Tuberoinfundibular Dopamine Neurons Research Articles

Use of Fos-related antigens (FRAs) as markers of neuronal activity: FRA changes in dopamine neurons during proestrus, pregnancy and lactation

This manuscript describes the use of staining of Fos-related antigens (FRAs) as markers for changes in neuronal activity. The model system consisted of the tuberoinfundibular dopamine (TIDA) neurons located in the arcuate nucleus of the hypothalamus. Under normal conditions, these neurons are devoid of c-Fos staining even though the neurons are tonically active and can express FRAs. During specific neuroendocrine states the neurons undergo changes in activity, as described by other studies. At times when the activity is relatively high as in pregnancy and during proestrus, approximately 50%–60% of the TIDA neurons expressed FRA immunoreactivity. Moreover changes over the course of proestrus paralleled known shifts in TIDA activity (declining as the day progressed). At times when TIDA activity is suppressed, such as during lactation, FRA staining in TIDA neurons was markedly reduced or absent. Upon removal of the suckling stimulus, FRA staining rose to reach peak expression 12–24 h after pup removal (without coordinate induction of c-Fos). These data suggest that FRA staining can serve as a useful marker of activity in the TIDA neurons which permits not only assessment of stimulated activity but also suppressed function in the neurons. A cautionary note in using this approach along with acquisition of serial blood samples for hormone measurement is that surgical procedures for monitoring plasma hormone levels are associated with strong long-lived induction of FRAs (and c-Fos) in many neurons (including the TIDA neurons) that can canfound interpretation of FRA staining.

The effect of short-photoperiod exposure on tuberoinfundibular dopamine neurons in male and female Syrian hamsters.

In male hamsters, exposure to a short photoperiod results in a significant decrease in median eminence (ME) dopamine (DA) concentrations. The mechanism responsible for this decrease in DA is unknown. The experiments described in this paper were designed to examine the effects of photoperiod on DA metabolism and synthesis in the ME to determine if a change in these processes is responsible for the short-photoperiod-induced decrease in ME DA concentrations. In the first experiment, the metabolism of DA in tuberoinfundibular dopamine (TIDA) neuronal terminals was determined by measuring ME concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC; a major metabolite of DA) and DA in male and female hamsters housed in long and short photoperiods. In both males and females, exposure to the short photoperiod induced a collapse of the reproductive system and a reduction in circulating prolactin. In males, but not in females, exposure to the short photoperiod reduced ME DA concentrations; however, DOPAC concentrations were not affected by photoperiod. Thus, the decrease in ME DA seen in males is not the result of an increase in DA metabolism. In the second experiment, tyrosine hydroxylase (TH) activity in the ME of males was determined by injecting animals housed in long and short photoperiods with a L-aromatic amino acid decarboxylase inhibitor (NSD 1015) and measuring 3,4-dihydroxyphenylalanine (DOPA). Consistent with Experiment 1, ME DA concentrations were significantly decreased in gonadally regressed males housed in a short photoperiod; however, ME DOPA accumulation was not affected. Thus, the observed decrease in DA is not the result of a decrease in TH activity in the ME. The results of the experiments presented here indicate that (1) in males but not females, the decrease in circulating prolactin seen in animals housed in a short photoperiod for 12 weeks is associated with a decrease in ME DA concentrations, and (2) the decrease in ME DA seen in males housed in a short photoperiod is not the result of an increase in DA metabolism or a decrease in synthesis by TIDA neurons.

Effects of morphine on hypothalamic tyrosine hydroxylase mRNA levels in dopaminergic neurons and on preoptic DOPAC levels measured by microdialysis

