A comparative study of the behavioral effects of d-amphetamine and apomorphine in the rat

The novel dehydroepiandrosterone (DHEA) derivative BNN27 counteracts cognitive deficits induced by the D1/D2 dopaminergic receptor agonist apomorphine in rats

RATIONALE

Schizophrenia is a devastating mental disease that affects nearly 1% of the population worldwide. It is well documented that the dopaminergic (DAergic) system is compromised in schizophrenia. It is of note that the mixed dopamine (DA) D₁/D₂ receptor agonist apomorphine induces schizophrenia-like symptoms in rodents, including disruption of memory abilities. Neuroactive steroids, comprising dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulphate (DHEAS), were shown to affect brain DAergic system and to be involved in schizophrenia. BNN27 is a novel DHEA derivative, which is devoid of steroidogenic activity. It has recently been reported that BNN27 counteracted schizophrenia-like behavioural deficits produced by glutamate hypofunction in rats.

OBJECTIVES

The aim of the present study was to investigate the ability of BNN27 to attenuate non-spatial, spatial recognition and discrete memory deficits induced by apomorphine in rats.

METHODS

To this end, the object recognition task (ORT), the object location task (OLT) and the step-through passive avoidance test (STPAT) were used.

RESULTS

BNN27 (3 and 6 mg/kg, i.p.) attenuated apomorphine (0.5 mg/kg, i.p.)-induced non-spatial, spatial recognition and discrete memory deficits. Interestingly, the effects of compounds on memory cannot be ascribed to changes in locomotor activity.

CONCLUSIONS

Our findings suggest that BNN27 is effective to DA dysfunction caused by apomorphine, attenuating cognitive impairments induced by this D1/D2 receptor agonist in rats. Additionally, our findings illustrate a functional interaction between BNN27 and the DAergic system that may be of relevance for schizophrenia-like behavioural symptoms.

RGS4 overexpression in the rat dorsal striatum modulates mGluR5- and amphetamine-mediated behavior and signaling

RATIONALE

Regulator of G-protein signaling 4 (RGS4) is a brain-enriched negative modulator of G-protein-coupled receptor signaling. Decreased availability of RGS4 in the frontal cortex and striatum has been described in animal models of schizophrenia and drug addiction. However, cellular and behavioral consequences of dysregulated RGS4-dependent receptor signaling in the brain remain poorly understood.

OBJECTIVE

This study aims to investigate whether RGS4, through inhibiting the function of mGluR5 receptors in the dorsal striatum (dSTR), regulates cellular and behavioral responses to acute amphetamine.

METHODS

After herpes simplex virus-RGS4 was infused into the dSTR, RGS4 overexpression as well as binding of recombinant RGS4 to mGluR5 was assessed. The effect of RGS4 overexpression on behavioral activity induced by the intrastriatal mGluR5 agonist, DHPG, or amphetamine was recorded. Activation of extracellular signal-regulated kinase (ERK) and Akt (protein kinase B) was measured in the dSTR tissue at the end of each behavioral experiment.

RESULTS

RGS4 overexpressed in the dSTR coimmunoprecipitated with mGluR5 receptors and suppressed both behavioral activity and phospho-ERK levels induced by DHPG. RGS4 overexpression or the mGluR5 antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP), attenuated amphetamine-induced phospho-ERK (but not phospho-Akt) levels. RGS4 suppressed amphetamine-induced vertical activity and augmented horizontal activity over 90 min. Similarly, MTEP augmented amphetamine-induced horizontal activity, but did not affect vertical activity.

CONCLUSIONS

The present data demonstrate that RGS4 in the dSTR attenuates amphetamine-induced ERK signaling and decreases the behavioral efficacy of acute amphetamine likely by limiting mGluR5 function.

