Psychomotor stimulation by dopamine D₁-like but not D₂-like agonists in most mouse strains

Many neurological and psychiatric disorders are treated with dopamine modulators. Studies in mice may reveal genetic factors underlying those disorders or responsiveness to various treatments, and species and strain differences both complicate the use of mice and provide valuable tools. We evaluated psychomotor effects of the dopamine D₁-like agonist R-6-Br-APB and the dopamine D₂-like agonist quinelorane using a locomotor activity procedure in 15 mouse strains (inbred 129S1/SvImJ, 129S6/SvEvTac, 129X1/SvJ, A/J, BALB/cByJ, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, SJL/J, SPRET/EiJ, outbred Swiss Webster, and CD-1) and Sprague-Dawley rats, using groups of both females and males. Both D₁ and D₂ stimulation produced hyperactivity in the rats, and surprisingly, only two mouse strains were similar in that regard (C3H/HeJ, SPRET/EiJ). In contrast, the majority of mouse strains exhibited hyperactivity only with D₁ stimulation, whereas D₂ stimulation had no effect or decreased activity. BALB substrains, A/J and FVB/NJ mice showed only decreased activity after either D₁ or D₂ stimulation. CAST/EiJ mice exhibited hyperactivity exclusively with D₂ stimulation. Sex differences were observed but no systematic trend emerged: For example, of the five strains in which a main factor of sex was identified for the stimulant effects of the D₁ agonist, responsiveness was greatest in females in three of those strains and in males in two of those strains. These results should aid in the selection of mouse strains for future studies in which D₁ or D₂ responsiveness is a necessary consideration in the experimental design.

Contributions of dopamine D1, D2, and D3 receptor subtypes to the disruptive effects of cocaine on prepulse inhibition in mice

Deficits in prepulse inhibition (PPI) of startle, an operational measure of sensorimotor gating, are characteristics of schizophrenia and related neuropsychiatric disorders. Previous studies in mice demonstrate a contribution of dopamine (DA) D(1)-family receptors in modulating PPI and DA D(2) receptors (D2R) in mediating the PPI-disruptive effects of amphetamine. To examine further the contributions of DA receptor subtypes in PPI, we used a combined pharmacological and genetic approach. In congenic C57BL/6 J wild-type mice, we tested whether the D1R antagonist SCH23390 or the D2/3R antagonist raclopride would attenuate the effects of the indirect DA agonist cocaine (40 mg/kg). Both the D1R and D2/3R antagonists attenuated the cocaine-induced PPI deficit. We also tested the effect of cocaine on PPI in wild-type and DA D1R, D2R, or D3R knockout mice. The cocaine-induced PPI deficit was influenced differently by the three DA receptor subtypes, being absent in D1R knockout mice, partially attenuated in D2R knockout mice, and exaggerated in D3R knockout mice. Thus, the D1R is necessary for the PPI-disruptive effects of cocaine, while the D2R partially contributes to these effects. Conversely, the D3R appears to inhibit the PPI-disruptive effects of cocaine. Uncovering neural mechanisms involved in PPI will further our understanding of substrates of sensorimotor gating and could lead to better therapeutics to treat complex cognitive disorders such as schizophrenia.

Effects of selective dopamine D1-like and D2-like agonists on prepulse inhibition of startle in inbred C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice

RATIONALE

Prepulse inhibition (PPI) and locomotor activity have been used to investigate the effects of antipsychotic and stimulant drugs and their underlying dopaminergic mechanisms. Whereas D2-like agonists consistently decreased PPI and increased locomotion in rats in previous studies, we recently reported that these hallmark behavioral effects were not observed in several mouse strains. Nevertheless, we recently identified three mouse strains (C3H/HeJ, SPRET/EiJ, and CAST/EiJ) that exhibited locomotor hyperactivity after administration of a selective D2-like agonist.

OBJECTIVES

We hypothesized that, similar to rats, C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice would exhibit decreased PPI after administration of a D2-like agonist.

RESULTS

Administration of the D2-like agonist quinelorane dose-dependently decreased PPI in C3H/HeJ and SPRET/EiJ mice. In agreement with previous reports in rats and other strains of mice, the D1-like agonist R-6-Br-APB also decreased PPI in C3H/HeJ and SPRET/EiJ mice. In contrast, CAST/EiJ mice had low levels of baseline PPI in our standard test session and quinelorane and R-6-Br-APB had no effect on PPI under those conditions. Through the optimization of session parameters, we obtained higher baseline PPI in CAST/EiJ mice and found that quinelorane but not R-6-Br-APB decreased PPI. In summary, similar to rats and unlike previous published reports on several strains of mice, we have now identified three strains of mice in which a D2-like agonist decreased PPI.

