L-2-chloropropionic acid inhibits glutamate and aspartate release from rat cerebellar slices but does not activate cerebellar NMDA receptors: implications for L-2-chloropropionic acid-induced neurotoxicity.

L-2-Chloropropionic acid (L-CPA), when orally administered at single high dose to rats produces a selective lesion in the cerebellum involving destruction of a high proportion of granule cells by a mechanism which involves N-methyl-D-aspartate (NMDA) receptors. Receptor binding studies demonstrated that L-CPA a had low affinity at the glutamate and glycine binding sites at NMDA receptors (530-660 microM), respectively, whereas L-CPA did not displace [3H]AMPA, [3H]NBQX or [3H]kainate from AMPA or kainate receptors. Whole cell-patch clamp experiments using cultured granule cells failed to demonstrate changes in membrane potential of cultured granule cells when either L-CPA (0.25 or 1 microM) was added alone to the bathing solution, or in combination with glycine (10 microM). Furthermore L-CPA did not alter the magnitude of the inward current produced by application of NMDA (100 microM)) to cultured granule cells, in the presence of glycine, as measured by patch clamp techniques. Experiments were also performed to discover whether L-CPA may alter the release of the excitatory amino acids from the cerebellum, which may then indirectly alter activity at glutamate receptors, leading to neuronal cell death. L-CPA (2 mM) did not affect either basal or stimulated (electrical or high potassium) endogenous aspartate release from superfused cerebellar slices nor did it alter the basal or stimulated release of [3H]aspartate from preloaded slices when introduced into the superfusion medium over 30 min. However, when cerebellar slices were preincubated with 2 mM L-CPA for 2 h at concentrations that are known to be neurotoxic to the brain in vivo, but not in vitro, the stimulated endogenous glutamate and aspartate net release was significantly attenuated, as compared to controls. Basal release was not significantly affected by the introduction of L-CPA-induced cerebellar neurotoxicity may be related to the inhibition of excitatory amino acid release from the cerebellum. In conclusion, although L-CPA does not appear to directly alter NMDA receptor activity the L-CPA-induced cerebellar neurotoxicity may be related to the inhibition of excitatory amino acid release from the cerebellum.

Effects of Zeneca ZD7288 in comparison with alinidine and UL-FS 49 on guinea pig sinoatrial node and ventricular action potentials.

ZENECA ZD7288 (4-(N-ethyl-N-phenyl-amino)-1,2-dimethyl-6-(methylamino) pyrimidium chloride) is a novel compound which we compared with alinidine and UL-FS 49 (zatebradine) in guinea pig sinoatrial node (SAN) and papillary muscle preparations, using conventional microelectrode techniques. At low concentrations (1 x 10(-8)-1 x 10(-6) M), ZD7288 caused slowing of the diastolic depolarisation rate of SAN pacemaker cells, thus prolonging the diastolic interval and slowing the beating rate. Alinidine and UL-FS 49 also had qualitatively similar effects on diastolic depolarisation rate, but ZD7288 caused least prolongation of the action potential duration (APD) of SAN cells at the concentrations that had "selective bradycardic actions." ZD7288 affected the APs of ventricular cells in guinea pig papillary muscle only at relatively high concentrations (3 x 10(-6)M-1 x 10(-4) M), which reduced plateau potential duration, although total APD was less affected. Reduced force of contraction (FOC) was also observed at these high concentrations; significant effects on AP Vmax were noted only at concentrations > or = 3 x 10(-5)M. Alinidine also had negative inotropic effects on papillary muscle, but its effects were noted at concentrations similar to those with bradycardic actions; in contrast, UL-FS 49 had marked positive inotropic actions and also increased ventricular APD within the bradycardic concentration range. These data provide a basis for the selective actions of ZD7288 on heart rate (HR).

Inhibitory actions of ZENECA ZD7288 on whole-cell hyperpolarization activated inward current (If) in guinea-pig dissociated sinoatrial node cells.

1. ZENECA ZD7288 (4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride) is a sinoatrial node (SAN) modulating agent which produces a selective slowing of the heart rate. Its effects have been studied in single, freshly dissociated guinea-pig SAN cells, by standard patch clamp procedures. 2. Whole-cell inward currents were evoked by hyperpolarizing voltage clamp steps from a holding potential of -40 mV. ZD7288 inhibited the hyperpolarization activated cationic current (If) in a concentration-dependent manner. The 'selective bradycardic agents' alinidine and UL-FS 49 (zatebradine) both also inhibited If. 3. The activation of If was investigated by measuring tail current amplitudes at +20 mV after hyperpolarizing steps to different potentials to activate the current. The reduction in If resulted from both a shift in the If current activation curve in the negative direction on the voltage axis, and also a reduction in the activation curve amplitude. 4. ZD7288 did not affect the ion selectivity of the If channel, since the tail current reversal potential was unchanged in the presence of the drug. 5. With ZD7288 the inhibition of If was not use-dependent, whereas UL-FS 49 displayed use-dependence in the block of the If current. 6. Whereas ZD7288 had no significant effect on the delayed rectifier current (Ik) in these cells, both alinidine and UL-FS 49 significantly reduced Ik at the same concentrations which reduced If. 7. The data show that ZD7288 reduces If by affecting the activation characteristics of the If current; this inhibition may account for this agent's selective bradycardiac properties.