Functional recovery after destruction of dopamine systems in the nucleus accumbens of rats. III. Further analysis of the facilitating effect of the ACTH-(4-9) analog ORG 2766

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.

Alpha-melanocyte stimulating hormone promotes regrowth of injured axons in the adult rat spinal cord

Peptides related to melanotropin (alphaMSH) and corticotropin (ACTH), collectively termed melanocortins, are known to improve the postlesion repair of injured peripheral nerves. In addition, melanocortins exert trophic effects on the outgrowth of neurites from central nervous system neurons in vitro. Here we report, for the first time, the stimulation by alpha-MSH of spinal neurite outgrowth in vivo after injury. In the in vivo model, spinal cord trauma was produced at lower thoracic spinal levels of adult rats. Under a surgical microscope a laminectomy was performed exposing the dorsum of the spinal cord. Then the dura was cut longitudinally and the dorsal columns were identified. Iridectomy scissors were used to transect the dorsal half of the spinal cord bilaterally, thereby completely lesioning the main corticospinal tract component. Then the lesion gap was immediately filled with a solid collagen matrix. Ingrowth of fibers was quantified using an advanced image analyser using a video image of sections transmitted by a camera. In the control situation virtually no ingrowth of sprouting injured fibers into the collagen implant in the lesion gap was seen. However, when the collagen matrix contained 10(-8) M alpha-MSH, a profound and significant stimulation of fiber ingrowth into the implant was observed (alpha-MSH, 21.5 +/- 2.9%; control, 1.4 +/- 0.6% p < 0.01). A small percentage of these ingrowing fibers was CGRP-immunoreactive (17.0 +/- 4%), whereas no serotonergic ingrowth was observed. Furthermore, we found that local application of alpha-MSH directs a substantial amount of lesioned anterogradely labelled corticospinal tract axons to regrow into the collagen implant (alpha-MSH, 15.2 +/- 5.2%; control, 0.5 +/- 0.3%, p < 0.01). The observed fiber ingrowth is not accompanied by an invasion of astroglial or reactive microglial cells into the implant. In conclusion, inclusion of alpha-MSH in the collagen implant stimulates the regrowth of injured axons in the adult rat spinal cord.

Melanocortin-4 receptor messenger RNA expression is up-regulated in the non-damaged striatum following unilateral hypoxic-ischaemic brain injury

Melanocortin peptides (alpha-melanocyte-stimulating hormone, adrenocorticotropin and fragments thereof) have been shown to have numerous effects on the central nervous system, including recovery from nerve injury and retention of learned behaviour, but the mechanism of action of these peptides is unknown. A family of five melanocortin receptors have recently been discovered, two of which (melanocortin-3 and melanocortin-4 receptors) have been mapped in the rat brain. We have tested the hypothesis that the expression of one or more of the messenger RNAs for three melanocortin receptors (melanocortin-3, melanocortin-4 and melanocortin-5 receptors) would be altered in rat brain following unilateral transient hypoxic-ischaemic brain injury. In this study, using in situ hybridization, we show that melanocortin-4 receptor messenger RNA was up-regulated in the striatum in the non-damaged hemisphere within 24 h after severe hypoxic-ischaemic injury compared with control brains (P<0.05). In a small group of animals, this induction was not blocked by treatment with the anticonvulsant, carbamazepine. Expression of melanocortin-3 receptor messenger RNA in the brain was not altered in this hypoxic-ischaemic injury model and melanocortin-5 receptor messenger RNA was not detected in either control or hypoxic-ischaemic injured rat brains. We hypothesize that the up-regulation of melanocortin-4 receptor messenger RNA expression in the contralateral striatum may be involved in transfer of function to the uninjured hemisphere following unilateral brain injury.

