Similar Depressant Effects Research Articles

Acute Administration of Gut Microbial Metabolites Reduces Blood Pressure and Cardiac Contractility

Metabolites produced by the gut microbiota can affect host physiology; one class of these metabolites are short chain fatty acids (SCFAs). Prior work by our laboratory has shown that intravenous administration of SCFAs causes acute hypotension (Pluznick, Protzko et al. 2013) and that treating vessels with SCFAs ex vivo results in vasorelaxation (Natarajan, Hori et al. 2016). To expand upon these findings, we performed IP injections of acetate in mice that were implanted with DSI telemetry transmitters to monitor mean arterial pressure (MAP) and heart rate (HR) in real time. Sodium acetate (acetate is the most abundant SCFA in the circulation) was given at a 1g/kg dose via IP, which elevates plasma acetate by 3–4 fold over baseline (Shubitowski, Poll et al. 2019). Upon sodium acetate IP injection, we observed a rapid and reproducible drop in MAP (−49.2 +/− 10.7 mmHg maximum drop, n=10 males + females, p<0.05) when compared to an IP injection of saline. Surprisingly, in addition to a significant depression of MAP, mice injected with acetate also showed a significant depression in HR (−257.6 +/− 55.8 bpm maximum drop, p<0.05). IP injections of propionate and butyrate, two other SCFA microbial metabolites, had similar depressive effects on MAP and HR, but lactate (control) had no effect. Had SCFAs been solely acting on the vasculature, we would have expected the baroreflex to increase HR. To determine if acetate was having a direct cardiac effect, we used isolated perfused Langendorff hearts from mice. During acetate infusion, Langendorff hearts showed a significant decrease in HR, suggesting a direct mechanism of action (353.8 +/− 39.5 bpm baseline vs 274.6 +/− 21.3 bpm acetate n=5, p<0.05), however the HR drop was on a much slower time scale than seen in vivo. To further probe the effects of acetate on cardiac contractility, pressure volume loops were performed on anesthetized mice and acetate was administered by IP. Inferior vena cava occlusion after acetate IP showed a significant decrease in Preload Recruitable Stroke Work (PRSW, 73.06 +/− 20.98 vs. 49.06 +/− 22.11 mmHg*μl baseline vs acetate, n=7 males + females, p<0.05) and end‐systolic elastance (Ees, 3.493 +/−1.139 vs. 1.519 +/− 0.5781 mmHg/μl baseline vs. acetate, n=7 males + females, p<0.05), two parameters of load‐independent cardiac contractility. Finally, pre‐treatment of conscious mice with cardioselective β antagonist metoprolol blocked the acetate mediated drop in HR in mice implanted with telemetry devices, but the drop in MAP remained intact. We hypothesize that acetate acts independently on the peripheral vasculature and the cardiac muscle, with direct and indirect effects on cardiac function. Both components are likely contributing to the MAP and HR effects seen in whole animals. Together these data demonstrate a previously unknown and potentially detrimental effect of short chain fatty acids on cardiac function.Support or Funding InformationF31HL144061, R01HL128512, R01DK107726

Opposing effects of cervical spinal cold block on spinal itch and pain transmission.

Inactivation of descending pathways enhanced responses of spinal dorsal horn neurons to noxious stimuli, but little is known regarding tonic descending modulation of spinal itch transmission. To study effects of cervical spinal cold block on responses of dorsal horn neurons to itch-evoking and pain-evoking stimuli, single-unit recordings were made from superficial dorsal horn wide dynamic range and nociceptive-specific-type neurons in pentobarbital-anesthetized mice. Intradermal histamine excited 17 units. Cold block starting 1 minute after intradermal injection of histamine caused a marked decrease in firing. The histamine-evoked response during and following cold block was significantly lower compared with control histamine-evoked responses in the absence of cold block. A similar but weaker depressant effect of cold block was observed for dorsal horn unit responses to chloroquine. Twenty-six units responded to mustard oil allyl isothiocyanate (AITC), with a further significant increase in firing during the 1-minute period of cold block beginning 1 minute after AITC application. Activity during cold block was significantly greater compared with the same time period of control responses to AITC in the absence of cold block. Ten units’ responses to noxious heat were significantly enhanced during cold block, while 6 units’ responses were reduced and 18 unaffected. Cold block had no effect on mechanically evoked responses. These results indicate that spinal chemonociceptive transmission is under tonic descending inhibitory modulation, while spinal pruriceptive transmission is under an opposing, tonic descending facilitatory modulation.

