Behavioural effects of neonatal systemic 6-hydroxydopamine

Ectopic noradrenergic hyperinnervation does not functionally compensate for neonatal forebrain acetylcholine lesion

Adult rats who have undergone neonatal 192 IgG-saporin induced lesions of forebrain acetylcholine (ACH) neurons are normal on many behavioral tasks. In this study we determined whether ectopic hippocampal ingrowths, a documented consequence of these neonatal cholinergic lesions, functionally compensate for ACH denervation in these rats. Neonatal rats underwent systemic 6-hydroxydopamine (6-OHDA) injections on postnatal days (PND) 1-3 to prevent the ingrowths, and/or intraventricular 192 IgG-saporin injections on PND 7. The 192 IgG-saporin profoundly reduced basal forebrain p75 neurotrophin receptor (p75(NTR)) immunoreactive (IR) neurons. The 6-OHDA treatment abolished hippocampal and cortical dopamine-beta-hydroxylase (DBH) IR terminals, indicating the absence of normal norepinephrine (NE) innervation. Ectopic DBH IR and p75(NTR) IR varicosities which occurred in the hippocampus of 192 IgG-saporin treated rats were also eliminated by 6-OHDA treatment. Behavioral testing in adulthood indicated no effect of the treatments on the Morris water maze. 192 IgG-saporin treatment caused perseveration during delayed spatial alternation (DSA) and increased working but not reference memory errors on the radial arm maze (RAM). The 6-OHDA plus 192 IgG-saporin treated rats did not differ from the 192 IgG-saporin only rats on any task. These results indicate that ectopic hippocampal NE ingrowths do not functionally compensate for neonatal ACH lesions. Neonatal forebrain ACH lesion impairs working memory on the RAM but the absence of an effect on DSA contraindicates a basic dysfunction of short term memory. Despite severe combined neonatal loss of forebrain ACH and NE innervation, behavior is remarkably intact.

6-Hydroxydopamine treatment of neonatal rats. II. Effects on the development of the hindlimb flexor reflex

The flexor reflex (FR) of the hindlimb was measured in rats at various ages between postnatal days (PND) 12-45, in vehicle-treated control rats and in rats treated with 6-hydroxydopamine (6-OHDA) (100 mg/kg, i.p.) on the first and second days after birth. The hindlimb FR was elicited by graded electrical stimulation of the footpad (0.1-2.0 mA) and quantified by the amplitude of the flexion in grams. The half maximal FR response was evoked by 1.2 mA and was 2-3 g in animals at PND 12-17 and 9-10 g in animals on PND 30 and 45. A similar age dependency was evident in the maximum hindlimb FR evoked by 2.0 mA; the maximum FR was 4.7 +/- 0.5 g on PND 12 and 24 +/- 2 g on PND 45. In rats treated with 6-OHDA, the strength of the FR was similar to that of the controls up to PND 15. However, the FR was increased 25% by PND 17 and 200% by PND 45 in the 6-OHDA-treated animals, versus controls. Pretreatment with clonidine (100 micrograms/kg, i.p.), which activates alpha 1 receptors under the current experimental conditions, did not enhance the FR in control animals. However clonidine pretreatment caused an increase of 400-800% in the FR in the 6-OHDA-treated animals at PND 15 and beyond. Our companion paper demonstrated that as early as PND 5, there was a significant increase (P < 0.05) in alpha 1 receptors in the spinal cord of 6-OHDA-treated animals, versus the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic and serotonergic mediation of the locomotor and antinociceptive effects of clonidine in infant and adult rats

This experiment examined the necessity for intact noradrenergic and serotonergic function for the locomotor and nociceptive effects of clonidine in 10- and 100-day-old rats. Newborn rats were administered systemically 6-hydroxydopamine (100 micrograms/g; 12 and 24 hours after birth) to deplete norepinephrine (NE), and at 10 or 100 days they were injected with para-chlorophenylalanine (300 mg/kg PCPA; 5 and 24 hours before testing) to deplete serotonin (5-HT). They were then tested for the locomotor and analgesic effects of one of various clonidine doses (0, 10, 100 or 1000 micrograms/kg). Clonidine enhanced locomotion at 10 days. This effect was potentiated by NE depletion and reduced by 5-HT depletion. Clonidine reduced locomotion at 100 days, and again this was augmented by NE depletion but reduced by 5-HT depletion. NE depletion did not have an enduring effect on clonidine antinociception whereas 5-HT depletion reduced it at both ages. It is concluded that the locomotor effects of clonidine in both infant and adult rats, despite reversing with maturation, reflect its agonist action at postsynaptic alpha2 adrenoceptors. The results also add to the accumulating evidence for an early maturing and behaviorally relevant serotonergic system(s).

