Changes In Tyrosine Hydroxylase Research Articles

Effects of long-term treatment with antidepressant drugs on proopiomelanocortin and neuropeptide Y mRNA expression in the hypothalamic arcuate nucleus of rats.

Antidepressant drugs have in common a delayed onset of clinical efficacy. In rats, long-term, daily administration of four different types of clinically effective antidepressant drugs results in decreased corticotropin releasing hormone (CRH) mRNA expression levels in the hypothalamic paraventricular nucleus (PVN). Because a subpopulation of neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons in the hypothalamic arcuate nucleus (Arc) projects to the PVN, we measured NPY and POMC mRNA expression in the Arc using in situ hybridization histochemistry at several time points following daily administration of four different antidepressant drugs. After 14 and 56 days of imipramine treatment, Arc NPY mRNA levels are decreased to 85% and 75% of control levels, but are unchanged compared to control after one or five days of treatment. Arc POMC mRNA levels are unchanged compared to controls at 1, 5, 14, or 56 days following imipramine treatment. Unlike after imipramine, Arc NPY and POMC mRNA levels are increased significantly to 134-172% of control following 56-day treatment with the antidepressant drugs fluoxetine, phenelzine, or idazoxan. The divergent effects of imipramine vs the other 3 antidepressant drugs on Arc NPY mRNA expression are similar to the pattern of changes in tyrosine hydroxylase (TH) mRNA expression levels in the locus coeruleus (LC) using the same experimental paradigm, but are different from the unidirectional depressive effects of all four drugs on CRH mRNA expression in the PVN. Thus, the Arc NPY and LC noradrenergic systems may act coordinately in mediating antidepressant effects. The present data are consistent with the delayed onset of clinical efficacy for antidepressant drugs, and suggest that Arc NPY and POMC neurotransmitter systems play a role in the pathophysiology of depression.

Effects of unilateral 6-hydroxydopamine lesions on neuropeptide immunoreactivity in the basal ganglia of the common marmoset, Callithrix jacchus, a quantitative immunohistochemical analysis

Previous immunocytochemical studies in rats have indicated that striatal dopamine depletion leads to an increase in enkephalin-immunoreactivity and a decrease in substance P-immunoreactivity in the striatum. Similar studies in primates have lead to contradictory results. In the present study changes in tyrosine hydroxylase-, met-enkephalin- and substance P-immunoreactivity were determined in the basal ganglia of 6 common marmosets Callithrix jacchus following dopamine depletion by unilateral intracerebral 6-hydroxydopamine (6-OHDA) injections using three different survival times. The non-lesioned side served as an intra-individual control. Tyrosine hydroxylase immunoreactivity was strongly reduced in the entire ipsilateral striatum. Enkephalin-immunoreactivity was increased throughout the striatum. Substance P-immunoreactivity was significantly increased in only one case in the caudate nucleus and in two cases in the putamen, while in other cases either a non-significant increase or decrease was found. Therefore, the results of the present study indicate that in marmosets dopamine has a inhibiting effect on the levels of striatal enkephalin, while its effect on substance P (SP) appears to be absent.

A point mutation in the tyrosine hydroxylase gene associated with Segawa's syndrome.

We have examined the molecular basis of Segawa's syndrome in six families with seven affected children. In one family two siblings with this disease carried a point mutation in exon 11 of the tyrosine hydroxylase gene, resulting in an amino acid exchange of Gln381 to Lys381. These results suggest that a change in tyrosine hydroxylase causes this form of Segawa's syndrome.

Inactivation of phosphorylated rat tyrosine hydroxylase by ascorbate in vitro.