Effects of epinephrine exposure on contractile performance of compact and spongy myocardium from rainbow trout (Oncorhynchus mykiss) during hypoxia

Hypoxia results in elevated circulating epinephrine for many fish species, and this is likely important for maintaining cardiac function. The aims of this study were to assess how hypoxia impacts contractile responses of ventricular compact and spongy myocardium from rainbow trout (Oncorhynchus mykiss) and to assess how and if epinephrine may protect myocardial performance from a depressive effect of hypoxia. Work output and maximum contraction rate of isolated preparations of spongy and compact ventricular myocardium from rainbow trout were measured. Tissues were exposed to the blood PO2 that they experience in vivo during environmental normoxia and hypoxia and also to low (5nM) and high (500nM) levels of epinephrine in 100% air saturation (PO2 20.2kPa) and during hypoxia (PO2 2kPa, 10% air saturation). It was hypothesized that hypoxia would result in a decrease in work output and maximum contraction rate in both tissue types, but that epinephrine exposure would mitigate the effect. Hypoxia resulted in a decline in net work output of both tissue types, but a decline in maximum contraction rate of only compact myocardium. Epinephrine restored the maximum contraction rate of compact myocardium in hypoxia, appeared to slightly enhance work output of only compact myocardium in air saturation but surprisingly not during hypoxia, and restored net work of hypoxic spongy myocardium toward normoxic levels. These results indicate hypoxia has a similar depressive effect on both layers of ventricular myocardium, but that high epinephrine may be important for maintaining inotropy in spongy myocardium and chronotropy in compact myocardium during hypoxia.

Downregulation of the cough reflex by aclidinium and tiotropium in awake and anesthetized rabbits

Long-acting muscarinic receptor antagonists (LAMAs) have been reported to attenuate cough in preclinical and clinical studies. The present study was performed on rabbits to compare aclidinium and tiotropium efficacy in the downregulation of the cough reflex. This reflex was evoked by citric acid inhalation in unanesthetized animals and by both citric acid inhalation and mechanical stimulation of the tracheobronchial tree in anesthetized animals 90 min following the inhalation of each drug (nebulizer output always at 1 mL/min). Aclidinium 4 mg/mL and tiotropium 200 μg/mL inhaled in 1 min proved to have similar protective effect on methacholine-induced bronchoconstriction in anesthetized animals. The total dosage employed for aclidinium and tiotropium was 4 mg and 200 μg, respectively. In awake animals, similar reductions in the cough number were observed following 10-min inhalation of each drug with a slight, not significant tendency to higher antitussive effects for aclidinium. In anesthetized animals, 1-min inhalation of each drug caused similar depressant effects on cough responses induced by both mechanical and chemical stimulation. A complete suppression of cough responses to mechanical stimuli was seen in some preparations. The results strongly suggest that the LAMA-induced downregulation of cough may be mediated not only by transient receptor potential vanilloid type 1 channels, as already reported, but also by acid-sensing ion channels and mechanoreceptors. The route of administration along with the more rapid hydrolysis of aclidinium into inactive metabolites minimize potential systemic side effects and give to this drug a very favorable safety profile.

Antiarrhythmic effects of dehydroevodiamine in isolated human myocardium and cardiomyocytes

