Using psychometric ability to improve education in ultrasound-guided regional anaesthesia: a multicentre randomised controlled trial.

The learning curve for novices developing regional anaesthesia skills, such as real-time ultrasound-guided needle manipulation, may be affected by innate visuospatial ability, as this influences spatial cognition and motor co-ordination. We conducted a multinational randomised controlled trial to test if novices with low visuospatial ability would perform better at an ultrasound-guided needling task with deliberate practice training than with discovery learning. Visuospatial ability was evaluated using the mental rotations test-A. We recruited 140 medical students and randomly allocated them into low-ability control (discovery learning), low-ability intervention (received deliberate practice), high-ability control, and high-ability intervention groups. Primary outcome was the time taken to complete the needling task, and there was no significant difference between groups: median (IQR [range]) low-ability control 125 s (69-237 [43-600 s]); low-ability intervention 163 s (116-276 [44-600 s]); high-ability control 130 s (80-210 [41-384 s]); and high-ability intervention 177 s (113-285 [43-547 s]), p = 0.06. No difference was found using the global rating scale: mean (95%CI) low-ability control 53% (95%CI 46-60%); low-ability intervention 61% (95%CI 53-68%); high-ability control 63% (95%CI 56-70%); and high-ability intervention 66% (95%CI 60-72%), p = 0.05. For overall procedure pass/fail, the low-ability control group pass rate of 42% (14/33) was significantly less than the other three groups: low-ability intervention 69% (25/36); high-ability control 68% (25/37); and high-ability intervention 85% (29/34) p = 0.003. Further research is required to determine the role of visuospatial ability screening in training for ultrasound-guided needle skills.

Association of obesity with failure of ultrasound-guided axillary brachial plexus block: a two-centre, prospective, observational, cohort study.

The objective of this study was to evaluate whether the failure rate of ultrasound-guided axillary brachial plexus block is similar in obese patients compared with non-obese patients when performed as the primary anaesthetic technique. We recruited 105 obese (body mass index ≥ 30 kg.m-2 ) and 144 non-obese patients to this prospective, observational, cohort study conducted at two Canadian centres. A perineural technique of axillary brachial plexus block was performed using 30 ml ropivacaine 0.5% under real-time ultrasound guidance. Sensory and motor block assessment was carried out every 5 min until 30 min after block completion in all four terminal nerve distributions (radial, median, ulnar and musculocutaneous nerve). A composite score consisting of three sensory points and three motor points was used for assessment in each nerve distribution. A failed block was defined as a score of less than 14 points out of a possible 16 points, or a sensory block score less than 7 out of 8 points 30 min after block completion. Thirty minutes after block completion, obese patients had a higher failure rate of 33.7% (34/101) compared with 17.8% (24/135) for non-obese patients, with a failure rate difference (95%CI) of 15.9% (6.4-27.1%) between the groups. The median (IQR [range]) time to achieve a successful block in obese patients was 25 (20-30 [5-30]) min, compared with non-obese patients at 20 (15-30 [5-30]) min (p = 0.003). Despite a higher sensory-motor failure rate as per the composite score, the axillary brachial plexus block provided adequate surgical anaesthesia as indicated by a low need for conversion to general anaesthetic in obese (8.6%) and non-obese patients (7.0%; p = 0.656). This study showed that despite ultrasound guidance, obese patients had a slower onset time and higher axillary brachial plexus block failure rate at 30 min compared with non-obese patients.

Effect of ammonium ions on synaptic transmission in the mammalian central nervous system.