CONCLUSIONS

The C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice may more closely mirror the Sprague Dawley rat than most other mouse strains and may confer advantages in cross-species behavioral pharmacology studies related to D2 receptor function.

Effects of repeated maternal separation on prepulse inhibition of startle across inbred mouse strains

A growing body of research implicates genetic factors and childhood trauma in the etiology of neuropsychiatric diseases such as schizophrenia. However, there remains little understanding of how genetic variation influences early life stress to affect later disease susceptibility. Studies in rats have shown that postnatal maternal separation (MS) results in later deficits in prepulse inhibition of the acoustic startle response (PPI), an impairment in sensorimotor gating found in schizophrenic patients. In the present study, genetic differences in the effects of repeated MS on PPI were examined in eight inbred strains of mice (129S1/SvImJ, 129P3/J, A/J, BALB/cJ, BALB/cByJ C57BL/6J, DBA/2J and FVB/NJ). Mice were assigned to either MS (180 min/day on postnatal days P0-P13), 'handling' (15 min/day, P0-P13) or facility-reared conditions and tested for PPI at 12 weeks of age. Results demonstrated major strain differences in the production of viable offspring irrespective of MS, leading to the exclusion of 129P3/J, A/J and BALB/cJ from the study. Pups from the five remaining strains exhibited marked differences in the acoustic startle response and PPI, confirming previous strain comparisons. However, MS produced no significant effects on PPI in any of the strains tested. A second form of postnatal stress (repeated footshock) also failed to alter PPI in the one strain studied, C57BL/6J. Present results demonstrate that the form of MS studied herein does not provide a robust model of early life stress effects on PPI in the mouse strains tested. The development and validation of a reliable mouse model of early life stress remains an important research goal.

Dopamine D1 and D2 agonist effects on prepulse inhibition and locomotion: comparison of Sprague-Dawley rats to Swiss-Webster, 129X1/SvJ, C57BL/6J, and DBA/2J mice

D2 receptors have been studied in relation to therapeutic uses of dopaminergic drugs, and psychomotor stimulant effects [as manifested by decreased prepulse inhibition (PPI) of startle and increased locomotor activity] are hallmark behavioral effects of D2 agonists in rats. Genetic studies with mutant mice might be useful in this line of investigation; however, recent studies suggest that mice differ from rats with respect to D2 agonist effects. Accordingly, we studied a wide range of doses of the D2-like agonist quinelorane (0.0032-5.6 mg/kg) and the D1-like agonist R-6-Br-APB [R(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide] (0.032-5.6 mg/kg) in outbred Sprague-Dawley rats, outbred Swiss-Webster mice, and inbred 129X1/SvJ, C57BL/6J, and DBA/2J mice. Whereas the D2 agonist dose-dependently decreased PPI and increased locomotion in rats, neither of these effects was observed in outbred or inbred mice. In contrast, the D1 agonist reduced PPI and increased locomotion in Sprague-Dawley rats and in Swiss-Webster, 129X1/SvJ, and C57BL/6J mice. Neither agonist decreased PPI in DBA/2J mice, although PPI was increased in this strain by a D2 antagonist. Pretreatment with either the D2 antagonist eticlopride (1 mg/kg) or the D1 antagonist SCH39166 [(-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo[d]naptho-(2,1-b)azepine] (1 mg/kg) prevented the PPI-disruptive effects of quinelorane in rats and R-6-Br-APB in mice, suggesting receptor interactions in both species. In summary, psychomotor stimulant effects of a D2 agonist that were robustly observed in outbred rats were absent in several outbred and inbred strains of mice. These results may have implications for the study of mutant mice to investigate genes involved in psychomotor function in humans.

Strain-specific effects of amphetamine on prepulse inhibition and patterns of locomotor behavior in mice

Several reports describe substantive behavioral differences between strains of mice both at baseline and in response to pharmacological manipulations. For example, mouse strain differences have been reported in prepulse inhibition (PPI) and patterns of locomotor activity, two behavioral processes that are altered by dopamine (DA) agonists such as amphetamine. Here, we characterized acoustic and tactile startle reactivity, acoustic PPI, and both the amounts and spatial patterns of locomotor activity in C57BL/6J, 129SvEv (129S6), and 129SvJ (129X1) mice at baseline and in amphetamine dose-response studies. Because hearing loss is common in numerous strains of mice, we also assessed cross-modal PPI using a light prepulse with an airpuff startle stimulus. The results establish that these three inbred strains of mice display both intra- and cross-modal PPI, and that amphetamine decreases PPI and startle reactivity in a dose-, sensory modality-, and strain-specific manner. Furthermore, the amount of locomotor activity and the spatial pattern of motor sequences are altered differentially after treatment with amphetamine in C57BL/6J and 129X1 mice, but not in 129S6 mice. Given that amphetamine releases presynaptic DA, these findings are consistent with the role of DA in the modulation of PPI and motor patterns in mice. These findings highlight the importance of selecting appropriate strains of mice for behavioral, pharmacological, and genetic studies.