Tyrosine hydroxylase activity is reversibly controlled by the actions of several protein kinases. Previous studies showed that, following phosphorylation by protein kinase A, physiological concentrations of ascorbate irreversibly inactivate tyrosine hydroxylase. Several studies were performed to establish the mechanism of inactivation. We found that inactivation occurred under oxygen-free conditions. The results of this and other experiments suggest that oxygenated species such as superoxide or hydrogen peroxide were not required for inactivation by ascorbate. Inhibition of tyrosine hydroxylase by low concentrations of ascorbate raised the question concerning the mechanism for maintaining enzyme activity under physiological conditions. We report that tyrosine, N alpha-methyl tyrosine, 3-iodotyrosine, and phenylalanine protected the phosphorylated enzyme against ascorbate inactivation. Catecholamines (dopamine, norepinephrine, and some of their analogues) also protected the enzyme against ascorbate inactivation. We performed studies to assess conformational changes of tyrosine hydroxylase by measuring the extrinsic fluorescence using 8-anilino-1-naphthalenesulfonic acid as a reporter group. Phosphorylation of tyrosine hydroxylase by protein kinase A decreased the extrinsic fluorescence. Treatment of tyrosine hydroxylase with ascorbate produced a further decrease in fluorescence. These results provide evidence for conformational changes following these treatments. In contrast to extrinsic fluorescence, the circular dichroic spectrum of tyrosine hydroxylase failed to change following phosphorylation by protein kinase A or inhibition by ascorbate. The spectrum was consistent with a secondary structure of tyrosine hydroxylase with 55% alpha helix, 20% beta sheet, 2% beta turn, and 23% random coil.

Decreased tyrosine hydroxylase mRNA but not cholecystokinin mRNA in the pars compacta of the substantia nigra and ventral tegmental area of aged rats

Quantitative in situ hybridization histochemistry was used to determine the age-related changes in tyrosine hydroxylase (TH) mRNA and cholecystokinin (CCK) mRNA in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) of the rat. Coronal sections (10 μm) were cut in a cryostat through the VTA and SNc of brains from 3 month and 33 month old Sprague-Dawley rats and immediately adjacent sections hybridized with 35S-labelled 45-mer oligonucleotide probes specific for either the rat TH or CCK genes. The mRNA levels of each gene were estimated by computerised densitometric analysis of the signal on X-ray film autoradiograms and estimation of the number of mRNA expressing cells as well as the density of expression per cell (grain density) was made from high resolution emulsion autoradiograms. Analysis of the TH mRNA on X-ray film autoradiograms indicated that the levels averaged 25% lower in the SNc ( P < 0.01) and 18% lower in the VTA ( P < 0.05) of the old rats. However, analysis of the emulsion autoradiograms showed that this reduction in TH mRNA in the VTA and SNc in the old rats was not due to a loss of TH mRNA expressing cells but due to a reduction in the hybridization signal per expressing cell. Analysis of the CCK mRNA signal on X-ray film autoradiograms showed that the levels averaged 9 and 15% higher (but non - significant) in the VTA and SNc respectively of the old rats while analysis of the emulsion autoradiograms showed no increase in the number of expressing cells but an averaged 20% (though not significant) increase in the hybridization signal per CCK mRNA expressing cell in the VTA and SNc of the old rats. The results demonstrate that in the aged rat there is no significant loss of dopamine (DA)-containing neurones in the VTA and SNc but there is a clear decrease in the synthetic activity of the neurones and this may be responsible for the neurochemical and behavioural deficits associated with the ageing process. Since in the rat CCK is extensively colocalized with DA in neurones in both the VTA and SNc, the trend of increased activity of the CCK gene (with a concomitant decrease in TH gene activity) suggests a complex regulation of colocalized neurotransmitters possibly via autofeedback mechanisms at the transcriptional level.

