Prepulse inhibition (PPI) of tactile startle response in recombinant congenic strains of mice: QTL mapping and comparison with acoustic PPI

JOURNAL OF

GENETICS AND GENOMICS J. Genet. Genomics 35 (2008) 139151

www.jgenetgenomics.org

Prepulse inhibition (PPI) of tactile startle response in recombinant congenic strains of mice: QTL mapping and comparison with acoustic PPI Adam Torkamanzehi a, Patricia Boksa b, c, Ridha Joober b, c, d, * a

University of Sistan and Baluchestan, Zahedan P.O. Box 98155-987, Iran b Douglas Hospital Research Centre, Montreal H4H 1R3, Canada c Department of Psychiatry, McGill University, Montreal H3A 1A1, Canada d Department of Human Genetics, McGill University, Montreal H3A 1B1, Canada Received for publication 1 May 2007; revised 25 September 2007; accepted 17 October 2007

Abstract Prepulse inhibition (PPI) of the startle response is a psychophysiological measure of sensorimotor gating believed to be cross-modal between different sensory systems. We analyzed the tactile startle response (TSR) and PPI of TSR (tPPI), using light as a prepulse stimulus, in the mouse strains A/J and C57BL/6J and 36 recombinant congenic strains derived from them. Parental strains were significantly different for TSR, but were comparable for tPPI. Among the congenic strains, variation for TSR was significant in both genetic backgrounds, but that of tPPI was significant only for the C57BL/6J background. Provisional mapping for loci modulating TSR and tPPI was carried out. Using mapping data from our previous study on acoustic startle responses (ASR) and PPI of ASR (aPPI), no common markers for aPPI and tPPI were identified. However, some markers were significantly associated with both ASR and TSR, at least in one genetic background. These results indicate cross-modal genetic regulation for the startle response but not for PPI, in these mouse strains. Keywords: tactile startle response; prepulse inhibition (PPI); acoustic startle response; recombinant congenic strains of mice; QTL mapping; microsatellite markers

Introduction Prepulse inhibition (PPI) of the startle response has been extensively used as an endophenotype measuring sensorimotor gating and is known to be abnormal in a number of psychiatric disorders including schizophrenia (Braff et al., 1992; Braff et al., 1999; Braff et al., 2001; Grillon et al., 1992; Koch et al., 1996; Kumari et al., 2005; Meincke et al., 2004; Ornitz et al., 1992; Perry et al., 1999; Swerdlow et al., 1995). PPI is a phenomenon wherein the startle response is reduced when a startling stimulus is preceded by a low intensity prepulse (Graham, 1975; Hoffman and Ison, 1980). It is believed that PPI is a multi-modal mechanism to the extent that both prepulse and startle stimuli can be presented in either the same or different sensory modalities. Previous 2

* Corresponding author. Fax: +1-514- 888 4064. E-mail address: [email protected]

reports on mouse PPI showed that acoustic prepulses combined with either acoustic or tactile (airpuff) startle stimuli produced parallel results (Bullock et al., 1997; Logue et al., 1997; Paylor and Crawley, 1997). A more recent report (Ralph et al., 2001), aimed to assess cross-modal PPI tests, used a light prepulse preceding a tactile airpuff startle stimulus in three mouse strains. These results showed that all three strains displayed light PPI of the tactile startle response (tPPI) with interstrain differences analogous to acoustic PPI of the acoustic startle response (aPPI). However some intermodal differences were observed when the animals were challenged with amphetamine. Amphetamine significantly disrupted both tPPI and aPPI in the C57BL/6J and 129S6 mice. However in the 129X1 mice, there was a trend toward an increase in tPPI while aPPI was decreased in response to amphetamine. The authors suggested that the findings may be due to differential sensitivities to amphetamine between the strains.

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Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

In a previous experiment with recombinant congenic strains (RCSs), derived from C57BL/6J and A/J mouse strains, we compared acoustic startle response (ASR) and aPPI between parental and the congenic strains in the two genetic backgrounds (Joober et al., 2002). Only male mice aged 810 weeks were tested in this experiment. Although mean ASR did not differ between the two parental strains, there were significant differences in the magnitude of aPPI, at most prepulse intensities. We also found significant differences for ASR and for aPPI at various prepulse intensities between some RCSs and their parental strains. Subsequent quantitative trait locus (QTL) analyses also revealed association of several marker loci with ASR and acoustic PPI in these young adult male mice, some of which may be related to auditory mechanisms. With ASR and aPPI in mice, an important consideration is the fact that many mouse strains suffer from age-related high frequency hearing loss (HFHL). Using the mouse strain CBA/CaJ, as a reference for normal hearing, Zheng et al. (1999) reported normal auditory brainstem thresholds for C57BL/6J mice until 33 weeks of age. With the A/J strain, there was a mild impairment at higher frequencies at 810 weeks of age. When compared to their own performance at 1 month of age, C57BL/6J mice also showed a mild HFHL by two months of age, which became more pronounced by 67 months (Willott, 1986; Zheng et al., 1999). However, more recent studies (McCaughran et al., 1999) indicate that such high frequency hearing deficits do not affect ASR and aPPI, if white noise is used as the acoustic stimulus. McCaughran et al. (1999) separated the BXD recombinant inbred series derived from C57BL/6J and DBA/2J parental strains into groups with juvenile-, intermediate-, and adult-onset HFHL. They reported that ASR was independent of HFHL in juvenile mice (68 weeks) and there was no effect of age on ASR amplitudes for high frequency stimuli. Furthermore, aPPI in the juvenile-onset HFHL group was similar to that in the adultonset group, when a white noise prepulse stimulus was used. In comparison to the RCSs derived from C57BL/6J and A/J parents, that were used in our current and previous studies, it should be noted that the BXD recombinants used by McCaughran et al. (1999) were derived from C57BL/6J parents crossed with DBA/2J, which suffers from greater HFHL than does A/J (Zheng et al., 1999). Although it does not appear as if hearing deficits have a major influence on acoustic PPI tested in young mice using white noise stimuli, it has been reported in previous QTL mapping experiments that the attribution of a QTL to a behavioral trait may in fact be driven by a sensory deficit (Cohen et al., 2001; Flint, 2003). The use of non-acoustic PPI may avoid experimental complications arising from possible differential hearing loss in mice strains. There are no published studies, to date, on the measurement of tactile startle responses (TSR) and PPI of the tactile startle response (tPPI), in recombinant congenic

