Serge Gobaille

Extracellular Events Induced by γ‐Hydroxybutyrate in Striatum: A Microdialysis Study

Abstract: The modification of dopamine release and accumulation induced by γ‐hydroxybutyrate (GHB) was studied using both striatal slices and in vivo microdialysis of caudate‐putamen. GHB inhibited dopamine release for ∼5–10 min in vitro, and this was associated with an accumulation of dopamine in the tissue. Subsequently, there was an increase in dopamine release. In the microdialysis experiments, low doses of GHB inhibited dopamine release, whereas higher doses strongly increased release; the initial decrease seen in slices could not be detected in vivo. Thus, GHB had a biphasic effect on the release of dopamine: An initial decrease in the release of transmitter was followed by an increase. A time‐dependent biphasic effect was observed when GHB was added to brain slices, and a dose‐dependent biphasic effect was seen in dialysate after systemic administration of GHB. Naloxone blocked GHB‐induced dopamine accumulation and release both in vitro and in vivo. GHB also increased the release of opioid‐like substances in the striatum. A specific antagonist of GHB receptors completely blocked both the dopamine response and the release of opioid‐like substances. These data suggest that GHB increases dopamine release via specific receptors that may modulate the activity of opioid interneurons.

Gamma hydroxybutyrate distribution and turnover rates in discrete brain regions of the rat

Gamma-hydroxybutyric acid and trans-gamma-hydroxycrotonic acid levels have been determined in 24 regions of the rat brain after sacrifice by microwave irradiation. Concentration ranges are from 4 pmol/mg protein (frontal cortex) to 46 pmol/mg protein (substantia nigra) for gamma-hydroxybutyric acid and from 0.4 pmol/mg protein (striatum) to 11 pmol/mg protein (hypothalamus) for trans-gamma-hydroxycrotonic acid. It appears that gamma-hydroxybutyric acid levels correlate well with GABA distribution in the same region. However this correlation is not evident with regard to the distribution of the gamma-hydroxybutyric acid synthesizing enzyme, specific succinic semialdehyde reductase. Using the antiepileptic drug, valproate which strongly inhibits gamma-hydroxybutyric acid release and degradation, we estimated the turnover rate of this compound in six regions of the rat brain. Turnover numbers ranged from 6.5 h(-1) in hippocampus to 0.76 h(-1) in cerebellum.

3′–5′ cyclic-guanosine monophosphate increase in rat brain hippocampus after gamma-hydroxybutyrate administration. Prevention by valproate and naloxone

An increase (123%) of cyclic GMP (cGMP) was observed in the hippocampus of the rat killed by microwave irradiation 45 min after administration of 500 mg/kg gamma-hydroxybutyrate (GHB) IP. This increase is time and dose dependent. No modification in cyclic nucleotide content was observed in striatum and in cerebellum. As the role of GHB has been implicated in neurotransmission, the fact that this compound increases cyclic GMP accumulation in hippocampus in vivo may represent a mechanism by which the actions of GHB are mediated at the cellular level. Valproate (400 mg/kg) or naloxone (10 mg/kg) pretreatment completely abolish the cGMP increase due to GHB. A GABAergic and/or opiate phenomenon may be involved in the mechanism of GHB induced increase of cGMP.

Xanthurenic acid distribution, transport, accumulation and release in the rat brain

Tryptophan metabolism through the kynurenine pathway leads to several neuroactive compounds, including kynurenic and picolinic acids. Xanthurenic acid (Xa) has been generally considered as a substance with no physiological role but possessing toxic and apoptotic properties. In the present work, we present several findings which support a physiological role for endogenous Xa in synaptic signalling in brain. This substance is present in micromolar amounts in most regions of the rat brain with a heterogeneous distribution. An active vesicular synaptic process inhibited by bafilomycin and nigericin accumulates xanthurenate into pre‐synaptic terminals. A neuronal transport, partially dependant on adenosine 5′‐triphosphate (ATP), sodium and chloride ions exists in NCB‐20 neurons which could participate in the clearance of extracellular xanthurenate. Both transports (neuronal and vesicular) are greatly enhanced by the presence of micromolar amounts of zinc ions. Finally, electrical in vivo stimulation of A10‐induced Xa release in the extracellular spaces of the rat prefrontal cortex. This phenomenon is reproduced by veratrine, K+ ions and blocked by EGTA and tetrodotoxin. These results strongly argue for a role for Xa in neurotransmission/neuromodulation in the rat brain, thus providing the existence of specific Xa receptors.

