A quantitative study of the effects of some muscarinic antagonists on the guinea-pig olfactory cortex slice.

1. Muscarinic depression of the electrically-evoked surface-negative field potential (N-wave) was measured in guinea-pig olfactory cortex slices maintained in vitro. 2. The effects of three muscarinic receptor antagonists, pirenzepine, atropine and gallamine on this muscarinic response were analysed in detail. 3. Pirenzepine was a potent competitive antagonist of carbachol (CCh)-evoked responses. Schild plot analysis yielded a pA2 value of 7.9 (Schild slope constrained to unity). A similar analysis for atropine versus CCh responses gave a pA2 of 8.9. 4. Combination experiments using pirenzepine and atropine produced dose-ratio shifts close to those expected for two antagonists competing for a similar receptor site. 5. Gallamine was only a weak antagonist of responses to CCh. 6. Oxotremorine behaved as a competitive antagonist at this muscarinic receptor (pA2 = 6.1). 7. It is concluded that the presynaptic muscarinic receptor mediating depression of the N-wave in the olfactory cortex slice is of the M1-subtype.     

Muscarinic suppression of the evoked N-wave by oxotremorine-M recorded in the guinea-pig olfactory cortex slice

The effect of the muscarinic agonist oxotremorine-M has been studied on the surface-negative field potential (N-wave) evoked by orthodromic stimulation of the lateral olfactory tract in slices of guinea-pig olfactory cortex. Bath-application of oxotremorine-M (5-80 microM) or carbachol (10-300 microM) produced a reversible depression of the N-wave amplitude without affecting the lateral olfactory tract compound action potential. Oxotremorine-M was approximately 5 times more potent than carbachol in this respect, and the effects of both agonists were competitively blocked by telenzepine (5-100 nM), a selective M1-receptor antagonist. In contrast, methoctramine or AF-DX 116, two 'cardioselective' M2-receptor antagonists, had little or no blocking effect on the agonist responses. It is suggested that oxotremorine-M (like carbachol) inhibits the evoked field potential by activating presynaptic M1-type muscarinic receptors in the olfactory cortex slice.     

Responses of the guinea-pig isolated olfactory cortex slice to gamma-aminobutyric acid recorded with extracellular electrodes.

1. Potential changes between the pial and cut surfaces of slices of guinea-pig olfactory cortex in vitro produced by gamma-aminobutyric acid (GABA) were recorded with extracellular electrodes. 2. GABA, superfused over the pial surface (0.1 to 10 mM), produced a pial-negative potential deflection, accompanied by inhibition of the postsynaptic response to lateral olfactory tract (LOT) stimulation. 3. This effect was replicated by the following compounds (potency relative to GABA = 1, in brackets): 3-aminopropanesulphonic acid (5.3), epsilon-aminovaleric acid (0.07), beta-alanine (0.07), beta-amino-nibutyric acid 0.05), epsilon-aminocaproic acid, alpha-amino-isobutyric acid, L-leucine (less than 0.02). 4. L-Glutamate (1 to 10 mM) produced a very large surface negative shift, with relatively less synaptic inhibition. Glycine (1 to 10 mM) produced less surface negatively, accompanied by synaptic inhibition. 5. Responses to GABA were antagonized more effectively than those to glycine by bicuculline (3 to 30 micrometer) and picrotoxin (1 to 30 micrometer). Strychnine (1 to 10 micrometer) incompletely inhibited responses to glycine. 6. It is concluded that, while the locus within the slice of these effects is uncertain, the preparation may be useful for testing the interaction of drugs with cerebral GABA receptors.     

Effects of halothane on evoked field potentials recorded from cortical and subcortical nuclei

The effects of four incremental doses of halothane on photic and acoustic evoked responses from four brain locations in freely behaving rats were assessed. Chronically implanted stainless-steel semi-microelectrodes (60 μ diameter) were implanted stereotaxically under sodium pentobarbital anesthesia in the inferior colliculus (IC), the lateral geniculate body (LGB), auditory (ACx) and visual cortex (VCx) in each rat. Three to five days after surgery, averaged photic and acoustic evoked responses were taken, four sets for each modality, before exposure to halothane and during exposure to increasing concentrations of halothane (0.25, 0.5 1.0 and 2.0%) in each rat. Measurement of changes in response amplitudes compared to pre-drug controls were used to assess the effect of the halothane. A difference in the effect of halothane effect based on sensory modality was observed. The average auditory evoked response (AAER) was affected by halothane in a dose-related fashion and was more sensitive to halothane than was the average visual evoked response (AVER), especially in the auditory cortex which demonstrated a biphasic (initial increase, then decrease with increasing doses of halothane) pattern of response to halothane. The recordings obtained from the sensory relay nuclei (IC and LGB) each produced early components (P₁-N₁) of the averaged evoked response which were resistant to halothane effects.     

