Effect of calcium ions on the sensitivities of cultured cerebellar neurons to glutamate and aspartate

Excitatory actions of serotonin on GABAergic neurons of the medial septum and diagonal band of Broca

The physiological and pharmacological actions of serotonin (5-HT) on neurons in the medial septum and diagonal band of Broca (MSDB) were examined using extracellular and intracellular recording techniques in an in vitro rat brain-slice preparation. In addition to previously described inhibitory effects, novel excitatory actions of 5-HT on GABA-type cells were observed. In intracellular recordings with KCl-containing electrodes, bath-applied 5-HT induced a bicuculline and tetrodotoxin-sensitive increase in the number of reverse IPSPs in both cholinergic- and noncholinergic-type neurons (presumably GABAergic). In brain slices where all structures neighboring the MSDB, including the lateral septum, had been excised, a similar increase in 5-HT-induced IPSPs occurred, indicating that 5-HT-induced IPSPs in both cholinergic- and noncholinergic-type neurons originate from GABAergic neurons within the MSDB itself. Accordingly, GABA-type neurons in the MSDB were found to be directly excited by 5-HT. MDL 100,907, a selective 5-HT2A antagonist, blocked 5-HT-induced excitations in a majority of neurons (58%). ICS 205-930, a 5-HT3/5-HT4 antagonist, or mianserin, a nonselective 5-HT antagonist, blocked most MDL-resistant responses, indicating a role for multiple 5-HT receptor subtypes. This study also provides the first electrophysiological evidence for synaptic interactions between 5-HT-activated GABAergic neurons and cholinergic neurons and amongst GABAergic neurons in the MSDB. The implications of the findings vis-à-vis intraseptal circuitry and septohippocampal circuitry are discussed.

Subtypes of adrenergic receptors and intracellular mechanisms involved in modulatory effects of noradrenaline on glutamate

We have previously reported that the response of cultured chick cerebellar neurons to glutamate is enhanced by noradrenaline (NA) or isoproterenol and suppressed by clonidine. The present study was carried out to further specify the adrenergic receptor subtypes involved in the facilitatory effect of NA or isoproterenol and the suppressive effect of clonidine, and to examine the intracellular mechanisms underlying these modulatory effects of NA. The clonidine effect, which was mimicked by NA iontophoresed with large ejecting currents, was blocked by yohimbine and tolazoline (alpha 2 antagonists) and also by dibutyryl cyclic AMP or forskolin which augmented the glutamate response by itself. Prazosin, an alpha 1 receptor antagonist did not block the clonidine effect. NA- or isoproterenol-induced facilitation, which was mimicked by denopamine (beta 1 agonist), was antagonized by acebutolol (beta 1 antagonist) and not by ICI 118,551 (beta 2 antagonist). Pretreatment of neurons with pertussis toxin for more than 24 h blocked the suppressive action of clonidine without affecting the facilitatory action of isoproterenol. Furthermore, intracellular injection of GDP beta S inhibited the modulatory effects of either clonidine or isoproterenol. These results indicate that the facilitatory and inhibitory modulatory effects of NA may be mediated by beta 1 and alpha 2 receptors linked to cAMP systems, respectively, and the former is coupled with the stimulatory G protein (Gs) and the latter is with the inhibitory G protein (Gi).

Organization of the septal region in the rat brain: cholinergic-GABAergic interconnections and the termination of hippocampo-septal fibers

This study deals with two characteristic cell types in the rat septal complex i.e., cholinergic and GABAergic neurons, and their synaptic connections. Cholinergic elements were labeled with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Antiserum against glutamate decarboxylase (GAD), the GABA synthesizing enzyme, was employed to identify GABAergic perikarya and terminals, by using either the peroxidase-antiperoxidase (PAP) technique or a biotinylated second antiserum and avidinated gold or ferritin. With these contrasting immunolabels we have studied the cholinergic-GABAergic interconnections in double-labeled sections of intact septal regions and the GABAergic innervation of medial septal area cholinergic neurons in sections taken from animals 1 week following lateral septal area lesion. In other electron microscopic experiments we have studied cholinergic and GABAergic neurons in the septal complex for synaptic contacts with hippocamposeptal fibers, which were identified by anterograde degeneration following fimbria-fornix transection. Our results are summarized as follows: (1) GAD-positive terminals form synaptic contacts on ChAT-immunoreactive dendrites in the medial septum/diagonal band complex (MSDB), (2) surgical lesion of the lateral septal area resulted in a dramatic decrease of the number of GABAergic boutons on MSDB cholinergic neurons, (3) cholinergic terminals establish synaptic contacts with GAD immunoreactive cell bodies and proximal dendrites in the MSDB as well as in the lateral septum (LS), (4) degenerated terminals of hippocampo-septal fibers were mainly observed in the LS, where they formed asymmetric synaptic contacts on dendrites of GABAergic neurons and on nonimmunoreactive spines. We did not observe degenerated boutons in contact with ChAT-positive dendrites or cell bodies in the MSDB. From these results and from data in the literature we conclude that excitatory hippocampo-septal fibers activate GABAergic cells, and as yet unidentified spiny neurons in the LS, which may control the discharge of medial septal cholinergic neurons known to project back to the hippocampal formation.