Ethnopharmacological relevanceDehydroevodiamine alkaloid (DeHE), a bioactive component of the Chinese herbal medicine Wu-Chu-Yu (Evodiae frutus), exerted antiarrhythmic effect in guinea-pig ventricular myocytes. We further characterize the electromechanical effects of DeHE in the human atrial and ventricular tissues obtained from hearts of patients undergoing corrective cardiac surgery or heart transplantation. Materials and methodsThe transmembrane potentials of human myocardia were recorded with a traditional microelectrode technique while sarcolemmal Na+ and Ca2+ currents in single human cardiomyocytes were measured by a whole-cell patch-clamp technique. The intracellular pH (pHi) and Na+–H+ exchanger (NHE) activity were determined using BCECF-fluorescence in human atria. ResultsIn human atria, DeHE (0.1–0.3μM) depressed upstroke velocity, amplitude of action potential, and contractile force, both in slow and fast response action potential. Moreover, the similar depressant effects of DeHE were found in human ventricular myocardium. Both in isolated human atrial and ventricular myocytes, DeHE (0.1–1μM) reversibly, concentration-dependently decreased the Na+ and Ca2+currents. Moreover, DeHE (0.1 and 0.3μM) suppressed delayed afterdepolarizations and aftercontractions, induced by epinephrine and high [Ca2+]o in atria. In human ventricular myocardium, the strophanthidin-induced triggered activities were attenuated by pretreating DeHE (0.3μM). The resting pHi and NHE activity were also significantly increased by DeHE (0.1–0.3μM). ConclusionsWe concluded for the first time that, in the human hearts, DeHE could antagonize triggered arrhythmias induced by cardiotonic agents through a general reduction of the Na+ and Ca2+ inward currents, while increase of resting pHi and NHE activity.

CrossTalk opposing view: The pre‐Bötzinger complex is not essential for respiratory depression following systemic administration of opioid analgesics

Opioid receptors and enkephalinergic nerve terminals are widely distributed throughout respiratory-related regions of the brainstem and in the phrenic motor nucleus of the spinal cord (Xia & Haddad, 1991; Laferriere et al. 1999; Wang et al. 2002; Haji et al. 2003a; Lonergan et al. 2003a,b; Strornetta et al. 2003). Since opiate drugs given systemically will act on opioid receptors with conjoint selectivity in all respiratory regions, respiratory depression is unlikely to be dependent on actions at a single site. Therapeutic doses of opioids given to most mammalian species depress respiratory rate, minute ventilation, alveolar–arterial gas exchange and respiratory responsiveness to hypoxia and hypercapnia (Jaffe & Martin, 1990). Opioid-mediated depression of respiration is due at least in part to direct effects on the brainstem respiratory network, which includes several sites of action in medullary and pontine regions (reviewed by Pattison, 2008; Lalley, 2008). The degree of opioid-mediated respiratory depression depends on agonist dose, opioid receptor density and the subtypes of opioid receptor in various respiratory regions. Species variability and stage of development are also factors (Santiago & Edelman, 1985). In the paragraphs to follow, we review results of studies that indicate that the pre-Botzinger complex (preBotC) is not essential for respiratory depression by systemically administered opioid analgesics.

The Large Role of Prescription Drugs in Accidental Drug Deaths

Drug abuse may involve illicit drugs, prescription drugs, or the combination of illicit and prescription drugs, with or without the use of alcohol. Historically, illicit drugs have been responsible for many of the drug-related deaths investigated by medical examiner and coroner offices. However, in more recent years, deaths resulting from prescription drugs have become increasingly more common. This study reviewed all accidental (unintentional) drug deaths that were investigated at a medical examiner's office over a one-year time frame. The study revealed that prescription drugs made up the largest category of drug deaths, followed by prescription drug/illicit drug combinations, followed by illicit drugs. Drugs capable of causing or contributing to significant respiratory insufficiency such as opioids, benzodiazepines, relaxants (defined as muscle relaxants or sleep medications), and alcohol, or some combination thereof, were detected in 197 out of 256 (77%) of all accidental drug deaths, and were detected in 132 out of 138 (95%) of all prescription drug deaths (cases without any illicit drugs detected). These prescription drugs were most often found in combination, and their similar respiratory depressant effects can be cumulative and deadly.

