Abstract
1. The acute toxic effects of kainic acid (KA) (16mg/kg; intraperitoneal) upon rat brain dopamine D2 and μ-opioid receptors were investigated by homogenate receptor binding and receptor autoradiography 4 hrs after treatment. Distribution studies were carried out using [3H]-KA administered intraperitoneally with 16mg/kg KA to determine the percentage of KA reaching brain areas. KA was also studied for its effect on [3H]-dopamine release from striatal slices in vitro. 2. Homogenate receptor binding studies carried out with the D2 antagonist [125I]-iodosulpiiide indicated the presence of a single saturable binding site in all regions from saline-treated rats. In saline-treated rats there was good quantitative agreement between levels of [125I]-iodosulpiride binding determined using homogenate receptor binding and receptor autoradiography. KA produced a 2-fold decrease in receptor affinity for [125I]-iodosulpiride and a 2-fold increase in receptor density in cerebellum, cortex, hippocampus, medulla and pons and midbrain and hypothalamus but not striatum. Quantitative autoradiography showed similar increased D2 labelling in these regions following KA treatment whilst caudate putamen, nucleus accumbens and olfactory tubercle displayed equivalent numbers of sites in saline and KA-treated rats. 3. Homogenate receptor binding studies carried out with the D2 antagonists [3H]-raclopride and [3H]-spiperone indicated the presence of single saturable binding sites in all regions from saline-treated rats. [3H]-raclopride binding in striatum was twice [3H]-spiperone binding in saline-treated rats but in the rest of the brain Bmax values for dopamine D2 binding where approximately equivalent. KA produced a significant increase in receptor number for [3H]-raclopride in. striatum and whole brain minus striatum but no change in [3H]-spiperone binding. Furthermore, KA produced a 2-fold increase in receptor affinity for [3H]-raclopride in striatum and a small but significant increase in the whole brain minus striatum but had no effect on KD for [3H]-spiperone. In brains of saline-treated rats [3H]-raclopride and [125I]-iodosulpiride may bind to the long form of the D2 receptor and [3H]-spiperone may bind with higher affinity to the short form. It is postulated that KA effects on D2 receptors may be a reflection of an increase in gene transcription of the D2 receptor gene or a change in its mRNA stability. KA may also alter the affinity state or ligand binding site of D2 long form transcripts which differentially affects the binding affinity of radioligands because of the tricyclic structure and bulky phenyl ring substituents of [125I]-iodosulpiride. 4. Homogenate receptor binding studies carried out with the selective μ-agonist [3H]-DAGO indicated the presence of a single saturable binding site in all regions from saline-treated rats. In saline-treated rats there was quantitative agreement between levels [3H]-DAGO binding determined using homogenate receptor binding and receptor autoradiography. KA produced a 2-fold increase in receptor affinity for [3H]-DAGO in all regions except cerebellum and an increase in receptor number in cerebellum, hippocampus, midbrain and hypothalamus which was significant in cortex, medulla and pons and striatum. Quantitative autoradiography showed similar increased μ-labelling of structures comprising these regions which was significant throughout the cortex except for the temporal region. The effects of KA on [3H]-DAGO binding may reflect a compensatory increase in the rate of μ-receptor mRNA transcription as a result of μ-receptor stimulation by enkephalin produced from elevated levels of its precursor proenkephalin. In addition, the effects of KA on binding affinity of [3H]-DAGO may be a reflection of increased expression of Go G-protein mRNA and/or increased functionality of Gi/o G-proteins postischaemia. 5. Distribution studies showed that after 4 hrs over 60% of intraperitoneally administered KA enters the brain. The high percentage of KA in rat brain in this study may be attributable to uptake across the blood-brain barrier by the glutamate carrier and disruption of the blood-brain barrier caused by KA-induced seizures. Highest labelling of [3H]-KA was observed in the hippocampus and striatum and this correlates with regions where the densest population of KA receptors are found. 6. KA (10 μM-10mM) was studied for its effect on [3H]-dopamine release from rat brain striatal slices in vitro. KA produced a dose-dependent increase in the release of [3H]-dopamine from striatal slices in vitro which was calcium-dependent at low KA concentrations (10μM and 50μM) and calcium-independent at concentrations above 100μM. It is suggested that a dual action is responsible for striatal KA-induced release of [3H]-dopamine in vitro, a KA receptor-mediated effect involved in calcium-dependent dopamine release and a calcium-independent action representing sodium-dependent dopamine release from cytoplasmic stores. KA (50μM-10mM) was studied for its effect on potassium-stimulated [3H]-dopamine release from rat brain striatal slices in vitro. KA produced a dose-dependent inhibition of potassium-stimulated [3H]-dopamine release from rat striatal slices in vitro, possibly due to inactivation of voltage channels on dopaminergic terminals. The effects of intermediate doses of KA on basal and evoked [3H]-dopamine release from striatal slices in vitro were both blocked by the α-amino-3-hydroxy-5-methyl-isoxazole/KA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (500μM) suggesting mediation of dopamine release by KA and α-amino-3-hydroxy-5-methyl-isoxazole receptors.