RP 67580, a selective antagonist of neurokinin-1 receptors, modifies some of the naloxone-precipitated morphine withdrawal signs in rats.

In order to clarify the participation of substance P in the expression of opiate withdrawal, we have investigated the effects induced by the new selective neurokinin-1 antagonist RP 67580 on naloxone-induced morphine withdrawal syndrome in rats. Intracerebroventricular administration of RP 67580 elicited a decrease in 7 of the 13 withdrawal signs evaluated. Mastication, salivation and signs related to the motor component of withdrawal (jumping, rearing and locomotor activity) were particularly reduced. One sign, wet dog shakes, was increased, but it was also enhanced by the inactive enantiomer RP 68651. Our results indicate that blockade of NK1 receptors induces a decrease in the expression of naloxone-precipitated morphine abstinence in rats, and support the participation of substance P in the opiate withdrawal response.

Differential effects of general anaesthetics on identified molluscan neurones in situ and in culture.

1. The only unifying principle of general anaesthesia is that general anaesthetics interact with membrane components and no single cellular mechanism appears to explain their widespread effects in the central nervous system. 2. The gastropod mollusc, Lymnaea stagnalis, provides an excellent model system for studies on general anaesthetics because it has large, uniquely identifiable nerve cells. Several of these cells are interneurones with identified neurotransmitters and monosynaptic connections to other cells. 3. Recent work on Lymnaea neurones suggests that calcium currents are depressed by volatile general anaesthetics applied in the clinical range, whilst evidence from other preparations indicates that there is a rise in intracellular calcium concentration following application of these substances. 4. Identified Lymnaea neurones have different responses to applied anaesthetics, irrespective of the anaesthetic used. Following application of halothane, barbiturates and several other anaesthetic agents, some cells gradually become quiescent after a short period, whilst in others a series of paroxysmal depolarizing shifts occur prior to quiescence. 5. Cultured neurones of Lymnaea, Helisoma and related species retain their characteristic action potential types and neurotransmitter identity. Their responses to anaesthetics are similar to those in the intact brain. They may also form synapses in culture. Thus, they are a useful tool for studying the cellular and subcellular actions of general anaesthetics.

Pharmacological characterization of RP 62203, a novel 5-hydroxytryptamine 5-HT2 receptor antagonist.

1. RP 62203 (2-[3-(4-(4-fluorophenyl)-piperazinyl)propyl]naphto[1,8- ca]isothiazole-1,1-dioxide) is a novel naphtosultam derivative which shows very high affinity for 5-HT2 receptors in the rat cerebral cortex (Ki = 50.0 pM). 2. RP 62203 is relatively selective for this sub-type of 5-hydroxytryptamine (5-HT) receptor, having lower affinity for the 5-HT1A receptor and very low affinity for the 5-HT, receptor. RP 62203 displayed low to moderate affinity for alpha 1-adrenoceptors, dopamine D2 receptors and histamine H1 receptors. 3. In vivo binding experiments demonstrated that oral administration of low doses of RP 62203 led to a long-lasting (greater than 6 h) occupation of cortical 5-HT2 receptors (ID50 = 0.39 mgkg-1). 4. In cortical slices from the neonatal rat, RP 62203 potently inhibited inositol phosphate formation evoked by 5-HT, with an IC50 of 7.76 nM. 5. The activity of neurones in the raphé and their responses to microiontophoretically applied 5-HT were studied with extracellular recording electrodes in the anaesthetized rat. RP 62203 potently and dose-dependently blocked excitations evoked by 5-HT when administered at doses of 0.5-4.0 mg kg-1, i.p. In contrast, neither 5-HT-evoked depressions nor glutamate-evoked excitations of raphé neuronal firing were blocked by RP 62203 at doses as high as 8.0 mg kg-1, i.p. 6. Head twitches induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) could be abolished by low doses of RP 62203 in mice (ED50 = 0.44 mg kg-1, p.o.) and in rats (ED50 = 1.54 p.o.). Similar results were obtained with mescaline and 5-hydroxytryptophan (5-HTP). 7. The potency of RP 62203 was compared with that of three other 5-HT2 receptor antagonists, ritanserin, ICI 169,369 and ICI 170,809. In all models, RP 62203 showed similar activity to ritanserin, whilst either ICI 169,369 or ICI 170,809 was several fold less active. 8. It is concluded that RP 62203 is a potent and selective antagonist at 5-HT2 receptors in the rodent central nervous system.

The actions of the novel anti-aggressive drug eltoprazine on central neurones in the anaesthetised rat.