Assessment of different induction protocols to elicit long-term depression (LTD) in the rat visual cortex in vivo

Changes in synaptic efficacy, including long-term potentiation (LTP) and long-term depression (LTD), provide mechanisms for experience-induced plasticity of cortical and subcortical circuits. LTP is readily induced under drastically different experimental conditions (e.g., in vitro and in vivo). However, few studies have compared the effectiveness of different induction protocols to elicit synaptic depression, especially under in vivo conditions. Here, we assessed the effectiveness of four different low frequency stimulation (LFS) protocols, applied to the lateral geniculate nucleus, to induce LTD-like changes of local field postsynaptic potentials (fPSPs) recorded on the surface of the primary visual cortex (V1) of urethane-anesthetized rats. Three LFS protocols (900 pulses at 1 Hz; 1800 pulses at 1 Hz, 1800 pulses at 1 Hz, repeated three times), known to induce LTD in neocortical and hippocampal slice preparations, failed to induce synaptic depression. In contrast, strong low frequency burst stimulation (3 pulses/burst at 20 Hz, 900 bursts repeated at 1 Hz) resulted in significant, but transient (∼20 min) depression of fPSPs in V1. This effect was resistant to systemic treatment with MK 801 (0.5 mg/kg) or local, cortical application of either APV (10 mM) or MCPG (10 mM), indicative of non-essential roles of N-methyl- d-aspartate and metabotropic glutamate receptors. A similar depressant effect was also observed under sodium pentobarbital anesthesia. These experiments emphasize the resistance of the in vivo neocortex to express the long-lasting down-regulation of synaptic strength, observations that require integration into current models and theories regarding the functions of LTD as a homeostatic and experience-dependent plasticity mechanism.

Non-competitive metabotropic glutamate 1 receptor antagonists block activity of slowly adapting type I mechanoreceptor units in the rat sinus hair follicle

Previous studies suggested that Group I metabotropic glutamate (mGlu) receptors play a role in mechanotransduction processes of slowly adapting type I mechanoreceptors. Using an isolated rat sinus hair follicle preparation we tested a range of compounds. Surprisingly, only non-competitive mGlu1 receptor antagonists produced profound and long-lasting depression of mechanically evoked firing. 6-Amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-α]benzimidazole-2-carboxamide hydrochloride (YM-298198) had an IC 50 of 8.7 μM (95% CI 5.7 to 13.2 μM), representing the most potent known blocker of type I mechanoreceptors. The derivative 6-amino-N-cyclohexyl-3-methylthiazolo[3,2-α]benzimidazole-2-carboxamide hydrochloride (desmethyl YM-298198) had a comparable potency. Another compound 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) had a similar depressant effect, although it was less potent with an approximate IC 50 of 100 μM. Between three and seven times the concentration of CPCCOEt and YM-298198 respectively was required to produce similar depressions in slowly adapting type II units. No depression, and some weak excitatory effects, were observed using the following ligands: the competitive mGlu1 receptor antagonist α-amino-5-carboxy-3-methyl-2-thiopheneacetic acid (3-MATIDA) (300 μM), the phosphoserine phosphatase inhibitor dl-2-amino-3-phosphonopropionic acid ( dl-AP3) (2 mM), non-competitive mGlu5 receptor antagonists 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine; (S)-3,5-DHPG, (S)-3,5-dihydroxyphenylglycine (MTEP) (10 μM) and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (100 μM), the mGlu1 receptor agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) (500 μM), and the mGlu5 receptor agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) (1 mM). The results suggest that the non-competitive mGlu1 receptor antagonists are not acting at conventional mGlu1 receptors but at other binding sites, possibly those directly associated with mechanogated channels or on any of a number of indirect biochemical pathways. YM-298198 and related compounds may prove to be useful ligands to identify mechanosensitive channel proteins. The selective interference of type I units may provide further evidence that Merkel cells are mechanotransducers. Finally such compounds may deliver insights or treatments for Merkel cell carcinoma.

Differential effects of GABA B autoreceptor activation on ethanol potentiation of local and lateral paracapsular GABAergic synapses in the rat basolateral amygdala

