Acute restraint stress decreases tuberoinfundibular dopaminergic neuronal activity: evidence for a differential response in male versus female rats

The basal activity o f tuberoinfundibular dopaminergic (TIDA) neurons is higher and the response of these neurons to the stimulatory actions of prolactin is greater in the female than in the male rat. In the female rat, the restraint-stress-induced increase in serum prolactin concentrations is accompanied by a concurrent decrease in the activity of TIDA neurons. The purpose of the present study was to compare these effects of restraint in male and female rats. TIDA neuronal activity was estimated by measuring the rate of dopamine (DA) synthesis (DOPA accumulation after the administration of a decarboxylase inhibitor, NSD 1015) and the rate of DA turnover (decline of DA after administration of a tyrosine hydroxylase inhibitor; alpha-methyltyrosine) in the median eminence. Thirty minutes of restraint increased serum prolactin concentrations in both male and female rats, but a greater response was observed in the females. Restraint also decreased the rates of synthesis and turnover of DA in the median eminence of the female but not the male rat. The difference in the response of TIDA neurons in male and female rats to restraint is not the consequence of neuronal differentiation resulting from neonatal androgen exposure, because restraint aso decreased the activity of TIDA neurons in androgen-sterilized female rats. The inability of restraint stress to reduce TIDA neuronal activity in the male rat appears to be the consequence of testosterone, since TIDA neurons were responsive to restraint following castration of the males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estimation of in vitro activity of tuberoinfundibular dopaminergic neurons by measurement of DOPA synthesis in the median eminence of hypothalamic slices

A new method for estimation of in vitro neurosecretory activity of tuberoinfundibular dopaminergic (TIDA) neurons was developed by measuring the rate of synthesis of dihydroxyphenylalanine (DOPA) in the median eminence of hypothalamic slices. Sagittal hypothalamic slices of ovariectomized rats were incubated in a medium containing 3-hydroxybenzylhydrazine (NSD 1015), an inhibitor of DOPA decarboxylase. DOPA accumulated in the median eminence following incubation with NSD 1015 was determined by high-performance liquid chromatography with electro-chemical detection. The amount of DOPA accumulated in vitro in the median eminence was maximal in a medium containing 10 mM NSD 1015 and linear up to 120 min at 37 degrees C. Increasing the concentration of tyrosine in medium stimulated the synthesis of DOPA in the median eminence. The synthesis of DOPA was blocked by 1 mM alpha-methyltyrosine, an inhibitor of tyrosine hydroxylase. The rate of in vitro synthesis of DOPA in the median eminence was 33% of that of in vivo synthesis. Incubation in a medium containing 50 mM K+ to depolarize neurons caused a 2.4-fold increase in DOPA synthesis in the median eminence. The high K+-induced increase in DOPA synthesis was blocked by omission of Ca2+ and addition of 1 mM EGTA into the medium, suggesting Ca2+ dependency of depolarization-activated DOPA synthesis. These results indicate that this in vitro assay is a useful means to study the regulatory mechanisms of TIDA neurons.     

Direct inhibitory effect of long term estradiol treatment on dopamine synthesis in tuberoinfundibular dopaminergic neurons: in vitro studies using hypothalamic slices

