Studies on memory: spontaneous return of memory in 6-hydroxydopamine-treated mice and its relation to cycloheximide-induced transient amnesia

We suggested previously that cycloheximide-induced transient amnesia was due in part to side-effects of the antibiotic on the central adrenergic system at the time of testing and that spontaneous return of memory depended upon recovery of the adrenergic system. To test these possibilities, mice chronically depleted of brain catecholamines (CAs) by 6=hydroxydopamine (6-OHDA) were trained in an avoidance-discrimination task and tested for spontaneous return of memory. Contrary to our hypothesis, amnesia at 24 hr after training was followed by recovery of memory by 72 hr, which indicates that recovery of memory can occur independently of adrenergic recovery. Injection of alpha-methyl-para-tyrosine immediately after training prevented return of memory at 72 hr, suggesting that the residual CAs remaining after 6-OHDA are necessary for memory to spontaneously return at this time.     

Impaired performance by post-trial injections of cycloheximide in a passive avoidance task

Summary Cycloheximide, injected subcutaneously after training in a passive avoidance task resulted in impaired performances on the retest given 7 days later. A gradient was observed for this effect, injections given immediately after training producing most impairment and injections given 2 h after training producing none.This research was supported by USPHS Research Grant MH 05319, 5T1 MH 6418, MH 01225, and MH 12773 and Career Development Award K3 MH 18232.     

Comparisons of the effects of guanethidine, 6-hydroxydopamine and diethyldithiocarbamate on retention of passive avoidance

Depending on the dose levels, guanethidine and diethyldithiocarbamate produced time dependent amnesia for a one-trial passive avoidance task. 6-Hydroxydopamine given at any dose level or at any time interval before or after training did not result in retention deficits. The results were discussed in terms of the effects of these agents on peripheral biogenic amines and the possible role of these amines in memory formation.     

Comparison of the cycloheximide and food satiation effects on a discrimination task.

The effects of intraventricular cycloheximide (CHX) pretreatment were examined on the ability of rats to recall a light position discrimination task. The CHX pretreatment used inhibited brain protein synthesis at the time of training by 71%. Following treatment with CHX the animals demonstrated no retention of the task 18 hr later. Free feeding for 24 hr prior to treatment with cerebro-spinal-like fluid (CSF) produced similar effects as CHX. It is proposed that the amnesic effect of CHX in this behavioral paradigm may be a nonspecific action resulting from decreased motivation. Both the CHX-treated and satiated animals exhibited low responding levels during training, despite comparable behavioral activity levels for the CHX- and CSF-treated groups.     

Effect of corticosteroids in the hippocampus on passive avoidance behavior in the rat

The site of action of corticosteroids in avoidance learning was investigated in 110 rats. Injection of cycloheximide, 30 min before one-trial training on a passive avoidance task suppressed corticosteroid secretion in response to footshock, and produced an avoidance deficit in a test 6 days later. However, an additional injection of hydrocortisone, either subcutaneously or intra-hippocampally within 5 min of training, restored the avoidance response in the test. Septal and hypothalamic injections of the hormone were ineffective in reversing the cycloheximide effect, whereas the effect of hormone injection into the amygdala was equivocal because of an increased level of activity. Corticosteroids secreted following an aversive experience appear to act upon the steroid-sensitive neurons in the hippocampus to influence the animal's later performance of passive avoidance response.     

Differential effects of intraventricular administration of 6-hydroxydopamine on behavior of rats in approach and avoidance procedures: Reversal of avoidance decrements by diazepam.

The administration of 6-hydroxdopamine (6-HD) and pargyline to rats produced similar selective decreases in responding during the conditioned stimulus (CS) on a discriminated avoidance test where the unconditioned stimulus (US) was shock and on an analogous conditioned approach test where the US was water. Approach behavior during the CS generally recovered, however, while avoidance decrements in the same rats remained for the duration of testing. This suggested that 6-HD-induced avoidance decrement was a result of two independent but interacting processes: (1) a decrease in conditioned behavior as reflected by the similar decrease in responding on both tests; and (2) a hyper-reaction to aversive stimuli that resulted in a tendency to selectively suppress avoidance behavior after the animal received shock. In support of this hypothesis, it was found that 6-HD-induced avoidance decrements could be reversed (1) by treatment with diazpam, a drug that releases suppressed responses; or (2) by delaying avoidance testing until conditioned responding had recovered, thus minimizing the interaction of the two processes.     

