Human brain somatostatin release from isolated cortical nerve endings and its modulation through GABAB receptors

1. The release of somatostatin-like immunoreactivity (SRIF-LI) in the human brain was studied in synaptosomal preparations from fresh neocortical specimens obtained from patients undergoing neurosurgery to remove deeply sited tumours. 2. The basal outflow of SRIF-LI from superfused synaptosomes was increased about 3 fold during exposure to a depolarizing medium containing 15 mM KCl. The K(+)-evoked overflow of SRIF-LI was almost totally dependent on the presence of Ca2+ in the superfusion medium. 3. The GABAB receptor agonist, (-)-baclofen (0.3 - 100 microM), inhibited the overflow of SRIF-LI in a concentration-dependent manner (EC50 = 1.84 +/- 0.20 microM; maximal effect: about 50%). The novel GABAB receptor ligand, 3-aminopropyl(difluoromethyl)phosphinic acid (CGP 47656) mimicked (-)-baclofen in inhibiting the SRIF-LI overflow (EC50 = 3.06 +/- 0.52 microM; maximal effect: about 50%), whereas the GABAA receptor agonist, muscimol, was ineffective up to 100 microM. 4. The inhibition by 10 microM (-)-baclofen of the K(+)-evoked SRIF-LI overflow was concentration-dependently prevented by two selective GABAB receptor antagonists, 3-amino-propyl (diethoxymethyl)-phosphinic acid (CGP 35348) (IC50 = 24.40 +/- 2.52 microM) and [3-[[(3,4-dichlorophenyl) methyl]amino]propyl] (diethoxymethyl) phosphinic acid (CGP 52432) (IC50 = 0.06 +/- 0.005 microM). 5. The inhibition of SRIF-LI overflow caused by 10 microM CGP 47656 was abolished by 1 microM CGP 52432. 6. When human synaptosomes were labelled with [3H]-GABA and depolarized in superfusion with 15 mM KCl, the inhibition by 10 microM (-)-baclofen of the depolarization-evoked [3H]-GABA overflow was largely prevented by 10 microM CGP 47656 which therefore behaved as an autoreceptor antagonist. 7.

IN CONCLUSION

(a) the characteristics of SRIF-LI release from synaptosomal preparations of human neocortex are compatible with a neuronal origin; (b) the nerve terminals releasing the neuropeptide possess inhibitory receptors of the GABAB type; (c) these receptors differ pharmacologically from the GABAB autoreceptors present on human neocortex nerve terminals since the latter have been shown to be CGP 35348-insensitive but can be blocked by CGP 47656.

Possible involvement of primary motor cortex in mentally simulated movement: a functional magnetic resonance imaging study

The role of the primary motor cortex (M1) during mental simulation of movement is open to debate. In the present study, functional magnetic resonance imaging (fMRI) signals were measured in normal right-handed subjects during actual and mental execution of a finger-to-thumb opposition task with either the right or the left hand. There were no significant differences between the two hands with either execution or simulation. A significant involvement of contralateral M1 (30% of the activity found during execution) was detected in four of six subjects. Premotor cortex (PM) and the rostral part of the posterior SMA were activated bilaterally during motor imagery. These findings support the hypothesis that motor imagery involves virtually all stages of motor control.

Characterization of the glutamate receptors mediating release of somatostatin from cultured hippocampal neurons

L-Glutamate, NMDA, DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and kainate (KA) increased the release of somatostatin-like immunoreactivity (SRIF-LI) from primary cultures of rat hippocampal neurons. In Mg(2+)-containing medium, the maximal effects (reached at approximately 100 microM) amounted to 737% (KA), 722% (glutamate), 488% (NMDA), and 374% (AMPA); the apparent affinities were 22 microM (AMPA), 39 microM (glutamate), 41 microM (KA), and 70 microM (NMDA). The metabotropic receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylate did not affect SRIF-LI release. The release evoked by glutamate (100 microM) was abolished by 10 microM dizocilpine (MK-801) plus 30 microM 1-aminophenyl-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466). Moreover, the maximal effect of glutamate was mimicked by a mixture of NMDA+AMPA. The release elicited by NMDA was sensitive to MK-801 but insensitive to GYKI 52466. The AMPA- and KA-evoked releases were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX) or by GYKI 52466 but were insensitive to MK-801. The release of SRIF-LI elicited by all four agonists was Ca(2+) dependent, whereas only the NMDA-evoked release was prevented by tetrodotoxin. Removal of Mg2+ caused increase of basal SRIF-LI release, an effect abolished by MK-801. Thus, glutamate can stimulate somatostatin release through ionotropic NMDA and AMPA/KA receptors. Receptors of the KA type (AMPA insensitive) or metabotropic receptors appear not to be involved.

