Inhibition of tryptophan hydroxylase by benserazide and other catechols.

Tryptophan hydroxylase (L-tryptophan, tetrahydropteridine:oxygen oxidoreductase [5-hydroxylating]; EC 1.14.16.4; TPH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin, was inhibited directly by benserazide, an inhibitor of aromatic-L-amino-acid decarboxylase (3,4-dihydroxy-L-phenylalanine carboxy-lyase; EC 4.1.1.28; AAAD). Benserazide was a competitive inhibitor for the pterin cofactor tetrahydrobiopterin and an uncompetitive inhibitor for the substrate tryptophan. NSD 1015, another decarboxylase inhibitor, did not directly inhibit TPH. Other compounds with catechol moieties in their structures such as 3,4-dihydroxyphenylalanine (DOPA), dopamine, apomorphine, and SKF 38393 were also found to be potent inhibitors of TPH. These results indicate that drugs or neurotransmitters with catechol structures directly inhibit the activity of TPH and add to a growing body of evidence indicating that endogenous dopamine can exert untoward effects on serotonin neurons, including inhibition of TPH. Furthermore, the use of decarboxylase inhibitors to cause the accumulation of 5-hydroxytryptophan as an in vivo measure of TPH activity could be problematic, particularly when drugs with catechol structures or dopamine-releasing compounds are also administered.

Identification of 2-amino-2-methyl-4-phosphonobutanoic acid as an antagonist at the mGlu4a receptor.

2-Amino-2-methyl-4-phosphonobutanoic acid (MAP4) was tested for interactions with the mGlu4a receptor which when expressed in baby hamster kidney (BHK570) cells couples to inhibition of forskolin-stimulated cAMP production. MAP4 had no agonist activity at this receptor and caused a concentration-dependent inhibition of the reduction in forskolin-stimulated cyclic AMP formation elicited by L-2-amino-4-phosphonobutanoic acid (L-AP4). Inhibition by MAP4 was consistent with a competitive mechanism of action (Schild slope = 1.2) with a Ki of 190 microM. MAP4 is the first antagonist identified for the mGlu4a receptor.

Immobilization of tryptophan hydroxylase by immune adsorption: a method to study regulation of catalytic activity.

Tryptophan hydroxylase (TPH) can be immobilized by adsorption to Pansorbin after binding to the monoclonal antibody PH8. This method yields recoveries of 35%-40% of total TPH activity in crude extracts and can be completed in 1.5 h. The immobilized form of TPH retains the essential kinetic properties of the native enzyme and responds to activators (phosphatidylserine) and inhibitors (catechol compounds) as does the native enzyme. Unlike TPH in brain extracts, immobilized TPH is not activated by calcium-stimulated phosphorylating conditions. When extracts from which TPH has been precipitated, and which contain calcium-calmodulin dependent protein kinase are added to immobilized TPH, the activation of TPH is restored. This method of immobilization of TPH via immune-adsorption allows for the highly specific and rapid preparation of affinity purified TPH that can be used to study the regulation of this enzyme by a variety of effectors, especially protein kinases.

B-HT 920 stimulates postsynaptic D2 dopamine receptors in the normal rat: electrophysiological and behavioral evidence.

The putative autoreceptor-selective dopamine (DA) agonist B-HT 920 was tested using electrophysiological and behavioral models thought to reflect actions at postsynaptic D2 DA receptors. Direct iontophoretic application of B-HT 920 onto nucleus accumbens neurons caused a current-dependent inhibition of firing which could be attenuated by pretreatment with alpha-methyl-p-tyrosine (to deplete DA) and reinstated (enabled) by concurrent administration of the selective D1 DA receptor agonist SKF 38393. These findings suggest that, like other selective D2 DA receptor agonists, the postsynaptic effects of B-HT 920 require concurrent stimulation of D1 DA receptors. Behavioral indices of postsynaptic D2 DA receptor stimulation (stereotyped sniffing and rearing) were also evident following combined treatment with B-HT 920 and SKF 38393. Moreover, similar "low-level" stereotyped behaviors were also observed when B-HT 920 was administered alone following pretreatment with the alpha-2 adrenoceptor antagonists idazoxane and piperoxane, suggesting that alpha-2 agonist actions of B-HT 920, in some way, mask the expression of D2 receptor-mediated stereotyped responses. When B-HT 920 was combined with SKF 38393 following pretreatment with idazoxane, both the intensity and form (continual licking and gnawing) of stereotyped behavior was enhanced. Taken together, these electrophysiological and behavioral findings indicate that B-HT 920 possesses the properties of a selective D2 DA receptor agonist and cannot be considered as a DA autoreceptor-selective compound.