Morphine not only suppresses norepinephrine-induced increases in LHRH mRNA levels but, in these same animals, it simultaneously amplifies norepinephrine (NE)-induced LH release. These observations suggest that NE may activate parallel mechanisms which independently increase LHRH mRNA levels and LHRH release and suggest that some of these effects could be mediated indirectly via morphine's action on different components of the hypothalamic dopamine (DA) system. Accordingly, in the present studies we examined the effects of morphine on various components of this dopamine system using as our index of altered DA neuronal activity, the changes which occur in tyrosine hydroxylase (TH) mRNA levels following morphine. As an ancillary index of changes which occur in dopamine neuronal activity, we measured, by microdialysis, the changes which occur in preoptic dihydroxyphenylacetic acid (DOPAC) levels after either subcutaneous injections or following microinfusions of morphine into the zona incerta (ZI). In a final study, we evaluated whether DA when given alone (icv infusion) or prior to icv NE would altered LH release. Single cell levels of TH mRNA in preoptic A15 and periventricular anterior hypothalamic A14 DA neurons were not affected by morphine 1, 5 and 24 h later. In contrast, within 1 h after morphine, TH mRNA levels in ZI A13 neurons were significantly elevated and they remained high at 5 and 24 h compared to controls. Morphine also resulted in a significant rise in TH mRNA levels in tuberoinfundibular DA neurons (TIDA) (A12) within 1 h and these levels remained high to 5 h. Thereafter, by 24 h, message levels had returned to control values. Morphine also resulted in a rapid rise in plasma prolactin (Prl) with peak values occurring at 20 min and then returning to baseline by 90 min. When morphine was given sc it resulted, within 15 min, in a rapid rise in preoptic DOPAC levels and these levels continued to rise such that they were 217% higher than pretreatment values by 105 min. Preoptic 5-hydroxyindoleacetic acid (5-HIAA) levels also increased by 25–75% after sc morphine. The microinfusion of morphine into ZI also resulted in elevated preoptic DOPAC but not 5-HIAA levels within 15 min. The icv infusion of DA alone had no effect on plasma LH whereas, NE (icv) produced a modest but significant increase in plasma LH. When DA was given 15 min prior to the infusion of NE, neither amplification nor inhibition of NE-induced LH release occurred. From these and other studies we conclude that the morphine-induced suppression of TIDA neuronal activity may allow NE to release greater amounts of LHRH from axon terminals in the median eminence. On the other hand, we doubt that the rise in preoptic DA (DOPAC) which occurs after morphine plays a role in augmenting the action of NE on LHRH release since icv DA does not alter the patterns or concentrations of plasma LH which occur after NE alone.

Use of neuroendocrine effects in discrimination between CNS‐active drugs in the rat

AbstractDifferent psychotropic drugs were investigated in order to determine their effect on the release of prolactin and corticosterone and their influence on the tuberoinfundibular dopamine (TIDA) neuron activity. The results were used in a principal component analysis, which grouped the psychotropic drugs into different clusters. In the plot showing these clusters the anxiolytic drugs were found to be grouped together and differ from the antidepressant drugs by their potent ability to increase plasma corticosterone. The antipsychotic drugs formed a separate group being clustered together. Typical neuroleptic and atypical antipsychotic drugs could be separated within the cluster by their different effects on plasma prolactin and corticosterone and on TIDA neuron activity. The results indicate that the neuroendocrine profiles of antidepressant and anxiolytic drugs are different from those of antipsychotic drugs and that the neuroendocrine measurements could be a useful tool in the early classification of psychotropic drugs. © 1993 Wiley‐Liss, Inc.

Developmental assessment of hypothalamic tuberoinfundibular dopamine in prolactin-deficient dwarf mice.

Development of the hypophysiotropic hypothalamus in PRL-deficient Ames dwarf (df/df) mice was examined for steady state dopamine (DA) by visualization using formaldehyde-induced catecholamine histofluorescence and by quantification using catecholamine HPLC at selected postnatal ages (7, 14, 21, 30, and 90 days). Phenotypically normal (DF/?) littermate mice were compared with dwarfs by both methods at each age. The studies were designed to investigate whether the known deficiency in hypothalamic tuberoinfundibular DA in adult dwarfs is present neonatally or develops over the postnatal period. The anterior pituitary of each mouse was processed for GH and PRL immunocytochemistry. At 7 days of age, GH immunostaining was robust, and scattered PRL-positive cells were noted in DF/? pituitary. Homogeneously distributed PRL cells increased in number through 30 days of age in normal mice. Neither GH nor PRL immunoreactivity was present in df/df mice at any age. At 7, 14, and 21 days of age, hypothalamic DA tuberoinfundibular histofluorescence was comparable in df/df and DF/? mice. At 90 days of age, tuberoinfundibular histofluorescence in normal mice remained intense, but was virtually undetectable in dwarfs. The developmental change affected only tuberoinfundibular neurons, since DA histofluorescence in nonhypophysiotropic areas, such as substantia nigra (SN), was qualitatively comparable for df/df and DF/? for all ages examined. Norepinephrine (NE) fluorescence in hypothalamus was also comparable for df/df and DF/?. Catecholamine HPLC provided quantitative confirmation of histofluorescence observations. DA and NE levels in both hypothalamus and ventral midbrain, including SN, increased during development in both df/df and DF/? brains. NE levels were not different between dwarf and normal animals at any age in either medial basal hypothalamus (MBH) or SN. The DA concentration in SN was not different between df/df and DF/? at any age examined. MBH DA was comparable in df/df and DF/? mice at 7, 14, and 21 days of age; at 30 and 90 days, MBH DA was markedly lower (P < 0.001) in dwarf than in normal mice. Although MBH DA in dwarfs was comparable to that in normal mice at 21 days, the increase in dwarfs between 14 and 21 days was not statistically significant. Thus, the hypothalamic DA deficit that exists in adult dwarf mice is not present neonatally and represents a failure to increase DA compared with normal mice after 14 days of age. The failure of continued development of hypophysiotropic tuberoinfundibular DA neurons in dwarf mice is correlated chronologically with absent pituitary PRL production.