Decreased platelet 3H-paroxetine binding in obsessive-compulsive patients

The similarities between the serotonin (5-HT) transporter in both human platelets and human brain permit us to investigate this structure in patients with different psychiatric disorders. Several reports have shown abnormalities of the 5-HT transporter, by means of the measurement of the 5-HT uptake or of the 3H-imipramine binding, in platelets of patients with obsessive-compulsive disorder (OCD). The availability of the ligand 3H-paroxetine, a selective 5-HT reuptake inhibitor, to label the 5-HT transporter, promoted us to evaluate the binding of 3H-paroxetine in platelets of 18 drug-free patients with OCD. The results, showing that the patients had a lower number of 3H-paroxetine sites, which is inversely correlated with the Yale Brown Obsessive-Compulsive Scale total score, than a similar group of controls, add supporting evidence to the involvement of 5-HT in OCD. In addition, the decreased functionality of the 5-HT transporter seems to be linked to the severity of OC symptoms.

Pharmacological discrimination between gamma-aminobutyric acid type B receptors regulating cholecystokinin and somatostatin release from rat neocortex synaptosomes

The gamma-aminobutyric acid (GABA)B receptors modulating the depolarization-evoked release of somatostatin (SRIF) or cholecystokinin (CCK) from superfused rat cerebrocortical synaptosomes have been characterized pharmacologically. GABA inhibited the 15 mM KCl-evoked overflow of both SRIF and CCK; the EC50 values were 1.3 microM and 1.4 microM, respectively. The GABAB receptor agonist (-)-baclofen also diminished the release of SRIF (EC50 = 1.9 microM) and CCK (EC50 = 2.6 microM). The novel compound CGP 47656, a highly selective GABAB receptor ligand, inhibited the release of SRIF, with its affinity and efficacy being similar to those of GABA or (-)-baclofen; however, the compound was unable to affect CCK release even when tested at 300 microM. The GABAB receptor antagonist phaclofen prevented, with identical affinities, the effects of (-)-baclofen on SRIF (pKb = 4.9) and CCK (pKb = 4.8) release. The same was true for CGP 35348, another GABAB receptor antagonist, which blocked (-)-baclofen with a pKb value of 6.1 at both the GABAB receptors regulating SRIF and CCK release. The effects of (-)-baclofen were also counteracted by the novel GABAB receptor antagonist CGP 52432. However, the affinity of the drug at the GABAB receptors modulating SRIF release (pKb = 6.2) was about 30-fold lower than that at the receptors regulating CCK release (pKb = 7.6). The data suggest that the GABAB receptors situated on nerve terminals releasing SRIF and CCK display pharmacological heterogeneity and may represent different subtypes of GABAB receptors.

[Treatment of insomnia related to depressive disorders. Effects of zolpidem versus flunitrazepam administration and withdrawal evaluated in a double-blind study]

The effects of a 15 day treatment with zolpidem (10 mg) and with flunitrazepam (1 mg) on Insomnia Disorders Related to Depressive Disorders (DSM-III-R) have been evaluated on 30 depressive in-patients (mean age 42.3 +/- 9.8). The trial has been carried out on double blind condition after 5 days of single blind placebo administration. Withdrawal effects have been evaluated in single blind condition on a 10 day period after drugs discontinuation. Patient's diagnosis was Major Depression or Dysthymia according to DSM-III-R; inclusion criteria were insomnia (total sleep time < or = 6 h, sleep latency > or = 30 min, wake after sleep onset > or = 30 min, No of awakenings > or = 3) refractory to clomipramine administration at constant dose (75-150 mg/day among patients). Both drugs have been followed by a rapid, significant diminution of insomnia as demonstrated by significant changes at Stanford Sleepiness Scale and Saint Mary Hospital Sleep Questionnaire and by a significant reduction of HDRS total scores. No clinical phenomena of rebound insomnia were detected after zolpidem and flunitrazepam withdrawal. Drug discontinuation however was followed by the slow increase of the score on insomnia items, approximating basal values at the end of the 10 day period after zolpidem and flunitrazepam withdrawal. A parallel increase of HDRS total score was also detected; HDRS changes were mainly due to the increase of the items anxiety somatic, general somatic symptoms, gastrointestinal somatic symptoms, hypochondriasis. The study confirms the therapeutic efficacy of zolpidem and of flunitrazepam in the treatment of insomnia resistant to antidepressant drugs in depressed patients. They also suggest that early drug discontinuation is frequently associated with clinical relapse of insomnia and of several other symptoms correlated with the affective pathology.

