Antisense oligonucleotides in the study of neuroendocrine systems

Recognition memory reconsolidation requires hippocampal Zif268

Object recognition memory (ORM) serves to distinguish familiar items from novel ones. Reconsolidation is the process by which active memories are updated. The hippocampus is engaged in ORM reconsolidation through a mechanism involving induction of long-term potentiation (LTP). The transcription factor Zif268 is essential for hippocampal LTP maintenance and has been frequently associated with memory processes. However, its possible involvement in ORM reconsolidation has not been determined conclusively. Using Zif268 antisense oligonucleotides in combination with behavioural, biochemical and electrophysiological tools in rats, we found that hippocampal Zif268 is necessary to update ORM through reconsolidation but not to retrieve it or keep it stored. Our results also suggest that knocking down hippocampal Zif268 during ORM reconsolidation deletes the active recognition memory trace.

Loss of synaptonemal complex protein-1, a synaptonemal complex protein, contributes to the initiation of follicular assembly in the developing rat ovary

In the rat ovary, germ and somatic cells become organized into primordial follicles 48-72 h after birth. Although several genes have been implicated in the control of early follicular growth, less is known about the factors involved in the formation of primordial follicles. Using the method of differential display of mRNAs, we found several genes differentially expressed at the time of follicular assembly. One of them encodes synaptonemal complex protein-1 (SCP1), a core component of the protein complex that maintains recombining chromosomes together during prophase I of the first meiotic division in germ cells. This association, evident during the pachytene stage, ends when chromosomal desynapsis begins in the diplotene stage at the end of prophase I. Oocytes become arrested in the diplotene/dictate stage before becoming enclosed into primordial follicles, suggesting that oocytes must complete meiotic prophase I before becoming competent to direct follicle assembly. We now show that attainment of the diplotene stage results in follicular formation. In developing rat ovaries, SCP1 mRNA expression is confined to oocytes and decreases precipitously within 24 h after birth, preceding the organization of primordial follicles. The premature loss of SCP1, achieved via treatment with an antisense oligodeoxynucleotide targeting SCP1 mRNA, resulted in more oocytes reaching the diplotene stage, as evidenced by a decrease in the number of oocytes containing germ cell nuclear antigen-1 (a nuclear protein whose expression ceases in diplotene) and an increase in the number of oocytes expressing MSY2 (a cytoplasmic Y box protein expressed in oocytes that have become arrested in diplotene). SCP1-deficient ovaries exhibited an increased number of newly formed follicles, suggesting that completion of meiotic prophase I endows oocytes with the ability to orchestrate follicular assembly.

Ten years of antisense inhibition of brain G-protein-coupled receptor function

Antisense oligonucleotides (AOs) are widely used as tools for inhibiting gene expression in the mammalian central nervous system. Successful gene suppression has been reported for different targets such as neurotransmitter receptors, neuropeptides, ion channels, trophic factors, cytokines, transporters, and others. This illustrates their potential for studying the expression and function of a wide range of proteins. AOs may even find therapeutic applications and provide an attractive strategy for intervention in diseases of the central nervous system (CNS). However, a lack of effectiveness and/or specificity could be a major drawback for research or clinical applications. Here we provide a critical overview of the literature from the past decade on AOs for the study of G-protein-coupled receptors (GPCRs). The following aspects will be considered: mechanisms by which AOs exert their effects, types of animal model system used, detection of antisense action, effects of AO design and delivery characteristics, non-antisense effects and toxicological properties, controls used in antisense studies to assess specificity, and our results (failures and successes). Although the start codon of the mRNA is the most popular region (46%) to target by AOs, targeting the coding region of GPCRs is almost as common (41%). Moreover, AOs directed to the coding region of the GPCR mRNA induce the highest reductions in receptor levels. To resist degradation by nucleases, the modified phosphorothioate AO (S-AO) is the most widely used and effective oligonucleotide. However, the end-capped phosphorothioate AOs (ECS-AOs) are increasingly used due to possible toxic and non-specific effects of the S-AO. Other parameters affecting the activity of a GPCR-targeting AO are the length (mostly an 18-, 20- or 21-mer) and the GC-content (mostly varying from 30 to 80%). Interestingly, one-third of the AOs successfully targeting GPCRs possess a GC/AT ratio of 61-70%. AO-induced reductions in GPCR expression levels and function range typically from 21 to 40% and 41 to 50%, respectively. In contrast to many antisense reviews, we therefore conclude that the functional activity of a GPCR after AO treatment correlates mostly with the density of the target receptors (maximum factor 2). However, AOs are no simple tools for experimental use in vivo. Despite successful results in GPCR research, no general guidelines exist for designing a GPCR-targeting AO or, in general, for setting up a GPCR antisense experiment. It seems that the correct choice of a GPCR targeting AO can only be ascertained empirically. This disadvantage of antisense approaches results mostly from incomplete knowledge about the internalisation and mechanism of action of AOs. Together with non-specific effects of AOs and the difficulties of assessing target specificity, this makes the use of AOs a complex approach from which conclusions must be drawn with caution. Further antisense research has to be carried out to ensure the adequate use of AOs for studying GPCR function and to develop antisense as a valuable therapeutic modality.

