Evidence for opioid-induced release of glutamate in guinea pig longitudinal muscle–myenteric plexus strip

By the use of longitudinal muscle–myenteric plexus (LMMP) strips of the guinea pig ileum it should be investigated whether opioids can contribute to an excitatory component of the intestinal smooth muscle contractions. LMMP preparations were set up in Tyrode solution with 1 μmol/l naloxone present or without naloxone from the beginning of tissue preparation. After a 30-min equilibration period the twitch contractions evoked by the first and the fifth electrical 3-s-stimulus in an 80-s-sequence were significantly higher in the tissues prepared and tested without naloxone present. When after continuous electrical twitch responses (+) MK-801, an NMDA receptor antagonist (10–100 μmol/l) was added to the Tyrode solution, the electrically evoked contractions were reduced. The inhibition of the twitch response by (+) MK-801 was less pronounced in the preparations treated continuously with naloxone as compared to the otherwise untreated LMMP preparations. Morphine, (1 and 5 μmol/l), evoked a release of glutamate from this nerve–muscle-preparation. In conclusion these experiments provide evidence that endogenously released opioids or exogenously applied morphine can release glutamate as an excitatory component within the LMMP preparation.     

Inhibition of the cholinergic neurotransmission in human airways via prejunctional alpha-2-adrenoceptors

Previous investigations on guinea-pig airways indicate that the excitatory, cholinergic neurotransmission can be inhibited via prejunctional alpha-2-adrenoceptors. The aim of this study was to investigate whether a similar inhibition of the cholinergic neurotransmission is present in human airways. Ring preparations of human bronchi were mounted for recording of isometric tension and immersed in oxygenated Krebs solution. Electrical field stimulation of the preparations elicited atropine-sensitive twitch contractions. Exogenous noradrenaline (in the presence of cocaine and propranolol) inhibited the electrically evoked contractions. The noradrenaline-induced inhibition could be antagonized by yohimbine whereas prazosin was ineffective, indicating that the inhibition was mediated by alpha-2-adrenoceptors. Contractions evoked by exogenous acetylcholine (in the presence of cocaine and propranolol) was unaffected by the addition of noradrenaline, which suggests that the alpha-2-adrenoceptors have a prejunctional localization. In conclusion, this report gives evidence that the human bronchial, cholinergic neurotransmission can be inhibited by stimulation of prejunctional alpha-2-adrenoceptors.     

Role of agonist-dependent receptor internalization in the regulation of mu opioid receptors

Organotypic cultures and ileal neuromuscular preparations were used to determine (i) whether endogenous release of opioids by electrical stimulation induces mu receptor endocytosis, and (ii) whether and under which conditions ligand-induced mu receptor endocytosis influences the responsiveness of neurons expressing native mu receptors. In longitudinal muscle-myenteric plexus preparations, electrical stimulation at 20 Hz induced a prominent endocytosis of mu receptors in enteric neurons, indicating endogenous release of opioids. A similar massive endocytosis was triggered by exogenous application of the mu receptor agonist, [D-Ala(2),MePhe(4), Gly-ol(5)] enkephalin, whereas exogenous application of morphine was ineffective. [D-Ala(2),MePhe(4),Gly-ol(5)] enkephalin and morphine induced a concentration-dependent inhibition of neurogenic cholinergic twitch contractions to electrical stimulation at 0.1 Hz. beta-Chlornaltrexamine shifted to the right the inhibitory curve of both agonists with a concentration-dependent reduction of the maximum agonist response, which is consistent with the existence of spare mu opioid receptors. Under these conditions, the induction of mu receptor endocytosis by exogenously applied [D-Ala(2), MePhe(4),Gly-ol(5)] enkephalin diminished the inhibitory effect of this agonist on twitch contractions and tritiated acetylcholine release. In contrast, there was no reduction of the inhibitory effect of morphine, which failed to induce mu receptor endocytosis, on neurogenic cholinergic response.These results provide the first evidence for the occurrence of mu receptor endocytosis in neurons by endogenously released opioids and show that agonist-dependent mu receptor endocytosis could serve as a mechanism to regulate mu opioid receptor responsiveness to ligand stimulation when the opioid receptor reserve is reduced.     