The mechanism of the inhibitory effect of long term treatment with estradiol on dopamine synthesis in tuberoinfundibular dopaminergic (TIDA) neurons was studied by using hypothalamic slices from ovariectomized rats. Treatment with 2 mg estradiol valerate (EV) at a 3-week interval increased the weight of the anterior pituitary gland and the concentration of serum PRL. In vivo and in vitro dopamine synthesis in TIDA neurons were estimated in EV-treated animals by 3,4-dihydroxyphenylalanine (DOPA) accumulation in the median eminence after injections of 3-hydroxybenzylhydrazine (NSD 1015), a DOPA decarboxylase inhibitor, and after incubation of hypothalamic slices with NSD 1015, respectively. In vivo DOPA accumulation in the median eminence was less in EV-treated rats than in control rats. The basal rate of in vitro DOPA accumulation in the median eminence of hypothalamic slices from EV-treated rats was lower than that in control rats. Ca2+-dependent DOPA accumulation in the median eminence, determined by incubation in medium containing depolarization agents such as 50 mM K+ and veratridine, was decreased in EV-treated rats. Furthermore, cAMP-dependent DOPA accumulation, determined by incubation with Bu2cAMP or forskolin, was also suppressed in EV-treated rats. The decreased depolarization-induced DOPA accumulation in the median eminence recovered after cessation of EV treatment. Hyperprolactinemia lasting for 6 weeks, achieved by transplantation of anterior pituitaries under the kidney capsule, increased the rate of depolarization-induced DOPA accumulation in the median eminence. On the other hand, EV treatment was effective in inhibiting depolarization-induced DOPA accumulation in hypophysectomized rats regardless of the presence of anterior pituitary transplants. These results suggest that chronically administered estradiol inhibits dopamine synthesis in TIDA neurons via a direct action on the hypothalamus and overcomes the facilitatory action of PRL on dopamine synthesis; and estradiol inhibits all three distinct systems that regulate basal, Ca2+-dependent, and cAMP-dependent dopamine synthesis in TIDA neurons.     

Comparison of the effects of ether and restraint stress on the activity of tuberoinfundibular dopaminergic neurons in female and male rats

The purpose of the present study was to characterize the acute inhibitory effects of restraint stress on the activity of tuberoinfundibular dopamine (DA) neurons as estimated by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence. The time course of the effects of two types of physical restraint (immobilization in the supine position or confinement in an acrylic cylindrical tube) was determined in unanesthetized and diethylether (ether)-exposed female and male rats. The combination of brief (2 min) exposure to ether followed by 10 and 20 min of supine restraint increased concentrations of prolactin in plasma and decreased DOPAC concentrations in median eminence of both female and male rats. Thirty minutes of supine restraint decreased DOPAC concentrations in the median eminence of female rats that were not exposed to ether, and brief exposure to ether enhanced this effect. By contrast, 30 min of supine restraint failed to alter DOPAC concentrations in the median eminence in either unanesthetized or ether-exposed male rats. Tube restraint in the absence of ether failed to alter DOPAC concentrations in the median eminence of either female or male rats; but in female rats preexposed to ether, 30 min of tube restraint decreased DOPAC concentrations in the median eminence. On the other hand, in the absence of physical restraint, 2 min ether exposure caused a transient increase in prolactin secretion and a concurrent decrease of DOPAC concentrations in median eminence of both female and male rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Restraint stress decreases the neurosecretory activity of tuberoinfundibular dopaminergic neurons in young but not in aged female rats

The regulation of prolactin secretion by tuberoinfundibular dopamine (DA) neurons appears to be altered in the aged rat: the concentration of prolactin in the serum increases and the activity of the tuberoinfundibular DA neurons decreases. In the young female rat a brief period of stress reduces the tuberoinfundibular DA neurosecretory activity and increases the secretion of prolactin. The purpose of the present study was to determine if the responsiveness of tuberoinfundibular DA neurons to restraint stress is altered in the aged female rat. The activity of these neurons was estimated from the rate of DA synthesis in their terminals in the median eminence, as measured by the rate of accumulation of dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor. Thirty minutes of restraint stress increased serum prolactin concentrations in both young (3 months) and aged (26 months) constant estrous rats, but reduced the rate of DOPA accumulation in the median eminence of only the young rats. Restraint also decreased the rate of DOPA accumulation in the median eminence of intermediate-aged rats (14 months) independently of whether the rats were exhibiting normal ovarian cycles (measured on the day of estrus) or were in a constant estrus. This suggests that the loss of ovarian cyclicity per se is not associated with the age-related change in the response of tuberoinfundibular DA neurons to restraint stress.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypoprolactinemia induced by hypophysectomy and long-term bromocriptine treatment decreases tuberoinfundibular dopaminergic neuronal activity and the responsiveness of these neurons to prolactin

