Paul Y. Sze

Steroid binding to synaptic plasma membrane: Differential binding of glucocorticoids and gonadal steroids

The specific binding of [3H]-corticosterone, [3H]-17 beta-estradiol, [3H]-testosterone, and [3H]-progesterone to synaptic plasma membrane (SPM) prepared from rat brain has been characterized. The dissociation constant is estimated as on the order of 1 x 10(-7) M for corticosterone and 1 x 10(-8) M for the other three steroids. In a competition experiment, none of the 3H-steroids was displaced by the other steroids at 500-fold excess, indicating the presence of specific binding sites on the membrane for each type of steroid. Moreover, pre-incubation of the SPM with phospholipase A2 or phospholipase C totally destroys the membrane binding of corticosterone and testosterone, but the binding of estradiol and progesterone remains intact. Since the SPM prepared from brain tissue is derived from many different neuronal cell types, it is possible that the membrane binding sites for glucocorticoids and for gonadal steroids are present in different neurons.

GLUCOCORTICOIDS AS A REGULATORY FACTOR FOR BRAIN TRYPTOPHAN HYDROXYLASE

—The normal developmental rise of tryptophan hydroxylase levels in neonatal rat brain was blocked by adrenalectomy. Similarly, adrenalectomy prevented the rescrpine‐induced elevation of tryptophan hydroxylase activity in brain stem of adult mice. In both cases, the effects of adrenalectomy could be reversed by replacement injections of corticosterone. Repeated injections of corticosterone (5 mg/kg daily) in fact induced a rise of brain tryptophan hydroxylase levels in neonatal brain. However, neither adrenalectomy nor repeated injections of large doses of the hormone (20 mg/kg, daily) was found to be effective in affecting the normal enzyme levels in adult brain. Apparent Km of the enzyme for substrate was unchanged by corticosterone in vivo or in vitro. These results indicate that glucocorticoids have a significant role in the regulation of brain tryptophan hydroxylase: possibly as an inducing signal during neonatal development and as a permissive factor at adult age.

Regulation of 5-hydroxytryptamine metabolism in mouse brain by adrenal glucocorticoids

The effects of glucocorticoid hormone on the metabolism of brain 5-hydroxytryptamine (5-HT) were studied in mice. A single injection of hydrocortisone acetate (HCA; 20 mg/kg, i.p.) accelerated the accumulation of 5-HT in whole brain after inhibition of monoamine oxidase activity by paragyline. The hormone did not appear to change brain tryptophan hydroxylase or 5-hydroxytryptophan decarboxylase activity. However, tryptophan levels in brain were elevated by 50% within 1 h after treatment with HCA. The effect of HCA on brain tryptophan levels was localized mainly in the nerve endings. In vitro synaptosomal preparations, HCA at 10(-5)-10(-7)M or corticosterone at 10(-5) M was found to stimulate the uptake of L-[3H]-tryptophan by the synaptosomes while androgenic and progesterone-like steroids were ineffective. These results demonstrate that glucocorticoids may directly act on nerve terminals in the regulation of 5-HT synthesis through an action on the uptake of tryptophan.

Effects of acute and chronic ethanol administration on ribosomal protein synthesis in mouse brain and liver.

Abstract The effect of ethanol on protein synthesis was studied in mouse brain and liver ribosomal preparations after acute and chronic administration of ethanol in vivo as well as after addition of ethanol in vitro . Incorporation of amino acid into protein was reduced in both brain and liver ribosomes 1.5–3 hours after a single administration of ethanol. The reduction was not temporally correlated with the elevation of blood level of ethanol and the degree of central depression. The ribosomal protein synthesis is rather insensitive to the in vitro addition of ethanol, as compared with the in vivo administration. The inhibition of protein synthesis in ribosomal preparations from acutely intoxicated animals may be due to transient alteration of either the ribosomal structure or the content of the associated messenger RNA. Chronic and continuous ingestion of ethanol (7–14 days) induces a stimulation of protein synthesis in brain and liver ribosomal systems, as well as an elevation of plasma corticosterone level.

Maternal Influences on Tryptophan Hydroxylase Activity in Embryonic Rat Brain

Maternal treatment with p-chlorophenylalanine strongly inhibits brain tryptophan hydroxylase (TPH) activity in the embryo, whereas administration of tryptophan significantly elevates the activity of this enzyme. Maternal stress, glucocorticoid injections, or a corn diet (low in tryptophan) do not change embryonic TPH activity, however. Since TPH is thought to be rate-limiting for serotonin (5-HT) synthesis in the developing animal as well as in the adult altered TPH activity as a result of maternal influences could change 5-HT synthesis in the embryo. If 5-HT is a developmental signal in the early differentiation of specific neurons during embryogenesis, as previously suggested, such maternal influences could affect this process, resulting in altered neuronal genesis in those brain regions receiving a serotonergic innervation.