Many studies have demonstrated that GABAergic inhibition within the basolateral amygdala (BLA) plays an integral role in the regulation of anxiety, an important behavioral component in the etiology of alcoholism. Although ethanol has recently been shown to enhance BLA GABAergic inhibition via two distinct populations of inhibitory cells, local and lateral paracapsular (lpcs) interneurons, little is known about the mechanisms underlying ethanol potentiation of these two inhibitory pathways. Ethanol is known to enhance GABAergic inhibition in many brain regions via a complex array of pre- and postsynaptic mechanisms. In addition, ethanol's presynaptic effects are often subject to GABA B autoreceptor (GABA B-R) modulation. Therefore, in this study, we characterized GABA B-R function and modulation of ethanol actions at local and lpcs GABAergic synapses. At local synapses, we found significant paired-pulse depression (PPD, 250 ms inter-pulse interval) which was abated by SCH-50911 (GABA B-R antagonist). No significant PPD was detected at lpcs synapses, but SCH-50911 significantly potentiated lpcs-evoked IPSCs. Baclofen (GABA B-R agonist) had similar depressant effects on local- and lpcs-evoked IPSCs, however baclofen pretreatment only reduced ethanol potentiation at local synapses. Ethanol also significantly enhanced the frequency of spontaneous and miniature IPSCs, and these effects were also sensitive to GABA B-R modulators. Collectively, these data suggest that stimulus-independent inhibitory responses recorded from BLA principal neurons primarily reflect the activity of local GABAergic interneurons and provide additional evidence that ethanol potentiates local BLA inhibitory synapses primarily via a presynaptic enhancement of GABA release that is tightly regulated by GABA B-Rs. In contrast, ethanol potentiation of lpcs GABAergic synapses is not sensitive to GABA B-R activation and does not appear to involve increased presynaptic GABA release.

Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

Group II metabotropic (mGlu) receptors are known to play an important role in regulating the release of excitatory transmitter in a number of brain areas. Previous experiments demonstrated that (1 S,3 R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3 R-ACPD) depressed excitatory transmission in the adult rat barrel cortex. Here we show, using in vivo extracellular single unit recordings and iontophoretic application of drugs, that selective activation of Group II mGlu receptors depresses excitatory but not inhibitory transmission. The selective Group II receptor agonist (2 R,4 R)-4-aminopyrrolidine-2,4-dicarboxylate (2 R,4 R-APDC) had similar depressant effects to 1 S,3 R-ACPD on tactile evoked responses of rapidly adapting neurons. The depressant effects were seen on shorter latency (<12 ms) responses, were most pronounced in layers 3–4 (and 5b for 2 R,4 R-APDC only), and were reversibly antagonized by the Group II receptor antagonist (2 S)-α-ethylglutamic acid (EGLU) relative to depressions produced by iontophoretic GABA. Where 1 S,3 R-ACPD and 2 R,4 R-APDC depressed excitatory transmission, there was little or no effect on postsynaptic excitations produced by iontophoretic AMPA—a result that supports a presynaptic location of Group II receptors on excitatory terminals. To assess the possible involvement of Group II mGlu receptors in the modulation of inhibition, we studied the effect of iontophoretic 1 S,3 R-ACPD in a condition-test protocol. The results contrasted markedly from those previously observed using the Group III agonist l(+)-2-amino-4-phosphonobutyric acid in that activation of Group II receptors using 1 S,3 R-ACPD did not modulate inhibition. Therefore our results show that Group II mGlu receptors play an important role in modulating excitatory, but not inhibitory, transmission. We propose that the Group II mGlu receptors are located on excitatory terminals, and act as autoreceptors. Their role appears to be important in the early stages of cortical processing, by keeping excitatory inputs within specified physiological limits, and possibly by mediating depression evidenced during synaptic plasticity.

Neural mechanism of acupuncture-modulated gastric motility

To investigate the acupuncture-modulated gastric motility and its underlying neural mechanism. Intragastric pressure and/or waves of gastric contraction in rats were recorded by intrapyloric balloon and changes of gastric motility induced by acupuncture stimulation were compared with the background activity before any stimulation. Gastro-vagal or splanchnic-sympathetic nerves were recorded or cut respectively for investigating the involvement of autonomic nerve pathways. Spinalization experiment was also performed. Acupuncture-stimulation by exciting A delta and/or C afferent fibers, could only modulate gastric motility. Acupuncture-stimulation on fore- and hind-limbs evoked a moderate gastric motility followed by increased vagus discharges with unchanged sympathetic activity, while the same stimulus to the acupoints in abdomen resulted in reversed effects on gastric motility and autonomic nervous activities. The inhibitory gastric response was completely abolished by splanchnic denervation, but the facilitative gastric response to stimulation of acupoints in limbs was not influenced, which was opposite to the effect when vagotomy was performed. The similar depressive effects were produced by the stimulation at the acupoints homo-segmental to the gastric innervation in the animals with or without spinalization. However, the facilitation induced by the stimulation at the acupoints hetero-segmental to the gastric innervation was not observed in the spinalized animals. Facilitative effects of stimulating hetero-segmental acupoints are involved in the intact preparation of vagal nerves and spinal cord, while the inhibitory response induced by stimulating homo-segmental acupoints is involved in the intact preparation of sympathetic nerves. Only the acupuncture-stimulation with intensity over the threshold of A delta and/or C afferent fibers can markedly modulate gastrointestinal motility.

