beta-Adrenergic response in mouse CNS reaggregate cultures

Presence of light-responding neurons in the reaggregate cultures of rat retinae

Since histological studies on retinal reaggregates have given evidence of lamination similar to that in the intact retina, the presence of retinal function, i.e. light response, in vitro has been expected as well. Therefore, neurons in the outermost layers of reaggregates, mostly consisting of amacrine cells, were studied with cell-attached recording. About one fifth of the neurons in that layer showed spontaneous discharges. Interestingly, the frequency of spontaneous discharges of some neurons increased with light stimulation but decreased to the basal level under dark conditions. They were reproducible and lasted more than 10 min in the best case. These results suggest that functional connections between photoreceptors which are exclusively present in the central region and amacrine cells in the outermost layer might be reconstituted in reaggregate cultures.

Paradoxical and subtype-specific effects of opiate antagonists on the expression of opioid receptors in rat brain cultures

Cultures of aggregating fetal rat brain cells express mu, delta, and kappa opioid receptors. The potent and long-lasting opioid antagonist naltrexone was used to investigate whether different regulatory mechanisms are involved in the expression of the three receptor subtypes. In cultures treated for seven days, naltrexone increased dose-dependently the binding of 3H-diprenorphine to the three receptor subtypes, with the mu sites being affected at a lower concentration than the other two; A Scatchard analysis indicated that this receptor up-regulation was obtained by an increase in the BMax, with no significant change in the affinity of the ligand to the receptors. In contrast to these effects in cultures treated for 7 days, it was surprising to find that a 48 hr treatment with naltrexone had an apparent converse and subtype-specific influence; the antagonist decreased significantly the binding of 3H-diprenorphine to both mu and delta receptors but had no effect on kappa sites. Two other opioid antagonists, naloxone and levallorphan, had a similar effect. Further analysis of naltrexone's mode of action was obtained by studying its effect on the adenylate cyclase activity. Of several inducers of this enzyme, the beta-adrenergic inducer isoproterenol gave the highest increase in cyclic AMP. Naltrexone had no significant effect on the basal adenylate cyclase activity but it altered the pattern of cyclic AMP formation in isoproterenol-stimulated cultures. Overall, the results indicate that in addition to its classic antagonistic activity, naltrexone exhibits in fetal brain aggregates some properties associated with opiate agonists.

Selective localization of glycine-accumulating cells in reaggregate culture of rat retina

Uptake of [3H]glycine into the cells either in monolayer or reaggregate cultures of retina from two-day-old rat pups was studied. The glycine-accumulating cells (glycine cells) had short processes with several branches. Only 5% of process-bearing cells were labelled in the monolayer cultures. The major cell type, previously identified as photoreceptor cells, was unlabelled. In reaggregate cultures, the glycine cells were localized mainly in the outermost layer of the reaggregate. But the proportion of positive cells among all the cells in that layer was not so large. Although the cell type of the glycine cells has not yet been unambiguously identified, these results demonstrate a possible example of selective sorting out of a group of biochemically distinct cells from a cell mixture.

Developmental study on the regulation of neurotransmitter-sensitive adenylate cyclase systems in primary cerebral cell cultures from embryonic mice

An ontogenetic study of the effect of various neurohormones and other activators on adenylate cyclase systems was carried out using cultures of cells from 15-d-old embryonic mouse brain. Dopamine stimulated the enzyme activity at earlier culture ages (i.e. 4 and 10 d) but had little stimulatory effect at later ages (i.e. 20 and 33 d). Further, this stimulation at the earlier ages was blocked by the dopaminergic blocker, fluphenazine, but not by alpha and beta-adrenergic antagonists. In contrast to dopamine, isoproterenol (a beta-adrenergic agonist) had little stimulatory effect at earlier ages, but its ability to stimulate cyclase activity increased with age. This increase in all age groups was blocked by propranolol (a beta-adrenergic antagonist). Epinephrine-sensitive enzyme activity showed a steady increase with age, which could be blocked with propranolol except in 4-d-old cultures, where it was blocked instead by fluphenazine. Because the cultures are relatively enriched in neurons at earlier ages and in glia in later ages, the results suggest a predominantly neuronal localization for the dopamine sensitive adenylate cyclases and a glial localization of the isoproterenol and epinephrine sensitive adenylate cyclases. Histamine, serotonin, calcium/calmodulin and chloroadenosine were either only slightly or not at all stimulatory.