Developmental assessment of hypothalamic tuberoinfundibular dopamine in prolactin-deficient dwarf mice [published erratum appears in Endocrinology 1993 Jul;133(1):327

Development of the hypophysiotropic hypothalamus in PRL-deficient Ames dwarf (df/df) mice was examined for steady state dopamine (DA) by visualization using formaldehyde-induced catecholamine histofluorescence and by quantification using catecholamine HPLC at selected postnatal ages (7, 14, 21, 30, and 90 days). Phenotypically normal (DF/?) littermate mice were compared with dwarfs by both methods at each age. The studies were designed to investigate whether the known deficiency in hypothalamic tuberoinfundibular DA in adult dwarfs is present neonatally or develops over the postnatal period. The anterior pituitary of each mouse was processed for GH and PRL immunocytochemistry. At 7 days of age, GH immunostaining was robust, and scattered PRL-positive cells were noted in DF/? pituitary. Homogeneously distributed PRL cells increased in number through 30 days of age in normal mice. Neither GH nor PRL immunoreactivity was present in df/df mice at any age. At 7, 14, and 21 days of age, hypothalamic DA tuberoinfundibular histofluorescence was comparable in df/df and DF/? mice. At 90 days of age, tuberoinfundibular histofluorescence in normal mice remained intense, but was virtually undetectable in dwarfs. The developmental change affected only tuberoinfundibular neurons, since DA histofluorescence in nonhypophysiotropic areas, such as substantia nigra (SN), was qualitatively comparable for df/df and DF/? for all ages examined. Norepinephrine (NE) fluorescence in hypothalamus was also comparable for df/df and DF/?. Catecholamine HPLC provided quantitative confirmation of histofluorescence observations. DA and NE levels in both hypothalamus and ventral midbrain, including SN, increased during development in both df/df and DF/? brains. NE levels were not different between dwarf and normal animals at any age in either medial basal hypothalamus (MBH) or SN. The DA concentration in SN was not different between df/df and DF/? at any age examined. MBH DA was comparable in df/df and DF/? mice at 7, 14, and 21 days of age; at 30 and 90 days, MBH DA was markedly lower (P < 0.001) in dwarf than in normal mice. Although MBH DA in dwarfs was comparable to that in normal mice at 21 days, the increase in dwarfs between 14 and 21 days was not statistically significant. Thus, the hypothalamic DA deficit that exists in adult dwarf mice is not present neonatally and represents a failure to increase DA compared with normal mice after 14 days of age. The failure of continued development of hypophysiotropic tuberoinfundibular DA neurons in dwarf mice is correlated chronologically with absent pituitary PRL production.

Effects of acute and chronic cocaine on the activity of tuberoinfundibular dopamine neurons in the rat

We investigated the effects of acute and chronic cocaine on dopamine (DA) synthesis in tuberoinfundibular dopamine (TIDA) neurons by measuring the accumulation of 3,4-dihydroxyphenylalaine (DOPA) in the median eminence (ME). Cocaine-induced prolactin (PRL). Chronic cocaine did not alter were evaluated for comparison. Acute cocaine inhibited DA synthesis in the ME and decreased circulating PRL. Chronic cocaine did not alter basal DOPA levels in TIDA nerve terminals or PRL levels in plasma. After repeated cocaine injections, cocaine's inhibitory effect on DA synthesis was abolished in the ME while the PRL response to drug was unchanged. These results suggest that TIDA neurons become tolerant to cocaine-induced suppression of DA synthesis after chronic cocaine exposure, but the altered sensitivity of TIDA neurons is not revealed by plasma PRL measures.