Effects of muscarinic agonists in the guinea-pig prostate

1. The contractile response to transmural stimulation of the guinea-pig prostate is largely due to activation of noradrenergic neurons but there is a small contribution from cholinergic neurons. Carbachol and acetylcholine have been reported to act via muscarinic M(1) cholinoceptors to facilitate contractions produced by neuronal stimulation of the tissue. This action of cholinomimetics was further investigated in isolated ventral lobes of the prostate. 2. Oxotremorine-M, bethanecol, pilocarpine, xanomeline and McN-A-343 produced facilitation of the response to transmural stimulation of the prostate. When carbachol was applied as the first agonist, the facilitatory response to the latter four agonists above was absent or reduced, compared with the effect observed when the other agonist was applied first, indicating that the effect of these agonists is readily desensitized. Only oxotremorine-M was unaffected by prior exposure of the tissue to carbachol. When applied first, pilocarpine and xanomeline produced a smaller degree of facilitation than carbachol indicating they were partial agonists for the effect. The facilitation produced by McN-A-343, when applied as the first agonist, was variable. In some preparations the facilitation was less than that of carbachol but in others it exceeded that produced by a subsequent application of carbachol. 3. The release of endogenous choline from the prostate was measured at rest and during transmural stimulation using a chemiluminescent technique. A statistically significant negative correlation existed between pmol mg(-1) of endogenous choline released during transmural stimulation and the mass of the ventral lobe of the prostate. As the guinea-pig prostate is known to undergo postpubertal stromal hypertrophy the finding suggests that endogenous choline release from the prostate is largely from epithelial, rather than stromal tissue. 4. The possible involvement of facilitatory M(1) autoreceptors on cholinergic neurons in the effect of cholinomimetics was investigated. Tissue was incubated with (3)H-choline to label neuronal stores of acetylcholine and the subsequent release of (3)H-choline from the tissue was measured. Carbachol per se increased the release of (3)H-choline. 5. Transmural stimulation usually increased the release of (3)H-choline but in c. 30% of preparations there was a decrease. In the presence of carbachol there was a significant increase in the release of (3)H-choline during transmural stimulation of prostate lobes. However, there was no correlation between the two effects of carbachol; facilitating contractions produced by transmural stimulation and enhancing (3)H-choline release during transmural stimulation. The finding provides no evidence that facilitatory M(1) autoreceptors on cholinergic neurons play a major role in the facilitation of contractions.     

Comparison of the effects of some muscarinic agonists on smooth muscle function and phosphatidylinositol turnover in the guinea-pig taenia caeci.

1. The effects of the muscarinic agonists acetylcholine (ACh), carbachol (CCh), AHR-602, and McN-A-343 on contractility and on inositol phosphate accumulation in the presence of lithium were compared in the taenia of the guinea-pig caecum. 2. Compared to CCh, ACh was a full agonist for contraction but AHR-602 and McN-A-343 were partial agonists producing 80-85% of the maximal response to CCh. Similar to previous findings with CCh, tonic contractions produced by AHR-602 and McN-A-343 were less sensitive to inhibition by nifedipine or verapamil than tonic contractions to ACh. 3. CCh and ACh produced similar increases in inositol phosphate accumulation and the effect of CCh (0.1 mM) was inhibited by atropine (IC50 8.5 nM) and pirenzepine (IC50 450 nM). The accumulation of inositol phosphates in the presence of AHR-602 or McN-A-343 was not significantly different (P greater than 0.05) from basal levels. 4. A concentration of 0.2 mM AHR-602 produced a parallel shift of the concentration-response curve to CCh on inositol phosphate accumulation. The IC50 value for inhibition of CCh (0.1 mM) was greater than 50 fold higher than the EC50 value for contraction produced by the partial agonist. McN-A-343 (20 microM) produced a flattening of the concentration-response curve to CCh for inositol phosphate accumulation. 5. The results suggest that the increase in phosphatidylinositol turnover produced by muscarinic agonists, like the contractile response, involves an M2-muscarinic receptor. AHR-602 and McN-A-343 are partial agonists for the contractile response and while producing no significant increase in phosphatidylinositol turnover inhibit the response to CCh.     

