1B KO Mice Research Articles

5-HT1B autoreceptors differentially modulate the expression of conditioned fear in a circuit-specific manner

Located in the nerve terminals of serotonergic neurons, 5-HT1B autoreceptors are poised to modulate synaptic 5-HT levels with precise temporal and spatial control, and play an important role in various emotional behaviors. This study characterized two novel, complementary viral vector strategies to investigate the contribution of 5-HT1B autoreceptors to fear expression, displayed as freezing, during contextual fear conditioning. Increased expression of 5-HT1B autoreceptors throughout the brain significantly decreased fear expression in both wild-type (WT) and 5-HT1B knockout (1BKO) mice when receptor levels were increased with a cell-type-specific herpes simplex virus (HSV) vector injected into the dorsal raphe nucleus (DRN). Additional studies used an intersectional viral vector strategy, in which an adeno-associated virus containing a double-floxed inverted sequence for the 5-HT1B receptor (AAV-DIO-1B) was combined with the retrogradely transported canine adenovirus-2 expressing Cre (CAV-Cre) in order to increase 5-HT1B autoreceptor expression only in neurons projecting from the DRN to the amygdala. Surprisingly, selective expression of 5-HT1B autoreceptors in just this circuit led to an increase in fear expression in WT, but not 1BKO, mice. These results suggest that activation of 5-HT1B autoreceptors throughout the brain may have an overall effect of attenuating fear expression, but activation of subsets of 5-HT1B autoreceptors in particular brain regions, reflecting distinct projections of serotonergic neurons from the DRN, may have disparate contributions to the ultimate response.

Presynaptic serotonergic modulation of 5-HT and acetylcholine release in the hippocampus and the cortex of 5-HT 1B-receptor knockout mice

Lesioning of serotonergic afferents increases hippocampal ACh release and attenuates memory deficits produced by cholinergic lesions. Improved memory performance described in 5-HT 1B-knockout (KO) mice might thus be due to a weaker 5-HT 1B-mediated inhibitory influence of 5-HT on hippocampal ACh release. The selective delay-dependent impairment of working memory observed in these KO mice suggests, however, that cortical regions also participate in task performance, possibly via indirect influences of 5-HT on ACh release. To provide neuropharmacological support for these hypotheses we measured evoked ACh and 5-HT release in hippocampal and cortical slices of wild-type (WT) and 5-HT 1B KO mice. Superfused slices (preincubated with [ 3H]choline or [ 3H]5-HT) were electrically stimulated in the absence or presence of 5-HT 1B receptor ligands. In hippocampus and cortex, 5-HT 1B agonists decreased and antagonists increased 5-HT release in WT, but not in 5-HT 1B KO mice. In 5-HT 1B KO mice, 5-HT release was enhanced in both structures, while ACh release (in nCi) was reduced. ACh release was inhibited by 5-HT 1B agonists in hippocampal (not cortical) slices of WT but not of 5-HT 1B KO mice. Our data (i) confirm the absence of autoinhibition of 5-HT release in 5-HT 1B-KO mice, (ii) demonstrate a reduced release of ACh, and the absence of 5-HT 1B-receptor-mediated inhibition of ACh release, in the hippocampus and cortex of 5-HT 1B-KO mice, and (iii) are compatible with an indirect role of cortical ACh in the working memory impairment observed in these KO mice.

Light-induced Fos expression is attenuated in the suprachiasmatic nucleus of serotonin 1B receptor knockout mice

The hypothalamic suprachiasmatic nucleus (SCN) is a circadian oscillator that receives a dense serotonergic innervation from the median raphe nucleus. Serotonin (5-HT) modulates the effects of light on circadian behavior by acting on 5-HT 1B receptors on retinohypothalamic (RHT) terminals in the SCN. Activation of 5-HT 1B presynaptic receptors on RHT terminals inhibits glutamate release. However, 5-HT 1B receptor knockout (5-HT 1B KO) mice have attenuated behavioral responses to light [P.J. Sollars, M.D. Ogilvie, A.M. Simpson, G.E. Pickard, Photic entrainment is altered in the 5-HT 1B receptor knockout mouse, J. Biol. Rhythms 21 (2006) 21–32]. To assess the cellular response of the 5-HT 1B KO SCN to light, light-induced Fos expression was analyzed in 5-HT 1B KO and wild-type (WT) mice. In addition, the distribution of melanopsin containing retinal ganglion cells that contribute the majority of axons to the RHT was examined in 5-HT 1B KO mice and compared to that of WT mice. Light-induced Fos expression in the SCN was reduced in 5-HT 1B KO mice compared to WT mice at circadian time (CT) 16 and CT 23 in a manner similar to the reduction previously described in light-induced behavioral phase shifts. The number of melanopsin retinal ganglion cells was similar in WT and 5-HT 1B KO mice. These data taken together with previous data suggest that functional removal of the 5-HT 1B receptor results in reduced functional light input to the SCN.

