Acceleration of norepinephrine synthesis in the rat submaxillary gland in vivo during sympathetic nerve stimulation

A quest for antipsychotic drug actions in the brain: Personal experiences from 50 years of neuropsychiatric research at Karolinska Institutet

The exploration of physiological and molecular actions of psychoactive drugs in the brain represents a fundamental approach to the understanding of emerging psychological phenomena. The author gives a personal account of his medical training and research career at Karolinska Institutet over the past 50 years. The paper aims at illustrating how a broad medical education and the integration of basic and clinical neuroscience research is a fruitful ground for the development of new methods and knowledge in this complicated field. Important aspects for an optimal research environment are recruitment of well-educated students, a high intellectual identity of teachers and active researchers, international input and collaboration in addition to good physical resources. In depth exploration of specific signaling pathways as well as an integrative analysis of genes, molecules and systems using multivariate modeling, and bioinformatics, brain mechanisms behind mental phenomena may be understood at a basic level and will ultimately be used for the alleviation and treatment of mental disorders.

Role of N- and L-type calcium channels in depolarization-induced activation of tyrosine hydroxylase and release of norepinephrine by sympathetic cell bodies and nerve terminals

Multiple types of voltage-activated calcium (Ca(2+)) channels are present in all nerve cells examined so far; however, the underlying functional consequences of their presence is often unclear. We have examined the contribution of Ca(2+) influx through N- and L- type voltage-activated Ca(2+) channels in sympathetic neurons to the depolarization-induced activation of tyrosine hydroxylase (TH), the rate-limiting enzyme in norepinephrine (NE) synthesis, and the depolarization-induced release of NE. Superior cervical ganglia (SCG) were decentralized 4 days prior to their use to eliminate the possibility of indirect effects of depolarization via preganglionic nerve terminals. The presence of both omega-conotoxin GVIA (1 microM), a specific blocker of N-type channels, and nimodipine (1 microM), a specific blocker of L-type Ca(2+) channels, was necessary to inhibit completely the stimulation of TH activity by 55 mM K(+), indicating that Ca(2+) influx through both types of channels contributes to enzyme activation. In contrast, K(+) stimulation of TH activity in nerve fibers and terminals in the iris could be inhibited completely by omega-conotoxin GVIA alone and was unaffected by nimodipine as previously shown. K(+) stimulation of NE release from both ganglia and irises was also blocked completely when omega-conotoxin GVIA was included in the medium, while nimodipine had no significant effect in either tissue. These results indicate that particular cellular processes in specific areas of a neuron are differentially dependent on Ca(2+) influx through N- and L-type Ca(2+) channels.

Tyrosine hydroxylase and DOPA decarboxylase activities in the medical preoptic area and arcuate nucleus during the estrous cycle: effects of aging

HPLC and Palkovits' microdissection technique were used to measure activities of two catecholamine-synthesizing enzymes, tyrosine hydroxylase (TH) and dopa decarboxylase (DD), in the medial preoptic area (MPA) and arcuate nucleus (AN), both of which are involved in LH regulation. The measurements were made during an 8-h period at 1200, 1400, 1600, 1800, and 2000 h on the days of proestrus and diestrus in young (4-5-month-old) rats. Similar measurements were made at 1400, 1600, 1800, and 2000 h in middle-aged (8-10-month-old) proestrous rats and in 18-22-month-old persistently diestrous rats. For each hour (1200, 1400, etc.), five to seven rats were used. In the young proestrous rats, TH activity in the MPA increased progressively to maximum levels at 1800 h, which is approximately the time when the proestrous surge of LH is known to occur. In contrast, in the young diestrous rats, in which serum LH is known to remain stable, TH activity remained unchanged throughout the afternoon. As in the young proestrous rats, in the middle-aged proestrous rats TH activity reached a peak at 1800 h followed by a precipitous decline at 2000 h. As in the young diestrous rats, in the old persistently diestrous rats no changes in TH activity were observed. The profiles of TH activity in the AN of the four groups were essentially similar to those in the MPA. The cyclic changes in TH activity observed in this study provide a basis for the reported cyclic changes in NE activity, which, in turn, are believed to be responsible for cyclic changes in LH release. The marked deficiency and absence of changes in TH activity in the acyclic old animals corresponded to the reported marked decrease and absence of fluctuations in catecholamine activity in old age. A correlation between DD activities and catecholamine activities was not obvious, most probably due to the large number of compounds that are known to be substrates for this enzyme.

