Joseph M. Masserano

Tyrosine hydroxylase regulation in the central nervous system

SummaryTyrosine hydroxylase is considered to be the rate-limiting enzyme in the synthesis of catecholamines in both the central and peripheral nervous system. Increased or decreased neuronal activity, stress, lesions, drug effects, endocrinological manipulations and experimental models of hypertension are associated with alterations in tyrosine hydroxylase activity in the central nervous system. In many of these instances, the changes in the activity of tyrosine hydroxylase in the central nervous system that occur are localized to discrete catecholaminergic pathways and nuclei in the brain. The purpose of this review is to summarize and assess this information and to provide insight into the function of catecholamine systems in the brain and their interactions with other putative neurotransmitter systems.

Evaluation of methods for the isolation of plasma membranes displaying guanosine 5′-triphosphate-dependence for the regulation of adenylate cyclase activity: Potential application to the study of other guanosine 5′-triphosphate-dependent transduction systems

The GTP-dependence for stimulatory and inhibitory regulation of plasma membrane adenylate cyclase activity was measured in plasma membrane fractions isolated from a variety of cell types (platelets, lymphocytes, PC12 cells, GH3 cells, NBP2 cells, and hepatocytes). This report shows that the isolation of plasma membranes for the study of GTP-dependent adenylate cyclase activity was, for some cells, enhanced by the exposure of the cells to glycerol prior to cell lysis. The isolation of plasma membranes from other cells, which did not appear to be sensitive to glycerol pretreatment, was enhanced by the removal of heavy particulate matter prior to fractionation of the cell lysate. The regulation of enzyme activity by various agents was found to be dependent upon the presence of (exogenous) GTP to varying degrees, indicating variable contamination of membrane preparations with GTP. It is concluded that (i) exposure of platelets and lymphocytes to glycerol prior to cell lysis decreases subsequent contamination of the plasma membrane preparation with GTP, and (ii) although glycerol pretreatment of other cells does not ensure the subsequent isolation of plasma membrane adenylate cyclase activity displaying high requirements for (exogenous) GTP, it is a reasonable first approach to be used during the development of procedures for the isolation of plasma membranes.

The effects of reserpine and haloperidol on tyrosine hydroxylase activity in the brains of aged rats

Many neurotransmitter systems appear to be altered with aging. The effects of aging on the regulation of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of catecholamines in the brain has been examined. The endogenous basal activity of tyrosine hydroxylase was lower in the hypothalamus of 24 month old Fisher 344 rats than in the hypothalamus of 3 month old or 6 month old animals. There was no difference in the basal activity of tyrosine hydroxylase in the locus ceruleus, frontal cortex, hippocampus, substantia nigra, or the striatum of rats of ages 3 months, 6 months and 24 months. Tyrosine hydroxylase activity was increased in the striatum of 3 month old (60%) and 6 month old (28%) rats after treatment with haloperidol or reserpine, whereas no change in enzyme activity followed administration of these drugs to 24 month old animals. In conclusion, increases in tyrosine hydroxylase activity in the brain that normally occur in the striatum of 3 month old rats after haloperidol or reserpine treatment are significantly decreased in 6 month old rats and not apparent in 24 month old rats.

Bradykinin activates tyrosine hydroxylase in rat pheochromocytoma PC-12 cells

Tyrosine hydroxylase is activated in PC‐12 cells by bradykinin in a concentration‐dependent manner with maximal stimulation occurring at 1 μM. This stimulatory effect occurs within 15 s and is maximal at 5 min. This stimulation is due to an increase in the affinity of tyrosine hydroxylase for its pterin cofactor, and can be blocked by a specific bradykinin receptor antagonist. These data indicate that bradykinin can regulate the activity of tyrosine hydroxylase in PC‐12 cells.

Effects of Electroconvulsive Shock on Tetrahydrobiopterin and GTP-Cyclohydrolase Activity in the Brain and Adrenal Gland of the Rat

Abstract: The effects of a single and repeated electroconvulsive shock (ECS) (300 mA, 0.2 s) on tetrahydrobiopterin (BH4) levels and GTP‐cyclohydrolase activity in the brain and adrenal glands of rats were examined. Twenty‐four hours after the last ECS treatment (one/day for 7 days), biopterin levels were significantly elevated in the locus coeruleus, hippocampus, frontal cortex, hypothalamus, ventral tegmental area, and adrenal gland. There were no changes in biopterin levels after a single application of ECS. GTP‐cyclohydrolase activity was significantly increased in the locus coeruleus, frontal cortex, hippocampus, hypothalamus, and adrenal gland 24 h after repeated ECS and remained elevated in certain tissues up to 8 days after the last treatment. Kinetic analysis of adrenal and locus coeruleus GTP‐cyclohydrolase 1 day after 7 days of ECS showed significant changes in both Km and Vmax values. These data suggest that the long‐term increases in BH4 levels and GTP‐cyclohydrolase activity after repeated ECS may play a part in the mediation of the antidepressant effects of ECS.