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 1B receptor knockout mice show no adaptive changes in 5-HT 1A receptor function as measured telemetrically on body temperature and heart rate responses

Two presynaptic receptors play an important role in the regulation of serotonergic neurotransmission, i.e., the 5-HT 1A and 5-HT 1B receptor. The present study focuses on putative adaptive changes in the 5-HT 1A receptor system in mice that lack 5-HT 1B receptors (5-HT 1B KO). 5-HT 1A receptor sensitivity was assessed in vivo in two models of presynaptic 5-HT 1A receptor activity: agonist-induced hypothermia and prevention of stress-induced hyperthermia. The effects of 5-HT 1A receptor activation by flesinoxan (0.1–3.0 mg/kg s.c.) were determined telemetrically on body temperature and heart rate in 5-HT 1B KO and wild-type (WT) mice. Flesinoxan induced hypothermia dose-dependently without affecting heart rate and prevented stress-induced hyperthermia and tachycardia equipotently in both genotypes. Specificity of these responses was confirmed by blockade with the selective 5-HT 1A receptor antagonist WAY100635 (1.0 mg/kg s.c.). The importance of continuous sampling in freely moving subjects to improve appropriate characterization of mutants is discussed. 5-HT 1B KO mice showed no shift in 5-HT 1A receptor sensitivity compared to WT mice. This study found no indications for adaptive changes in presynaptic 5-HT 1A receptor function in 5-HT 1B KO mice as measured telemetrically on body temperature and heart rate responses.

Amphetamine-induced locomotor activation in 5-HT(1B) knockout mice: effects of injection route on acute and sensitized responses.

Knockout mice lacking serotonin(1B) (5-hydroxytryptamine(1B); 5-HT(1B)) receptors (1BKO) exhibit increased sensitivity to the stimulant and reinforcing effects of indirect dopamine (DA) agonists and are more reactive to mild stressors such as handling and the injection procedures that commonly occur when using the intraperitoneal (i.p.) route of drug administration. Since the intravenous (i.v.) route of administration allows minimal handling and injection-induced stress, the present study was designed to evaluate the effect of the administration route on amphetamine-induced locomotor activation and behavioural sensitization in 1BKO mice. For this purpose, 1BKO and wild-type (WT) control mice were administered i.p. or i.v. amphetamine in a within-session design, which allows evaluation of a complete dose-response curve within a single session. The locomotor stimulant effects of i.p. (0.5-4.0 mg/kg) and i.v. (0.6-1.2 mg/kg) amphetamine were investigated both acutely and following repeated treatments (four treatments at 48 h intervals). The results showed that acute i.p. amphetamine injection induced a significant higher horizontal activity peak effect in 1BKO mice, while this difference was less profound after acute i.v. injection. However, repeated i.p. or i.v. administration of amphetamine induced significantly higher locomotion in 1BKO mice. We conclude that the stimulant effects of amphetamine can be influenced by the route of administration in a genotype-dependent manner, and that route of drug administration (and associated variables) should be considered an important factor in studies of psychostimulant action in knockout mice.

