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

Electrophysiological characterization of laminar synaptic inputs to the olfactory tubercle of the rat studied in vitro: modulation of glutamatergic transmission by cholinergic agents is pathway-specific

We have exploited the complementary arrangement of afferents in a coronal slice (300-400 microm) of the rat olfactory tubercle (OT) maintained in vitro to investigate transmission in two separate synaptic pathways. We recorded extracellular responses within the OT dense cell layer in slices and stimulated either the outermost layer to activate primary olfactory fibres or deeper to activate secondary input. Superficial stimulation produced a synaptic potential with superimposed population spike. This interpretation was based on blockade by calcium removal from the bathing medium and the use of the glutamate antagonist DNQX (10 microM); the spike was found to be selectively suppressed by tetrodotoxin applied near the cells. The spike, but not the synaptic wave, was depressed by 12 mM Ca2+ and enhanced by 1 mM Ba2+ in the bathing medium. Deep stimulation to activate association and intrinsic fibres elicited a nerve volley followed by a later response, also blocked by Ca2+ removal or 10 microM DNQX. It was unaffected by high Ca2+ or Ba2+, hence resulting from synaptic and not action current flow. Removal of Mg2+ from the bathing medium revealed an NMDA component of synaptic transmission at both loci that was selectively blocked by D-AP-5. The deep synaptic response, only, was depressed by carbachol IC50 7 microM or muscarine IC50 13 microM. This depression was also induced by AChE inhibitors eserine or tacrine and was antagonized by 1 microM atropine or 5-10 microM clozapine. These results characterize transmission in the OT and demonstrate a role for muscarinic modulation of deeper synapses in the OT that is influenced by psychotherapeutic drugs.

Muscarinic agonist carbachol depresses excitatory synaptic transmission in the rat basolateral amygdala in vitro

Intracellular recordings in slice preparations of the basolateral amygdala were used to test which excitatory amino acid receptors mediate the excitatory postsynaptic potentials due to stimulation of the external capsule. These recordings were also used to examine the action of muscarinic agonists on the evoked excitatory potentials. Intracellular recordings from amygdaloid pyramidal neurons revealed that carbachol (2-20 microM) suppressed, in a dose-dependent manner, excitatory postsynaptic responses evoked by stimulation of the external capsule (EC). This effect was blocked by atropine. The estimated effective concentration to produce half-maximal response (EC(50)) was 6.2 microM. Synaptic suppression was observed with no changes in the input resistance of the recorded cells, suggesting a presynaptic mechanism. In addition, the results obtained using the paired-pulse protocol provided additional support for a presynaptic action of carbachol. To identify which subtype of cholinergic receptors were involved in the suppression of the EPSP, four partially selective muscarinic receptor antagonists were used at different concentrations: pirenzepine, a compound with a similar high affinity for muscarinic M1 and M4 receptors; gallamine, a noncompetitive antagonist for M2; methoctramine, an antagonist for M2 and M4; and 4-diphenylacetoxy-N-methylpiperidine, a compound with similar high affinity for muscarinic receptors M1 and M3. None of them independently antagonized the suppressive effect of carbachol on the evoked EPSP completely, suggesting that more than one muscarinic receptor subtype is involved in the effect. These experiments provide evidence that in the amygdala muscarinic agonists block the excitatory synaptic response, mediated by glutamic acid, by acting on several types of presynaptic receptors.

Muscarinic presynaptic inhibition of neostriatal glutamatergic afferents is mediated by Q-type Ca2+ channels

Cholinergic presynaptic inhibition was investigated on neostriatal glutamatergic transmission. Paired pulse facilitation (PPF) of orthodromic population spikes (PS) were used to construct a concentration-response relationship for muscarine on presynaptic inhibition. Muscarine had an effect proportional to its extracellular concentration with an EC50 (mean +/- standard estimation error) of: 2.5 +/- 1.5 nM, and a maximal effect (saturation) of 245 +/- 16%. Several peptidic toxins against some voltage-gated Ca2+-channels increased PPF indicating that the Ca2+-channels they block participate in transmitter release. However, neither 1 microM omega-conotoxin GVIA, a specific blocker of N-type Ca2+-channels, nor 10-30 nM omega-agatoxinTK, a selective blocker of P-type Ca2+-channels, were able to occlude muscarine's effect on presynaptic inhibition. Nevertheless, 100-400 nM omega-agatoxinTK occluded muscarine's action on PPF in a dose-dependent manner. These results are consistent with Q-type Ca2+-channels mediating muscarinic presynaptic inhibition of neostriatal afferents.