Age‐Related Changes in Tyrosine Hydroxylase and Choline Acetyltransferase in Sympathetic Ganglia of a Rat Strain with Reduced Sympathetic Neuron Numbers

In Wistar rats, a subpopulation of sympathetic ganglionic neurons dies during ageing, but in the GH strain, these same neurons die during the period of perinatal maturation. We have compared tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) in superior cervical ganglia of GH and control rats at different ages. Ganglionic TH rose to near adult levels between postnatal weeks 1 and 2. No significant differences in TH values were seen between GH and control ganglia at any age, indicating that reduced neuron numbers are compensated for by increased cellular activity. Ganglionic ChAT rose initially in parallel with TH and then more slowly over postnatal weeks 3-4, reaching adult levels that were about 20% lower in GH than in normal ganglia. During ageing, TH remained constant but ChAT continued to rise slowly in GH ganglia, whereas ChAT in normal ganglia fell by about 10%. Both the strain difference in ChAT during development and the fall in ChAT during ageing in normal animals parallel the differences in ganglion cell numbers seen under these circumstances.

The vigilance-promoting drug modafinil counteracts the reduction of tyrosine hydroxylase immunoreactivity and of dopamine stores in nigrostriatal dopamine neurons in the male rat after a partial transection of the dopamine pathway

We studied the ability of the vigilance-promoting drug modafinil to modulate the anterograde and retrograde changes in tyrosine hydroxylase (TH) immunoreactivity and in dopamine (DA) stores in the nigro-neostriatal DA neurons, following a partial hemitransection of this ascending DA system, using a combined morphometrical, biochemical and behavioural analysis. Modafinil was given daily i.p. in doses of 10-100 mg/kg, starting 15 min after the lesion, and the partially hemitransected rats were killed 2 weeks later. Changes in TH-immunoreactive nerve cell bodies and nerve terminals induced by the partial hemitransection were studied in the substantia nigra and neostriatum in combination with image analysis. The substantia nigra and neostriatum were also subjected to biochemical analysis of DA, 3,4-dihydroxyphenylacetic acid and homovanillic acid levels. Modafinil treatment dose-dependently (10-100 mg/kg) counteracted the hemitransection-induced disappearance of nigral TH-immunoreactive nerve cell body profiles and neostriatal TH-immunoreactive nerve terminal profiles. A 2-week treatment with 100 mg/kg of modafinil also counteracted the hemitransection-induced depletion of DA stores in the neostriatum and the ventral midbrain. Moreover, the repeated daily treatment with modafinil (100 mg/kg) protected against the hemitransection-induced disappearance of striatal 5-hydroxytryptamine, 5-hydroxyindoleacetic acid and noradrenaline levels. Striatal DA function was analysed by studying apomorphine-induced (1 mg/kg, s.c.) ipsilateral rotational behaviour 4 and 11 days after the operation. A marked dose-dependent reduction of ipsilateral rotational behaviour was demonstrated after the daily modafinil treatment in the partially hemitransected rats. In another model involving unilateral nigral microinjections of 6-hydroxydopamine, acute (one single dose) modafinil (100 mg/kg) did not affect the contralateral rotational behaviour induced by apomorphine (0.05 mg/kg s.c.), when given 30 min before the apomorphine. Taken together, morphological, neurochemical and behavioural evidence has been obtained that anterograde and retrograde changes induced in the DA stores and TH immunoreactivity of the nigro-neostriatal DA neurons by a partial hemistransection are counteracted by modafinil in a dose dependent way with 100 mg/kg producing a significant protective action against impairment of DA transmission. The results of this study open up the possibility that modafinil may protect against the anterograde and retrograde degeneration of nigrostriatal DA neurons seen after mechanically induced injury.

