Effects of serotonin depletion byp-chlorophenylalanine,p-chloroamphetamine or 5,7-dihydroxytryptamine on central dopaminergic neurons: Focus on tuberoinfundibular dopaminergic neurons and serum prolactin

Three serotonin (5-HT) neurotoxins,p-chlorophenylalanine (PCPA, 125 and 250 mg/kg, i.p.),p-chloroamphetamine (PCA, 10 mg/kg, i.p.) and 5,7-dihydroxytryptamine (5,7-DHT, 200 µg/rat, i.c.v.) were used to examine whether depletion of central 5-HT has an effect on central dopaminergic (DA) neuronal activities or on prolactin (PRL) secretion. Adult ovariectomized Sprague-Dawley rats primed with estrogen (polyestradiol phosphate, 0.1 mg/rat, s.c.) were treated with one of three neurotoxins and then decapitated in the morning after 3–7 days. Blood sample and brain tissues were collected. The acute effect of PCA (from 30 to 180 min) was also determined. The concentrations of 5-HT, DA and their metabolites, 5-hydroxyindoleacetic acid and 3,4-dihydroxyphenylacetic acid, in the median eminence, striatum and nucleus accumbens were determined by HPLC-electrochemical detection. All three toxins significantly depleted central 5-HT stores by 11–20%. Except for PCPA, neither PCA nor 5,7-DHT had any significant effect on basal DA neuronal activities or PRL secretion. PCA also exhibited an acute effect on the release and reuptake of 5-HT and DA. In summary, depletion of central 5-HT stores to a significant extent for 3–7 days did not seem to affect basal DA neuronal activity and PRL secretion.     

Stable interaction between G-actin and neurofilament light subunit in dopaminergic neurons

Excessive accumulation of neurofilaments in the cell bodies and proximal axons of motor neurons is a major pathological hallmark of motor neuron diseases. In this communication we provide evidence that the neurofilament light subunit (68 kDa) and G-actin are capable of forming a stable interaction. Cytochalasin B, a cytoskeleton disrupting agent that interrupts actin-based microfilaments, caused aggregation of neurofilaments in cultured mesencephalic dopaminergic neurons, suggesting a possible interaction between neurofilaments and actin; which was tested further by using crosslinking reaction and affinity chromatography techniques. In the cross-linking experiment, G-actin interacted with individual neurofilament subunits and covalently cross-linked disuccinimidyl suberate, a homobifunctional cross-linking reagent. Furthermore, G-actin was extensively cross-linked to the light neurofilament subunit with this reagent. The other two neurofilament subunits showed no cross-linking to G-actin. Moreover, neurofilament subunits were retained on a G-actin coupled affinity column and were eluted from this column by increasing salt concentration. All three neurofilament subunits became bound to the G-actin affinity column. However, a portion of the 160 and 200 kDa neurofilament subunits did not bind to the column, and the remainder of these two subunits eluted prior to the 68 kDa subunit, suggesting that the light subunit exhibited the highest affinity for G-actin. Moreover, neurofilaments demonstrated little or no binding to F-actin coupled affinity columns. The phosphorylation of neurofilament proteins with protein kinase C reduced its cross-linking to G-actin. The results of these studies are interpreted to suggest that the interaction between neurofilaments and actin, regulated by neurofilament phosphorylation, may play a role in maintaining the structure and hence the function of dopaminergic neurons in culture.     

Involvement of central GABAergic neurons in basal and diurnal changes of tuberoinfundibular dopaminergic neuronal activity and prolactin secretion