strains of mice, and QTL mapping for these behaviors has not been reported. Given this background, the present study had two main aims. The first was to establish strain distribution patterns for TSR and tPPI in the C57BL/6J and A/J parental strains and the RCSs derived from them, and to map provisional QTLs associated with these behaviors. The second aim was to compare strain distributions and mapping data for TSR and tPPI with our previously published data for ASR and aPPI, under the assumption that loci associated with two different modalities are more likely to be fundamental to intrinsic brain mechanisms rather than to sensory mechanisms. To achieve these aims, we assessed TSR and tPPI in the same parental and recombinant congenic strains used by (Joober et al., 2002) to study aPPI, and using similar conditions, except that visual and tactile stimuli were used as prepulse and pulse, respectively.

Materials and methods Recombinant congenic strains (RCSs) We purchased recombinant congenic strains developed by Dr. E. Skamene (McGill University, Montreal, Quebec) through the Montreal General Hospital Research Institute and Emirillon Therapeutics Inc. (Montreal, Quebec). These strains were derived from A/J (A) and C57BL/6J (B) mouse strains originally purchased from Jackson Laboratories (Bar Harbor, ME). The breeding scheme for congenics, first introduced by Demant, and Hart (1986), involved production of an F1 followed by two generations of reciprocal backcrosses [(F1hA)hA and (F1hB)hB] and 15 to 30 generations of sib-mating. The procedure produced 15 strains with the A/J background (also called AcB or A strains) and 22 strains with C57BL/6J background (also called BcA or B strains). Originally, 625 microsatellite markers, informative for C57BL/6J and A/J, were genotyped. However, 5 of these markers were omitted from mapping analysis in this work due to high level of missing genotypic data. Markers were spaced with an average coverage of 2.6 cM throughout the genome. The average representation of the donor genomes in each RCS was 13% with total coverage of 79% and 84% in the AcB and BcA panels, respectively. The average degree of homozygosity within the RCSs was greater than 99%. One of the AcB strains later died out due to poor fertility and was not available for testing. For detailed explanation on the RCS production and genotyping see Fortin et al. (2001).

Animal care and maintenance Breeding and maintenance of animals were carried out

Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

at the animal care facility of the Montreal General Hospital Research Institute using standard practices. Pups were weaned at three weeks of age and identified by ear punching. Post-weaning housing, until adulthood (8 weeks), was in groups of 5 mice of the same strain and sex in each cage. Temperature was kept at 22 f 1eC with lights on and off at 6:00 h and 18:00 h, respectively. Animals were fed with standard mouse chow and water ad libitum. All procedures with animals were in accordance with the guidelines from the Canadian Council on Animal Care and were approved by the McGill University Animal Care Committee. Apparatus and testing Behavioral tests were performed at the Douglas Hospital Research Center. Mice were allowed to recover from the stress of transport to the new facility for 35 days prior to testing. Tactile startle response was measured using two startle chambers (SR-LAB-ABS, San Diego Instruments, San Diego, CA, USA). Each chamber consisted of a clear non-restrictive Plexiglas cylinder resting on a platform inside a ventilated box. Tactile stimuli were delivered as air puffs via copper tubing with an ending in a hole at the top of each animal cylinder. Pressured air was available from a regulated air tank. Prepulses consisted of light flashes from two puck lights, located in front of the animal cylinder in each chamber. Vibrations of the Plexiglas cylinder caused by the whole-body startle response of the animal were transduced into analog signals by a piezoelectric unit attached to the platform. These signals were then digitized and stored by a computer. Sixty-five readings were taken at 1-ms interval, starting at stimulus onset, and the average amplitude was used to determine the startle response. The SR-LAB calibration unit was used routinely to ensure consistent stabilimeter sensitivity between test chambers and over time. Testing occurred between 9:00 h and 17:00 h, on male mice aged 810 weeks. After mice were placed in the startle chambers, a 70 db background noise level was presented for a 5 min acclimatization period and continued throughout the test session. Tactile stimulus startle trials consisted of a 40-ms 20-psi air puff. Trials testing light prepulse inhibition of tactile startle (tPPI) consisted of a 20-ms puck light flash, 100-ms delay, then a 40-ms 20-psi air puff. Each session began with five air puff only trials, followed by 10 trials each of air puff only or light + air puff in pseudorandom order, and concluding with five air puff only trials. There was an average of 15 s (range: 12–30 s) between trials. Light prepulse inhibition of the tactile startle response (tPPI) was calculated as: tPPI % = 100 {[(average startle response on light + puff trials)/(average startle response on puff only trials)] h100}. Tactile startle response (TSR) was calculated as the av-