Effects of baclofen on the olfactory cortex slice preparation

Baclofen has been shown to be ineffective against first order excitatory synaptic transmission in the olfactory cortex slice, whereas it is known that GABA-mediated inhibition depresses this transmission in a bicuculline-sensitive manner. In contrast, second and third order synaptic transmission, excitatory and inhibitory respectively, were depressed by baclofen. This suggests that the distribution of baclofen (GABAB) receptors differs from that of bicuculline-sensitive GABAA receptors in the olfactory cortex.     

Morphine excitation: Effects on field potentials recorded in the in vitro hippocampal slice

The hippocampal slice preparation was used to study the effects of morphine on CA1 field potentials recorded in vitro. Morphine sulfate (10^?3 M) produced two distinct excitatory effects when added to the bathing media or applied directly to the slice via a pressure pipette injection system. First, morphine produced an increase in amplitude and a reduction in latency of the CA1 population spike elicited by orthodromic stimulation of stratum radiatum, without any change in the amplitude of the synaptic field potential. This was accompanied by a pronounced shift to the left of the population spike input-output curve indicating an increase in excitability of the CA1 neurons. At higher stimulus intensities, orthodromic stimulation produced a series of multiple peaks following the original population spike where only a single spike had been present before morphine application. The increase in population spike amplitude was partially reversed by application of the morphine antagonist naloxone, but naloxone did not affect the number of peaks elicited in morphine-treated slices. Pentobarbital was effective in reducing the number of stimulus-elicited peaks but did not reduce the increase in the population spike. The possibility that these effects are mediated by morphine-induced inhibition of GABAergic synapses in the hippocampus is discussed.     

NMDA antagonists increase recovery of evoked potentials from slices of rat olfactory cortex after anoxia.

1. The role of glutamate in producing tissue damage during cerebral anoxia was investigated in brain slices using antagonists to the NMDA and AMPA receptor types. 2. Tissue function was assessed by field recordings of the synaptically evoked potentials elicited by stimulating the main afferent input to the olfactory cortex, the lateral olfactory tract. Anoxia was produced by bathing the slice in glucose-free solution equilibrated with 95% N2/5% CO2. 3. The amount of recovery of the evoked potential was inversely dependent on the period of anoxia and temperature: at 24 degrees C, 15 min of anoxia followed by reoxygenation produced a 14.6 +/- 4.1% recovery whereas there was no recovery at 35 degrees C. 4. Dizocilpine and ketamine had no effect on synaptic transmission in oxygenated media but following anoxia they produced an increased recovery of the responses: from 14.6 +/- 4.1% to 48.3 +/- 7.8% for dizocilpine (10 microM) and 21.6 +/- 7.7% to 87.2 +/- 7.1% for ketamine (200 microM); the tissue endurance to anoxia was increased by around 5 min. 5. Blockade of the AMPA receptors did not influence recovery in spite of the depressed synaptic transmission. A similar synaptic attenuation produced by lignocaine provided some increase in post-anoxic recovery. 6. The NMDA receptor antagonist, AP5, antagonized NMDA at 50 microM by 3.7 fold and at 200 microM by 15 fold but only 200 microM increased post-anoxic recovery. This suggests that a substantial degree of NMDA antagonist is required before anoxic tissue damage due to NMDA receptor activation can be nullified. The antagonist to the glycine binding site, 7-chlorokynurenic acid also increased recovery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cocaine on sensory evoked potentials recorded from hypothalamus and limbic structures.

1. The present investigation was intended to examine the effects of the acute administration of several doses of cocaine on sensory evoked responses of freely moving rats. 2. Each structure and each component of the average sensory evoked potentials were affected by cocaine differently. 3. The changes induced by cocaine differ from those observed following other drug treatments. Therefore, it may be assumed that the changes observed are related directly to cocaine. 4. The present observation demonstrated a greater degree of sensitivity to low doses of cocaine than other types of electrophysiological recordings. In general cocaine in the lower doses (0.25 or 0.5 mg/kg) induced an increase in the sensory amplitude response while in the highest dose (10.0 mg/kg) attenuation of the sensory responses was obtained. 5. We assume that the euphoric effect induced by cocaine is due to the potentiation of sensory input within the limbic-hypothalamic system. In higher doses, the fatality caused by the drug may be due to the depression of the sensory input.     