Central administration of gamma-aminobutyric acid (GABA) has been shown to stimulate the secretion of prolactin (PRL). Whether GABA acts via dopamine, the major PRL-inhibiting hormone, and which GABA receptor type(s) is involved have not been ascertained. Both GABA(A) and GABA(B) receptor agonists and/or antagonists were administered centrally in this study and their effects on both basal and diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activity were determined by measuring the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME). Serum PRL level was determined by RIA. Ovariectomized, estrogen-primed Sprague-Dawley rats implanted with intracerebroventricular (icv) cannulae were used. Muscimol (1 ng/3 microl/rat, icv), a GABA(A) receptor agonist, but not baclofen (1-100 ng/3 microl/rat, icv), a GABA(B) receptor agonist, injected in the morning significantly lowered and elevated ME DOPAC and serum PRL levels, respectively at 15 and 30 min. Lower and higher doses of muscimol were not effective. The effects of muscimol could also be prevented by co-administration of bicuculline (0.1-10 ng/3 microl, icv), a GABA(A) receptor antagonist. When bicuculline (10-500 ng/3 microl, icv) was given in the afternoon (at 1500 h), it significantly reversed the lowered ME DOPAC level in the afternoon and prevented the concurrent PRL surge. We conclude that endogenous GABA acting through GABA(A) receptors may play a significant role in the control of basal and diurnal changes of TIDA neuronal activity, and in turn, PRL secretion.     

Evidence that histamine-stimulated prolactin secretion is not mediated by an inhibition of tuberoinfundibular dopaminergic neurons.

The effects of histamine on prolactin secretion and the activity of tuberoinfundibular dopaminergic (DA) neurons were examined in male rats. Tuberoinfundibular DA neuronal activity was estimated in situ by measuring the metabolism [concentration of 3,4-dihydroxyphenylacetic acid (DOPAC)] and synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor] of dopamine in the median eminence. Intracerebroventricular (icv) injection of histamine produced a dose- and time-dependent increase in plasma prolactin levels but had no effect on DOPA accumulation or DOPAC concentrations in the median eminence. These results indicate that the stimulation of prolactin secretion following icv histamine is not mediated by an inhibition of tuberoinfundibular DA neurons.     

瘦素与中枢葡萄糖感受性神经元在摄食调节中的交互作用

瘦素（lepfin）是新近发现的一种重要的摄食调节因子,主要通过中枢途径参与调节摄食和能量平衡。下丘脑是瘦素中枢作用的主要靶点,含有多种摄食相关神经元;其中,葡萄糖感受性神经元可感测细胞外葡萄糖水平的变化,参与摄食调控。本文即对瘦素调节摄食的中枢途径及其对中枢葡萄糖感受性神经元作用的研究进展做简要综述。     

Interactions between melatonin and prolactin during gestation in mink (Mustela vison)

Immature female mink, 8 weeks of age in July, were treated with implants releasing melatonin. Mating, which induced ovulation, took place during the normal breeding season in the following March. Circulating prolactin and progesterone concentrations did not undergo the expected gestational increases, and no embryos implanted. A similar absence of gestational changes in prolactin and progesterone values ensued in primiparous mink treated with the melatonin implant 2-3 days after the second of 2 matings. Administration of exogenous sheep prolactin (0.5 mg/day) by minipump induced precocious elevation of progesterone concentrations in mated mink. Prolactin administration overcame the effects of melatonin, in that the corpora lutea were activated and embryos implanted, but exogenous prolactin resulted in degeneration of implanted embryos both in the presence and absence of chronic melatonin. The results suggest that melatonin has a single effect in alteration of gestation in mink; i.e. the prevention of prolactin secretion. Hyperprolactinaemia may inhibit embryo development in this species.     

Temperature effects on serum prolactin concentrations and activity of dopaminergic neurons in the infundibulum/pituitary stalk of calves

The effects of ambient temperature on serum concentrations of prolactin and neurochemical estimates of activity of dopaminergic neurons projecting to the infundibulum/pituitary stalk were investigated in Holstein bull calves. Accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the infundibulum/pituitary stalk after intravenous injection of a decarboxylase inhibitor was used to estimate activity of these dopaminergic neurons. Increasing ambient temperature from 21 to 33 degrees C for 22 hr increased serum concentrations of prolactin and decreased activity of the dopaminergic neurons. Conversely, reducing ambient temperature from 22 degrees C to 11 degrees C decreased serum concentrations of prolactin and increased activity of these dopaminergic neurons. It is suggested that alterations in activity of dopaminergic neurons terminating in the infundibulum/pituitary stalk of bull calves may mediate acute temperature-induced changes in secretion of prolactin.     