It is not surprising that a compound with such unique properties as NH3/NH4+, should have a large variety of biochemical and neurological effects and to find itself implicated in many pathological conditions. Its undissociated (NH3) or dissociated (NH4+) forms, having different physicochemical properties, enter neurons and other cells through differing pathways. These two forms then change internal pH in opposite directions, and initiate a variety of regulatory processes that attempt to overcome these pH changes. In addition, ammonia has a central role in normal intermediary metabolism, and when present in excess, it can disturb reversible reactions in which it participates. The challenge in interpreting these various observations lies in the difficulty in assigning to them a role in the generation of symptoms seen in experimental and clinical hyperammonemias. In this review we have attempted to summarize information available on the effects of ammonium ions on synaptic transmission, a central process in nervous system function. Evidence has been presented to show that ammonium ions, in pathologically relevant concentrations, interfere with glutamatergic excitatory transmission, not by decreasing the release of glutamate, but by preventing its action on post-synaptic AMPA receptors. Furthermore, NH4+ depolarizes neurons to a variable degree, without consistently changing membrane resistance, probably by reducing [K+]i. A decrease in EK+ may also be responsible for decreasing the effectiveness of the outward chloride pump, thus explaining the well known inhibitory effect of NH4+ on the hyperpolarizing IPSP. There is a consensus of opinion that chronic hyperammonemia increases 5HT turnover and this may be responsible for altered sleep patterns seen in hepatic encephalopathy. There does not seem to be a consistent effect on catecholaminergic transmission in hyperammonemias. However, chronic hyperammonemia causes pathological changes in perineuronal astrocytes, which may lead to a reduced uptake of released glutamate and a decreased detoxification of ammonia by the brain. Chronic moderate increase in extracellular glutamate results in a down-regulation of NMDA receptors, while the decreased detoxification of ammonia makes the central nervous system more vulnerable to a sudden hyperammonemia, due, for instance, to an increased dietary intake of proteins or to gastrointestinal bleeding in patients with liver disease. Clearly, data summarized in this review represent only the beginning in the elucidation of the mechanism of ammonia neurotoxicity. It should help, we hope, to direct future investigations towards some of the questions that need to be answered.

The effects of cortical ablation on multiple unit activity in the striatum following dexamphetamine.

Bilateral removal of the fronto-parietal cortex of the rat resulted in decreased spontaneous multiple-unit activity recorded in the striatum of freely-moving rats. Cortical ablations changed the neuronal response in the striatum to systemic administration of dexamphetamine (2.5 mg/kg i.p.) from excitation in control animals (88%) to inhibition in ablated animals (61%). Furthermore, catalepsy, induced by haloperidol, but not by morphine, was markedly attenuated after cortical ablation. These changes were accompanied by a 23% decrease in the specific binding of [3H]spiperone in the striatum. The binding of [3H]met-enkephalin was unaffected by the cortical lesions. Levels of glutamate in the striatum decreased from 8.88 +/- 0.5 mumols/g in control animals to 6.93 +/- 0.37 mumols/g after bilateral cortical ablation. On the other hand, cortical ablations did not alter the content of either the gamma-aminobutyric acid or glutamine of the striatum. It is concluded that the excitatory response, observed in striatal neurons in freely-moving animals, is dependent upon an intact cerebral cortex and requires intact cortico-striatal afferents. The results further suggest that neurons in the striatum are under the tonic influence of glutamate, released from cortico-striatal afferents. Lastly, some dopamine D2 binding sites in the striatum are located on cortico-striatal afferent terminals and blockade of these striatal D2 sites may be involved in the induction of catalepsy by neuroleptic drugs.

Modification of neocortical acetylcholine release and electroencephalogram desynchronization due to brainstem stimulation by drugs applied to the basal forebrain.

Acetylcholine released from the cerebral cortex was collected using microdialysis while stimulating the region of the pedunculopontine tegmentum in urethane-anesthetized rats. Electrical stimulation in the form of short trains of pulses delivered once per minute produced a 350% increase in acetylcholine release and a desynchronization of the electroencephalogram, as measured by relative power in the 20-45 Hz range (low-voltage fast activity). Perfusion of the region of cholinergic neurons believed to be responsible for the cortical release of acetylcholine, the nucleus basalis magnocellularis, was carried out using a second microdialysis probe. Exposure of the nucleus basalis magnocellularis to blockers of neural activity (tetrodotoxin or procaine) or to blockers of synaptic transmission (calcium-free solution plus magnesium or cobalt) produced a substantial decrease in the release of acetylcholine and desynchronization evoked by brainstem stimulation. Exposure of the nucleus basalis magnocellularis to the glutamate antagonist, kynurenate, resulted in a decrease in evoked acetylcholine release and electroencephalogram desynchronization similar in magnitude to that produced by nonspecific blockers, whereas application of muscarinic or nicotinic cholinergic blockers to the nucleus basalis magnocellularis did not reduce acetylcholine release or electroencephalogram desynchronization. Application of tetrodotoxin to the collection site in the cortex abolished the stimulation-evoked acetylcholine release, but not the low baseline release indicating that cholinergic nucleus basalis magnocellularis neurons have a low spontaneous firing rate in urethane-anesthetized animals. The results of this study suggest that the major excitatory input to the cholinergic neurons of the nucleus basalis magnocellularis from the pedunculopontine tegmentum is via glutamatergic and not cholinergic synapses.