The effects of purified botulinum neurotoxin type A on cholinergic, adrenergic and non-adrenergic, atropine-resistant autonomic neuromuscular transmission

The twitch response observed during low frequency electrical stimulation of postganglionic cholinergic neurones supplying the longitudinal smooth muscle of the guinea-pig ileum was markedly reduced by incubation with an homogeneous preparation of botulinum type A neurotoxin (4.3-8.6 nM). This intoxication of the autonomic cholinergic neurones was long-lasting, irreversible by washing, but readily reversed by 4-aminopyridine (50-1000 microM). The noradrenergic motor response of the rat anococcygeus following field stimulation was partially antagonised by the neurotoxin. The non-adrenergic inhibitory response of the guinea-pig taenia coli, elicited by field stimulation, was not antagonised by botulinum toxin, suggesting that a source of a non-adrenergic inhibitory transmitter exists, other than intramural cholinergic neurones. However, the neurogenic excitatory responses of the guinea-pig bladder, elicited by field stimulation in the presence of atropine and guanethidine, were virtually abolished by botulinum toxin. It is suggested that the parasympathetic neurones which supply the smooth muscle of the guinea-pig urinary bladder co-release acetylcholine and a non-cholinergic excitatory transmitter; ATP or polypeptides are possible candidates.     

Inhibitory actions of indomethacin on electrical and mechanical responses produced by nerve stimulation in circular smooth muscle of the guinea-pig gastric fundus.

The effects of indomethacin on electrical and mechanical responses produced by transmural nerve stimulation (TNS) were investigated in isolated circular smooth muscle of the guinea-pig gastric fundus. TNS evoked a cholinergic excitatory junction potential (e.j.p.). The e.j.p.s were inhibited by 1-10 microM indomethacin, in a concentration-dependent manner, with no marked alteration of the resting membrane potential. Exogenously applied acetylcholine caused a depolarization of the membrane that was not altered by indomethacin. TNS evoked a cholinergic twitch contraction at low frequencies (0.1 Hz). A train of TNS's at high frequency (1 Hz) produced a transient contraction with a subsequent sustained relaxation. Indomethacin reduced the resting tension and inhibited these TNS-induced contractions. Application of Nomega-nitro-L-arginine (NOLA), an inhibitor of nitric oxide (NO) synthesis, increased the amplitude of twitch contractions, and altered transient contractions to tetanic contractions during TNS at a frequency of 1 Hz, also with an increased amplitude. In the presence of NOLA, indomethacin (5 microM) again reduced the resting tension and inhibited TNS-induced contractions. This inhibition was greater for twitch contractions than for tetanic contractions. Nifedipine reduced the TNS-induced contractions, while addition of indomethacin further reduced the amplitude of contractions. Contractions produced by low concentrations of acetylcholine (0.1 microM) were inhibited by indomethacin, while those produced by 1 microM were not. These results indicate that the inhibitory actions of indomethacin on TNS-induced contractions do not involve enhanced production of NO or selective inhibition of voltage-gated Ca-channels. Prejunctional autoregulatory mechanisms may also not be altered by indomethacin. As indomethacin inhibits the enzyme cyclooxygenase, it is speculated that endogenously produced prostaglandins exert excitatory actions on gastric smooth muscle, and act mainly postjunctionally to facilitate spontaneous and neurogenic electrical and mechanical activity.     

The neurotransmitter role of calcitonin gene-related peptide in the rat and guinea-pig ureter: effect of a calcitonin gene-related peptide antagonist and species-related differences in the action of omega conotoxin on calcitonin gene-related peptide release from primary afferents.