Conditioned fear inhibits c-fos mRNA expression in the central extended amygdala

We have shown previously that unconditioned stressors inhibit neurons of the lateral/capsular division of the central nucleus of the amygdala (CEAl/c) and oval division of the bed nucleus of the stria terminalis (BSTov), which form part of the central extended amygdala. The current study investigated whether conditioned fear inhibits c-fos mRNA expression in these regions. Male rats were trained either to associate a visual stimulus (light) with footshock or were exposed to the light alone. After training, animals were replaced in the apparatus, and 2 h later injected remotely, via a catheter, with amphetamine (2 mg/kg i.p.), to induce c-fos mRNA and allow inhibition of expression to be measured. The rats were then presented with 15 visual stimuli over a 30 minute period. As expected, fear conditioned animals that were not injected with amphetamine, had extremely low levels of c-fos mRNA in the central extended amygdala. In contrast, animals that were trained with the light alone (no fear conditioning) and were injected with amphetamine had high levels of c-fos mRNA in the CEAl/c and BSTov. Animals that underwent fear conditioning, and were re-exposed to the conditioned stimulus after amphetamine injection had significantly reduced levels of c-fos mRNA in both the BSTov and CEAl/c, compared to the non-conditioned animals. These data suggest that conditioned fear can inhibit neurons of the central extended amygdala. Because these neurons are GABAergic, and project to the medial CEA (an amygdaloid output region), this may be a novel mechanism whereby conditioned fear potentiates amygdaloid output.

CRF type 1 receptors in the dorsal periaqueductal gray modulate anxiety-induced defensive behaviors

The dorsal periaqueductal gray (dPAG) is involved in defensive coping reactions to threatening stimuli. Corticotropin releasing factor (CRF) is substantially implicated as a direct modulator of physiological, endocrine and behavioral responses to a stressor. Previous findings demonstrate a direct role of the central CRF system in dPAG-mediated defensive reactions toward a threatening stimulus. These include anxiogenic behaviors in the elevated plus maze (EPM) in rats and defensive reactions in both the mouse defense test battery (MDTB) and rat exposure test (RET) paradigms in mice. Furthermore, CRF was shown to directly and dose-dependently excite PAG neurons in vitro. The aim of the present series of experiments was to directly evaluate the role of the CRF1 receptor (CRF1) in dPAG-induced defensive behaviors in the MDTB and the RET paradigms. For this purpose, cortagine, a novel CRF1-selective agonist, was directly infused into the dPAG. In the RET the high dose of cortagine (100 ng) significantly affected spatial avoidance measures and robustly increased burying behavior, an established avoidance activity, while having no effects on behaviors in the MDTB. Collectively, these results implicate CRF1 in the dPAG as a mediator of temporally and spatially dependent avoidance in response to controllable and constant stimuli.

A reverse-translational approach to bipolar disorder: rodent and human studies in the Behavioral Pattern Monitor

Mania is the defining feature of bipolar disorder (BD). There has been limited progress in understanding the neurobiological underpinnings of BD mania and developing novel therapeutics, in part due to a paucity of relevant animal models with translational potential. Hyperactivity is a cardinal symptom of mania, traditionally measured in humans using observer-rated scales. Multivariate assessment of unconditioned locomotor behavior using the rat Behavioral Pattern Monitor (BPM) developed in our laboratory has shown that hyperactivity includes complex multifaceted behaviors. The BPM has been used to demonstrate differential effects of drugs on locomotor activity and exploratory behavior in rats. Studies of genetically engineered mice in a mouse BPM have confirmed its utility as a cross-species tool. In a "reverse-translational" approach to this work, we developed the human BPM to characterize motor activity in BD patients. Increased activity, object interactions, and altered locomotor patterns provide multi-dimensional phenotypes to model in the rodent BPM. This unique approach to modeling BD provides an opportunity to identify the neurobiology underlying BD mania and test novel antimanic agents.