Central cardiovascular effects of 6-hydroxydopamine

Effects of 6-hydroxydopamine (6-OHDA) on the vasomotor loci of medulla oblongata, hypothalamus and spinal cord have been investigated in anaesthetized (chloralosed) cats. Intracerebroventricular (i.c.v.) and intrathecal (i.t.) administration of 6-OHDA, 0.5–2.0 mg, inhibited reflex, as well as direct, excitability of vasomotor loci. The blood pressure and the heart rate were reduced for a prolonged period. Central tyramine response was blocked while peripheral noradrenaline response remained unaffected. The adrenergic neurones appear to have an excitatory function in central cardiovascular regulation.     

Operant behavioural and neurochemical effects after neonatal 6-hydroxydopamine treatment

Newborn rats were treated at 1 and 2 days after birth with 100 mg/kg 6-hydroxydopamine (6OHDA), s.c. Testing on several operant behavioural tasks was begun at 6 months of age. On a fixed ratio 30 (FR 30) schedule of food reinforcement, the neonatal 6OHDA treated rats responded at a significantly higher rate. Further analysis of the FR 30 response pattern indicated that the higher rate was due to a decrease in the amount of time spent pausing after the receipt of each reinforcer. The 6OHDA treatment failed to alter the rat's behaviour during the extinction of the FR 30 response and on the progressive ratio or variable interval schedules of food reinforcement. Biochemical analysis of several brain areas at 9 months of age showed a decrease in noradrenaline (NA) levels in the cerebral cortex and hippocampus, while in the pons-medulla NA, content was doubled. The tyrosine hydroxylase activity in these same brain areas was not significantly altered, but there appeared to be some decrease in the activity of this enzyme in the hippocampus. Comparison of the operant behavioural effects seen after various lesioning procedures in this and other studies, suggest the effects on FR performance are a result of destruction of NA neurons in the hippocampus and/or the apparent regeneration of neurons in the pons-medulla.     

Alteration of avoidance and ingestive behavior after destruction of central catecholamine pathways with 6-hydroxydopamine

Alterations of shuttle-box avoidance acquisition, ingestive behavior, and catecholamine content in 4 different parts of brain were determined following bilateral infusion of 6-hydroxydopamine into the ventral tegmental area containing A-10 dopamine cell bodies, the tegmental segment of the ascending norepinephrine pathways, the globus pallidum, or the caudate-putamen. The maximum antagonism of active avoidance acquisition occurred following placement of 6-hydroxydopamine into the ventral tegmental and caudate areas. No effect on either avoidance or ingestive behavioral measures occurred after infusion of 6-hydroxydopamine into the norepinephrine pathways. Factor analysis of behavioral and biochemical data suggested that only striatal dopamine content bore a high relationship to avoidance behavior, while ingestive behavioral measures were highly related to both striatal and limbic dopamine content. Results suggest a functional—anatomical differentiation of dopamine pathways in brain.     

Amnesia due to beta-antagonists in a passive avoidance task in the chick

The beta-adrenergic antagonists sotalol, nadolol and timolol (which act at both beta 1- and beta 2-receptors) induce amnesia in the domestic chick when given systemically after a one trial passive avoidance task. State dependent learning and effects on performance at test (e.g. interference with recall) almost certainly do not cause the observed amnesia. All three effective beta-antagonists induce amnesia only when injected within a limited period after training, suggesting effects on memory formation. Sotalol differs markedly from nadolol and timolol in: (a) showing a sharp and markedly earlier loss of effectiveness as administration is moved to progressively later times after training (25-30 min rather than 40-50 min), (b) producing when given after training a delayed and gradual loss of retention rather an immediate and rapid loss. These differences seem not to reflect a lesser effectiveness of sotalol, nor a greater delay in the onset of its action, but instead qualitative differences in effects on memory formation.     

Intraventricular 6-hydroxydopamine lowers isolation-induced fighting behavior in male mice

Male mice with high isolation-induced fighting tendencies were administered 200 μg 6-OHDA or vehicle intraventricularly and tested for fighting tendency for up to 10 weeks until sacrifice, and assayed for brain NE levels. A strong correlation was found between NE depletion and reduced fighting tendency after 6-OHDA treatment. The depressed fighting by mice with less than 200 ng NE/g persisted throughout a series of test fights, indicating no recovery in fighting behavior throughout the survival time.     

