Contribution of presynaptic GABA-B receptors to paired-pulse depression of GABA-responses in the hippocampus.

The synaptic release of gamma-aminobutyric acid (GABA) is thought to be regulated by presynaptic GABA receptors of the B-type. It was the goal of this study to validate this concept electrophysiologically using four selective antagonists of GABA-B receptors. Experiments were performed in hippocampal slices exposed to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX 30 microM) and D-2-amino-5-phosphonopentanoate (AP5 40 microM) in order to block excitatory transmission. Consequently, electrical stimulation of the Schaffer collateral/commissural fibers evoked monosynaptic inhibitory potentials (IPSP) recorded intracellularly from CA 1 pyramidal neurons. In a test called paired-pulse paradigm two identical stimuli were applied at intervals ranging from 350 to 4000 ms. The IPSP evoked by the second stimulation was smaller in its amplitude over the entire interval range. This reduction of the second GABA-response is thought to result from the activation of presynaptic GABA receptors. The GABA-uptake inhibitor SKF 89976 (100 microM) increased the amplitude of the IPSP's and increased the ratio of the first to the second IPSP amplitude. These findings indicate that the drug increases the GABA content in the synaptic cleft leading to a facilitation of paired-pulse depression. The actions of four bath-applied GABA-B receptor antagonists were examined in the paired-pulse paradigm. None of these compounds abolished paired-pulse inhibition completely even at concentrations higher than those required to block postsynaptic GABA-B responses. The potent GABA-B antagonists CGP 55845 and CGP 52432 reduced paired-pulse depression by 80% at 10 microM (maximal effect).(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of orally active GABAB receptor antagonists on GABAergic transmission in vivo and in vitro.

The goal of this report is to present the results obtained with three new GABAB receptor antagonists. CGP 54062 has an IC50 in a GABAB binding test of 0.013 microM which is roughly 2500-fold lower than one of the most potent blockers known so far, CGP 35348 (IC50 = 34 microM). CGP 46381 and CGP 36742 have IC50s of 4.9 and 36 microM respectively. The latter two compounds are the first orally active GABAB receptor antagonists. All three compounds bind to the GABAB receptor selectively, and are inactive in a number of binding tests assessing the compounds' affinity to various other receptor sites. The effect of these blockers on GABAergic transmission was investigated in the CA1 area of hippocampal slices. The Schaffer collateral/commissural fibers were stimulated and the evoked postsynaptic potentials were recorded intracellularly in pyramidal neurons. The three antagonists blocked the late inhibitory postsynaptic potential with the following rank order of potency CGP 54062 > 46381 > 36742 approximately 35348. These findings support the hypothesis that these potentials are mediated by GABAB receptors. Orally administered CGP 36742 and CGP 46381 block the neuronal depression induced by iontophoretically applied baclofen in anaesthetised rats. Up to a dose of 10 mg/kg i.v. CGP 54062 was inactive and thus does not appear to cross the blood-brain barrier at this dose. In anaesthetised rats the effects of the three new GABAB antagonists and of CGP 35348 were investigated on the paired-pulse inhibition of the population spikes evoked in the CA1 area of the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid receptors in rat locus coeruleus. An extracellular in vitro study.

The goal of this study was to investigate whether locus coeruleus neurons of the rat are sensitive to agonists of the different excitatory amino acid receptors. All experiments were performed on a midpontine rat slice preparation. Bath-applied L-glutamate, kainate, N-methyl-D-aspartate (NMDA) and quisqualate induced concentration-dependent activations of all neurons which were reflected in an increase of the neurons' mean discharge rate. The rank order of cell activation was kainate approximately quisqualate greater than NMDA greater than L-glutamate. None of the agonists induced a bursting-type of discharge. The NMDA-receptor blocker DL-2-amino-5-phosphonovaleric acid (APV, 30 microM) selectively antagonized the NMDA-induced increase in cell firing. Kynurenic acid (100 microM) non-selectively attenuated the response to NMDA, kainate and quisqualate. Neither APV nor kynurenic acid per se had any effect on the spontaneous firing rate. If the Mg2+ concentration in the superfusion medium was lowered from 2 mM to nominally zero the response to NMDA was selectively increased. In conclusion, locus coeruleus neurons share with other neurons their sensitivity to agonists of all three types of excitatory amino acid receptors. However, in contrast to other neurons, they do not respond with a bursting type of discharge.

Valproate enhances GABA-A mediated inhibition of locus coeruleus neurones in vitro.