Multivariate Analysis of Temporal Descriptions of Open-field Behavior in Wild-derived Mouse Strains

The open-field test is a commonly used apparatus in many behavioral studies. However, in most studies, temporal changes of details of behavior have been ignored. We thus examined open-field behavior as measured by both conventional indices and 12 ethograms supported by detailed temporal observation. To obtain a broader understanding, we used genetically diverse mouse strains: 10 wild-derived mouse strains (PGN2, BFM/2, HMI, CAST/Ei, NJL, BLG2, CHD, SWN, KJR, MSM), one strain derived from the so-called fancy mouse (JFI), and one standard laboratory strain, C57BL/6. Conventional measurements revealed a variety of relationships: some strains did not show the hypothesized association between high ambulation, longer stay in the central area, and low defecation. Our ethological approach revealed that some behaviors, such as freezing and jumping, were not observed in C57BL/6 but were seen in some wild-derived strains. Principal component analysis which included temporal information indicated that these strains had varied temporal patterns of habituation to novelty.

Acute global anoxia during C-section birth affects dopamine-mediated behavioural responses and reactivity to stress

Perinatal asphyxia may induce major neurological deficits shortly after birth as well as neurological/behavioural disorders later in development. We used a rat model of global perinatal asphyxia to model acute intrauterine asphyxia around the time of birth. Caesarean section was performed in rats and their pups, still in uterus horns, were placed into a water bath at 37 degrees C for periods of 0, 10 or 20 min. Pups were then given to surrogate mothers, and examined for long-term behavioural effects of the perinatal asphyctic insult. Behavioural assessment included analysis of novelty seeking behaviour at adolescence, while spatial discrimination abilities, response to both an acute and a chronic stress, and the effects of the full D1 receptor agonist SKF 82958 on open field behaviour were assessed at adulthood. Overall, no marked abnormalities were found in the novelty seeking test, in the ability to discriminate spatial changes in the test environment and in physiological response to stress. However, adult rats subjected to severe perinatal asphyxia (20 min) showed lower activity level and lower stereotyped behaviour after the administration of SKF 82958 in an open field test. These results support the observations from human and animal studies that perinatal insult can produce long-term dysfunction of dopaminergic neurotransmission, and points to the need of more thorough examination of the potential effects of perinatal asphyxia on hypothalamic-pituitary-adrenal (HPA) axis. Altogether, the present findings suggest that the present 20 min perinatal asphyxia model might serve for the study of neurodevelopmental disorders associated with perinatal insults.

Cholecystokinin modulation of apomorphine- or amphetamine-induced stereotypy in rats: opposite effects

Stereotyped behavior can be induced by the dopamine agonist apomorphine or by the releasing agent amphetamine. Cholecystokinin influence on dopamine-mediated behaviors has been extensively studied but a real controversy remains. Our purpose was to further characterize the dopamine-cholecystokinin interaction in apomorphine- and amphetamine-induced stereotyped behavior using sulphated cholecystokinin octapeptide (CCK8) and cholecystokinin tetrapeptide (CCK4) treatments. The results showed that CCK8 decreases apomorphine-induced stereotyped behavior and CCK4 has no effect. CCK4 and CCK8 increased the amphetamine-induced stereotyped behavior; CCK4 was more effective. The results confirm the opposite modulation of apomorphine or amphetamine-induced stereotyped behavior by CCK. These data suggest that this modulation is mediated by both CCK receptors on apomorphine-induced and only by CCK(2) receptors on amphetamine-induced stereotyped behavior.

Facilitation of preparatory behavior in an artificial prey paradigm by D1-subfamily dopamine receptor activation

