Antisense strategy unravels a dopamine receptor distinct from the D2 subtype, uncoupled with adenylyl cyclase, inhibiting prolactin release from rat pituitary cells

The role of prolactin in andrology: what is new?

Prolactin (PRL) has been long deemed as a hormone involved only in female reproduction. However, PRL is a surprising hormone and, since its identification in the 1970s, its attributed functions have greatly increased. However, its specific role in male health is still widely unknown. Recently, low PRL has been associated with reduced ejaculate and seminal vesicle volume in infertile subjects. In addition, in men consulting for sexual dysfunction, hypoprolactinemia has been associated with erectile dysfunction and premature ejaculation, findings further confirmed in the general European population and infertile men. Several metabolic derangements, recapitulating metabolic syndrome, have also been associated with low PRL both in men with sexual dysfunction and from the general European population. In men with sexual dysfunction, followed-up for more than 4 years, low PRL was identified as an independent predictor of the incidence of major adverse cardiovascular events. Finally, an association with anxiety or depressive symptoms has been found in men with sexual dysfunction and from the general European population. While a direct role for impaired PRL function in the pathogenesis of these reproductive, sexual, metabolic and psychological disorders is conceivable, the possibility that low PRL is a mirror of an increased dopaminergic or a decreased serotonergic tone cannot be ruled-out. Hyperactivity of the dopaminergic system can explain only a few of the aforementioned findings, whereas a hypo-serotonergic tone fits well with the clinical features associated with low PRL, and there is significant evidence supporting the hypothesis that PRL could be a mirror of serotonin in the brain.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

New insights into dopaminergic receptor function using antisense and genetically altered animals

Dopaminergic receptors are widespread throughout the central and peripheral nervous systems, where they regulate a variety of physiological, behavioral, and endocrine functions. These receptors are also clinically important drug targets for the treatment of a number of disorders, such as Parkinson's disease, schizophrenia, and hyperprolactinemia. To date, five different dopamine receptor subtypes have been cloned and characterized. Many of these subtypes are pharmacologically similar, making it difficult to selectively stimulate or block a specific receptor subtype in vivo. Thus, the assignment of various physiological or behavioral functions to specific dopamine receptor subtypes using pharmacological tools is difficult. In view of this, a number of investigators have--in order to elucidate functional roles--begun to use highly selective genetic approaches to alter the expression of individual dopamine receptor subtypes in vivo. This review discusses recent studies involving the use of genetic approaches for the study of dopaminergic receptor function.

The antisense strategy applied to the study of dopamine D3 receptor functions in rat forebrain

1. The authors have investigated the effects of a dopamine D3 receptor antisense oligodeoxynucleotide (ODN), on neuropeptides (neurotensin and dynorphin) and transcription factor (c-fos) mRNA levels in rat forebrain. 2. Intracerebroventricular injections of ODNs were made into the lateral ventricle (5 and 10 micrograms/h, for 5 days). Effect of antisense administration on dopamine D2 and D3 receptor binding were measured by means of receptor autoradiography. Neuropeptides and c-fos mRNA levels were evaluated by in situ hybridization using specific complementary RNA probes. 3. Dopamine D3 receptor densities were dose-dependently reduced in the shell of nucleus accumbens of rats that received the D3 antisense ODN. Sense and missense controls remained without effect. No significant effect was observed on D2 receptor binding in any of the ODN groups studied, as measured with [3H]raclopride binding. Concomitant reductions of dynorphin and neurotensin mRNA levels were observed in the shell of nucleus accumbens after D3 antisense ODN administration. Interestingly, the D3 antisense administration also reduced c-fos mRNA levels in the cingulate cortex of these animals. 4. The results show that D3 receptors may tonically regulate basal transcription factor, as well as neuropeptides, gene expression in the rat forebrain. These results clearly demonstrate that an antisense strategy could be useful to identify molecular targets under control of specific dopamine receptor subtypes.