Forebrain norepinephrine and neurobehavioral plasticity: neonatal 6-hydroxydopamine eliminates enriched-impoverished experience effects on maze performance

Newborn male rats were depleted of forebrain norepinephrine (NE) by systemic 6-hydroxydopamine injection and then reared from 25 to 60 days under either isolated or enriched conditions. They were subsequently tested for acquisition of either the Lashley III maze or the Hebb-Williams maze problems. Isolated rearing impaired Lashley maze performance of the controls but not the 6-OHDA injected rats. Similarly, for the Hebb-Williams maze, the isolation-reared controls made more errors than their enriched-reared counterparts while no differences were observed between the isolated and enriched reared, 6-OHDA injected rats. These results are consistent with the hypothesis that forebrain NE is permissive to the deleterious behavioral consequences of restricted experience during maturation.

Neonatal 6-hydroxydopamine lesion of spinal noradrenergic terminals: nociception, clonidine analgesia and spinal alpha two adrenoceptors

Newborn rats received intraspinal injections of 6-hydroxydopamine to enduringly deplete spinal norepinephrine (NE). When tested in adulthood for pain sensitivity with a hot water-tail immersion procedure, this neonatal spinal NE lesion lowered tail flick latencies of females but not males. It was postulated that this sexually dimorphic sparing or recovery of function reflected the development of denervation supersensitivity for males but not females. Contrary to expectation from such an hypothesis, females, not males, showed exaggerated sensitivity to the analgesic effects of a test dose of clonidine. Furthermore, neither males nor females showed an increased number of spinal cord binding sites for (3H)para-amino-clonidine [(3H)PAC]. These receptor binding data failed to indicate proliferation of the spinal alpha two adrenoceptor in either sex. That the lesioning of spinal NE terminals did not reduce (3H)PAC binding sites suggests that the spinal alpha two adrenoceptor does not reside exclusively on NE terminals. This is consistent with current conclusions concerning the alpha two adrenoceptor in the cerebral cortex.

Neonatal 6-hydroxydopamine lesion of spinal norepinephrine terminals fails to affect the ontogeny of swimming behavior

Newborn rats were injected either systemically or intraspinally with 6-hydroxydopamine. Both treatments drastically reduced spinal norepinephrine (NE), while the former also reduced forebrain NE. Spinal NE loss did not affect the ontogeny or the adult expression of swimming, a sensitive test of complex patterned locomotion. Spinal NE is not essential to either the ontogeny of complex motor behavior or to the generation of such behavior in adult rats.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Neonatal 6-hydroxydopamine prevents adaptation to chemical disruption of the pituitary-adrenal system in the rat

Three days after the subcutaneous implant of a dexamethasone pellet, which depletes both corticosterone and ACTH, normal rats showed impaired acquisition of a two-way avoidance task. Rats who had received systemic 6-hydroxydopamine at birth to lesion the forebrain noradrenergic terminals from the locus coeruleus did not show this impairment. After a single injection of metyrapone, which inhibits corticosterone synthesis and increases ACTH release, both intact and norepinephrine (NE)-depleted rats showed impaired avoidance acquisition. After the seventh injection, however, acquisition in normal rats was no longer impaired by the drug while the NE-depleted rats were still deficient. These results indicate that the simple combination of forebrain NE loss with reduced corticosterone levels does not necessarily retard avoidance acquisition. Rather, they suggest that the NE efferents from the locus coeruleus are essential for the brain's adaptation to at least some behavioral consequences of changes in the circulating level of ACTH.

The neurochemistry and pharmacology of extinction behavior

The role of various neurotransmitter systems in the brain in extinction behavior is examined. An attempt is made to suggest psychological mechanisms (such as attention, secondary reinforcement or internal inhibition) by which the neurotransmitter systems or drugs act to produce the observed alteration in extinction behavior. The putative neurotransmitters acetylcholine, noradrenaline, dopamine, serotonin, endorphins and the peptides are reviewed, as are pharmacological agents such as the benzodiazepines, the barbiturates, the psychodelics, the neuroleptics, the psychomotor stimulants and cannabinoids. Other treatments and factors are considered such as peripheral hormones and the adrenal-pituitary axis. It is suggested that the noradrenergic system may be involved in the expression of extinction behavior by a role in selective attention, the dopamine system via an involvement with secondary reinforcement, the cholinergic system by a mechanism of response inhibition and the barbiturates and benzodiazepines by a block of nonreward.

