Involvement of glutamate neurotransmission and N-methyl-d-aspartate receptor in the activation of midbrain dopamine neurons by 5-HT1A receptor agonists: an electrophysiological study in the rat.

Systemic administration of selective 5-HT1A agonists, such as 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT), stimulates the electrical activity of ventral tegmental area (VTA) dopamine neurons by a mechanism which remains unknown. We have examined if this activation is dependent on glutamatergic, serotonergic and GABAergic neurotransmission and if 5-HT1A receptors located within the VTA or within the prefrontal cortex (PFC) could contribute. In vivo electrophysiological recordings were obtained from VTA dopamine neurons from anesthetized rats. The i.v. administration of the 5-HT1A agonist 8-OHDPAT induced a strong stimulation of burst and firing activity of dopamine neurons. This activation remained unchanged in rats pre-treated with the 5-HT depleting agent parachlorophenylalanine. However, pre-administration of the GABAB receptor antagonist phaclophen, but not of the GABAA antagonist picrotoxin, significantly reduced the 8-OHDPAT-induced activation. The N-methyl-d-aspartate (NMDA) antagonist MK 801 (dizocilpine), but not the AMPA/kainate antagonist [1,2,3,4-tetrahydro-7-morpholinyl-2,3-dioxo-6-(fluoromethyl)quinoxalin-1-yl] methyl-phosphonate (ZK 200775), partially prevented or reversed the effects of 8-OHDPAT. However, only the combined pre-administration of the two glutamate antagonists did completely prevent the activatory response to 8-OHDPAT and even converted the effect of 8-OHDPAT into an inhibition, in half of the dopamine neurons tested. Inactivation of the local 5-HT1A receptors by the microinfusion within the VTA of the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY 100,635), or of pertussis toxin, reduced the ability of 8-OHDPAT to stimulate the firing of dopamine neurons but not their burst activity. On the other hand, burst activation elicited by 8-OHDPAT was strongly reduced following the inactivation of prefrontal 5-HT1A receptors achieved by the microinfusion of WAY 100,635 within the PFC. These results show that activation of midbrain dopamine neurons by the systemic administration of 5-HT1A agonists does involve the inactivation of a tonic GABAergic tone, involving mainly the GABAB receptors, probably leading to the stimulation of a glutamatergic excitatory drive from the PFC to the VTA and an increase in glutamate release. This will excite dopamine neurons, preferentially through NMDA receptors. Furthermore, our results suggest that some 5-HT1A receptors located within the VTA may also participate in this activation.

Activation of the mesocorticolimbic dopaminergic system by stimulation of muscarinic cholinergic receptors in the ventral tegmental area.

OBJECTIVE: To investigate the function of muscarinic receptors in the ventral tegmental area in vivo, the release of endogenous monoamines was simultaneously measured in the somatodendritic (ventral tegmental area) and terminal (frontal cortex and nucleus accumbens) regions of the mesocorticolimbic dopaminergic system in rats, using dual probe microdialysis. METHODS: Rats were implanted with dual microdialysis probes ipsilaterally into the ventral tegmental area (VTA) and nucleus accumbens (NAC) or frontal cortex (FC). RESULTS: Intrategmental infusion of the muscarinic agonist oxotremorine M (OXO M, 0.1 and 1 mM) increased extracellular levels of dopamine and serotonin, but not noradrenaline, in the VTA to a maximum of 200% over baseline in both urethane-anaesthetized and unanaesthetized rats. In freely moving animals, this effect was accompanied by strong motor agitation. Both VTA dopamine and serotonin levels dropped to 60% or less of baseline when the perfusion medium was replaced by a calcium-free medium containing OXO M. In the NAC and FC, a similar increase in extracellular dopamine, but not serotonin and noradrenaline, was observed during OXO M infusion in the VTA. The removal of calcium during OXO M infusion in the VTA did not cause a decrease in NAC dopamine levels. Activation of serotonin and dopamine release by OXO M in the VTA and FC was dramatically reduced or prevented by the co-infusion of the muscarinic antagonist N-methylscopolamine (0.1 mM). CONCLUSION: These data demonstrate that VTA dopamine cells possess functional muscarinic receptors whose activation stimulates the release of dopamine in the VTA, NAC and FC. These results also suggest that muscarinic receptors may modulate the synaptic release of serotonin in the VTA.