In the rat and guinea-pig isolated ureter electrical field stimulation of intrinsic nerves (10 Hz for 10 s) produces transient inhibition of evoked (20 mM KCl or 0.1-1 microM neurokinin A) rhythmic contractions by releasing transmitter(s) from peripheral endings of capsaicin-sensitive primary afferents. The C-terminal fragment of human calcitonin gene-related peptide (8-37) blocked the inhibitory effect of electrical field stimulation as well as that produced by exogenous calcitonin gene-related peptide, while leaving unaffected the inhibitory response to isoprenaline. Human calcitonin gene-related peptide (8-37) was devoid of any inhibitory activity of its own but enhanced the amplitude and frequency of KCl-evoked rhythmic contractions in the rat ureter, probably by antagonizing the inhibitory effect of endogenous calcitonin gene-related peptide released by KCl. Omega conotoxin fraction GVIA, a peptide which possesses a potent blocking activity of N-type voltage-sensitive calcium channels, prevented the inhibitory response to electrical stimulation in the guinea-pig ureter, while leaving the response unaffected in the rat ureter. Conotoxin had no effect toward the inhibition produced by exogenous calcitonin gene-related peptide indicating its prejunctional site of action, demonstrated previously in the guinea-pig ureter [Maggi et al. (1990) Neurosci, Lett. 114, 203-206]. Dermorphin, an amphibian peptide with potent agonist activity on mu-type opioid receptors, inhibited the response to electrical stimulation in the guinea-pig ureter but had no effect in the rat ureter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that ATP released from the postsynaptic site by noradrenaline, is involved in mechanical responses of guinea-pig vas deferens: cascade transmission.

The release of endogenous ATP and [3H]noradrenaline, and the mechanical response of the guinea-pig vas deferens to field stimulation of its motor nerves were examined using a perfusion system. The release of ATP at rest was 0.83 +/- 0.13 pmol/g per min, and ATP released by field stimulation (8 Hz, 480 shocks) was 5.47 +/- 1.23 pmol/g. The evoked release was completely inhibited when Ca2+ was removed and 1 mM EGTA was added, or by 1 microM tetrodotoxin. The release of ATP and [3H]noradrenaline in response to field stimulation was constant with an S2/S1 ratio of 1.10 +/- 0.11 for ATP and 0.92 +/- 0.03 for [3H]noradrenaline, respectively (where S1 and S2 are stimulation periods). Prazosin (1 microM), a potent alpha 1-adrenoceptor antagonist, significantly reduced the stimulation-evoked release of ATP by 75% and significantly reduced both mechanical twitch and tonic responses, but enhanced the release of [3H]noradrenaline. This finding indicates that there is an alpha 1-adrenoceptor-mediated release of endogenous ATP. However, the prazosin-insensitive portion of ATP release (25%) is considered to be of presynaptic origin. The stimulation of alpha 1-adrenoceptors by 1-noradrenaline or methoxamine in concentrations ranging from 10 to 100 microM resulted in a concentration-dependent release of ATP and a biphasic contraction of the vas deferens: a twitch response was followed by a tonic contraction. Prazosin (1 microM) completely prevented the effect of 1-noradrenaline or methoxamine on both ATP release and mechanical response. When Ca2+ was omitted and EGTA (1 mM) was added, 1-noradrenaline was still able to release ATP but failed to produce contraction. Nifedipine, a Ca-channel and ATP receptor antagonist, reduced the twitch contraction and enhanced the release of ATP from muscle in response to noradrenaline administration. This finding indicates that the release of ATP from the muscle is not linked to mechanical contraction. When the vas deferens was made deficient in noradrenaline by 6-hydroxydopamine pretreatment (100 + 250 mg/kg, i.p.), electrical field stimulation failed to release [3H]noradrenaline and ATP. Under these conditions, exogenous 1-noradrenaline was much more effective in releasing ATP from the smooth muscle, and producing twitch responses, followed by a tonic contraction. After reserpine pretreatment (2 x 5 mg/kg, i.p.), the field stimulation-evoked release of ATP and both phases of contraction were markedly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of neuroeffector transmission in guinea-pig pulmonary artery and vas deferens by exogenous nitric oxide