The effect of long-term decreases in circulating concentrations of prolactin was determined on the responsiveness of tuberoinfundibular dopamine (DA) neurons to this hormone. The activity of these neurons in ovariectomized rats was estimated by measuring the rate of DA synthesis (DOPA accumulation after the administration of a decarboxylase inhibitor) in the median eminence at various times after serum concentrations of prolactin had been reduced by hypophysectomy or the chronic administration of a DA agonist (bromocriptine, 3 mg/kg/day). The concentration of DA in the median eminence, but not in striatum, declined progressively up to 12 days after hypophysectomy, but did not change at any time during bromocriptine treatment. On the other hand, norepinephrine concentrations in the median eminence were increased 12 days after both treatments. Within 24 h after hypophysectomy or the first injection of bromocriptine the rate of DA synthesis in the median eminence was decreased; this decrease was maintained for at least 12 days suggesting that tuberoinfundibular DA neuronal activity is normally maintained by endogenous prolactin. Intracerebroventricular (ICV) injections of prolactin (10 micrograms, 12 h prior to sacrifice) increased the rate of DA synthesis in the median eminence of control, 24-hour hypophysectomized and 24-hour bromocriptine-treated rats. After longer periods (6-12 days) of bromocriptine treatment or after hypophysectomy the responsiveness of tuberoinfundibular DA neurons to prolactin was reduced. Dose-response studies revealed that the sensitivity and magnitude of response to ICV prolactin was markedly reduced in 12-day hypophysectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rapid effects of hyperprolactinemia on basal prolactin secretion and dopamine turnover in the medial and lateral median eminence

Subcutaneous injections of ovine PRL (oPRL; 4 mg/kg) were used to study the negative feedback of PRL on its own secretion in the adult male rat. A single injection of oPRL significantly suppressed the endogenous secretion of rat PRL within 3-4 h, an effect that persisted until the oPRL was substantially cleared from the circulation some 4-6 h later. In rats injected every 8 h, rat PRL levels were suppressed for 48 h, while LH titers increased significantly at some time points, and FSH levels varied in the same direction as LH. LHRH concentrations in 10 brain structures containing cell bodies, axons, and terminal boutons were not affected by 48 h of oPRL treatment. Dopamine turnover in both medial and lateral aspects of the median eminence increased as early as 2 h after the first oPRL injection and remained elevated after 10 and 26 h of oPRL exposure. The results are consistent with the hypothesis that the tuberoinfundibular dopamine neurons mediate the negative feedback action of PRL on its own secretion. Further, under these experimental conditions, neurons projecting to both medial and lateral aspects of the median eminence are equally sensitive to elevated PRL levels.     

Lack of evidence for an inhibitory role played by tuberoinfundibular dopaminergic neurons on TSH secretion in the rat.

The role of dopamine (DA) in the control of thyroid stimulating hormone (TSH) secretion in basal or cold stimulated conditions was investigated by using pharmacological or neurosurgical tools. The intraventricular injection of DA (5 micrograms/animal) or the subcutaneus (s.c.) injection of a dopaminomimetic agent failed to induce changes of TSH plasma levels in normal or in cold stimulated conditions. The same results were obtained by intraperitoneal (i.p.) administration of haloperidol, a blocker of dopaminergic receptors. The complete deafferentation of hypothalamus, which causes degeneration of norepinephrinergic nerve endings and leaves the DA tuberoinfundibular system unaffected, prevented the TSH release evoked by cold exposure. alpha-Methyl-p-tyrosine (alpha-MpT) (250 mg/kg i.p.), which causes a remarkable reduction of DA in the median eminence (ME) of deafferented animals, was unable to restore the TSH response to cold. Collectively these results seem to suggest that DA does not play a significative role in the control of TSH secretion in the rat.     