Modification of the beta-adrenergic sensitivity of rat submandibular gland by constant light

Rats exposed for 2, 5, 10, 20, 35 and 50 days to constant light (CL) showed beta-adrenergic desensitization of the submandibular gland as indicated by dose-response curves to 1.5, 2.0, 3.0, 10.0, 30.0 and 50.0 micrograms/kg isoproterenol. The phenomenon, evident with 1.5, 2.0 and 50.0 micrograms/kg after 2 days at CL, was more intense as time of exposure increased to reach values of about 50% those for controls after 20 days. After 35 and 50 days, desensitization showed some reversion, but the secretory responses were still lower than for controls maintained under a typical photoperiod. In the groups of rats exposed to CL for 20 days, gland wet and dry weights were higher (10%) than those of controls. These changes probably indicate an adaptation of the sympathetic control of salivary secretion induced by environmental illumination.

Nerve stimulation in vivo acutely increases tyrosine hydroxylase activity in the superior cervical ganglion and its end organs

Previous studies have shown that preganglionic nerve stimulation in vitro increases acutely the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, in sympathetic neuronal cell bodies in the rat superior cervical ganglion (SCG). In the present study, we have examined whether a similar increase occurs after nerve stimulation in vivo, and whether this enzyme activity also increases in sympathetic nerve terminals in autonomic end organs. Immediately following stimulation at 10 Hz for 15 min in vivo, TH activity was found to have increased 4- to 8-fold in the SCG and in 3 of its end organs: the iris, the pineal gland and the submaxillary gland. These results indicate that increases in sympathetic nerve activity in vivo can lead to increases in TH activity both in adrenergic cell bodies/dendrites in the SCG and in adrenergic nerve terminals/fibers in various autonomic end organs.

Phosphorylation of purified rat striatal tyrosine hydroxylase by Ca2+/calmodulin-dependent protein kinase II: effect of an activator protein

The phosphorylation of tyrosine hydroxylase, purified from rat striatum, was investigated using purified Ca2+/calmodulin (CaM)-dependent protein kinase II. This kinase catalyzed the Ca2+-dependent incorporation of up to 0.8 mol 32PO4/mol tyrosine hydroxylase subunit (62 kilodaltons). Reverse-phase high-performance liquid chromatography mapping of tryptic 32P-peptides established that the Ca2+/CaM-dependent protein kinase II phosphorylated a different serine residue than was phosphorylated by the cyclic AMP-dependent protein kinase. Limited proteolysis sequentially reduced the subunit Mr from 62 to 59 kilodaltons and finally to 57 kilodaltons, resulting in loss of the site phosphorylated by the Ca2+/CaM-dependent protein kinase II, but not the site phosphorylated by the cyclic AMP-dependent protein kinase. Phosphorylation by the Ca2+/CaM-dependent protein kinase II had little direct effect on the kinetic properties of tyrosine hydroxylase, but did convert it to a form that could be activated twofold by addition of an activator protein. This heat-labile activator protein increased the Vmax without affecting the Km for the pterin cofactor. This effect was specific in that the activator protein was without effect on nonphosphorylated tyrosine hydroxylase or on tyrosine hydroxylase phosphorylated by the cyclic AMP-dependent protein kinase. These results are consistent with the hypothesis that the "Vmax-type" activation of tyrosine hydroxylase observed upon depolarization of neural and adrenal tissues may be mediated by the Ca2+/CaM-dependent protein kinase II.

Synthesis and release of catecholamines by the cat carotid body in vitro: effects of hypoxic stimulation

The role of catecholamines (CAs) in cat carotid body chemoreception has been controversial. On the basis of pharmacological experiments, it would appear that endogenous dopamine (DA) may act either as an inhibitory or excitatory transmitter. Neurochemical studies on the effects of natural stimulation on the release of carotid body CAs in the cat have also been inconclusive. In the present study, we have characterized the synthesis and release of CAs in the in vitro cat carotid body preparation in response to different levels of hypoxic stimulation and have correlated these measures with the chemosensory activity of the carotid sinus nerve. The synthesis of [3H]DA and [3H]norepinephrine was linear for at least 4 h in carotid bodies incubated with their natural precursor [3H]tyrosine. Synthesis of both [3H]CAs plateaued when the [3H]tyrosine concentration in the media reached 40 microM, which is a concentration similar to that found in cat plasma. Exposure of the animals to an atmosphere of 10% O2 in N2 for 3 h prior to removal and incubation of the carotid bodies with [3H]tyrosine resulted in an approximately 100% increase in the rate of [3H]DA synthesis but no change in [3H]norepinephrine synthesis. This selective increase in [3H]DA synthesis was not detected when [3H]dihydroxyphenylalanine was used as precursor. Carotid bodies first incubated with [3H]tyrosine and later superfused with solutions equilibrated with different gas mixtures (0-100% O2 in N2) exhibited an increase in [3H]DA release and carotid sinus nerve discharge which were inversely related to the oxygen concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Reverse trans-synaptic regulation of catecholamine synthesis in adrenergic neurones