Intraventricular administration of histidyl-proline-diketopiperazine [Cyclo(His-Pro)] suppresses prolactin secretion and synthesis: a possible role of Cyclo(His-Pro) as dopamine uptake blocker in rat hypothalamus.

Histidyl-proline-diketopiperazine [Cyclo (His-Pro) (CHP)] was discovered to be one of the metabolites of TRH. To understand the specific role of CHP in rat hypothalamic dopamine neurons, we examined the in vivo effects of intraventricular (icv) infusion of CHP on the release and synthesis of PRL in the rat pituitary and the 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine ratio in the rat hypothalamus. We also examined the in vitro effects of CHP on the release of [3H]dopamine from dispersed tuberoinfundibular dopamine neurons, [3H]dopamine reuptake in hypothalamic membrane fractions, and PRL release from rat pituitary cultured cells. Female rats were treated by icv infusion of 1 microM CHP daily for 1, 3, and 7 days, using Alzet osmotic pumps. After 1 day of treatment, the serum PRL concentration was significantly decreased. Northern blot analysis of the total RNA isolated from the pituitary glands of control animals using 32P-labeled PRL cDNA as a probe indicated the presence of PRL gene transcript, 1.0 kilobase in size, and its amount was decreased by CHP treatment. CHP did not affect [3H]dopamine release from dispersed tuberoinfundibular dopaminergic neurons at any concentration up to 1 microM. CHP did not inhibit PRL release from cultured pituitary cells at low concentrations (1-100 nM), but it stimulated PRL release at high concentrations (1 and 10 microM). We also examined the concentrations of dopamine and DOPAC in the rat hypothalamus when CHP was administered icv for 1 or 7 days. There was a significant decrease in the DOPAC/dopamine ratio after CHP treatment for 1 day. Furthermore, CHP caused dose-dependent inhibition of [3H]dopamine uptake by the rat hypothalamus similar to other dopamine uptake blockers, such as benztropine and GBR12909. These data suggest that icv administration of CHP might decrease both PRL secretion and accumulation of PRL gene transcripts in the pituitary by decreasing the DOPAC/dopamine ratio and inhibiting dopamine reuptake in the rat hypothalamus.

Effect of the TRH analogue, CG 3703, on prolactin secretion in the pseudopregnant rat

The effect of the TRH analogue, CG 3703 (AG), on secretion of prolactin (Prl) was studied in pseudopregnant rats. AG stimulated Prl secretion of incubated pituitary glands in the presence of 5.10 −8 M dopamine (DA) but not in the presence of 0 or 10 −6 M DA. AG did not inhibit the in vitro secretion of Prl. AG postponed the peaks of Prl which were generated between 0100 and 1200 h (delay after 500 μg AG: 3–4 h); the height of these nocturnal Prl peaks was not affected by AG. AG did not induce Prl secretion during the phase of low Prl secretion (the interphase) between 2000 and 0100 h. The DA-release blocking drug, HA 966, induced peaks of Prl during the interphase. These peaks were as high as spontaneous nocturnal Prl peaks and were blocked by AG. The results show that AG has two opposite effects on Prl secretion: a direct, stimulating effect, which is modulated by DA, and an indirect inhibitory effect, which is probably executed by DA, released by AG from the tuberoinfundibular DA neurons.

Neurochemical evidence that the inhibitory effect of galanin on tuberoinfundibular dopamine neurons is activity dependent.

The purpose of the present study was to examine the effects of galanin on the basal and stimulated activity of tuberoinfundibular dopaminergic (TIDA) neurons in male and female rats. TIDA neuronal activity was estimated by measuring dopamine (DA) synthesis [accumulation of 3,4-dihydroxyphenylalanine(DOPA) after administration of a decarboxylase inhibitor] and metabolism [ratio of 3,4-dihydroxyphenylacetic acid (DOPAC) to DA concentrations] in terminals of these neurons in the median eminence. Central administration of galanin (2 micrograms/rat, i.c.v.) produced a rapid (by 15 min) increase in plasma prolactin concentrations, but had no effect on the ratio of DOPAC to DA in the median eminence of either male or female rats. In contrast, galanin decreased the ratio of DOPAC to DA in the median eminence of both male and female rats whose TIDA neuronal activity was stimulated following administration of the DA antagonist haloperidol (1 mg/kg, s.c., 12 h). The galanin receptor antagonist galanin-(1-13)-bradykinin-(2-9)-amide had no effect on the accumulation of DOPA in the median eminence of male rats per se, but blocked the inhibitory effects of either exogenous or endogenous galanin on median eminence DOPA accumulation in haloperidol-treated rats. These results indicate that in both male and female rats, galanin-induced activation of prolactin secretion is not mediated by changes in tonic inhibition of hormone release by TIDA neurons, and that TIDA neurons are responsive to the inhibitory effects of galanin only under activated conditions.