Electrophysiological effects of cocaine in the mesoaccumbens dopamine system: studies in the ventral tegmental area.

Extracellular single-cell recording and microiontophoretic techniques were used to characterize the effects of cocaine on the activity of mesoaccumbens A10 dopamine (DA) neurons in the rat ventral tegmental area (VTA), which have been implicated in the rewarding effects of this and other drugs of abuse. Because cocaine inhibits the reuptake of DA, norepinephrine (NE), and serotonin (5-HT), and exerts local anesthetic actions, the possible involvement of each of these various mechanisms in the effects of cocaine on A10 DA neurons was investigated. Intravenous administration of cocaine caused a significant, dose-dependent, partial inhibition (50-70%) of the firing of antidromically identified mesoaccumbens DA neurons. Similar partial inhibition of A10 neurons was observed following intravenous administration of nomifensine, GBR-12909, and norcocaine, all of which inhibit DA reuptake. Neither the selective 5-HT uptake inhibitor fluoxetine nor the selective NE uptake inhibitor desmethylimipramine (DMI) inhibited the firing of A10 DA neurons. The local anesthetic agent procaine, which lacks DA uptake blocking efficacy, caused a slight, transient increase in firing rate. These results suggest that the effects of cocaine on A10 DA neurons are due to inhibition of DA reuptake, a conclusion that has been supported by several other findings. Pretreatment with reserpine to deplete vesicular stores of DA significantly reduced the ability of intravenous cocaine to suppress A10 DA neuronal activity. Microiontophoretic administration of cocaine caused only a weak (15-20%) inhibition of the activity of A10 DA neurons, but significantly increased and prolonged the inhibition produced by iontophoretic DA. This effect was not observed with iontophoretically administered procaine iontophoresis of cocaine also significantly potentiated the inhibition of A10 DA activity caused by electrical stimulation of the nucleus accumbens (NAc). Both unilateral ibotenic acid lesions of the NAc and hemitransections of the brain rostral to the VTA significantly reduced the inhibitory effects of intravenous cocaine on A10 DA neurons, suggesting that both somatodendritic impulse-regulating DA autoreceptors and inhibitory NAc-VTA feedback processes are involved in the effects of intravenous cocaine on A10 DA neurons. Therefore, it is hypothesized that the relatively weak inhibitory effects of cocaine on A10 DA neurons may represent a poor compensatory response to enhanced DA neurotransmission within the NAc, and may help to explain the extremely potent rewarding effects of this important drug of abuse.

Relationship between D1 dopamine receptors, adenylate cyclase, and the electrophysiological responses of rat nucleus accumbens neurons.

The electrophysiological effects of three selective D1 dopamine (DA) receptor agonists, which exhibit different potencies and efficacies for stimulation of adenylate cyclase, were compared in the rat nucleus accumbens (NAc) using single unit recording and microiontophoretic techniques. The partial agonists SKF75670 and SKF38393, and the full agonist SKF81297 produced nearly identical current-response curves for the inhibition of firing of NAc neurons. In rats acutely depleted of DA by alpha-methyl-p-tyrosine (AMPT) pretreatment, all three D1 agonists enabled the inhibition of firing produced by the selective D2 receptor agonist quinpirole, with SKF38393 exerting the greatest efficacy, followed by SKF81297 and SKF75670. Thus, no apparent relationship was found between the previously reported ability of these compounds to stimulate cyclic adenosine monophosphate (cAMP) production and their ability either to inhibit the firing of NAc neurons or to enable quinpirole-mediated inhibition of firing in DA-depleted rats. In addition, the membrane-permeable cAMP analog 8-bromo-cAMP also caused a current-dependent inhibition of the firing of NAc neurons, but failed to enable quinpirole-mediated inhibition in AMPT-pretreated animals. These results suggest either that only a small percentage of D1 receptors need to be stimulated to produce these electrophysiological effects, or that D1 receptors exist within the rat NAc which are linked to transduction mechanisms other than, or in addition to, adenylate cyclase.

