Wolfgang Frostl

Developmental changes of agonist affinity at GABABR1 receptor variants in rat brain

Recently, two N-terminal splice variants of the metabotropic receptor for GABA (gamma-amino-butyric acid) were cloned. Here, we describe an antiserum that recognizes the two receptor variants. We demonstrate that these proteins are identical with GABAB receptors that are photoaffinity labeled with [125I]CGP71872 in rat brain. The C-terminal epitopes recognized by the antiserum are conserved in several vertebrate species but not in chicken. No hints for the existence of additional closely related receptor subtypes or variants are found in double-labeling experiments with antibody and photoaffinity ligand. Western blot analysis reveals widespread expression of the GABABR1 receptor proteins in rat brain with the highest level of expression at early postnatal stages. The binding affinity of the GABAB receptor agonist L-baclofen at native R1a and R1b variants is similar. In early postnatal development the affinity at R1a and R1b is 10-fold lower than in adult brain and gradually increases with aging.

Effects of the putative P-type calcium channel blocker, R,R-(−)-daurisoline on neurotransmitter release

The alkaloid and medicinal herb constituent, R,R-(−)-daurisoline, was originally reported to be a N-type Ca2+ channel blocker, but newer evidence indicates that it is a blocker of P-type Ca2+ channels. To clarify its specificity with respect to N- and P-channels, we compared its effects on the electrically induced release of endogenous glutamate, 3H-GABA and 3H-noradrenaline, from brain slices with those of ω-agatoxin IVA and ω-conotoxin GVIA. Like ω-agatoxin IVA (but with about 1000-fold lower potency), and unlike ω-conotoxin GVIA, R,R-(−)-daurisoline inhibited the release of 3H-GABA and glutamate, with IC50 values of 8 and 18 μM. However, inhibition particularly of 3H-GABA release was more complete than by ω-agatoxin IVA, indicating interaction with one or more additional voltage-sensitive Ca2+ channels, possibly the Q-type. Its potency to inhibit glutamate release elicited either electrically, by veratrine or by high concentrations of K+ was similar, in contrast to sodium channel blockkes. The effects of R,R-(−)-daurisoline on the release of 3H-noradrenaline, 3H-dopamine and 3H-acetylcholine were in agreement with previous knowledge from experiments with ω-agatoxin IVA suggesting an involvement of P-channels. A weak inhibition of 3H-noradrenaline release at 10 μM, similar to that by ω-agatoxin IVA at 0.03 μM, was occluded by α2-antagonistic properties and could be unmasked in presence of rauwolscine. At 10 μM, it also inhibited electrically evoked 3H-dopamine and 3H-5-hydroxytryptamine release and caused a marked spontaneous release of all three monoamines in a reserpine-like manner. Spontaneous and evoked release of 3H-acetylcholine was inhibited by about 25% at 10 μM.In radioligand binding studies, R,R-(−)-daurisoline interacted with α1 and α2-adrenoceptors, 5-HT2 and muscarinic cholinergic receptors with IC50 values close to 1 μM, and with μ opiate receptors even with 0.18 μM. Atropine reduced the weak inhibitory effect of R,R-(−)-daurisoline on 3H-acetylcholine release somewhat, suggesting that it was brought about by both P channel blockade and cholinergic agonist activity. The effect on 3H-GABA release was unaffected by naloxone, indicating that the interaction of R,R-(−)-daurisoline with μ opiate receptors is antagonistic.The pattern of effects on neurotransmitter release observed with R,R-(−)-daurisoline resembles that of ω-agatoxin IVA and supports previous electrophysiological data suggesting that the compound blocks P-type voltage-sensitive Ca2+ channels. However, the more complete blockade of amino acid release by R,R-(−)-daurisoline suggests interaction with additional Ca+ channel subtypes. Although it does also possess other pharmacological properties, we think that the compound is suitable to test whether blockade of glutamate release via voltage-sensitive Ca2+ channels is a viable concept to obtain novel neuroprotective and/or anticonvulsant compounds.