Pertussis toxin treatment differentially affects cholinergic and dopaminergic receptor stimulation of midbrain dopaminergic neurons.

Extracellular single-unit recordings and iontophoresis were used to compare the effect of a single administration of pertussis toxin (PTX, 1 microg), into midbrain dopamine (DA) nuclei (A9 and A10 regions), on the muscarinic, NMDA and DA receptor responses of midbrain DA cells in the anesthetized rat. Iontophoretic applications of DA, or apomorphine (50 microg/kg, i.v.), markedly reduced the firing of DA cells in control rats. In PTX-treated animals, these inhibitory responses were totally abolished, indicating that, in both DA nuclei, the inhibitory DA receptors are coupled to Gi/o proteins. In parallel, there was a significant decrease in the number of active DA cells per track which returned to baseline 5 weeks after the treatment. Applications of the muscarinic agonist oxotremorine M (OXO M) or of NMDA produced a potent increase in the firing of DA cells in control rats. DA neurons treated with PTX were still responsive to OXO M, although their sensitivity to the agonist was significantly reduced by 40%. In contrast, NMDA-induced activation remained unchanged, indicating that PTX did not non-selectively dampen all excitatory responses. Applications of cell-permeable cAMP derivatives did not change the basal firing of DA neurons. On the other hand, the phospholipase C inhibitors neomycin and ET-18-OCH3 (200 microg, i.c.v.), reduced significantly the activation of DA cells induced by OXO M. These data suggest that muscarinic activation of DA cells involves an M1-like receptor, possibly coupled to Gq/11 proteins, but also the participation of a PTX substrate.

Involvement of sigma receptors in the modulation of the glutamatergic/NMDA neurotransmission in the dopaminergic systems.

Extracellular single-unit recordings and iontophoresis were used to examine the effects of different selective sigma receptor ligands on dopaminergic and glutamatergic N-methyl-D-aspartate (NMDA) neurotransmissions both in origin (A10 and A9 areas) and terminal (nucleus accumbens and caudate nucleus) regions of the rat mesolimbic and nigrostriatal dopaminergic systems. The selective sigma1 receptor ligands 2-[4-(4-methoxy-benzyl)piperazin-1-yl-methyl]4-oxo[4H]-benzo-th iazolin-2-one (S-21377), systemically administered (1.2 mg/kg, i.v., cumulative dose), and 2[(4-benzyl piperazin-1-yl) mothyl] naphthalene, dichiorydrate (S-21378), iontophoretically applied, slightly increased the spontaneous firing rate and potentiated the NMDA-induced neuronal activation of dopaminergic neurons in the A9 and A10 regions. (+)N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-butyl-2-N (JO-1784), another selective sigma1 receptor ligand produced no or little effect in these areas. The systemic administration of the selective sigma2 receptor ligand 1,4-bis-spiro[isobenzofuran-1(3H), 4'-piperidin-1'yl]butane (Lu 29-252) (2 mg/kg, i.v., cumulative dose) did not modify the firing activity of A9 and A10 dopaminergic neurons, but significantly potentiated the NMDA-induced increase in firing activity of A10 dopaminergic neurons. None of the sigma receptor ligands tested had any effects on the dopamine-induced suppression of firing. In the nucleus accumbens, the systemic administration of (JO-1784), (40 microg/kg, i.v.), (+)-pentazocine (30 microg/kg, i.v.), another selective sigma1 receptor ligand, and of the non selective sigma1 receptor ligand di-tolyl-guanidine (DTG) (20 microg/i.v.) produced a significant increase of NMDA-induced neuronal activation. Microiontophoretic applications of JO-1784 also potentiated the NMDA response. They also increased significantly the suppressant effect of dopamine on NMDA and kainate-induced activations of accumbens neurons. In the caudate nucleus, (+)-pentazocine, but not JO-1784, potentiated slightly the neuronal response to NMDA. None of the sigma receptor ligands tested did modify significantly the responses of caudate and accumbens neurons to kainate. These findings suggest that at least two subtypes of sigma1 receptors may affect differentially the glutamate NMDA neurotransmission in the terminal and origin regions of the mesolimbic and nigrostriatal dopaminergic systems. These results also demonstrate the existence of a functional interaction between sigma2 and NMDA receptors in the A10 region.