Cederqvist , B. & Gustafsson , L. E. 1994. Modulation of neuroeffector transmission in guinea-pig pulmonary artery and vas deferens by exogenous nitric oxide. Acta Physiol Scand 150, 75–81. Received 11 March 1993, accepted 30 July 1993. ISSN 0001–6772. Department of Physiology and Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden. Blockade of nitric oxide (NO) synthesis enhances contractile responses to transmural nerve stimulation in guinea-pig pulmonary artery, indicating neuromodulation by endogenous nitric oxide. In the present study, neuromodulatory effects of exogenous NO were examined in guinea-pig pulmonary artery and vas deferens. Application of NO as acid nitrite, to the guinea-pig pulmonary artery, inhibited the contractile response to transmural nerve stimulation as well as contractions to exogenous noradrenaline. The inhibition occurred in a dose-dependent manner. Acid nitrite did not affect stimulation-induced release of [³H]noradrenaline in guinea-pig pulmonary artery. In the guinea-pig vas deferens, contractile ‘twitch’ responses to brief (25 pulses) transmural nerve stimulation and stimulation-induced release of [³H]noradrenaline was unaffected by acid nitrite. However, ‘twitch’ responses to prolonged stimulation were inhibited by acid nitrite, and tonic ‘hump’ responses were either enhanced or unaffected by acid nitrite. In conclusion, exogenous nitric oxide exerted an inhibition of adrenergic neurotransmission post-junctionally in guinea-pig pulmonary artery. In guinea-pig vas deferens, exogenous nitric oxide affected adrenergic and/or non-adrenergic non-cholinergic neurotransmission in a complex fashion, however without alteration in noradrenaline release. The data support a role for NO in modulation of neuroeffector transmission, especially by modulation of effector sensitivity, whereas modulation of noradrenaline release seems an unlikely role for NO in these and similar peripheral tissues.     

Opioidergic inhibition of reflexes evoked by selective stimulation of sural nerve C fibres in the rabbit.

Reflexes were evoked in the gastrocnemius medialis (GM) muscle nerve by selective electrical stimulation of the non-myelinated C fibres of the ipsilateral sural nerve of decerebrated, spinalized rabbits. The opioid antagonist (-)-quadazocine (555 micrograms/kg i.v.) enhanced responses to sural C fibre stimulation to an average of 236% of pre-drug levels. In addition, C fibre-evoked reflexes were depressed for 7-9 min after repetitive activation of the high threshold axons of the common peroneal nerve, and this effect was reversed after quadazocine. Thus, GM responses to stimulation of non-myelinated sural afferent fibres are suppressed by endogenous opioid peptides, but the degree of inhibition does not appear to be as profound as that previously reported for reflexes evoked by myelinated fibres.     

Role of protein kinase C in the excitatory action of cholinergic nerve stimulation on spontaneous activity of circular smooth muscle isolated from the guinea-pig stomach antrum

Following inhibition of NO production with nitroarginine, circular muscle isolated from the guinea-pig gastric antrum generated periodic slow potentials and unitary potentials. Transmural nerve stimulation (TNS) during the interval between slow potentials evoked an apamin-sensitive inhibitory junction potential (IJP) followed by an atropine-sensitive depolarization; the latter was either a transient depolarization with enhanced generation of unitary potentials or a slow potential. After inhibition of unitary potentials and slow potentials with 1 mM caffeine, TNS evoked an IJP and subsequent cholinergic depolarization, the latter developing slowly and lasting for about 10 s. TNS was unable to elicit a slow potential until a certain period of time had elapsed following the cessation of a slow potential. The period during which TNS could not evoke slow potentials (termed the high-threshold period) was about 10 s, and this period was increased by chelerythrine and decreased by phorbol esters. It is concluded that cholinergic nerve-mediated excitation of gastric muscle involves the activation of protein kinase C (PKC), and that the high-threshold period, during which the generation of slow potentials by TNS is inhibited, may be a consequence of reduced activity of PKC.     