It has previously been claimed that the anticonvulsant valproate acts by augmenting GABA-ergic transmission, however, the data supporting this claim is controversial. Here we demonstrate that valproate strongly and reversibly potentiates the depressant effects of the GABA-A receptor agonist muscimol on locus coeruleus neurones recorded extracellularly from a midpontine slice preparation of the rat. The depressant effect of muscimol (2 microM) is augmented by bath applied valproate at concentrations of 50 microM, 100 microM and 1 mM. The effect of GABA is also potentiated by valproate. The potentiating effect is selective since the cell inhibition elicited by the GABA-B receptor agonist baclofen is not affected. Valproate on its own had no effect on the firing frequency.

GABAA and GABAB receptors in locus coeruleus: effects of blockers.

Racemic baclofen, (-)-baclofen and muscimol depressed all spontaneously firing locus coeruleus neurons tested in a slice preparation. Racemic phaclofen (100 microM; 1 mM) moderately antagonized the effects of racemic baclofen without antagonizing those of muscimol. Bicuculline (10, 30, 100 microM) potently antagonized the action of muscimol without affecting the inhibition of baclofen. Phaclofen and bicuculline had no pronounced effect on the spontaneous discharge rate of cells. The results suggests that there are GABAA and GABAB receptors in the locus coeruleus.

Comparative investigations on the actions of ACTH1-24, somatostatin, neurotensin, substance P and vasopressin on locus coeruleus neuronal activity in vitro.

A considerable number of neuropeptides have been localized immunohistochemically in the area of the locus coeruleus of the rat. The objective of this study was to assess the actions of some of these transmitter candidates on spontaneously active locus coeruleus neurons in vitro. The effects of bath-applied peptides on the discharge rate of individual locus coeruleus neurons were investigated. A midpontine slice preparation of the gerbil brain was used. Excitatory dose-dependent effects were found with four peptides with the following rank order of potency: Substance P, (Arg8)-vasopressin, neurotensin, ACTH1-24. Somatostatin hyperpolarized all neurons tested. Given the pronounced effects seen with substance P, somatostatin and vasopressin in the nanomolar range, it is conceivable that these peptides may have a role in regulating neuronal activity in locus coeruleus.

Substance P depresses neuronal activity in the rat olfactory bulb in vitro and in vivo: possible mediation via gamma-aminobutyric acid release.

The rat olfactory bulb is an area displaying a particularly high density of substance P receptors in the glomerular cell layer whose functions are unknown. In pilot in vivo experiments we discovered that iontophoretically administered substance P potently depressed the spontaneous firing rate of most unidentified neurons of the rat olfactory bulb. To further elucidate the mechanism of this unexpected depressant effect, we studied the peptide's action in vitro on coronal sections of this brain region. Bath applied and microiontophoretically administered substance P depressed the spontaneous discharge of unidentified glomerular neurons in a dose-dependent fashion. This inhibiting effect is mediated indirectly via the release of another transmitter because it was abolished completely if the standard perfusion medium was replaced by a medium containing zero calcium and high magnesium. It appears that substance P acts by means of releasing GABA which in turn evokes the observed cell depression because the depressant effects were completely abolished by bath-applied bicuculline (10 microM) and picrotoxin (100 microM). In conclusion we propose that substance P indirectly depresses neuronal activity in the glomerular cell layer of the rat olfactory bulb by releasing gamma-aminobutyric acid.

Reduced activity of locus coeruleus neurons in hypertensive rats.

The effect of blood pressure on the neuronal activity of the locus coeruleus (LC) was investigated by means of electrophysiological techniques in rats anaesthetized with chloral hydrate. The mean neuronal discharge rate of noradrenergic neurons of the LC was reduced by 19% in spontaneously hypertensive rats (SHR) and by 25% in deoxycorticosterone acetate (DOCA)-salt hypertensive rats compared with their corresponding controls. The cellular activity of 27 out of 40 neurons was reversibly suppressed by acute, peripherally induced blood pressure increases in normotensive rats. Five neurons were reversibly activated and eight neurons were not affected. Conversely, acute decreases in blood pressure stimulated the neuronal activity of the LC in 10 out of 16 neurons. These findings support the hypothesis of the LC having a role in both short and long term regulation of blood pressure.

Reduced activity of locus coeruleus neurons in hypertensive rats.

The influence of blood pressure on locus coeruleus (LC) neuronal activity was investigated in chloral hydrate anesthetized rats. Mean spontaneous firing rate of LC neurons was reduced by 25% in deoxycorticosterone acetate (DOCA-salt) treated Sprague-Dawley rats and by 19% in spontaneously hypertensive rats (SHR) as compared to their corresponding control animals. In acute experiments performed on normotensive rats, peripherally induced blood pressure changes elicited reciprocal changes in cell firing of the majority of LC neurons. The findings suggest that LC may have a role in long-term regulation of blood pressure.