The effect of the adrenocorticotropin-(4-9) analogue, ORG 2766, and of dizolcipine (MK-801) on infarct volume in rat brain

The purpose of this study was to evaluate whether the synthetic adrenocorticotropin-(4-9) (ACTH-(4-9)) analogue ORG 2766, HMet(O2)-Glu-His-Phe-D-Lys-Phe-OH, which has been shown to have beneficial effects on both the recovery from experimentally induced lesions of the central nervous system and peripheral nerve degeneration, has a protective effect on focal ischemic neuronal damage. The NMDA receptor antagonist dizolcipine (MK-801), a very potent neuroprotective drug, was used as positive reference compound. Isoflurane-anesthetized rats had the middle cerebral artery occluded using either an intravasal or an extravasal technique, because pilot experiments had shown differences in the severity of ischemia for the two middle cerebral artery occlusion techniques. MK-801, 500 microg kg(-1) min(-1), or saline was administered i.v. 30 min after occlusion of the middle cerebral artery. In the ACTH-(4-9) analogue/saline group, 10 and 150 microg/kg of the analogue, or saline was injected s.c. both directly after and 24 h after occlusion. The ACTH-(4-9) analogue treatment had no effect on the infarction volume in either model of middle cerebral artery occlusion, whereas MK-801 caused a significant reduction in the volume of cortical infarction in both models. We conclude that, although ORG 2766 is known to enhance the recovery from experimentally induced lesions of the central nervous system through a neurotrophic action and has proven to have significant beneficial effects on peripheral nerve regeneration, it did not prevent ischemic neuronal damage after intravasal or extravasal middle cerebral artery occlusion in rats. The results with MK-801, which caused significant reductions in the volume of cortical infarction in both models of middle cerebral artery occlusion, with clearly the largest reduction in the intravasal middle cerebral artery occlusion model, again indicate that there are differences in the severity of the cerebral ischemia which the two models produce in the rat brain.

Melanocortins and opiate addiction

Adrenocorticotropic hormone (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) are centrally acting melanocortin peptides with numerous reported functions, including induction of excessive grooming and antipyresis, among others. Also reported is a role for melanocortins in aspects of opiate action. Although early work examined the effects of ACTH and MSH on opiate-induced behaviors, further progress has been limited. Recently, however, advances in the identification and characterization of melanocortin receptor (MC-R) subtypes have provided novel tools with which to study interactions between melanocortins and addiction. The present review discusses the effects of ACTH and MSH on opiate-induced behaviors and relates these findings to more recent reports on the regulation of melanocortin systems by exogenous opiates. Emerging from these data is the possibility that melanocortin receptor activation, specifically at the MC4-R subtype, may act to antagonize certain properties of exogenous opiates, including perhaps addiction.

Beneficial effects of the melanocortin alpha-melanocyte-stimulating hormone on clinical and neurophysiological recovery after experimental spinal cord injury

OBJECTIVE

Melanocortins, peptides related to melanocyte-stimulating hormone (MSH) and corticotropin (ACTH), exhibit neurotrophic and neuroprotective activity in several established models of peripheral and central nervous system damage. The beneficial effects of melanocortins on functional recovery after experimental brain damage and central demyelinating diseases have prompted us to investigate alpha MSH treatment in a weight drop model of traumatic spinal cord injury in rats.

METHODS

In two independent randomized blinded experiments, treatment with either alpha MSH (75 micrograms/kg of body weight administered subcutaneously every 48 h for 3 weeks after trauma) or single high-dose (30 mg/kg, 30 min after injury) methylprednisolone was compared with saline treatment in rats subjected to a moderately severe 20-gcm weight drop injury. Spinal cord function was monitored using behavioral, electrophysiological, and histological parameters.

RESULTS

In both experiments, alpha MSH significantly improved recovery, as illustrated by Tarlov scores, thoracolumbar height, and amplitude of rubrospinal motor evoked potentials. The magnitude of the alpha MSH effect on motor performance was comparable with the one observed after treatment with methylprednisolone.

CONCLUSION

The reproducible neurological and electrophysiological improvement in spinal cord function of animals treated with alpha MSH suggests a new lead in the treatment of traumatic spinal cord injury.

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.