5-Hydroxytryptamine (5-HT) and the novel anti-aggressive drug eltoprazine (1-(2,3-dihydro-1,4-benzodioxin-5-yl) piperazine hydrochloride) were applied by microiontophoresis to spinal motorneurones and also to neurones in the brainstem which gave two distinctly different responses to 5-HT. In vitro microiontophoretic release studies showed that the electrophoretic mobility of eltoprazine and 5-HT were similar and that similar amounts of each drug would be applied by similar iontophoretic currents. Cells in the brainstem have been shown previously to be excited by 5-HT, acting at a 5-HT2 receptor. Eltoprazine only occasionally and weakly mimicked the excitatory effect of 5-HT on these cells. Although a potent antagonism of the 5-HT excitation by eltoprazine was observed, this was a non-selective effect, as responses to glutamate and D,L-homocysteic acid were also reduced. Cells in the lateral brainstem are depressed by 5-HT, acting on a receptor which has previously been shown to be of the 5-HT1-like group. At this receptor, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) and 5-carboxamidotryptamine, are potent agonists. Eltoprazine was a more potent depressant agonist than 5-HT on these brainstem neurones. The antagonist metergoline did not antagonise responses to either 5-HT or eltoprazine. It is suggested however that both drugs act at the same receptor to depress these cells because desensitizing the receptor by repeated, frequent applications of 5-HT abolished responses to 5-HT and eltoprazine, without altering responses to GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

A local circuit neocortical synapse that operates via both NMDA and non-NMDA receptors.

1. In slices of rat neocortex, spike triggered averaging was employed to record in one neurone the excitatory postsynaptic potential (e.p.s.p.) generated by a spike in another, neighbouring neurone. When recorded at different membrane potentials, some of these e.p.s.ps exhibited a voltage relation typical of neuronal responses to N-methyl-D-aspartate (NMDA). 2. Selective NMDA antagonists reduced the amplitude of these e.p.s.ps, but had little effect on their early rising phase. In contrast, a less selective excitatory amino acid antagonist reduced all phases of the e.p.s.p. 3. By analyzing single axon e.p.s.ps we have been able to establish that the synaptic input to one cortical cell, delivered by a single presynaptic cortical cell, operates simultaneously via NMDA and non-NMDA amino acid receptors.

The actions of halothane on spontaneous activity, action potential shape and synaptic connections of the giant serotonin-containing neurone of Lymnaea stagnalis (L.).

1. Cerebral giant cells (CGCs) in the isolated central nervous system (CNS) of the pond snail Lymnaea stagnalis (L.) exhibit bursting activity when superfused with anaesthetic concentrations of halothane. 2. Calcium-dependent components of the CGC action potential appear more sensitive to halothane than do other ionic mechanisms. 3. Higher concentrations of halothane block the chemical synaptic connection between CGC and buccal motoneurone B1, but have no effect on the strong electrotonic coupling between the CGCs. 4. The mechanisms underlying CGC bursting and synaptic block in the presence of halothane are discussed.

A system for the application of general anaesthetics and other volatile agents to superfused, isolated tissue preparations.

1. A delivery system for the application of general anaesthetics or other gaseous and volatile agents to superfused, isolated preparations is described in detail. 2. This system delivers known concentrations of anaesthetic and controls for evaporation and absorption of volatile agents, whilst allowing intracellular electrophysiological recordings to be made from the tissue with minimal disturbance. 3. In particular, this delivery system permits accurate, controlled experiments to be carried out on the neuronal actions of general anaesthetics.

The actions of three volatile general anaesthetics on withdrawal responses of the pond-snail Lymnaea stagnalis (L.).

1. The gastropod mollusc Lymnaea stagnalis (L.) is an ideal model system for studies on anaesthesia. It is reversibly anaesthetized by the general anaesthetics halothane, enflurane and isoflurane. 2. Criteria for "anaesthesia" in Lymnaea were established. The reflex used in ED50 trials was the whole animal withdrawal reflex. 3. ED50 values for halothane, enflurane and isoflurane were, 0.83% v.v. (volume for volume), 1.01% v.v. and 1.09% v.v. respectively. 4. Relationships between anaesthetic concentrations, weights of animals and mortality rates are reported.

Voltage-dependent currents prolong single-axon postsynaptic potentials in layer III pyramidal neurons in rat neocortical slices.