Comparative electrophysiological investigations with Hydergine and other psychogeriatric agents on corticol and subcortical structures.

The action of systemically administered Hydergine on the spontaneous neuronal firing of neurons located in various brain areas of the anaesthetized rat was investigated using conventional extracellular single cell recording techniques. Whereas locus coeruleus neurons were activated by both acute and repetitive injections of the drug, spontaneously active neurons of the medial septal nucleus, pyramidal cells of the hippocampus and unidentified neurons of the cingulate cortex were not affected. The possible functional consequences of the drug's activating action on locus coeruleus noradrenergic neurons are discussed. In view of the finding that noradrenaline affects the signal-to-noise ratio in various brain areas and given the widespread innervation pattern of noradrenergic fibers arising from locus coeruleus, Hydergine through its action on this brain area may potentially affect many brain regions.

Decreased activity of locus coeruleus neurons in the rat after DSP-4 treatment.

The activity of noradrenergic neurons of the rat locus coeruleus was investigated at 10 and 50 days after the administration of DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine), a selective noradrenergic neurotoxin. The mean neuronal firing rate in control animals was 2.4 Hz. In contrast, DSP-4 animals had lower rates of 1.2 Hz at 10 days and 1.7 Hz at 50 days. Histological examinations revealed no morphological changes of locus coeruleus cell bodies at either the 10- or 50-day time points. These results suggest that DSP-4 can impair neuronal activity of the locus coeruleus without altering the structural appearance of locus coeruleus perikarya.

The locus coeruleus: actions of psychoactive drugs.

The locus coeruleus is one of the most thoroughly investigated mammalian brain areas. Its fibers innervate large parts of the neuraxis, in particular, areas involved in cognitive functions such as the cortex and the hippocampus. A role of locus coeruleus has been proposed in such processes as memory, the control of vigilance, blood pressure and others. Results obtained in this and other laboratories demonstrate that the firing rate of locus coeruleus neurons is affected by a great number of psychoactive agents such as antidepressants, minor tranquillizers, neuroleptics, psychostimulants and certain psychogeriatric drugs. We have attempted to correlate the data obtained on the cell bodies of locus coeruleus with studies reporting effects on terminal areas and thereby gain an overall view of the action of the above mentioned drugs on this cell system. The activity of noradrenergic neurons in locus coeruleus is thought to correlate with the level of cortical vigilance. Special emphasis is placed on the finding that a number of drugs which exert a positive effect on cognitive functions in man and animals increase the firing rate of the rat locus coeruleus neurons.

The effect of vincamine, hydergine and piracetam on the firing rate of locus coeruleus neurons.

Vincamine, hydergine and piracetam are used for the treatment of memory disturbances as well as for the treatment of various other brain dysfunctions seen in the elderly patient. The action of these three psychogeriatric compounds on the activity of noradrenergic neurons in the rat locus coeruleus was investigated. All three compounds, when administered intraperitoneally, increased the firing rate of noradrenergic neurons in the chloral hydrate anaesthetized animals. Vincamine and hydergine had a similar potency producing a maximal mean increase of about 70% at a dose of 1 mg/kg. Likewise, piracetam was significantly less potent eliciting a 30 to 40% increase in firing at doses of 300 and 1000 mg/kg, respectively. A role for the locus coeruleus in processes related to attention, cortical and behavioral arousal, learning and memory has been proposed on the basis of behavioral and electrophysiological experiments. In the awake rat, the firing rate of noradrenergic neurons in this brain nucleus has previously been shown to relate directly to the level of vigilance. The activating action of vincamine, hydergine and piracetam on these neurons may be linked with some of the drugs' beneficial therapeutic effects.

Differential actions of mianserin and iprindole on the sensitivity of cortical neurons to noradrenaline: effect of chronic treatment.

The sensitivity of cingulate cortical neurons to microiontophoretically administered noradrenaline (NA) was compared in groups of rats treated with one or other of the two antidepressants mianserin (30 mg/kg s.c.) or iprindole (10 mg/kg i.p.) either acutely or chronically, for 10 and 28 days respectively. In the rats treated chronically with mianserin, the neurons were significantly less sensitive to NA than those of the acutely treated animals. By contrast, chronic administration of iprindole did not significantly affect neuronal sensitivity to NA.