The data provided in the present experiments demonstrate that similar neurochemical and physiological effects following MA treatment may be observed in mice as are seen in rats after a neurotoxic dosing regimen [8
]. Both rats and mice show increased body temperature and GFAP levels following MA treatment, along with reductions in brain monoamines. However, in terms of behavioral effects, rats and mice differ (at least in so far as egocentric and object recognition learning are concerned) despite similar changes in markers of neurotoxicity. MA causes substantial deficits in egocentric learning in the CWM in rats [30
]whereas this effect was absent in mice. Neither rats nor mice showed differences in spatial learning in the MWM following MA, which is consistent across species. Rats show hypolocomotion for ~3 days after treatment and exhibit a modest differential response to a pharmacological challenge dose of MA [30
], compared to mice that showed recovery of locomotor levels to those of SAL-treated controls after one day and showed an exaggerated hyperlocomotion following a pharmacological challenge dose of MA. Hence, despite neurochemical similarities between rats and mice following a binge/neurotoxic regimen of MA, functionally there are more differences than similarities.
We verified that mice given MA (10 mg/kg) at 2 h intervals 4 times on a single day had increased neostriatal GFAP protein levels 72 h post-treatment, demonstrating increased reactive gliosis as a marker of neurotoxicity as seen in rats. This marker has reliably been shown in rats and mice to reflect MA-induced neurotoxicity [30
As in rats, MA-induced hyperthermia is seen in mice, along with monoamine reductions. Furthermore, the degree of hyperthermia is sensitive to ambient temperature; i.e., increases in ambient temperature heighten neurotoxicity and decreased temperature reduces neurotoxicity [3
]. We observed increased core body temperatures in MA-treated C57BL/6 mice compared to controls, which, combined with increased GFAP levels and previous data [44
], demonstrate that the dose regimen used here was neurotoxic. Higher doses typically cause sharp increases in mortality, placing practical limits on testing higher doses.
Despite evidence of neurotoxicity, MA-treated mice showed no impairments in recognition memory, spatial learning, reference memory, or in egocentric learning. We previously observed deficits in rats in egocentric learning in the CWM [30
]. There was also impairment in novel object recognition memory in rats [30
] that was not observed here, although this effect was not replicated in another study conducted in rats (Herring et al., unpublished). Other groups have observed deficits in novel object learning in mice following MA treatment [5
]. However, these studies used a single, low (1 mg/kg) dose given on multiple days (7 days). These are not neurotoxic doses and are not comparable to the model used here. In addition to differences in dosing regimens, we used a different mouse strain.
In rats, novel object recognition deficits have been reported after binge/neurotoxic MA treatment [7
] however, despite the number of reports, this effect has proven difficult to reliably replicate. No novel object recognition deficits were observed here in C57BL/6 mice, perhaps indicating a species difference or perhaps because the effect on novel object recognition is itself variable. It has also been demonstrated that rats treated with MA using an escalating dose + binge paradigm (14 days) or a single 1 mg/kg MA dose do not demonstrate object recognition deficits [7
There is mounting evidence that recognition memory involves a multi-component system, consisting of contributions from the hippocampus (allocentric) and perirhinal cortex (discrimination of object familiarity and recency)[13
] as well as the prefrontal cortex [52
]. The glutamatergic system in the perirhinal cortex [6
] and the dopaminergic system in prefrontal cortex (D1 receptors) [52
] are important for encoding and/or retrieval in recognition memory tasks. Neither perirhinal nor prefrontal cortices were examined in this experiment and it may be that the current dose did not significantly affect these regions, although the doses used here would be predicted to affect these regions. Another consideration is time since treatment. Perhaps the extended period between treatment and the later tests allowed neurotransmitters to partially recover thereby eliminating behavioral differences on these tests. Prior behavioral testing may also have contributed to the absence of differences on tasks given later in the testing sequence.