The pharmacology of excitatory transmission in the rat olfactory cortex slice

The pharmacology of excitatory transmission in slices of olfactory cortex of the rat, perfused with solution containing picrotoxin, has been studied by assessing the effects of cis-2,3-piperidine dicarboxylate, a nonselective antagonist of excitatory amino acid receptors, 2-amino-5-phosphonopentanoate, a selective antagonist of N-methyl-D-aspartate (NMDA) receptors and 2-amino-4-phosphonobutyrate (APB) and baclofen, which act at APB and GABAB sites, respectively, on evoked surface field potentials. Monosynaptic excitatory transmission was monitored by measuring the amplitude of the N'a'-wave, evoked on stimulation of the lateral olfactory tract, whilst di-/polysynaptic excitatory transmission was evaluated by calculating the areas of the potentials evoked on direct stimulation of the superficial and deep-lying association fibre systems. On the basis of the effects of the drugs in this and earlier studies, it is concluded that: (i) transmission at the lateral olfactory tract-pyramidal cell synapse is mediated by kainate/quisqualate but not NMDA receptors and is regulated by inhibitory APB receptors, located on the tract terminals; (ii) NMDA receptors are involved in mediating excitatory transmission at the synapses of superficial association fibres with the proximal apical dendrites of pyramidal cells with inhibitory APB receptors playing a regulatory role; (iii) transmission at synapses of association fibres with basal dendrites of pyramidal cells, is mediated in part by NMDA receptors with (presynaptic?) GABAB receptors exerting a strong inhibitory influence. These proposed roles of NMDA receptors have been confirmed in experiments in which the effects of magnesium ions on field potentials evoked in slices perfused in magnesium-free solution were monitored.     

Adenosine A1 receptors mediate the inhibitory effects of exogenous adenosine in the rat olfactory cortex slice

A study has been undertaken to identify the category of receptors mediating the inhibitory effects of adenosine on evoked activity in slices of olfactory cortex in the rat. The approach has been to measure the relative potencies of adenosine and a range of structural analogues [2-chloroadenosine, 2' deoxyadenosine, cyclohexyladenosine, (-)-5'N-ethyl-carboxamide adenosine and N6(L-2-phenylisopropyl)adenosine] required to: inhibit excitatory transmission at the lateral olfactory tract-pyramidal cell synapse; inhibit the specific binding of [3H]cyclohexyladenosine to membrane preparations and evoke formation of cyclic AMP. In contrast to the relative concentrations of the analogues necessary to increase levels of cyclic AMP, those required to inhibit synaptic transmission were characteristic of a selectivity for adenosine A1 receptors. The presence of adenosine A1 receptors has been demonstrated directly by characterizing the binding of [3H]cyclohexyladenosine to membranes prepared from slices of olfactory cortex. It is concluded that inhibition of transmission at the lateral olfactory tract-pyramical cell synapse by adenosine is mediated by receptors of the A1 category.     

An analysis of the action of pentobarbitone on the excitatory postsynaptic potentials and membrane properties of neurones in the guinea-pig olfactory cortex.

Intracellular recordings were made from neurones in slices of guinea-pig olfactory cortex maintained in vitro at 37 degrees C. The average membrane potential was 63 +/- 12 mV and the input resistance of these cells was 42 +/- 20 M omega (mean +/- s.d.). Stimulation of the lateral olfactory tract (l.o.t.) generated a transient depolarization in these cells which had the characteristics of an excitatory postsynaptic potential (e.p.s.p.). If the e.p.s.p. was of sufficient amplitude it culminated in an action potential. The e.p.s.p. was potentiated by repetitive stimulation at 10-50 Hz and showed post-tetanic potentiation after a prolonged period of high frequency stimulation (50-100 Hz for 30-60 s). Pentobarbitone (0.1-0.5 mM) depressed the e.p.s.p. reversibly but was without effect on the resting membrane potential, input resistance or time constant of the neurones. Pentobarbitone did not inhibit potentiation of the e.p.s.p. by a preceding conditioning shock. It is concluded that pentobarbitone does not affect the passive membrane properties of neurones in the olfactory cortex. The depressant action of pentobarbitone on synaptic transmission results from a decrease in the amount of transmitter released in response to a nerve impulse, or a decrease in the sensitivity of the postsynaptic membrane to the transmitter or a combination of both effects.     