Neurochemical and electrophysiological studies on the inhibitory effect of ammonium ions on synaptic transmission in slices of rat hippocampus: evidence for a postsynaptic action.

To elucidate the mechanisms involved in the inhibition of synaptic transmission by ammonium ions, the effects of NH4Cl on glutamate release and on synaptic transmission from Schaffer collaterals to CA1 pyramidal cells were measured in fully submerged slices of rat hippocampus. The large, Ca(2+)-dependent release of glutamate evoked by electrical-field stimulation or by 56 mM K+ was not reduced by 5 mM NH4Cl. In contrast, 5 mM NH4Cl decreased the smaller, field stimulation-induced release of glutamate observed in the presence of low concentrations of Ca2+ (0.1 mM), as well as the spontaneous release of glutamate both in normal and low Ca2+. Unlike the Ca(2+)-dependent release of glutamate, synaptic transmission was reversibly depressed even by 1 mM NH4 Cl. Firing of CA1 pyramidal cells evoked by iontophoretically applied glutamate was significantly inhibited by 2 or 5 mM NH4Cl. This depression was increased in the presence of 25 microM bicuculline. Results suggest that ammonium ions do not depress the Ca(2+)-dependent release of glutamate originating from synaptic vesicles, which is involved in synaptic transmission. Rather, ammonium ions inhibit synaptic transmission by a postsynaptic action, a conclusion strengthened by the inhibitory effect of NH4Cl on glutamate-induced firing. However, NH4Cl may inhibit the formation of cytoplasmic glutamate, the source of spontaneous and Ca(2+)-independent release.

The release of labelled acetylcholine and choline from cerebral cortical slices stimulated electrically.

1 In order to establish the origin of the increased efflux of radioactivity caused by electrical stimulation of cerebral cortical slices which had been incubated with [(3)H]-choline, labelled choline and acetylcholine (ACh) collected by superfusion were separated by gold precipitation.2 In the presence of physostigmine electrical stimulation (1 Hz, 10 min) increased the release of only [(3)H]-ACh which was greatly enhanced by the addition of atropine.3 Continuous stimulation in the presence of physostigmine resulted in an evoked release of [(3)H]-ACh which declined asymptotically. This evoked release appeared to follow first-order kinetics with a rate constant which remained stable over the course of prolonged stimulation.4 The rate constant for the evoked release of [(3)H]-ACh with 1 Hz stimulation was three times greater in the presence of physostigmine and atropine than in the presence of physostigmine alone, while the size of the store from which [(3)H]-ACh was released was nearly identical under these two conditions.5 In the absence of physostigmine and atropine, stimulation caused the appearance of only [(3)H]-choline in the samples.6 Reduction of [(3)H]-ACh stores before the application of physostigmine resulted in a reduced evoked release of total radioactivity, both in the absence or presence of physostigmine and atropine, and decreased the evoked release of [(3)H]-ACh without affecting the release of [(3)H]-choline.7 Results suggest that electrical stimulation of cortical slices which had been incubated with [(3)H]-choline causes the release of only [(3)H]-ACh, both in the presence or absence of an anticholinesterase. The evoked increase in the efflux of total radioactivity is therefore a good measure of the release of [(3)H]-ACh.

Kinetics of morphine-sensitive [3H]-acetylcholine release from the guinea-pig myenteric plexus.

1 Longitudinal muscle-myenteric plexus preparations from the guinea-pig ileum were superfused at a constant rate while isotonic contractions were monitored. 2 The preparations were superfused with [3H]-choline while stimulated supramaximally at 0.1 Hz followed by washout in the presence of hemicholinium-3. The evoked release of the label due to a second 0.1 Hz stimulation in the absence of an anticholinesterase was measured. 3 Evoked efflux of the label was initially fast followed by a slower phase. 4 Morphine reduced the size of the pool and the rate of the initial fast efflux and the size of the pool but not the rate of the slow efflux evoked by supramaximal stimulation. 5 Submaximal stimulation reduced only the size of the pools from which the fast and slow efflux originated. 6 Naloxone reversed the depression of contractions and evoked release produced by morphine. 7 Results suggest that 0.1 Hz stimulation releases [3H]-acetylcholine simultaneous from two pools. The fast release may originate from spontaneously firing units whose rate of discharge is depressed by morphine, while the slow release originates from neurones which do not fire spontaneously and whose threshold to field stimulation is increased by morphine.