Enkephalin inhibits dopamine synthesis in vitro in the median eminence portion of rat hypothalamic slices

The effect of enkephalin on dopamine synthesis in vitro in tuberoinfundibular dopaminergic (TIDA) neurons was investigated in rat hypothalamic slices. Dopamine synthesis in vitro in TIDA neurons was estimated by 3,4-dihydroxyphenylalanine (DOPA) accumulation in the median eminence after incubation of slices with a DOPA decarboxylase inhibitor. The enkephalin agonist [D-Ala2]Met-enkephalinamide (ENKamide) decreased the rate of basal DOPA accumulation in the median eminence portion of hypothalamic slices from ovariectomized rats at concentrations over 2 microM. The inhibitory action of ENKamide was more pronounced in hypothalamic slices from haloperidol-treated rats in which basal DOPA accumulation in the median eminence was stimulated by increased PRL secretion. In contrast, ENKamide decreased neither the rate of depolarization- induced CA2+-dependent DOPA accumulation nor the rate of (Bu)2cAMP- or forskolin-induced DOPA accumulation in the median eminence of normal or haloperidol-treated rats. The rank order of the potencies of enkephalins and their analogs for inhibition of DOPA accumulation in the median eminence was similar to that of their binding capacities for opioid receptors. ENKamide inhibited basal DOPA accumulation even when hypothalamic slices were incubated in Ca2+-free medium to which tetrodotoxin was added or when the median eminence was incubated alone without the remainder of the hypothalamic slice. These results suggest that enkephalin, by acting directly on axon terminals of TIDA neurons in the median eminence, inhibits basal dopamine synthesis.     

Characterization of in vitro dopamine synthesis in the median eminence of rats with haloperidol-induced hyperprolactinemia and bromocriptine-induced hypoprolactinemia

In order to investigate the mechanism of PRL action on dopamine synthesis in tuberoinfundibular dopaminergic (TIDA) neurons, in vitro dopamine synthesis in the median eminence of hypothalamic slices was compared between hyperprolactinemic and hypoprolactinemic rats, Hyper- and hypoprolactinemia were induced in ovariectomized rats by repetitive injections of the dopamine antagonist haloperidol (Halo) and the dopamine agonist bromocriptine (Bromo), respectively. In vitro dopamine synthesis in TIDA neurons was estimated by measuring 3,4-dihydroxyphenylalanine (DOPA) accumulated in the median eminence after incubation of hypothalamic slices with a DOPA decarboxylase inhibitor. Treatment with Halo or Bromo produced increases or decreases, respectively, in the concentration of PRL in serum and in in vivo DOPA accumulation in the median eminence, as compared with vehicle treatment. The basal rate of in vitro DOPA accumulation in the median eminence was increased in Halo-treated rats and was decreased in Bromo-treated rats. The increase in basal DOPA accumulation after Halo treatment was inhibited by Ca2+ removal from medium or tetrodotoxin addition. A CA2+ -dependent increase in DOPA accumulation in the median eminence by depolarization was greater in Halo-treated rats than in Bromo-treated rats. This difference in DOPA accumulation was due to the changes in PRL secretion after Halo and Bromo treatments, since hypophysectomy abolished it. Incubation of hypothalamic slices in Na+-free media to increase the intracellular concentration of Ca2+ through inhibition of Na+-Ca2+ exchange caused an increase in DOPA accumulation. The rate of DOPA accumulation in Na+-free media was increased in Halo-treated rats and was decreased in Bromo-treated rats. On the other hand, neither Halo nor Bromo treatment altered the increase in DOPA accumulation induced by (Bu)2cAMP or forskolin. These results support the view that PRL stimulates dopamine synthesis in TIDA neurons by mechanisms which include an increase in the firing rate of TIDA neurons and increased depolarization-induced synthesis due to an enhanced response of the component that regulates dopamine synthesis to intracellular Ca2+.     