Biogenic amines and adenosine-sensitive adenylate cyclases in primary cultures of striatal neurons

Primary cultures of virtually pure striatal neurons from 16-day-old mouse embryos can be obtained using a serum-free chemically defined medium. Membranes prepared from these cells contain dopamine, beta-adrenergic, serotonin and adenosine sensitive adenylate cyclases. The pharmacological properties of the dopamine receptors are similar to those found for D1 receptors in adults except for the apparent affinities for agonists which were 5-10 times higher in fetal neurons. Beta-adrenergic receptors of striatal and cerebellar fetal neurons are of the beta 1-subtype as indicated by their identical affinity for adrenaline and noradrenaline and by their homogeneous, high affinity for practolol (Ki = 1.3 X 10(-6)M). Adenosine and serotonin sensitive adenylate cyclases present classical characteristics. An extensive study of the additive effects of the 4 neurotransmitter-sensitive adenylate cyclases indicates that: (1) part of the neurons bear more than one type of biogenic amine receptors; (2) the serotonin receptors are always associated with adenosine receptors on the same neurons; (3) adenosine- and dopamine-sensitive adenylate cyclases are additive. From this it can be concluded that as far as their adenylate cyclases-linked amine receptors are concerned, a maximal number of 15 types of neurons are present in these striatal cell cultures.

beta-Adrenergic receptors of brain cells. Membrane integrity implies apparent positive cooperativity and higher affinity

Beta-Adrenergic receptors were studied in intact cells of chick, rat and mouse embryo brain in primary cultures, by the specific binding of [3H]dihydro-L-alprenolol ([3H]DHA). The results were compared to the receptor binding of broken cell preparations derived from the cell cultures or from the forebrain tissues used for the preparation of the cultures. Detailed analysis of [3H]DHA binding to living chick brain cells revealed a high-affinity, stereoselective, beta-adrenergic-type binding site. Equilibrium measurements indicated the apparent positive cooperativity of the binding reaction. By direct fitting of the Hill equation to the measured data, values of Bmax = 12.01 fmol/10(6) cells (7200 sites/cell), Kd = 60.23 pM and the Hill coefficient n = 2.78 were found. The apparent cooperative character of the binding was confirmed by the kinetics of competition with L-alprenolol, resulting in maximum curves at low ligand concentrations. The rate constants of the binding reaction were estimated as k+ = 8.31 X 10(7) M-1 X min-1 and k- = 0.28 min-1 from the association results, and k- = 0.24 min-1 from the dissociation data. The association kinetics supported the cooperativity of the binding, providing a Hill coefficient n = 1.76; Kd, as (k-/k+)1/n was found to be 101 pM. Analysis of the equilibrium binding of [3H]DHA to rat and mouse living brain cells resulted in values of Bmax = 13.04 fmol/10(6) cells (7800 sites/cell), Kd = 43.85 pM and n = 2.52, and Bmax = 8.08 fmol/10(6) cells (4800 sites/cell), Kd = 46.70 pM and n = 1.63, respectively, confirming the apparent cooperativity of the beta-receptor in mammalian objects, too. The [3H]DHA equilibrium binding to broken cell preparations of either chick, rat or mouse brain cultures or forebrain tissues was found to be non-cooperative, with a Hill coefficient n = 1, Kd in the range 1-2 nM, and a Bmax of 10(3) - 10(4) sites/cell. Our findings demonstrate that cell disruption causes marked changes in the kinetics of the beta-receptor binding and in the affinity of the binding site, although the number of receptors remains unchanged.