduced from 3849 +/- 460 to 1534 +/- 31 cpm indicating that the inhibition was non-competitive. U-50,488H also inhibited guanosine 5'-[beta, gamma-imido]triphosphate (Gpp[NH]p)-stimulated PI turnover in rat hippocampal membranes in a concentration-dependent manner with an IC50 value of 33 +/- 12 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic opioid treatment attenuates carbachol-mediated polyphosphoinositide hydrolysis in chick embryo neuronal cultures

Opiate binding sites on cultured neurons derived from 6-day-old (E6) chick embryo cerebral hemispheres (CH), shown to be cholinergic by choline acetyltransferase immunostaining, were labeled with [3H]etorphine (mu and delta opiate receptors expression) and [3H]morphine (mostly mu). When examined by light microscope autoradiography, opiate receptors were found to be expressed by most neurons, and were distributed predominantly on neuronal perikarya. Muscarinic and opiate receptors in E6CH cultured neurons were found to be functionally coupled when the effects of opiate receptor occupancy on the inositol phosphate-linked muscarinic receptors was studied. Carbachol stimulated the release of [3H]inositol phosphates (InsP) from cultures preincubated with [3H]inositol and LiCl, in a dose-dependent manner, and the functional expression of muscarinic receptors peaked in number at day 7 in culture, declining thereafter. Short-term (less than 1 h) treatment of E6 neuronal cultures with 1 microM opioid peptides such as morphiceptin or D-Ala2-D-Leu5-enkephalin (DADLE) did not inhibit the release of inositol phosphates in response to 1 mM carbachol whereas forskolin, which also activates adenylate cyclase and raises cAMP levels, inhibited InsP release by about 25%. In contrast, long-term (48 h) opioid treatment with either morphiceptin or DADLE (1-10 microM) inhibited the carbachol-stimulated inositol phosphate release by greater than or equal to 50%. Prolonged treatment with morphiceptin also inhibited the bradykinin-mediated release of InsP from E6CH cells. In both cases, the inhibition was partially blocked by the continuous presence of naloxone, suggesting that the inhibition was mediated through opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin regulation of endogenous acetylcholine release from rat striatal slices is independent of dopaminergic tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.

Evidence that somatostatin enhances endogenous acetylcholine release in the rat hippocampus

The present experiments show that somatostatin (SS)-like immunoreactive material is present in the hippocampus and that its release can be increased by K+ stimulation of rat hippocampal slices, suggesting that SS-like peptides may be of significance to neurotransmission in the hippocampus. Exogenous SS-28 and SS-14 enhanced the K(+)-evoked release of endogenous acetylcholine (ACh) from rat hippocampal slices, whereas amino-terminal fragments of SS-28 did not. The increased ACh release in the presence of either peptide appeared to be mediated by an interaction with SS receptors because cyclo-SS, a putative SS antagonist, abolished the effects of both SS-28 and SS-14. In addition, the increase in ACh release induced by SS-14 or SS-28 was antagonized by the calcium channel antagonists omega-conotoxin GVIA, nifedipine, and cinnarizine, implicating voltage-sensitive calcium channels in this effect. Moreover, the effect was sensitive to tetrodotoxin, suggesting an indirect action of the peptides at a site distal to cholinergic nerve terminals. Cysteamine, which has been reported to deplete SS content and to increase SS release in brain, augmented the basal and evoked release of ACh from hippocampal slices, without affecting SS-like content and release. Finally, neuropeptide Y, which is colocalized with SS in many neurons of the hippocampal formation, did not alter ACh release, nor did it facilitate the SS-induced increase. The results suggest that in the rat hippocampus, both SS-28 and SS-14 interact with SS receptors to regulate ACh release indirectly by a mechanism that involves alterations of calcium influx during depolarization.

Neurotensin regulation of endogenous acetylcholine release from rat cerebral cortex: effect of quinolinic acid lesions of the basal forebrain

The effects of neurotensin (NT) on endogenous acetylcholine (ACh) release from basal forebrain, frontal cortex, and parietal cortex slices were tested. The results show that NT differentially regulates evoked ACh release from frontal and parietal cortex slices without altering either spontaneous or evoked ACh release from basal forebrain slices. In the frontal cortex, NT significantly inhibited evoked ACh release by a tetrodotoxin (TTX)-insensitive mechanism, suggesting an action directly on cholinergic terminals. In the parietal cortex, NT enhanced evoked ACh release by a TTX-sensitive mechanism, suggesting an action of NT on the cholinergic neuron or in close proximity to the cholinergic neuron. The effects of NT on ACh release were confined to evoked ACh release; that is, spontaneous ACh release was not affected. NT did not affect spontaneous or potassium-evoked ACh release from occipital cortex slices. The second set of experiments tested the effects of quinolinic acid (QUIN) lesions of the basal forebrain cell bodies on the NT-induced regulation of evoked ACh release in the cerebral cortex. QUIN lesions of basal forebrain cell bodies caused decreases in choline acetyltransferase activity (27 and 28%), spontaneous ACh release (14 and 21%), and evoked ACh release (38 and 44%) in frontal and parietal cortex, respectively. In addition, 11 days following QUIN lesions of basal forebrain cell bodies, the action of NT to regulate evoked ACh release in frontal cortex or parietal cortex was no longer observed. The results suggest that in the rat frontal and parietal cortex, NT differentially regulates the activity of cholinergic neurons by decreasing and increasing evoked ACh release, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

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.