Hypoprolactinemia decreases tyrosine hydroxylase activity in the tuberoinfundibular dopaminergic neurons acutely by protein dephosphorylation and chronically by changes in gene expression

This study evaluated the roles of protein dephosphorylation or suppressed gene expression in reducing tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons after acute or chronic bromocriptine (BROMO) administration. Diestrous or ovariectomized rats were injected with BROMO (3 mg/kg, s.c.) at 1000 h and were sacrificed 4 h later or were injected with BROMO at 12 h intervals for 3 days.In vitro tyrosine hydroxylase activity was assessed by incubating hypothalamic explants with brocresine, an L-aromatic amino acid decarboxylase inhibitor, and measuring 3,4-dihydroxyphenylalanine (DOPA) accumulation in the stalk-median eminence (SME). The incubation medium also contained either 2 μM okadaic acid, a protein phosphatase 1 and 2A inhibitor, or its vehicle (0.25% dimethylsulfoxide). Acute (4 h) and chronic (3 days) BROMO treatment suppressed circulating PRL levels from 10–12 ng/ml to<1 ng/ml and reduced tyrosine hydroxylase activity in the SME by 60% or 40% in diestrous or ovariectomized rats, respectively. Okadaic acid increased tyrosine hydroxylase activity in the SME 2-fold in control diestrous or ovariectomized rats. The reduced tyrosine hydroxylase activity in the SME after acute BROMO treatment was increased by okadaic acid 5-or 3-fold in diestrous or ovariectomized rats respectively, to a value similar to the controls. In sharp contrast, after chronic BROMO treatment, the response to okadaic acid was blunted. As assessed byin situ hybridization, tyrosine hydroxylase mRNA signal levels in the arcuate nucleus of diestrous rats were not altered after acute BROMO treatment, but were reduced by 70% after chronic BROMO treatment. The acute BROMO-induced decrease in tyrosine hydroxylase activity was reversed by co-administration of oPRL or rPRL, indicating that the action of BROMO is via a reduction in PRL. The data suggest that protein dephosphorylation may be a primary mechanism for the rapid BROMO-dependent suppression of tyrosine hydroxylase activity, whereas suppression of tyrosine hydroxylase gene expression may contribute to the lower tyrosine hydroxylase activity after chronic BROMO treatment.     

The lateral and medial median eminence: distribution of dopamine, norepinephrine, and luteinizing hormone-releasing hormone and the effect of prolactin on catecholamine turnover

In the adult male rat, we have succeeded in microdissecting the median eminence into a LHRH-rich lateral region (MEl) and a LHRH-poor medial region (MEm). Dissected in this manner, the MEm has a 4.8-fold lower LHRH concentration and a 1.5-fold higher DA concentration than the MEl. The concentration of norepinephrine (NE) is not different in the two regions. Estimates of the rate constants for dopamine (DA) and NE decay after synthesis inhibition revealed no significant differences between the MEl and MEm kinetics for either amine. Hyperprolactinemia, produced by ovine PRL administration, resulted in marked increases in DA turnover rates in both the MEm (2.7-fold) and the MEl (4.7-fold). These effects of PRL were specific to the tuberoinfundibular DA neurons, as DA turnover was unaffected in nigrostriatal, mesolimbic, and incertohypothalamic DA neurons, and no changes were observed in NE turnover in any study area. The results support the involvement of PRL in the short loop feedback regulation of its own secretion in the MEm. The data further demonstrate a hypothalamic mechanism in the MEl by which hyperprolactinemia could inhibit LHRH release.     

Thyroidectomy and central catecholamine neurons of the male rat. Evidence for the existence of an inhibitory dopaminergic mechanism in the external layer of the median eminence and for a facilitatory noradrenergic mechanism in the paraventricular hypothalamic nucleus regulating TSH secretion