Comparison of the effects of M1 and M2 muscarinic receptor activation in the absence of GABAergic inhibition in immature rat hippocampal CA3 area

We have reported previously that addition of carbachol (CCh, 25 microM) in the presence of the GABAA antagonist bicuculline (BMI, 10 microM), depresses evoked synaptic responses and significantly increases the frequency of spontaneous synchronous discharges in immature (postnatal days 10-20, P10-P20) hippocampus in vitro. The present experiments were designed in order to compare the activity of CCh with that of the selective muscarinic agonists McN-A-343 (M1 agonist) and oxotremorine-M (M2 agonist), in inducing the above-mentioned effects. Cholinergic agonists were tested in concentrations ranging from 2.5 nM to 5 microM and synaptic field potentials (evoked, spontaneous) were recorded from the CA3 pyramidal layer of hippocampal slices in the presence of BMI (10 microM). Depression of the evoked responses was measured as decrease (i) in their duration, (ii) in the number of population spikes/response, and (iii) in the amplitude of the first population spike of each sequence. Sigmoid dose-response curves were plotted, and the IC50s were calculated. For all three indexes, the profile of effectiveness was Oxotremorine-M>carbachol=McN-A-343. CCh and oxotremorine-M induced or increased the frequency of spontaneous synchronous discharges in the presence of BMI in a concentration-dependent manner. At the range of concentrations tested, McN-A-343 did not mimic this effect. In conclusion, our results suggest that M2 muscarinic receptors play a major role in both the depression of synaptic responses and the increase in the frequency of spontaneous synchronous discharges observed in the absence of GABAergic inhibition.

Metabotropic glutamate receptor subtypes mediating slow inward tail current (IADP) induction and inhibition of synaptic transmission in olfactory cortical neurones

1. The pharmacological features of the pre- and postsynaptic metabotropic glutamate receptors (mGluRs) present in the guinea-pig olfactory cortex, were examined in brain slices in vitro by use of a conventional intracellular current clamp/voltage clamp recording technique. 2. Bath-application of trans-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) (50 microM) produced a sustained membrane depolarization, increase in cell excitability and induction of a post-stimulus inward (after depolarizing) tail current (IADP) (measured under 'hybrid' voltage clamp) similar to those evoked by the muscarinic receptor agonist oxotremorine-M (OXO-M, 2 microM). 3. L-Glutamate (0.25 1 mM. in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM-DL-amino-5-phosphono valeric acid (DL-APV)) or the broad spectrum mGluR agonists 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), 1S,3S-ACPD (50 microM), ibotenate (Ibo; 25 microM. in the presence of 100 microM DL-APV), the selective mGluR I agonists (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG, 10 microM), (S)-3-hydroxyphenylglycine ((S)-3HPG, 50 microM), or quisqualate (10 microM, in the presence of 20 microM CNQX), but not the mGluR II agonist 2S,1'S,2'S-2-(2'-carboxycyclopropyl)-glycine (L-CCG1,1 microM) or mGluR III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 1 mM), were all effective in producing membrane depolarization and inducing a post-stimulus IADP. Unexpectedly, the proposed mGluR II-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG-IV, 10 microM, in the presence of 100 microM DL-APV) was also active. 4. The excitatory effects induced by 10 microM 1S,3R-ACPD were reversibly antagonized by the mGluR I/II antagonist (1)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG, 0.5 1 mM), as well as the selective mGluR I antagonists (S)-4-carboxyphenylglycine ((S)-4CPG) and (S)-4-carboxy-3-hydroxyphenyl glycine ((S)-4C3HPG) (both at 1 mM), but not the nonselective mGluR antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP3, 1 mM) or the selective mGluR III antagonist (S)-alpha-methyl-L-AP4 (MAP4, 1 mM). 5. The excitatory postsynaptic potentials (e.p.s.ps), induced by single focal stimulation of cortical excitatory fibre tracts, were markedly reduced by 1S,3R-ACPD or L-AP4 (both at 10 microM), and by the selective mGluR II agonists (mGluR 1 antagonists) (S)-4CPG or (S)-4C3HPG (both at 1 mM) but not (S)-3,5-DHPG or (S)-3HPG (both at 100 microM). 6. The inhibitory effects of 1S-3R-ACPD, but not L-AP4, were reversibly blocked by (+)-MCPG (1 mM), whereas those produced by L-AP4, but not 1S,3R-ACPD, were blocked by the selective mGluR III antagonist MAP4 (1 mM). 7. It is concluded that a group I mGluR is most likely involved in mediating excitatory postsynaptic effects, whereas two distinct mGluRs (e.g. group II and III) might serve as presynaptic inhibitory autoreceptors in the guinea-pig olfactory cortex.

