Martin W. Steinmann

CGP 35348 : a centrally active blocker of GABAB receptors

The biochemical, electrophysiological and pharmacological properties of the new GABAB receptor blocker CGP 35348 are described. In a variety of receptor binding assays CGP 35348 showed affinity for the GABAB receptor only. CGP 35348 had an IC50 of 34 microM at the GABAB receptor. The compound antagonized (100, 300, 1000 microM) the potentiating effect of L-baclofen on noradrenaline-induced stimulation of adenylate cyclase in rat cortex slices. In electrophysiological studies CGP 35348 (10, 100 microM) antagonized the effect of L-baclofen in the isolated rat spinal cord. In the hippocampal slice preparation CGP 35348 (10, 30, 100 microM) blocked the membrane hyperpolarization induced by D/L-baclofen (10 microM) and the late inhibitory postsynaptic potential. CGP 35348 appeared to be 10-30 times more potent than the GABAB receptor blocker phaclofen. Ionophoretic and behavioural experiments showed that GABAB receptors in the brain were blocked after i.p. administration of CGP 35348. This compound may be of considerable value in elucidating the roles of brain GABAB receptors.

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.

Age-related decline in the activity of noradrenergic neurons of the rat locus coeruleus

Abstract Spontaneous neuronal activity of noradrenergic neurons located in the rat locus coeruleus was compared in 3-, 10- and 22-month-old rats. A statistically significant change in the frequency distribution pattern between the 3-month and the older rats was noticed. The mean firing rate was reduced by 21% and 25% in the 10- and 22-month-old rats, respectively.

Comparison of antagonist potencies at pre- and post-synaptic GABAB receptors at inhibitory synapses in the CA1 region of the rat hippocampus

Synaptic activation of γ‐aminobutyric acid (GABA)B receptors at GABA synapses causes (a) postsynaptic hyperpolarization mediating a slow inhibitory postsynaptic potential/current (IPSP/C) and (b) presynaptic inhibition of GABA release which depresses IPSPs and leads to paired‐pulse widening of excitatory postsynaptic potentials (EPSPs). To address whether these effects are mediated by pharmacologically identical receptors the effects of six GABAB receptor antagonists of widely ranging potencies were tested against each response. Monosynaptic IPSPBs were recorded in the presence of GABAA, AMPA/kainate and NMDA receptor antagonists. All GABAB receptor antagonists tested depressed the IPSPB with an IC50 based rank order of potency of CGP55679CGP56433=CGP55845A=CGP52432>CGP51176> CGP36742. Paired‐pulse EPSP widening was recorded as an index of paired‐pulse depression of GABA‐mediated IPSP/Cs. A similar rank order of potency of antagonism of paired‐pulse widening was observed to that for IPSPB inhibition. Comparison of the IC50 values for IPSPB inhibition and paired‐pulse EPSP widening revealed a close correlation between the two effects in that their IC50s lay within the 95% confidence limits of a correlation line that described IC50 values for inhibition of paired‐pulse EPSP widening that were 7.3 times higher than those for IPSPB inhibition. Using the compounds tested here it is not possible to assign different subtypes of GABAB receptor to pre‐ and post‐synaptic loci at GABAergic synapses. However, 5–10 fold higher concentrations of antagonist are required to block presynaptic as opposed to postsynaptic receptors when these are activated by synaptically released GABA.

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.

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

The synaptic release of γ-aminobutyric acid (GABA) is thought to be regulated by presynaptic GABA receptors of the 13-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 μM) and d-2-amino-5-phosphonopentanoate (AP5 40 μM) 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 GABAresponse is thought to result from the activation of presynaptic GABA receptors. The GABA-uptake inhibitor SKF 89976 (100 μM) increased the amplitude of the IPSPs 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 GABAB 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 μM (maximal effect). The other two compounds, CGP 46381 and CGP 36742 had no significant or only a subtle effect respectively.The adenosine receptor antagonist, caffeine (100 μM) and the metabotropic excitatory amino acid receptor antagonist (1SS, 3 R)-1-aminocyclopentane-t,3-dicarboxylic acid (MCPG, 1 mM) had no effect on paired-pulse depression in the presence of CGP 55845 (10 μM)In conclusion since all CGP compounds are GABA-B antagonists at postsynaptic sites these findings suggest that there may be differences between the pre- and postsynaptic GABA-B receptors. Apart from presynaptic GABA-B receptors there appear to exist additional mechanisms involved in paired-pulse depression that remain to be elucidated.