The permissive role of glucocorticoids in the development of ethanol dependence and tolerance

In mice chronically treated with ethanol (in a liquid diet containing 6% ethanol ad libitum for 2 weeks), brain tryptophan hydroxylase (TPH) activity was increased (by 30-45% in whole brain), while brain tyrosine hydroxylase activity remained unchanged. Such chronic ethanol treatment also induced susceptibility to audiogenic seizures during withdrawal (60% incidence). When ethanol treatment was given to adrenalectomized (Adx) mice, the increase of brain TPH activity and the development of withdrawal audiogenic seizures were both prevented. In Adx mice receiving daily injections of corticosterone (0.5 mg/mouse), the ethanol-induced increase of brain TPH activity and the occurrence of withdrawal audiogenic seizures were both restored. Similarly, the ethanol-induced increase of liver alcohol dehydrogenase activity (by 60%) was prevented in Adx mice and restored by corticosterone replacement. It was noted that in all three cases replacement with such large doses of the corticoid did not enhance the ethanol effects, but merely restored the effects to the levels observed in intact mice. Apparently, glucocorticoids are required in a permissive role in order for the ethanol effects to occur.

Influence of ACTH on Tyrosine Hydroxylase Activity in the Locus coeruleus of Mouse Brain

Tyrosine hydroxylase activity in noradrenergic neurons of the locus coeruleus was found to rise in bilaterally adrenalectomized adult mice, with maximum increase of the enzyme activity (52% above control) occurring 7 days after the surgery. No such increase of tyrosine hydroxylase activity was found in dopaminergic neurons of the substantia nigra. The increase of enzyme activity in the locus coeruleus following adrenalectomy was totally prevented by corticosterone replacement. Moreover, the adrenalectomy effect was abolished by hypophysectomy, indicating the involvement of the pituitary rather than the adrenocortical function. Chronic administration of ACTH (20 IU/kg, i.p., daily) resulted in an increase of tyrosine hydroxylase activity in the locus coeruleus, with a time course and magnitude similar to those found after adrenalectomy. Additionally, the effects of four ACTH analogs were determined. ACTH 1-24 and ACTH 4-10 were as effective as the whole ACTH molecule, whereas ACTH 4-10,7-D-Phe and ACTH 11-24 were ineffective. The effect of ACTH 4-10, a peptide fragment with no adrenocorticotrophic activity, further indicates that glucocorticoids are not involved. From these data, it appears that tyrosine hydroxylase in the locus coeruleus neurons is under the regulatory influence of pituitary ACTH. It remains to be determined whether the hormone can be transported from its pituitary origin to the locus coeruleus and exerts a direct action on the noradrenergic neurons. Regardless of the mechanism, the response of the noradrenergic neurons to pituitary activity may be an important component in physiological adaptation of the central nervous system to chronic stress.

Effects of hydrocortisone and electric footshock on mouse brain tyrosine hydroxylase activity and tyrosine levels.

Tyrosine hydroxylase (TH) activity and levels of tyrosine were measured in whole brains from mice subjected to brief electric foot shock or to single or multiple injections of hydrocortisone acetate (HCA; 20 mg/kg). Adult brain tyrosine levels showed a rapid increase after either foot shock or a single injection of HCA; TH activity was also rapidly increased by foot shock but not by HCA. Multiple injections of HCA over three days increased brain TH activity in neonatal mice, but had no effect in adults. These results suggest that glucocorticoid hormone may have a regulatory influence on brain TH during the neonatal period, and that the hormone may also affect brain tyrosine. The acute effect of foot shock stress on brain TH activity is not a glucocorticoid-mediated event, but can be interpreted as enzyme activation due to neural stimulation.

Neurochemical Factors in Auditory Stimulation and Development of Susceptibility to Audiogenic Seizures

Susceptibility to audiogenic seizures has been observed in several higher animal species, including man. Several inbred strains of mice are known that show either resistance or susceptibility to convulsive seizures during the presentation of an auditory stimulus, and these differences have been attributed to specifically defined genetic backgrounds (1, 2). Mice of the C57BL/6 strain are known to be highly resistant to such seizures (1, 2). In recent studies, Henry (3) and Jumonville (4) reported, independently, that the genetically seizure-resistant C57BL/6 mice can be induced to develop high susceptibility by prior exposure to auditory stimulation during a sensitive period of postnatal development. Similar effects of priming by sound in inducing susceptibility to audiogenic seizures have also been found in other resistant strains of mice (5, 6, 7). It has now been made clear that susceptibility to audiogenic seizures is not only determined by genetic differences but that it is also inducible by prior auditory input, at least in some strains of mice. The findings on this phenomenon of acoustic priming from several behavioral studies and their neurological implications have been reviewed and discussed in the article by K. R. Henry and R. Bowman in this volume.

Effects of Aminergic Drugs and Glutamic Acid on Audiogenic Seizures Induced by Early Exposure to Ethanol

Adult C57BL/lOBg mice, normally resistant to audiogenic seizures, became susceptible when the mothers drank 10%ethanol in water during pregnancy and for 14 days postpartem. Reserpine enhanced the incidence of seiqres, and the effect was reversed by 5‐hydrOxYtryp tophan but not by dihydroxyphenylalanine p‐Chlorophenylalanine also enhanced the inci dence of seizures, whereas a‐methyl‐p‐tyrosine did not effect. Monosodium glutamate almost completely prevented seizures. These results are consistent with the interpretation that the serotonergic systems may be among those involved in the seizure mechanism induced by fetal and early exposure to ethanol.