Factors regulating central serotonergic receptor sensitivity

Sl16 FACTORS REGULATING

CENTRAL SEROTONERGIC

RECEPTOR SENSITIVITY

TOMIO SEGAWA, HIROAKI NISHI0*, MUTSUKO FUJITA* and AKEMI KIMOTO*, Department of Pharmacology, Institute of Pharmaceutical Sciences, Hiroshima University School of Medicine, Kasumi 1-2-3, Minami-ku, Hiroshima 734, Japan Several factors have been known to regulate ligand binding to 5-hydroxytryptamine (5-HT) receptors. We investigated, in the present experiments, the mechanism of Mn 2- and HCO~- to increase central 5-HTz recsptor sensitivity in the rat. Mn 2 (i0-~-i0 -3 M) increased specific [3H]5-HT binding to 5-HTI receptors from rat frontal cortex, the effect being enhanced by the addition of HC0~(i0~3-2 x 10 -2 M)~ In the presence of HC03-, other divalent cations, Ca 2~ Mg 2 , Cu 2+ and Fe + had no effect on [3H]5-HT binding. Futhermore, in the presence of Mn=+(10 -3 M) other anions, H=P04-, HPO4- and CH3CO0- increased [3H]5-HT binding, but the effect was less than that of HC03-. Scatchard analysis of Mn2+-HC03 - effect on [3H]5-HT binding indicated that only a number of binding sites increased. Mn~+-HCO~ - effect was specific for synaptosomal membrane fractions, and was temperature dependent. SH group inhibitors, N-ethylmaleimide (NEM) and iodoacetic acid ([AA), decreased Mn2+-HCO3 - effect; while the effect was completely abolished by reducing agents, 1,4-dithiothreitol (DTT), 2-mercaptoethanol and ascorbate. Mn2+-HC03 - induced enhancement was not influenced by the addition of guanosine triphosphate (GTP) and guanyl-5'-yl imidodiphosphate (GppNHp). Pretreatment with Triton X-100 increased Mn2+-HC03 - enhanced [3H]5-HT binding. Prom these results it is suggested that Mn 2+ specifically interacts with HC03- and selectively increases [3H]5-HT bindin~ to 5-HT~ receptors in the central nervous system. THE INHIBITORY EFFECT OF SEROTONIN AUTORECEPTOR OF TRYPTOPHAN HYDROXYLASE

STIMULATION ON THE ACTIVITY

IN RAT RAPHE SLICES

MAKOTO SAWADA * and TOSHIHARU N A G A T S U § ' * Laboratory of Cell Physiology, Department of life Chemistry, Graduate School at Nagatsuta, ~okyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, and Department of Biochemistry, Nagoya University School of Medicine, Nagoya 466, Japan.

The role of serotonin autoreceptor tryptophan hydroxylase

was investigated

of tryptophan hydroxylase 5-hydroxytryptophan

in the regulation

(NSD-I055)

Serotonin and its agonists,

of

by HPLC with fluorescence

N,N-dimethylmethoxytryptamine

reduced the formation of 5-hydroxytryptophan

The effect of serotonin was reversed by methiothepin,

of serotonin autoreceptor.

tion of 5-hydroxytryptophan

and to

an antagonist

N-(6-Aminohexyl)-5-chloro-l-naphthalenesulfonamide

(W-7), an antagonist of calmodulin,

by A-23187.

The activity

the accumulation

under the inhibition of aromatic L-amino acid decarboxylase

m-chlorophenylpiperadine, 60-50%.

in rat raphe slices.

was estimated by measuring

using 3-hydroxy-4-bromobenzyloxyamine detection.

of the activity of

dose-dependently

reduced the basal forma-

to 50%, which also reduced the activated formation

Since serotonin did not enhance the inhibitory effect of W-7,

the effect of serotonin was thought to be related to the inhibition of the calcium-calmodulin-dependent

mechanism.