Behavior and receptor changes after kainate lesioning of nodular cerebellum

Metabotropic actions of kainate receptors modulating glutamate release

Presynaptic kainate (KA) receptors (KARs) modulate GABA and glutamate release in the central nervous system of mammals. While some of the actions of KARs are ionotropic, metabotropic actions for these receptors have also been seen to modulate both GABA and glutamate release. In general, presynaptic KARs modulate glutamate release through their metabotropic actions in a biphasic manner, with low KA concentrations producing an increase in glutamate release and higher concentrations of KA driving weaker release of this neurotransmitter. Different molecular mechanisms are involved in this modulation of glutamate release, with a G-protein independent, Ca²⁺-calmodulin adenylate cyclase (AC) and protein kinase A (PKA) dependent mechanism facilitating glutamate release, and a G-protein, AC and PKA dependent mechanism mediating the decrease in neurotransmitter release. Here, we describe the events underlying the KAR modulation of glutamatergic transmission in different brain regions, addressing the possible functions of this modulation and proposing future research lines in this field.

Kainate Receptor-Mediated Depression of Glutamate Release Involves Protein Kinase A in the Cerebellum

Kainate (KA) receptors (KAR) have important modulatory roles of synaptic transmission. In the cerebellum, the action mechanisms of KAR-mediated glutamatergic depression are unknown. We studied these mechanisms by recording evoked excitatory postsynaptic currents (eEPSCs) from cerebellar slices using the whole-cell configuration of the patch-clamp technique. We observed that 3 μM KA decreased the amplitude of eEPSCs and increased the number of failures at the synapses established between parallel fibers (PF) and Purkinje neurons, and the effect was antagonized by NBQX under the condition where AMPA receptors were previously blocked. The inhibition of protein kinase A (PKA) suppressed the effect of KAR activation on eEPSC, and effect was not prevented by protein kinase C inhibitors. Furthermore, in the presence of Pertussis toxin, the depression of glutamate release mediated by KAR activation was prevented, invoking the participation of a G_i/o protein in this modulation. Finally, the KAR-mediated depression of glutamate release was not prevented by blocking calcium-permeable KARs or by treatments that affect calcium release from intracellular stores. We conclude that KARs present at these synapses mediate an inhibition of glutamate release through a mechanism that involves the activation of G-protein and protein kinase A.

Postnatal expression of V2 vasopressin receptor splice variants in the rat cerebellum

The V(2) vasopressin receptor gene contains an alternative splice site in exon-3, which leads to the generation of two splice variants (V(2a) and V(2b)) first identified in the kidney. The open reading frame of the alternatively spliced V(2b) transcript encodes a truncated receptor, showing the same amino acid sequence as the canonical V(2a) receptor up to the sixth transmembrane segment, but displaying a distinct sequence to the corresponding seventh transmembrane segment and C-terminal domain relative to the V(2a) receptor. Here, we demonstrate the postnatal expression of V(2a) and V(2b) variants in the rat cerebellum. Most importantly, we showed by in situ hybridization and immunocytochemistry that both V(2) splice variants were preferentially expressed in Purkinje cells, from early to late postnatal development. In addition, both variants were transiently expressed in the neuroblastic external granule cells and Bergmann fibers. These results indicate that the cellular distributions of both splice variants are developmentally regulated, and suggest that the transient expression of the V(2) receptor is involved in the mechanisms of cerebellar cytodifferentiation by AVP. Finally, transfected CHO-K1 expressing similar amounts of both V(2) splice variants, as that found in the cerebellum, showed a significant reduction in the surface expression of V(2a) receptors, suggesting that the differential expression of the V(2) splice variants regulates the vasopressin signaling in the cerebellum.

Immunosuppressive effect of vestibulo-cerebellar lesion in rats

Kainate lesion of the vestibulo cerebellum induces sympathetic hyperactivity, but the mechanism of immunosuppression observed as a result is not yet clarified. Here we report that vestibulo cerebellum lesioned (VCL) rats have depressed secretion of haematopoietic cytokines (bioimmunomodulator or BIM, a 12.7 kD peptide and thymosin FrV) in tissue cultures of bone marrow and thymus, respectively, compared with controls (P < 0.01). Peripheral blood leukocyte concentration, neutrophil myeloperoxydase response, T-SRBC rosette and antibody titre to sheep red blood cells (SRBC) are also significantly less, compared with control (P < 0.01). Injection of BIM (concentration 0.01 microg/g body weight) in VCL rats corrected the immunodeficiency. Partial restoration of immune competence is observed after injection of thymosin FrV (0.01 microg/g body weight) or after prolonged vestibular stimulation (18 rpm for 15 min/day for 21 days). The results indicate that the vestibular nodule (VN) through autonomic nerves (AN) can modulate the immune function of rats by regulating the secretion of cytokines from bone marrow and thymus.

