Anxiolytic effects of steroid hormones during the estrous cycle. Interactions with ethanol

Behavioral differences in anxiety have been observed between both males and females and across the ovarian cycle in females. However, the data are not entirely consistent and the mechanisms of this potential interaction are largely unexplored. It appears that the GABA/BZ receptor complex is a site of action for steroids as well as for many anxiolytic drugs. Both natural steroids, such as progesterone and its metabolites, and synthetic steroids, such as alphaxalone, reduce anxiety-like behavior in rats. Alphaxolone also reverses the behavioral effects of potent anxiogenic agents in the conflict test of anxiety. Studies reported here found that ethanol administered to rats in different phases of the estrous cycle was more effective as an anxiolytic when hormone levels were high. The anticonflict response to chlordiazepoxide also was examined in ovariectomized and steroid-replaced female rats. Insight into the mechanisms and sites of action for these steroids can be gained from such an approach.

Dopamine production in the caudate putamen restores feeding in dopamine-deficient mice

Dopamine-deficient (DD) mice cannot synthesize dopamine (DA) in dopaminergic neurons due to selective inactivation of the tyrosine hydroxylase gene in those neurons. These mice become hypoactive and hypophagic and die of starvation by 4 weeks of age. We used gene therapy to ascertain where DA replacement in the brain restores feeding and other behaviors in DD mice. Restoration of DA production within the caudate putamen restores feeding on regular chow and nest-building behavior, whereas restoration of DA production in the nucleus accumbens restores exploratory behavior. Replacement of DA to either region restores preference for sucrose or a palatable diet without fully rescuing coordination or initiation of movement. These data suggest that a fundamental difference exists between feeding for sustenance and the ability to prefer rewarding substances.

Amiloride-sensitive signals and NaCl preference and appetite: a lick-rate analysis

Rats prefer hypotonic and isotonic NaCl solutions to water in long-access drinking paradigms. To focus on the role of taste signals in NaCl preference, licking patterns of rats with 30-s exposure to NaCl solutions (0-0.5 M) were examined when they were either water deprived, sodium depleted, or not deprived (NaCl mixed in dilute sucrose). In all three conditions, rats displayed a preference for NaCl. The addition of 100 microM amiloride, a sodium channel blocker, to NaCl did not change rats' licking when they were sodium replete but dramatically reduced licking when they were deplete. Transection of the chorda tympani (CT) nerve, an afferent pathway for amiloride-sensitive Na(+) signals, had no effect on NaCl preference in nondeprived rats and only a modest effect on those that were Na(+) deplete. Amiloride was found to exert significant suppression of NaCl intake in Na(+)-depleted rats with transection of the CT, supporting the existence of other afferent pathways for transmission of amiloride-sensitive Na(+) signalling. Together, these studies argue for the involvement of different neural signalling mechanisms in NaCl preference in the presence and absence of explicit Na(+) need.

Neurotransmitter transporters: target for endocrine regulation

Aminergic signaling in the CNS is terminated by clearance from the synapse via high-affinity transporter molecules in the presynaptic membrane. Relatively recent sequence identification of these molecules has now permitted the initiation of studies of regulation of transporter function at the cellular and systems levels. In vitro studies provide evidence that the transporters for dopamine, serotonin, and gamma-aminobutyric acid are substrates for regulation by protein kinase C signaling. In vivo studies provide evidence that insulin and adrenal and gonadal steroid hormones may regulate the synthesis and activity of the transporters. Future directions should permit evaluation of the role of endocrine regulation in neurotransmitter clearance, and thus in the maintenance of normal CNS aminergic signaling.

