Adrenergic agonists have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study examined the loci of alpha(1)- and beta-adrenoceptor agonists, which have opposite effects on excitatory neural transmission, involved in modulation of glutamatergic transmission in layer V pyramidal cells of rat cerebral cortex. Phenylephrine, an alpha(1)-adrenoceptor agonist, suppressed the amplitude of AMPA receptor-mediated excitatory postsynaptic currents evoked by repetitive electrical stimulation (eEPSCs, 10 pulses at 33 Hz). The coefficient of variation (CV) of the 1st eEPSC amplitude and paired-pulse ratio (PPR), which were sensitive to extracellular Ca(2+) concentration, were not affected by phenylephrine. Phenylephrine suppressed miniature EPSC (mEPSC) amplitude without changing its frequency. In contrast, isoproterenol, a beta-adrenoceptor agonist, strongly increased the amplitude of the 1st eEPSC compared with that of the 2nd to 10th eEPSCs, which resulted in a decrease in PPR. Isoproterenol-induced enhancement of eEPSC amplitude was accompanied by a decrease in CV. Isoproterenol increased the frequency of mEPSCs without significant effect on amplitude. Phenylephrine suppressed inward currents evoked by puff application of glutamate, AMPA, or NMDA, whereas isoproterenol application was not accompanied by significant changes in these inward currents. These findings suggest that phenylephrine decreases eEPSCs through postsynaptic AMPA or NMDA receptors, while the effects of isoproterenol are mediated by facilitation of glutamate release from presynaptic terminals without effect on postsynaptic glutamate receptors. These two different mechanisms of modulation of excitatory synaptic transmission may improve the "signal-to-noise ratio" in cerebral cortex.

beta-Adrenoceptors play a crucial role in the regulation of taste aversion learning in the insular cortex (IC). However, beta-adrenergic effects on inhibitory synaptic transmission mediated by gamma-aminobutyric acid (GABA) remain unknown. To elucidate the mechanisms of beta-adrenergic modulation of inhibitory synaptic transmission, we performed paired whole cell patch-clamp recordings from layer V GABAergic interneurons and pyramidal cells of rat IC aged from postnatal day 17 (PD17) to PD46 and examined the effects of isoproterenol, a beta-adrenoceptor agonist, on unitary inhibitory postsynaptic currents (uIPSCs). Isoproterenol (100 microM) induced facilitating effects on uIPSCs in 33.3% of cell pairs accompanied by decreases in coefficient of variation (CV) of the first uIPSC amplitude and paired-pulse ratio (PPR) of the second to first uIPSC amplitude, whereas 35.9% of pairs showed suppressive effects of isoproterenol on uIPSC amplitude obtained from fast spiking (FS) to pyramidal cell pairs. Facilitatory effects of isoproterenol were frequently observed in FS-pyramidal cell pairs at > or =PD24. On the other hand, isoproterenol suppressed uIPSC amplitude by 52.3 and 39.8% in low-threshold spike (LTS)-pyramidal and late spiking (LS)-pyramidal cell pairs, respectively, with increases in CV and PPR. The isoproterenol-induced suppressive effects were blocked by preapplication of 100 microM propranolol, a beta-adrenoceptor antagonist. There was no significant correlation between age and changes of uIPSCs in LTS-/LS-pyramidal cell pairs. These results suggest the presence of differential mechanisms in presynaptic GABA release and/or postsynaptic GABA(A) receptor-related assemblies among interneuron subtypes. Age- and interneuron subtype-specific beta-adrenergic modulation of IPSCs may contribute to experience-dependent plasticity in the IC.

Yamamoto K, Koyanagi Y, Koshikawa N, Kobayashi M. Postsynaptic cell type-dependent cholinergic regulation of GABAergic synaptic transmission in rat insular cortex. J Neurophysiol 104: 1933-1945. First published August 4, 2010 doi:10.1152/jn.00438.2010. The cerebral cortex consists of multiple neuron subtypes whose electrophysiological properties exhibit diverse modulation patterns in response to neurotransmitters, including noradrenaline and acetylcholine (ACh). We performed multiple whole cell patch-clamp recording from layer V GABAergic interneurons and pyramidal cells of rat insular cortex (IC) to examine whether cholinergic effects on unitary inhibitory postsynaptic currents (uIPSCs) are differentially regulated by ACh receptors, depending on their presynaptic and postsynaptic cell subtypes. In fast-spiking (FS) to pyramidal cell synapses, carbachol (10 M) invariably decreased uIPSC amplitude by 51.0%, accompanied by increases in paired-pulse ratio (PPR) of the second to first uIPSC amplitude, coefficient of variation (CV) of the first uIPSC amplitude, and failure rate. Carbachol-induced uIPSC suppression was dose dependent and blocked by atropine, a muscarinic ACh receptor antagonist. Similar cholinergic suppression was observed in non-FS to pyramidal cell synapses. In contrast, FS to FS/non-FS cell synapses showed heterogeneous effects on uIPSC amplitude by carbachol. In roughly 40% of pairs, carbachol suppressed uIPSCs by 35.8%, whereas in a similar percentage of pairs uIPSCs were increased by 34.8%. Non-FS to FS/non-FS cell synapses also showed carbacholinduced uIPSC facilitation by 29.2% in about half of the pairs, whereas nearly 40% of pairs showed carbachol-induced suppression of uIPSCs by 40.3%. Carbachol tended to increase uIPSC amplitude in interneuron-to-interneuron synapses with higher PPR, suggesting that carbachol facilitates GABA release in interneuron synapses with lower release probability. These results suggest that carbachol-induced effects on uIPSCs are not homogeneous but preiotropic: i.e., cholinergic modulation of GABAergic synaptic transmission is differentially regulated depending on postsynaptic neuron subtypes.

Background: P/Q-and N-type voltage-gated calcium channels (VGCC) are the principal subtypes mediating synaptic vesicle (SV) exocytosis. Both the degree of isoflurane inhibition of SV exocytosis and VGCC subtype expression vary between brain regions and neurotransmitter phenotype. We hypothesised that differences in VGCC subtype expression contribute to synapse-selective presynaptic effects of isoflurane. Methods: We used quantitative live-cell imaging to measure exocytosis in cultured rat hippocampal neurones after transfection of the fluorescent biosensor vGlut1-pHluorin. Selective inhibitors of P/Q-and N-type VGCCs were used to isolate subtype-specific effects of isoflurane. Results: Inhibition of N-type channels by 1 mM u-conotoxin GVIA reduced SV exocytosis to 81±5% of control (n¼10). Residual exocytosis mediated by P/Q-type channels was further inhibited by isoflurane to 42±4% of control (n¼10). The P/ Q-type channel inhibitor u-agatoxin IVA at 0.4 mM inhibited SV exocytosis to 29±3% of control (n¼10). Residual exocytosis mediated by N-type channels was further inhibited by isoflurane to 17±3% of control (n¼10). Analysis of isoflurane effects at the level of individual boutons revealed no difference in sensitivity to isoflurane between P/Q-or N-type channelmediated SV exocytosis (P¼0.35). There was no correlation between the effect of agatoxin (P¼0.91) or conotoxin (P¼0.15) and the effect of isoflurane on exocytosis. Conclusions: Sensitivity of SV exocytosis to isoflurane in rat hippocampal neurones is independent of the specific VGCC subtype coupled to exocytosis. The differential sensitivity of VGCC subtypes to isoflurane does not explain the observed neurotransmitter-selective effects of isoflurane in hippocampus.