Expression of the nuclear receptor coactivator, cAMP response element-binding protein, is sexually dimorphic and modulates sexual differentiation of neonatal rat brain

Recent studies indicate that the transcriptional activity of steroid receptors is governed by proteins called nuclear receptor coactivators. Using immunocytochemistry, we found that on the day of birth (postnatal d 0) males express higher levels of the nuclear receptor coactivator, cAMP response element binding protein-binding protein (CBP), within the ventromedial hypothalamus, medial preoptic area, and arcuate nucleus. Using Western immunoblots, we confirmed that males have higher levels of CBP on postnatal d 0, 1, and 5; however, there was no sex difference on postnatal d 11. To examine the functional role of CBP, we infused oligodeoxynucleotides that were antisense to CBP mRNA or a scrambled sequence as a control into the hypothalamus of female rats on postnatal d 0, 1, and 2. On postnatal d 1, all rats were injected with 100 microg testosterone propionate to both masculinize (increase male) and defeminize (decrease female) sexual behavior. Rats were ovariectomized in adulthood and tested for adult sexual behavior. Neonatal CBP antisense oligodeoxynucleotides treatment interfered with the defeminizing, but not the masculinizing, actions of testosterone. These results indicate that CBP expression in developing rat brain is sexually dimorphic and an important modulator for steroid hormone action.

AT(1) antisense distinguishes receptors mediating angiotensin II actions in solitary tract nucleus

Angiotensin (Ang) II receptors in the solitary tract nucleus (nTS) are located on vagal sensory-afferent fiber terminals as well as on neuronal cell bodies. Results from in vitro slice preparations indicate that approximately 50% of the neuronal excitatory actions of Ang II result from actions at presynaptic receptors. The differential contribution of actions on fiber terminals versus neuronal cell soma to the cardiovascular effects of Ang II in the nTS is not known. We used antisense oligonucleotides to the angiotensin type 1 (AT(1)) receptor, which should reduce receptors on neurons within the injection site but not those on fiber terminals projecting to the nTS. Ang II injections (250 fmol/30 nL) into the nTS reduced blood pressure by 14+/-1 mm Hg and heart rate by 13+/-1 bpm (n=8) in male Sprague-Dawley rats anesthetized with chloralose/urethane. Although there was still a significant fall in pressure that was induced by Ang II at 90 and 150 minutes after AT(1) antisense (164 pmol/120 nL) was injected into the nTS, the response was blunted 50% (P<0.01). Heart rate responses were completely blocked at the 150-minute time point. Scrambled sequence oligonucleotides did not alter Ang II responses at any time. There was a 40% reduction in (125)I[Sar(1)Thr8]-Ang II binding when antisense-injected and noninjected sides of the nTS were compared with receptor autoradiography. This finding is consistent with the continued presence of AT(1) receptors on afferent fibers. This unique strategy illustrates that both presynaptic fiber terminals and nTS neurons are involved in the blood pressure lowering actions of Ang II, whereas heart rate responses are largely due to actions directly on nTS neurons and activation of vagal efferent pathways.

