Increased locomotor activity in mice lacking the low-density lipoprotein receptor

Abstract

While the low-density lipoprotein receptor (LDLR) is best known for its role in regulating serum cholesterol, LDLR is expressed in brain, suggesting that it may play a role in CNS function as well. Here, using mice with a null mutation in LDLR (LDLR-/-), we investigated whether the absence of LDLR affects a series of behavioral functions. We also utilized the fact that plasma cholesterol levels can be regulated in LDLR-/- mice by manipulating dietary cholesterol to investigate whether elevated plasma cholesterol might independently affect behavioral performance. LDLR-/- mice showed no major deficits in general sensory or motor function. However, LDLR-/- mice exhibited increased locomotor activity in an open field test without evidence of altered anxiety in either an open field or a light/dark emergence test. By contrast, modulating dietary cholesterol produced only isolated effects. While both C57BL/6J and LDLR-/- mice fed a high cholesterol diet showed increased anxiety in a light/dark task, and LDLR-/- mice fed a high cholesterol diet exhibited longer target latencies in the probe trial of the Morris water maze, no other findings supported a general effect of cholesterol on anxiety or spatial memory. Collectively these studies suggest that while LDLR-/- mice exhibit no major developmental defects, LDLR nevertheless plays a significant role in modulating locomotor behavior in the adult.

Fig. 1

Plasma cholesterol in C57BL/6J wild type and LDLR-/- mice. C57BL/6J and LDLR-/- mice (N = 12 per genotype and diet condition) were fed either low (C57BL-L, LDLR-L) or high (C57BL-H, LDLR-H) cholesterol diets and blood was drawn for plasma cholesterol levels after completion of behavioral testing.

Fig. 2

General observations, assessment of spontaneous activity and elicited behavior. Body weights (A) were obtained on the first day of behavioral testing as well as on the subsequent days indicated during the testing period that lasted seven weeks. LDLR-L mice weighted consistently less than mice in the other groups while body lengths (B) did not differ among the groups. The score in the bar hanging test (C) was determined by the following scale: 0 = mouse did not grasp the bar; 2 = mouse grasped the bar but fell off in less than 60 seconds; 3 = mouse grasped the bar and held on for longer than 60 seconds. Score represents the sum of three trials. LDLR-L mice scored better than the other groups. No differences were found in total time spent hanging on the bar (D). In negative/positive geotaxis (E) C57BL-L mice righted more quickly than the other groups. All groups contained 12 mice.

Fig. 3

Activity in open field-testing. General locomotor activity and anxiety were assessed in an open field chamber (N = 10, C57BL-L and C57BL-H; N = 11, LDLR-L; N = 9, LDLR-H). Shown are total distance moved (A, F) time spent in motion (B, G), velocity (C, H), as well as time spent in the center (D, I) vs. the periphery (E, J) of the open field on days 1 (A-E) and 2 (F-J) of testing.

Fig. 4

Activity in the initial minutes of the open field test. Shown is time spent in the center of the open field for the first ten minutes of open field-testing on day 1 (A) for the animals tested in figure 3. Also shown is total center time (B, D, F) and distance moved (C, E, G) analyzed in three minute intervals (minutes 1-3 in B, C; minutes 4-6 in D, E; minutes 7-9 in F, G) during the first 9 minutes of testing on day 1.

Fig. 5

Performance in light/dark emergence test. Shown are light edge latency (A), light edge entries (B), light edge duration (C), light center latency (D), light center entries (E), light total duration (F), dark edge distance (G) and dark total distance (H). All groups contained 12 mice.

Fig. 6

Anxiety as assessed by an elevated zero maze. Shown is the amount of time spent in the open (A, B) or closed (B, D) portions of the arc as well as the number of open arm entries (E, F) on days 1 (A, C, E) and 2 (B, D, F) of testing (N = 11, C57BL-L and C57BL-H; N = 9, LDLR-L; N = 8, LDLR-H). There were no differences among the groups.

Fig. 7

Spatial memory assessed by Morris water maze. Shown are escape latencies (A) during the training phase with the platform visible for days 1 and 2 and hidden for days 3-8. In the probe trial (B-H), results are shown for the latency to reach the target site (B), total swimming distance (C), percent time spent in the target quadrant (D) and time spent in the individual quadrants (E-H). All groups contained 12 mice.

Fig. 8

Prepulse inhibition. Shown is the response to a 125 db tone without (A) or with a 79 db prepulse (B), the difference between pulse and prepulse (C), the pulse to prepulse ratio (D) and the percent prepulse inhibition (E). There were no differences among the groups (N = 11, C57BL-L; N = 12, C57BL-H, LDLR-L and LDLR-H).