Time course of interictal EEG patterns induced by a penicillin injection into the olfactory cortex

In awake rats, the time courses of behavioural and cortical interictal hyperactivity patterns were investigated following an injection of the GABA antagonist Na-penicillin (PCN) (125 IU/0.5 microliters) into the olfactory or motor cortex. The cortical EEG was recorded by means of 6 AgAgCl-electrodes, behaviour was videotaped simultaneously. Behavioural hyperactivity developed immediately after PCN injection. It lasted longer than 2.5 to 4 h in the olfactory but less than 30 min in the motor cortex group of rats. The interictal EEG pattern of the olfactory group was characterized by a slow establishing of three centers of high cortical activity. They were located in the ipsilateral central and the contralateral frontal and occipital cortex. In the motor cortex group, however, a cortical center of high activity developed immediately after the PCN injection near the injection site only, or additionally, over the homotopic area of the contralateral hemisphere. The results indicate different susceptibility properties in the underlying neuronal networks. Ongoing epileptiform activity obviously modifies this susceptibility in a site-specific manner. Moreover, the time-correlated occurrence of high activity in the frontal motor and occipital cortex evoked by a PCN injection into the olfactory cortex suggests a close coupling of these three areas. A coupling between the frontal motor, occipital and focal area could not be shown, if PCN was injected into the motor cortex.

Muscarinic receptor agonist-mediated modulation of neuronal activity in rat cerebral cortex

Multiple cortical neuronal responses were elicited by the iontophoretic application of muscarinic receptor agonists and antagonists in the rat cerebral sensorimotor cortex in vivo. (1) The muscarinic receptor agonist, oxotremorine-M induced a biphasic effect on spontaneous firing. This was evident as an early brief increase in the firing rate over the spontaneous discharge followed by secondary inhibition of spontaneous activity. The excitation could be blocked by the muscarinic receptor non-selective antagonist atropine and by both the M1 receptor antagonist pirenzepine and the M2 receptor antagonists gallamine or methoctramine. Oxotremorine-M inhibition of spontaneous activity was not affected by the M1 receptor antagonist pirenzepine, while evaluation of its sensitivity to gallamine and methoctramine was not possible since these two M2 receptor antagonists also depressed spontaneous activity, unlike pirenzepine. Of the other two muscarinic receptor agonists, oxotremorine had inconsistent and weak excitatory effects whilst McN-A-343 had only weak excitatory or inhibitory effects on spontaneous activity. (2) Oxotremorine-M, oxotremorine and McN-A-343 had a depressant action on neuronal discharges evoked by glutamate or acetylcholine. A depressant effect of oxotremorine-M was also demonstrated on the early excitation evoked by subsequent applications of oxotremorine-M itself. Of the three muscarinic receptor agonists tested, oxotremorine-M was the most potent in evoking a long-term depression of evoked discharges, lasting from several minutes (greater than 5 min) to as long as 40 min. Oxotremorine-M-induced depression of evoked responses was most sensitive to the M2 receptor antagonists, whereas oxotremorine-induced depression was more sensitive to the M1 receptor antagonist pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic inhibition of excitatory neurotransmission in guinea-pig olfactory cortex slices: weak antagonism by M3-muscarinic receptor antagonists

Dose-dependent depression of the electrically evoked surface-negative field potential (N-wave) produced by bath-superfusion of carbachol was measured in guinea-pig olfactory cortex slices maintained in vitro. The possible involvement of M3 (smooth muscle/glandular) type muscarinic receptors in partly mediating this response was investigated by testing the effectiveness of the muscarinic M3 receptor antagonists hexahydro-sila-difenidol (HHSiD) and p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD). Low doses of HHSiD (10-100 nM) or p-F-HHSiD (up to 1 microM), pre-applied for 30 min, produced no obvious antagonism of carbachol responses. However, a clear competitive-type inhibition of carbachol effects was observed in 250 nM-1 microM HHSiD or 10-50 microM p-F-HHSiD respectively. Schild plot analysis (regression slope constrained to unity) of pooled data yielded pA2 values of 6.6 for HHSiD (n = 6 slices) and 5.5 for p-F-HHSiD (n = 6 slices) respectively, suggesting a weak competitive antagonism by both compounds. In addition, combination experiments using either HHSiD or p-F-HHSiD with atropine, produced dose-ratio shifts close to those predicted for two antagonists competing for a common receptor site. By comparison, another suggested M3-receptor antagonist, 4-diphenyl-acetoxy-N-methyl-piperidine methiodide (4-DAMP) was a potent competitive blocker of carbachol responses. Schild analysis for 4-DAMP versus carbachol gave a pA2 of 7.9 (n = 6 slices). It is concluded that the muscarinic receptors involved in the suppression of the olfactory cortical N-wave possess a low affinity for HHSiD and particularly for p-F-HHSiD, but not 4-DAMP.(ABSTRACT TRUNCATED AT 250 WORDS)