Opiate-enhanced toxicity and noradrenergic sprouting in rats treated with 6-hydroxydopa

Perinatal Lesioning and Lifelong Effects of the Noradrenergic Neurotoxin 6-Hydroxydopa

6-hydroxydopa (6-OHDOPA) was synthesized with the expectation that it would be able to cross the blood-brain barrier to be enzymatically decarboxylated to 6-hydroxydopamine (6-OHDA), the newly discovered neurotoxin for noradrenergic and dopaminergic neurons. In part, 6-OHDOPA fulfilled these criteria. When administered experimentally to rodents, 6-OHDOPA destroyed peripheral sympathetic noradrenergic nerves and did exert neurotoxicity to noradrenergic nerves in brain-in large part, from its conversion to 6-OHDA. However, the efficacy of 6-OHDOPA was less than that of 6-OHDA; also, 6-OHDOPA was relatively selective for noradrenergic neurons; near-lethal doses of 6-OHDOPA were required to damage dopaminergic nerves; and ultimately, 6-OHDOPA was found to be an agonist at AMPA receptors, thus accounting for more non-specificity. Nevertheless, 6-OHDOPA was found to be a particularly valuable tool in uncovering processes and mechanisms associated with noradrenergic nerve regeneration and sprouting, particularly when administered to perinatal rodents. Also, 6-OHDOPA was a good tool for selective mapping of noradrenergic nerve tracts in brain, since dopaminergic tracts were unaffected and did not interfere with the histofluorescent methodology used for this purpose in the early 1970s. As an experimental research tool, 6-OHDOPA was valuable in a short time-window, but its utility is largely limited because of newer research technologies that provide better means today for nerve tract mapping, and for experimental approaches engaged toward study of processes and mechanisms attending nerve regeneration. AMPA actions of 6-OHDOPA have not been extensively studied, so this avenue may enliven use of 6-OHDOPA in the future.

Destruction of catecholamine-containing neurons by 6-hydroxydopa, an endogenous amine oxidase cofactor

The amino acid, 6-hydroxydopa (6-OHDOPA), found at the active site of amine oxidases, exists as a keto-enol. Exogenously administered 6-OHDOPA is an excitotoxin like beta-N-oxalylamino-L-alanine (BOAA) and beta-N-methylamino-L-alanine (BMAA), acting at the non-N-methyl-D-aspartate (non-NMDA) alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor. BMAA and BOAA are causal factors of neurolathyrism in humans. Much exogenously administered 6-OHDOPA is biotransformed by aminoacid decarboxylase (AADC) to the highly potent and catecholamine-(CA) selective neurotoxin, 6-hydroxydopamine (6-OHDA). 6-OHDOPA destroys locus coeruleus noradrenergic perikarya and produces associated denervation of brain by norepinephrine-(NE) containing fibers. Opiopeptides and opioids enhance neurotoxic effects of 6-OHDOPA on noradrenergic nerves, by a naloxone-reversible process. An understanding of mechanisms underlying neurotoxic effects of 6-OHDOPA can be helpful in defining actions of known and newfound amino acids and for investigating their potential neurotoxic properties.

