Johannes C. Stoof

Stimulation of D2-dopamine receptors in rat neostriatum inhibits the release of acetylcholine and dopamine but does not affect the release of γ-aminobutyric acid, glutamate or serotonin

D1-dopamine receptor stimulation did not affect the K+-induced release of [3H]GABA, [3H]glutamate, [3H]serotonin, [3H]dopamine and [14C]acetylcholine from slices of rat neostriatum. D2-dopamine receptor stimulation did not change the release of [3H]GABA, [3H]glutamate and [3H]serotonin, but inhibited the release of both [3H]dopamine and [14C]acetylcholine; this inhibition was antagonized by (-)-sulpiride. The release-inhibiting dopamine autoreceptors and the post synaptic dopamine receptors mediating the inhibition of acetylcholine release appear to be pharmacologically similar and can be classified as D2-receptors.

9-Amino-1,2,3,4-tetrahydroacridine (THA), an alleged drug for the treatment of Alzheimer's disease, inhibits acetylcholinesterase activity and slow outward K+ current

The in vitro release of acetylcholine in rat brain tissue was inhibited by 9-amino-1,2,3,4-tetrahydroacridine (THA). Atropine antagonized this effect of THA. As THA does not display an affinity for muscarinic receptors, we conclude that THA inhibits acetylcholinesterase activity. In electrophysiological studies with neurons of Lymnaea stagnalis, THA inhibited the slow outward K+ current and consequently increased the duration of the action potentials. It is discussed that both effects of THA possibly contribute to its reported effect in the treatment of patients with Alzheimers disease.

ASTROCYTE-ENHANCED NEURONAL SURVIVAL IS MEDIATED BY SCAVENGING OF EXTRACELLULAR REACTIVE OXYGEN SPECIES

The survival of cultured neurons is promoted by the presence of antioxidants or astrocytes. This indicates that extracellular reactive oxygen species (ROS) impair neuronal survival and suggests that astrocytes exert their survival-enhancing effect through inactivation of these toxicants. However, to our knowledge, data supporting this hypothesis are lacking. Previously, we showed that loss of the antioxidant glutathione abolishes the neuronal survival-stimulating action of astrocytes in cocultures, consisting of rat striatal astrocytes and mesencephalic, dopaminergic neurons. Using uptake of [3H]dopamine as marker of neuronal survival, we presently investigated whether this effect of glutathione depletion is mediated by extracellular ROS. For this purpose, we incubated glutathione-depleted cocultures with superoxide dismutase, catalase or both. Whereas superoxide dismutase had no effect and catalase only partially protected, addition of the enzymes together completely prevented the impairment of neuronal survival caused by glutathione loss. No change in neuronal survival occurred upon exposure of control cocultures to superoxide dismutase and/or catalase. These data strongly implicate scavenging of extracellular ROS in astrocyte-stimulated neuronal survival and moreover suggest a crucial role for glutathione in this process.

The anti-parkinsonian drug amantadine inhibits the N-methyl-D-aspartic acid-evoked release of acetylcholine from rat neostriatum in a non-competitive way

The anti-viral drug amantadine is used in the treatment of Parkinsons disease without the drug having a well established mechanism of action. Amantadine is reported to displace the non-competitive NMDA receptor antagonist MK-801 from its binding site in the central nervous system. We show that amantadine inhibits, in a non-competitive way, the NMDA receptor-mediated stimulation of acetylcholine release from rat neostriatum in vitro in therapeutic (i.e. low micromolar range) concentrations. Moreover, contrary to previous reports, amantadine, in this concentration range, did not affect the in vitro release of dopamine from neostriatal tissue.

GABA modulates the release of dopamine and acetylcholine from rat caudate nucleus slices.

The effects of GABA on depolarization-induced (26 mM K+) release of radiolabeled dopamine (DA) and acetylcholine (ACh) from slices of rat caudate nucleus were examined with a superfusion method. GABA, in concentrations of 10(-5)--10(-3) M, dose-dependently enhanced the release of DA, either accumulated by high-affinity uptake or synthesized from 14-C-tyrosine. In contrast, the release of ACh was reduced by GABA. This reduction appeared to be caused by the increase in DA-release. These effects of GABA decreased from the caudal to rostral part within the caudate nucleus, an order which parallels the distribution of endogenous GABA and glutamic acid decarboxylase. However, GABA had little, if any, effect in the nucleus accumbens. Since it was difficult to antagonize the effects of GABA on DA and ACh release with bicuculline or picrotoxin, it remains uncertain whether these effects were mediated via GABA receptors. In view of the high endogenous GABA level in the caudate nucleus it is concluded that GABA may be one of the local factors involved in the control of the amount of transmitter that will be released from dopaminergic varicosities upon depolarization.

Stimulation of D2-receptors in rat nucleus accumbens slices inhibits dopamine and acetylcholine release but not cyclic AMP formation

We compared some functional responses of D1- and D2-receptor stimulation in tissue slices of rat neostriatum with those in slices of the nucleus accumbens. In both brain regions D2-receptor stimulation inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine. In both brain regions D1-receptor stimulation and forskolin increased the cyclic AMP formation. Only in the neostriatum, stimulation of D2-receptors inhibited the formation of cyclic AMP, brought about by forskolin or by D1-receptor stimulation. It is concluded from these experiments that, although functional responses of D2-receptor stimulation can be demonstrated in the nucleus accumbens, D2-receptors in this brain region are apparently uncoupled to adenylate cyclase.