Changes in tyrosine hydroxylase mRNA levels in medullary A1 and A2 neurons and locus coeruleus following castration and estrogen replacement in rats

Temporal changes in tyrosine hydroxylase (TH) mRNA levels in medullary A1 and A2 neurons and locus coeruleus (LC) cells were studied 6, 12 and 24 h following orchidectomy in rats. Brains from intact controls and sham castrated rats also were evaluated at these same time periods. In situ hybridization histochemistry and quantitative image analysis techniques were used to quantitate levels of cytoplasmic TH mRNA. Neither the time of day nor the stress of sham castration affected TH mRNA levels in A1, A2 and LC neurons. In contrast, 6 h following castration, TH mRNA levels in A1 neurons had declined significantly. Thereafter, there was a linear increase in A1 message levels such that, by 24 h, TH mRNA values were significantly higher than those obtained in intact controls. Placement of Silastic estrogen capsules immediately after castration prevented the 6 h decline in A1 message levels. At 12 h, TH mRNA levels in A1 neurons were significantly higher in estrogen-treated rats compared to those of the castrate or intact control groups. By 24 h, message levels in A1 neurons of steroid-treated rats were comparable to the intact control. Neither castration nor estrogen treatment altered TH mRNA levels in A2 neurons. TH mRNA levels in LC neurons increased significantly 6 h after castration and estrogen produced a further significant increase in message levels. Six hours later (12 h), TH mRNA values were still higher than controls but, in the estrogen-treated group, these levels had declined to those observed in the 12 h castrate group. By 24 h after castration, there were no differences in TH mRNA levels in any of the groups studied. Plasma LH concentrations increased significantly between 8 and 24 h and estrogen treatment completely blocked this post-castration rise. Moreover, we observed that drugs which disrupt the synthesis of NE (MPT, DDC) or which selectively block α 1-adrenoreceptors (prazosin) also prevented the post-castration rise in plasma LH. Earlier observations have shown that following castration, hypothalamic norepinephrine concentrations and turnover rates increase. The present studies suggest that, concomitant with changes in hypothalamic norepinephrine secretion, there is an increase in TH mRNA in A1 and LC neurons which may be a required preamble for the synthesis of the rate-limiting enzyme which is required to replenish the secreted norepinephrine.

Temporal changes in tyrosine hydroxylase mRNA levels in A1, A2 and locus ceruleus neurons following electrical stimulation of A1 noradrenergic neurons

We examined the effects of electrical stimulation (ES) of right A1 noradrenergic cells on temporal changes in tyrosine hydroxylase (TH) mRNA levels in A1, A2 and locus ceruleus (LC) neurons by in situ hybridization histochemistry and quantitative image analysis methods. The stimulation parameters used previously have been shown to increase hypothalamic norepinephrine (NE) release. Within 1 h after beginning A1 stimulation, TH mRNA levels were significantly increased and they continued to rise to reach plateau by 6 h. TH message levels at 12 h were not difference from 6 h values. A1-ES did not affect TH mRNA levels in contralateral A1 or in A2 or locus ceruleus neurons. These data suggest that changes in TH mRNA levels may serve as an index of increased A1 neuronal activity in circumstances when increases in hypothalamic NE secretion occur.

Quantitation of tyrosine hydroxylase and neuropeptide Y immunoreactivity in single rat sympathetic neurons: Effects of preganglionic nerve activity

Using computerised densitometry to measure immunocytochemical reaction product in a model system, we established conditions that produced a linear relationship between the logarithm of antigen concentration and the measured intensity of staining. We then applied the densitometric technique to assess the changes in tyrosine hydroxylase (TH) and neuropeptide tyrosine (NPY) within sympathetic neurons of rat superior cervical ganglion following chronic decentralization and following reserpine treatment. One week after surgical or pharmacological decentralization, there was appreciable reduction of neuronal levels of both TH and NPY. However, there remained considerable variation in the immunoreactivities of individual cells. Three days of treatment with reserpine elevated TH levels but substantially reduced NPY. Both these effects were prevented by prior decentralization of the ganglia. No differences were seen between normotensive and the Otago strain of genetically hypertensive rats, either in basal TH or NPY immunoreactivities or in responses to the maneuvers performed. Comparison of our findings with previous biochemical data indicate that densitometric immunocytochemistry provides an accurate index of neuronally localised antigen concentrations but also allows analysis of interneuronal differences that are not otherwise apparent.