141

erage response to all of the puff only trials, excluding the first and last blocks of five puff trials presented. Statistical analysis Out of the range values from each strain (strain mean ± 2SD) were excluded. Data from a total of 383 animals from the two parental, 14 AcB and 22 BcA congenic strains (815 animals/strain) were included in the analyses. One-way analyses of variance (ANOVA) was used to compare tactile startle response (TSR) and tPPI % between RCSs within A/J and C57BL/6J backgrounds, using strains as grouping factor. Pearson’s correlation test was used to estimate pair-wise phenotypic correlation coefficients for TSR and tPPI within each genetic background. Intra-class correlation coefficients (t) and coefficients of genetic determination (g2), which estimate broad sense heritability, were calculated from the analysis of variance of strain phenotypes for TSR and tPPI within each background. The formulae used for these estimations were: t = (MS% MSW)/[(MSB+(n1)MSW)] and g2 = (MSBMSW)/ [(MSB+(2n1)MSW)], where MSB and MSW are the between and within strain variances, respectively, and n is the number of animals per inbred strain (Falconer, and Mackay, 1996; Lerman et al., 2002; Lightfoot et al., 2001). MSB includes additive (VA) and dominant (VD) genetic effects, along with common environment variation (VEC). Within strain variance (MSW) is an indication of the residual varianceümainly general environmental effects. With unequal number of animals per strain, n = (1/a1) (NȈni2/N), where a is the number of strains, ni is the number of individuals in the ith strain and N is the total number of animals within each genetic background. QTL analysis The significance of the association of TSR with each genetic marker was determined by comparing mean TSR in all strains with the AA genotype with mean TSR in all strains with the BB genotype at that marker, using one way analyses of variance (ANOVA). ANOVA was performed within each background group, in order to determine the effect of donor genes on a given genetic background. Similar analyses were performed to determine markers associated with tPPI. Genotypes were available for 620 microsatellite markers on most of the strains. A small number of microsatellite markers which were not genotyped unambiguously were considered as missing values. Markers with significantly different phenotypic values were considered associated with the trait. In this provisional mapping study the aim was to identify as many associated markers as possible, by keeping the significance level at 0.05. At the expense of some false positive signals

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Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

due to type I errors associated with multiple testing, this would reduce the type II error and the risk of missing a true association. Any falsely associated marker could be eliminated at a second stage of mapping using F2 analysis in the informative strains.

Results Tactile startle response and PPI Fig.1A shows strain distribution patterns of mean tactile startle responses (TSR) for the AcB (left) and BcA (right) congenics along with their parental strains. There was a significant difference between parental strains for TSR (P < 0.01), with C57BL/6J showing a higher response than A/J. Most congenics with significantly different TSR from

the parental strains, in both backgrounds, showed lower levels of startle compared to the relevant background parent. Within the A background only AcB57 showed a significant difference in TSR from the parental strain (P < 0.05). One other strain, AcB62, had a near significant decrease in TSR (P < 0.07). Within the B background, more congenics were found to be significantly different from the parental C57BL/6J for TSR. These included congenic strains BcA71, BcA73, BcA80, BcA79 and BcA84 (P < 0.01), BcA68 and BcA75 (P < 0.05). BcA72 in this background was the strain with the highest startle response for TSR. However, the difference from the parental strain was only nearly significant (P < 0.07). Strain distribution patterns for tactile PPI (tPPI), calculated as described in the materials and methods, are shown in Fig.1B. Although there was no significant difference between the parental strains for tPPI, some congenics with

Fig. 1. Strain distribution patterns of (A) tactile startle responses (TSR) and (B) prepulse inhibition of the tactile startle response (tPPI) for AcB (left) and BcA (right) recombinant congenic strains. Parental strains are identified by arrows. Strains significantly different from their respective parental are indicated by * (P < 0.05) or ** (P < 0.01).

Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

the B background departed significantly from their parental (BcA77 and BcA68: P < 0.01; BcA73: P < 0.05). All these strains showed higher tPPI when compared to their parental strain. None of the congenic strains, within the A background, departed significantly from the parental A/J for tPPI (although strains AcB51 and AcB60 had close to significant P values). Tactile PPI was negative (i.e. higher startle following prepulse) for a considerable number of congenic strains from both backgrounds. However, none of these strains were significantly different from their respective parental strain. Phenotypic correlations were calculated between TSR and tPPI in both genetic backgrounds. The correlation between tPPI % and TSR was positive and significant in the A background (r = 0.33; P < 0.01). There was no correlation between these two traits in the B background. The intra-class correlation coefficient (t) and the coefficient of genetic determination (g2) are two measures commonly used for estimation of broad-sense heritability in inbred strains. These were estimated for the tactile startle response and tPPI in each genetic background, as described in the materials and methods, and are presented in Table 1. The coefficient of genetic determination (g2) is a more stringent estimate of heritability than t and is much closer to the narrow sense heritability which is an indication of the relative importance of additive genetic variance to the phenotypic variance. Thus, as expected, the t estimates for TSR and tPPI were higher than the g2 estimates, with the estimates being numerically larger in the B background. Overall, these estimates show that the contribution of genetic variation to TSR and tPPI is generally low in the congenics from both background strains. Marker association Table 2 shows the genotypes of the AcB congenic panel for genetic markers significantly associated with TSR in the A background. These markers appeared on chromosomes 1, 2, 4, 6, 8, 10, 12, 14, 18 and 20. Examination of the marker association indicates that except for one marker on chromosome 4 (D4M178) none of the other markers can be assigned, specifically, to influence TSR in these strains. Marker D4M178 is associated with TSR in the A