Mmu and D2 receptor antisense oligonucleotides injected in nucleus accumbens suppress high alcohol intake in genetic drinking HEP rats

Numerous pharmacological and other studies have implicated both Mmu and dopamine receptor subtypes in alcohol consumption. In the genetic drinking rat as well as those chemically induced to drink, evidence has accrued that the abnormal intake of alcohol is underpined by these receptors in the brain. The purpose of this investigation was to demonstrate unequivocally that a biological impairment by antisense oligodeoxynucleotide (ODN) targeted specifically to these two receptor subtypes would disrupt ongoing alcohol drinking. In this project, a new strain of female and male high-ethanol preferring (HEP) rats was used that had free access to preferred concentrations of alcohol over water in a two choice paradigm. A guide cannula for a microinjection needle was first implanted bilaterally above the nucleus accumbens (NAC) of each rat. Following recovery, a dose of either 250 or 500 ng of the Mmu ODN or 500 ng D2ODN was microinjected into the NAC of the rat in a volume of 0.8-1.0 microl. A standard temporal sequence was used in which microinjections were given four times at successive 12-h intervals over a 2-day interval. The control mismatch ODNs corresponding to both the Mmu or D2 receptor antisense were microinjected identically at homologous sites in the NAC. Following the experiments, the brain of each rat was removed and sectioned in the coronal plane for histological analysis so that each microinjection site was identified. The results showed that the Mmu receptor antisense caused a significant dose dependent fall in free access alcohol drinking within 12 to 24 h following the initial microinjection. This decline often persisted for 1 to 2 days in terms of both g/kg intake and proportion of alcohol to water consumed. Similarly, the D2 receptor ODN likewise induced an intense and significant decline in both g/kg and proportion measures of alcohol intake. Since the corresponding mismatch ODN for both Mmu and D2 receptors exerted no effect on either of these measures of alcohol consumption, the specificity of molecular action of the respective antisense molecules on drinking behavior of the HEP rats was confirmed. Thus, these results provide the first unequivocal evidence that the genes for D2 and Mmu receptors are fundamentally involved in abnormal alcohol drinking in the genetically predisposed individual. Finally, important new anatomical evidence is introduced for the critical role of the NAC in the genetic basis of aberrant drinking of alcohol.

Water-absorbent polymer as a carrier for a discrete deposit of antisense oligodeoxynucleotides in the central nervous system

One of the problems of introducing antisense oligodeoxynucleotides (ODN) into the central nervous system (CNS) is their rapid disappearance from the target site due to their dispersion and diffusion, which results in poor uptake and/or retention in cells (M. Morris, A.B. Lucion, Antisense oligonucleotides in the study of neuroendocrine systems, J. Neuroendocrinol. 7 (1995) 493-500; S. Ogawa, H.E. Brown, H.J. Okano, D.W. Pfaff, Cellular uptake of intracerebrally administrated oligodeoxynucleotides in mouse brain, Regul. Pept. 59 (1995) 143-149) [2,5]. Recently, we adapted a new method using water-absorbent polymer (WAP; internally cross-linked starch-grafted-polyacrylates) as a carrier for antisense ODN. The polymer forms a hydro-gel after absorbing water which is chemically and biologically inert. In these studies, the polymer (powder-form) is fully swollen by physiological saline containing antisense ODN (0.2 micromol/ml) to make 80-fold volume gel. Hydro-gel (1 microliter) is injected into the target site, and water solutes are assumed to be diffused stoichiometrically into CNS from the surface of the gel. Histological studies indicate that 24 h after the injection, antisense ODN (5'biotinylated-S-oligos of 15 mer) are distributed to within 800 micrometer from the edge of the area where the gel is located and then gradually disappear from this area within days, but still remain within 300-micrometer distance 7 days later. Antisense ODN are effectively incorporated by all the cell types examined, i.e., neurons, astrocytes and microglias, and suppress the synthesis of the target protein. This method can be adapted to slow delivery of antisense ODN and other water soluble substances into the CNS.