Using the Falck-Hillarp methodology in combination with quantitative microfluorimetry, catecholamine (CA) levels and utilization in discrete hypothalamic CA nerve terminal systems in the male rat have been analyzed 24 h, 1, 2 and 4 weeks following thyroidectomy as well as after T3 or T4 restitution therapy 4 weeks after thyroidectomy. By means of high pressure liquid chromatography (HPLC), dopamine (DA) and noradrenaline (NA) levels and utilization 4 weeks after thyroidectomy have been analyzed in various brain regions. Triiodothyronine treatment (10 micrograms/kg, s.c., twice daily during 10 days) of 4-week athyroidic rats increased serum T3 levels, but not T4 serum levels. Thyroxine treatment (36 micrograms/kg, s.c., twice daily during 10 days) of 4-week athyroidic rats increased serum T4 levels and the T3 levels were found to be even slightly higher than those found in normal animals. Triiodothyronine or T4 restitution therapy reversed the changes induced by thyroidectomy on the anterior pituitary hormones (TSH, prolactin and growth hormone) and corticosterone secretion. It is suggested that removal of thyroid hormones may be responsible for the changes in the anterior pituitary hormones and corticosterone secretions. In the quantitative microfluorimetrical analysis 24 h, 1, 2 and 4 weeks after thyroidectomy, decreases in DA levels and utilization and increases in NA levels and utilization were found in the lateral palisade zone (LPZ) of the median eminence and in the parvocellular and magnocellular parts of the paraventricular hypothalamic nucleus (PA), respectively. In addition, 1-,2- and 4-week decreases were found in DA levels and turnover in the medial palisade zone (MPZ) as well as in NA turnover in the dorsomedial hypothalamic nucleus (DM) and in the 'border zone' (BZ) between the medial and lateral hypothalamus ventral to the fornix. In the HPLC analysis it could be demonstrated that 4 weeks after thyroidectomy decreases in DA levels and utilization in the mediobasal hypothalamus and increases in DA levels as well as NA levels and utilization in the hypothalamus had developed. The T3 or T4 restitution therapies after 4 weeks of thyroidectomy counteracted the effects on CA levels and utilization in all hypothalamic CA nerve terminal systems except for the reduced NA utilization found in the DM following 4 weeks after thyroidectomy.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence for protein kinase-C mediation of the neurotensin-induced activation of tyrosine hydroxylase in tuberoinfundibular dopaminergic neurons.

The purpose of the present study was to determine whether neurotensin acts within the arcuate nucleus/median eminence to activate tyrosine hydroxylase (TH) within tuberoinfundibular dopamine neurons. The role of Ca2+/phospholipid-dependent protein kinase (protein kinase-C) in the regulation of TH and its involvement in the neurotensin-induced activation of TH within tuberoinfundibular dopamine (TIDA) neurons also was investigated. The activity of TH within TIDA neurons was assessed by quantification of the formation of 3,4-dihydroxyphenylalanine in the arcuate nucleus/median eminence after inhibition of 3,4-dihydroxyphenylalanine decarboxylase. Neurotensin (0.1-10 nM) increased the activity of TH within the arcuate nucleus/median eminence under in vitro conditions by approximately 80%. The activity of TH in the arcuate nucleus/median eminence also was increased approximately 55% by the phorbol ester 12-O-tetradecanoyl(phorbol-13-acetate) (1-100 nM), which activates protein kinase-C. Sphingosine (10 microM), an inhibitor of protein kinase-C, attenuated the activation of TH within TIDA neurons that was induced by both 12-O-tetradecanoyl(phorbol-13-acetate) and neurotensin. Sphingosine alone did not alter the activity of TH, nor did it alter the (Bu)2cAMP-induced activation of TH in the arcuate nucleus/median eminence. It is concluded that neurotensin acts directly within the arcuate nucleus/median eminence to activate TIDA neurons. Furthermore, it is suggested that the activity of TH within these neurons is enhanced after the activation of protein kinase-C and that protein kinase-C may mediate the neurotensin-induced activation of TH within these hypothalamic dopamine neurons.     