To date, little data exist examining the effects of neurotoxic MA doses on spatial learning in the MWM in mice. One study found that mice given 10 mg/kg MA i.p. on a single day demonstrated increased latencies in MWM when examined one week after treatment and these deficits could be attenuated by pseudoginsenoside-F11+
is a saponin-like compound found in ginseng [79
]. Unfortunately, this study did not show whether the single MA dose was neurotoxic or not. No similar MWM deficits were observed in the current study; however the current experiment differed in that an increased interval (2 weeks vs. 1) was imposed between the time of treatment and MWM testing. The reason for our experimental design was to match the approach typically used with rats following a neurotoxic dosing regimen [30
Egocentric learning is the ability of an animal to navigate to a destination using cues based on self-motion, without relying on distal landmarks. This form of learning is used by vertebrates and invertebrates to find their way in their environment and back again [20
]. In our experiment, we did not observe egocentric deficits in MA-treated animals in the CWM. An analysis of escape latencies also showed no effect, suggesting that egocentric learning was not affected in MA-treated animals given neurotoxic doses. While we did not measure swimming speed in the CWM, it was captured by the tracking software during MWM testing and no differences in swimming speed were detected. We have noted that mice vary widely in how they perform in the CWM. For example, some mice search actively to escape, some swim rapidly but enter few cul-de-sacs, others swim slowly, and still others spend intervals not searching. This creates larger variations in performance than are seen when rats are tested in this maze and may result in the test being less sensitive in mice than in rats.
We observed hypoactivity 1 day after MA treatment, but activity levels in MA-treated mice returned to those of SAL-treated controls on the second day. It has been established that mice, as well as other species, become hyperactive shortly following MA treatment [34
], but little is known about the effects on locomotor activity days following treatment with a neurotoxic regimen. We have previously demonstrated hypoactivity 1–3 [30
] and 7 days [74
] following a neurotoxic regimen of MA treatment in rats. The decreased initial locomotion observed in the MA-treated mice in the present experiment may be caused by drug-induced DA reductions in the neostriatum. Neostriatal monoamine levels measured following behavioral testing and those from published studies suggest that DA reductions were likely present shortly after drug treatment. Such monoamine reductions are also evident in rats treated with a neurotoxic MA regimen measured 3 days later; and have been shown to remain reduced several weeks later [30
]. Although hypoactivity was observed in MA-treated mice, swimming ability/speed, assessed by measuring swim velocity in the MWM, was not affected.
Locomotor activity following a 1 mg/kg MA challenge produced a biphasic response in the MA-treated neurotoxic group. MA-treated animals were initially more hyperactive than SAL-treated animals after challenge, but later they became hypoactive. We previously observed this biphasic response in rats [30
], however in mice the duration of the hyperactive phase was longer and the hypoactive phase less pronounced. It is unclear why such a biphasic response occurs. Aside from the reductions in neostriatal DA, it is possible that alterations in DA receptors are involved, but further testing will be needed to test this possibility.
Other neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) have also shown inconsistencies in behavioral effects between rats and mice despite robust depletions of DA. One group demonstrated that MPTP produced increased akinesia and catalepsy [47
], whereas others have not [76
]. Similar observations have been made for spontaneous locomotion in the open field in MPTP models (see [65
] for review). Reduced [60
] and no differences [64
] have been observed in mice on the rotorod test following MPTP. In 6-OHDA-treated mice, deficits in rotorod performance are partially restored by postnatal (P) day 28 [4
]. Consistent MPTP-induced deficits in rotorod performance can only be achieved by chronic MPTP combined with co-administration of an adjuvant such as probenecid [58
]. In the MPTP/probenecid model, motor deficits persist [58
] while in other MPTP models they are transient [26
] if seen at all [58
]. Additionally, MPTP/probenecid mice have altered gait, MWM cued learning, motor deficits, and some of these (gait, balance, and movement deficits) are reversed by exercise [59
]. Inconsistencies have also been reported in the spatial version of MWM following MPTP [18
]. Therefore, it may not be surprising that we find differing effects of MA-induced DA depletions compared to what are seen in rats.
C57BL/6 mice are widely used in genetic studies and the present data demonstrate some similarities in response to a neurotoxic dose regimen of MA between rats and mice, but also differences. The C57BL/6 mouse does not appear to be an appropriate species for examining egocentric learning despite similar neostriatal DA and DOPAC reductions, increased basal corticosterone levels, and increased GFAP as in the rat, and this is congruent with differences between mice and rats in response to other dopaminergic depleting treatments (e.g. MPTP and 6-OHDA).