Tyrosine hydroxylase, tryptophan hydroxylase, biopterin, and neopterin in the brain of anorexia nervosa

The activities of tyrosine hydroxylase and tryptophan hydroxylase and contents of biopterin and neopterin were measured for the first time in various regions of human brain from a patient with anorexia nervosa (AN). In AN as compared with controls, tyrosine hydroxylase activity was markedly reduced in all brain regions analyzed, while tryptophan hydroxylase activity and biopterin content had a tendency to increase. Neopterin content did not change dramatically. The opposite changes of tyrosine hydroxylase and tryptophan hydroxylase suggest an imbalance between the activity of catecholaminergic neurons and that of serotonergic neurons, and may be related to pathogenesis of AN.

Modulation of tyrosine hydroxylase gene expression in rat brain and adrenals by exposure to cold.

The long-term changes in tyrosine hydroxylase (TH) activity induced by chronic exposure to cold in brain noradrenergic neurons of the locus coeruleus (LC) were analyzed and compared to those measured in a peripheral tissue such as adrenals. This analysis was made possible at the level of one single tissue corresponding to one animal by the use of sensitive methods that allow assay of TH activity, protein, and mRNA levels in parallel from the same homogenate. The three parameters were measured in brain structures and adrenals of rats maintained at 4 degrees C during 4 days and were compared to those of control animals kept at normal housing temperature (22 degrees C). LC of rats exposed to cold contained 200% more TH mRNA than controls. The amount of TH protein in this area rose to as much as 164% that of controls. Similarly, the activity of the enzyme increased to 140% of the normal value. Thus, these observations show that 1) the increase in TH mRNA was much higher than the increase in protein levels, and that 2) the newly synthesized molecules have about the same activity as that present under normal conditions. In contrast to the LC, no variation of these parameters was observed in the substantia nigra. In the adrenals, the variations in the different parameters were qualitatively similar to that observed in the LC, although they were quantitatively higher: TH mRNA, TH protein, and TH activity levels were respectively 330%, 182%, and 167% that of control adrenals. Altogether, these results demonstrate that exposure to cold induces an alteration in TH synthesis in brain noradrenergic neurons as well as in adrenals.

Changes in tyrosine hydroxylase, glutamic acid decar☐ylase and choline acetyltransferase after local microinoculation of scrapie agent into the nigrostriatal system of the golden hamster

A microinjection of a homogenate of scrapie agent-infected brain (strain 263 K) into the nigrostriatal system in the golden hamster is followed by the progressive development of the disease which terminates by the death of animals around the 4th month postinoculation. These intracerebral inoculations induce more rapid changes in neuronal activity which can be revealed by the assessment of the specific synthesizing enzymes of neurotransmitter systems. The microinoculation of a homogenate of an infected brain unilaterally into the substantia nigra (SN) provokes a decrease in tyrosine hydroxylase (TH), the synthesizing enzyme for dopamine in the dopaminergic neurones, in the striatum ipsilateral to the injected SN. This biochemical response, specifically induced by the active pathogen, is detectable as soon as the 5th day postinoculation and is detectable towards the 80th day. A return of TH levels to control values is detected after this period. At the end of the incubation period and towards the death of the animals, TH is not different from control TH measured from intact animals. The decrease in TH is concomitant with an increase in striatal glutamic acid decar☐ylase (GAD), the synthesizing enzyme for γ-aminobutyric acid (GABA), measured 20 days postinoculation with no change in choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine. Studies of the biochemical responses associated locally to the scrapie agent inoculation have been performed at the striatal level. The intrastriatal administration of the infective agent induces 20 days postinoculation an increase in GAD with no change in TH and ChAT. Ninety days postinoculation, a decrease in GAD was detected associated with an increase in TH with no change in ChAT. The decrease in striatal TH 5 days after inoculation of the scrapie agent in the ipsilateral SN represents in the hamster an early marker of the slow progressive pathologic process which can only be revealed about 80 days postinoculation. The influence of the pathogen in the nigrostriatal system could be specific for certain categories of neurones. The cholinergic neurones are apparently not affected. The dopaminergic neurones seem not to be permanently damaged, as seen by the return of TH to control values during the development of the disease. The GABAergic neurones could represent a preferential target for the pathogen as shown by the early GAD activation and tardive decrease activity.