Male and female 5-HT 1B receptor knockout mice have higher body weights than wildtypes

5-HT 1B receptors have a regulatory role in serotonergic activity and influence feeding behavior and body weight. Because the absence of 5-HT 1B receptors may cause changes in this regulation, body weight was measured in male and female 5-HT 1B receptor knockout (5-HT 1B KO) and wildtype (WT) mice from weaning until the age of 30 weeks. In both genders, 5-HT 1B KO mice had a higher body weight than WT mice (17% and 9%, respectively). Body weight was significantly higher for males over the entire period and for females from Week 18 onwards. Absolute food and water consumption were related to body weight. However, relative to body weight, males consumed more than females. 5-HT 1B KO males drank strikingly more water. Housing mice singly reduced food and water intake in males, but not in females. Plasma leptin levels and most organ weights did not differ between genotypes, indicating that higher body weight in 5-HT 1B KO mice is not related to obesity. Relative to body weight, brains and adrenals were larger in females, while heart and liver were smaller. Kidneys were smaller in females, but larger in 5-HT 1B KO mice, while lungs showed opposite effects. Spleen and testes were smaller in 5-HT 1B KO mice. Although 5-HT 1B KO males are more aggressive, testosterone levels were not different from WT mice. Basal corticosterone levels were similar in all groups and increased in response to mild stress, particularly in females. Lifelong absence of 5-HT 1B receptors in mice resulted in clear phenotypic differences in body weights and food and water intake. Lacking this receptor increases body growth, without signs of obesity. A potential genetic background effect influencing this phenotype is discussed.

Absence of 5-HT 1B receptors is associated with impaired impulse control in male 5-HT 1B knockout mice

Background: Serotonin (5-HT) plays a complex regulatory role in processes like anxiety, depression, aggression, and impulse control. Due to the large amount of serotonergic receptors, knockout mice offer an important opportunity to investigate the role of specific receptors. The 5-HT 1B receptor is thought to mediate aggression and impulse control. This was studied here in mice lacking 5-HT 1B receptors (5-HT 1B KO). Methods: Wild type and 5-HT 1B KO mice were exposed to several types of entrained and nonentrained stimuli. With telemetry, body temperature, heart rate, and locomotor activity were measured continuously during the different experiments. Results: To nonentrained stimuli like disturbance stress and confrontation with an intruder, 5-HT 1B KO mice showed exaggerated physiologic and behavioral responses. These mice displayed behavioral disinhibition, measured as increased social interest and aggression to an intruder mouse. However, in response to well-entrained stimuli like daily light transitions, responses were smaller in 5-HT 1B KO than in wild type mice, suggesting that hyperreactivity is stimulus specific. Conclusions: Serotonin 1B receptors are essential in impulse control by inhibiting responses to nonentrained stimuli. Therefore, the 5-HT 1B KO mouse might be an important additional model for studying aspects of disinhibition in aggression and impulse control.

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.

Serotonin releasers increase prepulse inhibition in serotonin 1B knockout mice.

Prepulse inhibition (PPI) is the normal reduction of the startle response which occurs when an abrupt startling stimulus is preceded by a weak pre-stimulus and is decreased in several neuropsychiatric disorders. The role of the serotonin 1B (5-HT(1B)) receptor in modulating PPI was investigated using 5-HT-releasing agents in wild-type (WT) and 5-HT1B knockout (1BKO) mice. Whether the differential effects of 5-HT-releasing agents on PPI in WT and 1BKO mice resulted from lack of the 5-HT1B receptor or altered development was also assessed. PPI was assessed in WT and 1BKO mice treated with the 5-HT-releasing agents (+)3,4-methylenedioxy-N-methylamphetamine (MDMA: 0, 10 mg/kg) or (+/-)N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (MBDB: 0, 10 mg/kg). Additionally, intact 129 Sv mice received pre-treatments of the 5-HT1B/1D antagonist GR 127935 (0, 0.75, 1.5, 3.0 mg/kg) and treatments of MDMA (10 mg/kg). MDMA and MBDB increased PPI in 1BKO mice, but did not alter PPI in WT mice. Intact 129 Sv mice receiving 3.0 mg/kg GR 127935 and 10 mg/kg MDMA exhibited increases in PPI. The ability of GR 127935 to increase PPI in intact MDMA-treated mice suggests that lack of the 5-HT1B receptor, and not altered development, is responsible for the PPI-increasing effects of 5-HT releasers in 1BKO mice. 5-HT release activates multiple 5-HT receptor subtypes, which individually may increase or decrease PPI and together have a combined effect on PPI. Our finding that MDMA and MBDB increase PPI in 1BKO, but not WT mice, indicates that the activation of 5-HT1B receptors by 5-HT disrupts PPI.