Hypothalamic paraventricular nucleus modulates maternal aggression in rats: effects of ibotenic acid lesion and oxytocin antisense

Central oxytocin (OT) appears to be crucial for maternal behavior. OT, through the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN), can exert its physiological and behavioral effects by acting on OT receptors in nonpituitary projections of the PVN. The purpose of the present study was to analyze the role of the PVN and OT on maternal aggressive behavior in two different periods after delivery: on the fifth day (period of high aggressiveness) and on the eighteenth day postpartum (period of low aggressiveness). In the first experiment, ibotenic acid was injected into the PVN in order to lesion the parvocellular neurons. A second experiment was designed to study more specifically the effects of OT using the antisense technique. On the fifth day postpartum, both the PVN lesion by the ibotenic acid and a possible acute reduction of OT synthesis by the antisense administration in that nucleus increased maternal aggressive behavior, while on the eighteenth day postpartum no effect was recorded. We may conclude that central projections of the PVN modulate maternal aggression during a restricted period after delivery, only when lactating females show naturally high levels of aggressive behaviors.

Antisense oligomers of cfos and cjun block glucocorticoid stimulation of thyrotropin-releasing hormone (TRH) gene expression in cultured anterior pituitary cells

The proto-oncogenes, cfos/cjun, are co-localized with thyrotropin-releasing hormone (TRH) in cultured anterior pituitary cells and increase following exposure to dexamethasone (Dex). To assess the role of cfos and cjun in the Dex stimulation of TRH gene expression, we used antisense oligonucleotides to block cfos and cjun expression in order to reduce formation of activating protein-1 (AP-1). The results showed that the antisense oligonucleotides together effectively reduced cfos/cjun gene expression and consequently the glucocorticoid stimulation of TRH peptide and mRNA. The findings indicate that cfos/cjun are involved in the glucocorticoid activation of TRH gene expression.

Oxytocin pathways mediate the cardiovascular and behavioral responses to substance P in the rat brain

Stimulation of brain periventricular and hypothalamic substance P receptors induces a pressor response and tachycardia associated with mesenteric and renal vasoconstriction and hindlimb vasodilation resembling thus the classical defense reaction. This cardiovascular response is brought about by the activation of the sympathoadrenal system and is accompanied by grooming behavior. To address the role of oxytocinergic pathways in the brain in the mediation of these responses, we investigated the effects of central pretreatment of rats with oxytocin antisense, mixed base, and sense oligodeoxynucleotides on mean arterial pressure, heart rate, and grooming behavior induced by intracerebroventricular injections of substance P (50 pmol). Central pretreatment of conscious rats with the oxytocin antisense oligodeoxynucleotide (intracerebroventricular injections, 8 and 4 hours before administration of substance P) attenuated the mean arterial pressure (by 55%) and heart rate responses (by 58%) as well as grooming behavior induced by the peptide. A complete recovery of all substance P-induced responses was observed 28 hours after antisense oligodeoxynucleotide pretreatment. Intracerebroventricular pretreatment of rats with mixed base and sense oligodeoxynucleotides did not affect the cardiovascular and behavioral responses to substance P. The signal for oxytocin mRNA in the paraventricular nucleus was reduced only in rats pretreated with the antisense oligodeoxynucleotide. These results demonstrate that oxytocin neurons in the paraventricular nucleus, which innervate the cardiovascular centers in the hindbrain and the spinal cord, mediate the increases in blood pressure and heart rate induced by stimulation of substance P receptors in the forebrain. These neurons may also transmit signals, which are generated by substance P in the hypothalamus and are responsible for the sympathoadrenal activation in response to stress.