Interactions between neurons and their targets during in vivo synaptogenesis.

In vivo synaptogenesis is described in a simple vertebrate system, the chick ciliary ganglion, a parasympathetic autonomic ganglion. An attempt is made to integrate anatomical, physiological and biochemical observations during synapse formation in the ganglion and in the peripheral target structures; the iris, ciliary muscle, and smooth muscle of the choroid coat. The relationship between synaptogenesis and neuron survival is explored, and it is shown that a critically timed interaction between the neuron and target organ is necessary for full neuronal maturation and survival. The existence of an active competition between neurons for survival is documented, and the possible relationship between neuronal cell death and specificity of connections is discussed.     

Neurotensin-induced activation of hypothalamic dopaminergic neurons is accompanied by a decrease in pituitary secretion of prolactin and alpha-melanocyte-stimulating hormone.

The effects of neurotensin on the activity of hypothalamic tuberoinfundibular and periventricular-hypophysial dopaminergic (DA) neurons, and on the secretion of pituitary hormones that are tonically regulated by these neurons (i.e. prolactin and alpha-melanocyte-stimulating hormone [alpha MSH], respectively) were examined in estrogen-primed ovariectomized rats. The activity of tuberoinfundibular and periventricular-hypophysial DA neurons was estimated by measuring concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the terminals of these neurons in the median eminence and intermediate lobe of the posterior pituitary, respectively. Intracerebroventricular administration of neurotensin caused a dose- and time-related increase in DOPAC concentrations in both the median eminence and intermediate lobe, and a concurrent decrease in plasma levels of prolactin and alpha MSH. These results suggest that neurotensin-induced inhibition of secretion of prolactin and alpha MSH from the pituitary may be due to the stimulatory action of this neuropeptide on the release of dopamine from tuberoinfundibular and periventricular-hypophysial neurons.     

Spontaneous activity of dopaminergic retinal neurons

Dopaminergic local circuit neurons in the retina (DA cells) show robust, spontaneous, tetrodotoxin-sensitive pacemaking. To investigate the mechanism underlying this behavior, we characterized the sodium current and a subset of the potassium currents in the cells in voltage-clamp experiments. We found that there is a persistent component of the sodium current in DA cells which activates at more depolarized potentials than the transient component of the current. The transient component was completely inactivated at -50 mV, but DA cells remained able to fire spontaneous action potentials when potassium channels were partially blocked and the membrane potential remained above -40 mV. Based on these electrophysiological data, we developed a reduced computer model that reproduced the major features of DA cells. In simulations at the physiological resting potential, the persistent component of the sodium current was both necessary and sufficient to account for spontaneous activity, and the major contribution of the transient component of the sodium current was to initiate the depolarization of the model cell during the interspike interval. When tonic inhibition was simulated by lowering the input impedance of the model cell, the transient component played a larger role.     

Interactions of antisense peptides with ovine prolactin

Two antisense peptides were synthesized to a sense peptide corresponding to amino acid residues 23-35 of ovine prolactin. Both of the antisense peptides formed a saturable complex with the sense peptide and ovine prolactin. The sense peptide inhibited the interaction of ovine prolactin with the antisense peptides. Both of the antisense peptides have a common core sequence VMNV which can bind to ovine prolactin. The lactogenic hormones, rat prolactin and human growth hormone, compete with the binding of ovine prolactin to an antisense peptide whereas a nonlactogen, ovine growth hormone, does not compete indicating a degree of specificity in the interaction.     

