Pharmacologically induced neural plasticity in the prefrontal cortex of adult gerbils (Meriones unguiculatus)

Plasticity in the prefrontal cortex of adult rats

We review the plastic changes of the prefrontal cortex of the rat in response to a wide range of experiences including sensory and motor experience, gonadal hormones, psychoactive drugs, learning tasks, stress, social experience, metaplastic experiences, and brain injury. Our focus is on synaptic changes (dendritic morphology and spine density) in pyramidal neurons and the relationship to behavioral changes. The most general conclusion we can reach is that the prefrontal cortex is extremely plastic and that the medial and orbital prefrontal regions frequently respond very differently to the same experience in the same brain and the rules that govern prefrontal plasticity appear to differ for those of other cortical regions.

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.

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.

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

BACKGROUND

The aim of the study was to test long-term effects of (+)-methamphetamine (MA) on the dopamine (DA) innervation in limbo-cortical regions of adult gerbils, in order to understand better the repair and neuroplasticity in disturbed limbic networks.

METHODS

Male gerbils received a single high dose of either MA (25 mg/kg i.p.) or saline on postnatal day 180. On postnatal day 340 the density of immunoreactive DA fibres and calbindin and parvalbumin cells was quantified in the right hemisphere.

RESULTS

No effects were found in the prefrontal cortex, olfactory tubercle and amygdala, whereas the pharmacological impact induced a slight but significant DA hyperinnervation in the nucleus accumbens. The cell densities of calbindin (CB) and parvalbumin (PV) positive neurons were additionally tested in the nucleus accumbens, but no significant effects were found. The present results contrast with the previously published long-term effects of early postnatal MA treatment that lead to a restraint of the maturation of DA fibres in the nucleus accumbens and prefrontal cortex and a concomitant overshoot innervation in the amygdala.

CONCLUSION

We conclude that the morphogenetic properties of MA change during maturation and aging of gerbils, which may be due to physiological alterations of maturing vs. mature DA neurons innervating subcortical and cortical limbic areas. Our findings, together with results from other long-term studies, suggest that immature limbic structures are more vulnerable to persistent effects of a single MA intoxication; this might be relevant for the assessment of drug experience in adults vs. adolescents, and drug prevention programs.

A novel pharmacological concept in an animal model of psychosis

OBJECTIVE

We have analysed pharmacologically induced perturbation of functional and structural neurogenesis in the prefrontal cortex (PFC) and hippocampus.

METHOD

Juvenile gerbils received a single dose of methamphetamine (METH, 50 mg/kg, i.p.). In adults the following parameters were quantitatively investigated: prefrontal dopaminergic and GABAergic innervation densities (immunocytochemistry), morphogenesis of pyramidal cells (Golgi), dentate granule cell proliferation (BrdU-labelling), working memory and behavioural inhibition (delayed response, open-field).

RESULT

A single challenge of METH continuously suppresses granule cell proliferation in adult gerbils and initiates rewiring of neuronal networks in the PFC which run concurrently with the development of severe deficits in PFC-related behaviours.

CONCLUSION

It appears that a continuous remodelling of neuronal circuits is an inherent property of the brain, the biological significance of which seems to be to ascertain adaptive interaction between brain and environment. Learning more about drug-induced neuronal reorganization might be basic for understanding the genesis of psychotic conditions in the brain. This presentation is based both on own research and on a review of the literature.

Profound astrogenesis in the striatum of adult mice following nigrostriatal dopaminergic lesion by repeated MPTP administration

Neural progenitor cells are present in the rodent brain throughout adulthood, and can proliferate and differentiate into new neurons and/or glia to repair injury. To explore the repair processes mediated by brain progenitor cells, a selective lesion of the nigrostriatal dopaminergic pathway was induced in young adult mice by repeated administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). A thymidine analog, bromodeoxyuridine (BrdU), was used as a tracer for DNA synthesis to label the dividing cells and their terminal progeny following injury. Three days after MPTP treatments (25 mg/kg, once daily for 5 days), an 8-fold increase in the number of BrdU-labeled newborn cells was observed in the dorsal striatum. A 5-fold increase was also seen in the substantia nigra (SN). Newborn cells in the striatum survived beyond 60 days after their birth whereas newborn cells in the SN survived for less than 31 days. The vast majority of newborn cells in the striatum differentiated into astroglia according to their radial morphology and co-expression with an astroglial marker, S100beta, within 10 days after birth. In contrast, most BrdU-positive cells in the SN failed to co-express S100beta. Little or none of BrdU-labeled cells in both the striatum and SN were found to co-localize with a neuronal marker, neuronal nuclear antigen, or tyrosine hydroxylase during the full course of survival days surveyed (3 to 60 days). Repeated MPTP also decreased dopamine content and uptake in the striatum, which showed a significant recovery 31 days after MPTP lesion. These results demonstrate a rapid and profound astrogenesis in the striatum of young adult mice in response to toxic dopaminergic insult. The lack of neurogenesis in the two affected brain areas indicates the relative importance of glial cell regeneration in repairing MPTP injury.

