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The NMDA receptor/nitric oxide (NO)/cyclic GMP pathway and its modulation by 5‐hydroxytryptamine (5‐HT) was studied in slices of neocortical samples obtained from patients undergoing neurosurgery.The cyclic GMP elevation produced by 100 μMNMDA was blocked by 100 μMof the NO synthase inhibitor NG‐nitro‐L‐arginine (L‐NOARG) or by 10 μMof the soluble guanylate cyclase inhibitor 1H‐[1,2,4]oxadiazolo[4,3,‐α] quinoxaline‐1‐one (ODQ).The NMDA effect was prevented by 5‐HT or by the 5‐HT2agonist (±)‐1‐(2,5‐dimethoxy‐4‐iodophenyl)‐2‐aminopropane ((±)‐DOI; EC50=22 nM). The (±)‐DOI inhibition was insensitive to the 5‐HT2Areceptor antagonist MDL 100907 or the 5‐HT2Bantagonist rauwolscine; it was largely prevented by 1 μMof the non‐selective 5‐HT2Cantagonists mesulergine (5‐HT2A,B,C), ketanserin (5‐HT2A,C) or SB 200646A (5‐HT2B,C); it was completely abolished by 0.1 μMof the selective 5‐HT2Creceptor antagonist SB 242084.The NMDA‐induced cyclic GMP elevation also was potently inhibited by the selective 5‐HT2Cagonist RO 60‐0175 and by the antidepressant trazodone, both added at 1 μM, in an SB 242084‐sensitive manner.Finally, the 5‐HT1Aagonist 8‐hydroxy‐2‐(di‐n‐propylamino) tetralin (8‐OH‐DPAT; 1 μM) inhibited the NMDA‐evoked cyclic GMP response, an effect blocked by the selective 5‐HT1Areceptor antagonist WAY 100635.In conclusion, the NMDA receptor/NO/cyclic GMP pathway in human neocortex slices can be potently inhibited by activation of 5‐HT2Cor 5‐HT1Areceptors.

N‐methyl‐D‐aspartate (NMDA) receptors exist on noradrenergic axon terminals and mediate enhancement of noradrenaline (NA) release. We here investigated modulation by somatostatin (SRIF, somatotropin release inhibiting factor) of the NMDA‐induced release of NA using superfused hippocampal synaptosomes.The NMDA response was increased by SRIF‐28 and SRIF‐14, but not SRIF‐28(1–14), whereas the release of [3H]‐NA elicited by α‐amino‐3‐hydroxy‐5‐methylisoxazide‐4‐propionic acid (AMPA) was unaffected. SRIF‐14 did not mimic glycine at the NMDA receptor but activated SRIF receptors sited on noradrenergic terminals.The SRIF‐14 effect was blocked by pertussis toxin but mimicked by mastoparan, a G‐protein activator. BIM‐23056, but not Cyanamid 154806, antagonized the SRIF‐14 effect. This effect was mimicked by L362855, a partial agonist at the sst5subtype, but not by the new selective sst1–sst4receptor agonists L797591, L779976, L796778 and L803087.Protein kinase C (PKC) inhibitors (H7, staurosporine, GF 209103X, cheleritrine and sphingosine) prevented the SRIF‐14 effect, while phorbol 12‐myristate 13‐acetate enhanced the NMDA response.SRIF‐14 permitted NMDA receptor activation in the presence of 1.2 mMMg2+ions, both in hippocampal synaptosomes and slices. Blockade of inositol‐1,4,5‐trisphosphate (InsP3) receptors with heparin abolished the effect of SRIF‐14.It is concluded that SRIF receptors, possibly of the sst5subtype, can exert a permissive role on NMDA receptors colocalized on hippocampal noradrenergic terminals: activation of sst5receptors is coupled to pertussis toxin‐sensitive G proteins enhancing phosphoinositide metabolism with activation of InsP3receptors and PKC; NMDA receptor subunits might be phosphorylated with consequent removal of the Mg2+block in absence of depolarization.