It was established that the blocking agent of beta-adrenoceptors, propanolol (1 X 10(-6)M), activates [3H] catecholamine synthesis from [3H] tyrosine in isolated rat organs (vas deferens and atrium) by 40-80%. The blocking agent of alpha-adrenoceptors, phentolamine (1 X 10(-6)M) activates [3H] catecholamine synthesis by 30-60% only in the organ possessing postsynaptic alpha-adrenoceptors (vas deferens). The activator of beta-adrenoceptors, isopropylnoradrenaline (1 X 10(-6)M), was shown to produce a decrease in [3H] catecholamine synthesis by 30-40% in both organs investigated. The substance activating alpha-adrenoceptors, phenylephrine (1 X 10(-5)M), inhibits [3H] catecholamine synthesis in the organ with postsynaptic alpha-adrenoceptors by 40-50%. Activation and inhibition of [3H] catecholamine synthesis induced by adrenotropic drugs is due to the release of chemical factors from he effector cell and their influence on the adrenergic neurone. The formation of chemical factors changing the intensity of catecholamine synthesis is related to the activation of protein synthesis in the effector cell. The processes which proceed in the adrenergic neurone are not connected with protein synthesis de novo. Existence of the common mechanism for trans-synaptic regulation of noradrenergic synthesis and uptake via the adrenoceptors of the effector cell is discussed.

Catecholamine synthesis in rabbit carotid body in vitro

1. Catecholamine synthesis in rabbit carotid body was studied in vitro using [(3)H]DOPA and [(3)H]tyrosine as precursors. The effects of sympathectomy and transection of the carotid sinus nerve on [(3)H]dopamine ([(3)H]DA) and [(3)H]noradrenaline ([(3)H]NA) synthesis were investigated in chronically denervated carotid bodies.2. When [(3)H]DOPA was used as precursor, the synthesis of [(3)H]DA was linear for more than 6 hr. The carotid body synthesized larger amounts of [(3)H]catecholamines than when [(3)H]tyrosine was used as precursor, but most of this excess was liberated into the incubation media. Using 10 muM-[(3)H]DOPA as precursor, the synthesis rates were 6.76 and 1.51 n-mole/g per hr for [(3)H]DA and [(3)H]NA, respectively; with 40 muM-[(3)H]DOPA, these values increased to 19.22 and 3.23 n-mole/g per hr, respectively.3. The relationship between [(3)H]DOPA concentration and [(3)H]DA synthesis was linear throughout the range 5-40 muM-[(3)H]DOPA.4. Sympathectomy reduced the synthesis of [(3)H]NA by 90% and [(3)H]DA by 37% when [(3)H]DOPA was used as precursor.5. When [(3)H]tyrosine (40 muM) was used as precursor, synthesis of [(3)H]catecholamines was linear for at least 4 hr, with rates of 12.10 and 0.85 n-mole/g per hr for [(3)H]DA and [(3)H]NA, respectively.6. [(3)H]DA and [(3)H]NA synthesis from [(3)H]tyrosine exhibited the characteristics of saturable processes, with K(m) values of 16.8 and 17.6 muM, respectively.7. 6-methyltetrahydropterine (6-MPH(4), 100 muM), a synthetic analogue of the natural co-factor for tyrosine hydroxylase, increased [(3)H]DA and [(3)H]NA synthesis from [(3) H]tyrosine in both the carotid body and superior cervical ganglion, with the greatest effect seen in the carotid body.8. When [(3)H]tyrosine was used as precursor, sympathectomy of the carotid body reduced [(3)H]NA synthesis by 80%, but did not alter [(3)H]DA or [(3)H]tyrosine levels in the tissue. Transection of the carotid sinus nerve had no effect on [(3)H]catecholamine synthesis in the carotid body.