Neonatal 6-hydroxydopamine attenuates the neural and behavioral effects of enriched rearing in the rat

Newborn male rats were administered subcutaneous 6-hydroxydopamine (6-OHDA) to deplete forebrain norepinephrine and after weaning were reared in normal or enriched environments. Subsequently the 6-OHDA treated rats and their vehicle controls were trained in a Lashley type III maze and then sacrificed for assay of regional brain weights and brain catecholamines. Whereas for the control rats, enriched rearing was found to: (1) increase hypothalamic and posterior cortical dopamine; (2) increase forebrain and decrease hypothalamic weight; and (3) to enhance maze acquisition, none of these consequences of enriched rearing was found in the 6-OHDA treated rats. We conclude that forebrain norepinephrine plays a permissive role in the neuroanatomical, neurochemical and behavioral alterations induced by the enriched rearing of weanling rats and that it is essential to at least some aspects of the shaping of the brain by experiential factors.

Failure to replicate the dorsal bundle extinction effect: telencephalic norepinephrine depletion does not reliably increase resistance to extinction but does augment gustatory neophobia

Depletion of telencephalic noradrenaline (Ne), caused by lesion of the dorsal tegmental bundle, has been reported to increase persistence of non-reinforced responding in various operant tasks. This has been referred to as the dorsal bundle extinction effect (DBEE). In an effort to reproduce this effect, rats receiving 6-hydroxydopamine lesions of the dorsal Ne bundle (DB-6-OHDA) were compared to controls during extinction of a continuous food rewarded (CRF) lever-press response. While the lesion group showed an increase in responding during initial extinction, no significant difference in resistance to extinction using a 2-min non-response criterion was obtained. Moreover, no differences in reinforced response rates were observed with CRF, fixed ratio (FR-15, FR-30, FR-60) or variable interval (VI-30, VI-60, VI-120 s) schedules of reinforcement. In order to test the hypothesis that the DBEE is dependent on time of behavioral testing after surgery, subsequent experiments were performed where rats began CRF operant training 5, 17, 31 or 110 days post-lesion. No differences in resistance to extinction were observed between lesion and control rats at any post-lesion interval. Neonatal treatment with 6-OHDA which permanently lesions forebrain Ne terminals also failed to prolong extinction. Finally, when both DB-6-OHDA and neonatal rats were given a choice between water and saccharin the lesioned animals exhibited a neophobic reaction whereby they drank significantly less saccharin. We conclude that while the DBEE is not a reliably reproducible phenomenon other effects of forebrain Ne lesions, such as neophobia, appear to be robust.

Chlordiazepoxide-induced released responding in extinction and punishment-conflict procedures is not altered by neonatal forebrain norepinephrine depletion

The effects of chlordiazepoxide (CDZ) in extinction and punishment-conflict tasks were examined in rats after neonatal systemic administration of 6-hydroxydopamine (6-OHDA) to deplete forebrain norepinephrine (NE). At about 70 days of age the rats were water deprived and trained for three days to drink in a novel apparatus. On the fourth day (test day) drinking was either extinguished by elimination of water from the drinking tube or punished by lick-contingent shock. Just prior to this test session half of the vehicle and half of the 6-OHDA treated rats were given an injection of CDZ (8 mg/kg IP). Both the injection of CDZ and forebrain NE depletion prolonged responding during extinction and reduced the suppressant effects of punishment in male rats, and these effects were of similar magnitude. Furthermore, CDZ was as effective in neonatal 6-OHDA treated male rats as in vehicle treated rats indicating that decreased transmission is ascending NE fibers caused by CDZ is not solely responsible for its behavioral effects in extinction and conflict tasks. Rather, these effects may involve cooperative mediation by both noradrenergic and serotonergic forebrain terminals. Unexpectedly, CDZ's anti-extinction effect was absent in female rats and its anti-conflict effect observed only in NE depleted females.

Infant rat hyperactivity elicited by home cage bedding is unaffected by neonatal telencephalic dopamine or norepinephrine depletion