Unilateral sensorineural hearing loss in children and auditory performance with respect to right/left ear differences.

A group of children suffering from unilateral hearing loss (UHL) were evaluated. Serological tests were performed and vaccination against mumps was offered if negative. In 43.6% of the cases mumps was the plausible cause but a further 33.3% had positive serological tests for mumps without having had the clinical disease. Due to the long delay in the diagnosis of UHL, the question of mumps as major causative agent can be solved only when the results from prophylactic vaccination become apparent in the future. Thirty children aged 10-16 years underwent further audiological examination in order to detect any difference between right and left UHL. The results of word discrimination scores (PB lists) show the well known superiority of binaural v. monaural hearing but was too crude for a detailed analysis of ear difference. An interrupted speech test in background noise shows, however, that right ear impaired children perform significantly poorer than the left ear impaired and are at risk whenever reverberation conditions are poor.

Unilateral sensorineural hearing loss in children: cognitive abilities with respect to right/left ear differences.

Thirty children (age 10 16 years) suffering from unilateral hearing loss (UHL) were matched with a control group and examined by a battery of psychological tests (verbal and non-verbal subtests) in order to investigate a possible right or left ear difference on cognitive functions. The results confirm that right ear impaired children perform significantly poorer than their left ear impaired counterparts especially in verbal subtests that are sensitive to minor input/processing damages. The data obtained suggest that right ear impaired children are at risk in the educational system.

Tryptophan hydroxylase is phosphorylated by protein kinase A.

The effect of protein kinase A on the catalytic activity and phosphorylation of brain tryptophan hydroxylase was examined. Stimulation of endogenous protein kinase A by cyclic AMP or its analogues, dibutyryl-cyclic AMP and 8-thiomethyl-cyclic AMP, failed to activate tryptophan hydroxylase. The activation of tryptophan hydroxylase by calcium/calmodulin-phosphorylating conditions was not modified by cyclic AMP. Endogenous protein kinase A phosphorylated a large number of proteins and tryptophan hydroxylase could be identified as one substrate by sucrose gradient centrifugation, immunoprecipitation, and immunoblotting. These results indicate that tryptophan hydroxylase is phosphorylated by protein kinase A in brain and question whether this protein kinase exerts direct regulatory influence over tryptophan hydroxylase activity via phosphorylation.

Electrophysiological effects of putative autoreceptor-selective dopamine agonists on A10 dopamine neurons.

The dopamine (DA) hypothesis of schizophrenia proposes hyperactivity of the mesocorticolimbic DA system, originating within the A10 DA cells of the ventral tegmental area (VTA), as a pathophysiological mechanism. Thus, reduction of activity in this system, including that produced by putative "autoreceptor-selective" DA agonists, may be of clinical utility. The present studies compared the ability of eight D2 DA receptor agonists to inhibit the firing of rat A10 DA neurons after i.v. administration. Both N-n-propyl-N-phenylethyl-p(3-hydorxyphenyl)ethylamine hydrochloride (RU 24213) and 2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]- azepine dihydrochloride (B-HT 920) were potent, high-efficacy agonists which completely inhibited the firing of A10 DA cells. The putative autoreceptor-selective DA agonists 3-(4-(4-phenyl-1,2,3,6-tetrahydropyridyl-(1)-butyl)-indole hydrochloride (EMD 23,448) and (+)-3-(3-hydroxy-phenyl)-N-n-propylpiperidine [(+)-3-PPP] were considerably weaker than RU 24213 and B-HT 920, but also exhibited "full" efficacy (i.e., they completely suppressed cell firing). The putative autoreceptor agonist preclamol [(-)-3-PPP] and its trans-fused congener (-)-HW 165 were weak partial agonists that failed to completely inhibit A10 DA cells. The new putative autoreceptor agonist N-[(8-alpha)-2-chloro-6-methylergoline-8-yl]-2,3]dimethylopropa namide (SDZ 208-911) was also a weak partial agonist that exhibited partial antagonist effects (reversed inhibition produced by the D2 agonist quinpirole), whereas its structural analog N-[(8-alpha)-2-chloro-6-methylergoline-8-yl]-2,2-dimethylopropa namide (SDZ 208-912) was nearly inactive as an agonist, but was an effective antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation and activation of tryptophan hydroxylase by exogenous protein kinase A.