Prepulse inhibition deficits and perseverative motor patterns in dopamine transporter knock-out mice: differential effects of D1 and D2 receptor antagonists

Dopamine is known to regulate several behavioral phenomena, including sensorimotor gating and aspects of motor activity. The roles of dopamine D1 and D2 receptors in these behaviors have been documented in the rat literature, but few reports exist on their role in mice. We used dopamine transporter (DAT) (-/-) mice to examine the behavioral consequences of a chronically hyperdopaminergic state, challenging them with the preferential dopamine D2 receptor antagonist raclopride and D1 receptor antagonist SCH23390. At baseline, DAT (-/-) mice exhibited deficient sensorimotor gating as measured by prepulse inhibition (PPI) of the startle response, exhibited nonfocal preservative patterns of locomotion, and were hyperactive in a novel environment. Pretreatment with raclopride significantly increased PPI in the DAT (-/-) mice, whereas SCH23390 had no significant effect. Blockade of D2 receptors did not affect the predominantly straight patterns of motor behavior produced by the DAT (-/-) mice, but antagonism of D1 receptors significantly attenuated the preservative patterns, producing more of a meandering behavior seen in the DAT (+/+) control mice. Both D1 and D2 receptor antagonists decreased the hyperactivity seen in the DAT (-/-) mice. These findings support the role of the D2, but not the D1, receptor in the modulation of PPI in mice. Furthermore, D1 receptor activation appears to be the critical substrate for the expression of preservative patterns of motor behavior, whereas both D1 and D2 receptors appear to regulate the amount of motor activity.

Behavioral organization is independent of locomotor activity in 129 and C57 mouse strains

Assessing locomotor behavior is a standard methodology to characterize the behavioral phenotype of a genetic manipulation. Typically, levels of locomotor activity are measured using various methods that are based on the frequency of photobeam breaks or distance traveled as assessed by video-tracking systems. Locomotor behavior, however, is multi-dimensional and reflects the combined influences of multiple processes. Here, we examine the number of independent dimensions of locomotor behavior in mice based on measures derived from a video-tracking system. In addition, we test the hypothesis that locomotor behavior varies substantially across mouse strains. 84 mice were tested for 30 min in a 41 x 41 cm enclosure. Based on previous investigations in rats, we also assessed the spatial and dynamical aspects of locomotor behavior using the spatial scaling exponent, d, and the dynamical entropy, h. A principal component analysis and a one-way repeated measure ANOVA were conducted. C57 mouse strains differ substantially from 129 mouse strains on almost all measures of locomotor behavior. The principal component analysis revealed that two independent factors influence this set of measures. The first factor reflects the amount or level of locomotor activity, the second factor quantifies the degree of spatial and dynamical organization of behavior. These strain differences and the existence of at least two independent dimensions when measuring locomotor behavior may help to parse the effects of gene manipulations relative to strain differences in mutant mice.

The dopamine D2, but not D3 or D4, receptor subtype is essential for the disruption of prepulse inhibition produced by amphetamine in mice

Brain dopamine (DA) systems are involved in the modulation of the sensorimotor gating phenomenon known as prepulse inhibition (PPI). The class of D2-like receptors, including the D2, D3, and D4 receptor subtypes, have all been implicated in the control of PPI via studies of DA agonists and antagonists in rats. Nevertheless, the functional relevance of each receptor subtype remains unclear because these ligands are not specific. To determine the relevance of each receptor subtype, we used genetically altered strains of "knock-out" mice lacking the DA D2, D3, or D4 receptors. We tested the effects of each knock-out on both the phenotypic expression of PPI and the disruption of PPI produced by the indirect DA agonist d-amphetamine (AMPH). No phenotypic differences in PPI were observed at baseline. AMPH significantly disrupted PPI in the D2 (+/+) mice but had no effect in the D2 (-/-) mice. After AMPH treatment, both DA D3 and D4 receptor (+/+) and (-/-) mice had significant disruptions in PPI. These findings indicate that the AMPH-induced disruption of PPI is mediated via the DA D2 receptor and not the D3 or D4 receptor subtypes. Uncovering the neural mechanisms involved in PPI will further our understanding of the substrates of sensorimotor gating and could lead to better therapeutics to treat gating disorders, such as schizophrenia.