Presynaptic inhibition by kainate receptors converges mechanistically with presynaptic inhibition by adenosine and GABA B receptors

Kainate receptors are widely reported to regulate the release of neurotransmitter in the CNS, but the mechanisms involved remain controversial. Previous studies have found that the kainate receptor agonist ATPA, which selectively activates Glu K5-containing kainate receptors, depresses glutamate release at Schaffer-collateral synapses in the hippocampus. In the present study, we provide pharmacological evidence that this depressant effect is mediated by Glu K5-containing heteromers, but is distinct from a similar depressant effect engaged by the kainate receptor agonist domoate. The depressant effect of ATPA is insensitive to antagonists for GABA A, GABA B, and adenosine receptors, and is also unaffected by lowering the release probability by reducing extracellular calcium. However, the effect of ATPA is partly occluded by prior activation of GABA B receptors and completely occluded by prior activation of adenosine receptors, suggesting a mechanistic convergence of heteromeric Glu K5 kainate receptor signaling with GABA B receptors and adenosine receptors. The effects of domoate are partially occluded by both adenosine and GABA B receptor agonists, indicating at least a partial convergence of Glu K5-lacking kainate receptor signaling with these other pathways. The depressant effect of ATPA is not blocked by inhibition of serine/threonine protein kinases. These results suggest that ATPA and domoate inhibit glutamate release through mechanisms that converge with those of classical metabotropic receptor agonists, although they do so through different receptors.

Comparison of effects of octopamine and insecticidal essential oils on activity in the nerve cord, foregut, and dorsal unpaired median neurons of cockroaches

Essential oil constituents were tested for their neurophysiological effects in Periplaneta americana and Blaberus discoidalis. Eugenol depressed spontaneous and stimulus-evoked impulses recorded extracellularly in the abdominal nerve cord, with an almost complete block of spikes at 2×10 −3 M. Geraniol and citral had similar depressive effects but increased spontaneous firing at lower doses (threshold 2.5×10 −4 M). Similar effects occurred in dorsal unpaired median (DUM) neurons, recorded intracellularly in the isolated terminal abdominal ganglion of P. americana. Spontaneous firing was progressively reduced by increasing concentrations of eugenol, whereas geraniol and citral produced biphasic effects (excitation at 10 −4 M, depression at 2×10 −3 M). All three oils decreased excitability of silent DUM neurons that were depolarised by applied current, but eugenol (at 10 −3 M) also changed the firing pattern from single spikes to bursts driven by plateau potentials. All oils reduced spike undershoot. Low doses of citral and geraniol (threshold ca. 10 −4 M) reversibly increased the frequency of spontaneous foregut contractions and abolished them at 2×10 −3 M (together with response to electrical stimulation). Eugenol reversibly reduced spontaneous activity at 10 −4 M and above. Eugenol has been reported to exert its insecticidal properties via a low-dose activation of octopamine receptors. In our studies, however, octopamine was found to have opposing effects to eugenol on DUM neurons and foregut activity (excitatory in both). Furthermore, eugenol did not affect the response to octopamine in DUM neurons. These results suggest that reported effects of eugenol were on a different sub-type of octopamine receptor.