Activation of midbrain presumed dopaminergic neurones by muscarinic cholinergic receptors: an in vivo electrophysiological study in the rat.

1. Extracellular single-unit recording and iontophoresis were used to examine the effects of different cholinoceptor agonists and antagonists on the firing rate and firing pattern of A9 and A10 presumed dopaminergic neurones in the anaesthetized rat. 2. Administration of low currents (1-5 nA) of the selective muscarinic agonists oxotremorine M (Oxo M) and muscarine and of the non-selective muscarinic/nicotinic agonist carbamylcholine (CCh) produced a dose-dependent increase in firing rate in most of the A9 and A10 presumed dopaminergic neurones tested. Oxo M-induced activation could be completely blocked by iontophoretic application of the muscarinic antagonist butyl-scopolamine or systemic administration of the muscarinic antagonist scopolamine (300 microg kg(-1), i.v.). 3. Iontophoretic application of the selective nicotinic agonist methylcarbamylcholine (MCCh), but not nicotine, induced a consistent increase in firing rate. Surprisingly, the excitatory effect of MCCh was significantly reduced by the selective muscarinic antagonist scopolamine (300 microg kg(-1), i.v.), but not by the selective nicotinic antagonist mecamylamine (2.2 mg kg(-1), i.v.). Mecamylamine (3 mg kg(-1), i.v.) was also ineffective in reducing the CCh-induced activation of presumed dopamine neurones, suggesting that both CCh and MCCh increased the activity of dopamine neurones via an interaction with muscarinic receptors. 4. Iontophoretic application of the endogenous agonist acetylcholine (ACh) had no or little effect on the firing activity of A10 presumed dopaminergic neurones. However, concomitant application of neostigmine, a potent cholinesterase inhibitor, with acetylcholine induced a substantial activation of these neurones. This activation consisted of two components; one, which was prevalent, was scopolamine (300 microg kg(-1), i.v.)-sensitive, and the other was mecamylamine (2 mg kg(-1), i.v.)-sensitive. 5. In addition to their effect on firing activity, Oxo M, muscarine and concomitant neostigmine/ACh caused a significant increase in burst firing of A10 neurones, but not of A9 neurones. 6. These data suggest that dopamine cells, both in the A9 and A10 regions, possess functional muscarinic receptors, the activation of which can increase their firing rate and, for A10 neurones, their amount of burst activity. These cholinoceptors would be able to influence the activity of the midbrain dopamine system greatly and may play a role in, and/or be a therapeutic target for, brain disorders in which dopamine is involved (e.g., Parkinson's disease, drug addiction and schizophrenia).

Electrophysiological evidence for the implication of cholecystokinin in the modulation of the N-methyl-D-aspartate response by sigma ligands in the rat CA3 dorsal hippocampus.

Previous studies from our laboratory have demonstrated that low doses of selective sigma (sigma) ligands potentiate the response of pyramidal neurones to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. It has also been found that the neuropeptide cholecystokinin (CCK) is involved in the effects induced by sigma ligands on colonic motility. The present experiments were undertaken to determine if this interaction is also present in the rat dorsal hippocampus. Using microiontophoresis and in vivo extracellular recordings of rat CA3 dorsal hippocampus pyramidal neurones, we assessed the effects of CCKA and CCKB receptor antagonists on the potentiation of the NMDA response, induced by the intravenous administration of low doses of the sigma ligands 1,3-di(2-tolyl)guanidine (DTG), (+)-pentazocine and JO-1784. The potentiation of the NMDA response induced by these sigma ligands was abolished by the selective CCKA receptor antagonist SR 27897, but not by the CCKB antagonist Cl-988. CCK-8S, applied with a low current, insufficient to induce by itself an increase of the firing activity, markedly potentiated the response of NMDA without affecting significantly that of quisqualate. SR 27897, but not Cl-988, significantly reduced the potentiation of the NMDA response by CCK-8S. These results suggest the existence of a functional interaction between CCK and sigma receptor-mediated effects in the dorsal hippocampus.