Modulation of apomorphine-induced rotations in unilaterally 6-hydroxydopamine lesioned rats by cholinergic agonists and antagonists

In the present study, we examined the effects of the agonists and antagonists of cholinergic receptors on central dopaminergic function using the 6-hydroxydopamine model of dopamine receptor supersensitivity. Unilateral lesioning of the substantia nigra with 6-hydroxydopamine was carried out in Wistar rats. Two weeks after surgery, the rats were tested for the presence of dopaminergic supersensitivity by their response to the dopamine receptor agonist, apomorphine. Apomorphine-induced rotations were significantly reinforced by the muscarinic receptor antagonist, atropine. In contrast to atropine, the muscarinic receptor agonist oxotremorine attenuated apomorphine's effects. Acute treatment of nicotine significantly reduced apomorphine-induced rotations. However, when increasing doses of nicotine were given for nine days, the rotations of the nicotine-dependent rats were significantly enhanced. So the fact that both muscarinic and nicotinic cholinergic activity could modulate apomorphine-induced rotations was readily apparent in these experiments.

Altered excitatory amino acid function and morphology of the cerebellum of the spastic Han-Wistar rat

A mutant strain of Han-Wistar rat carries an autosomal recessive gene producing spastic paresis which is characterized by ataxia, tremor and hind limb rigidity. Brains of affected rats and unaffected littermate controls were transected at the mesencephalon into rostral and caudal portions (the caudal portion contained the cerebellum and brainstem). Poly(A)+ mRNA was isolated from pooled rostral or caudal portions and injected into Xenopus oocytes. The oocytes were voltage-clamped and exposed to 1 mM L-glutamate, 500 microM kainate, 500 microM quisqualate, 200 microM N-methyl-D-aspartate (NMDA) or 1 mM gamma-aminobutyric acid (GABA). Oocytes injected with mRNA isolated from the caudal portions of the affected rat brains exhibited statistically significant increases in glutamate and kainate peak current responses compared to oocytes injected with mRNA from other brain samples. No differences were noted in the responses of the groups when exposed to quisqualate, NMDA or GABA. Cerebellar and brain stem mRNA were also isolated separately in different groups of mutants and unaffected littermates. Only oocytes injected with cerebellar mRNA from mutants displayed statistically significant increases in responses to glutamate and kainate. In parallel morphological studies changes in the cerebellum of mutants were also observed. These consisted of a loss of Purkinje cells and an asymmetrical disarrangement of the granule cell layer of cerebellar cortex. Taken together, the physiological and morphological results suggest that alterations in glutamate/kainate receptors in the cerebellum are phenotypic manifestations of the Han-Wistar mutation. The results are consistent with the hypothesis that this mutant rat might serve as a model of glutamate/kainate excitotoxicity in the brain.

Quantitative autoradiography of nicotinic receptors in large cryosections of human brain hemispheres

In vitro quantitative autoradiography was used to visualize nicotinic receptors in large cryosections of human brain hemispheres. Sections 80 microns thick of human brain hemispheres were incubated with (-)-[3H]nicotine and the distribution of nicotinic receptors in human brain was studied. Increasing numbers of nicotinic binding sites were observed in the: hippocampus less than cortex less than cerebellum less than substantia nigra less than putamen less than periaqueductal gray. The cartography of nicotinic receptors in the normal human brain will hopefully be of use in the study of the alteration of these receptors in diseased brain.

Receptor binding characteristics of a 3H-labeled azetidine analogue of nicotine

A new radioligand, (+/-)-[3H]-1-methyl-2-(3-pyridyl)-azetidine, which is an analogue of nicotine, has been used to investigate the binding characteristics of the nicotine receptor in rat brain membranes. By Scatchard analysis, the azetidine analogue yielded a curvilinear plot with Kd values of 7 X 10(-11) and 1.7 X 10(-9) M and Bmax values of 0.3 X 10(-14) and 2.5 X 10(-14) mol/mg protein respectively. Thermodynamic analyses yielded negative free enthalpy values for both sites, a decrease in the Bmax of only the lower affinity site, and no effect on either Kd. The psychotropic potency (prostration in rats following intraventricular injection) of the azetidine analogue was about 5-fold greater than (-)-nicotine, being among the greatest of any known nicotine analogues tested to date. Since only the higher affinity Kd differed from that of (-)-nicotine, 3-fold greater, the psychotropic potency appears to be correlated with the higher affinity site. Insofar as [3H]methylcarbamylcholine, a nicotinic ligand resembling acetylcholine, exhibits a linear Scatchard with a Kd of 1 X 10(-9) M, the higher affinity site appears to be characteristic of nicotine analogues.

Barrel rotation and prostration by vasopressin and nicotine in the vestibular cerebellum

The aim of this study was to determine whether the primary sites for the action of vasopressin and nicotine in producing barrel rotation and prostration in rats were located in the modular cerebellum, i.e., lobule X. When arginine vasopressin was administered into either the fourth ventricles or directly into the nodular cerebellum via chronically implanted cannulae, the rats displayed intermittent barrel rotation and clonic convulsions. The administration of nicotine into the same areas resulted in prostration, atonia and, occasionally, clonic convulsions. A few days after the nodular cerebellum was lesioned with kainic acid, the motor disturbances resulting from either agent were virtually abolished. Histologic studies revealed that kainic acid had destroyed Purkinje and other large neurons, but had left the granular neurons relatively intact. The administration of procaine into either the fourth ventricles or nodular cerebellum blocked the behavioral responses of either vasopressin or nicotine given into the fourth ventricles. It was concluded that the nodular cerebellum is a primary site for the motor disturbances produced by vasopressin and nicotine.