Food deprivation decreases mRNA and activity of the rat dopamine transporter

We have hypothesized that the midbrain dopamine (DA) neurons are a target for insulin action in the central nervous system (CNS). In support of this hypothesis, we have previously demonstrated that direct intracerebroventricular infusion of insulin results in an increase in mRNA levels for the DA reuptake transporter (DAT). In this study, 24- to 36-hour food deprivation was used as a model of decreased CNS insulin levels, to test whether DAT mRNA levels, DAT protein concentration or DAT functional activity would be decreased. DAT mRNA levels, assessed by in situ hybridization, were significantly decreased in the ventral tegmental area/substantia nigra pars compacta (VTA/SNc) (77 +/- 7% of controls, p < 0.05) of food-deprived (hypoinsulinemic) rats. Binding of a specific high-affinity DAT ligand (125I-RTI-121) to membranes from brain regions of fasted or free-feeding rats provided an estimate of DAT protein, which was unchanged in both of the major terminal projection fields, the striatum and nucleus accumbens (NAc). In addition, we utilized the rotating disk electrode voltametry technique to assess possible changes in the function of the DAT in fasting (hypoinsulinemic) rats. The Vmax of DA uptake was significantly decreased (87 +/- 7% of control, p < 0.05), without a change in the Km of uptake, in striatum from fasted rats. In vitro incubation with a physiological concentration (1 nM) of insulin resulted in an increase of striatal DA uptake to control levels. We conclude that striatal DAT function can be modulated by fasting and nutritional status, with a contribution by insulin.

Deficits in discriminated learning remain despite clearance of long-term persistent viral infection in mice

Mice persistently infected with lymphocytic choriomeningitis virus (LCMV) exhibit impaired learning ability. In this report, we determined whether clearance of the virus was associated with restoration of behavioral function. Neonatal Balb/cByJ mice were persistently infected with LCMV and tested as adults in a nonconditional spatial discrimination task. The presence of viral proteins in neurons was confirmed immunohistochemically and infectious virus was quantified in the blood by plaque assay. LCMV-infected adult mice made more errors in a Y-maze avoidance task compared to sham-inoculated controls. After the initial behavioral analysis, infected and control mice received a dose of cytotoxic T-lymphocytes sufficient to clear virus from these mice. Following complete clearance of the virus, mice were re-tested in the behavioral task, 5 months after the original test. No reversal of the learning deficit was seen following viral clearance; mice that had been cleared of the virus and those that remained persistently infected behaved similarly. These data indicate that persistent LCMV infection of the CNS lasting up to 7 months results in discriminated learning impairments that are not reversed by subsequent anti-viral immunocytotherapy.

The anxiolytic-like effects of the neurosteroid allopregnanolone: interactions with GABA(A) receptors

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) was administered systemically to rats which were tested in the Geller-Seifter conflict paradigm, an established animal model of anxiety. Allopregnanolone was found to produce significant anxiolytic-like effects at a dose of 8 mg/kg. When three ligands that function at different sites on the gamma-aminobutyric acid/benzodiazepine receptor-chloride ionophore complex (GABA(A) receptors) were examined in conjunction with allopregnanolone, the anti-conflict effects of allopregnanolone were effectively reversed only by the benzodiazepine receptor inverse agonist RO15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-alpha]-[1,4]benzodiazepine-3-carboxylate). Since this inverse agonist has been reported to inhibit the GABA(A)-activated chloride flux in neuronal membranes, it is likely that the stimulation of the chloride channel in GABA(A) receptors is an important component of the effects of allopregnanolone. In contrast, the benzodiazepine receptor antagonist flumazenil (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-alpha]-[1,4]benzodiazepine-3-carboxylate) did not block the anxiolytic-like actions of allopregnanolone, indicating that allopregnanolone does not bind at the benzodiazepine site directly. Isopropylbicyclophosphate, which binds at the picrotoxinin site on the GABA(A) receptors and blocks the behavioral actions of ethanol, also dose-dependently reversed the anti-conflict effect of this neurosteroid. The results suggest that allopregnanolone may be working either at a site specific for the benzodiazepine receptor inverse agonist RO15-4513 or at the picrotoxinin site to produce its potent anxiolytic-like behavioral effects.

Diabetes causes differential changes in CNS noradrenergic and dopaminergic neurons in the rat: a molecular study

We have previously reported that chronic elevation of insulin in the CNS of rats results in opposing changes of the mRNA expression for the norepinephrine transporter (NET; decreased) and the dopamine transporter (DAT; increased). In the present study we tested the hypothesis that a chronic depletion of insulin would result in opposite changes of NET and DAT mRNA expression, from those observed with chronic elevation of insulin. Rats were treated with streptozotocin to produce hypoinsulinemic diabetes. One week later, steady state levels of mRNA were measured by in situ hybridization for NET in the locus coeruleus (LC) and for DAT in the ventral tegmental area/substantia nigra compacta (VTA/SNc). The mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme for NE and DA synthesis, was measured in these same brain regions. In the diabetic animals, NET mRNA was significantly elevated (159 +/- 22% of average control level) while DAT mRNA was non-significantly decreased (78 +/- 9% of average control level). Additionally, TH mRNA was significantly altered in both the LC (131 +/- 11% of average control level) and VTA/SNc (79 +/- 5% of average control level). We conclude that endogenous insulin is one physiological regulator of the synthesis and re-uptake of NE and DA in the CNS.