Differential regulation of peptide and catecholamine characters in cultured sympathetic neurons

Mechanisms regulating peptidergic, noradrenergic and cholinergic development were compared in dissociated cell cultures of neonatal rat sympathetic ganglia. The majority of cultured neurons contained at least two neurotransmitters and many neurons contained three or more. These studies were undertaken to determine whether co-existing transmitters were co-ordinately regulated by the environment. Co-culture of sympathetic neurons with ganglion non-neuronal cells increased substance P and choline acetyl- transferase activity but decreased somatostatin and tyrosine hydroxylase activity. Conversely, elimination of non-neuronal cells virtually abolished neuronal expression of substance P and choline acetyltransferase and increased somatostatin and tyrosine hydroxylase. Consequently, under these conditions, somatostatin and tyrosine hydroxylase were similarly regulated, whereas substance P was associated with choline acetyltransferase. By contrast, stimulation of adenylate cyclase or treatment with membrane-permeable adenosine 3′,5′-phosphate analogs increased tyrosine hydroxylase and decreased choline acetyltransferase, but had no effect on substance P or somatostatin levels. Moreover, potassium- or veratridine-induced membrane depolarization increased tyrosine hydroxylase but decreased substance P, somatostatin and norepinephrine levels. However, inhibition of neurotransmitter release with magnesium or calcium-free medium prevented the decrease in norepinephrine levels but not the decrease in substance P and somatostatin. Consequently, the effects of membrane depolorization on peptide levels cannot be ascribed to release and subsequent depletion of substance P and somatostatin and must result from decreased net synthesis (synthesis minus catabolism) of the transmitters. Nerve growth-factor treatment also differentially regulated transmitter metabolism; nerve growth factor increased protein-specific activities of tyrosine hydroxylase and choline acetyltransferase but did not increase the protein-specific content of substance P and somatostatin. Quantitative transmitter expression was also influenced by neuron density; increasing density elevated substance P and choline acetyltransferase activity but decreased somatostatin and tyrosine hydroxylase activity per neuron. Finally, culture of sympathetic neurons in a defined (serum-free) medium also altered some but not all traits, decreasing substance P. somatostatin and choline acetyltransferase without any change in tyrosine hydroxylase. Our observations suggest that there is no fixed relationship between any pair of the four phenotypes examined and that each phenotype appears to be independently regulated in cultured sympathetic neurons. Nevertheless, under many circumstances noradrenergic and somatostatin development were co-ordinately regulated, while substance P and choline acetyltransferase development were similar, possibly indicating some functional relationship between these pairs of transmitters.

Early changes in tyrosine hydroxylase and glutamate decarboxylase activity in the golden hamster striatum after intracerebral inoculation of the nigrostriatal system with scrapie agent (strain 263 K)

Unilateral inoculation of hamster substantia nigra (SN) with scrapie agent led to an early decrease in tyrosine hydroxylase (TH) activity in the corresponding striatum, which was detectable by the 5th day. This decrease was accompanied by an increase in glutamate decarboxylase (GAD) observed on the 20th day. Local phenomena related to administration of the agent were investigated by intrastriatal inoculation followed by local measurement of TH, GAD and choline acetyltransferase (ChAT) activities. A rise in GAD activity was observed 20 days later. The decrease in TH activity which occurred 5 days after inoculation of the substantia nigra with scrapie agent constitutes an extremely early indication in hamsters of the slow pathological processes at work: at clinical and behavioural levels, these can be detected at best only 80 days after the intracerebral inoculation.