143

background (P = 0.02) and segregates only in AcB57 and AcB62. AcB57 is the only strain in this panel which had a significantly decreased TSR (P = 0.02) compared to the parental A/J. However, the difference for AcB62 was nearly significant (P = 0.07). These results suggest that D4M178 may be associated with TSR in this background. The rest of the markers, showing association with TSR in this background, may be considered false positives, as they appear in some congenics whose mean TSR did not differ significantly from that of the parental strain (Table 2). Table 3 shows microsatellite markers significantly associated with TSR in the B background. These were located on chromosomes 1, 2, 6, 9, 10, 14, 19 and 20. Markers associated with decreased TSR included a group of three markers on chromosome 9 (D9M67, D9M247 and D9M254), which segregated in BcA80, BcA84 and BcA75, and a single marker on chromosome 20 (DXM79) which segregated in BcA73, BcA79 and BcA75. All of these strains had significantly decreased TSR compared to the parental C57BL/6J (Table 3). Surprisingly, the strain with the lowest TSR (BcA71) did not share any of these markers. Closer examination of the donor genome in this strain revealed two segments of sizes 7.5 cM and 17 cM on chromosomes 1 and 5, respectively, which segregate uniquely in this strain. It is possible that loci in one or both of these segments are responsible for the lowering TSR in this line. However, effects of these loci could not be tested by ANOVA since no variance can be calculated when only one line contains the donor parental genotype. Further analyses, within F2 (or backcross) populations, are needed to verify the possible association with these donor genes. Significant associations were identified between tPPI and 34 markers, located on chromosomes 2, 4, 6, 10, 11 and 18, in the A background (Table 4). Overall, none of these strains harbored a marker with an effect strong enough to generate a significant difference in tPPI when compared to the parental A/J, an observation compatible with the fact that the heritability of this trait was only 5% in this background (Table 1). Thus most of these markers may be considered QTLs with very small effects or false positives. Markers on chromosomes 1, 8, 11, 17, 19 and 20, were associated with tPPI in strains with the B background (Table 5). Most markers in this background, except for two

Table 1 Broad sense heritability estimates for tactile startle responses (TSR) and prepulse inhibition of the tactile startle response (tPPI) in the recombinant congenic strains with A and B genetic backgrounds Variable

Background

a

N

TSR

A

15

154

tPPI%

A

15

154

TSR

B

23

228

tPPI%

B

23

228

Within strains MS

Between strains df MS

                    

10.26

14

48772.82

137

10.26

14

309.13

137

9.89

22

97143.43

197

9.89

22

274.31

197

n

                   

df

P

14573.88

3.347

0.000

0.186

0.103

200.90

1.539

0.106

0.050

0.026

22352.68

4.346

0.000

0.253

0.145

114.89

2.388

0.001

0.123

0.066

a = number of strains, N = total number of animals, and n = number of animals/strain corrected for an unequal number of animals/strain. t = intra-class correlation coefficient and g2 = coefficient of genetic determination (calculated as described in the materials and method).

t

g2

F

b

a

10 13 10 10 10 10 10 10 11 10 9 11 10 10 11

495.79 483.33 450.33 449.06 430.90 426.17 406.05 398.29 398.17 367.21 338.89 326.80 309.36 299.07 276.42

TSR

426.17 406.05 398.29 398.17 367.21 338.89 326.80 309.36 299.07 276.42

430.90

495.79 483.33 450.33 449.06

TSR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Rank

6 7 8 9 10 11 12 13 14 15

5

1 2 3 4

Rank

Chr. cM

Markerb

Chr. cM

Markerb

1 1 1 1 1 2 1 1 2 2

1 1 1 1 1 1 2 1 1 2 2

1

2 40.4 1 1 1 1 1 1 1 1 1 2 1 1 2 2

1

2 41.0 1 1 1 1 1 1 1 1 1 1 2 1 2 2

1

4 8.6 1 1 1 1 1 1 1 1 1 1 2 1 2 2

1 1 1 1 1 1 2 1 2 2

1

4 4 10.5 12.1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 2 2

1

4 13.3 1 1 1 1 1 1 1 1 1 1 2 1 2 2

1

4 17.9 1 1 1 1 1 1 1 1 1 1 2 1 2 2

1

4 19.8 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1

4 21.9 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1

4 21.9 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1

4 28.6 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1

4 28.6 1 1 1 1 1 1 1 1 1 1 1 1 2 2

1

4 35.5 1 1 1 1 1 1 1 2 1 1 1 1 1 1

1

6 45.5 2 2 1 2 1 1 1 2 1 1 1 1 1 1

1

6 46.0 2 2 1 2 1 1 1 2 1 1 1 1 1 1

1

6 46.3 2 2 1 2 1 1 1 2 1 1 1 1 1 1

1

6 48.2 2 2 1 2 1 1 1 2 1 1 1 1 1 1

1

6 49.0 2 2 1 2 1 1 1 1 2 2 2 1 1 2

1

8 6.0 1 1 1 1

8 57.0 1 1 1 1 1 1 1 1 1 1 2 1 2 1 2

8 58.0 1 1 1 1 1 1 1 1 1 1 2 1 2 1 2

8 59.0 1 1 1 1 1 1 1 1 1 1 2 1 2 1 2

8 67.0 1 1 1 1 1 1 1 1 2 1 2 1 2 1 2

8 71.0 1 1 1 1 1 1 1 1 2 1 2 1 2 1 2

10 36.0 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1

10 44.0 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

10 49.0 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

10 50.0 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

10 51.0 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

10 51.5 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

10 52.0 2 2 2 1 2 1 1 1 1 1 1 1 1 1 1

12 52.0 1 1 1 1 1 1 1 1 1 2 1 1 2 1 2

2 1

1 1 1 1 1 1 1 1 1 2

14 28.4

18 4.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

18 5.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

18 8.7 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

18 11.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

18 20.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

18 16.0 2 2 1 1 1 1 2 1 1 1 1 1 1 1 1

20 45.0 2 1 2 2 1 2 1 1 2 1 1 1 1 1 1

D8M D8M D8M D8M D8M D10M D10M D10M D10M D10M D10M D10M D12M D14M D18M D18M D18M D18M D18M D18M DXM 271* 200* 186* 14* 22* 111* 17* 120* 64* 42* 31** 42** 230** 264** 10** 68** 231** 233* 262* 110* 20*