Effects of hypoxia on catecholamine synthesis in rabbit carotid body in vitro

1. Unanaesthetized, unrestrained rabbits were exposed for 3 hr in a chamber to either air, hypoxic gas mixtures (10% or 14% O(2) in N(2)) or a hyperoxic gas mixture (50% O(2) in N(2)). The carotid bodies were then removed and incubated for 3 hr in modified Tyrode media equilibrated with 100% O(2) and containing either [(3)H]tyrosine or [(3)H]DOPA. The contents of [(3)H]DA and [(3)H]NA in the tissue were determined as described in the preceding paper.2. When [(3)H]DOPA was used as precursor, neither labelled dopamine (DA) or noradrenaline (NA) synthesis was increased in carotid bodies from rabbits exposed to 10% O(2) in N(2). Following exposure to 10% O(2) in N(2) and incubation with [(3)H]tyrosine, however, [(3)H]DA synthesis was increased by 72% above control (air) values while [(3)H]NA synthesis was unchanged. Less severe hypoxia, 14% O(2) in N(2), resulted in a smaller increase in [(3)H]DA synthesis, i.e. 53% above control value. Again, [(3)H]NA synthesis was unchanged. Similar experiments with the superior cervical ganglion involving exposure of the animals to either 10% or 14% O(2) in N(2) did not produce any change in the amounts of [(3)H]DA or [(3)H]NA synthesized from [(3)H]tyrosine when compared to control animals breathing air.3. Sympathectomy of the carotid body or transection of the carotid sinus nerve 12-15 days prior to hypoxic exposure (10% O(2) in N(2)) did not alter the increase in [(3)H]DA synthesis compared to normally innervated carotid bodies.4. Carotid bodies incubated with [(3)H]tyrosine for 2 hr in an alternating O(2)/N(2) sequence (5 min in media equilibrated with 100% O(2) followed by 3 min in media equilibrated with 100% N(2)) synthesized 37% more [(3)H]DA than control carotid bodies similarly exposed to an alternating O(2)/O(2) sequence. [(3)H]NA synthesis was unchanged. However, tissue levels of non-metabolized [(3)H]tyrosine were reduced by 19% in the carotid bodies exposed to the O(2)/N(2) sequence.5. Exposure of rabbits for 3 hr to 50% O(2) in N(2), followed by incubation of their carotid bodies in [(3)H]tyrosine, resulted in a 19% decrease in the absolute value for [(3)H]DA synthesis compared to control carotid bodies, but this difference was not significant (P > 0.05). However, [(3)H]NA synthesis was significantly reduced (51%; P < 0.05) in the hyperoxic carotid bodies. Similar experiments with the superior cervical ganglion showed that [(3)H]DA and [(3)H]NA synthesis were unchanged under control vs. hyperoxic conditions.6. Carotid bodies incubated with [(3)H]tyrosine for 3 hr, then transferred for 1 hr to unlabelled media equilibrated with 10% O(2) in N(2), released 81% more [(3)H]DA, and contained 38% less [(3)H]DA, than similarly treated carotid bodies exposed to 100% O(2). [(3)H]NA was not detectable in the media, and tissue levels of [(3)H]NA were the same in both hypoxic and control carotid bodies.

Whole brain tyrosine hydroxylase activity during the development of deoxycorticosterone acetate-1% sodium chloride-induced hypertension in rats

Water was chosen as the optimal medium for the extraction of tyrosine hydroxylase (TH) from rat brain. Determination of TH activity in crude homogenates failed to exhibit a linear relationship between enzyme concentration and measured activity, however, when a supernatant was used, a linear relationship existed. At a time when TH activity is maximally increased in the locus coeruleus after reserpine treatment (Reis et al 1975; Zigmond.1979) (2.5 mg kg-1 day-1 for 3 days, kill 24 h after last dose) we could detect no alterations in whole brain TH, however if treatment continued for 4 days and animals were killed 72 h after the last dose it was possible to detect increases in TH activity in various brain regions. These results suggest that local changes in brain TH activity are not revealed in measurements made on whole brain. The early rise in blood pressure, following the administration of deoxycorticosterone acetate (doca) and 1% NaCl to male Wistar rats, was accompanied by bradycardia. Whole brain Th activity was determined in these hypertensive animals 6-21 days after the commencement of treatment and the results failed to confirm the reported elevation of TH activity (Rylett et al 1976). The results are discussed with reference to the TH assay employed.

Effect of cold exposure on peroxidase and iodinase activities of rat submandibulary gland

Exposure of rats to 6 +/- 2 degrees C for 3 h caused an increase in peroxidase activity (55%) and tyrosine iodinase activity (40%) of the submandibulary glands. Thiocyanate, hydrogen peroxide and formation of triiodide from iodide were also elevated under the same conditions. Administration of alpha-receptor blockers and indomethacin prevented the rise of peroxidase activity during cold stress whereas beta-receptor blockers were less effective.

Adrenal depletion during hypoglycemia without participation of the Golgi apparatus

Adrenal depletion in rats due to hypoglycemia was effected by prolonged administration of adrenaline, without artificial increase in insulin levels. In spite of significantly enhanced catecholamine turnover in this stadium Golgi complexes - which are usually reported to develop hypertrophy during adrenal depletion using insulin induced hypoglycaemia - could never be observed. This leads to the conclusions, that firstly the presence of hypertrophic Golgi complexes is not necessarily linked with increased catecholamine turnover and secondly that permanent adrenalin application suppresses and insulin application promotes the formation of Golgi systems. Moreover, our electromicrographs do strongly suggest, that catecholamine turnover is linked with the rough endoplasmatic reticulum.