Critical issues in the antisense inhibition of brain gene expression in vivo: experiences targetting the 5-HT1A receptor

There have been many recent reports of receptor down-regulation in the brain by antisense oligodeoxynucleotides (ODNs) administered in vivo. However, the literature is inconsistent regarding the experimental criteria that are necessary or sufficient to demonstrate a true antisense effect. Here we review some of the critical conceptual and methodological issues. We highlight the problems of specificity and toxicity encountered in our attempts to down-regulate the 5-HT1A receptor using a phosphorothioate-modified ODN. We also present preliminary data suggestive of a decreased hippocampal 5-HT1AR expression induced by the antisense ODN, but it is a reduction which is of limited extent and which does not provide unequivocal evidence for an antisense-mediated effect. We conclude that antisense ODNs are not yet suitable as tools for routine in vivo neuropharmacological use, although they show considerable promise.

An antisense oligodeoxynucleotide to lipocortin 1 reverses the inhibitory actions of dexamethasone on the release of adrenocorticotropin from rat pituitary tissue in vitro

Our previous studies have demonstrated that lipocortin 1 (LC1, also called annexin 1) is an important mediator of glucocorticoid action in the neuroendocrine system, particularly with regard to the powerful inhibitory actions of the steroids on the secretion of ACTH and its hypothalamic releasing hormones. In the present study, we have used an antisense oligodeoxynucleotide (ODN) unique to LC1 to investigate further the role of this protein in the regulatory effects of dexamethasone on ACTH release in vitro from rat anterior pituitary cells. Pituitary cells dispersed with collagenase retained their functional and morphological integrity in vitro and sequestered ODNs in a time-dependent manner from the incubation medium. LC1 was readily detected in the cells by Western blot analysis or by immunoprecipitation/autoradiography after preloading with 35S-methionine/cysteine; the bulk of the protein was contained within an intracellular pool but a small amount was attached to the outer cell surface (pericellular). Dexamethasone (100 nm, 2.5 h) initiated de novo synthesis of LC1; it also increased the amount of LC1 in the pericellular pool detected by either method and caused a concomitant decrease in intracellular LC1. The responses to the steroid were prevented by the inclusion in the medium of an LC1 antisense ODN (50 nM, 3.5 h) but the corresponding sense and scrambled ODN sequences were inert. None of the ODN sequences tested influence the expression of annexin 5 in the pituitary tissue. CRH-41 (100 pM-1 mM), forskolin (1 nM-1 mM) and an L-Ca2+-channel opener BAY K8644 (100 pM-1 microM) initiated concentration dependent increases in immunoreactive- (ir-) ACTH release from the pituitary cells that were reduced (P < 0.01) by preincubation with dexamethasone (100 nM, 2.5 h). The inhibitory effects of the steroid were reversed by the LC1 antisense ODN (50 nM, P < 0.01), whereas the LC1 sense and scrambled control sequences (50 nM) were both ineffective in this respect (P > 0.05). The results add further support to the view that the acute inhibitory effects of glucocorticoids on the secretion of ACTH by the pituitary gland are dependent on the generation of lipocortin 1.

An antisense oligodeoxynucleotide complementary to corticotropin-releasing hormone mRNA inhibits rat splenocyte proliferation in vitro

Expression of neuropeptides in immune tissues has been implicated in the paracrine control of immune functions. The effects of the endogenous splenic neuropeptide corticotropin-releasing hormone (CRH) on immune cell proliferation were investigated by incubating splenocytes from adult male Wistar rats in vitro with a specific antisense oligodeoxynucleotide probe complementary to CRH mRNA. Incubation of cells with 1 microgram/ml phosphodiester antisense probe for 24 h prior to stimulation with concanavalin A (Con A) resulted in a 30-65% decrease in 3H-thymidine uptake compared to controls. In spleen cells incubated with a random base sequence (nonsense) probe the uptake of 3H-thymidine was not different to that in control cells. Incubation of cells with either antisense or nonsense phosphorothioate-protected probes resulted in variable uptake of 3H-thymidine, demonstrating that these probes, unlike the phosphodiester probes, have non-specific effects on cells. Addition of synthetic CRH to the cells incubated with the antisense phosphodiester probe partially restored the proliferative response of splenocytes to Con A. Immunoreactive (ir) CRH measured by radioimmunoassay in splenocytes incubated with the antisense probe was significantly less than ir-CRH in splenocytes incubated with the nonsense probe or without probe, indicating that the expression of splenic CRH mRNA was specifically impaired. This attenuation of the cell proliferative response following reduced expression of splenic ir-CRH provides functional evidence for the involvement of endogenously synthesised immune ir-CRH in splenocyte activation.