The quinolinic acid hypothesis in Huntington's chorea

In the central nervous system and particularly in the striatum of patients with Huntingtons disease (HD) a dramatic cell loss can be observed. Animal models of HD are based on intrastriatal injection of excitatory amino acids (EAAs). Stimulation of EAA receptors for a prolonged period of time degenerates the cells on which the EAA receptors are located, a phenomenon known as excitotoxicity. Several categories of EAA receptors, viz. quisqualate, kainate and N-methyl-D-aspartate (NMDA), have been identified in the central nervous system. Interestingly, quinolinic acid, a metabolite of tryptophan along the kynurenine pathway, appeared to be an agonist on the NMDA receptor and a potent excitotoxin. Indications have been reported, although still controversial, for derangements in the formation of quinolinic acid to occur in the brains of patients with HD. Based on these studies the likeliness of a role for quinolinic acid in the etiology of HD is evaluated.

Stimulation of D-2 dopamine receptors decreases the evoked in vitro release of [3H]acetylcholine from rat neostriatum: role of K+ and Ca2+

Abstract: Reportedly, stimulation of D‐2 dopamine receptors inhibits the depolarization‐induced release of acetylcholine from the neostriatum in a cyclic AMP‐independent manner. In the present study, we investigated the role of K+ and Ca2+ in the D‐2 receptor‐mediated inhibition of evoked [3H]acetylcholine release from rat striatal tissue slices. It is shown that the D‐2 receptor‐mediated decrease of K+‐evoked [3H]acetylcholine release is not influenced by the extracellular Ca2+ concentration. However, increasing extracellular K+, in the presence and absence of Ca2+, markedly attenuates the effect of D‐2 stimulation on the K+‐evoked [3H]acetylcholine release. Furthermore, it is shown that activation of D‐2 receptors in the absence of Ca2+ also inhibits the veratrine‐evoked release of [3H]acetylcholine from rat striatum. These results suggest that the D‐2 dopamine receptor mediates the decrease of depolarization‐induced [3H]acetylcholine release from rat striatum primarily by stimulation of K+ efflux (opening of K+ channels) and inhibition of intracellular Ca2+ mobilization.

The effect of intrastriatal application of directly and indirectly acting dopamine agonists and antagonists on the in vivo release of acetylcholine measured by brain microdialysis

SummaryThe effect of intrastriatal application of D-1, D-2 and indirect dopaminergic drugs on the release of striatal acetylcholine as a function of the post-implantation intervals was studied using in vivo microdialysis. The dopamine, D-2 agonists LY 171555 and (−)N0437 inhibited the release of striatal acetylcholine to 40% of control values 16–24 h after implantation of the dialysis cannula. When LY 171555 was infused 40–48 h after implantation of the dialysis cannula, the response was attenuated to 20% of control values. Meanwhile, the effectiveness of infusions of the antagonists (−)sulpiride and haloperidol was augmented from a non significant effect at 16–24 h to a 150% increase 40–48 h after implantation of the cannula.Infusions of the dopamine releasing agent amphetamine or the dopamine uptake inhibitor nomifensine resulted in a dose-dependent increase in the overflow of dopamine. Not until a sevenfold increase in the level of dopamine was seen, the release of acetylcholine was significantly affected. This hyporesponsiveness of the striatal cholinergic interneurons to endogenous dopamine could not be attributed to dopamine D-1 receptor activation, since no effects on striatal acetylcholine release were found by intrastriatal infusions of the selective D-1 agonist CY 208-243 or the selective D-1 antagonist SCH 23390.The results indicate that dopamine D-2 receptors are involved in the regulation of striatal acetylcholine release and that these receptors are tonically occupied by endogenous dopamine under the present experimental conditions 40–48 h after probe implantation. The fact that cholinergic responses to intrastriatally applied dopaminergic agents are dynamic with respect to the time between implantation surgery of the dialysis tube and the experimental measurements suggests that the striatal neuronal system is perturbated by the implantation of a dialysis probe for a considerable length of time.

Dopamine inhibits the release of immunoreactive β-endorphin from rat hypothalamus in vitro

Mediobasal hypothalamus tissue (MBH) from adult male rats was incubated in Krebs-Ringer bicarbonate medium (KRB). KRB was changed at 15 min intervals and the concentration of immunoreactive beta-endorphin (beta-ENDi) in the medium was measured by radioimmunoassay. Incubation of MBH tissue in normal KRB resulted in a constant release rate of beta-ENDi of approximately 1% of the tissue content per h. KRB containing 45 mM K+ causes a two fold increase in the release rate of beta-ENDi which was Ca2+ dependent. Dopamine (0.01-1.0 microM) inhibits both the spontaneous and the K+-stimulated release of beta-ENDi in a dose related manner. The dopamine receptor blocking agent haloperidol prevents this inhibitory effect of dopamine. The selective D-1 receptor agonist SKF 38393 does not affect the release rate of beta-ENDi; whereas the selective D-2 receptor agonist LY 141865 inhibits both the spontaneous and K+-stimulated release of beta-ENDi. The effects of LY 141865 can be blocked by (-)-sulpiride, a selective D-2 receptor antagonist. Norepinephrine only weakly inhibits the K+-stimulated release of beta-ENDi, an effect that can be blocked by haloperidol but not by the alpha-adrenoceptor blocker phentolamine. At concentrations tested (0.01-1.0 microM), isoproterenol, 5-hydroxytryptamine, carbachol and 8-Br-cAMP (1.0 microM) do not affect beta-ENDi release. It is concluded that dopamine can inhibit the release of beta-ENDi from hypothalamic neurons via a D-2 receptor mechanism.