Chronic and intra-amygdala administrations of the ACTH(4-9) analog ORG 2766 modulate behavioral changes after manipulation of NMDA-receptor activity

Microinjection of N-methyl-D-aspartic acid (NMDA, 300 ng/3 microliters) into the left lateral ventricle causes a substantial increase in locomotor activity which can be significantly reduced by a chronic pretreatment with the ACTH(4-9) analogue ORG 2766(1 microgram/0.5 ml saline, subcutaneous (s.c.) every day for 7 days, last injection 24 h before the NMDA-injection). A single dose of ORG 2766 (1 ng/1 microliter) injected into the left central amygdaloid nucleus 30 min before the NMDA-injection was equally effective in reducing the increase in locomotion. Furthermore it counteracted the predominance of contralateral turning induced by the NMDA-injection. The data give support for the idea that ORG 2766 excerts its effects on behavior and neural recovery by modulating NMDA receptor activity in the brain.

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.

The effect of the adrenocortical axis upon recovery from closed head injury

Fragments and analogs of the hormone ACTH were previously shown to have beneficial effect on the outcome of head injury, while elevated levels of corticosterone (CS) exacerbate it. In the present study we investigated the role of the hypothalamo-pituitary-adrenal (HPA) axis in the pathophysiology of closed head injury (CHI). CHI was produced in ether-anesthetized rats by a calibrated weight-drop device. After evaluating the functional status according to a set of criteria, at 1 and 24 h, the rats were sacrificed and cortical tissue was removed to determine its water content. CHI was also produced in rats that underwent surgical procedures to remove their adrenal gland (ADEX) or the pituitary (HypoX), thus altering the levels of their circulating HPA hormones. Given after CHI, to rats with intact HPA axis, ACTH reduced edema and improved recovery. ADEX rats (6 days postsurgery) had 10-fold higher levels of plasma ACTH. ADEX rats subjected to CHI showed improved functional outcome (p = 0.008) and reduced edema (p = 0.02). We then produced CHI in three groups of rats: HypoX (15 days postsurgery), HypoX treated with ACTH, and controls. In HypoX rats, CHI resulted in increased mortality (35% vs 0) and edema in the surviving rats, and a slower recovery, as compared with the control. Mortality was prevented, edema slightly reduced, and recovery significantly improved after administration of 1-24-ACTH to HypoX rats with CHI. Our results suggest that ACTH has a cerebroprotective effect on the outcome of CHI.

The influences of fragments and analogs of ACTH/MSH upon recovery from nervous system injury

Post-injury treatment with some fragments and analogs of the adrenocorticotropic hormone (ACTH) can influence recovery after nervous system injury. This review considers both the successful and unsuccessful attempts to facilitate neural and behavioral recovery from nervous system damage via post-injury administration of these compounds. To date no single unifying explanation for the mixed results observed in animals prepared with forebrain injuries has been achieved. Several possible explanations for the variety of observations reported and several potentially productive avenues for future research are suggested.

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

This study examines whether treatment with GM1 ganglioside or the corticotropin (ACTH)(4-9) analogue ORG2766 can facilitate the behavioural recovery of adult rats with medial prefrontal cortex (mPFC) lesions, as animals are impaired in their food hoarding and spatial delayed alternation performance following mPFC lesions. No ameliorating effects of GM1 treatment on performance of these behaviours were observed. Although treatment with ORG2766 somewhat improved the hoarding performance of lesioned animals, the intermediate amount of pellets hoarded was not significantly different from that of either sham-operated or vehicle-treated lesioned rats. No effect of ORG2766 treatment was observed in the spatial delayed alternation test. Further, no changes were detected in the mesocortical dopamine innervation, presumed to be involved in the neural mechanism of behavioural sparing, in response to either treatment.

Mechanism of action of melanocortin peptides. Possible role in astrocyte regulation

Melanocortin peptides exert pleiotropic effect in numerous cell types, controlling processes ranging from adrenal steroidogenesis and melanocyte pigmentation to lacrimation and nerve regeneration. The binding of melanocortins to specific cell surface receptors initiates cellular responses via GTP binding proteins (G-proteins). The affinity of these peptides to the receptor is modulated by extracellular Ca2+ ions, a property unique to melanocortin receptors. In astrocyte cultures derived from the rat brain, melanocortin stimulation elevates cAMP levels that appear to induce morphological changes. However, a transient proliferative response to melanocortins in these cells appears to be cAMP independent. The presence of melanocortin receptors in brain tissue and their unique Ca2+ dependence are discussed in relation to their putative role as regulators of astrocytes.