Differential decrease in the rate of dopamine synthesis in several dopaminergic neurons of aged rat brain

The purpose of this study was to investigate the in vivo rate of dopamine (DA) synthesis in dopaminergic neurons of the brain related to motor disturbances observed in aged rats. Aged rats of both sexes showed a decrease in spontaneous motor activity during a dark period as compared with those of mature rats. The in vivo rate of DA synthesis, as reflected in DOPA accumulation, in the striatum and nucleus accumbens, was slower in aged than in mature rats, whereas in the olfactory tubercle there was no significant difference between them. This suggests a differential vulnerability of dopaminergic neurons in the extrapyramidal motor areas of aged animals. After DA receptor blockade by haloperidol, increases in DA synthesis in the striatum, nucleus accumbens and olfactory tubercle were similar in both aged and mature rats, suggesting that there may be no age-related change in inhibitory regulation of in vivo DA synthesis mediated by presynaptic DA autoreceptors and/or a neuronal feedback mechanism via postsynaptic DA receptors in the striatum and mesolimbic DA regions.     

Regulation of the tuberoinfundibular and nigrostriatal systems. Evidence for different kinds of dopaminergic neurons in the brain.

The tuberoinfundibular dopaminergic (TIDA) neurons appear to present remarkable differences from the nigrostriatal and mesocortical dopaminergic (DA) systems. The hormonal feedback of prolactin on TIDA neurons differs from the neuronal feedback system operating in the striatum and olfactory tubercles. The uptake of dopamine appears to present the characteristics of a high affinity process in the terminals of the striatum and olfactory tubercle, whereas in the median eminence this process exhibits a much lower affinity. The pharmacological properties of postsynaptic receptors in the different DA pathways appear to be similar. However, the intracellular events which are ultimately manifested as physiological responses to dopamine may be completely different in the postsynaptic cells in the neostriatum and in the lactotrophs of the anterior pituitary. In addition, there is evidence suggesting that dopamine receptors in the anterior pituitary are more sensitive to the actions of agonists and antagonists than are dopamine receptors in the striatum.     

Identification of dopamine and growth hormone-releasing factor-containing neurons projecting to the median eminence of the rat by combined retrograde tracing and immunohistochemistry.

The topographical location of tyrosine hydroxylase (TH) neurons, a marker for dopamine neurons in the hypothalamus, that project to the median eminence was identified with immunofluorescence staining in combination with retrograde transblue, indicating that they projected to the median eminence. Only a few retrogradely labeled TH cells were observed in the periventricular nucleus and the lateral basal hypothalamus. The elution restaining procedure revealed that an average of 32% of the labeled TH cells in the ventrolateral portion of the ARC contained GRF. These findings support the hypothesis of co-expression of dopamine with GRF from the ARC.     

Calmodulin and a cyclic nucleotide-dependent protein kinase facilitate the prolactin-induced increase in tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons

Many aspects of tuberoinfundibular dopaminergic neuronal function are increased by elevated prolactin (PRL) levels, including the activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of dopamine. This study evaluated the roles of calmodulin, cyclic nucleotide-dependent protein kinase, and calcium/calmodulin-dependent protein kinase II in the PRL-induced increase in tyrosine hydroxylase activity. Ovariectomizerd rats were treated with haloperidol or ovine PRL (oPRL) for 20–30 h before the experiment, respectively. Treatment with haloperidol increased circulating PRL levels 8-fold and tyrosine hydroxylase activity in the stalk-median eminence 1.8-fold. Treatment with oPRL increased tyrosine hydroxylase activity 1.9-fold. W-7, a calmodulin antagonist, reversed both the haloperidol- and oPRL-induced increase in tyrosine hydroxylase activity to control levels. H-8, a cyclic nucleotide-dependent protein kinase inhibitor, also reversed the haloperidol induced increase in tyrosine hydroxylase activity. KN62, a selective calcium/calmodulin-dependent protein kinase II inhibitor, attenuated the haloperidol-induced increase in tyrosine hydroxylase activity, but KNO4, a structurally related control compound, had no effect. By contrast, the oPRL- and haloperidol-induced increases in tyrosine hydroxylase activity were not altered by KN93, a selective calcium/calmodulin-dependent protein kinase II inhibitor. These data indicate that calmodulin and a cyclic nucleotide-dependent protein kinase contribute to the PRL-induced increase in tyrosine hydroxylase activity, but the role of calcium/calmodulin-dependent protein kinase II is still unclear.