Effects of morphine on sprouting of locus coeruleus fibers in the neonatal rat

Acutely (by 48 h), as well as after one month survival, morphine (20 mg/kg) significantly potentiated the extent of noradrenergic degeneration in the forebrain of newborn rats caused by 50 mg/kg of 6-hydroxydopamine (6-OHDA) s.c. This was associated with increased regenerative sprouting in the cerebellum and pons-medulla as determined by measurements of norepinephrine (NE) concentration and [3H]NE uptake. This potentiating effect of morphine was not seen in either forebrain, brainstem, or cerebellum if it was administered to newborn rats with a 24-h delay following 6-OHDA. Clonidine (0.1 mg/kg) which, like morphine, decreases locus coeruleus activity, acutely tended to enhance the acute degenerative effects of 6-OHDA, whereas yohimbine (5 mg/kg), an alpha 2-antagonist which increases locus coeruleus activity, significantly attenuated the acute effect of 6-OHDA in the forebrain. In order to determine whether morphine directly alters the regenerative growth of neurons independently of its acute interaction with 6-OHDA, we explored its effect on the collateral sprouting of locus coeruleus projections induced by midcollicular hemisection. Unilateral midcollicular hemisection in neonates resulted, at 2 months of age, in a 91% reduction of NE concentration in the ipsilateral forebrain and an increase in the ipsilateral cerebellar hemisphere to over 200% of control. Significant increases of a slightly smaller magnitude were found in the cerebellar vermis, pons-medulla and the colliculi. Morphine given simultaneously with the lesion significantly attenuated this collateral sprouting in the cerebellar hemisphere and colliculi. These results indicate that morphine potentiates the initial toxicity of 6-OHDA, perhaps by altering activity of locus coeruleus neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological modifications of the neurotoxic action of the noradrenaline neurotoxin DSP4 on central noradrenaline neurons

Systemic treatment with the noradrenaline neurotoxin DSP4 (N-[2-chloroethyl]-N-ethyl-2-bromobenzylamine; 7 days) led to a marked and quantitatively similar reduction (-80%) of endogenous noradrenaline, [3H]noradrenaline uptake in vitro and [3H]desipramine binding in the frontal cortex of adult rats. Inhibition of monoamine oxidase, and/or 1-dopa administration 1 week after DSP4 produced very small changes in brain noradrenaline and dopamine levels. These results are all consistent with the view that DSP4 produces an acute and selective degeneration of central noradrenaline nerve terminals. Pretreatment with the noradrenaline uptake blocker desipramine prevented the action of DSP4 almost completely, while treatment after DSP4 had minute effects on DSP4-induced reduction of endogenous noradrenaline and [3H]noradrenaline uptake. The data suggest that the irreversible neurotoxic actions of DSP4 are very rapid and largely complete within 0.5 h after DSP4 administration. Measurement of catecholamine turnover using monoamine oxidase inhibition by pargyline indicated an increased noradrenaline turnover in the remaining nerve terminals innervating cerebral cortex and hippocampus after DSP4, while dopamine turnover appeared to be decreased. Pretreatment with d-amphetamine and clonidine or subsequent treatment with oxotremorine were without effect on the DSP4-induced reductions of the regional brain noradrenaline levels. Morphine pretreatment was also ineffective, while repeated morphine administration after DSP4 produced a significant potentiation of the DSP4-induced noradrenaline depletion in the frontal cortex, cerebellum and the spinal cord. Pretreatment with the monoamine oxidase inhibitor pargyline led to a very pronounced counteraction of the DSP4-induced noradrenaline depletion in all brain regions analysed, in particular in the occipital cortex. The data suggest that morphine can potentiate the neurotoxic action of DSP4 while pargyline can counteract it.

Opiates, endorphins and the developing organism: a comprehensive bibliography

A comprehensive bibliography of the literature concerned with opiates, endorphins, and the developing organism is presented. A total of 1378 clinical and laboratory references, with citations beginning in 1875, are recorded. A series of indexed accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics. The clinical section is subdivided into: age of subject examined; maternal aspects; effects on the fetus; pharmacology, physiology, and the withdrawal syndrome; and "other" effects on the offspring. The laboratory section is subdivided into: type of opiate/endorphin studied; species utilized; and major subject areas explored.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Substance P modifies the 6-hydroxydopamine induced alteration of postnatal development of central noradrenaline neurons