Differential effects on D2 dopamine receptor and prolactin gene expression by haloperidol and aripiprazole in the rat pituitary

[3H]Spiperone-binding assay to D2 receptors and quantitative ribonuclease protection assay for both isoforms (D2L and D2S receptor) of the D2 receptor mRNA and the prolactin mRNA were performed on pituitaries from the control rat and from the rat injected orally daily with either haloperidol (2 mg/kg) or aripiprazole (24 mg/kg) for 21 days. Haloperidol treatment increased the [3H]spiperone-binding by 28%, the levels of D2L and D2S receptor mRNA by 41% and 38%, respectively, and the level of prolactin mRNA by 26%. In contrast, the treatment with aripiprazole, a newly developed atypical antipsychotic with reduced side effects, decreased the [3H]spiperone-binding by 24% and the levels of D2L and D2S receptor mRNA by 23% and 23%, respectively, and did not have any effect on the level of prolactin mRNA. The same treatment with sulpiride (100 mg/kg) increased the levels of D2L and D2S receptor mRNA by 59% and 62%, respectively, but treatment with clozapine (25 mg/kg) did not cause any effect. Neither treatment changed the ratio of the level of D2S receptor mRNA to the level of D2L receptor mRNA in the pituitary. These findings indicate that D2 receptor densities in the pituitary are influenced differentially by the treatment with these antipsychotics, which could be induced at least partly by the changes in the levels of mRNA without any effects on the splicing mechanisms and thus affect the plasticity of the prolactin mRNA expression. The inhibitory effects of chronic aripiprazole treatment on D2 receptors in the pituitary might underlie this drug's clinical property of reduced hyperprolactinemia side effect.

Dopamine induces apoptosis through an oxidation-involved SAPK/JNK activation pathway

Dopamine (DA) is a neurotransmitter, but it also exerts a neurotoxic effect under certain pathological conditions, including age-related neurodegeneration such as Parkinson's disease. By using both the 293 cell line and primary neonatal rat postmitotic striatal neuron cultures, we show here that DA induces apoptosis in a time- and concentration-dependent manner. Concomitant with the apoptosis, DA activates the JNK pathway, including increases in JNK activity, phosphorylation of c-Jun, and subsequent increase in c-Jun protein. This DA-induced JNK activation precedes apoptosis and is persistently sustained during the process of apoptosis. Transient expression of a dominant negative mutant SEK1(Lys --> Arg), an upstream kinase of JNK, prevents both DA-induced JNK activation and apoptosis. A dominant negative c-Jun mutant FLAGDelta169 also reduces DA-induced apoptotic cell death. Anti-oxidants N-acetylcysteine and catalase, which serve as scavengers of reactive oxygen species generated by metabolic DA oxidation, effectively block DA-induced JNK activation and subsequent apoptosis. Thus, our data suggest that DA triggers an apoptotic death program through an oxidative stress-involved JNK activation signaling pathway. Given the fact that the anti-oxidative defense system declines during aging, this molecular event may be implicated in the age-related striatal neuronal cell loss and age-related dopaminergic neurodegenerative disorders, such as Parkinson's and Huntington's diseases.

Antisense strategies in neurobiology

The use of antisense oligodeoxynucleotides, targeted to the transcripts encoding biologically active proteins in the nervous system, provides a novel and highly selective means to further our understanding of the function of these proteins. Recent studies of these agents also suggest the possibility of their being used therapeutically for a variety of diseases involving neuronal tissue. In this paper we review studies showing the in vitro and in vivo effects of antisense oligodeoxynucleotides as they relate to neurobiological functions. Particular attention is paid to the behavioral and biochemical effects of antisense oligodeoxynucleotides directed to the various subtypes of receptors for the neurotransmitter dopamine. An example is also provided showing the effects of a plasmid vector expressing an antisense RNA targeted to the calmodulin mRNAs in the PC12 pheochromocytoma cell line. The advantages of antisense oligodeoxynucleotides over traditional pharmacological treatments are assessed, and the advantages of using vectors encoding antisense RNA over the use of antisense oligodeoxynucleotides are also considered. We also describe the criteria that should be used in designing antisense oligodeoxynucleotides and several controls that should be employed to assure their specificity of action.