CGP 52432: a novel potent and selective GABAB autoreceptor antagonist in rat cerebral cortex

As previously reported GABAB receptors are heterogeneous. Three pharmacologically distinct receptor subtypes mediating inhibition of gamma-aminobutyric acid (GABA), glutamate or somatostatin release, respectively, exist on axon terminals of rat cerebral cortex. We investigated the novel GABAB receptor antagonist, [3-[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxy-methyl) phosphinic acid (CGP 52432), on the above receptor subtypes. The effects of (-)-baclofen on the K(+)-evoked release of GABA, glutamate or somatostatin from rat cortical synaptosomes were antagonized by CGP 52432. The IC50 of the drug at GABA autoreceptors (0.085 microM) was 35- and 100-fold lower than at the receptors regulating somatostatin and glutamate overflow, respectively. At the autoreceptor the calculated pA2 for CGP 52432 amounted to 7.70, which makes the drug about 1000-fold more potent than phaclofen at this receptor. The potency and selectivity characteristics of CGP 52432 indicate that the drug is by far the most appropriate tool to investigate the terminal GABAB autoreceptors of the rat cerebral cortex.

gamma-Aminobutyric acidB receptors mediate inhibition of somatostatin release from cerebrocortex nerve terminals

The effects of gamma-aminobutyric acid (GABA) and of various GABA receptor agonists and antagonists on the calcium-dependent depolarization-evoked release of somatostatin (SRIF) from rat cerebrocortex synaptosomes have been studied by a superfusion technique. GABA (0.3-30 microM) decreased the K+ (15 mM)-evoked overflow of SRIF-like immunoreactivity (SRIF-LI) in a concentration-dependent manner (EC50 = 1.3 microM; maximal inhibition, 45% reached at 10 microM GABA). The effect of the amino acid was insensitive to the GABAA receptor antagonist bicuculline. Accordingly, the K(+)-evoked SRIF-LI release was not affected by muscimol, a GABAA receptor agonist, up to 100 microM. The effect of GABA was mimicked by the GABAB receptor agonist (-)-baclofen (EC50 = 1.2 microM; maximal effect, about 45% reached at 10 microM). The effect of baclofen was stereoselective, the (+)-enantiomer being inactive up to 100 microM. The inhibition of SRIF-LI release brought about by GABA was sensitive to the GABAB receptor antagonists 2-hydroxy-saclofen and CGP 35348 [3-aminopropyl(diethoxymethyl)phosphinic acid]. Also, the effect of (-)-baclofen was antagonized by CGP 35348 (IC50 = 4.8 microM). It is concluded that GABA can inhibit the depolarization-evoked release of SRIF by activating receptors which are located on SRIF-releasing nerve terminals and belong to the GABAB type.

gamma-Aminobutyric acid (GABA) stimulates somatostatin release following activation of a GABA uptake carrier located on somatostatin nerve endings of rat cerebral cortex

The effect of gamma-aminobutyric acid (GABA) on the release of somatostatin-like immunoreactivity (SRIF-LI) was studied in synaptosomes prepared from rat cerebral cortex and exposed in superfusion to the amino acid. GABA (1-300 microM) increased the spontaneous outflow of SRIF-LI in a concentration-dependent manner. The effect of GABA was not prevented by the GABAA receptor antagonists bicuculline or picrotoxin. The GABAA receptor agonist muscimol (10-100 microM) did not affect SRIF-LI release. Similarly ineffective was the GABAB receptor agonist (-)-baclofen (100 microM). The GABA-induced SRIF-LI release was counteracted by the GABA uptake inhibitors N-(4,4-diphenyl-3-butenyl)-nipecotic acid (SK&F 89976A) and nipecotic acid. When used as a GABA carrier substrate, nipecotic acid mimicked GABA and increased SRIF-LI release; its effect was antagonized by SK&F 89976A. The mechanism involved appears to be selective for GABA inasmuch as neutral amino acids such as leucine, alpha-aminobutyric acid or valine, tested at 100 microM, had little or no effect on the release of SRIF-LI. Neither GABA (100 microM) nor nipecotic acid (300 microM) enhanced the release of cholecystokinin-like immunoreactivity. The GABA-evoked somatostatin release was calcium-dependent and tetrodotoxin-insensitive. It is concluded that a carrier for the uptake of GABA exists on somatostatin-releasing terminals of rat cerebral cortex and that GABA uptake may regulate somatostatin release. This conclusion would be compatible with the reported coexistence of GABA and somatostatin in cerebrocortical neurons.