Behavioral effects of persistent lymphocytic choriomeningitis virus infection in mice

Lymphocytic choriomeningitis virus (LCMV) is a nonlytic murine virus that provides a valuable model system for studying the behavioral correlates of CNS viral infection. Newborn or immunosuppressed mice infected with LCMV develop a persistent tolerant infection characterized by continuous viral production. Virus can be found in various body organs including lung, liver, kidney, and brain. In brain, neurons are the predominant CNS cells infected and the greatest number of persistently infected neurons are found in the cerebral cortex, hippocampus, other limbic structures and parts of the hypothalamus. Despite continuous infection throughout the animal's life, neurons show no structural injury or dropout. Mice from the DBA/2J strain were infected with LCMV (1000 plaque-forming units) within 18 h of birth and tested for behavioral function as adults. Plaque assays indicated persistent infection in virus-injected mice. Mice were tested for their ability to learn a Y-maze spatial discrimination to avoid the onset of a mild footshock (0.43 mA). The number of correct avoidance responses made during training was taken as a measure of acquisition performance. The virus-infected mice showed a deficit in acquisition of the Y-maze discrimination compared to that seen in vehicle-injected and noninjected controls. Following additional training to reach control levels of performance, the infected mice and the controls were injected with the cholinergic antagonist scopolamine. Scopolamine (2.0 mg/kg) disrupted the performance of the infected mice significantly more than control performance, suggesting that a cholinergic dysfunction accounted for some of the learning deficit. A separate group of virus-infected mice exhibited hypoactivity during the first exposure to a locomotor testing apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

Local implants of testosterone metabolites regulate vasopressin mRNA in sexually dimorphic nuclei of the rat brain

Biosynthesis of the neuropeptide vasopressin (VP) in extrahypothalamic neurons is dependent on circulating levels of testosterone (T). However, the mechanism by which endogenous or peripherally administered T induces VP gene expression in the brain has not been established. This study investigated the effects of androgens and estrogen in the steroid-dependent expression of VP mRNA in the bed nucleus of the stria terminalis (BNST). Testosterone, estrogen, and the T metabolite, dihydrotestosterone (DHT), were either peripherally administered or locally implanted in cannula into the BNST of castrated male rats to determine whether these steroids influence VP gene expression through a local effect within the nucleus itself. The results indicate that T does act locally within the BNST, since complete restoration of VP mRNA levels occurred in BNST neurons in the vicinity of T-containing cannulas but not on the contralateral side. In addition, both DHT and estrogen were partially effective in stimulating VP gene expression in the BNST, and in combination, synergized to produce the full complement of VP gene expression induced by T itself.

Vasopressin deficiency abolishes a sexually dimorphic behavior in Brattleboro rats

The role of vasopressin (VP) in a sexually dimorphic behavior, the extinction of a conditioned taste aversion, was investigated in male and female rats of three different genotypes. This behavior was examined with a two bottle test in the wild-type Long-Evans (LE) rats, and then in partially VP deficient heterozygous (HET-BB) and completely VP deficient homozygous (HO-BB) Brattleboro rats. In Experiment 1, non-deprived male and female LE rats were given aversions to a sucrose solution by pairing it with a LiCl injection. The rate of extinction of the aversion upon reexposure to ad lib sucrose solution was examined and observed to be sexually dimorphic. Female LE rats extinguished at a significantly more rapid rate than males. Experiment 2 compared HET-BB and HO-BB male and female rats using the same paradigm. Neither of these VP-deficient groups showed sexual dimorphism of the extinction behavior. The data suggest that intact VP levels are necessary to observe the expression of this sexually dimorphic behavior.