Dopamine D4 receptor-mediated inhibition of cyclic adenosine 3',5'-monophosphate production does not affect prolactin regulation

Under physiological conditions, PRL synthesis and secretion are predominantly under negative control by dopamine acting through dopamine D2 receptors present in the pituitary lactotroph cells. To investigate the role of D4 receptors in the regulation of PRL synthesis and secretion, we stably transfected the human D4 receptor complementary DNA into the somatomammotrophic cell line GH4C1. The pharmacological characteristics of D4 expressed in GH4C1 were in close agreement with previous D4 receptor studies in Chinese hamster ovary and COS-7 cells. In GH4C1 cells, activation of D4 receptor variants (D4.2, D4.4, and D4.7) resulted in a similar level of reduction in forskolin- and vasoactive intestinal peptide (VIP)-stimulated cAMP levels (33% and 50%, respectively). In addition, the forskolin-stimulated activity of cAMP response elements fused to the VIP promoter driving the lacZ reporter gene could be blocked by D4 activation. However, quinpirole treatment had a minimal effect on transiently expressed luciferase reporter gene driven by a proximal PRL promoter in one of the D4-expressing cell lines. In contrast, the dopamine D2short receptor expressing GH4ZR7 cells treated with quinpirole displayed a significant decrease (51.3 +/- 4.1%) in PRL promoter activity. VIP-stimulated PRL release was not affected by D4 receptor activation, whereas in GH4ZR7 cells, a significant decrease in VIP-stimulated PRL levels was observed. Neither PRL promoter activity nor PRL secretion levels were affected in control untransfected GH4C1 cells. From this study it appears that although the D4 receptor may be expressed in the anterior pituitary, it does not have a major effect on PRL promoter activity or PRL secretion in GH4C1 cells despite its ability to reduce cAMP production. This might explain why D4- over D2-preferring antipsychotics such as clozapine do not cause hyperprolactinemia.

Antisense strategies in dopamine receptor pharmacology

Recent advances in molecular biology have provided pharmacologists the opportunity of developing an entirely new type of agent for studying and treating a variety of biological disorders. These agents, termed antisense oligodeoxynucleotides, have as their target the messenger RNAs encoding specific proteins. They act by binding to selected portions of these mRNAs through complimentary interactions and thereby prevent the synthesis of these proteins. These novel pharmacological tools have the promise of being easier to design and being more selective and predictable in their actions. In addition, insofar as agents targeted to receptors for neurotransmitters are concerned, unlike the classical pharmacological agents, these new compounds may not lead to the upregulation of the very receptors the drugs are designed to inhibit. The present review summarizes briefly studies on the effect of oligodeoxynucleotides antisense to the mRNAs encoding the various subtypes of the dopamine receptor. The studies show that oligodeoxynucleotides antisense to the D2 dopamine receptor when intracerebroventricularly into brains of rodents are rapidly taken up into the brain tissue, distributed to brain cells, and produce effects characteristic of highly selective D2 dopamine antagonists. The compounds also produced specific reductions in the levels of D2 dopamine receptor mRNA and D2 dopamine receptors. Similarly, injecting an antisense oligodeoxynucleotide targeted to the D1 dopamine receptor mRNA produces effects characteristic of D1 dopamine receptor antagonists. Other studies using these agents has produced evidence that there is a small pool of receptors that turn over very rapidly and which constitute the functional pool of these receptors. The evidence suggests further that antisense oligodeoxynucleotides inhibit the synthesis of this small functional pool of dopamine receptors, thereby providing an explanation of why there is often a discordance between changes in dopaminergic function and changes in the levels of dopamine receptors. Studies of antisense oligodeoxynucleotides targeted to the other subtypes of dopamine receptor may help reveal the biological roles that these and other newly discovered subtypes of neurotransmitter receptors have. They may also provide an entirely new and potentially more selective therapeutic regimen for altering the functions of these receptors.

Use of transgenics, null mutants, and antisense approaches to study ethanol's actions

Behavioral and biochemical responses mediating ethanol's actions have been difficult to study in humans and animals because of their complex polygenic nature. Recent progress in the creation of new animal models using recombinant DNA technology has provided a set of genetic tools by which the role of specific candidate genes in ethanol's actions can be examined. These techniques include the creation of transgenic and null mutant mice, as well as manipulation of protein synthesis with antisense treatments. These techniques are reviewed, and their potential applications to alcohol research are discussed.