1. Using isolated slices of rat cingulate and sensorimotor cortex, intracellular recordings were obtained from pyramidal neurons in layer III. Simultaneous extracellular recordings were obtained from neurons in ventral layer III and layer IV. Spike-triggered averaging was employed to investigate synaptic connections from neurons in layers III/IV to pyramidal cells in layer III. 2. Of 701 simultaneously recorded pairs of neurons, comprising 699 extracellularly and 128 intracellularly recorded neurons, synaptic connections were demonstrated in 30 pairs. Of these, 29 were excitatory postsynaptic potentials (EPSPs) and 1, an inhibitory postsynaptic potential (IPSP). Single-axon EPSPs with a wide variety of amplitudes were recorded: the range recorded at membrane potentials between -68 and -72 mV was 0.079-2.3 mV. Comparing recordings obtained from different cells, EPSP amplitude was found to be independent of both the membrane resistance of the postsynaptic neuron and the EPSP time course; i.e., the largest EPSPs were not necessarily those recorded from neurons with the highest input resistance, nor those with the briefest time course. 3. Shape indices: width at half amplitude and rise-time, indicative of both proximal and distal synaptic locations were obtained. Normalized rise-times were between 0.1 and 2 times the membrane time constant and half-widths between 0.8 and 20 times. 4. The majority of postsynaptic neurons displayed nonlinear voltage relations typical of pyramidal neurons, and the contribution to EPSP shape of voltage-dependent currents was investigated. EPSP amplitude and duration were found to be dependent on membrane potential. The majority of single-axon EPSPs (26 of 29), increased in amplitude and duration with membrane depolarization over the range -95 - -50 mV, despite the significant decrease in driving force for the EPSP that would be expected to accompany such large depolarizations. This increase coincided with an increase in the amplitude of voltage responses to small injected current pulses. 5. It is concluded that the amplitude and time course of single-axon EPSPs recorded in cortical pyramidal somata are affected not only by the amplitude of the postsynaptic current and the location(s) of the synapse(s) relative to the soma, but also by voltage-dependent currents. The possibility that the increase in amplitude and duration of these EPSPs with membrane depolarization is due to N-methyl-D-aspartate receptor involvement is discussed.

Effects of 5-hydroxytryptamine agonists and antagonists on the responses of rat spinal motoneurones to raphe obscurus stimulation.

1. The excitability of lumbar spinal motoneurones was studied in halothane-anaesthetized rats by recording with microelectrodes the amplitude of the population spike evoked antidromically by stimulation of the cut ventral roots. 2. Electrical stimulation of the nucleus raphe obscurus for 1 min at 20 Hz increased the population spike amplitude and, as shown by intracellular recording, depolarized motoneurones. This response could be mimicked by microinjection of DL-homocysteic acid into raphe obscurus but the response was not present in animals pretreated with the 5-hydroxytryptamine (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). 3. Microiontophoretically applied 5-HT had very similar effects on the extracellularly recorded population spike to those caused by stimulation of the raphe obscurus. These responses to 5-HT were larger in 5,7-DHT-pretreated animals. 4. The effects of 5-HT were potently mimicked by iontophoretically applied 5-carboxamidotryptamine but 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) was without effect. 5. Antagonists were applied by microiontophoresis and also by intravenous injection. Ketanserin, the selective 5-HT2 antagonist, did not antagonize the effects of 5-HT. Neither did the 5-HT3-receptor antagonist MDL 72222 or the selective 5-HT1 binding ligand cyanopindolol. 6. The non-selective 5-HT1/5-HT2-receptor antagonist methysergide was an effective antagonist of both the effects of 5-HT and the response to raphe obscurus stimulation. Methysergide did not reduce the excitatory effects of noradrenaline. 7. It is concluded that 5-HT application and stimulation of raphe obscurus increase the excitability of motoneurones by an action on a 5-HT1-like receptor which appears to be different from the 5-HT1A-and the 5-HT1B-binding sites characterized by others.

Transneuronal transport of wheat germ agglutinin-conjugated horseradish peroxidase into trigeminal interneurones of the rat.

Intramuscular injections of either horseradish peroxidase (HRP) or wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) were made into the masseter muscle of rats. Both tracers labeled primary sensory neurones in the V mesencephalic nucleus, motoneurones in the V motor nucleus, and some motoneurones in the facial motor nucleus. WGA-HRP labeled additional neurones in the V main sensory nucleus and the rostral pole of the V nucleus oralis. These were classed as interneurones because they lay in areas outside those known to contain either first-order afferent or motoneurone somata. We argue that these were labeled by retrograde transport of tracer because they lay close to the V motor nucleus, and from some of them processes could be followed into the region of the V motor nucleus.

Effects of halothane on feeding motor activity in the snail Lymnaea stagnalis.

Snails exposed to the general anaesthetic halothane show an increase in biting plus mouthing movements. Perfusion of the isolated CNS with halothane leads to a period of increased spiking activity, followed by suppression of activity in identified feeding motoneurones in the buccal ganglia. Synaptic inputs to motoneurones from interneurones of the buccal feeding pattern generator are differentially affected. Possible mechanisms underlying the generation of motoneuronal bursting in the presence of halothane are examined.