Tyrosine hydroxylase activation and inactivation by protein phosphorylation conditions

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, catalyzes the conversion of tyrosine to DOPA, Cyclic AMP-dependent protein phosphorylation conditions alter tyrosine hydroxylase activity in rat striatal homogenates. In agreement with other laboratories, we find that short-term pre-incubation (3 min) of extracts under phosphorylating conditions (Mg . ATP, cAMP) increases enzyme activity two- to tenfold over control as measured during a subsequent 15-min assay. We now report that preincubation under phosphorylating conditions for longer periods (30 min) results in a loss of activity to levels equal to or below that of the control enzyme. Addition of purified bovine brain protein kinase catalytic subunit and Mg . ATP enhances activation and increases the rate of inactivation. To demonstrate that inactivation is not associated with proteolytic degradation or irreversible denaturation, the inactivated form of the enzyme can be reactivated. The protein kinase inhibitor protein decreases the activation process and prevents inactivation of the enzyme to below control values. The sedimentation coefficient is not changed by phosphorylation conditions (S = 8.8 +/- 0.1). Although the apparent Km of the enzyme for the 6-methyltetrahydropterine (6-MPH4) cofactor is reduced (0.86 mM, control; 0.32 mM, activated), it is also reduced in the inactivated form (0.38 mM). The Ki for dopamine is increased from 4.5 microM for the control to 28 microM for the activated enzyme, whereas the inactivated form of the enzyme exhibits a Ki of 10 microM. Removal of catecholamines by gel filtration fails to alter activity and the apparent cofactor Km. Moreover, both the activated and the inactivated states persist following gel filtration. It therefore appears that the activation-inactivation process is not mediated solely by the modulation of enzyme feedback inhibition or changes in the Km for 6-MPH4. We also describe a coupled decarboxylase assay in which labeled dopamine is resolved from the precursors tyrosine and DOPA by low-voltage paper electrophoresis.

Synthesis of catecholamines from [3H]tyrosine in brain during the performance of operant behavior

Changes in the synthesis rates of dopamine (DA) and norepinephrine (NE) in brain were found to accompany the performance of positively reinforced operant behavior in rats. Catecholamine synthesis rates were estimated at short time intervals after the intraventricular injection of [3H]tyrosine with use of a conversion index which expressed the levels of [3H]DA or [3H]NE accumulated in various brain regions as a function of the specific activity of [3H]tyrosine. In rats lever pressing on a fixed ratio 5 schedule of water reinforcement, the DA conversion index in the caudate putamen was 66% higher than in control rats but was not different from control values in the mesolimbic area and hypothalamus. The NE conversion index in operant-performing animals was 48% higher in the hypothalamus than in controls but was unchanged in the mesolimbic area, telencephalon and brainstem. The changes in the CA conversion indices seen during operant performance were associated with increases in the brain levels of [3H]DA or [3H]NE, but were not accompanied by alterations in either the specific activity of [3H]tyrosine or the endogenous levels of catecholamines. The relative increment in the DA conversion index seen in the caudate putamen of operant-performing rats was directly proportional to the number of lever presses emitted following [3H]tyrosine administration. These findings indicate that the performance of operant behavior is associated with increases in rate of catecholamine synthesis within select populations of central neurons.

Thyroid cold acclimation influences on norepinephrine metabolism in brown fat

Norepinephrine turnover rates and tyrosine hydroxylase activities were determined in the interscapular brown fat pad of the rat during cold acclimation, hyperthyroxinism, and after thyroidectomy. Rats were cold acclimated by placement in a cold room, one rat to a cage, for a period of 6 wk. Hyperthyroxinism was induced by daily subcutaneous injections of L-thyroxine (1 mg/kg) for 6 days. Norepinephrine turnover rate and enzyme activity were determined at the end of each experimental period and at 8 wk after thyroidectomy. The rate of norepinephrine turnover increased during cold acclimation and hyperthyroxinism and decreased after thyroidectomy. Cold acclimation resulted in a significant increase in tyrosine hydroxylase activity, whereas no significant effect on enzyme activity was observed in hyperthyroxinism or after thyroidectomy. None of the conditions produced a change compared to controls in the apparent Km of tyrosine hydroxylase for L-tyrosine. Cold acclimation resulted in a significant decrease in the apparent Km of tyrosine hydroxylase for pterin cofactor, whereas thyroxine treatment and thyroidectomy had no effect.

