Long-term effects of a single adult methamphetamine challenge: minor impact on dopamine fibre density in limbic brain areas of gerbils

Developmental neuroplasticity and the origin of neurodegenerative diseases

OBJECTIVES

Neurodegenerative diseases like Alzheimer's and Parkinson's Disease, marked by characteristic protein aggregations, are more and more accepted to be synaptic disorders and to arise from a combination of genetic and environmental factors. In this review we propose our concept that neuroplasticity might constitute a link between early life challenges and neurodegeneration.

METHODS

After introducing the general principles of neuroplasticity, we show how adverse environmental stimuli during development impact adult neuroplasticity and might lead to neurodegenerative processes.

RESULTS

There are significant overlaps between neurodevelopmental and neurodegenerative processes. Proteins that represent hallmarks of neurodegeneration are involved in plastic processes under physiological conditions. Brain regions - particularly the hippocampus - that retain life-long plastic capacities are the key targets of neurodegeneration. Neuroplasticity is highest in young age making the brain more susceptible to external influences than later in life. Impacts during critical periods have life-long consequences on neuroplasticity and structural self-organization and are known to be common risk factors for neurodegenerative diseases.

CONCLUSIONS

Several lines of evidence support a link between developmental neuroplasticity and neurodegenerative processes later in life. A deeper insight into these processes is necessary to design strategies to mitigate or even prevent neurodegenerative pathologies.

Anxiety Assessment in Methamphetamine - Sensitized and Withdrawn Rats: Immediate and Delayed Effects

OBJECTIVE

The anxiety profile in the stimulant-sensitized animals is not clear. Thus, this study was conducted to elucidate the effects of acute and chronic administration of methamphetamine (METH) on the anxiety profile. The aim of this study was to examine whether METH-sensitized rats would show an increase in the expression of anxiogenic-like behaviors and to determine whether a low dose of METH elicits behavioral sensitization.

METHODS

Rats were repeatedly given METH (2 mg/kg, s.c., once a day for 14 days), and the immediate and delayed effects of METH on the anxiety profile was compared considering 30 minutes (min) and 120 min after injections in METH-sensitized, withdrawn and intact rats using the elevated plus-maze (EPM), also, to re-challenge with a low dose of METH (0.5 mg/kg) in withdrawn groups.

RESULTS

RESULTS have shown that METH-sensitized rats exhibited an increase in the open arm time and entries 120 min after injection compared to the control group. We found a reduction in the time spent in open arms for the immediate effects of METH (30 min after injection) in METH-sensitized rats as compared to the control group. In withdrawn rats, METH/METH groups exhibited an increase in the open arm time and entries than METH/Sal and Sal/METH groups.

CONCLUSION

It was found that unlike delayed effects, an immediate effect of METH exhibited anxiogenic-like behaviors in METH-sensitized rats using the EPM. Also, results indicated that a low dose of METH is a potent stimulus for reinstatement of methamphetamine behavioral sensitization in a long withdrawn period.

A single high dose of methamphetamine increases cocaine self-administration by depletion of striatal dopamine in rats

Psychostimulant addicts often take high doses of drugs, and high doses of psychostimulants such as methamphetamine (METH) are neurotoxic to striatal dopamine (DA) terminals. Yet, the effects of high doses of METH on drug-seeking and drug-taking behavior have not been examined. In the present study, we found that single high doses of METH in rats (10-20 mg/kg) dose-dependently increased cocaine self-administration under fixed-ratio 2 (FR2) reinforcement conditions, while higher doses (40 mg/kgx1 or 10 mg/kg/2 hx4) caused high mortality among rats maintained on daily cocaine self-administration. The increased cocaine self-administration appeared to be a compensatory response to reduced cocaine reward after METH, because the same doses of METH caused a dose-dependent reduction both in "break-point" levels for cocaine self-administration under progressive-ratio reinforcement and in nucleus accumbens DA response to acute cocaine. Further, METH (10-20 mg/kg) produced large DA release (4000%-6000% over baseline), followed by a significant reduction in striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) contents, but without significant changes in striatal DA transporter levels. These findings suggest that the present high doses of METH caused striatal DA depletion or hypofunction without severe damage in DA terminals, which may contribute to the increased cocaine-taking behavior observed in the present study. Provided that the present doses of METH may mimic METH overdose incidents in humans, the present findings suggest that METH-induced DA depletion or neurotoxicity may lead to an increase in subsequent drug-taking and drug-seeking behavior.

Repeated neonatal separation stress alters the composition of neurochemically characterized interneuron subpopulations in the rodent dentate gyrus and basolateral amygdala

Emotional experience during early life has been shown to interfere with the development of excitatory synaptic networks in the prefrontal cortex, hippocampus, and the amygdala of rodents and primates. The aim of the present study was to investigate a developmental "homoeostatic synaptic plasticity" hypothesis and to test whether stress-induced changes of excitatory synaptic composition are counterbalanced by parallel changes of inhibitory synaptic networks. The impact of repeated early separation stress on the development of two GABAergic neuronal subpopulations was quantitatively analyzed in the brain of the semiprecocial rodent Octodon degus. Assuming that PARV- and CaBP-D28k-expression are negatively correlated to the level of inhibitory activity, the previously described reduced density of excitatory spine synapses in the dentate gyrus of stressed animals appears to be "amplified" by elevated GABAergic inhibition, reflected by reduced PARV- (down to 85%) and CaBP-D28k-immunoreactivity (down to 74%). In opposite direction, the previously observed elevated excitatory spine density in the CA1 region of stressed animals appears to be amplified by reduced inhibition, reflected by elevated CaPB-D28k-immunoreactivity (up to 149%). In the (baso)lateral amygdala, the previously described reduction of excitatory spine synapses appears to be "compensated" by reduced inhibitory activity, reflected by dramatically elevated PARV- (up to 395%) and CaPB-D28k-immunoreactivity (up to 327%). No significant differences were found in the central nucleus of the amygdala, the piriform, and somatosensory cortices and in the hypothalamic paraventricular nucleus. Thus during stress-evoked neuronal and synaptic reorganization, a homeostatic balance between excitation and inhibition is not maintained in all regions of the juvenile brain.