Hypothalamic paraventricular nucleus, oxytocin, and maternal aggression in rats

Both the lesion of the parvocellular region of the PVN (FIG. 1) and the acute reduction of OT synthesis in that nucleus (FIG. 2) increase maternal aggression in rats. Previous work showed that ibotenic acid as well as the OT antisense in the PVN reduced the level of OT in the brainstem, but not in the pituitary. Therefore, the oxytocinergic parvocellular neurons of the PVN appear to exert an inhibitory effect on the aggressive behavior of the lactating female rat against an adult intruder. In a relationship of a different nature, mother-infant, a facilitatory effect of OT has been shown. Previous work showed a significant decrease of OT mRNA levels in the PVN of female rats during the first 10 days after delivery compared to late pregnancy, which is the inverse ratio of the natural temporal evolution of maternal aggressive behavior. Furthermore, in the present work, a functional decrease of OT mRNA was probably the effect of the antisense in the PVN. In conclusion, OT cells in the PVN appear to play different roles on maternal care and maternal aggression.

Antisense oligodeoxynucleotides as specific tools for studying neuroendocrine and behavioral functions: some prospects and problems

Synthetic antisense oligodeoxynucleotides can inhibit the expression of a gene in a sequence-specific manner at the translational level. Their potential use to understand the role of neuropeptides or neurotransmitters in neuroendocrine and behavioral functions, and perhaps for therapeutic gene suppression, has become of great interest in neuroscience, especially in the cases of absence of available specific antagonists. Whether their action can be fully specific to the target gene and not only sequence-specific is, however, the main question about their application to brain studies. A number of factors such as the mode of action, specificity and chemistry of antisense molecules as well as the carrier vehicle and the time course of antisense treatment, must be carefully considered for the design and successful application of antisense oligonucleotides. Assay systems and controls must be chosen so as to ensure that the observed biological effects of antisense oligodeoxynucleotides do in fact reflect the result of a specific target gene inhibition. This article discusses these biochemical factors with the emphasis on the use of phosphodiester or phosphorothioate oligodeoxynucleotides in neuroendocrine or behavioral studies.

Oxytocin antisense reduces salt intake in the baroreceptor-denervated rat

Experiments were performed to evaluate the role of central oxytocin (OT) in the inhibition of salt intake produced by sinoartic denervation (SAD). The effect of OT antisense treatment on 24 h intake of 2% NaCl in SAD and sham-operated (SO) rats was determined. PVN injection of unmodified antisense oligodeoxynucleotides (ODNs) to OT mRNA decreased intake of 2% NaCl in SAD, but not SO rats. Salt consumption was 22 +/- 4 ml after the injection of control ODN as compared to 8 +/- 4 ml after the OT antisense injection (P < 0.05). SAD animals also demonstrated an increased plasma OT response to salt loading, an elevation from 3.2 +/- 0.7 to 6.9 +/- 0.8 pg/ml. In contrast, salt ingestion produced no significant change in plasma OT in the SO group. The increased endocrine response in the SADs occurred even though salt intake was lower in this group. There were no group differences in plasma electrolytes or posterior pituitary OT content. Results show that OT antisense specifically inhibits salt intake in the denervated rat, suggesting that the central oxytocinergic axis stimulates sodium drive in this experimental model.