Ineffectiveness of GM1 and ORG2766 on behavioural recovery after prefrontal cortical lesions in adult rats

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.

Short and long term plasticity after lesioning of the cell body or terminal field area of the dopaminergic mesocorticolimbic system in the rat

To investigate within one study regenerative capacities of dopaminergic axons and cell bodies, short and long term recovery of behavioral and biochemical impairments following a bilateral 6-hydroxydopamine (6-OHDA) lesion of the ventral tegmental area (VTA)-nucleus accumbens (NAc) pathway was investigated in rats. Novelty-induced motility, presynaptic functions and the levels of dopamine (DA) and its metabolites were reduced when cell bodies in the VTA or axons in the NAc were lesioned. Spontaneous recovery of the behavioral deficit was observed 4 weeks after a lesion of the NAc. Subsequently presynaptic functions recovered as shown by the reappearance of low dose apomorphine (50 mg/kg)-induced hypomotility, normalization of [(3)H]dopamine uptake, reinnervation of the NAc and normalization of levels of DA and its metabolites within 24 weeks. In contrast, after a VTA lesion no recovery was observed during 48 weeks, neither from hypomotility and loss of the low dose apomorphine response nor from decreased [(3)H]dopamine uptake and levels of DA in the NAc. Short term postsynaptic supersensitivity (hypermotility upon a higher dose of apomorphine (125 mg/kg)) was present 1 and 4 weeks after the lesion but not thereafter. A near total absence of dopaminergic neurons in the VTA and axons in the NAc were found 24 weeks postlesion. Treatment with the ACTH-(4-9) analog ORG 2766 (10 mg/kg s.c., 6 days once daily) facilitated recurrence of presynaptic functions after a lesion of axons but had no short or long term effect when cell bodies were lesioned. These findings substantiate the postulate that the peptide facilitates recovery processes.

Chronic administration of ORG 2766 for 6 months produces a subtle impairment in strategic learning by rats

Rats were administered saline or 10 micrograms of the ACTH4-9 analog ORG 2766 on alternate days for 160 days (i.e. 80 injections total). Behavioral assessments began 1 week later. Locomotor competence was assessed by examining the number of slips and falls made by the animals on a rotating rod. The rats were also trained on a position task and 10 subsequent position reversals in a 'T' shaped water maze. Exposure to ORG 2766 failed to affect either locomotor competence or the overall number of errors committed while learning the original position habit and 10 reversals. However, the response accuracy of the ORG 2766-treated animals on trial 2 of the reversals was equivalent to that expected by chance (58% correct choices), whereas saline-treated animals effectively altered their behavior after experiencing nonreinforcement on the initial trial of a reversal (77% correct choices). This result is consistent with other observations revealing that ORG 2766 can influence attention and, therefore, some cognitive functions.

The ACTH(4-9) analog ORG 2766 and recovery after brain damage in animal models--a review

Treatment with adrenocorticotrophic hormone (ACTH), as well as with ACTH fragments and analogues, can influence behaviour of animals and humans. Furthermore it facilitates recovery of damaged peripheral nervous tissue. The question whether ACTH/MSH peptides affect recovery processes after injury to the central nervous system as well is addressed in the present review. The effects of administration of the ACTH(4-9) analog ORG 2766 after brain lesions has been studied frequently. However, the interpretation of the available data is confused by the variability of the results. Several factors can be identified which influence the efficacy of the peptide: (i) not all behavioural tests are equally suitable to reveal a peptide effect on behavioural recovery; (ii) the affected brain area; (iii) whether cell bodies or terminals are affected; (iv) the post-operative housing conditions; and (v) the onset and duration of peptide administration. Two possible explanations of peptide efficacy on functional recovery are considered: first, the peptide may accelerate spontaneously occurring recovery processes and second, the peptide may induce compensatory mechanisms underlying functional recovery without recuperation of the damaged neurons. These compensatory mechanisms seem to rely mainly on enhanced non-selective attention by activation of limbic structures. It is as yet unknown to which receptor system ORG 2766 binds; the analog lacks affinity for the known melanocortin (MC) receptors in brain, yet ORG 2766 is able to modulate the activity of endogenous opioids and the NMDA-receptor. A modulating influence of the peptide on NMDA-receptor activity might indirectly account for both enhanced attention--with ensuing behavioural recovery--and the acceleration of spontaneous recovery.

Ontogeny of PFC-related behaviours is sensitive to a single non-invasive dose of methamphetamine in neonatal gerbils (Meriones unguiculatus)

A single dose of methamphetamine (50 mg/kg; i.p.) was administered to neonatal male gerbils (Meriones unguiculatus) aged 14 days, and adult prefrontal cortex (PFC)-related behaviours were analysed and compared with saline-treated controls at the age of postnatal day 90. For that purpose, animals were tested for open-field activities and y-maze delayed alternation. This solitary and non-invasive drug challenge, which has recently been found to initiate serious restraint in maturation of the mesoprefrontal dopamine (DA)-system (Dawirs et al., 1994), induces a significant delayed alternation impairment as well as significant increases in open-field motor activity and emotionality. Since an undisturbed development of the prefrontal DA-innervation seems to be a precondition for the maturation of normal PFC-related behaviours, a single early methamphetamine impact may be a suitable animal model for further investigation of structural and functional aspects of non-invasively induced behavioural deficits in rodents. The present results are discussed with regard to the assumption that hypofunctional mesoprefrontal DA-systems might be basic to schizophrenic behaviours in man.