Is the muscarinic receptor that mediates potentiation of dopamine release negatively coupled to the cyclic GMP system?

Dopamine (DA) terminals in rat corpus striatum and frontal cortex possess muscarinic receptors that mediate enhancement of the depolarization-evoked release of the catecholamine. The effects of the membrane-permeating cyclic guanosine monophosphate (cyclic GMP) analog 8-Br-cyclic GMP and of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) on the muscarinic-induced increase of DA release were investigated in striatal synaptosomes prelabeled with [3H]DA and exposed in superfusion to 15 mM KCl and to acetylcholine (ACh). Preincubation of synaptosomes with 8-Br-cyclic GMP (10-200 microM) or with IBMX (200 microM) prevented the ACh-induced enhancement of [3H]DA release, without affecting the K+-evoked release of the [3H]amine. No significant decrease of the ACh effect was observed when 8-Br-cyclic GMP or IBMX were added concomitantly with ACh to the superfusion medium. The data suggest that stimulation of presynaptic muscarinic receptors on DA terminals may produce enhancement of 3H DA release through a decrease of the intraterminal cyclic GMP content.

Alpha 2-adrenoceptors in rat hypothalamus and cerebral cortex: functional evidence for pharmacologically distinct subpopulations

The presynaptic alpha 2-adrenoceptors regulating, respectively, [3H]noradrenaline and [3H]5-hydroxytryptamine release were compared in experiments with noradrenaline and clonidine as agonists and the two enantiomers of mianserin as antagonists in rat hypothalamic and cortical synaptosomes depolarized with 15 mM KCl. The affinity of clonidine was 10 times higher at the alpha 2-autoreceptors than at the alpha 2-heteroreceptors. (-)Mianserin antagonized noradrenaline at the heteroreceptors but not at the autoreceptors. In contrast, (+)mianserin did not discriminate between the two receptors. The results support the existence in the rat brain of subtypes of alpha 2-adrenoceptors having different neuronal location, function and pharmacological properties.

Differential blockade by (-)mianserin of the alpha 2-adrenoceptors mediating inhibition of noradrenaline and serotonin release from rat brain synaptosomes

Mianserin and its two enantiomers were studied as antagonists of the alpha 2-adrenoceptors mediating inhibition of noradrenaline and 5-hydroxytryptamine release in rat brain cortex. The inhibitory effect of exogenous noradrenaline on the release of 3H-noradrenaline evoked by 15 mM KCl from superfused cortical synaptosomes was antagonized by racemic mianserin and by (+)mianserin; the (-)enantiomer was ineffective. In contrast, both (+)mianserin and (-)mianserin antagonized the inhibitory effect of noradrenaline on the release of 3H-5-hydroxytryptamine. The results suggest that the alpha 2-autoreceptors on noradrenergic nerve endings differ from the alpha 2-adrenoceptors located on serotoninergic terminals.

Noradrenaline inhibits central serotonin release through alpha 2-adrenoceptors located on serotonergic nerve terminals

The effect of noradrenaline on the depolarization-evoked release of 3H-5-hydroxytryptamine was investigated in superfused synaptosomes prepared from rat cortex and hippocampus and prelabelled with the radioactive indoleamine. Noradrenaline reduced in a concentration-dependent way the release of 3H-5-hydroxytryptamine elicited by 15 mM KCl. The inhibition was counteracted by the alpha-adrenoceptor antagonists phentolamine or yohimbine, but not by prazosin. The results indicate that, in rat brain, the inhibition of 5-hydroxytryptamine release by noradrenaline is mediated by adrenoceptors of the alpha 2-type localized on the terminal serotonergic fibres.