Serotonergic fibres degenerating in the aging rat brain or sprouting from grafted fetal neurons are not affected by the neurotrophic ACTH analogue Org 2766

The effects of chronic treatment with the purported neurotrophic factor ACTH(4-9) analogue Org 2766 were studied on age-related degeneration of serotonergic fibres and on gliosis in the rat hippocampus and caudate putamen complex. In addition, the potential growth-promoting effects of Org 2766 were investigated on fetal serotonergic cells implanted in a previously denervated hippocampus of young adult rats. Chronic treatment of rats from the age of 11 months to 17-18 months did not affect the incidence of aberrant serotonergic fibres in the caudate-putamen complex or the fibres densities in the hippocampus or the caudate-putamen complex. Gliosis was unaffected by Org 2766 treatment as indicated by increased number and staining intensity of glial fibrillary acidic protein-immunoreactive cell bodies in both brain areas. Grafting of fetal raphe cells in young adult rats caused a time-dependent reinnervation of the previously denervated hippocampus. The reinnervation was not affected by treatment of the rats with Org 2766 for 4 weeks following implantation.

Melanocortins stimulate proliferation and induce morphological changes in cultured rat astrocytes by distinct transducing mechanisms

Melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), and several peptides derived from pro-opiomelanocortin, are present in the dorsolateral hypothalamus and arcuate nucleus of several vertebrate species. These peptides affect central nervous system (CNS) functions including behavior, memory, and foetal brain development. In this study we investigated the effects of ACTH1-24, ACTH1-17, ACTH4-10, alpha-MSH, beta-MSH, and a potent analog (Nle4,D-Phe7)-alpha-MSH (melanocortins) on immunocytochemically defined astroglial cells prepared from primary cultures of 1-2-day-old rat brains. A cyclic adenosine 3',5'-monophosphate (cAMP) response to the melanocortins was only detected in astrocytes and not in other cell types in the culture. The extent of the cAMP response was greatest on day 21, the latest time tested. On the other hand, (methyl3H)-thymidine incorporation in astrocytes was significantly stimulated (1.5-2-fold) by melanocortins only in 7 and not in 14 and 21 day cultures. This mitogenic activity of melanocortins was not mimicked by other agents such as forskolin or isoproterenol which efficiently stimulate cAMP production in astrocytes. ACTH1-17 as a melanocortin representative induced significant morphological changes in 7 and 14 day cultures which included rounding of the cell body and process extension. This response, however, resembled that induced by forskolin and hence appears to be cAMP mediated. These findings suggest that astrocytes in the CNS may serve as a target for melanocortins. These peptides appear to affect differentiation and proliferation of these cells during certain developmental periods. While the morphological effects of melanocortins seem to be cAMP mediated, induction of proliferation of the astrocytes by melanocortins appears to involve an alternative signal transduction pathway.

Effects of BIM-22015, an analog of ACTH4-10, on functional recovery after frontal cortex injury

Male rats, 90-100 days old, with frontal cortex lesions were given either subcutaneous sterile water (SW) as a vehicle control or 1, 10, or 100 micrograms of BIM-22015 every other day for 20 days. Brain-injured subjects tested in the Morris water maze with either 10 micrograms BIM-22015 or SW took significantly more trials than sham-operated rats to locate a submerged platform eight consecutive times within 60 s. The animals given 1 or 100 micrograms BIM-22015 took significantly fewer trials to reach criterion than brain-injured animals in the other drug treatment groups. On a percentage of savings, measured 8 days after reaching criterion, the brain-injured subjects given 1, 10, or 100 micrograms BIM-22015 did not differ from sham-operated rats. In contrast, the brain-injured animals given SW took longer to find the submerged platform than they did during the initial training. To assess long-term effects of the ACTH analog treatment, rats were trained on a delayed spatial alternation task 30 days after receiving the last injection. On this task, brain-injured rats treated with the 10-micrograms dose performed significantly better than those given sterile water. Acetylcholinesterase (AChE)-labeled neurons counted in the nucleus basalis magnocellularis indicated that rats with frontal cortex damage given the 10-micrograms treatment did not differ from the sham controls and had significantly more AChE-positive neurons than injured counterparts treated with SW or 100 micrograms.