Release and modulation of release of serotonin in rabbit superior colliculus

The release of previously incorporated [3H]serotonin and its presynaptic modulation were studied in slices of rabbit superior colliculus. Electrical stimulation at frequencies of 0.017-3 Hz greatly increased the outflow of tritiated compounds; this response was almost abolished by tetrodotoxin and in a low calcium medium. Unlabelled serotonin, when added in the presence of nitroquipazine, an inhibitor of high-affinity neuronal serotonin uptake, reduced the electrically evoked overflow of tritium, an effect antagonized by metitepin. Given alone, metitepin caused an increase. The evoked overflow was also decreased by clonidine, and the effect of clonidine was counteracted by phentolamine. Phentolamine itself increased the overflow response. However, this was probably not due to antagonism against an inhibitory effect of endogenous noradrenaline because, first, the selective alpha 2-adrenoceptor antagonist idazoxan did not share with phentolamine the overflow-enhancing effect, second, phentolamine continued to increase the overflow after noradrenergic axons had been destroyed by 6-hydroxydopamine, and third, the facilitatory effects of metitepin and phentolamine were not additive. Phentolamine, like metitepin, antagonized the presynaptic inhibitory effect of serotonin, indicating that it may increase the evoked overflow of tritium by blocking serotonin receptors rather than alpha-adrenoceptors. Ethylketocyclazocine decrease the electrically evoked overflow, and its effect was prevented by naloxone: peptides selective for opioid mu- or delta-receptors caused no change. Nicotine increased the basal outflow of tritium (in the absence of electrical stimulation); the increase was attenuated by hexamethonium and low calcium medium. No or minimal changes in tritium outflow were obtained with beta-adrenoceptor, dopamine receptor, muscarine receptor and GABA receptor ligands or with substance P and glutamate. In conjunction with our previous studies, these results indicate that serotonin is a neurotransmitter in the superior colliculus. Its release is modulated through presynaptic autoreceptors (probably 5-HT1), alpha 2-adrenoceptors, opioid kappa-receptors and nicotine receptors, of which only the autoreceptors receive an endogenous input, at least under the experimental conditions chosen. Each of the three groups of collicular monoamine axons that we have studied recently (cholinergic, noradrenergic, serotoninergic) possesses a specific pattern of presynaptic, release-modulating receptors. A physiological role seems likely only for the alpha 2-autoreceptors at the noradrenergic and the 5-HT1-autoreceptors at the serotoninergic axons.     

Spinal opioid receptor-sensitive muscle afferents contribute to the fatigue-induced increase in intracortical inhibition in healthy humans

We investigated the influence of spinal opioid receptor-sensitive muscle afferents on cortical changes following fatiguing unilateral knee-extensor exercise. On separate days, seven subjects performed an identical five sets of intermittent isometric right-quadriceps contractions, each consisting of eight submaximal contractions [63 ± 7% maximal voluntary contraction (MVC)] and one MVC. The exercise was performed following either lumbar interspinous saline injection or lumbar intrathecal fentanyl injection blocking the central projection of spinal opioid receptor-sensitive lower limb muscle afferents. To quantify exercise-induced peripheral fatigue, quadriceps twitch force (Q(tw,pot)) was assessed via supramaximal magnetic femoral nerve stimulation before and after exercise. Motor evoked potentials and cortical silent periods (CSPs) were evaluated via transcranial magnetic stimulation of the motor cortex during a 3% MVC pre-activation period immediately following exercise. End-exercise quadriceps fatigue was significant and similar in both conditions (Q(tw,pot) -35 and -39% for placebo and fentanyl, respectively; P = 0.38). Immediately following exercise on both days, motor evoked potentials were similar to those obtained prior to exercise. Compared with pre-exercise baseline, CSP in the placebo trial was 21 ± 5% longer postexercise (P < 0.01). In contrast, CSP following the fentanyl trial was not significantly prolonged compared with the pre-exercise baseline (6 ± 4%). Our findings suggest that the central effects of spinal opioid receptor-sensitive muscle afferents might facilitate the fatigue-induced increase in CSP. Furthermore, since the CSP is thought to reflect inhibitory intracortical interneuron activity, which may contribute to central fatigue, our findings imply that spinal opioid receptor-sensitive muscle afferents might influence central fatigue by facilitating intracortical inhibition.     

Non-quantal release of acetylcholine in guinea-pig airways: role of choline transporter

In the resting state, motor neurons continuously release ACh through quantal and non-quantal mechanisms, the latter through vesicular ACh transporter (VAChT) and choline transporter (ChT). Although in skeletal muscle these mechanisms have been extensively studied, the non-quantal release (NQR) from parasympathetic neurons of airway smooth muscle has not been described. Here we corroborated that the organophosphate paraoxon (acetylcholinesterase inhibitor) induced a contraction blocked by atropine (muscarinic antagonist) in guinea-pig tracheal rings. This contraction was not modified by two blockers of evoked quantal release, tetrodotoxin (voltage-dependent Na(+) channel blocker) and ω-conotoxin GVIA (N-type Ca(2+) channel blocker), nor by the nicotinic blocker hexamethonium, suggesting that acetylcholine NQR could be responsible of the paraoxon-induced contraction. We confirmed that tetrodotoxin, and to some extent -conotoxin, abolished the evoked quantal ACh release induced by electrical field stimulation. Hemicholinium-3 (ChT inhibitor), but not vesamicol (VAChT inhibitor), caused a concentration-dependent inhibition of the response to paraoxon. The highest concentration of hemicholinium-3 left ～75% of the response to electrical field stimulation, implying that inhibition of paraoxon-induced contraction was not due to depletion of neuronal vesicles. Non-neuronal sources of ACh released through organic cation transporters were discarded because their inhibition by quinine or corticosterone did not modify the response to paraoxon. Calcium-free medium abolished the effect of paraoxon, and NiCl(2), 2-aminoethyl diphenyl-borate and SKF 96365 partly inhibited it, suggesting that non-specific cation channels were involved in the acetylcholine NQR. We concluded that a Ca(2+)-dependent NQR of ACh is present in cholinergic nerves from guinea-pig airways, and that ChT is involved in this phenomenon.     

