As MA use is of growing concern, understanding the initial response to the drug may help in understanding why only some individuals become addicted to it. There are many factors to consider including age and genetic predisposition. This study examined older mice (12-month-old) to compare to previous studies in < 6 month-old mice (Brien et al., 1978
; Fukushima et al., 2007
; Itzhak, 1997
; Kuribara et al., 1996
; Milesi-Halle et al., 2005
; Watanabe and Yanai, 2001
). Not every individual develops the same side effects related to their MA exposure, even those who report anxiety effects vary in the type and degree of symptoms (Anglin et al., 2000
). Commonly, rodent research has focused on behavioral sensitization effects in open field paradigm to use as a model for human sensitization; however, there are several other behavioral paradigms that may help to understand MA-related responses. Therefore, we also assessed measures anxiety and activity in the elevated zero maze and the shock-startle response.
During the first part of the 5-day paradigm, we examined 10 min of open field behavior after daily MA injections over 3 consecutive days. We injected MA (1mg/kg) expecting only a slight increase in activity and distance moved, as previously reported in younger mice (Mori et al., 2004
). Our data indicated differences in active time and distance moved in the open field when mice were tested at 30 or 60 min post-injection. At 30 min post-injection, active time and distanced moved increased by day 3 indicating the start of behavioral sensitization. Unexpectedly, at 60 min post-injection on day 1, the mice were less active and slower in MA-related responses. Activity and distance moved increased over the days as expected in this behavioral sensitization paradigm. The lack of MA-related differences 60 min post-injection response on day 3 appears to be caused by variability or agitation in the SA-injected group movement and activity after the injection itself, as all other SA groups steadily declined over the days. Overall, these data suggests that 12-month-old Hdc+/+
wild type mice are hyposensitive to MA compared to younger mice.
Compared to 12-month-old Hdc+/+
mice, 12-month-old Hdc−/−
mice were more sensitive to the effects of MA in terms of open-field behavior at 60 min similar to previous studies in younger mice (Kubota et al., 2002
). Compared to the center of the open field, MA had dramatic genotype-dependent effects on the amount of time spent in the open areas of the elevated zero maze. Compared to genotype-matched mice treated with SA, MA-treated Hdc+/+
mice spent more time in the open areas of the elevated zero-maze than did HDC−/−
mice showed a lower shock-startle response than Hdc+/+
mice, but both genotypes showed an enhanced shock-startle response following MA treatment. Although MA increased both active times and distance moved in the open field in both genotypes, our results show that the responses of 12 month-old male mice are similar to those of younger mice, suggesting that MA-induced hyperactivity is not age-dependent (Iwabuchi et al., 2004
; Kubota et al., 2002
; Watanabe and Yanai, 2001
In the elevated zero maze, MA treatment affected both velocity and where the mice spent their time in a genotype-dependent fashion. While MA reduced the velocity of the Hdc+/+ mice, it increased the velocity of Hdc−/− mice. In contrast to velocity, MA increased the time spent in the open areas in Hdc+/+ mice, while it decreased this measure in Hdc−/− mice. As MA-injected Hdc+/+ mice spent almost 50% of their time in the open areas of the elevated zero-maze, it is possible that they do not find these open areas to be anxiety-provoking. Results showing that the opposite effects of MA on time spent in the open areas are seen in the 12 month-old Hdc−/− mice, indicate that histamine is required for the effects of MA to reduce velocity and increase time spent in the open areas of the elevated zero maze. Follow-up studies aimed to identify whether this effect is similar in younger Hdc−/− mice may help to determine whether this effect is age-dependent.
To determine if a particular histamine receptor is responsible for MA-related behavioral changes or whether it is simply due to the levels of histamine in the brain in older mice, further studies will be needed. Studies indicate that young H1
R knockouts (H1R−/−
) display decreased locomotion in the open field (Inoue et al., 1996
) and young H3R−/−
are more active and less anxious in both the open field and the elevated zero maze (Rizk et al., 2004
). With respect to MA effects, young H1R−/−
exposed to MA show increased locomotor activity, young H2
R knockouts (H2R−/−
) are not affected, but that young H1R−/−
double knockouts show hyperactivity similar to that of Hdc−/−
mice (Iwabuchi et al., 2004
). These data support the idea that the level of histamine in the brain alters the MA response and not necessarily a particular histamine receptor.
In contrast to what has been found in the open field and elevated zero maze, our data indicate that MA increases the shock-startle response in both Hdc+/+
mice similarly. This suggests that the effects seen after MA injections are not related to histamine neurotransmission. The overall lower shock-startle response of the 12 month-old Hdc−/−
mice compared to that of the Hdc+/+
mice suggests that histamine modulates the shock startle response. Our data is consistent with evidence that H3R−/−
mice display hypersensitivity to shock startle (Rizk et al., 2004
). The number of injections (not the time at which they were tested post-injection) was a factor in our paradigm with the SA groups being more sensitive to shock on day 5, which is likely due to repeated injections or handling. Overall, shock startle response may not be a sensitive measure for MA-related behavioral responses.
We recognize that low levels of HA might still be present in the brains of the animals due to HA-containing fish extracts in the diet and HA-producing bacteria in the gut. However, this study does offer a contribution to the field, as it is the first to show that at 12 months of age behavioral responses to MA in male mice are HA-dependent. In addition, this study is the first to examine the effect of MA treatment on zero maze performance. Additional MA doses females mice need to be examined in the future. Sex differences have been indicated in other MA behavioral sensitization paradigms (Milesi-Halle et al., 2005
). The short 5 to 7 day paradigm offers the ability to study acute response to MA with additional open field behavior without excessive handling and fatigue issues. However, more tests need to be included for the development of an appropriate paradigm. Injections at additional time intervals would also be beneficial in the future to examine the MA response before and during peak levels of histamine occurrence.
In summary, MA affects performance in the open field and elevated zero maze in a HA-dependent fashion but MA affects the shock-startle response in a HA-independent fashion.