Participation of cyclic adenosine monophosphate and beta-adrenergic receptors in the facilitatory effect of noradrenaline on the response of cultured cerebellar neurons to glutamate

For the purpose of examining possible involvement of the cyclic adenosine monophosphate (cAMP) system and adrenergic receptors in the modulatory effect of noradrenaline (NA) on the glutamate-induced depolarizing response, the effects of dibutyryl cAMP (DBcAMP), forskolin, theophylline, clonidine, isoproterenol and propranolol were intracellularly investigated in the cerebellar neurons cultured from chick embryos. Not only NA-induced hyperpolarization and increase in input resistance but also the facilitatory effect of NA on the glutamate response were mimicked by DBcAMP and isoproterenol. This facilitatory effect of DBcAMP was enhanced by theophylline or forskolin, while that of isoproterenol was antagonized by propranolol. Clonidine suppressed glutamate-induced depolarization. These results that the enhancing action of NA on the responsiveness of cultured cerebellar neurons to excitatory amino acids is mediated by beta-adrenergic receptors and the intracellular cAMP system.

Characterization of excitatory amino acid receptors in cultured chick cerebellar neurons

In order to characterize excitatory amino acid receptors in cultured chick cerebellar neurons, the effects of amino acid agonists on the input resistance and the antagonist specificity of the depolarization induced by each agonist were intracellularly studied. In Mg-containing medium, glutamate (especially at low doses), aspartate and N-methyl-D-aspartate not only decreased the input resistance at depolarized membrane potentials but also increased it at around the resting potential. In Mg-free medium, glutamate (high and low doses) and all other agonists simply decreased the input resistance. The effects of antagonists on amino acid-induced depolarizations in Mg-free medium were as follows: Mg and alpha-aminoadipate blocked N-methyl-D-aspartate and aspartate most strongly, glutamate and kainate moderately, and quisqualate only slightly; 2-amino-4-phosphonobutyrate antagonized N-methyl-D-aspartate most strongly, aspartate moderately and others mildly; 2-amino-5-phosphonovalerate blocked aspartate most strongly and others mildly; gamma-D-glutamylglycine blocked kainate most strongly and others moderately; and kynurenate was rather nonselective but most strongly antagonized N-methyl-D-aspartate and aspartate. These results suggest that all receptor subtypes (N-methyl-D-aspartate-, quisqualate- and kainate-types) are present in cultured chick cerebellar neurons, but their antagonist specificities are different from those in other central neurons, and also that glutamate at a low dose activates N-methyl-D-aspartate receptors, while it acts on quisqualate receptors at a high dose.

Effects of noradrenaline on the responsiveness of cultured cerebellar neurons to excitatory amino acids

The effects of noradrenaline (NA) on the responsiveness of cultured cerebellar neurons to excitatory amino acids were intracellularly investigated. NA applied to external medium to a final concentration of 10 microM or lower slightly decreased the firing frequency of spontaneous spikes, induced a small hyperpolarization or slightly increased the input resistance of Purkinje cells. In addition, bath-applied NA was found to enhance the depolarizations induced by iontophoretically applied glutamate and aspartate but to a smaller extent for the latter. These direct and modulating effects of NA were also observed when NA was applied by iontophoresis. The sites sensitive to iontophoresed NA were found to be not uniformly distributed but localized in restricted regions on individual Purkinje cells. The enhancement by NA of the glutamate or aspartate response was blocked by beta-adrenergic antagonists, propranolol or pindolol, and extracellularly applied cAMP mimicked the NA action. These results suggest the possibility that NA physiologically modulates excitatory amino acid-mediating synaptic transmission in the cerebellum probably by acting on beta-rather than alpha-adrenergic receptors.

Glutamate-induced ionic currents in cultured neurons from the rat superior colliculus

The ionic currents induced in cultured rat superior colliculus neurons by rapid application of glutamate (Glut) and the glutamate receptor agonists quisqualate (Quis) and N-methyl-D-aspartate (NMDA) were examined using the whole-cell patch clamp technique. Dissociated cell cultures consisting exclusively of superficial gray layer neurons from rats aged E21-P2 were used. After 7-10 days in vitro, all neurons responded to Glut and the selective agonists, NMDA and Quis. Glut was a mixed agonist, and a variable fraction (10-100%) of Glut-activated currents was due to involvement of NMDA receptors. The NMDA response was strongly regulated by extracellular Ca and Mg levels and modified by exposure to Quis. Quis transiently removed the block of NMDA-activated currents by D-amino-phosphonovaleric acid (APV).

Electrophysiological and pharmacological actions of N-acetylaspartylglutamate intracellularly studied in cultured chick cerebellar neurons

The electrophysiological and pharmacological actions of N-acetylaspartylglutamate (NAAG) in cultured chick cerebellar neurons were intracellularly investigated in comparison with L-aspartate (ASP) and L-glutamate (GLU). Iontophoretically applied NAAG dose-dependently induced depolarizations associated with increases in spike discharge and changes in membrane conductance. Relative excitatory potencies seemed to be GLU greater than ASP greater than or equal to NAAG. The voltage-dependent increase in input resistance observable in the presence of Mg ions was most notable for ASP, moderate for NAAG and least for GLU. The reversal potential of NAAG-induced depolarization was at about 0 mV and similar to that for ASP or GLU, indicating primary concern of Na+/K+-conductances to the NAAG action. Mg ions depressed the actions of ASP and NAAG more strongly than the GLU action. 2-Amino-5-phosphonovalerate (APV) and D-alpha-aminoadipate antagonized the actions of ASP and NAAG more effectively than the GLU action. 2-Amino-4-phosphonobutyrate (APB) and glutamic acid diethylester showed rather non-selective antagonisms to NAAG, ASP and GLU. These results suggest that NAAG is excitatory to cultured chick cerebellar neurons and functionally resembles ASP or is intermediate between ASP and GLU, and may also support the suggested candidacy of NAAG for a neurotransmitter in the CNS.