Serotonin-induced decrease in hypothalamic tyrosine hydroxylase activity and corresponding increase in prolactin release are abolished at midpregnancy and by transplants of rat choriocarcinoma cells.

We investigated the effect of central serotonin (5-hydroxytryptamine, 5-HT) administration on hypothalamic tuberoinfundibular dopamine neurons and related changes in neuronal activity to circulating PRL levels in two physiological models: 1) pregnant rats expressing (day 8) or not expressing (days 11 and 16) PRL surges, and 2) ovariectomized rats transplanted with rat choriocarcinoma cells, which secrete functional placental lactogen-I. Over a 4-min period between 0900 and 1400 h, rats were administered either vehicle or 5-HT (20 micrograms/6 microliters) through lateral ventricular cannulae. Plasma PRL levels were determined by RIA. NSD 1015 (25 mg/kg intraarterial), a dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, was injected 20 min after initiation of ventricular infusion. Ten min later, the stalk-median eminence (SME) was dissected. The rate of DOPA accumulation, determined by measuring DOPA levels in the SME by HPLC, was used as an index of tyrosine hydroxylase catalytic activity, indicating tubero infundibular dopamine neuronal activity. In day-8 pregnant rats 5-HT reduced DOPA accumulation to 57% of vehicle-injected controls and increased circulating PRL levels 13-fold. In contrast, on days 11 and 16 of pregnancy 5-HT did not alter DOPA accumulation in the SME or plasma PRL levels. In nonpregnant rats ovariectomized for 24 h, 5-HT decreased DOPA accumulation in the SME to 43% of vehicle-infused controls and increased PRL levels approximately 26-fold. However, in nonpregnant rats with rat choriocarcinoma cells, 5-HT produced no changes in either DOPA accumulation in the SME or in circulating PRL levels. The inability of 5-HT to reduce tyrosine hydroxylase activity after mid-pregnancy may account for the lack of a PRL response. Placental lactogens secreted at midpregnancy, particularly placental lactogen-1, may induce this loss of 5-HT effect.

Interaction of amfonelic acid with antipsychotic drugs on dopaminergic neurons.

The effects of two inhibitors of dopamine (DA) reuptake, amfonelic acid and GBR 12909, on the clozapine- and haloperidol-induced increases in DA synthesis, release, and metabolism were investigated in the rat. In the striatum, as well as in the nucleus accumbens, the haloperidol-induced increase in tissue concentrations of dihydroxyphenylacetic acid (DOPAC) or the accumulation of dihydroxyphenylalanine (DOPA) was potentiated or unaltered, respectively, in rats treated with amfonelic acid. In contrast, amfonelic acid attenuated the stimulatory effects of clozapine on DOPAC concentrations and DOPA accumulation in both brain regions. GBR 12909 also differentially affected the haloperidol- and clozapine-induced increases in DOPAC concentrations. However, the clozapine-induced increase in DOPA accumulation in the median eminence was not significantly altered by treatment with amfonelic acid. The haloperidol-induced increase in the extracellular concentrations of DA and DOPAC in the striatum also was potentiated by amfonelic acid, whereas the increase elicited by clozapine was suppressed. The increase in extracellular DA produced by the administration of morphine or the coadministration of ritanserin, a 5-HT2 antagonist, and haloperidol also was potentiated by amfonelic acid. The ability of amfonelic acid to distinguish between the actions of clozapine and haloperidol on nigrostriatal and mesocorticolimbic DA neurons does not appear to be related to differences in the effects of the drugs on DA autoreceptors or 5-HT2 receptors. Moreover, the mechanism through which clozapine activates tuberoinfundibular DA neurons may differ from that which is involved in the activation of nigrostriatal or mesocorticolimbic DA neurons.