The output per stimulus of acetylcholine from cerebral cortical slices in the presence or absence of cholinesterase inhibition.

1 The release of endogenous acetylcholine (ACh) from cerebral cortical slices stimulated at 0.25, 1, 4, 16 and 64 Hz was measured in the presence either of physostigmine or of physostigmine and atropine.2 Atropine potentiated the evoked release of endogenous ACh especially at low frequencies resulting in an output per stimulus which sharply declined with increasing frequency of stimulation, while in the absence of atropine the output of ACh per stimulus was low and fairly constant.3 The evoked release of [(3)H]-ACh per stimulus following the incubation of the slices with [(3)H]-choline, as estimated by means of rate constants of the evoked release of total radioactivity, showed a frequency dependence similar to endogenous ACh when the two were tested under identical conditions.4 In the absence of an anticholinesterase the evoked release of [(3)H]-ACh per stimulus was dependent on frequency of stimulation in a similar way to that in the presence of physostigmine and atropine.5 Results suggest that under physiological conditions, i.e. in the absence of an anti-cholinesterase, the release of ACh per stimulus decreases with increasing frequency of stimulation and that this decrease is due to a lag in the mobilization of stored ACh rather than in the synthesis of new ACh.

Possible reasons for the failure of glutamine to influence GABA release in rat hippocampal slices; Effect of nipecotic acid and methionine sulfoximine.

Although labelled glutamine is readily incorporated into labelled releasable GABA, it has been shown recently that high concentrations (0.1-0.5 mM) glutamine do not increase the release of GABA from brain slices, while greatly enhancing that of glutamate. Two possible reasons for this discrepancy were investigated: (a) That released GABA, in contrast to glutamate is not freshly synthesized but derives from GABA taken up by terminals. The possibility was made unlikely by the present finding which showed that even in the presence of the uptake inhibitor nipecotic acid, glutamine failed to enhance GABA release. (b) That glutamine is transported into GABA-ergic terminals by a high-affinity transport system which is saturated even at low glutamine concentrations obtained without adding glutamine to the superfusion fluid. However, when glutamine efflux was further reduced by prolonging depolarization with 50 mM K(+) and by pretreatment with the glutamine synthetase inhibitor methionine sulfoximine, GABA release was depressed only very little and this decrease was related to the duration of depolarization and not to extracellular glutamine levels. These results can be reconciled with the ready incorporation of labelled glutamine into releasable GABA by assuming that GABA originates from a glutamate pool to which both glutamine and glucose contribute. The formation of releasable GABA however, is not governed by the supply of glutamate in this pool but by the activity of the rate-limiting enzyme glutamate decarboxylase.

Storage and release of endogenous and labelled GABA formed from [3H]glutamine and [14C]glucose in hippocampal slices: effect of depolarization.

To study the effect of depolarization on the synthesis, storage and release of GABA, hippocampal slices were incubated in 0.25 mM [3H]glutamine and 2.5 mM [14C]glucose in the presence of 3 or 50 mM K+. Total and labelled glutamine, glutamate and GABA contents were measured by high-performance liquid chromatography. Depolarization in the presence of Ca2+ led to a two-fold increase of labelled glutamate and a 3-fold increase of labelled GABA content originating from both labelled precursors. In the absence of Ca2+ and in the presence of 10 mM Mg2+, depolarization failed to increase labelled glutamate content and labelled GABA formation was increased by only 30%. Following superfusion with unlabelled 0.25 mM glutamine and 2.5 mM glucose a second depolarization with 50 mM K+ released twice as much labelled GABA from slices that had been incubated in the presence of 50 mM K+, than from those incubated in 3 mM K+. This difference remained unchanged in slices that were superfused with 1 mM aminooxyacetic acid, an inhibitor of GABA synthesis. The contribution of labelled GABA, especially of GABA derived from [3H]glutamine, to released GABA was significantly higher than to GABA stored in the slices. Results suggest that depolarization in the presence of Ca2+ results in increased glutamate and GABA synthesis from both glutamine and glucose and that part of GABA released by high K+ originates from preformed GABA stores.

Glutamate release and spreading depression in the fascia dentata in response to microdialysis with high K+: role of glia.