Secretory effects of 6-hydroxydopamine in normal and denervated submaxillary glands of the rat

Summary Injection of 6-hydroxydopamine (6-OH-DA) elicited a marked and sustained secretory response of control and surgically sympathectomized submaxillary glands of rats. These responses were diminished by previous treatment with reserpine 0.1 mg/kg 48 and 24h before the experiment and almost abolished by 5 mg/kg reserpine 6 h before the administration of 6-OH-DA. The responses to 6-OH-DA were potentiated in control glands by previous preganglionic denervation of either the parasympathetic or sympathetic nerves of the gland. Development of postjunctional supersensitivity in denervated glands also increased the responses to 6-OH-DA, while atropine had a feeble blocking action. For all these responses, the adrenal catecholamines played no role. After two consecutive doses of 6-OH-DA a third dose of the drug still elicited a secretory response that was 50% of that of the first dose.It is concluded that for the responses to 6-OH-DA the leakage of noradrenaline from the degenerating adrenergic nerve endings of the submaxillary gland plays a partial role. Noradrenaline released by the drug from other tissues and reaching the gland via the circulation also contributes to the responses observed. A muscarinic component may also participate in the secretory effects of 6-OH-DA.Members of Carrera del Investigador Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos AiresPartially supported by Grant no. 3211/74, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires     

Influence of 6-hydroxydopamine on the behavioral effects induced by apomorphine or clonidine in rats

The aim of this paper is to examine if central chemical sympathectomy induced by two injections of 6-hydroxydopamine (6-OHDA) in a dose of 250 g intracerebroventricularly (k.c.v.) affects behavioral phenomena elicited by apomorphine (AP) (1 or 1.2 mg/kg i.p.) or clonidine (CL) (0.1 mg/kg, 5 or 1 g/kg i.p.).Experiments were carried out on male Wistar rats. The time of duration of several components of behavior and the degree of irritability of rats were measured. Moreover, open field and hole test were performed. The lower dose of AP did not affected behavior of rats. The higher dose increased the locomotor and exploratory activity of animals. 6-OHDA potentiated these effects of AP. CL (0.1 mg/kg) had a depressive effect on the rats' behavior, which was potentiated by 6-OHDA. CL (5 g/kg) had no effect on the rats' behavior, but in a dose of 1 g/kg caused excitatory behavior. This type of behavior was abolished by 6-OHDA. In conclusion, central chemical sympathectomy caused increased sensitivity of the central nervous system on AP. Excitatory behavioral effects of CL in low dosage may be connected with stimulation of central adrenergic receptors. Depressive behavioral effect of CL in high dosage is unspecific. Central chemical sympathectomy affects by different methods the reactivity of dopaminergic and noradrenergic neurons.     

The effects of 6-hydroxydopamine and guanethidine on peripheral adrenergic nerves in the guinea pig

Chemical sympathectomy was achieved in guinea pig trachea 24 hr after a single dose of 50 mg/kg (i.v. or i.p.) 6-hydroxydopamine (6-OHDA). This was shown by (a) the dissapearance of fluorescent nerve terminals (b) decreased uptake-with-retention of radioactivity following incubation in (?)-³H-noradrenaline (50 nM) and (c) the supersensitive response of the isolated tracheal chain preparation to noradrenaline. The susceptibility of the adrenergic nerve terminals in the lung of 6-OHDA resembled that of trachea, in that complete dissapearance of fluorescent nerve terminals was achieved with the above doses, whereas doses higher than 100 mg/kg were required to produce this in heart and iris. Chronic treatment of guinea pigs with guanethidine (25 or 30 mg/kg daily by i.p. injection for up to 6 weeks) did not produce destruction of adrenergic nerves in peripheral tissues.     

Neonatal 6-hydroxydopamine potentiates clonidine's locomotor effects throughout maturation in the rat

Newborn rats were administered 6-hydroxydopamine (6-OHDA) systemically (2 X 100 micrograms/g) or vehicle and subsequently tested for their locomotor response to one of three doses of clonidine (10, 100 or 1000 micrograms/kg) at either 10, 20, 30 or 50 days of age. Clonidine caused a dose-related increase in activity in 10-day-old rats and this effect was potentiated in the 6-OHDA-treated rats. At 20 days, clonidine did not affect activity in the vehicle rats but at the highest dose, increased that of the 6-OHDA rats. At 30 days, clonidine again did not affect activity in the vehicle rats but the 1000 micrograms/kg (when compared to the 100 micrograms/kg) dose, significantly increased activity of the 6-OHDA rats. A biphasic effect of clonidine was apparent at 50 days. At this age the 6-OHDA treatment exaggerated the depressant effect of the 100 micrograms/kg dose. Thus, neonatal-OHDA generally potentiated the locomotor response to clonidine despite the fact that the effect of the drug changed with age. It is concluded that the locomotor effects of clonidine are mediated through alpha 2 adrenoceptors which are not located on noradrenergic terminals but rather are postsynaptic to them. With maturation, the "functional wiring" of these terminals appears to be altered.     