Pertussis toxin prevents adenosine receptor- and m-cholinoceptor-mediated sinus rate slowing and AV conduction block in the guinea-pig heart

The influence of pertussis toxin on the effects of adenosine, the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) and the m-cholinoceptor agonist carbachol on heart rate and atrioventricular (AV) conduction was investigated in spontaneously beating isolated perfused guinea-pig hearts. In addition, the effects of the agents on the electrocardiogram recorded from anesthetized guinea pigs were studied. Adenosine (0.1-100 mumol/l) and PIA (0.001-100 mumol/l) had concentration-dependent negative chronotropic and negative dromotropic effects. These effects were prevented by pretreatment of the animals with pertussis toxin (150 micrograms/kg; i.v.). Carbachol (0.001-100 mumol/l) had similar cardiac depressant effects. These effects were also abolished by pertussis toxin. In contrast, the negative chronotropic and negative dromotropic effects of the calcium antagonist verapamil which was investigated for comparison were not influenced by pretreatment with pertussis toxin. Since the cardiac depressant effects mediated via adenosine receptors or via m-cholinoceptors are most probably due to an activation of a K+ conductance, it is concluded that both receptors in the sinus node and in the AV node may be coupled via a common pertussis toxin-sensitive guanine nucleotide-binding protein to the K+ channel. It remains to be elucidated whether an additional inhibitory coupling to Ca2+ channels also plays a role.

Electrophysiological evidence for the role of substance P in retinohypothalamic transmission in the rat

The retinohypothalamic tract (RHT) is a neural pathway through which photic time cues are delivered directly to the mammalian circadian pacemaker in the suprachiasmatic nucleus (SCN). Although the excitatory amino acid glutamate is the primary neurotransmitter in the RHT, other substances such as substance P (SP) also have been suggested to play a role. The present study tested the hypothesis that SP participates in retinohypothalamic transmission and selectively modulates either N-methyl-d-aspartate (NMDA) or non-NMDA receptor-mediated neurotransmission. The SP antagonist L-703,606 depressed the excitatory postsynaptic current (EPSC) evoked by optic nerve stimulation in SCN neurons in rat hypothalamic slices. The SP antagonist also had a similar depressive effect on the NMDA and non-NMDA receptor-mediated components of the EPSC. These results suggest that SP is an excitatory neuromodulator contributing to the expression of both the NMDA and non-NMDA receptor-mediated components of retinohypothalamic transmission.

The NMDA receptor antagonist MK-801 differentially modulates μ and κ opioid actions in spinal cord in vitro

We have examined the interactions between NMDA receptors and opioid effects in isolated neonatal rat spinal cord. Electrical stimulation of a lumbar dorsal root evoked a nociceptive-related slow ventral root potential (sVRP) recorded at the corresponding ipsilateral ventral root. The κ opiate receptor agonist U69,593 (2.5 nM-1 μM) depressed sVRP area by a maximum of 80%; EC50 was approximately 33 nM. Both the non-specific antagonist naloxone and the κ-specific antagonist nor-binaltorphimine (nor-BNI) antagonized the effects of U69,593. Morphine, a μ agonist, (1 nM-1 μM) depressed sVRP area with an approximate EC50 of 90 nM. The effects of both μ and κ opioid agonists were selective for the very slow metabotropically mediated components of the sVRP, compared to the relatively fast NMDA receptor-mediated components. The non-competitive N-methyl- d-aspartate (NMDA) antagonist MK-801 (20 nM) had no effect on sVRP area when applied alone but co-applied with morphine significantly potentiated the depressant effects of morphine. In contrast, MK-801 either had no effect on or slightly antagonized the depressant effects of U69,593. Naloxone following morphine produced a significant increase in sVRP area above pre-morphine control values; the increase lasted 30 min or more. Neither naloxone nor nor-BNI was associated with an increase in sVRP area when given alone or following U69,593. MK-801 co-applied with morphine blocked the rebound increase in sVRP area following naloxone. These results suggest that (1) both μ and κ receptor agonists exert similar selective depressant effects on spinal nociceptive neurotransmission; (2) μ but not κ agonists exert prolonged excitatory effects that oppose the depression; and (3) NMDA receptors play a role in determining opioid analgesic potency and naloxone-precipitated hyperresponsiveness. The results may be related to initial steps in the development of acute tolerance to μ opioids, and suggest that tolerance to κ opioids may have a different mechanism.

Electrophysiological effects of monoamine oxidase inhibition on rat midbrain dopaminergic neurones: an in vitro study.