1 1 1 1 1 1 1 2 1 2

1

1 2 101.2 37.0 1 1 1 1 1 1 1 1

D1M D2M D2M D2M D4M D4M D4M D4M D4M D4M D4M D4M D4M D4M D4M D6M D6M D6M D6M D6M D8M 360* 90** 380** 56** 172** 41** 236* 237** 214** 89** 110*** 111*** 288*** 177*** 178* 105* 36* 149* 389* 287* 124*

In the first column, strains are rank ordered according to decreasing values for their mean TSR (shown in column 4). Strains whose mean TSR is significantly different from that of A/J at P < 0.05 are indicated by §. In the uppermost column, markers are arranged by chromosome number and their relative map distance (cM) within each chromosome. The significance of the association at each marker (* = P< 0.05; ** = P< 0.01; *** = P< 0.001) was determined by comparing mean TSR in all strains with the AA genotype with mean TSR in all strains with the BB genotype at that marker. Blocks of closely linked markers which showed the same mean TSR difference and the same P values are grouped together with differential shading. 1 indicates alleles from A (A/J) and 2 alleles from B (C57BL/6J). Dark cells indicate regions from the donor genome.

AcB52 AcB54 AcB51 AcB60 AcB56 AcB53 AcB64 A/J AcB55 AcB61 AcB63 AcB65 AcB58 AcB62 AcB57§

Strain

AcB53 AcB64 A/J AcB55 AcB61 AcB63 AcB65 AcB58 AcB62 AcB57§

# of animals

10

10 10 10 11 10 9 11 10 10 11

AcB56

a

10 13 10 10

# of animals

AcB52 AcB54 AcB51 AcB60

Strain

a

Table 2 Strain genotypes at markers significantly associated with tactile startle response (TSR) in the A/J and AcB recombinant congenic strains

10

11

10

BcA86

BcA69

BcA70

§§

4

306.93

341.19

343.22

344.16

348.87

400.51

400.56

429.71

443.17

454.61

477.22

493.90

497.59

531.98

542.01

547.52

548.00

656.29

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Rank

70

cM

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1

Chr.

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1

2

2

2

2

2

2

2

2

2

1

1

2

2

1

2

2

2

2

2

2

2

2

1 2

1

1

1

17

112 1

2

1

2

2

2

2

2

2

2

2

2

2

2

1

2

2

2

1

2

2

2

2

1

1

2

85.2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

2

2

2

1

15.6

6

2

2

1

2

1

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

17

9

2

2

1

2

1

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

17

9

2

2

1

2

1

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

25

9

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

44

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

49

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

50

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

51

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

51.5

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

52

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

59

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

59

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

69

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

70

10

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

3

14

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

1

2

0.5

19

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

1

2

5

19

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

1

2

8.7

19

2

1

2

1

2

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

50.5

20

In the first column, strains are rank ordered according to decreasing values for their mean TSR (shown in column 4). Strains whose mean TSR is significantly different from that of C57BL/6J at P < 0.05 and P < 0.01 are indicated by § and §§, respectively. b In the uppermost row, markers are arranged by chromosome numbers and their relative map distance (cM) within each chromosome. The significance of the association at each marker (* = P < 0.05) was determined by comparing mean TSR in all strains with the AA genotype with mean TSR in all strains with the BB genotype at that marker. Blocks of closely linked markers which showed the same mean TSR difference and the same P values are grouped together with differential shading. 1 indicates alleles from A (A/J) and 2 alleles from B (C57BL/6J). Dark cells indicate regions from the donor genome.

a

BcA71

BcA80

12

10

10

15

BcA78

BcA87

BcA73§§

5

BcA81

§§

10

BcA85

10

10

BcA74

11

10

BcA82

BcA79§§

10

BcA66

BcA84§§

5

BcA83

11

481.89

10

10

485.58

9

BcA67

C57BL/6J

BcA68§

568.92

12

BcA77

BcA75§

594.35

12

BcA76

668.53

11

BcA72

TSR

# of animals

Strainsa

D1M D2M D2M D6M D9M D9M D9M D10M D10M D10M D10M D10M D10M D10M D10M D10M D10M D14M D19M D19M D19M DXM Markerb D1M 445* 155* 295* 500* 268* 67* 247* 254* 42* 230* 264* 10* 68* 231* 133* 70* 205* 297* 98* 59* 56* 127* 79*

