1AKO Mice Research Articles

Acute 5-HT1A autoreceptor knockdown increases antidepressant responses and serotonin release in stressful conditions

Identifying the etiological factors in anxiety and depression is critical to develop more efficacious therapies. The inhibitory serotonin(1A) receptors (5-HT(1A)R) located on 5-HT neurons (autoreceptors) limit antidepressant responses and their expression may be increased in treatment-resistant depressed patients. Recently, we reported that intranasal administration of modified small interference RNA (siRNA) molecules targeting 5-HT(1A)R in serotonergic neurons evoked antidepressant-like effects. Here we extended this finding using marketed siRNAs against 5-HT(1A)R (1A-siRNA) to reduce directly the 5-HT(1A) autoreceptor expression and evaluate its biological consequences under basal conditions and in response to stressful situations. Adult mice were locally infused with vehicle, nonsense siRNA, and 1A-siRNA into dorsal raphe nucleus (DR). 5-HT(1A)R knockout mice (1A-KO) were also used. Histological approaches, in vivo microdialysis, and stress-related behaviors were performed to assess the effects of 5-HT(1A) autoreceptor knockdown. Intra-DR 1A-siRNA infusion selectively reduced 5-HT(1A)R mRNA and binding levels and canceled 8-OH-DPAT-induced hypothermia. Basal extracellular 5-HT in medial prefrontal cortex (mPFC) did not differ among treatments. However, 1A-siRNA-treated mice displayed less immobility in the tail suspension and forced swim tests, as did 1A-KO mice. This was accompanied by a greater increase in prefrontal 5-HT release during tail suspension test. Moreover, intra-DR 1A-siRNA infusion augmented the increase of extracellular 5-HT in mPFC evoked by fluoxetine, up to the level in 1A-KO mice. Together with our previous report, the present results indicate that acute suppression of 5-HT(1A) autoreceptor expression evokes robust antidepressant-like effects, likely mediated by an increased capacity of serotonergic neurons to release 5-HT in stressful conditions.

Serotonin1A receptor deletion does not interact with maternal separation-induced increases in startle reactivity and prepulse inhibition deficits

RationaleEarly life stress is a risk factor for the development of psychopathology in later life. Consequences of adverse life events, however, may depend on the genetic makeup of an individual. Reduced serotonin1A receptor function may predispose to the development of anxiety disorders.ObjectiveDetermine susceptibility of serotonin1A receptor knockout (1AKO) mice on different background strains to the effects of maternal separation (MS) by assessing startle plasticity in adulthood.Methods1AKO mice on a 129S6 and a Swiss Webster (SW) background were used. MS groups were separated daily from their mother for 180 min/day from postnatal days 2 to 14. Control groups underwent normal animal facility rearing. In adulthood, effects on acoustic startle response, habituation, prepulse inhibition (PPI), and foot shock sensitization were determined.ResultsMS increased startle reactivity and reduced PPI in 129S6 mice. These effects of MS were independent of genotype. MS had no effect on the other readouts. In SW mice, MS had no consistent effect on startle reactivity and did not alter startle plasticity in wild type or in 1AKO mice. 1AKO mice did not differ from wild-type mice in startle plasticity.ConclusionSerotonin1A receptor deletion does not enhance vulnerability to the effects of MS on startle plasticity. The life-long increase in startle reactivity and PPI deficit induced by MS are strain-dependent. Further, the use of startle reactivity and plasticity may have added value in translational studies relating to early life stress.

Early-Life Blockade of 5-HT 1A Receptors Alters Adult Anxiety Behavior and Benzodiazepine Sensitivity

Early-life stress may affect 5-HT(1A) receptor circuitry, which could result in increased anxiety in later life. An increased anxiety phenotype in 5-HT(1A) receptor KO mice (1AKO) mice has been ascribed to 5-HT(1A) receptor absence during the early postnatal period. Thus, subtle and transient serotonergic changes during the early postnatal period may lead to an increased risk for developing stress-related disorders during adulthood. Wildtype and 1AKO mice on a Swiss-Webster (SW) background were treated during the early postnatal period with vehicle or the 5-HT(1A) receptor antagonist WAY-100,635. Pharmacologic 5-HT(1A) receptor blockade during the early postnatal period induced long-lasting effects on anxiety and benzodiazepine sensitivity in adolescent and adult mice on a Swiss-Webster background and resembles the SW 1AKO phenotype. Furthermore, WAY-100,635-treated mice had increased cortical gamma-aminobutyric acid-A receptor (GABA(A)R) alpha(1) and alpha(3) subunit levels and increased hippocampal GABA(A)R alpha(2) subunit levels. Absence of 5-HT(1A)R signaling during early stages of brain maturation predisposes an organism to affective dysfunction later in life. Because early-life treatment with WAY-100,635 in Swiss-Webster mice reduced diazepam sensitivity and increased GABA(A)R alpha subunit levels in the prefrontal cortex and hippocampus, our data suggest a putative link between early-life disruption of the serotonergic system and the emergence of increased anxiety and decreased benzodiazepine responsivity at adult age. Moreover, early-life 5-HT(1A) receptor functionality appears to be essential for the development of normal GABA(A)R functionality. This study may have clinical implications for psychoactive drug use during pregnancy and for the pharmacogenetic background of benzodiazepine sensitivity.