Effect of the activation of alpha-adrenoreceptors on the synthesis and release of noradrenaline by peripheral adrenergic nerves in vivo

The synthesis and release of noradrenaline (NA) in the heart and submaxillary glands were studied in the rat following s.c. injections of oxymetazoline (50 mug/kg) or noradrenaline (500 mug/kg). NA release was evaluated from the decline in tissular specific radioactivity after administration of 3H-NA and NA synthesis by the estimation of the amounts of 3H-NA synthesized from 3H-tyrosine (TY) or 3H-Dopa, 30 min after the injection. Oxymetazoline treatment delayed the release of NA, the NA biological half-lives rising from 12 up to 36 hours in the heart and from 5.9 up to 21 hours in sub-maxillary glands. This inhibitory effect on NA release was interpreted as the consequence of the stimulation of alpha-adrenoreceptors. Thirty minutes after its injection, oxymetazoline increased both NA endogenous levels and 3H-NA amounts formed from 3H-TY: 3H-NA specific activities were not significantly altered. NA treatment led to an acceleration of NA release in the heart (NA biological half-life decreasing from 12 to 2.2 hours) but not in sub-maxillary glands. After injection of 3H-TY, the amounts of 3H-NA found in the heart and sub-maxillary glands were strongly reduced. Similar results were observed in the heart using 3H-Dopa as a precursor. These data are interpreted as the consequence of the removal of the newly synthesized 3H-NA by exogenous NA. The results obtained with oxymetazoline point out a dissociation between the NA release which is reduced and the NA synthesis which is unaltered. This indicates that NA synthesis rate by sympathetic nerve terminals is not immediately regulated by its release intensity. These data do not support the end-product feedback inhibition hypothesis according to which tyrosine hydroxylase is regulated by the intraneuronal NA concentration.

Endogenous levels of catecholamines in the rat myocardium following exposure to stress

Following exposure of male rats to irregular signalled footshock from which they could escape, the endogenous level of norepinephrine in the myocardium was significantly reduced. In atria the depletion of endogenous norepinephrine was significant following 1 day of stress. This significant depletion was maintained following 4 days of stress but began to return towards control levels by Day 10. A similar, but less pronounced pattern was seen with the ventricles. Little if any epinephrine was detected in both control and stress atria and ventricles. From the results presented it is postulated that irregular signalled footshock results in an inhibition of neuronal uptake.

Failure of decreased serotonin uptake or monoamine oxidase inhibition to block the acceleration in brain 5-hydroxyindole synthesis that follows food consumption

The acceleration in brain serotonin synthesis produced by injecting rats with tryptophan or allowing them to consume a carbohydrate diet was not blocked by the prior elevation of brain serotonin levels (by administration of a MAO inhibitor: Lilly 51641) or by a treatment (chlorimipramine administration) that decreases impulse flow along serotoninergic neurons.

Lipid droplet accumulation in cardiac muscle cells of the bat: potential auto-toxicity of the cardiac sympathetic innervation

The previously described ability of reserpine and parachlorophenylalanine to induce the accumulation of lipid droplets in ventricular cardiac muscle cells of the bat was investigated. Lipid droplet accumulation was assessed qualitatively by light microscopy and quantitatively by morphometric analysis of electron micrographs. An hypothesis that the action of the drugs was an indirect one, mediated by the cardiac adrenergic innervation, was framed and tested. Lipid droplet accumulation occurred during a time of intense sympathetic activity, that of arousal from hibernation. The ability of the two drugs to produce the effect was antagonized by prior sympathetectomy with 6-hydroxy-dopamine. The effect was mimicked by administration of exogenous norepinephrine together with inhibitors of its catabolic enzymes, monoamine oxidase and catechol-o-methyl transferase. These observations are all consistent with the initial hypothesis and raise the possibility that endogenous norepinephrine in the cardiac sympathetic innervation might be, at least potentially, auto-toxic.

Tyrosine hydroxylase: allosteric activation induced by stimulation of central noradrenergic neurons

Electrical stimulation of the rat locus coeruleus cases about a 300% increase in the activity of the tyrosine hydroxylase prepared from the hippocampus on the stimulated side and assayed in the presence of subsaturating concentrations of tyrosine and pteridine cofactor. Addition of calcium or cAMP to soluble preparations of tyrosine hydroxylase isolated from the hippocampus produces a similar activation of tyrosine hydroxylase. The activation of tyrosine hydroxylase produced by calcium is reversed by addition of the calcium chelator, EGTA, while the activation produced by cAMP addition or by electrical stimulation of the locus coeruleus is unaffected by addition of EGTA to the assay medium. The activation of tyrosine hydroxylase produced by electrical stimulation or by addition of calcium or cAMP to the assay medium appears to be mediated in part by alterations in the kinetic properties of the enzyme. All treatment causes the enzyme to have an increased affinity for substrate and pteridine cofactor and a decreased affinity for the endproduct inhibitor, norepinephrine. These results are suggestive that the activation of tyrosine hydroxylase which occurs during periods of increased impulse flow in noradrenergic neurons may be initiated by alterations in calcium fluxes or by changes in the steady state levels of cAMP which accompany neuronal depolarization.