Newborn rats received either stereotaxically guided bilateral injections of 6-hydroxydopamine (6-OHDA) in the neostriatum so as to deplete dopamine (DA) there, or subcutaneous 6-OHDA to deplete forebrain norepinephrine (NE). Both the DA and NE depleted rats as well as their respective control rats were significantly more active at 15-16 days of age when tested in a novel environment containing soiled bedding from their home cages, than when tested in the presence of clean bedding material. Furthermore, under both the home cage and clean bedding conditions the DA depleted rats were more active at this age than their controls. Thus while transiently elevated locomotor activity is one consequence of neonatal, neostriatal DA depletion, inattention to olfactory stimuli (which occurs after adult neostriatal DA depletion) is not another. The NE depleted rats in both home cage and clean bedding test conditions showed activity levels equivalent to that of their control groups. Furthermore, the NE depletion did not affect hyperactivity elicited by artificially (peppermint) scented bedding like that in which the rats had been reared. Thus, contrary to expectations based on the reported reduction of preference for conspecific odor after neonatal and adult forebrain NE depletion, these data show that the locomotor activating effects of neither conspecific nor artificial odors associated with the nest odors are attenuated by neonatal NE depletion. The mortality rates among NE depleted rats raised in the peppermint scented shavings was unusually high and overall, these rats were less active than similarly raised controls.

Sex differences in lung and adrenal neurosympathetic development in rabbits

To assess the possible mechanism for the increased incidence and severity of the respiratory distress syndrome (RDS) in male versus female infants, we studied neurosympathetic development in the lung and adrenal glands in male and female fetal and newborn rabbits. Tissue levels of catecholamines, norepinephrine (NE), epinephrine (E), and dopamine (DA) were measured using a sensitive radioenzymatic assay. Beta adrenergic receptor development in the lung was studied using the tritiated radioligand dihydroalprenolol. Neurosympathetic innervation of the lung was assessed by measuring tissue NE levels, which increased gradually from 27-day fetuses to 8-day-old newborns. Sex differences were not significant. Lung beta-receptor number was significantly elevated in females as compared with males at each gestational age and throughout the neonatal period (p less than 0.005). Adrenal gland content of E, NE, and DA increased exponentially with advancing developmental age (each p less than 0.005). Adrenal E was significantly (p less than 0.01 elevated in female as compared with male fetuses, as was the proportion of E. Adrenal NE and DA were similar in male and female fetuses. Sex differences were not significant in the newborn animals. The significant relative delay in adrenal medullary and lung beta-receptor matration may relate to the male susceptibility to neonatal morbidity and neonatal RDS.

Importance of the Locus coeruleus and involvement of alpha-adrenergic receptors in the post-decapitation reflex in the rat

The latency, duration, hindlimb kick frequency, and total activity components of the post-decapitation reflex (PDR) were measured in the rat using a movement-sensitive transducer. Reduction of brain and spinal cord norepinephrine (NE) caused by neonatal administration of 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine, which also reduced brain serotonin, decreased all components of the PDR. Depletion of serotonin or dopamine alone reduced the vigor of the reflex, suggesting that these pathways can influence the PDR but are not essential for the response. Lesions of neurons in the Locus coeruleus, made electrolytically or with 6-OHDA, decreased the intensity of the PDR, with the 6-OHDA-induced lesion being more effective. Depletion of forebrain NE terminals with 6-OHDA did not alter the PDR, consistent with a critical involvement of spinal noradrenergic fibers. The PDR was also decreased by phentolamine and prazosin, but not by propanolol, suggesting an involvement of alpha-adrenergic receptors in the response. This hypothesis was further supported by the finding that the efficacy of a variety of drugs (such as tricyclic antidepressants, phenothiazines, and anti-hypertensive compounds) for blocking the reflex was apparently related to their affinity for alpha-adrenergic receptors. Thus, the PDR is dependent on noradrenergic fibers in the spinal cord and may provide a simple screen for drugs with suspected alpha-adrenergic blocking properties or for agents that disrupt the function of central noradrenergic fibers.

Physiological function of descending noradrenaline projections to the spinal cord: role in post-decapitation convulsions

Destruction of the descending noradrenergic innervation to the spinal cord, but not that to the cerebellum or the forebrain, by the use of intracerebral injection of 6-hydroxydopamine completely prevented the occurrence of the usual itation convulsion. Depletion of brain noradrenaline by synthesis inhibition with DDC, FLA 57 or FLA 63 g reduced the duration of the post-decapitation convulsion. Blockade of alpha-noradrenergic receptors by phentolamine or phenoxybenzamine, but not of beta-receptors by propranolol, also reduced the duration of the convulsion. The presynaptic alpha-agonist, clonidine, at either 1 mg/kg or 0.05 mg/kg also reduced the magnitude of the convulsion but either blockade of dopamine receptors with pimozide or destruction of the ascending dopamine systems by 6-hydroxydopamine was without effect. It is concluded that dopamine systems are not involved in post-decapitation convulsions and that the noradrenergic involvement is by the descending spinal projections acting on a post-synaptic alpha-receptor in the spinal cord, but also modulated by presynaptic alpha-receptors possibly on the locus coeruleus perikarya.