1 The effects of the inhibition of monoamine oxidase (MAO) type A and B have been evaluated on the spontaneous firing activity of the dopaminergic (principal) neurones of the rat midbrain intracellularly recorded from a slice preparation. 2 The non-specific MAO inhibitor, pargyline, superfused at a concentration of 10-100 microM, decreased or abolished the spontaneous firing discharge of the principal neurons in the subtantia nigra pars compacta and ventral tegmental area. This effect had a slow onset and appeared to be sustained. 3 The administration of the dopamine D2/3 receptor antagonist, sulpiride (100-300 nM), antagonized the pargyline-induced effect, while the superfusion of the dopamine D1 receptor antagonist, SCH 23390 (1-3 microM) did not counteract the induced inhibition of the firing rate. 4 The inhibitor for the MAO A, clorgyline (30-100 microM), reduced the firing rate of the dopaminergic neurones. A similar depressant effect was also observed when a MAO B inhibitor, deprenyl (30-100 microM), was applied. Lower concentrations of both drugs (300 nM-10 microM) did not produce consistent effects on neuronal discharge. 5 Our data suggest that only the blockade of both types of MAO enzymes favours the inhibitory action of endogenous dopamine on somato-dendritic D2/3 autoreceptors.

EEG arousal during laryngoscopy and intubation: comparison of thiopentone or propofol supplemented with nitrous oxide

We studied EEG arousal after laryngoscopy and intubation with standardized bolus induction of anaesthesia. Twenty patients were prospectively allocated randomly to induction with propofol 3 mg kg-1 (n = 10) or thiopentone (6 mg kg-1 (n = 10) and 50% nitrous oxide in oxygen. Neuromuscular block was produced with vecuronium 0.2 mg kg-1 given 30 s after induction. Three minutes after induction, laryngoscopy was performed for 60 s, with intubation at 3 min 30 s, and study end at 5 min. Nociception to laryngoscopy and intubation was followed by loss of low (relative delta activity change: thiopentone -30%, propofol -7%; P < 0.05) and a shift to higher frequency EEG activity (beta activity change: thiopentone +647%, propofol +61%; P < 0.05). This EEG arousal was greater in the thiopentone group, despite the fact that EEG depression was similar to that produced by propofol before laryngoscopy and intubation. Propofol and thiopentone in combination with nitrous oxide had similar cortical depressant effects, but propofol appeared to depress subcortical nociceptive processing more than thiopentone. While the degree of cortical EEG depression seems less useful for predicting reaction to subsequent nociception, EEG arousal reactions may prove suitable for monitoring intra-anaesthetic nociception and its modulation.

Neural Tolerance of the Non-Ionic Dimers Iodixanol and Iotrolan and the Non-Ionic Monomer Iopamidol during Myelography in Non-Anaesthetised Rabbits

The neural tolerance of the recently introduced dimer iodixanol (320 g I/l) was compared with that of the dimer iotrolan (300 g I/l) and of the monomer iopamidol (300 g I/l), both used in clinical myelography. Non-anaesthetised rabbits were injected into the cisterna magna with Ringer's solution (control) or contrast media (CM) at doses of 1.0 or 0.5 ml/kg b.w. The behaviour of the animals (10 in each of 7 groups) was evaluated for signs of excitation and depression during the first 3 hours after injection. At the dose level of 1.0 ml/kg b.w., iodixanol produced no seizures but did cause focal twitching in 4/10 rabbits. Iopamidol produced grand mal seizures in 2/10 and hyperexcitability in 4/10 rabbits. Iotrolan produced generalised grand mal seizures in 8/10 rabbits, an incidence of excitation significantly greater than that of iodixanol (p < 0.01) and iopamidol (p < 0.05). The excitative effects of iodixanol were not significantly different from those of iopamidol and Ringer's solution. All 3 CM produced similar depressive effects on rabbit behaviour. Ringer's solution caused no depressive effects (p < 0.01). A clear dose response was produced with all 3 CM after treatment with a lower dose of 0.5 ml/kg b.w. The results indicate that the neural tolerance in the rabbit of iodixanol is higher than that of iotrolan, and is at least equal to that of iopamidol.