Tyrosine hydroxylase: activation by nerve stimulation

The synthesis of the sympathetic neurotransmitter, norepinephrine, is accelerated by electrical stimulation of the guinea pig vas deferens. The molecular mechanism responsible for this enhanced formation of transmitter is unknown but has been attributed to an increase in the activity of tyrosine hydroxylase (EC 1.14.16.2; tyrosine 3-monooxygenase) during nerve stimulation. In the present experiments, we found that crude preparations of tyrosine hydroxylase isolated from guinea pig vasa deferentia that were electrically stimulated or depolarized by potassium show an increase in activity compared with enzyme obtained from untreated paired control tissues. This increase in activity is partially antagonized by addition of the Ca(++) chelator, ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA), to the assay medium, and can be completely blocked if Ca(++) is removed from the potassium-rich medium used to depolarize the intact tissue, before preparation of the enzyme. A similar increase in enzyme activity occurs when Ca(++) ions are added directly to enzyme prepared from untreated vasa deferentia. In this instance, the activation is completely reversed by EGTA. The increase in activity produced by addition of Ca(++) to the isolated enzyme or by electrical stimulation or potassium depolarization of the tissue before isolation of the enzyme appears to be mediated by changes in the kinetic properties of tyrosine hydroxylase. All treatments appear to activate tyrosine hydroxylase by causing an increase in its affinity for substrate and pteridine cofactor and by decreasing its affinity for the end-product inhibitor, norepinephrine. These results provide direct evidence that the enhanced formation of norepinephrine seen during stimulation of sympathetically innervated tissues arises from an activation of tyrosine hydroxylase. The fact that the activation produced by nerve stimulation is mimicked by Ca(++) ions raises the intriguing possibility that the influx or mobilization of Ca(++) that accompanies nerve stimulation and that is intimately involved in release of transmitter may also participate in the activation of tyrosine hydroxylase.

Functional significance of synthesis of noradrenaline in adrenergic nerves of rat salivary glands

The secretory response and the noradrenaline content of the submaxillary gland due to continuous electrical stimulation of the cervical sympathetic trunk with 5 Hz (5 V, 2 ms) were determined in five differently treated groups of rats. Stimulation during 5 h in untreated control rats reduced the noradrenaline content to about 40% while the secretory response remained unchanged. Another group received DL-α-methyl-m-tyrosine 400 mg kg−1 i.p. 26 and 18 h before the stimulation and 200 mg kg−1 2 h before. This treatment displaced 93% of the noradrenaline content of the glands, and the secretory response stabilized after an initial decrease, at a level of about two-thirds of the normal response, possibly indicating a reduced rate of transmitter synthesis compared to normal rats. Thus, a nerve with greatly reduced amounts of active transmitter in the store can satisfy rather high demands of transmitter release for long time periods, provided that the synthesis continues. A third group of rats received DL-α-methyl-m-tyrosine in the same doses as above but 50, 42 and 26 h before the experiments, respectively. After this treatment, the noradrenaline level was reduced to about 35% of normal, yet the secretory response was even higher than normal. Probably the decreased transmitter synthesis supposed to occur following treatment with α-methyl-m-tyrosine has been reversed at this time interval into an increased synthesis rate. To a fourth group of rats 500 mg kg−1 DL-α-methyl-p-tyrosine methylester HCl (H 44/68) was injected i.p. 15 min before stimulation, resulting in a virtually complete inhibition of the tyrosine hydroxylase. Three h of stimulation reduced noradrenaline to 15%, and the secretory response disappeared. When treatment with both drugs was given, a very rapid disappearance of an initially normal response was seen. In conclusion, synthesis of new transmitter appears to be essential for a normal function of the nerve.