Enhanced phosphoinositol hydrolysis in response to vasopressin in the septum of the homozygous Brattleboro rat

Arginine8-vasopressin (AVP) receptors in the septum of the Long-Evans rat have been shown to be both pharmacologically (displacement profiles) and functionally (ability to stimulate phosphoinositide hydrolysis) similar to the peripheral V1-type receptor for AVP. Previous binding studies of AVP receptors in the septum of heterozygous (HE) and homozygous (vasopressin-deficient, HO) Brattleboro (BB) rats revealed an increased number of receptors with a lower affinity for AVP in the HO-BB rat when compared to the HE-BB rat. To determine the effect of these receptor changes in the HO-BB rat septum on the postreceptor response of the tissue to AVP, concentration-response relationships for AVP-stimulated phosphoinositide hydrolysis were examined in septal slices from age-matched, adult male HE- and HO-BB rats. AVP-stimulated accumulation of [3H]inositol-1-phosphate (IP1) was significantly greater in the HO-BB (43.7%) than in the HE-BB (13.7%) at AVP concentrations of 10(-08) to 10(-05) M. The two groups did not, however, differ in their ability to stimulate [3H]IP1 accumulation in response to 2.0 mM carbachol. When the AVP-stimulated phosphoinositide response in both genotypes was compared to that obtained for the Long-Evans (LE) rat (the parent strain of the Brattleboro rat) septum under the same assay condition, it was found that the response in the HE-BB was much lower than in the LE. AVP receptor binding capacity (Bmax) correlated (r = 0.975) with release of IP1 ([3H]IP1 accumulation) for all 3 groups studied (LE, HE, HO).(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of a vasopressin antagonist with vasopressin receptors in the septum of the rat brain

The ability of d(CH2)5-Tyr(Me)-arginine-8-vasopressin, an antagonist of peripheral pressoric (V1-type) vasopressin receptors, to label vasopressin binding sites in the septum of the rat brain was evaluated. Using crude membrane preparations from the septum, 3H-arginine-8-vasopressin (AVP) specifically labels a single class of binding sites with a Kd of 2.9 nM and maximum binding site concentration of 19.8 fmole/mg protein. 3H-Antag also labels a single class of membrane sites but with higher affinity (Kd = 0.47 nM) and lower capacity (10.1 fmole/mg protein) than 3H-AVP. The rank order of potency of various competitor peptides for 3H-AVP and 3H-Antag binding was similar. Oxytocin was 100-1,000 fold less potent than AVP in competing for binding with both ligands. 3H-AVP and 3H-Antag showed similar labeling patterns when incubated with septal tissue slices. Unlabeled Antag also effectively antagonized vasopressin-stimulated phosphatidylinositol hydrolysis in septal tissue slices.

Flavor, not postingestive, cues contribute to the salience of proteins as targets in aversion conditioning

Proteins have proven to be more salient targets for aversion conditioning than carbohydrates. The present studies examined the contribution of flavor and postingestive factors to the salience of proteins as targets in aversion conditioning in the rat. Two methods for separating flavor and postingestive cues were used, sham feeding and intragastric gavage. Both methods agreed in indicating that postingestive consequences of protein consumption were neither necessary nor sufficient for the development of more severe protein than carbohydrate aversions. Differences in palatability did not appear likely to be the basis of protein salience because when acceptability or palatability of the nutrient solutions was matched, aversions to protein continued to be more severe. Differences in odor intensity of nutrient solutions may be important because when an odorant was added to a carbohydrate solution, the severity of aversions to protein and carbohydrate was no longer different. These results indicate that the presence of both taste and odor cues in target nutrients may contribute importantly to their salience.

Flavor, not postingestive, cues contribute to the salience of proteins as targets in aversion conditioning

Proteins have proven to be more salient targets for aversion conditioning than carbohydrates. The present studies examined the contribution of flavor and postingestive factors to the salience of proteins as targets in aversion conditioning in the rat. Two methods for separating flavor and postingestive cues were used, sham feeding and intragastric gavage. Both methods agreed in indicating that postingestive consequences of protein consumption were neither necessary nor sufficient for the development of more severe protein than carbohydrate aversions. Differences in palatability did not appear likely to be the basis of protein salience because when acceptability or palatability of the nutrient solutions was matched, aversions to protein continued to be more severe. Differences in odor intensity of nutrient solutions may be important because when an odorant was added to a carbohydrate solution, the severity of aversions to protein and carbohydrate was no longer different. These results indicate that the presence of both taste and odor cues in target nutrients may contribute importantly to their salience.