Changes in tyrosine hydroxylase and dopamine-beta-hydroxylase activities but not in phenylethanolamine- N-methyltransferase activity within central adrenaline neurons after 6-hydroxydopamine administration

By using a new microdissection procedure allowing the noradrenaline (NA) and adrenaline (A) cell groups of the A2-C2 region to be sampled preferentially, it was possible to study the biochemical response of these two neuronal populations after 6-hydroxydopamine (6-OHDA) administration. Five days after an intraventricular 6-OHDA injection, tyrosine hydroxylase (TH) activity increased (+ 104%, P s 0.01) in the adrenergic C2 region, in the locus coeruleus (LC) and in the A1-C1 104 region, while the NA A2 region exhibited no significant increase. Twenty-one days after 6-OHDA administration, dopamine-beta-hydroxylase (DBH) activity had decreased in both the noradrenergic regions (LC, A1-C1 and A2 regions) and in the C2 adrenergic region. Conversely, phenylethanolamine- N-methyltransferase (PNMT) activity was not modified either in the cell bodies or in the terminals located in the tractus intermediolateralis of the spinal cord and in the hypothalamic nuclei. These data suggest: (i) that adrenaline-containing neurons could be sensitive to the neurotoxic action of 6-OHDA since they exhibit changes in TH and DBH activities; and (ii) that the determination of PNMT activity may not be sensitive enough to estimate the functional integrity of the A cell bodies or terminals.

Developmental patterns of tyrosine hydroxylase activity in discrete central nervous system regions and serum LH and prolactin in the prepubertal rat.

Changes in tyrosine hydroxylase (TH) activity in the whole hypothalamus and discrete nuclear regions of the limbic system were observed to occur from 10 to 40 days of age in the female rat. In the whole hypothalamus, TH activity increased between 10 and 30 days of age then decreased at 40 days of age. In the preoptic (POA) and suprachiasmatic nuclear areas (SCN), TH activity increased to peak values at 20 days then declined to baseline values at 25 days. However, an increase in TH activity of SCN was again noted at 39 days of age. TH activity in the amygdala area (AMYG) increased to peak values at 25 and 30 days with a return to baseline levels at 35 days, while in the median eminence (ME), TH activity increased at 20 days and remained elevated through 39 days of age. These results show that temporally distinct changes in TH activity occur during development in various areas of the brain. These changes may represent maturation of neuronal control systems known to be involved in adult reproductive function. Serum LH levels in the developing rat increased between days 10 and 25 then showed a precipitous drop at day 30 followed by increased values at days 35-39. Serum prolactin levels remained low through day 20, increased at days 25-30 then decreased on day 35 with a peak at day 37, prior to the onset of puberty. The inverse relationship of LH and prolactin at day 30 suggests a transient inhibitory (antigonadotropin) effect of increasing prolactin levels on LH at this time.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine hydroxylase regulation: apparent kinetic alterations following incubation of brain slices in a sodium-free medium.