Effect of drugs on the synthesis of noradrenaline in guinea-pig vas deferens

1. Reserpine in vitro (10(-5)M) caused a profound inhibition (>85%) of the formation of both (14)C-catecholamine ((14)C-CA) and (14)C-dihydroxyphenylalanine ((14)C-DOPA) (in the presence of the amino acid decarboxylase inhibitor brocresine) from (14)C-tyrosine in guinea-pig vas deferens. The magnitude of the inhibition was similar for both (14)C-CA and (14)C-DOPA suggesting that the inhibition occurred primarily at the tyrosine hydroxylase step.2. One hour after in vivo treatment with reserpine (1 mg/kg) when tissue stores of noradrenaline (NA) were depleted by 50%, there was a significant inhibition of the formation of (14)C-DOPA. Twenty-four hours after such treatment, when endogenous NA could no longer be detected, synthesis of (14)C-DOPA was indistinguishable from untreated controls. However a 45% inhibition of (14)C-DOPA synthesis from (14)C-tyrosine could be produced in tissues which had been depleted of NA for 24 h or 48 h by the addition of reserpine, 10(-5)M, to the incubation medium.3. Addition of pteridine cofactor, 2-amino-6,7,-dimethyl-4-hydroxy-5,6,7,8-tetrahydropteridine, to the incubation medium in a concentration of 5 x 10(-3)M enhanced the formation of both (14)C-CA and (14)C-DOPA from (14)C-tyrosine in guinea-pig vas deferens. In 52 mM KCl Krebs-Henseleit medium (14)C-CA formation increased from 2.58+/-0.20 (nmol/g)/h to 6.35+/-0.47 (nmol/g)/h whilst (14)C-DOPA formation increased from 5.04+/-0.88 (nmol/g)/h to 11.29+/-0.59 (nmol/g)/h.4. Pteridine cofactor (5 x 10(-3)M) did not reverse the inhibition of (14)C-DOPA formation seen with reserpine (10(-5)M) in previously untreated tissues or in vasa deferentia from animals pretreated with reserpine 1 mg/kg for 24 hours. However, the inhibition did disappear in the presence of pteridine cofactor when treatment with reserpine was prolonged to 48 h and included two doses of reserpine of 2 mg/kg.5. Tyramine (5.8 x 10(-5)M) and bretylium (10(-5)M) in vitro inhibited the formation of (14)C-CA and (14)C-DOPA from (14)C-tyrosine to the same extent in guinea-pig vas deferens again indicating that their major site of action is on tyrosine hydroxylase. The inhibitory effects were reversed by pteridine cofactor.6. Synthesis of (14)C-NA from (14)C-tyrosine in calf splenic nerve was not increased by incubating the tissue in 52 mM KCl-Krebs-Henseleit solution.

Studies of the interaction of 5-hydroxytryptamine and the perivascular innervation of the guinea-pig caecum

1. The action and interaction of noradrenaline (NA), 5-hydroxytryptamine (5-HT) and the sympathetic innervation was studied in the isolated taenia of the guinea-pig caecum.2. Addition of 5-HT led to a contraction of the taenia while addition of NA or perivascular nerve stimulation resulted in relaxation. Responses to 5-HT or perivascular nerve stimulation were abolished by tetrodotoxin. Tetrodotoxin did not affect responses to applied NA. Hexamethonium and hyoscine converted the 5-HT response to a relaxation and augmented the relaxation which followed low frequency perivascular nerve stimulation. Hexamethonium and hyoscine did not affect the dose-response relationship for NA.3. Fatigue of mechanical responses of the taenia to perivascular nerve stimulation was accelerated when nerves were stimulated in the presence of 5-HT or alpha-methyl-p-tyrosine (alpha-MPT). These two agents were additive in this action.4. Reserpine, 6-hydroxydopamine and alpha-MPT all reduced the NA content of the taenia. However, only after 6-hydroxydopamine could adrenergic activity be related to NA content.5. Segments of taenia were incubated with either tritiated NA or 5-HT. An increased rate of release of radioactivity followed perivascular nerve stimulation after incubation with either substance. This release did not occur when tissue was taken from animals given reserpine or 6-hydroxydopamine.6. It is concluded that 5-HT activates neural elements exclusively while NA has a direct effect on smooth muscle. 5-HT can apparently be taken up by adrenergic axons, and appears to enter the releasable neurotransmitter pool. Since none of the actions characteristic of 5-HT are seen when it is released by adrenergic axons as a false neurotransmitter, the released amine probably fails to reach neuronal receptors for 5-HT.

Catecholamine concentrations and the activity of tyrosine hydroxylase after an increase in the concentration of tyrosine in rat tissues

The concentrations of tyrosine in rat plasma and brain were increased 2-7 fold by the administration of either L-tyrosine or cycloheximide. Under these conditions catecholamine concentrations in the brain and the heart remained unchanged even when the rats were maintained in a cold environment to increase catecholamine turnover. The data are interpreted to mean that an increase in the tyrosine concentration in the tissues does not result in an in vivo substrate inhibition of tyrosine hydroxylase.