Systemic treatment of new-born rats with the catecholamine neurotoxin 6-hydroxydopamine leads to a permanent and selective alteration of the postnatal development of the central noradrenaline neurons, in particular of the locus coeruleus system. The changes involve a pronounced and permanent degeneration of distant nerve terminal projections (e.g. in the cerebral cortex and spinal cord) and a hyperinnervation of regions close to the noradrenaline perikarya (e.g. in the cerebellum and pons-medulla). Substance P administered intracisternally was found to counteract significantly both the 6-hydroxydopamine-induced denervation and hyperinnervation, as monitored by measuring endogenous noradrenaline levels and [3H]noradrenaline uptake in vitro. The counteracting effect of substance P disclosed a clear dose-response relationship and was most effective when injected on postnatal days one and two, while practically no effects were observed after injection on postnatal days three and four. Substance P treatment alone of new-born rats had no effect on the postnatal development of the regional monoamine levels. Binding studies employing radioligand technique showed substance P treatment to abolish the 6-hydroxydopamine-induced increase in beta-receptor binding in the frontal cortex, suggesting the 'spared' noradrenaline terminals to be functionally active. Substance P was shown to increase the utilization of noradrenaline in the neonatal stage. The results indicate that the counteracting effect of substance P may be due to a prevention of degeneration, growth stimulation and/or trophic influences on central noradrenaline neurons, possibly related to an excitatory effect of substance P on noradrenaline neurons.

Changes in the development of central noradrenaline neurons after neonatal axon lesions

The effects of early surgical lesions of ascending noradrenaline (NA) axons and 6-hydroxydopamine (6-OHDA) induced NA denervation of the spinal cord on the postnatal development of central NA neurons of the rat have been studied using histo- and neurochemical techniques. The lesions were performed during the first week after birth and analyzed at the age of one or two weeks or in the adult stage. Complete unilateral hemisection at the mesencephalic level in the neonatal stage produced marked reductions of the 3H-NA uptake in vitro in the cerebral cortex with concomitant significant increases (+30-50%) of the uptake on the ipsilateral side of the pons-medulla when determined in adulthood. Partial unilateral hemisection, restricted to produce an axotomy of the dorsal NA bundle, led to almost complete NA depletions in the cerebral cortex, while significant NA elevations were found in the mesencephalon (+25-40%), pons-medulla (+15%) and cerebellum (+90%) on the ipsilateral side. In the latter region a significant NA increase (+50%) was also observed in the controlateral side. Regional analysis of the 3H-NA uptake after partial hemisection gave similar results. Intraspinal injection of 6-OHDA on the day of birth resulted consistently in marked NA depletions (-90% or more) in the spinal cord, while the effects on NA in the cerebral cortex varied, being either unaffected or markedly reduced. Animals with unaffected cerebral cortex disclosed a significant NA increase in the pons-medulla only, whereas animals with cortical NA denervation displayed significant increases in endogenous NA levels both in the pons-medulla and the cerebellum. The present results give further support for the "pruning effect" concept explaining the growth response that central NA neurons exhibit after neonatal 6-OHDA treatment systemically or when one axonal branch is damaged neonatally.

Modulation of 6-hydroxydopamine induced alteration of the postnatal development of central noradrenaline neurons

The effect of substance P and morphine on the 6-hydroxydopamine (6-OHDA) induced alteration of the postnatal development of central noradrenaline (NA) neurons in the rat has been investigated using neurochemical techniques. Neonatal administration of 6-OHDA systemically leads to permanent NA denervations of distant NA projections, while the projections close to the NA cell bodies are increased, leading to NA hyperinnervation. Intracisternal injection of substance P was found to counteract both the NA denervation and hyperinnervation induced by 6-OHDA. The effect of substance P disclosed a clear dose-dependent relationship. Morphine, on the other hand, was observed to potentiate the alterations induced by 6-OHDA, both the NA denervation and hyperinnervation. The effect of morphine was dose-dependent and could be blocked by the morphine antagonist naloxone. The present results give further support for the view that the 6-OHDA induced alteration of the postnatal development of central NA neurons is related to a "pruning effect." The data furthermore imply that the end-result from a 6-OHDA induced degeneration of central NA neurons during ontogeny may be modulated by the functional state of the neurons.