The releases of [3H]5‐hydroxytryptamine ([3H]5‐HT) and of endogenous glutamic acid and their modulation through presynaptic h5‐HT1Bautoreceptors and h5‐HT1Dheteroreceptors have been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery.The inhibition by 5‐HT of the K+(15 mM)‐evoked overflow of [3H]5‐HT was antagonized by the 5‐HT1B/5‐HT1Dreceptor ligand GR 127935, which was ineffective on its own; this drug was previously found to behave as a full agonist at the h5‐HT1Dheteroreceptor regulating glutamate release.The recently proposed selective h5‐HT1Breceptor ligand SB‐224289 also prevented the effect of 5‐HT at the autoreceptor, being inactive on its own; in contrast, SB‐224289, at 1 μM, was unable to interact with the h5‐HT1Dheteroreceptor.The inhibitory effect of 5‐HT on the K+‐evoked overflow of glutamate was antagonized by the h5‐HT1Dreceptor ligand BRL‐15572; added in the absence of 5‐HT the compound was without effect. BRL‐15572 (1 μM) was unable to modify the effect of 5‐HT at the autoreceptor regulating [3H]5‐HT release.The selective 5‐HT1Areceptor antagonist (+)‐WAY 100135, previously found to be an agonist at the h5‐HT1Dheteroreceptor regulating glutamate release, could not interact with the h5‐HT1Bautoreceptor when added at 1 μM.It is concluded that native h5‐HT1Band h5‐HT1Dreceptors exhibit a hitherto unexpected pharmacological diversity.

1The in vivoeffects of nicotine on the nitric oxide (NO) synthase/cyclic GMP pathway of the adult rat hippocampus have been investigated by monitoring the levels of extracellular cyclic GMP during microdialysis in conscious unrestrained animals.2Intraperitoneal (i.p.) administration of nicotine caused elevation of cyclic GMP levels which was prevented by mecamylamine. The effect of nicotine was abolished by local infusion of the NO synthase inhibitor NG‐nitro‐L‐arginine (L‐NOARG) or by the soluble guanylyl cyclase blocker 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxaline‐1‐one (ODQ).3Local administration of the NMDA receptor antagonists cis‐4‐(phosphonomethyl)‐2‐piperidinecarboxylic acid (CGS19755) and dizocilpine (MK‐801) inhibited by about 60% the nicotine‐induced elevation of cyclic GMP. Nicotine was able to stimulate cyclic GMP outflow also when administered directly into the hippocampus; the effect was sensitive to mecamylamine, L‐NOARG, ODQ or MK‐801.4Nicotine, either administered i.p. or infused locally, produced augmentation of glutamate and aspartate extracellular levels, whereas the outflows of γ‐aminobutyric acid (GABA) and glycine remained unaffected. Following local administration of high concentrations of nicotine, animals displayed symptoms of mild excitation (sniffing, increased motor and exploratory activity) during the first 20–40 min of infusion, followed by wet dog shake episodes; these behavioural effects were prevented by mecamylamine or MK‐801, but not by L‐NOARG or by ODQ.5It is concluded that (a) nicotine stimulates the production of NO and cyclic GMP in the hippocampus; (b) this occurs, at least in part, through release of glutamate/aspartate and activation of NMDA receptors. Modulation of the NMDA receptor/NO synthase/cyclic GMP pathway may be involved in the cognitive activities of nicotine.

The release of endogenous γ‐aminobutyric acid (GABA) and glutamic acid in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply located tumours.The basal outflows of GABA and glutamate from superfused synaptosomes were largely increased during depolarization with 15 mmKCl. The K+‐evoked overflows of both amino acids were almost totally dependent on the presence of Ca2+in the superfusion medium.The GABABreceptor agonist (−)‐baclofen (1, 3 or 10 μm) inhibited the overflows of GABA and glutamate in a concentration‐dependent manner. The inhibition caused by 10 μmof the agonist ranged from 45–50%.The effect of three selective GABABreceptor antagonists on the inhibition of the K+‐evoked GABA and glutamate overflows elicited by 10 μm(−)‐baclofen was investigated. Phaclofen antagonized (by about 50% at 100 μm; almost totally at 300 μm) the effect of (−)‐baclofen on GABA overflow but did not modify the inhibition of glutamate release. The effect of (−)‐baclofen on the K+‐evoked GABA overflow was unaffected by 3‐amino‐propyl (diethoxymethyl)phosphinic acid (CGP 35348; 10 or 100 μm); however, CGP 35348 (10 or 100 μm) antagonized (−)‐baclofen (complete blockade at 100 μm) at the heteroreceptors on glutamatergic terminals. Finally, [3‐[[(3,4‐dichlorophenyl) methyl]amino]propyl] (diethoxymethyl) phosphinic aid (CGP 52432), 1 μm, blocked the GABABautoreceptor, but was ineffective at the heteroreceptors. The selectivity of CGP 52432 was lost at 30 μm, as the compound, at this concentration, inhibited completely the (−)‐baclofen effect both on GABA and glutamate release.It is concluded that GABA and glutamate release evoked by depolarization of human neocortex nerve terminals can be affected differentially through pharmacologically distinct GABABreceptors.