CCKB receptors mediate CCK-8S-induced activation of dorsal hippocampus CA3 pyramidal neurons: an in vivo electrophysiological study in the rat.

The sulphated octapeptide C-terminal fragment of cholecystokinin (CCK-8S) is present in high concentration in the mammalian brain, where it acts via two types of receptor denoted CCKA and CCKB. In the dorsal hippocampus, CCK-8S exerts a potent excitatory effect on pyramidal neurons. The present electrophysiological study was undertaken to determine which CCK receptor type mediates this neuronal activation. Using in vivo extracellular unitary recordings of CA3 pyramidal hippocampal neurons, we compared the effect of SNF-8702, a potent selective CCKB receptor agonist, to that of CCK-8S, and assessed the effects of selective CCKA and CCKB antagonists. CCK-8S and SNF-8702, microiontophoretically applied on the same neurons produced a similar degree and pattern of activation. Both CCK-8S- and SNF-8702-induced activations were suppressed by the microiontophoretic application of the CCKB antagonist CI-988, but not by that of the CCKA antagonist SR 27897. CCK-8S-induced activation was not significantly modified by the intravenous administration of the CCKA antagonists devazepide and SR 27897. However, it was reduced by the CCKB antagonist PD 135158, administered intravenously or intracerebroventricularly, and by the intravenous administration of the CCKB antagonist L-365,260. The intravenous administration of PD 135158 also reduced SNF-8702-induced activations. These results indicate that CCKB receptors mediate CCK-8S-induced activation of rat CA3 pyramidal neurons.

Kinetic characterization of a carrier-mediated transport system for L-tryptophan in human blood platelets.

We have characterized a membrane transport system on washed human blood platelets for tritiated L-tryptophan (L-TRP). This transport was extremely rapid, temperature dependent and markedly reduced by disruption of the platelet membranes. Kinetic studies within a large range of L-TRP concentrations have revealed the presence of a high affinity saturable transport system which follows simple Michaelis-Menten kinetics, with an apparent Km value of 10 microM and a Vmax of 200 pmol/min/10(8) cells. Platelet L-TRP accumulation was insensitive to changes in sodium concentrations and to the inclusion of ouabain in the incubation medium. Furthermore, uptake was unmodified by the presence of the metabolic inhibitor dinitrophenol, suggesting that it is mediated by facilitated diffusion. D-Tryptophan was a very poor inhibitor of L-TRP uptake. Transport was insensitive to serotonin and imipramine but was inhibited in a dose-dependent manner by L-tyrosine, L-phenylalanine and L-leucine, implying that it may be mediated by a system that is specific for aromatic and long chain amino acids. The results were compared to reports examining the L-TRP transport in other cell types.

The effects of sigma ligands and of neuropeptide Y on N-methyl-D-aspartate-induced neuronal activation of CA3 dorsal hippocampus neurones are differentially affected by pertussin toxin.

1. The in vivo effects of the high affinity sigma ligands 1,3-di(2-tolyl)guanidine (DTG), (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1- ethyl-but-3-en-1-ylamine hydrochloride (JO-1784), (+)-pentazocine and haloperidol, as well as of those of neuropeptide Y (NPY), on N-methyl-D-aspartate (NMDA)- and quisqualate (Quis)-induced neuronal activations of CA3 pyramidal neurones were assessed, using extracellular unitary recording, in control rats and in rats pretreated with a local injection of pertussis toxin (PTX), to evaluate the possible involvement of Gi/o proteins in mediating the potentiation of the neuronal response to NMDA by the activation of sigma receptors in the dorsal hippocampus. 2. Microiontophoretic applications as well as intravenous injections of (+)-pentazocine potentiated selectively the NMDA response in control rats as well as in PTX-pretreated animals. In contrast, the PTX pretreatment abolished the potentiation of the NMDA response by DTG, JO-1784 and NPY. Moreover, microiontophoretic applications of DTG induced a reduction of NMDA-induced neuronal activation. Neither in control nor in PTX-treated rats, did the sigma ligands and NPY have any effect on Quis-induced neuronal response. 3. In PTX-treated rats, the potentiation of the NMDA response induced by (+)-pentazocine was suppressed by haloperidol, whereas the reduction of the NMDA response by DTG was not affected by haloperidol. 4. This study provides the first in vivo functional evidence that sigma ligands and NPY modulate the NMDA response by acting on distinct receptors, differentiated by their PTX sensitivity.