Angiotensin II stimulates superoxide production via both angiotensin AT 1A and AT 1B receptors in mouse aorta and heart

The present study was conducted to determine the roles of angiotensin AT 1A and AT 1B receptors in angiotensin II-induced superoxide anion production in mouse aorta and heart. Superoxide anion production in aorta was determined by the lucigenin chemiluminescence method, and thiobarbituric acid reactive substances in heart tissues were measured by biochemical assay. The basal production rate of superoxide anion in aorta of wild type (WT) mice was significantly higher than in angiotensin AT 1A receptor knockout (AT 1A KO) mice. Angiotensin II (2.8 mg/kg/day, s.c. for 13 days) significantly increased superoxide anion production in aorta of both AT 1A KO and WT mice. However, the superoxide anion production rate in aorta of angiotensin II-infused AT 1A KO mice was significantly lower than in angiotensin II-infused WT mice. Valsartan (40 mg/kg/day in drinking water) prevented angiotensin II-induced superoxide anion production in aorta of WT and AT 1A KO mice. Similarly, thiobarbituric acid reactive substances levels in heart tissues of angiotensin II-treated WT and AT 1A KO mice were significantly higher than those in vehicle-infused WT and AT 1A KO mice, respectively. Valsartan prevented angiotensin II-induced increases of thiobarbituric acid reactive substances levels in heart tissue of both WT and AT 1A KO mice. These results indicate that angiotensin II stimulates superoxide anion production via both angiotensin AT 1A and AT 1B receptors, and that angiotensin AT 1A receptors appear to play a predominant role in angiotensin II-induced superoxide anion production in mouse aorta and heart.

5-HT1A receptor and 5-HT1B receptor knockout mice in stress and anxiety paradigms.

Generation of receptor knockout mice has offered a new approach to study processes underlying anxiety. In this paper, studies focusing on anxiety using 5-HT1A receptor knockout (1AKO) and 5-HT1B receptor knockout (1BKO) mice are reviewed. 1AKO mice on different genetic background strains have initially been described as more anxious. In 1AKO mice on the 129/Sv background strain, the initial findings could not always be replicated, although under certain conditions, mild anxiety-like responses were observed in these 1AKO mice. In 1BKO mice, some indications of reduced anxiety have been found, but these observations may be confounded partly with increased motor impulsivity of these mutants. To study whether the putative effects of the null mutations on anxiety were reflected in the autonomic nervous system, basal heart rate and body temperature of 1AKO and 1BKO mice were measured, as well as their autonomic responses to novel cage exposure and to reversal of the light-dark rhythm. 1AKO mice did not differ from wild-type mice in any parameter, neither under non-stress conditions, nor following novel cage exposure. In 1BKO mice, basal heart rate was reduced and body temperature was increased. 1BKO mice showed exaggerated autonomic responses to novel cage stress. Adaptation to the reversal of the light-dark cycle was comparable in the three genotypes. The stress-induced hyperthermia procedure showed no differential responses of the three genotypes to the stressor. Pharmacological responses to various psychotropic drugs in the stress-induced hyperthermia test were also comparable in 1AKO, 1BKO and wild-type mice. The present data illustrate the complexity of studying the behavioural and physiological consequences of deletion of genes coding for important receptors in the CNS.

Operant learning and differential-reinforcement-of-low-rate 36-s responding in 5-HT 1A and 5-HT 1B receptor knockout mice

Previous studies with mice lacking 5-HT 1A (1AKO) and 5-HT 1B (1BKO) receptors in hippocampus-dependent learning and memory paradigms, suggest that these receptors play an important role in learning and memory, although their precise role is unclear. In the present study, 1AKO and 1BKO mice were studied in operant behavioural paradigms of decision making and response inhibition, to further study the putative involvement of these receptors in prefrontal cortex-dependent learning and memory. Moreover, because 1AKO mice have been shown to exhibit an antidepressant-like phenotype and 1BKO mice to be more impulsive in ethological studies, mice were trained in a differential-reinforcement-of-low-rates (DRL) procedure. Overall, results indicate that 1AKO and 1BKO mice display subtle differences in operant paradigms of decision making and response inhibition compared to wild type (WT) mice. In addition, when responding under a DRL 36-s schedule had stabilised, 1BKO mice showed a phenotype indicative of increased impulsivity, whereas 1AKO mice did not differ from WT mice. In conclusion, 5-HT 1B receptors appear to play an important role in impulsivity and a minor role in prefrontal cortex-dependent learning and memory as shown by the results obtained in serial reversal learning and extinction. In contrast, 5-HT 1A receptors appear to be involved in facilitation of autoshaping, but their role in impulsivity and prefrontal cortex-dependent learning and memory appears to be limited.