Receptor desensitization and the transient nature of the secretory response of adrenal chromaffin cells

Transition of the cellular responsiveness to a cholinergic stimulation during a prolonged stimulation with acetylcholine (ACh) of adrenal chromaffin cells was studied by directly monitoring the secretory process in cell bed perfusion experiments. A pulsatile injection of hexamethonium during the prolonged stimulation caused an interruption of the response. Step-up of the ACh concentration in the declining phase of the primary response evoked an additional response. The magnitude of the response to the step-up was reduced by extracellular Ca2+ in a dose-dependent manner. However, neither the peak amplitude nor the declining rate of the primary response was affected by Ca2+ in such a direction. These results suggest that the transient nature of the ACh-evoked response is mainly attributable to the inactivation of transduction mechanisms between receptor stimulation and channel activation. Although Ca2+-facilitated desensitization of nicotinic receptors is in process during the stimulation, such a desensitization may account only insignificantly for the transient nature.     

Ontogeny of noradrenergic effects on ultrasonic vocalizations in rat pups

The ontogeny of noradrenergic effects and the interaction of opioid and noradrenergic systems on vocalizations in rat pups from Day 10 to Day 18 were evaluated. Day 10 pups given clonidine (0.05 or 0.5 mg/kg) ip showed a sustained high level of calling throughout a 25-min isolation period that was reversed with yohimbine (0.1 mg/kg). Day 15 pups showed identical profiles with a lower baseline rate. Day 17 pups' calls were differentially affected according to dose; Day 18 pups reduced vocalizing with clonidine. In addition, it was found that at all ages when clonidine increased calling during isolation, the pups vocalized in the nest as well. Naltrexone, an opioid antagonist, lost its effectiveness to increase vocalizations after Day 15 unless it was given subsequent to clonidine. These results suggest that pups' vocalizations are differentially affected by noradrenergic and opioid stimulation or inhibition with developmental changes.     

The mode of action of enkephalins in the guinea-pig myenteric plexus

Intracellular recording were made from ganglion cells of of guinea-pig ileum. Methionine-enkephalin and ? enke the nerve membrane were studied by adding them ? che perf? Both opioid pentapeptides elicit concentration-dependent the membranes of neurons; however, met-enkephalin was sho? two to four times as potent as leu-enkephalin. The membrane ? polarization caused by enkephalins was reduced by the narcotic antagonist? one and naltrexone. From these results, it may be speculated that the ? ensitive neurons may be the cholinergic efferents to the muscle layers. By hyperpolarizating these neurons, enkephalins may exert their inhibitory action. This may well explain why opiate analgesics reduce the output of acetylcholine and tile response of this preparation when it is excited.     

Electric muscle stimulation in the spontaneously hypertensive rat induces a post-stimulatory reduction in activity: role of different opioid receptors

We have previously shown that prolonged low-frequency muscle stimulation, inducing contractions of the gastrocnemius muscle, in conscious spontaneously hypertensive rats leads to an opioid-mediated post-stimulatory reduction in blood pressure and analgesia. In the present study we investigated whether muscle stimulation would also induce a post-stimulatory reduction in behavioural activity in the spontaneously hypertensive rats. Selective opioid receptor antagonists were used to analyse the involvement of endogenous opioids. Muscle stimulation, lasting 60 min, induced a post-stimulatory sedation that outlasted the stimulation for hours. Sniffing, locomotor activity and total behavioural activity were significantly reduced. The post-stimulatory reduction in activity was reversed back to control levels by a high dose of naloxone (15 mg kg-1 i.v.). The selective mu-receptor antagonist beta-funaltrexamine, given intracerebroventricularly before stimulation, did not influence the development of the post-stimulatory drop in activity. The delta-receptor antagonist ICI 154,129 had no effect at all on the already developed sedation, whereas MR 2266 BS, a kappa-receptor antagonist (3 mg kg-1 i.v.), completely reversed the drop in activity. These results show that muscle stimulation gives rise to an opioid-mediated post-stimulatory reduction in activity in spontaneously hypertensive rats. The results also indicate the involvement of the opioid kappa-receptor in the behavioural response.     