Gliogenesis in the striatum of the adult rat: alteration in neural progenitor population after psychostimulant exposure

Cytogenesis from proliferating progenitor cells is present in the rat brain throughout adulthood, and is regulated by a variety of environmental stimuli. To determine whether adult cytogenesis occurs in the intact rat striatum and to explore the possible regulatory role of psychostimulant exposure on striatal cytogenesis, immunohistochemistry with the thymidine analog bromodeoxyuridine (BrdU), a marker of DNA synthesis that labels dividing cells and their terminal progeny, was performed on the brain sections of normal adult rats and rats treated with a psychostimulant, amphetamine (AMPH). Scattered cells that incorporated BrdU were consistently seen throughout the dorsal (caudate putamen) and ventral (nucleus accumbens) striatum 24 h after BrdU injection. Three to four weeks after BrdU injection, approximately 10-20% of surviving newborn cells differentiated into astroglia according to their radial morphology of glia and co-expression of an astroglial marker, S100beta. However, none of BrdU-positive cells were found to co-localize with a neuronal marker, neuronal nuclear antigen (NeuN). Acute injection of AMPH at a behaviorally active dose (10 mg/kg, i.p.) produced a rapid and transient decrease in the number of BrdU-labeled cells in both the dorsal (70.6% of control) and ventral (66.7% of control) striatum, but not in the subventricular zone and the hippocampal dentate gyrus. However, the fraction of differentiated astrocytes was not altered 3-4 weeks after AMPH treatment. These results indicate an existence of active gliogenesis (both proliferation and differentiation) in the adult rat striatum. Vulnerability of striatal cytogenesis to psychostimulant exposure indicates a new approach to elucidate brain mechanisms responsible for addictive properties of drugs of abuse.

Neuronal degeneration and reorganization: a mutual principle in pathological and in healthy interactions of limbic and prefrontal circuits

Based on developmental principles and insights from animal research about neuroplasticity in cell assemblies, this article is to propose a view of plasticity that promotes a link between hippocampal and prefrontal structure and function. Both the mitotic activity (counting of BrdU-labeled cells) in hippocampal dentatus and the maturation of dopamine fibres (quantitative immunochemistry of mesoprefrontal projection) in the prefrontal cortex proved to be a measurable combination for investigating the complex chain of events that relate activity dependent neuroplasticity to normal as well as to pathological maturational processes. With our animal model we demonstrate that both rearing conditions and neuroactive substances can effectively interfere with developmental plasticity and induce a malfunctional adaptation of prefrontal structures and neurotransmitter systems (dopamine, GABA). In the hippocampal dentatus, where ontogenetic plasticity proved to be preserved by continued neuro- and synaptogenesis, serious damage can be internalized without simultaneous disruption of neural dynamics offering an approach to reverse dysfunctional reorganization in the prefrontal cortex.

Functional recovery after central infusion of alpha-melanocyte-stimulating hormone in rats with spinal cord contusion injury

Melanocortins, peptides related to alpha-melanocortin-stimulating hormone (alpha MSH) and adrenocorticotropic hormone (ACTH), are known to improve axonal regeneration following peripheral nerve injury and stimulate neurite outgrowth from central nervous system (CNS) neurons both in vitro and in vivo. The neurite outgrowth promoting capacity of alpha MSH has prompted us to investigate the effects of intrathecal application of alpha MSH on functional and electrophysiological recovery in a well-characterized model of spinal cord contusion injury. Different doses of alpha MSH were applied via osmotic minipumps into the cisterna magna for 10 days, thereby delivering the peptide directly into the CNS. Functional recovery was monitored during 8 postoperative weeks by means of the Basso, Beattie, and Bresnahan locomotor rating scale, and the thoracolumbar height test. At the end of the study, electrophysiological analysis of rubrospinal motor evoked potentials as performed. Our data showed that application of 3.75 micrograms/kg/h alpha MSH resulted in a marked functional recovery, accompanied by a decrease in the latency of the rMEP. This study demonstrates that intrathecal application of alpha MSH results in functional recovery after spinal cord contusion injury. These findings may initiate new treatment strategies and/or the use of melanocortins in human spinal cord injury.

[Not Available]

With the increasing application of imaging techniques, characteristic changes in the structure and functional activity of certain neuronal networks and transmitter Systems have been discovered in the brains of patients suffering from various psychiatric disorders. These findings have often been assumed to support biological concepts of the genetic background and causation of these disorders. However, several lines of research are converging to indicate that the initially established genetically programmed neuronal Connectivity is further elaborated, fine tuned and modified by usedependent neuronal and synaptic plasticity. In all socially organized species in general and in human subjects in particular, psychosocial experiences appear to represent the most important trigger of use-dependent adjustments of neuronal Connectivity through the facilitation, modification and reorganization of neuronal networks. In experimental animals, changes in psychosocial rearing conditions were shown to cause profound and persistent changes in the cytoarchitecture, dendritic arborization and synapse formation in individual brain regions as well as in the maturation of monoaminergic afferences. Based on these findings, the mechanisms of the biological affixation of psychosocial experiences are described and the implications of experience dependent neuronal and synaptic plasticity in the prevention and the therapy of mental disorders are outlined.