To see electrophysiological and neurochemical events during microdialysis with high [K+], direct current (DC) and excitatory postsynaptic field potentials (fEPSPs) due to perforant path stimulation were recorded in the granule cell layer of the fascia dentata, while 3, 25, 50 or 100 mM KCl was perfused through a microdialysis probe placed 1.5 mm from the recording electrode. Glutamate and glutamine content of the dialysate was measured by high performance liquid chromatography. Raising [K+] from 3 to 25 mM reduced the efflux of glutamine, without affecting that of glutamate or the electrical activity. In about 50% of experiments, 50 mM K+ induced large (20-30 mV) negative waves of spreading depression (SD), and a suppression of fEPSPs. In the other 50%, without SD, fEPSPs did not change. Glutamate efflux increased 3-fold in both groups. SD waves were produced in all experiments with 100 mM K+ which evoked a more than 10-fold increase in glutamate release. Glutamine efflux decreased equally, by about 50%, with the 3 concentrations of K+. Microdialysis with 20 mM fluoroacetate, a glial metabolic poison, decreased the spontaneous efflux of glutamine and glutamate and increased the incidence of SD waves. Results suggest that perfusion of 50 or 100 mM K+ through a microdialysis probe causes spreading depression which blocks surrounding electrical activity. The activity of glia partly protects against spreading depression caused by high [K+].

Increase in the stimulation-induced overflow of glutamate by fluoroacetate, a selective inhibitor of the glial tricarboxylic cycle.

Fluoroacetate is known to be taken up selectively by glia, where after forming fluorocitrate, it inhibits the tricarboxylic acid cycle. Since uptake into glia has a major role in the inactivation of synaptically released glutamate, the effect of fluoroacetate on the overflow of glutamate evoked by electrical field stimulation in slices of rat hippocampus was investigated. In agreement with previous reports, 1 mM fluoroacetate reduced the release and content of glutamine, but increased only slightly the overflow of glutamate induced by stimulation. If, however, 0.5 mM glutamine was added to the superfusion fluid, fluoroacetate nearly tripled the overflow of glutamate evoked by electrical field stimulation. The large glutamate overflow due to field stimulation in the presence of fluoroacetate was fully Ca2+ -dependent. Results confirm the major role of glia in the inactivation of glutamate. The absence of such an uptake may contribute to the in vivo convulsive effect of fluoroacetate.

Effects of ammonium ions on synaptic transmission and on responses to quisqualate and N-methyl-D-aspartate in hippocampal CA1 pyramidal neurons in vitro.

Effects of NH4Cl on CA1 pyramidal neurons and synaptic transmission were investigated with intracellular recording in fully submerged rat hippocampal slices. Superfusion with 1-4 mM NH4Cl reversibly depolarized the membrane by 15.1 +/- 1.4 mV, reduced the amplitude and broadened the duration of action potentials due to a slower rate of repolarization, without significant change in membrane conductance. When membrane potential was returned to control level by the injection of a steady outward current, action potential amplitude recovered but repolarization remained slow. The extent of depolarization was not dependent on the concentration of NH4Cl between 1 and 4 mM. NH4Cl greatly depressed orthodromic transmission evoked by the stimulation of Schaffer collateral/commissural fibers several minutes after depolarizing the CA1 neuron. Interruption of transmission began with a decrease in excitatory postsynaptic potential (EPSP) amplitude and eventually EPSPs were almost eliminated. When NH4Cl was removed, it took 2-3 min for membrane potential and 10-15 min for transmission to recover. Inward currents induced by bath application of quisqualate acting on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors were also depressed. In contrast, NH4Cl enhanced N-methyl-D-aspartate (NMDA)-induced currents. This potentiation disappeared in the absence of added Mg2+. A reduction in quisqualate-induced responses provided a possible explanation for the inhibition of excitatory transmission by NH4Cl.

The effect of cholinergic drugs on [3H]acetylcholine release from slices of rat hippocampus, striatum and cortex.

Slices from rat hippocampus, striatum or cortex were incubated with l mum [3H] choline and following 75 min superfusion with Krebs solution the efflux of radioactivity was measured. The slices were stimulated either electrically (1 Hz) or with 25 mM potassium and the rate constant of the evoked release and the size of the releasable pool were estimated. The spontaneous efflux of radioactivity and the releasable pool but not the rate of evoked release correlated with the reported endogenous ACh content of the 3 areas. Raised potassium released radioactivity at a lower rate but from a larger pool than electrical stimulation from all 3 areas. In all 3 areas atropine alone potentiated while physostigmine, oxotremorine and carbamylcholine decreased the rate of evoked release. This depression was fully antagonized by atropine. The drugs had no effect on the size of the releasable pool. Findings suggest that muscarinic receptors located on cholinergic axons or terminals have a physiological role in the autoregulation of ACh release from these 3 areas.