In an attempt to determine if alterations in intraneuronal Ca2+ may regulate tyrosine hydroxylase activity, brain slices were subjected to experimental manipulations known to increase the intraneuronal concentration of free Ca2+ ions. Incubation of either striatal or olfactory tubercle slices in a Na+-free medium for 15 min at 37 degrees resulted in a marked increase in the activity of tyrosine hydroxylase present in the 20,000 g supernatant fraction of homogenates prepared from the slices. Tyrosine hydroxylase isolated from slices previously incubated in a Na+-free, choline-enriched medium or in a Na+-free, sucrose-enriched medium exhibited maximal activities when assayed at pH 6.0 and 7.0, respectively. However, the percentage stimulation of enzyme activity induced by incubation of the slices in a Na+-free medium was maximal when the enzyme assays were performed at pH 7.0. The observed increase in enzyme activity seems to be mediated by a decrease in the apparent Km of the enzyme for pteridine cofactor, regardless of whether the kinetic enzyme analyses were conducted at pH 6.0 or 7.0, and by an increase in the Ki of the enzyme for end-product inhibitor dopamine. The apparent kinetic changes in the enzyme do not seem to result from alterations in the endogenous dopamine content of the slices, and they are independent of any increase in dopamine release that might have occurred as a response to the augmented intraneuronal Ca2+ concentration. Furthermore, the activation of tyrosine hydroxylase produced by incubating slices in a Na+-free medium is observed even in slices depleted of dopamine by pretreatment of rats with reserpine 90 min before preparation of brain slices. The activation of tyrosine hydroxylase observed under these experimental conditions does not seem to be mediated by cAMP or by a cAMP-dependent phosphorylation process. It is suggested that the changes in tyrosine hydroxylase reported are mediated primarily by a rise in the free Ca2+ concentration within the nerve tissue. These observations are consistent with the hypothesis that the kinetic activation of tyrosine hydroxylase produced after depolarization of central dopaminergic neurons may occur through a Ca2+-dependent even other than transmitter release.

Potentiation of apomorphine action in rats by l-prolyl-l-leucyl-glycine amide

Although the antiparkinsonian activity of l-prolyl-l-leucyl-glycine amide (PLG = MIF-I) has been previously observed in several clinical trials, little is known of the mechanism of action of this tripeptide on the brain. Our study demonstrates potentiation of the action of apomorphine by PLG on the rotational behavior of mature rats which received unilateral 6-OHDA (16 μg) lesions of the striatum as neonates. No change in tyrosine hydroxylase or dopa decarboxylase activities in rat striatal homogenates was found after addition of PLG (10 −8−10 −3 M). The results suggest that PLG modifies the dopamine receptor, making it more responsive to stimulation by the agonistic agent apomorphine and perhaps by the natural neurotransmitter dopamine.

The effect of castration, thyroidectomy and haloperidol upon the turnover rates of dopamine and norepinephrine and the kinetic properties of tyrosine hydroxylase in discrete hypothalamic nuclei of the male rat

Adult male rats were either castrated, thyroidectomized, or treated with haloperidol and the rates of turnover of dopamine (DA) and norepinephrine (NE) in the median eminence (ME), the arcuate and dorsomedial nuclei of the hypothalamus were estimated from the rate of decay of DA and NE concentrations as determined by radioenzymatic assay following blockade of catecholamine synthesis byalpha-methyl- p-tyrosine. The ME of animals similarly prepared was also examined for changes in the total activity and kinetic properties of tyrosine hydroxylase (TH). Four days following the administration of haloperidol (400 μg/kg) or 10 days after castration, there was a significant increase in the rate of turnover of DA but not NE in the ME accompanied by an increase in the V max but not K m for substrate or cofactor of TH. Furthermore, the administration of haloperidol to hypophysectomized rats also significantly increased the TH activity in the ME, indicating that such changes may occur independently of any changes in serum prolactin levels. Ten days after thyroidectomy, or three weeks after treatment with propylthiouracil, there was a significant increase in the turnover rate of DA in both the ME and dorsomedial nucleus but not in the arcuate nucleus. No changes in the turnover rates of NE in anu of the three areas were observed following thyroidectomy. In the ME, the increase in turnover of DA was accompanied by an increase in the total TH activity (V max) as well as a decrease in K m for tetrahydrobiopterin but not tyrosine. From these results 4 conclusions were drawn: (1) following haloperidol, castration, and thyroidectomy there are increases in the activity of dopaminergic terminals within the ME; (2) castration, haloperidol and thyroidectomy may influence the activity of dopaminergic terminals within the ME by different mechanisms; (3) changes in tyrosine hydroxylase and turnover of catecholamines within the ME may occur independently of changes in prolactin levels; and (4) local recurrent afferent circuits may exist in the arcuate nucleus region of the hypothalamus.