Environmental enrichment has no effect on the development of dopaminergic and GABAergic fibers during methylphenidate treatment of early traumatized gerbils

It is widely believed, that environmental factors play a crucial role in the etiology and outcome of psychiatric diseases such as Attention-Deficit/Hyperactivity Disorder (ADHD). A former study from our laboratory has shown that both methylphenidate (MP) and handling have a positive effect on the dopaminergic fiber density in the prefrontal cortex (PFC) of early traumatized gerbils (Meriones unguiculatus). The current study was performed to investigate if enriched environment during MP application has an additional influence on the dopaminergic and GABAergic fiber densities in the PFC and amygdala in this animal model.

Animals received a single early dose of methamphetamine (MA; 50 mg/kg; i.p.) on postnatal day (PD) 14, which is known to cause multiple changes in the subsequent development of several neurotransmitter systems including the dopaminergic systems, and were then treated with oral daily applications of MP (5 mg/kg) from PD30–60. Animals treated this way were either transferred to an enriched environment after weaning (on PD30) or were kept under impoverished rearing conditions.

There was no effect of an enriched environment on the dopaminergic or GABAergic fiber density neither in the PFC nor in the amygdala. With regard to former studies these results underline the particular impact of MP in the treatment of ADHD.

Density of dopaminergic fibres in the prefrontal cortex of gerbils is sensitive to aging

Mesencephalic dopamine (DA) projections are essential for cognitive and behavioral functions and believed to play a critical role during development and aging. The dopaminergic afferents of the rodent prefrontal cortex (PFC) show an extremely prolonged maturation which is very sensitive to epigenetic challenges. However, less is known about the long-term maturation and aging of these DA axons. Therefore, immunohistochemically stained DA fibres were quantitatively examined in the PFC of the Mongolian gerbil (Meriones unguiculatus) ranging from 6 to 24 months of age. Results show a decrease in DA fibre densities in the superficial layers of the PFC in 24 month old animals compared to 6 and 12 months.

Postnatal development of GABA and calbindin cells and fibers in the prefrontal cortex and basolateral amygdala of gerbils (Meriones unguiculatus)

The postnatal maturation of immunohistochemically stained gamma-amino-butyric acid (GABA) and calbindin (CB) cells and fibers were quantitatively examined in the prefrontal cortex (PFC) and the basolateral amygdala (BLA) of the Mongolian gerbil (Meriones unguiculatus). Animals of different ages, ranging from juvenile (postnatal day (PD)14, PD20, PD30), to adolescent (PD70), adult (PD180, PD540) and aged (PD720) were analyzed. Results reveal an increase in GABAergic fiber densities between PD14-20 in the PFC and the BLA with a concomitant decrease in cell density. After PD70 GABA fiber density slightly decreases again in the BLA, while there is a further slow but significant increase in the PFC between PD70 and PD540. Fibers immunoreactive for the calcium binding-protein CB, which is predominantly localized in particular GABAergic subpopulations, also accumulate between PD14 and PD20 in the PFC and BLA, while a concomitant decrease in cell density is only seen in the BLA. Both areas reveal a decrease of CB cells between PD30 and PD70, which parallels with a decrease of CB fibers in the PFC. However, there is no particular 'aging-effect' in the fiber or cell densities of GABA or CB in any of the investigated areas in old animals. In conclusion, we here demonstrate long-term dynamics in cell and fiber densities of the GABAergic system until late in development which might correspond to the prolonged maturation of other neuroanatomical and functional systems.

Postnatal development of dopamine innervation in the amygdala and the entorhinal cortex of the gerbil (Meriones unguiculatus)

Dopamine (DA) projections from the mesencephalon are believed to play a critical role during development and are essential for cognitive and behavioral functions. Since the postnatal maturation patterns of these projections differ substantially between various brain regions, cortical, limbic or subcortical areas might exhibit varying vulnerabilities concerning developmental disorders. The dopaminergic afferents of the rodent prefrontal cortex show an extremely prolonged maturation which is very sensitive to epigenetic challenges. However, less is known about the development of the DA innervation of caudal limbic areas. Therefore, immunohistochemically stained DA fibers were quantitatively examined in the basolateral (BLA) and central amygdaloid nucleus (CE) and the ventrolateral entorhinal cortex (EC) of the Mongolian gerbil (Meriones unguiculatus). Animals of different ages, ranging from juvenile [postnatal day (PD) 14, 20, 30)] to adolescent (PD70), adult (6, 18 months) and aged (24 months), were analyzed. Results show a significant increase of fibers between PD14 and PD20 in the BLA and lateral part of the CE, with a trend for a subsequent decline in fiber densities until PD30. The EC and medial part of the CE showed no developmental changes. Interestingly, none of the investigated areas showed significant reductions of DA fibers during aging.