Interrelationships between tyrosine hydroxylase-immunoreactive dopaminergic afferents and somatostatinergic neurons in the rat central amygdaloid nucleus

 Interrelationships between dopaminergic afferents and somatostatinergic neurons of the rat central amygdaloid nucleus were studied using tyrosine hydroxy-lase/somatostatin double immunolabeling for light and electron microscopy. Additionally, morphological features of somatostatin neurons in different subnuclei of the central nucleus were studied, and the results were complemented by codistribution studies of somatostatin and D₁ and D₂ dopamine receptor mRNA expression. Dense axonal immunolabeling for tyrosine hydroxylase was colocalized with somatostatin-immunoreactive or somatostatin mRNA-reactive neurons in the medial and the central lateral part of the central nucleus. The number of somatostatinergic neurons detected was higher using in situ hybridization than using immunolabeling. Somatostatin-immunoreactive neurons of the medial central nucleus possessed deeply indented nuclei, and immunoreaction product was confined to the Golgi apparatus and its vicinity. On the other hand, those in the central lateral subnucleus possessed nuclei without indentations and showed diffuse staining of the cytoplasm and/or in large vesicles. Double labeling showed that in the central lateral central nucleus, somatostatin-immunoreactive neurons were contacted by tyrosine hydroxylase-immunoreactive terminals, and on the electron microscopic level synaptic contacts between differently labeled structures were observed. D₁ and D₂ receptor mRNA-reactive neurons were differentially distributed in central nucleus subnuclei. D₁ receptor mRNA-expressing neurons were found only in the medial subnucleus, while D₂ receptor mRNA was expressed by a number of neurons in the lateral central and a few in the medial one. Thus, the study proves that somatostatin-immunoreactive neurons of the central lateral central nucleus are directly innervated by dopaminergic afferents and may express the D₂ dopamine receptor. Accepted: 2 July 1996     

Factors promoting survival of mesencephalic dopaminergic neurons

Growth factors promoting survival of mesencephalic dopaminergic neurons are discussed in the context of their requirement during development and adulthood. The expression of growth factors should be detectable in the nigrostriatal system during critical periods of development, i.e., during the period of ontogenetic cell death and synaptogenesis and during neurite extension and neurotransmitter synthesis. Growth factors discussed include members of the family of glial-cell-line-derived neurotrophic factors (GDNF), neurotrophins, transforming growth factors beta, and low molecular compounds mimicking growth factor activities. To date, the available data support the notion that GDNF is a highly promising candidate, although GDNF-null mice lack a dopaminergic phenotype. There remains a possibility that endogenous dopaminotrophic factors remain to be discovered.The authors work is supported by grants from the Deutsche Forschungsgemeinschaft through the DFG-Research Center for Molecular Physiology of the Brain     

Interactions of steroids with prolactin secretion in vitro

Estrogens prevent or diminish the sensitivity to dopamine of prolactin (PRL) secretion by cultured rat pituitary cells. Cultured tumor cells prepared from a transplantable rat PRL-secreting tumor were insensitive to dopamine and bromocriptine, while the anti-estrogen tamoxifen restored this sensitivity. Cultured normal human pituitary cells were shown to be more sensitive to dopamine, if they were preincubated with estradiol, while cultured human prolactinoma cells became insensitive to bromocriptine after they were exposed to estrogens. This sensitivity was restored, however, by tamoxifen. These results point to an important species difference between primates and rodents with regard to the normal regulation of PRL secretion.     

Punishment prediction by dopaminergic neurons in Drosophila

The temporal pairing of a neutral stimulus with a reinforcer (reward or punishment) can lead to classical conditioning, a simple form of learning in which the animal assigns a value (positive or negative) to the formerly neutral stimulus. Olfactory classical conditioning in Drosophila is a prime model for the analysis of the molecular and neuronal substrate of this type of learning and memory. Neuronal correlates of associative plasticity have been identified in several regions of the insect brain. In particular, the mushroom bodies have been shown to be necessary for aversive olfactory memory formation. However, little is known about which neurons mediate the reinforcing stimulus. Using functional optical imaging, we now show that dopaminergic projections to the mushroom-body lobes are weakly activated by odor stimuli but respond strongly to electric shocks. However, after one of two odors is paired several times with an electric shock, odor-evoked activity is significantly prolonged only for the "punished" odor. Whereas dopaminergic neurons mediate rewarding reinforcement in mammals, our data suggest a role for aversive reinforcement in Drosophila. However, the dopaminergic neurons' capability of mediating and predicting a reinforcing stimulus appears to be conserved between Drosophila and mammals.