Table 3 Strain genotypes at markers significantly associated with the tactile startle response (TSR) in the parental C57BL/6J and BcA recombinant congenic strains

b

a

6.42 6.37 4.15 4.14 1.70 0.28 1.61 1.92 2.07 5.07 6.09 6.21 6.46 9.72 10.43

PPI %

# of animals

10 10 13 10 10 11 11 10 9 10 10 10 10 11 10

6.42 6.37 4.15 4.14 1.70 0.28 1.61 1.92 2.07 5.07 6.09 6.21 6.46 9.72 10.43

PPI %

10 10 13 10 10 11 11 10 9 10 10 10 10 11 10

# of animals

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Rank

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Rank

Chr. cM

10 44.0 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

10 49.0 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

10 50.0 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

10 51.0 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

10 51.5 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

10 52.0 2 1 2 2 1 1 1 2 1 1 1 1 1 1 1

11 44.8 2 1 1 2 1 1 1 1 1 1 1 1 1 1 1

11 54.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

11 55.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D11M 288*

2 85.2 2 2 1 1 2 1 1 1 1 1 1 1 1 1 1

D2M 500*

D11M 70*

2 83.1 2 2 1 1 2 1 1 1 1 1 1 1 1 1 1

D2M 311*

D11M 245*

2 81.7 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1

D2M 280*

D10M 231*

2 79.7 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1

D2M 401*

D10M 68*

2 78.7 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D2M 452*

D10M 10*

2 77.6 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D2M 310*

D10M 264*

2 41.0 1 1 1 1 1 1 1 1 2 1 1 1 1 2 2

D2M 56*

D10M 230*

2 40.4 1 1 1 1 1 1 1 1 2 1 1 1 1 2 2

D2M 380*

D10M 42*

2 37.0 1 1 1 1 1 1 1 1 2 1 1 1 1 2 2

D2M 90*

Markerb

Chr. cM

Markerb

11 55.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D11M 289*

2 86.3 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D2M 456*

11 56.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D11M 54*

2 91.8 2 2 1 2 2 1 1 1 1 1 1 1 1 1 1

D2M 345**

11 57.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D11M 67*

D11M 263** 11 55.6 2 2 1 2 1 1 1 1 1 1 1 1 1 1 1

2 96.0 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D2M 226*

2 95.5 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

D2M 51*

18 27.0 1 1 1 1 1 1 1 1 1 2 1 2 2 2 1

D18M 53*

2 103.0 2 2 1 2 2 1 1 1 1 1 1 1 1 1 1

D2M 113**

18 31.0 1 1 1 1 1 1 1 1 1 2 1 2 2 2 1

D18M 122*

2 109.0 2 2 1 1 2 1 1 1 1 1 1 1 1 1 1

D2M 266*

18 31.0 1 1 1 1 1 1 1 1 1 2 1 2 2 2 1

D18M 123*

4 35.5 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2

D4M 178*

18 32.0 1 1 1 1 1 1 1 1 1 2 1 2 2 2 1

D18M 52*

6 72.0 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1

D6M 340*

In the first column, strains are rank ordered according to decreasing values for their mean tPPI % (shown in column 4). In the uppermost row, markers are arranged by chromosome numbers and their relative map distance (cM) within each chromosome. The significance of the association at each marker (* = P< 0.05; ** = P< 0.01) was determined by comparing mean tPPI in all strains with the AA genotype with mean tPPI in all strains with the BB genotype at that marker. Blocks of closely linked markers which showed the same mean tPPI difference and the same P values are grouped together with differential shading. 1 indicates alleles from A (A/J) and 2 alleles from B (C57BL/6J). Dark cells indicate regions from the donor genome.

AcB51 AcB60 AcB54 AcB56 AcB58 AcB55 AcB65 AcB52 AcB63 AcB64 A/J AcB53 AcB61 AcB57 AcB62

Strain

a

AcB51 AcB60 AcB54 AcB56 AcB58 AcB55 AcB65 AcB52 AcB63 AcB64 A/J AcB53 AcB61 AcB57 AcB62

Strain

a

Table 4 Strain genotypes at markers significantly associated with tactile PPI (tPPI) in the parental A/J and AcB recombinant congenic strains

11

BcA81

BcA75

5.88

3.34

2.47

2.18

1.77

0.60

0.05

0.51

0.58

1.40

1.64

1.90

2.16

2.89

3.30

5.29

6.20

7.14

7.38

8.13

9.58

12.42

16.09

PPI %

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Rank 81.6

cM

1

2

2

1

2

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

Chr.

D1M 267*

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1

1

2

1

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2.4

11

D11M 2*

2

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2 2

2

2

2

2

2

1.5

11

D11M 62*

1

2

2

2

1

52

8

D8M 113*

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1 2

1

2

2

2

2

1

29.5

17

D17M 88*

2

2

2

2

1

1

24.5

17

D17M 66**

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1

2

2

2

1

1

30.2

17

D17M 139***

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

1

2

2

2

2

1

32.3

17

D17M 7*

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

40

17

D17M 159**

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

2

2

2

2

1

41.5

17

D17M 193*

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

42

17

D17M 160**

2

2

2

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

2

1

1

26

19

D19M 10*

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1

2

2

1

1

43

19

D19M 53***

1

1

2

2

2

2

2

2

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

60

20

DXM 132*

1

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

62

20

DXM 197*

1

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

62.2

20

DXM 153*

In the first column, strains are rank ordered according to decreasing values for their meant PPI % (shown in column 4). Strains whose mean tPPI is significantly different from that of C57BL/6J at P < 0.05 and P < 0.01 are indicated by § and §§. b In the uppermost row, markers are arranged by chromosome numbers and their relative map distance (cM) within each chromosome. The significance of the association at each marker (* = P< 0.05; ** = P< 0.01; *** = P<0.001) was determined by comparing mean tPPI in all strains with the AA genotype with mean tPPI in all strains with the BB genotype at that marker. Blocks of closely linked markers which showed the same mean tPPI difference and the same P values are grouped together with differential shading. 1 indicates alleles from A (A/J) and 2 alleles from B (C57BL/6J). Dark cells indicate regions from the donor genome.