Hyperactivity in developing rats: Sex differences in 6-hydroxydopamine and amphetamine effects

Possible sex-related differences in the effects of 6-hydroxydopamine (6-OHDA) and d-amphetamine on activity were investigated in rats administered intracisternal injections of 6-OHDA or its vehicle at 5 days of age. Administration of the dopamine neurotoxin resulted in a significant depletion of whole brain dopamine to 62.2% of controls as indicated when the brains of animals sacrificed at 35–36 days of age were analyzed by high pressure liquid chromatography. In normal developing rat pups, activity for rats of both sexes increased from low levels at 12 days of age to fairly high levels at 21–24 days of age. Thereafter, activity levels in control male pups decreased to levels typical for adults, whereas female rats displayed little decrease from their peak level of activity. Hyperactivity was detected in 6-OHDA treated male rats at 24 and 27 days of age, after the decline in activity to adult levels in control (vehicle-treated) pups. Females treated with 6-OHDA, on the other hand, were hypoactive at 27 days of age relative to vehicle-injected female controls. Furthermore, d-amphetamine injections increased the activity of 6-OHDA treated males to a lesser extent than it did in their vehicle-injected controls, at 24 and 27 days of age, while just the opposite was true for female 6-OHDA treated rats administered d-amphetamine. This altered responsiveness to d-amphetamine associated with the 6-OHDA treatments was not related to body weight, which was virtually unaffected by the 6-OHDA treatment. Thus, no conclusive evidence was found for the ability of d-amphetamine to decrease 6-OHDA-induced hyperactivity. However, the results implicate the role of sex of subjects in modulating the effects of 6-OHDA and d-amphetamine on activity in developing rats and generally support clinical findings which suggest that hyperactivity may be largely confined to male, pre-adolescent organisms.     

Formation of 6-hydroxydopamine in caudate nucleus of the rat brain after a single large dose of methylamphetamine

We now report that 6-hydroxydopamine (0.39 +/- 0.31 nanograms/mg of tissue at 2 hr) is formed in the rat caudate nucleus after a single injection of methylamphetamine (100 mg/kg). The same dose of methylamphetamine causes approximately 50% depletion of caudate dopamine 2 weeks after the injection. We suggest that the formation of 6-hydroxydopamine from endogenous dopamine is responsible for the neurotoxicity to dopamine terminals seen after methylamphetamine administration.     

The neurosteroid pregnenolone sulfate blocks NMDA antagonist-induced deficits in a passive avoidance memory task

The neurosteroid pregnenolone sulfate (PS) has been recently shown to positively modulate NMDA receptors and to have memory enhancing properties in mice. In the present study, we examined the ability of PS to increase retention performance and to reduce deficits induced by a competitive NMDA receptor antagonist, the 3-((±)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), in a step-through passive avoidance task in rats. Pretraining administration of PS (0.84–1680 pmol, ICV) had minimal effects on retention performance assessed 24 h after training, while CPP significantly decreased retention performance at the doses of 1.2 and 1.6 nmol (ICV). However, when administered in combination with CPP (1.2 nmol), PS (0.84–840 pmol, ICV) dose-dependently blocked the deficit in passive avoidance response induced by the NMDA antagonist. At the dose of 840 nmol, PS also significantly reduced the motor impairment induced by CPP (1.2 nmol). The blockade of CPP-induced behavioral deficits by PS may result from its positive modulatory action at NMDA receptors.     

Physiological and behavioral effects of centrally-administered 6-hydroxydopamine in cats

Intraventricular injections of 6-hydroxydopamine (6-OHDA) into cats produced a greater reduction of brain norepinephrine than dopamine content while producing no change in brain serotonin. The effects immediately after the first injection of 6-OHDA included hypothermia, increase in respiratory rate, and in the presence of pargyline, sham rage. Following the first injection, a prolonged period of hypophagia and hypodipsia occurred. Subsequent injections produced less hypothermia and greater increases in respiratory rate. Measurement of sleep-wake ratios indicated that paradoxical sleep was initially reduced by 6-OHDA administration, but eventually recovered to occupy its normal percentage of the cycle. However, wake time was increased and slow-wave sleep time decreased. Animals treated with 6-OHDA had lower baseline heart rates and showed no evidence for acquiring a conditioned heart rate response, although heart rate response to shock was potentiated.     

Prevention of 6-hydroxydopamine neurotoxicity.

1-Phenyl-3-(2-thiazolyl)-2-thiourea (PTTU) prevented the neurotoxic actions of 6-hydroxydopamine on adrenergic nerves in the mouse atrium and iris. This is the first reported 6-hydroxydopamine antagonist that does not act by blocking uptake of catecholamines into nerve terminals. PTTU also prevented the diabetogenic action of alloxan.