The long-term effects of methamphetamine on depressive-like behaviour and neuroplasticity in stress-sensitive rats
Methamphetamine (METH) abuse has become a fast growing drug problem that has developed into a global epidemic. In fact, METH is one of the most commonly abused substances with an estimated 35 million abusers worldwide and is said to be the second most popular illicit drug. The Western Province of South Africa has seen a dramatic increase in drug abuse in recent years where METH is the primary or secondary drug of abuse. Interestingly, more than 50% of these individuals are under the age of 20 years. The longer duration of euphoric effects of METH has attracted many users away from cocaine in favour of METH. In addition to the rapid euphoric effect of METH, the direct short-term effects include arousal, reduced fatigue, an increase in blood pressure, reduced appetite as well as sustained attention. Chronic METH abuse may result in debilitating and long-lasting effects that includes mood disorders such as depression. Studies suggest a strong relationship between exposure to adverse environmental factors early in life and the later development of a neuropsychiatric disorder, such as depression. However, these severe consequences do not seem to invoke cessation of the drug. The euphoric and addictive properties of METH causes users to abuse the drug with an increase in frequency and dose, even though it might not have been their original intention. The primary objective of this study was to investigate the effect of early-life administration of METH to stress-sensitive (Flinders Sensitive Line - FSL) and control (Flinders Resistant Line - FRL) rats on depressive-like behaviour and regional brain monoamine levels later in life. The study implemented a sixteen-day period for administration of METH or a vehicle control from postnatal day 19 (PnD19) to postnatal day 34 (PnD34). The latter developmental stage corresponds to pre-adolescence in the rat when neurological development are similar to that seen in human adolescents, and represents the stage when drug abuse is most common in humans. Chronic dosing of METH and saline was performed twice daily at 09:00 and at 15:00. The animals received a sub-cutaneous (SC) escalating dose regimen of METH during the 16 day period (mimicking binging behaviour in humans), with every dose escalating in increments of 0.2 mg/kg from 0.2 mg/kg to 6.0 mg/kg. The study then investigated whether early-life administration of METH would cause depressive-like behaviours directly after the injection period (immediate drug effects before withdrawal on PnD35) or later in life (after the withdrawal period in early adulthood on PnD60). The behavioural effects were assessed in a battery of tests and thereafter the rats were sacrificed and the frontal cortex removed and snap frozen for later analyses of altered neurochemistry. The study demonstrated that chronic METH treatment during pre-adolescence induces significant behavioural changes related to depression in humans directly after the injection period (PnD35) and later in life (PnD60). The animals displayed antidepressant-like behaviour in the forced swim test (FST) before withdrawal, yet a depressogenic effect was observed 25 days post-withdrawal. This effect also seems to be additive to the congenital depressive-like phenotype of FSL rats, suggesting a role for genetic susceptibility. This observation would be in line with the two-hit hypothesis of depression, suggesting that the manifestation of depression will result when a genetic predisposition is followed by an environmental stressor (i.e. METH) later in life. The data suggests a working hypothesis that individuals that already have a predisposition to depression may be more susceptible to developing depression when abusing METH. The fact that the FSL control rats were more immobile than FRL control rats also confirmed the face validity of the FSL genetic rat model of depression. Locomotor activity assessment indicated that METH treatment decreased locomotor activity in FSL and FRL rats compared to their vehicle controls on PnD35 but not on PnD60. It is important to note that the effects observed in locomotor activity could not have contributed to the immobility observed in the FST, confirming that the immobility in the FST indeed reflects psychomotor and not locomotor effects. The study also demonstrated that METH significantly lowers social interaction behaviour in both FRL and FSL rats, both immediately following drug treatment (PnD35) and after withdrawal (PnD60). It is therefore clear that this effect of METH is long-lasting, putatively related to neurodevelopmental effects. In addition, the rats investigated the familiar object for a greater amount of time in the novel object recognition test (nORT) on PnD35 and PnD60 and may be the result of loss of recognition memory for the familiar object. This data confirms that METH results in cognitive memory deficits probably due to sustained adverse neurodevelopmental effects. Neurochemical analyses of the frontal cortex indicated decreased serotonin (5-HT) and norepinephrine (NE) levels on PnD35. METH is widely recognised for its pro-inflammatory effects, while the reduced 5-HT levels observed may have been the result of an increase in circulating pro-inflammatory cytokines. Neurochemical analyses provided thought-provoking data concerning the role of the permissive hypotheses of depression, indicating that dopamine (DA) is most likely not responsible for the behavioural effects observed, at least under the current study conditions, whereas 5-HT is decidedly more involved than expected. The data also suggest that depletion in NE plays a role in the development of depressive-like behaviours following METH exposure. Based on these findings, we propose that disturbances in 5-HT and NE are a crucial mechanism in how METH abuse may precipitate or worsen depressive-like symptoms in individuals who abuse METH. It should be noted that this study does not discard the role of DA in the development of depression after METH exposure, although under the current study conditions it appears that DA does not play a central role. The current study demonstrated that pre-adolescent exposure to METH can reproduce most of the behavioural changes seen in depressed individuals, and that these behavioural data can be used to identify causal neurochemical factors. Environmental stressors such as METH abuse should be regarded as an additional diagnostic criterion and is relevant to an accumulative risk factor hypothesis. Furthermore, although further study is required, the data suggests that early-life exposure to METH may predispose an individual to mood disorders and behavioural abnormalities later in life.
- Health Sciences