a

10

5

BcA66

11

10

BcA79

BcA72

10

BcA82

10

10

BcA85

10

15

BcA78

BcA80

10

BcA74

C57BL/6J

10

BcA70

11

10

BcA87

4

12

BcA76

BcA71

11

BcA69

BcA84

9

BcA83

10

5

BcA73§

BcA67

12

BcA68§§

BcA86

12

10

BcA77§§

# of animals

Strainsa

Markerb

Table 5 Strain genotypes at markers significantly associated with tactile PPI (tPPI) in the parental C57BL/6J and BcA recombinant congenic strains

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Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

markers on chromosome 17 (D17M159 and D17M160) and two markers on chromosome 20 (DXM195 and DXM153), are likely to be QTLs with very small effects or false positives, as they segregated in strains with both significant and non-significant mean tPPI differences compared to their parental strain. The markers on chromosome 17 had an apparently paradoxical effect on mean tPPI. Strains BcA77 and BcA68 were the only strains segregating the AA genotype at these chromosome 17 markers and they showed the highest tPPI of all the strains. However the A/J parental strain (i.e. the donor of the AA genotype) had lower tPPI compared to the C57BL/6J strain, although the difference between parental strains did not reach statistical significance. The two markers located on chromosome 20, segregated in strains with the lowest tPPI (BcA75 and BcA81). However, tPPI in these two strains was not significantly different from that of the parental strain (Table 5). Comparison of the marker association from this study with our previous results (Joober et al., 2002) found no markers commonly associated with both tactile and acoustic PPI, in either background. However, three markers on chromosome 9 (D9M67, D9M247 and D9M254) positioned at 1725 cM were found to be significantly associated with both tactile startle (TSR) and acoustic startle (ASR) responses, in the B background. Several markers in the A background also showed common association with ASR and TSR. However, these markers segregated in lines in which TSR was not significantly different from the value for the parental line, and hence were considered to be of very small effect or false positives.

Discussion Deficits in PPI have been reported in a number of psychiatric and neurological disorders including schizophrenia, obsessive compulsive disorder, Tourette’s syndrome and Huntington’s disease (Braff et al., 2001; Swerdlow et al., 1995). Thus it has been suggested that PPI deficits may play a role in several disorders with abnormalities in shared components of the specific neural circuitry regulating PPI. Although acoustic prepulse inhibition of the acoustic startle response is the paradigm most often employed in human studies on PPI, PPI of the tactile startle response is also used (Kumari et al., 2003; Swerdlow et al., 2001; Swerdlow et al., 2002) with the assumption that both access largely overlapping neural circuits and regulatory systems. Several attempts, using mouse and rat models, have focused on mapping genes for PPI in order to find suitable candidate loci for schizophrenia and other psychiatric disorders in humans (Joober et al., 2002; Palmer et al., 2003; Palmer et al., 2000; Watanabe et al., 2007). Previous stud-

ies testing the cross-modality of PPI in inbred mice have used acoustic prepulses combined with either acoustic or tactile (airpuff) startle stimuli and reported parallel PPI responses with the two types of startle stimuli (Bullock et al., 1997; Paylor and Crawley, 1997). However, in one of these works (Ralph et al., 2001), which compared light PPI of the tactile startle with acoustic PPI of the acoustic startle, in mice with and without amphetamine administration, it was found that amphetamine modulation of the tactile PPI and acoustic PPI differed between strains. A more recent work (Aubert et al., 2006) comparing auditory and visual PPI in C57BL/6J, 129S2 and BLAB/cByJ inbred mouse strains found opposite strain ranking for the two PPI modalities. All these are suggestive of evidence against cross-modality of PPI. Here we present further evidence supporting the hypothesis that acoustic and tactile PPI are under different modulatory mechanisms. In this study, we analyzed the tactile startle response (TSR) and light prepulse inhibition of the tactile startle response (tPPI) and performed provisional QTL mapping for this behavior in the parental mouse strains A/J and C57BL/6J and 36 recombinant congenic strains derived from them. Our results showed a significant difference for mean TSR, but no difference for tPPI between the two parental strains. In contrast, Joober et al. (2002) who performed the same analyses on the acoustic startle response (ASR) and acoustic prepulse inhibition of acoustic startle (aPPI), using the same panels of congenic strains, found no difference for acoustic startle response (ASR) between the parental strains but reported significant differences in aPPI at four out of the five prepulse intensities used in their experiment. Analysis of TSR and tPPI in the congenic strains revealed significant variation for TSR in both genetic backgrounds. However, tPPI varied significantly only in the B background. In comparison, Joober et al. (2002) found significant variation for ASR and aPPI in both genetic backgrounds. Taken together, the findings that the A panel showed variation in aPPI but not in tPPI suggests differences in genetic regulation of the two forms of PPI. Another difference between the present results on tPPI and previous results on aPPI relates to the direction of the changes in PPI in the B background. In the study by Joober et al. (2002), all RCSs with the B background, whose aPPI were significantly different from that of the parental C57BL/6J, showed decreased aPPI. In contrast, for tPPI, all B strains significantly deviating from the parental C57BL/6J had increased tPPI (Fig.1B). It might be suggested that the paradoxical effect of A genes to increase tPPI in the B background may be due to complex donor-recipient gene interactions. Broad sense heritability estimates from this study (Table 1) were also compared with estimates for acoustic startle reported by Joober et al. (2002). Even the more relaxed intra-class correlation coefficient (t) estimates of heritabil-

Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

ity for tactile PPI in this study had lower numerical values than those reported for acoustic PPI [(0.136 vs 0.184 for tactile and acoustic PPI, respectively. [Note: the t estimate for tPPI is the average from the two backgrounds, and that for aPPI is the average of all prepulse intensities regardless of the genetic background from Joober et al. (2002)]. This suggests that there may be a greater genetic contribution to variation in aPPI than in tPPI. Interestingly, the t estimates for TSR and ASR were comparable (0.218 vs 0.207 for TSR and ASR, respectively). Higher estimates of the broad sense heritability for TSR and tPPI in the B panel of congenics compared to the A panel were consistent with the larger variation between congenic strains within this panel. Coefficients of genetic determination (g2), which provide more conservative estimates of broad sense heritability, and are closer to the actual heritability (also called narrow sense heritability), indicated that the contribution of additive genetic variation to TSR and tPPI is generally low in both panels. This is consistent with the low and/or lack of significant genetic variability, at least within the A background. Another difference, observed between the present results on tactile PPI and the results of Joober et al. (2002) on acoustic PPI, was with respect to the informative strains identified in the two studies. There was only one strain within the A background, strain AcB60 (with a nearly significant difference in tPPI from its parental A/J), which showed changes in both acoustic and tactile PPI (and in the same direction) compared to A/J. Furthermore, within the B background, none of the strains significantly different from the parental C57BL/6J for aPPI (at least at three prepulse levels) were among the significantly different strains identified for tPPI. The unusual behavior of one other strain from the B background (BcA75) was also notable. This strain showed the highest aPPI (statistically significant at a prepulse level of 85dB) in the experiments of Joober et al. (2002), but the lowest tPPI (Fig. 1) in this study (although not statistically significantly different from the parental C57BL/6J). This observation is consistent with what reported by Aubert et al. (2006) and may be considered as further indication of a differential basis for different PPI modalities. We also compared marker association for tactile startle from this study with the results obtained for acoustic startle Joober et al. (2002). Three markers on chromosome 9 were found to show significant common association with both ASR and TSR in the B background. In contrast, out of 620 microsatellite markers which were analyzed, no marker was found to be commonly associated with acoustic PPI (at least, at 2 out of 5 prepulse intensities) and tactile PPI, in either background. Markers for aPPI and for tPPI had quite different chromosomal distributions. Markers associated with aPPI, were found on all 20 chromosomes except for chromosomes 9, 10, 11 and 12, taking

149

both backgrounds together. In comparison, markers associated with tPPI appeared on fewer chromosomes (chromosomes 2, 4, 6, 10, 11,18 and 1, 8, 11, 17, 19, 20 in the A and B backgrounds, respectively). More specifically, the QTLs on chromosome 16 associated with acoustic PPI and were originally reported by Joober et al. (2002), using the same RCSs as used in this study, and later mapped and confirmed by Petryshen et al. (2005), using chromosome substitution strains also derived from the same parental strains, did not show association with tPPI in the present study. Overall, these observations suggest that acoustic and tactile startle responses may share common genetic regulatory elements, but that aPPI and tPPI may not. Indeed, closer examination of the regions in the vicinity of the markers on chromosome 9, namely D9M67, D9M247 and D9M254 which were commonly associated with both ASR and TSR, showed some genes affecting behavior are positioned in these regions. Among these Brmth7 and Nrgn (1 cM apart form D9M67 and D9M254), as well as, Mfrp and Thy1 (less than 1 cM apart from D9M254) are notable (Mouse Genome Informatics, 2007). Brmth7 (behavioral response to methamphetamines 7) was identified as a QTL affecting drug abuse, and showed to influence many mouse behavioral traits in the presence of methamphetamine (Grisel et al., 1997). Nrgn (neurogranin) knock out mice exhibits abnormal behavior and show altered hippocampal short- and long-term plasticity (Mouse Genome Informatics, 2007). Mfrp (membrane-type frizzled-related protein) is involved in the degeneration of photoreceptors in mice. Thy1 (thymus cell antigen 1) is expressed in various regions of brain and inhibits Long term potentiation in the dentate gyrus (Mouse Genome Informatics, 2007). However, further research is needed to establish a definite link between any of the above mentioned loci with ASR and/or TSR. The main advantage of using non-acoustic PPI in mice has been to avoid the confound of high frequency hearing loss observed in many mouse strains. By combining light prepulses with tactile startle stimuli, PPI can be measured even if hearing loss has already begun. In this report we presented data, for the first time, on tactile startle responses and tPPI using congenic strains of mice. The use of RCSs facilitated QTL analysis and mapping for these traits. Further experiments are needed to genetically dissect the donor segments in the informative strains identified in this study. One such approach involves production and analysis of F2 populations followed by single gene (QTL) isolation and gene introgression. These would provide more accurate information on the genetic basis of the tactile startle response and tPPI. Comparison of QTL analyses for TSR and ASR revealed common genetic elements involved in the regulation of startle elicited by the two modalities. In contrast,

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Adam Torkamanzehi et al. / Journal of Genetics and Genomics 35 (2008) 139151

phenotypic analysis indicated that tactile PPI may be regulated independently of acoustic PPI and therefore the two may have different genetic bases. However, more stringent tests are needed to verify and resolve this issue. Indeed, if the genetic mechanisms implicated in PPI do not overlap across sensory modalities, as shown by this and other studies (Aubert et al., 2006), the assumption that PPI measured in any modality may be used as a general measure of brain gating capacities is challenged. Further research may be needed to dissect the complexity of the differential neural pathways regulating PPI elicited by different sensory modalities.

Acknowledgements This work was supported by a grant from Canadian Institute of Health Research (CIHR) to RJ. We thank Arash Torkamanzehi for developing the software ProDad which assisted in data analysis.

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