Electrical stimulation of the horizontal limb of the diagonal band of broca modulates population EPSPs in piriform cortex.

Electrical stimulation of the horizontal limb of the diagonal band of Broca (HDB) was coupled with recording of evoked potentials in the piriform cortex. Stimulation of the HDB caused an enhancement of the late, disynaptic component of the evoked potential elicited by stimulation of the lateral olfactory tract but caused a suppression of the synaptic potential elicited by stimulation of the posterior piriform cortex. The muscarinic antagonist scopolamine blocked both effects of HDB stimulation. The enhancement of disynaptic potentials could be due to cholinergic depolarization of pyramidal cells, whereas the suppression of potentials evoked by posterior piriform stimulation could be due to presynaptic inhibition of intrinsic fiber synaptic transmission by acetylcholine.     

Inhibition of synaptically evoked cortical acetylcholine release by adenosine: an in vivo microdialysis study in the rat

The release of cortical acetylcholine from the intracortical axonal terminals of cholinergic basal forebrain neurons is closely associated with electroencephalographic activity. One factor which may act to reduce cortical acetylcholine release and promote sleep is adenosine. Using in vivo microdialysis, we examined the effect of adenosine and selective adenosine receptor agonists and antagonists on cortical acetylcholine release evoked by electrical stimulation of the pedunculopontine tegmental nucleus in urethane anesthetized rats. All drugs were administered locally within the cortex by reverse dialysis. None of the drugs tested altered basal release of acetylcholine in the cortex. Adenosine significantly reduced evoked cortical acetylcholine efflux in a concentration-dependent manner. This was mimicked by the adenosine A(1) receptor selective agonist N(6)-cyclopentyladenosine and blocked by the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The A(2A) receptor agonist 2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosi ne hydrochloride (CGS 21680) did not alter evoked cortical acetylcholine release even in the presence of DPCPX. Administered alone, neither DPCPX nor the non-selective adenosine receptor antagonist caffeine affected evoked cortical acetylcholine efflux. Simultaneous delivery of the adenosine uptake inhibitors dipyridamole and S-(4-nitrobenzyl)-6-thioinosine significantly reduced evoked cortical acetylcholine release, and this effect was blocked by the simultaneous administration of caffeine.These data indicate that activation of the A(1) adenosine receptor inhibits acetylcholine release in the cortex in vivo while the A(2A) receptor does not influence acetylcholine efflux. Such inhibition of cortical acetylcholine release by adenosine may contribute to an increased propensity to sleep during prolonged wakefulness.     

The effects of mu, delta- and kappa-opioid receptor antagonists on the pain threshold increase following muscle stimulation in the rat

In a previous study, prolonged low-frequency muscle stimulation, inducing dynamic contractions in the hind leg of unanaesthetized rats, was shown to give rise to a hypoalgesia. The increase in pain threshold, measured as squeak threshold to noxious electric pulses, lasted 3 h. In the present study, the involvement of the endogenous opioid system in the post-stimulatory analgesia was investigated using selective opioid receptor antagonists. The post-stimulatory analgesia was completely reversed back to prestimulatory control levels by naloxone, 1 mg kg-1. ICI 154,129 and MR 2266 BS, selective delta- and kappa-receptor antagonists respectively, did not significantly influence the post-stimulatory analgesia, although ICI 154,129 had a minor pain threshold-lowering effect. Rats pretreated with beta-funaltrexamine, a mu-receptor antagonist, did not exhibit any post-stimulatory analgesia. These results suggest that opioid systems are involved in the increase in pain threshold after muscle stimulation and that the analgesic response is both elicited and maintained by the mu-receptor.