Acetylcholine release from visual and sensorimotor cortices of conditioned rabbits: the effects of sensory cuing and patterns of responding.

A technique was devised for the collection of acetylcholine (ACh) released from the cerebral cortex of awake rabbits while they were performing a previously learned operant task. Based on the assumption that ACh release is directly proportional to the activity of cholinergic synapses under the area of collection, two hypotheses of the functional role of cortical cholinergic mechanisms were examined: (1) that activity in cholinergic neurons is related to the inhibition of responding; (2) that cholinergic activity is related to the perception of a 'significant' stimulus. Five groups trained on different behavioral paradigms were used to test these hypotheses. ACh release was collected concurrently from visual and sensorimotor cortices to differentiate diffuse from specific cortical effects. A small (50-100%) increase in ACh release was found in all groups and from both cortical areas. In the case of one group (visually cued, reinforced for low response rates) a significantly greater increase occurred from sensorimotor cortex only. These findings do not support either hypothesis alone, and are interpreted as evidence for two cholinergic systems within, or projecting to the cortex. One is related to generalized behavioral arousal and desynchronization of the electroencephalogram. Activation of the second cholinergic system is dependent on both response inhibition and the presence of a significant stimulus of the visual (but not of the auditory) modality.

Is glutamate a co-transmitter in cortical cholinergic terminals? Effects of nucleus basalis lesion and of presynaptic muscarinic agents.

To obtain additional evidence in support of the co-transmitter role of glutamate in cortical cholinergic terminals proposed by Docherty et al., the right nucleus basalis in rats was lesioned with ibotenic acid; resulting changes in cortical acetylcholinesterase (AChE) staining, glutamate content, and the release of [3H]acetylcholine ([ 3H]ACh) and glutamate from cortical slices from the two sides were compared. While there was a profound reduction on the lesioned side in cortical AChE activity and in the size of the releasable pool of [3H]ACh, neither the content nor the evoked release of glutamate was reduced significantly on the lesioned side. Furthermore, while oxotremorine strongly depressed the evoked release of [3H]ACh, it had no effect on the evoked release of endogenous glutamate measured simultaneously. These results do not support the co-transmitter role of glutamate in cortical cholinergic terminals, although they cannot statistically exclude that a small fraction of glutamate has a co-transmitter role, as proposed by Docherty et al.

Frequency-dependent increase in cortical acetylcholine release evoked by stimulation of the nucleus basalis magnocellularis in the rat.

Acetylcholine was collected from the somatosensory cortex of anesthetized rats, using the microdialysis technique. Electrical stimulation of the nucleus basalis magnocellularis (NBM) with trains of 10 pulses at 100 Hz delivered every second produced a 3-4-fold increase in acetylcholine release. Stimulation with an intratrain frequency of 10, 50, 100 or 200 Hz demonstrated that 100 Hz trains produced the greatest increase, while the other frequencies were about half as effective. The cortical release of acetylcholine in this paradigm supports the hypothesis that the previously demonstrated enhancement by NBM stimulation of cortical sensory inputs is due to cholinergic activation.

Release of [3H]acetylcholine from rat hippocampal slices: effect of septal lesion and of graded concentrations of muscarnic agonists and antagonists.

To establish the existence and sensitivity of presynaptic muscarinic receptors on central cholinergic neurons, the electrically evoked release of [3H]ACh from hippocampal slices was measured after medial septal lesion or in the presence of graded concentrations of muscarinic agonists and antagonists. One week after septal lesion, the evoked release of [3H]ACh was abolished, indicating that septo-hippocampal cholinergic fibres are the source of this release. The muscarinic agonists, Oxotremorine, carbamylcholine and arecoline reduced the rate of evoked release of [3H]ACh with an ED50 similar to the ED50 required to displace specific [3H]quinuclidinyl benzilate (QNB) binding as found by Yamamura and Snyder. However, the antagonists QNB, antropine and scopolamine were 10 times weaker in increasing the rate of [3H]ACh release than in displacing [3H]QNB binding. Results suggest that the lower affinity of muscarinic antagonists to presynaptic receptors prevents the demonstration of the specific labelling of these receptors with [3H]QNB.