5-HT 1A receptor knockout mice and mice overexpressing corticotropin-releasing hormone in models of anxiety

Pharmacological experiments have implicated a role for serotonin (5-HT) 1A receptors in the modulation of anxiety. More recent is the interest in corticotropin-releasing hormone (CRH) system as a potential target for the treatment of anxiety disorders. However, selective pharmacological tools for the CRH system are limited, hampering research in this field. Gene targeting is a relatively new approach to study mechanisms underlying anxiety disorders. 5-HT 1A receptor knockout (1AKO) mice have been created on three different background strains, and two different lines of mice, overexpressing CRH (CRH-OE), have been generated. In the present review, behavioural and physiological findings reported for 1AKO mice and CRH-OE mice will be reviewed. As behavioural phenotyping is often limited to one or two approach avoidance paradigms, we extended these observations and also tested 1AKO and CRH-OE mice in a conditioned fear paradigm. This paradigm reflects essentially different aspect of anxiety than approach avoidance paradigms. 1AKO mice on a 129/Sv background strain showed similar freezing as wild-type (WT) mice. In CRH-OE mice, less freezing was observed than in the corresponding wild-type mice. The fact that the anxious phenotype of these genetically altered mice seems less clear than initially reported will be discussed. Rather than studying the direct consequences of alterations in the targeted gene, 1AKO and CRH-OE mice seem very valuable to study compensatory processes that seem to have taken place in reaction to life-long changes in gene expression.

GABA A–benzodiazepine receptor complex sensitivity in 5-HT 1A receptor knockout mice on a 129/Sv background

Previous studies in 5-HT 1A receptor knockout (1AKO) mice on a mixed Swiss Webster×129/Sv (SW×129/Sv) and a pure 129/Sv genetic background suggest a differential γ-aminobutyric acid (GABA A)–benzodiazepine receptor complex sensitivity in both strains, independent from the anxious phenotype. To further investigate these discrepancies, various GABA A–benzodiazepine receptor ligands were tested in different behavioral paradigms in 1AKO and wild type (WT) mice on a 129/Sv background. 1AKO and WT mice responded comparably to alprazolam, flumazenil, alcohol and pentylenetetrazol as measured in the stress-induced hyperthermia paradigm. In addition, sedative–anesthetic effects of pentobarbital measured via the righting reflex were similar and a selected dose of diazepam exerted similar anxiolytic effects in both genotypes in the elevated plus maze. In conclusion, 1AKO mice on a 129/Sv background have undisturbed GABA A–benzodiazepine receptor sensitivity in contrast to those described on a mixed Swiss Webster×129/Sv background. The anxious phenotype of 1AKO mice seems to occur independent of the GABA A–benzodiazepine receptor complex functioning.

5-HT 1B receptor knockout, but not 5-HT 1A receptor knockout mice, show reduced startle reactivity and footshock-induced sensitization, as measured with the acoustic startle response

To investigate whether the hyperreactivity to mild environmental and novel stimuli in 5-HT 1B receptor knockout (1BKO) mice, as suggested by measures of exploratory, aggressive, and impulsive behaviors, can be extended to phasic stimuli, 1BKO and wildtype mice were tested in acoustic startle reactivity and plasticity paradigms, including habituation, prepulse inhibition, and footshock-induced sensitization of the startle response. Furthermore, we compared 5-HT 1A receptor knockout (1AKO) and 1BKO mice to further test the suggested opposite behavioral profiles in these two genotypes. Results show that startle reactivity and footshock-induced sensitization was reduced in 1BKO mice, with no changes in habituation or PPI. In contrast, 1AKO mice did not differ from WT mice in any of the measures. These results indicate that an absence of 5-HT 1B receptors, but not of 5-HT 1A receptors, affects the modulation of startle reactivity and footshock-induced sensitization, without influencing startle plasticity. Moreover, this study suggests that 1AKO mice display a distinct, but not opposite behavioral profile from 1BKO mice. Furthermore, it is concluded that the hyperreactivity in 1BKO mice cannot be generalized to all stimuli, including the startling stimuli used in this study, but is probably restricted to mild environmental stimuli only.

Knockout Mice Reveal Opposite Roles for Serotonin 1A and 1B Receptors in Prepulse Inhibition

The serotonergic system is involved in the modulation of prepulse inhibition (PPI) and habituation of startle, which are deficient in schizophrenia patients. PPI is the reduction in startle amplitude that occurs when a weak “prepulse” precedes a startling stimulus by 30–500 msec. The roles of 5-HT1A and 5-HT1B receptors in modulating PPI and habituation were examined using wild-type (WT), 5-HT1A knockout (1AKO), and 5-HT1B knockout (1BKO) mice. The 5-HT1A/1B agonist RU24969 reduced PPI and habituation in WT and 1AKO, but not 1BKO mice, whereas the 5-HT1A agonist 8-OH-DPAT increased PPI in WT and 1BKO, but not in 1AKO mice. Similarly, the selective 5-HT1B agonist anpirtoline reduced PPI in WT, but not in 1BKO mice. In experiments using intact 129Sv mice, the 5-HT1A agonist flesinoxan increased PPI while anpirtoline decreased PPI and habituation. Findings suggest that 5-HT1B receptor activation decreases PPI and habituation, and 5-HT1A receptor activation increases PPI in mice.