Dopamine transporter is essential for the maintenance of spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation

Dopamine, a neurotransmitter released by the lateral olivocochlear efferents, has been shown tonically to inhibit the spontaneous and sound‐evoked activity of auditory nerve fibres. This permanent inhibition probably requires the presence of an efficient transporter to remove dopamine from the synaptic cleft. Here, we report that the dopamine transporter is located in the lateral efferent fibres both below the inner hair cells and in the inner spiral bundle. Perilymphatic perfusion of the dopamine transporter inhibitors nomifensine and N‐[1‐(2‐benzo[b]thiophenyl)cyclohexyl]piperidine into the cochlea reduced the spontaneous neural noise and the sound‐evoked compound action potential of the auditory nerve in a dose‐dependent manner, leading to both neural responses being completely abolished. We observed no significant change in cochlear responses generated by sensory hair cells (cochlear microphonic, summating potential, distortion products otoacoustic emissions) or in the endocochlear potential reflecting the functional state of the stria vascularis. This is consistent with a selective action of dopamine transporter inhibitors on auditory nerve activity. Capillary electrophoresis with laser‐induced fluorescence (EC‐LIF) measurements showed that nomifensine‐induced inhibition of auditory nerve responses was due to increased extracellular dopamine levels in the cochlea. Altogether, these results show that the dopamine transporter is essential for maintaining the spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation.

Evidence for a Role of the Parafascicular Nucleus of the Thalamus in the Control of Epileptic Seizures by the Superior Colliculus

Summary:  Purpose: The aim of this study was to investigate whether the nucleus parafascicularis (Pf) of the thalamus could be a relay of the control of epileptic seizures by the superior colliculus (SC). The Pf is one of the main ascending projections of the SC, the disinhibition of which has been shown to suppress seizures in different animal models and has been proposed as the main relay of the nigral control of epilepsy.

Endogenous lectin CSL is present on the membrane of cilia of rat brain ependymal cells

SummaryAn endogenous brain lectin, with a great affinity for oligomannosidic glycans, called CSL (for ‘cerebellar soluble lectin’), was detected on the surface of the cilia of ependymal cells both in cultures andin vivo. The lectin is not synthesized by the ependymal cells themselves.In vivo it is neither found in cerebrospinal fluid nor in cells of the choroid plexus. Probably, lectin CSL is produced by subependymal astrocytic cells. The membranes of ependymal cells seem to possess glycoprotein ligands for the lectin which explain the specific adhesion of CSL on the surface of these cells, particularly on the cilia. The localization of this adhesive molecule on cilia of ependymal cells suggests that it may play a role in trapping foreign cells, micro-organisms or debris.

Localization studies of γ-hydroxybutyrate receptors in rat striatum and hippocampus

Abstract Quantitative autoradiography using [ 3 H] γ-hydroxybutyrate was used in combination with anatomic and neurotoxic lesions to localize the γ-hydroxybutyrate (GHB) receptors in the striatum and hippocampus of rat brain. 6-Hydroxydopamine (6-OHDA) lesions of the nigro-striatal pathway failed to reduce [ 3 H] γ-hydroxybutyrate binding in the striatum. In contrast, kainic acid (KA) lesions of the caudate-putamen (CPu) resulted in about 45% loss of binding. For hippocampus, lesions of septo-hippocampal pathway did not modify receptor density but intrahippocampal kainic acid injection largely attenuated (50%) [ 3 H] GHB binding. These results demonstrate that γ-hydroxybutyrate receptors in the CPu and dorsal hippocampus are principally located on intrinsic neurons which may participate in the functional expression of the role γ-hydroxybutyrate has in these structures.

Regional distribution in rat brain of tryptophan hydroxylase apoenzyme determined by enzyme-linked immunoassay

Tryptophan hydroxylase apoenzyme was measured in 21 regions of the rat brain by a competitive enzyme-linked immunoassay (ELISA) technique using a recently developed antiserum from the sheep to this protein. Highest apoenzyme levels were found in the pineal gland and in the dorsal raphé. An insignificant level was observed in the cerebellum. In general, the distribution of tryptophan hydroxylase apoenzyme follows the distribution of serotonin previously detected by immunocytochemistry. A turnover number for tryptophan hydroxylase in a rat brain supernatant fraction of 7.5 s-1 was estimated, a value far higher than that estimated for serotonin turnover in vivo. This result confirms that serotonin biosynthesis is additionally regulated by factors other than tryptophan hydroxylase apoenzyme concentration.