Putative neurotrophic factors and functional recovery from peripheral nerve damage in the rat

1. In rats, recovery of sensory-motor function following a crush lesion of the sciatic or tibial nerve was monitored by measuring foot reflex withdrawal from a local noxious stimulation of the foot sole. 2. Putative neurotrophic compounds were tested on this functional recovery model: melanocortins (peptides derived from ACTH (corticotropin) and alpha-MSH (melanotropin], gangliosides and nimodipine were effective whereas isaxonine and TRH (thyrotropin releasing hormone) were not. 3. Structure-activity studies with melanocortins revealed a similar effectiveness of alpha-MSH, [N-Leu4, D-Phe7]-alpha-MSH, desacetyl-alpha-MSH and the ACTH analogue ORG 2766, questioning the validity of the previously suggested notion that the melanotrophic properties of these peptides are responsible for their neurotrophic effect. 4. As recovery of function after peripheral nerve damage follows a similar time course in hypophysectomized (five days post operation) and sham-operated rats, effective melanocortin therapy does not mimic an endogenous peptide signal in the repair process from pituitary origin. 5. Subcutaneous treatment with ORG 2766 (7.5 micrograms kg-1 48 h-1) facilitates recovery of function following peripheral nerve damage in young (6-7 weeks old), mature (5 month old) and old (20 month old) rats. 6. In view of the diversity in structure of the effective neurotrophic factors and the complexity of nerve repair, the present data support the notion that peripheral nerve repair may be facilitated by different humoral factors likely to be active on different aspects of the recovery process.

Structural modifications of the ACTH-(4-9) analog ORG 2766 yields peptides with high biological activity

The behavioral effects of two peptides (HOE 427) and ORG 31433) related to the ACTH-(4-9) analog ORG 2766 were investigated in Wistar rats in a number of tests in which Org 2766 is active. Subcutaneous administration of HOE 427 in a dose of 0.5 ng/kg or ORG 31433 in doses of 0.5-5.0 ng/kg facilitated passive avoidance behavior whereas these peptides attenuated the avoidance response in doses of 25 ng/kg and 250 ng/kg respectively. ORG 31433 (0.1 - 1.0 microgram/kg) decreased motor activity of group housed rats tested under low light conditions. Furthermore subcutaneous (1.0- 10.0 ng/kg) or oral (10 microgram/kg) administration of ORG 31433 accelerated functional recovery from 6-hydroxydopamine (6-OHDA)-induced lesions in the nucleus accumbens which cause motor hypoactivity. The experiments show that as compared to ORG 2766 the peptides HOE 427 and ORG 31433 induce qualitatively similar responses but are approximately 10 to 100 times more potent. These data may imply that substitution of the C-terminal COOH group of ORG 2766 yields neuropeptides with increased potency.

A single injection of a biodegradable microsphere formulation of the ACTH-(4-9) analogue ORG 2766 accelerates functional recovery after brain damage

The functional recovery from impaired motor activity induced by 6-hydroxydopamine lesions in rat nucleus accumbens was accelerated by subcutaneous treatment with the ACTH-(4-9) analogue Met/O2/-Glu-His-Phe-D-Lys-Phe (ORG 2766). Treatment was effective after daily injections of ORG 2766 dissolved in saline during the first 6 days following the lesion (ED50: 28.5 ng kg-1 day-1) or after a single injection of the peptide in a biodegradable microsphere formulation administered after the lesion (ED50: 8.9 ng kg-1 day-1). This study shows that a single injection of a microsphere preparation can replace multiple injections with ORG 2766 in order to facilitate functional recovery after brain damage.