Activation of tyrosine hydroxylase in the frontal cortex by phentolamine and prazosin

Tyrosine hydroxylase is considered to be the ratelimiting enzyme in the pathway concerned with synthesis of catecholamines. Short term activation of tyrosine hydroxylase can occur following nerve stimulation or stress. This has been demonstrated in a number of systems, including the guinea-pig vas deferens preparation after stimulation of the hypogastric nerve (Weiner et a1 1978), the rat hippocampus on stimulation of the locus coeruleus (Roth et a1 1975a), the rat striatum following electrical stimulation of the nigro-neostriatal pathway (Roth et a1 1975b), the rat adrenal gland and striatum after electroconvulsive shock (Masserano et a1 1981) and the rat striatum following the administration of dopamine receptor blocking agents (Zivkovic et a1 1974). In these studies the increased activity of tyrosine hydroxylase is associated with an increased affinity (reduced K,) of the enzyme for its pterin cofactor. The purpose. of the present experiments was to determine if a similar activation of tyrosine hydroxylase from a predominantly noradrenergic brain region (frontal cortex) can occur following the administration of the specific or-adrenoceptor blocking agents, phentolamine and prazosin.

Age-related changes in tetrahydrobiopterin and GTP-cyclohydrolase activity in the brain and adrenal gland of rats

In the present study, we observed that tetrahydrobiopterin (BH4) levels were decreased significantly in the striatum, substantia nigra, frontal cortex, hypothalamus, and cerebellum of 25-month-old and in the cerebellum only in case of 18-month-old rats as compared to 4-month-old rats. In contrast, BH4 levels in adrenal glands of 25-month-old rats were increased significantly. Kinetic analysis of GTP-cyclohydrolase revealed a decrease in the apparent Km along with an increase in Vmax value in the adrenal of 25-month-old rats compared to 4-month-old rats. Whereas, in cerebellum we observed that the apparent Km of the high affinity form of the enzyme was increased and a decrease in the Vmax value of the low affinity form only in case of 25-month-old rats compared to the young animals. These alterations in the BH4 levels and its synthesizing enzyme kinetics in specific brain areas and adrenal glands of aged rats are consistent with the reported changes in the catecholamine function in central and peripheral nervous system with aging.

Comparative effects of electroconvulsive shock and haloperidol on in vivo tyrosine hydroxylation and tetrahydrobiopterin in the brain of rats with 6-hydroxydopamine lesions

We have evaluated the effects of electroconvulsive shock (ECS) and haloperidol treatment on the in vivo tyrosine hydroxylation rate and the tetrahydrobiopterin levels in the nigrostriatal system of 6-OHDA-lesioned rats. The rate of DOPA accumulation was significantly decreased by 96% in the ipsilateral striatum and by 50% in substantia nigra of the 6-OHDA-lesioned rats compared to the control activity of contralateral non-lesioned striatum and substantia nigra. The loss of total biopterin was found to be 75% and 50% in the ipsilateral striatum and substantia nigra, respectively. Following administration of haloperidol, the rate of DOPA accumulation increased significantly in the striatum and substantia nigra on the lesioned side compared to that in the vehicle treatment group. Application of ECS also significantly increased the rate of DOPA accumulation in the ipsilateral striatum and substantia nigra compared to that obtained in the non-shocked rats. The biopterin levels in the nigrostriatal system of 6-OHDA-lesioned were elevated significantly in the striatum after haloperidol treatment; in contrast the biopterin levels were unchanged in response to ECS. Our results show that both haloperidol and ECS significantly enhanced the rate of in vivo tyrosine hydroxylation in the striatum and substantia nigra of rats with greater than 90% lesions. These results suggest that the nigrostriatal system, although up-regulated following 6-OHDA lesions, still maintains the potential for further up-regulation of dopaminergic function in response to haloperidol and ECS treatment.

The effects of electroconvulsive shock on catecholamine function in the locus ceruleus and hippocampus

Repeated electroconvulsive shock (ECS) treatment (once per day for 7 days) produced a significant increase in tyrosine hydroxylase activity, GTP-cyclohydrolase activity and tetrahydrobiopterin (BH4) levels in the locus ceruleus and hippocampus from 1 to 4 days after the last treatment. These changes may be responsible for, or contribute to, the antidepressant effect of ECS treatment.