Central Administration of Serotonin Decreases Tyrosine Hydroxylase Catalytic Activity and Messenger Ribonucleic Acid Signal Levels in the Hypothalamus of Female Rats

We investigated the effect of central serotonin (5-hydroxytryptamine, 5-HT) administration on hypothalamic tuberoinfundibular dopamine neurons and related changes in neuronal activity to circulating prolactin (PRL) levels. Ovariectomized rats were treated with either vehicle or 5-HT through a lateral ventricular cannula in one of two dose paradigms: 1) a bolus of 20 μg, with tissues taken at 30 min, or 2) the same bolus immediately followed by 20 μg/30 min via a syringe pump for 120 min, and tissues taken at 120 min. Blood samples were taken throughout experiments and plasma PRL determined by radioimmunoassay. Under both paradigms, NSD 1015, a dihydroxyphenylalanine (DOPA) decarboxylase inhibitor (25 mg/kg intraarterially) was injected 10 min before decapitation and brain excision followed by stalk-median eminence dissection. The rate of DOPA accumulation, determined by measuring DOPA levels in the stalk-median eminence by high-performance liquid chromatography with electrochemical detection was used as a measure of tyrosine hydroxylase (TH) catalytic activity. Stalk-median eminence DOPA accumulation in control rats was 29.9 ± 4.2 and 28.8 ± 4,4 ng/mg protein (30 and 120 min experiments, respectively). DOPA accumulation in 5-HT-treated rats was significantly reduced (P<0.05) after 30 min to 17.8 ± 1.2 ng/mg protein, but it was similar (21.7 ± 3.9) to controls after 120 min of 5-HT infusion. 5-HT levels in the stalk-median eminence of rats treated with 5-HT were 13- to 17-fold greater than controls (16.9 to 18.5 ng/mg protein). Plasma PRL levels in both groups increased 10-fold after 5-HT treatment with a peak at 5 min, returning to baseline by 120 min. TH mRNA levels were determined by in situ hybridization in a second group of rats which were treated with the 20μg bolus and subsequent 120 min infusion of 5-HT. TH mRNA signal levels in the arcuate nucleus of control rats averaged 144 ± 21 grains/cell. After treatment with 5-HT, TH mRNA levels in the arcuate nucleus were significantly lower (P<0.0001) with 69±14 grains/cell. In a third group of rats, the effects of the 30 min 5-HT treatment on TH catalytic activity and circulating PRL levels was challenged with two 5-HT(2) receptor antagonists, LY53857 (5 mg/kg intraperitoneally) or ketanserin (10 mg/kg intraperitoneally). Neither the 5-HT-induced decrease in TH catalytic activity nor the increase in PRL was altered by pretreatment (120 min) with 5-HT(2) antagonists. These data suggest that central 5-HT is capable of decreasing TH activity and TH mRNA levels in the tuberoinfundibular dopamine neurons and that the decrease in dopaminergic neuronal activity may contribute to the 5-HT-induced PRL rise. The changes in TH catalytic activity and PRL after intracerebroventricular administration of 5-HT do not appear to be mediated by 5-HT(2) receptors.

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.

Intraventricular administration of thyrotrophin-releasing hormone (TRH) suppresses prolactin secretion and synthesis: a possible involvement of dopamine release by TRH from rat hypothalamus.

The administration of thyrotrophin-releasing hormone (TRH) causes a variety of dopamine-related biological events. To understand the specific role of TRH on rat hypothalamic dopamine neurones, we examined the in-vivo effects of intraventricular (i.c.v.) infusion of TRH on the release and synthesis of prolactin in the rat pituitary gland and on the changes in binding of [3H]MeTRH and dopamine turnover rates in rat hypothalamus. We have also examined the in-vitro effects of TRH on the release of [3H]dopamine from dispersed tuberoinfundibular dopamine neurones. Female rats were treated with i.c.v. infusions of 1 mumol TRH/1 daily for 1, 3 and 7 days using Alzet osmotic pumps. Following 7 days of treatment the serum prolactin concentrations were significantly decreased. A reduction in hypothalamic TRH-binding sites (Bmax) was also apparent but the dissociation constant (Kd) was unaffected. Northern blot analysis of total RNA isolated from the pituitary glands of control animals using 32P-labelled prolactin cDNA as a probe indicated the presence of three species of prolactin gene transcripts of approximately 3.7, 2.0 and 1.0 kb in size, and these were decreased by TRH treatment. We examined the turnover rate of dopamine in the rat hypothalamus when TRH was administered i.c.v. for 7 days. There was a significant increase in 3,4-dihydroxyphenylacetic acid/dopamine ratio with TRH treatment. Moreover, exposure to TRH stimulated [3H]dopamine release from rat tuberoinfundibular neurones in a time- and dose-dependent manner. Dopamine receptor antagonists such as SCH23390 and (-)sulpiride, and other neuropeptides such as vasoactive intestinal peptide and oxytocin did not affect TRH-stimulated [3H]dopamine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroendocrinological and neurochemical effects of sigma ligands