Functional recovery after destruction of dopamine systems in the nucleus accumbens of rats. IV. Delay by intra-accumbal treatment with ORG 2766- or alpha-MSH antiserum

Rats with 6-OHDA lesions in the nucleus accumbens which were treated intra-accumbally with control serum during the first week following the lesion showed a similar level of motor activity 3 weeks after the lesion as sham-lesioned rats treated with control serum. In 6-OHDA-lesioned rats that were identically treated with antiserum against alpha-MSH or the ACTH-(4-9) analog ORG 2766 motor activity was decreased 3 weeks after the lesion. Intra-accumbal treatment with the antisera did not affect motor activity of sham-lesioned rats. The increased motor activity after apomorphine injection into the nucleus accumbens of control serum-treated 6-OHDA-lesioned rats was not observed in 6-OHDA-lesioned rats treated with the antisera. Furthermore, [3H]haloperidol binding studies showed that the changes in the DA receptor systems in the nucleus accumbens of 6-OHDA-lesioned rats treated with control serum, which may reflect denervation supersensitivity, were not observed in 6-OHDA-lesioned rats treated with the antisera. The present data indicate that the functional recovery and the concurrent development of supersensitive DA receptor systems in the nucleus accumbens of 6-OHDA-lesioned rats are delayed by intra-accumbal treatment with ORG 2766 or alpha-MSH antiserum. This suggests that endogenous ACTH/MSH-like factors may be mediating the recovery processes.

Functional recovery after destruction of dopamine systems in the nucleus accumbens of rats. II. Facilitation by the ACTH-(4-9) analog ORG 2766

Functional recovery from motor hypoactivity of rats with 6-OHDA lesions in the nucleus accumbens is accelerated by intra-accumbal or subcutaneous treatment with the ACTH-(4-9) analog ORG 2766. The spontaneous recovery period of 3 weeks is shortened to 7 days by daily treatment with this peptide during the first 6 days after the lesion. The 6-OHDA lesion induced a decrease of about 30-40% in the levels of dopamine, HVA and DOPAC as well as in the uptake of [3H]dopamine in nucleus accumbens tissue in vitro. Treatment with ORG 2766 during the first 6 days following the lesion did not affect the lesion-induced changes in these biochemical parameters. Binding studies with [3H]haloperidol in nucleus accumbens tissue of placebo or ORG 2766-treated sham-lesioned rats revealed a linear Scatchard plot 7 days after the sham lesion. In tissue of placebo-treated 6-OHDA lesioned animals a similar linear Scatchard plot was found but in tissue of ORG 2766-treated 6-OHDA-lesioned rats the Scatchard plot was curvilinear in shape indicating two types of binding sites. In the 6-OHDA-lesioned rats treated with ORG 2766 the behavioral response upon apomorphine challenge was enhanced suggesting the existence of functional supersensitivity of the DA system. Similar changes in Scatchard plots and apomorphine-induced behavioral changes have been previously reported after spontaneous recovery. The present study indicates that ORG 2766 accelerates the process of functional recovery from impaired motor behavior of rats with 6-OHDA lesions in the nucleus accumbens, which may be due to development of denervation supersensitivity.

Functional recovery after destruction of dopamine systems in the nucleus accumbens of rats. I. Behavioral and biochemical studies

Bilateral 6-hydroxydopamine (6-OHDA) lesions in the nucleus accumbens of rats induced motor hypoactivity 7 days after the lesion. Spontaneous functional recovery of this impaired behavior occurred in 3-4 weeks. Behavioral and biochemical studies suggest that the hypoactivity is due to damage of the dopamine systems in the nucleus accumbens. The 6-OHDA lesions induced a decrease in the nucleus accumbens levels of dopamine and its metabolites of about 30% both 7 and 20 days after the lesion. The in vitro uptake of [3H]dopamine in nucleus accumbens tissue of the 6-OHDA-lesioned rats was decreased to the same extent at 7, 14 and 28 days after the lesion. Scatchard analysis of [3H]haloperidol binding studies in nucleus accumbens tissue revealed a shift from one type of binding site in tissue of sham-lesioned rats to two types of binding sites in tissue of 6-OHDA-lesioned rats 29 days after the lesion. This shift was not present in nucleus accumbens tissue 8 days after a 6-OHDA lesion. The spontaneously recovered rats showed an enhanced behavioral response upon administration of the dopamine agonist apomorphine. The present data suggest that the spontaneous functional recovery of impaired motor activity is caused by the development of supersensitivity of the dopamine receptor systems in the nucleus accumbens. This supersensitivity may be the result of increased affinity of one type of binding site or an increased number of functional binding sites.