The potential long-term effect of third trimester MPD exposure in humans has received little attention. In a P11–20 rat model (analogous to the end of second trimester for cortical regions and third trimester for limbic regions, especially the hippocampus [12
]), rats were treated with MPD 4 times per day at 2 h intervals and the long-term effects on anxiety-related behavior and spatial learning and reference memory were evaluated in the offspring as adults. We found no long-term effects of higher doses of MPD on EZM performance, swimming ability, or spatial learning and reference memory in the MWM even though the doses given caused significant decreases in body weights during and after MPD treatment. For body weight, the data show the drug had dose-dependent effects that gradually diminished after the termination of treatment. Body weight differences completely disappeared in the three lower dose groups by the time behavioral testing commenced, but a residual deficit remained in the MPD-30 group. Despite these reductions in growth, there were no effects on the behavioral tests used herein.
To our knowledge, a model of late in utero
MPD exposure (which is postnatal in rodents) has not been previously studied in rats. There are data in mice. Pregnant mice were given a 5 mg/kg/day dose over a three day period during one of three treatment intervals: E8–10, E12–14, or E16–18. Offspring showed decreased anxiety in the elevated plus maze, but no other significant behavioral changes in a test battery which included a 3 day water maze task (visible platform) and a 4th
day hidden platform probe trial [25
]. The decreased anxiety may be explained by their use of short-term doses during gestation, whereas our regimen used longer exposure during later stages of brain development in order to model late second to third trimester development.
Most studies examining MPD expose juvenile animals to model ADHD treatment in children. As a result, rats are generally dosed from P20 onward. These studies have mixed results based on exposure length, but in general there are no effects on hippocampally-dependent learning [5
], something we also found in this study. Behaviors in adult animals that are affected after juvenile MPD exposure include the forced swim test and place conditioning [3
]. In another study, a longer dosing period (P15–45) produced negative effects on MWM performance, including increased latencies in both spatial and working memory versions of the task [31
]. It should be noted that in this study, behavioral testing began immediately after the end of MPD exposure. In addition, Bethancourt et al. noted acute effects of MPD on fear conditioning; effects which resolved 48 h later [5
]. In juvenile rats, MPD effects are mostly seen shortly after the end of treatment. Our study includes a significantly longer off-drug period before testing.
Results in animal experiments must be extrapolated to humans and this is complicated by the fact that animal models of ADHD were not used in these studies. Clinical studies in ADHD-afflicted children show improvement in a spatial memory task, but no improvement when distracting interference is introduced[30
]. Others have found effects of developmental cocaine exposure on non-spatial learning ability during fMRI in clinical subjects [22
], and using novel object recognition and delayed non-matching-to-sample in rats [29
Previously, we have has shown deficits in the MWM following methamphetamine and MDMA exposure during the same time period as used in the present experiment [34
]. Both methamphetamine and MDMA affect reuptake of monoamines via inhibiting DAT and/or SERT and cause these transporters to reverse direction cause DA and 5-HT efflux rather than normal reuptake. Both drugs also inhibit VMAT2-regulated vesicular reuptake and MAO metabolism. In combination, these effects lead to increases of monoamines in the presynaptic terminal and extracellularly in the synaptic cleft [35
]. The molecular mechanism of MPD differs in that it, similarly to cocaine, it inhibits DAT but does not reverse the direction of DAT flux, or inhibit VMAT2 or MAO [35
]. Interestingly, cocaine, while addictive, has a similar lack of effects on spatial learning in the MWM when animals are exposed during the same or nearly the same developmental period as in the present study [19
]. The MPD mechanism of action has been speculated to include increased catecholaminergic signaling in the prefrontal cortex in addition to increased DA and 5-HT at the synaptic cleft [9
]. During juvenile periods of exposure, increases in CREB, an important transcription factor, can also be seen after MPD exposure [3
]. Because of its integral role in synaptogenesis and maintenance [2
] regulating cellular processes and gene transcription[8
], CREB may be part of a neuroadaptive developmental mechanism activated during developmental MPD exposure. This may partially explain a lack of detrimental effects on learning and memory, although more research into mechanisms and behavioral outcomes are clearly needed.
Finally, treatment in previous studies has been 2–5 mg/kg dose, which is considered clinically relevant to children on a mg/kg basis. Our study differs in that a broader dose-response range was included. Although the 5 mg/kg dose is included here, the response curve was increased to include higher doses which model the upper end of the clinical extending into the abuse range. The higher doses, putatively in the maternal MPD abuse range, could cause detrimental effects on the offspring but apparently not on spatial learning and reference memory. Results in this study indicate that MPD use during late pregnancy causes presumptive evidence of harm to the extent tested in the elevated zero and Morris water mazes.
Limitations to the present study include evaluating behavior using only one test of learning and memory. Also, only one exposure period was employed although it was specifically chosen to match previous experiments with other stimulants, i.e., the exposure was during granule cell development in the rat hippocampus and dentate gyrus and the MWM assesses predominantly hippocampally-dependent learning and memory and this test is sensitive to exposure at these ages to other psychostimulants. We used a range of doses; therefore, it is unlikely that effects were missed because of an inadequate dose range but effects on other forms of learning and memory or other behaviors cannot be ruled out.