The potential antipsychotic agents BMY 14802, remoxipride, tiospirone and gevotriline (WY 47,384) have a relatively high affinity for sigma binding sites in brain tissue. In the present study, the effects of these sigma ligands on concentrations of prolactin and corticosterone in serum in the rat were investigated. In addition, the effects of these agents on the synthesis and/or release of dopamine from tuberoinfundibular and nigrostriatal neurons were determined. Concentrations of prolactin and corticosterone in serum were increased dose-dependently by BMY 14802, tiospirone, remoxipride and gevotriline. The activity of tyrosine hydroxylase within the terminals of tuberoinfundibular dopamine neurons in vivo also was increased by BMY 14802, tiospirone and gevotriline, but not by remoxipride. The extracellular concentrations of dopamine and dihydroxyphenylacetic acid in the striatum, as determined by in vivo microdialysis, were increased by BMY 14802 (5–20 mg/kg, s.c.) and remoxipride (3 mg/kg, s.c.). These data suggest the involvement of sigma receptors in the regulation of secretion of prolactin and corticosterone, as well as the activity of tuberoinfundibular and nigrostriatal dopamine neurons. Moreover, the pattern of neurochemical and neuroendocrinological responses to these sigma ligands resembled those which have been determined previously for atypical antipsychotics and support the contention that these sigma ligands may be antipsychotic agents with an atypical profile of action.

Evidence that hypothalamic periventricular dopamine neurons innervate the intermediate lobe of the rat pituitary.

The purpose of the present study was to provide neurochemical and endocrinological evidence that dopamine (DA) neurons terminating in the intermediate lobe of the rat pituitary originate in the periventricular nucleus of the hypothalamus. One week following surgical separation of the periventricular nucleus from the mediobasal hypothalamus, DA and 3,4-dihydroxyphenyl-acetic acid (DOPAC) concentrations in the intermediate lobe were reduced by 50%, and this was accompanied by an increase in plasma alpha-melanocyte-stimulating hormone (alpha-MSH) concentrations. In contrast, this procedure had no effect on concentrations of prolactin in the plasma, or DA or DOPAC in the median eminence, the region of the mediobasal hypothalamus containing terminals of tuberoinfundibular DA neurons. Electrical stimulation of the periventricular nucleus increased the ratio of DOPAC/DA in the intermediate lobe and reduced the concentrations of alpha-MSH in the plasma, whereas in these same animals the DOPAC/DA ratio in the median eminence and concentrations of prolactin in the plasma were unaltered. These results indicate that approximately 50% of all the DA neurons terminating in the intermediate lobe of the rat pituitary originate in or project through the periventricular nucleus of the hypothalamus, and that these DA neurons regulate the secretion of alpha-MSH from intermediate lobe melanotrophs.

Evidence that prolactin mediates the stimulatory effects of estrogen on tuberoinfundibular dopamine neurons in female rats.

The effects of ovariectomy and estrogen on prolactin secretion and/or the activity of tuberoinfundibular dopamine (TIDA) neurons were examined by either concurrently measuring concentrations of prolactin in plasma and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence of female rats or by determining the rate of DA synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor) in the median eminence. For comparison, concentrations of alpha-melanocyte-stimulating hormone (alpha MSH) in plasma and DOPAC in the intermediate lobe of the pituitary (an index of the activity of tuberohypophysial DA neurons) were also determined. Ovariectomy produced a time-dependent decrease in the accumulation of DOPA and the concentrations of DOPAC in the median eminence and prolactin in plasma with maximal effects occurring by 7 days. Estrogen administration to ovariectomized rats increased plasma prolactin and median eminence DOPAC concentrations to levels comparable to those in diestrous controls. In contrast, neither ovariectomy nor estrogen replacement altered the concentrations of alpha MSH in plasma or DOPAC in the intermediate lobe. Administration of the DA agonist bromocriptine blocked the ability of estrogen to increase plasma prolactin and median eminence DOPAC concentrations. Also, administration of antiserum to rat prolactin blocked the stimulatory action of estrogen on median eminence DOPAC concentrations. Taken together, these results indicate that the stimulatory effect of estrogen on the activity of TIDA neurons is mediated by prolactin.