The relative potencies of some agonists at M2 muscarinic receptors in guinea-pig ileum, atria and bronchi.

The effects of some agonists on isolated preparations of guinea-pig ileum, atria and bronchial muscle have been compared with those of carbachol. The concentrations producing comparable responses were used to estimate the equipotent molar ratio relative to carbachol. Arecaidine propargyl ester was 4 to 5 times as active as carbachol on the ileum but more than 10 times as active as carbachol on atrial rate or atrial force, so the results confirm that this compound has a 2 to 3 fold selectivity for receptors in atria. Ethoxyethyltrimethylammonium iodide was one-quarter to one-third as active as carbachol on ileum but only one-tenth as active as carbachol on atrial rate or atrial force and so shows a 3 to 4 fold selectivity for receptors in ileum. The other compounds tested, which included acetylcholine, methacholine, n-pentyltrimethyl-ammonium iodide and bethanechol showed less selectivity. There were no obvious differences between effects on atrial rate and effects on atrial force, though with esters it was often difficult to obtain effects on atrial rate in the absence of an inhibitor of cholinesterase. Activity on bronchial muscle was generally similar to activity on ileum.     

Characterization of electrically evoked field potentials in the medial prefrontal cortex and orbitofrontal cortex of the rat: Modulation by monoamines

Medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) play critical roles in cognition and behavioural control. Glutamatergic, GABAergic, and monoaminergic dysfunction in the prefrontal cortex has been hypothesised to underlie symptoms in neuropsychiatric disorders. Here we characterised electrically-evoked field potentials in the mPFC and OFC. Electrical stimulation evoked field potentials in layer V/VI of the mPFC and layer V of the OFC. The earliest component (approximately 2 ms latency) was insensitive to glutamate receptor blockade and was presumed to be presynaptic. Later components were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX (20 µM)) and were assumed to reflect monosynaptic (latency 4–6 ms) and polysynaptic activity (latency 6–40 ms) mediated by glutamate via AMPA/kainate receptor. In the mPFC, but not the OFC, the monosynaptic component was also partly blocked by 2-amino-5-phosphonopentanoic acid (AP-5 (50–100 µM)) indicating the involvement of NMDA receptors. Bicuculline (3–10 µM) enhanced the monosynaptic component suggesting electrically-evoked and/or glutamate induced GABA release inhibits the monosynaptic component via GAB_AA receptor activation. There were complex effects of bicuculline on polysynaptic components. In the mPFC both the mono- and polysynaptic components were attenuated by 5-HT (10–100 µM) and NA (30 and 60 µM) and the monosynaptic component was attenuated by DA (100 µM). In the OFC the mono- and polysynaptic components were also attenuated by 5-HT (100 µM), NA (10–100 µM) but DA (10–100 µM) had no effect. We propose that these pharmacologically characterised electrically-evoked field potentials in the mPFC and OFC are useful models for the study of prefrontal cortical physiology and pathophysiology.     

Dose effects of pentobarbital on evoked potentials in visual cortex and superior colliculus of the albino rat

This study examined the effects of pentobarbital on photic evoked potentials recorded from the primary visual cortex and superior colliculus of chronically implanted albino rats. Animals were given intraperitoneal injections of saline, and of 3, 6, 9, 18 and 40 mg pentobarbital/kg body weight on separate days. Evoked potentials were recorded at 5, 20, 40 and 60 min following injection. There were differential effects of pentobarbital on the amplitudes of individual visual cortex components. In general, the earliest components were augmented by pentobarbital, the late components were depressed, and the components in the middle of the waveform demonstrated biphasic effects. These middle components were increased in amplitude by one or more of the smaller doses, but were at or below control levels as a result of the 40 mg/kg dose. Increases in peak latency were also found in some components, beginning with the 6 mg/kg dose for the two earliest components, and beginning with the 18 mg/kg dose for the third component. In the superior colliculus, the peak amplitudes of two individual components of the early positive complex reacted in opposite directions to administration of pentobarbital. The first component was augmented, while the second was diminished (or even abolished by the 40 mg/kg dose). A later negative component demonstrated a biphasic effect, with increases at smaller doses and a return to control levels with the 40 mg/kg dose. Pentobarbital produced a reliable increase in latency for all three superior colliculus components, with the increase occurring at both the 18 and 40 mg/kg doses. A series of late oscillatory potentials recorded from the colliculus were increased in amplitude by pentobarbital.