Lack of association between platelet tritiated imipramine binding and clinical status of depressed patients on chronic antidepressant treatment.

Platelet tritiated imipramine binding (Bmax) was studied in 33 depressed patients, before and after 1 and 4 weeks of antidepressant treatment, and in 34 healthy volunteers. The Bmax was significantly lower (-21%) in drug-free depressed patients than in controls and increased significantly as early as the first week of treatment to reach the control value, in parallel with a 38% decrease in the Hamilton depression rating scale (HDRS) score. After 4 weeks of treatment, the Bmax was still normal and remained significantly higher than the baseline value, while the clinical state of the patients had greatly improved (a 63% decrease in the HDRS score). However, an increase in the Bmax also occurred in non-responders to treatment. In addition, we observed that the ability of the treatment to increase the Bmax depended on the pharmacological profile of the drug used. The present results show that, in patients on antidepressant medication, platelet tritiated imipramine binding normalization cannot be considered as a marker of clinical remission.

Evidence for a defective platelet L-tryptophan transport in depressed patients.

The kinetic characteristics of platelet L-tryptophan uptake were investigated in 23 untreated depressed patients and 18 healthy volunteers. A significant increase of 50% in the apparent Michaelis constant (Km) was observed in depressed patients compared with controls, without significant change in the maximal velocity (Vmax). After 1 month of successful antidepressant treatment the mean Km value decreased significantly and reached the control value. This result raises the possibility that the decrease in platelet tryptophan uptake affinity is a state-dependent marker for depression. It is likely that the transport alteration results in a decrease in platelet tryptophan accumulation. The effect of this peripheral membrane defect on brain serotonin function is discussed.

Rapid changes in 3H-imipramine platelet binding after chronic treatment with amineptine, a selective dopamine uptake blocker, in major depressed patients.

The effect of chronic treatment with amineptine (200 mg daily), a tricyclic antidepressant drug selectively blocking dopamine uptake, on 3H-imipramine binding, was investigated in platelets of major depressed patients in conjunction with changes in clinical state. Before treatment, depressed patients had a significantly lower Bmax (P less than 0.01) than age- and gender-matched healthy controls. After only 1 week of amineptine administration, Bmax values increased significantly (P less than 0.01) and reached the control value concomitantly with a large and significant clinical improvement (P less than 0.01). After 1 month, Bmax was still significantly different from the pretreatment value (P less than 0.05), and not significantly different from the control value, while the improvement in clinical status persisted. No significant changes in Kd values were observed during treatment. We also verified that amineptine did not displace 3H-imipramine binding from platelets either in depressed or in control subjects. The results show that the successful treatment with amineptine, an antidepressant drug devoid of affinity for the tritiated imipramine platelet binding site, can rapidly lead to its density normalization.

Platelet [3H]-imipramine binding according to DSM-III subtypes of depression.

The question of the previously reported changes in the density of high-affinity binding sites for [3H]-imipramine (IMI) in platelets from depressed patients was reexamined among the different diagnostic subtypes of depression according to the DSM-III classification and taking into account the possible influence of the low-affinity binding site. Using a least-square computer-assisted analysis, a precise determination of the [3H]-IMI binding parameters exclusively in relationship to the high-affinity site was performed in 46 untreated depressed patients and compared to 35 healthy controls. The results revealed a clear and highly significant 22% decrease in the maximal density (Bmax) of [3H]-IMI binding in all of the depressed patients compared to controls with no change in affinity values. Considering the diagnostic subgroups, we found that all the bipolar patients, the depressed as well as the euthymic or manic ones, had very low Bmax values and that some of them also exhibited an unusual low-affinity binding. Mean Bmax of the unipolar and dysthymic patients significantly decreased when compared to controls, although the Bmax values showed a large variability. Only dysthymic patients presented Bmax values which were significantly associated to symptom severity as assessed by the Hamilton Depression Rating Scale scores. Our results confirm the lower density of platelet [3H]-IMI binding in affective disorders, particularly in bipolar patients, and also suggest that this biological parameter is a trait marker in bipolar depression and a state marker in dysthmic disorder.