Activation of tuberoinfundibular and tuberohypophysial dopamine neurons following intracerebroventricular administration of bombesin

The effect of bombesin on the activity of dopamine (DA) neurons comprising the nigrostriatal, mesolimbic, tuberoinfundibular and tuberohypophysial systems in the male rat was determined by measuring: (1) the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor, and (2) the concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in brain (striatum, nucleus accumbens, median eminence) and pituitary regions (intermediate and neural lobes) containing terminals of these neurons. Intracerebroventricular (i.c.v.) injection of bombesin caused a dose- and time-related increase in the activity of DA neurons projecting to the median eminence and intermediate lobe of the pituitary, and a corresponding decrease in the concentrations of prolactin and α-melanocyte-stimulating hormone (αMSH) in the plasma. In contrast, doses of bombesin up to 10 ng i.c.v. failed to alter the activity of DA neurons terminating in the striatum, nucleus accumbens or neural lobe of the pituitary gland. Equimolar doses of bombesin and gastrin-releasing peptide (GRP), a bombesin-like peptide, increased the concentrations of DOPAC in the median eminence and intermediate lobe of the pituitary, suggesting that GRP-preferring receptors may be responsible for the stimulatory effects of bombesin on DA neuronal activity in these regions. The results of these studies suggest that bombesin increases the activity of tuberoinfundibular and tuberohypophysial DA neurons projecting to the median eminence and intermediate lobe of the pituitary, respectively, and thereby inhibits the secretion of prolactin and αMSH.

Endogenous Opioids Mediate the Nocturnal Prolactin Surge in the Pregnant Rat*

Endogenous opioids stimulate PRL release in the rat during proestrus, stress, and lactation. This study investigated the potential role of opioids in regulating the daily nocturnal PRL surges that occur during the first half of pregnancy. On day 8 of pregnancy, infusion of naloxone (NAL; 0.25, 1.0, or 2.0 mg/10 min), an opioid receptor antagonist, from 0100-0400 h blocked the nocturnal surge in a dose-dependent manner. PRL remained at basal levels throughout the infusion when the highest dose of NAL was used. A PRL surge was still present in this group, but its delayed onset occurred 1 h after termination of the NAL infusion. In another experiment NAL was used to determine whether opioids are needed to maintain the nocturnal surge or only to initiate the surge. On day 8 of pregnancy, infusion of NAL (2.0 mg/10 min) was started at 0300 h, after initiation of the surge had occurred, and was continued until 0600 h. After 1 h of infusion, PRL levels were less than 20 ng/ml, compared to more than 200 ng/ml in controls (P less than 0.002) and remained low. A PRL surge occurred approximately 2-3 h after the infusion was discontinued, at a time when the surge was completed in controls (P less than 0.01). Experimental animals did not differ in their surges relative to controls on the next day when no infusion was given. Another set of experiments was performed to determine whether blockade of endogenous opioids inhibited the nocturnal PRL surge by stimulating tuberoinfundibular dopamine (TIDA) neurons. Day 8 pregnant rats infused with NAL (2.0 mg/10 min) from 0100-0400 h demonstrated no nocturnal PRL surge. Subsequently, the medial basal hypothalamus in NAL-treated animals and saline controls was incubated in vitro with the DOPA decarboxylase inhibitor brocresine (100 microM). The greater amount of resultant DOPA accumulation in the stalk median eminence of NAL-treated animals reflected increased TIDA neuronal activity compared to that in SAL-infused controls. Similar results were obtained when rats were treated in vivo with the DOPA decarboxylase inhibitor NSD 1015 (25 mg/kg BW) just before termination of the NAL infusion. Based on these results, we conclude that endogenous opioid peptides are essential in initiating and maintaining nocturnal PRL surges in pregnant rats. Moreover, opioids appear to exert their effects by decreasing TIDA neuronal activity.