Locus Coeruleus as a Target for Psychogeriatric Agents

Today the noradrenergic-containing nucleus locus coeruleus is probably one of the best documented neuronal systems in brain. The locus coeruleus is thought to be involved in various functions such as the control of attention,’ central blood pressure,’ anxiety: and processes related to memory and learning.‘ No unifying concept is available to explain the relative contribution and exact role of this small nucleus in these diverse functions. The role of the locus coeruleus in processes related to memory and learning has gained a lot of attention and has been intensively investigated in the past fifteen years but the issue remains controversial. Whereas earlier behavioral studies strongly supported a role of locus coeruleus in these processes,” more recent investigations militate against this hypothesis? However, although behavioral studies remain controversial, recent electrophysiological investigations give a more coherent picture pointing to a basic role of the locus coeruleus in information processing. Single-cell recording studies performed in freely moving mammals have demonstrated that neuronal activity in the locus coeruleus correlates with the level of vigilance.’&’’ Highest levels of neuronal activity are observed during wakefulness and lower firing rates occur when the animals are drowsy and in slow wave and paradoxical sleep. The neurons respond with transient, biphasic changes in discharge to auditory, visual, and somatosensory stim~li.’~~’’ These findings suggest that the locus coeruleus affects target cell systems concerned with processing of external stimuli. In keeping with this notion, it has been shown that noradrenaline differentially affects the activity of auditory cortex neurons in the awake monkey.’’ Iontophoretically applied noradrenaline reduced both spontaneous and evoked (acoustic stimulus) firing but the latter was relatively less affected.’’ In line with these findings, it has been shown in various brain areas that noradrenaline enhances neuronal responsiveness to natural stimulation of afferent pathways’’ or to microiontophoretically applied GABA16 and ACh” in various brain areas. In contrast we have shown that the amine can reduce cortical neuron sensitivity to substance P.’* These electrophysiological findings point to a modulatory role of noradrenaline in central information processing. A large consensus of data indicates that the central noradrenergic system displays a functional decline with advancing age. Both preand postsynaptic markers of noradrenergic activity show such changes. The spontaneous firing rate of locus coeruleus neurons declines” and the sensitivity of target neurons to noradrenaline attenuates in various brain areas with advancin age.’0*’‘ Endogenous noradrenaline levelsz2 and the stimulation of adenylate cyclase by noradrenaline were found to be k

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

SummaryA 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.

Electrophysiological perspectives on locus coeruleus: Its role in cognitive versus vegetative functions

The hypothesis that the locus coeruleus (LC) may be involved in the control of cognitive and/or vegetative processes is reviewed, primarily focusing on electrophysiological experiments. With the impact of this nucleus on target neurons in different brain areas and with the activity pattern of LC neurons in conscious animals, the electrophysiological data are, to a large extent, in keeping with the postulated role of the LC in cognitive processes. A considerable body of evidence from lesion and stimulation experiments points to a possible function of the LC in the control of autonomic processes. In particular, many studies suggest a role in the central control of blood pressure. The electrophysiological data, although not entirely ruling out such a function, are not in keeping with this hypothesis. In conclusion, although there is a case for the LC’s having a role both in the cognitive sphere and in the vegetative sphere, the exact role of the LC in both of them remains to be elucidated.

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

SummaryIt has previously been claimed that the anti-convulsant 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 μM) is augmented by bath applied valproate at concentrations of 50 μM, 100 μM 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.