Different functional effect of Ro 15-4513 and Ro 19-4603 on synaptic responses of Purkinje cells in the rat cerebellar slice

GABAA Receptors of Cerebellar Granule Cells in Culture: Interaction with Benzodiazepines

GABAA receptor mediated inhibition plays an important role in modulating the input/output dynamics of cerebellum. A characteristic of cerebellar GABAA receptors is the presence in cerebellar granule cells of subunits such as α6 and δ which give insensitivity to classical benzodiazepines. In fact, cerebellar GABAA receptors have generally been considered a poor model for testing drugs which potentially are active at the benzodiazepine site. In this overview we show how rat cerebellar granule cells in culture may be a useful model for studying new benzodiazepine site agonists. This is based on the pharmacological separation of diazepam-sensitive α1 β2/3 γ2 receptors from those which are diazepam-insensitive and contain the α6 subunit. This is achieved by utilizing furosemide/Zn2+ which block α6 containing and incomplete receptors.

Dual action of the benzodiazepine receptor inverse agonist RU34347 on responses to exogenously applied GABA in the rat cerebellar slice

The benzodiazepine receptor inverse agonist has been shown to produce agonist-like effects at low concentrations. RU34347 has both inverse agonist (attenuation of GABA-responses) and agonist-like (reduction of spontaneous Purkinje cell firing rate) in the cerebellar slice preparation. The benzodiazepine antagonist flumazenil prevented the inverse agonist actions, but only partially reduced the agonist-like effects. Further, brief application of RU34347 to slices mimicked the response to GABA, and pharmacological investigation determined that this action was mediated through increased GABA through action at a site proximal to the parallel fiber-basket cell synapse, at an as yet undetermined receptor.

Biphasic actions of the benzodiazepine receptor inverse agonist RU34347 in the rat cerebellar slice

We have characterised the biphasic response of the benzodiazepine receptor inverse agonist RU34347 in the rat cerebellar slice preparation using extracellular electrophysiological recordings from Purkinje cells. RU34347 (10 fM-10 nM) produced an increase in GABA(A)-mediated inhibition at between 10 fM and 10 nM, a response normally associated with benzodiazepine agonists. This response was biphasic, being dose dependent between 10 fM and the peak effect at 10 pM, and inversely related to concentration between 100 pM and 10 nM. Associated with this increase in inhibition was a decrease in firing rate, also showing a biphasic concentration-response relationship. The agonist-like response was composed of two elements, an initial increase occurring after 5 min followed by a slow decline over the next 20 min. This second, declining, phase was more evident at higher concentrations of RU34347. The peak effect seen at 10 pM was fully antagonised by 1 microM flumazenil, a benzodiazepine receptor antagonist. This concentration of flumazenil also antagonised the decrease in firing rate induced by 10 pM RU34347. The response to 10 nM RU34347 was further characterised since the largest second phase decline was demonstrated at this concentration. 10 nM flumazenil fully antagonised the second, declining, phase of the response, but not the magnitude of the initial increase. A partial antagonism of the peak effect was seen with 1 microM flumazenil, and a full antagonism at 10 microM flumazenil. This effect was similar to that observed with Ro15-4513 in a previous study. Therefore, we investigated the binding of RU34347 to diazepam-insensitive benzodiazepine receptors. [3H]Ro15-4513 was displaced by Ro15-4513, flumazenil and Ro19-4603, but not by RU34347. Therefore, although the electrophysiological data correlate well with that previously reported for Ro15-4513, RU34347 does not displace Ro15-4513 binding at the diazepam-insensitive benzodiazepine receptor. Therefore, either both ligands bind to complimentary sites on the same receptor complex, or both induce a similar physiological response through an action on different receptors.

Reduction of cerebellar GABAA responses by interleukin-1 (IL-1) through an indomethacin insensitive mechanism

Recently, a role of IL-1 in the central nervous system has been described, principally a fever-inducing effect in the hypothalamus through a prostaglandin second messenger system. IL-1 has also been shown to potentiate gamma-aminobutyric acid (GABAA) responses in embryonic chick neurones. This study describes the investigation of the effect of IL-1 on GABAA responses within the in vitro rat cerebellar slice, a preparation containing intact neuronal circuitry. Stimulation of the area of passage of paralleled fibres produced a pure GABAA inhibition of the spontaneous firing of Purkinje cells. 5 and 10 ng/ml IL-1 produced a reduction in the duration of inhibition 10 min after beginning perfusion of IL-1. This effect reversed within 15 min of washing out the IL-1. 10 ng/ml IL-1 also reduced the effects of exogenously-applied GABA (0.1 mM) with the same time course. In the presence of 1 uM indomethacin, there was no change in the effect of the IL-1. It can therefore be concluded that the reduction in cerebellar GABAA responses by IL-1 is not mediated by the indomethacin-sensitive prostaglandin second messenger system.