The catalytic subunit of protein kinase A increases brain tryptophan hydroxylase activity. The activation is manifested as an increase in Vmax without alterations in the Km for either tetrahydrobiopterin or tryptophan. The activation of tryptophan hydroxylase by protein kinase A is dependent on ATP and an intact kinase and is inhibited specifically by protein kinase A inhibitors. Protein kinase A also catalyzes the phosphorylation of tryptophan hydroxylase. The extent to which tryptophan hydroxylase is phosphorylated by protein kinase A is dependent on the amount of kinase used and is closely related to the degree to which the hydroxylase is activated. These results suggest that a direct relationship exists between phosphorylation and activation of tryptophan hydroxylase by protein kinase A.

Type 4a metabotropic glutamate receptor: identification of new potent agonists and differentiation from the L-(+)-2-amino-4-phosphonobutanoic acid-sensitive receptor in the lateral perforant pathway in rats.

Before the discovery of the metabotropic glutamate receptors (mGluRs), the glutamate analogue L-2-amino-4-phosphonobutanoic acid (L-AP4) was identified as a potent presynaptic inhibitor of evoked synaptic transmission in the lateral perforant pathway in rats. The localization and L-AP4 sensitivity of the mGluR4a subtype of mGluRs were consistent with the hypothesis that this receptor mediates the synaptic depressant effects of L-AP4 in the lateral perforant pathway. In the present study, the pharmacology of mGluR4a expressed in baby hamster kidney 570 cells was characterized and compared with that previously reported for the lateral perforant pathway responses. The endogenous excitatory amino acid L-aspartate was inactive at mGluR4a, whereas L-homocysteic acid was only 5-fold less potent than L-glutamate. These data suggest that L-homocysteic acid may be an endogenous agonist at mGluR4a. Of the 30 L-AP4 analogues examined, several compounds were identified as agonists at mGluR4a. The cyclopropyl-AP4 analogue (Z)-(+/-)-2-amino-2,3-methano-4-phosphonobutanoic acid inhibited forskolin-stimulated cAMP production with an EC50 of 0.58 microM, which is comparable to that of L-AP4 (EC50 = 0.43 microM). Two other cyclic analogues of L-AP4 were approximately 10-fold less potent as agonists at mGluR4a, i.e., (+/-)-1-amino-3-(phosphonomethylene)cyclobutanecarboxylic acid (EC50 = 4.4 microM) and (E)-(+/-)-2-amino-2,3-methano-4-phosphonobutanoic acid (EC50 = 7.9 microM). Comparison of the potencies of the compounds for activation of mGluR4a with their potencies for inhibition of lateral perforant pathway responses demonstrates that some compounds have comparable activities in the two systems, whereas several compounds are at least 10-fold more potent in one of the systems. In addition, although the mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine blocked the effects of L-AP4 in the lateral perforant pathway, it did not block the effects of L-AP4 at the cloned receptor. These data provide evidence that mGluR4a does not mediate the effects of L-AP4 in the lateral perforant pathway, they provide new tools to identify the function of these receptors in the mammalian central nervous system, and they indicate that the effects of L-AP4 in the lateral perforant pathway are mediated by a (+)-alpha-methyl-4-carboxyphenylglycine-sensitive receptor.