Dopamine agonists facilitate, and antagonists inhibit, conditioned preparatory behaviors in rats. Similar effects are demonstrated on an unconditioned preparatory behavior: predatory search and contact of a moving artificial prey stimulus. Apomorphine (0.1, 0.2 mg/kg), a direct agonist, had no effect relative to a within-subject injection of saline vehicle but d-amphetamine (0.1 mg/kg), an indirect agonist, increased contact frequency without altering overall motor activation. To determine the relative importance of the D1 and D2 subfamilies of receptors in the amphetamine effect, separate groups of animals received amphetamine co-injected with either SCH23390 (0.01 and 0.005 mg/kg) or eticlopride (0.01 mg/kg), D1 and D2 antagonists, respectively. Whereas the eticlopride-amphetamine group showed no change in contact frequency from baseline, co-injections of either dose of SCH23390 and amphetamine led to near total suppression of contact, as did treatment with SCH23390 (0.005 mg/kg) alone. Treatment with 0.01 mg/kg eticlopride alone increased contact frequency while treatment with a higher dose (0.1 mg/kg) had no effect. Treatment with the D1-subfamily agonist SKF81297 (0.1 mg/kg) increased contact frequency. Collectively, these results support the hypothesis that dopamine mediates unconditioned preparatory behavior and suggest differing roles for the D1 and D2 receptor subfamilies.

Opposite modulation of apomorphine- or amphetamine-induced stereotypy by antagonists of CCK receptors

Stereotyped behavior is elicited by activation of dopaminergic systems with drugs such as apomorphine and amphetamine. In previous studies, we have reported that the sulfated cholecystokinin octapeptide (CCK-8) decreased apomorphine-induced stereotypy in animals with normal and supersensitive dopamine receptors. The aim of the present study was to evaluate the effects of CCK(1) and CCK(2) receptor antagonists on stereotyped behavior induced by apomorphine or amphetamine. Rats were pretreated with the CCK(1) (SR 27897B; 1-[[2-(4-(2-chlorophenyl) thiazol-2-yl) aminocarbonyl]indolyl]acetic acid; 500 microg/kg; i.p.) or CCK(2) (L-365,260; 3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5 phenyl-1H-1, 4-benzodiazepine-3-yl)-N'-(3-methyl phenyl)-urea; 500 microg/kg; i.p. ) receptor antagonists or saline 15 min before apomorphine (0.6 mg/kg; s.c.) or amphetamine (9.0 mg/kg; i.p.) injection. Both CCK(1) and CCK(2) receptor antagonists significantly increased apomorphine-induced stereotypy. In contrast, only the blockade of CCK(2) receptors significantly decreased amphetamine-induced stereotypy. The results suggest a dual opposite mechanism for CCK-dopamine interactions. These data also suggest that both apomorphine- and amphetamine-induced stereotypy should be used whenever effects of drugs acting on dopaminergic systems are being assessed.

Behavioral sensitization to apomorphine in adult rats exposed to cocaine during the preweaning period: a preliminary study

Sixty-day-old rats treated with cocaine (50 mg/kg SC) during postnatal days (PND) 11-20 received daily injections of apomorphine (2.0 mg/kg SC) for 10 consecutive days to examine the development of sensitization to a direct dopamine agonist. Behavior was monitored on days 1, 5, and 10, using a photobeam system, and on day 10 using the videotape assessments as well. Locomotor sensitization to apomorphine developed in the preweaning vehicle-treated males only. Neither the cocaine-treated males nor any females exhibited locomotor sensitization to repeated apomorphine injections at 2 mg/kg. There were no other treatment-related effects except for grooming, which showed an interaction between treatment and gender. Overall, every behavior analyzed showed significant apomorphine effects, except rearing. Margin time (wall hugging), grooming, and quiet were significantly decreased by apomorphine, while locomotion and the duration of sniffing were increased. In summary, these data indicate that with respect to locomotor activity, the development of sensitization to apomorphine at 2.0 mg/kg is prevented by preweaning cocaine administration in males. These data further suggest that developmental cocaine exposure produces long-term alterations in DA D1 receptor-mediated responses in male rats.

D-amphetamine, cocaine and caffeine: a comparative study of acute effects on locomotor activity and behavioural patterns in rats

Although open-field behaviour has been considered a valid and reliable index of locomotor activity in rodents, the simple measures traditionally recorded in this test do not readily allow for differentiation between compounds of the same general class, e.g. psychostimulants. The present methodology was developed to facilitate detailed and continuous observations on the behaviour of drug-treated rats. In addition to an automated (photocell) measure of general locomotor activity, ethological techniques were used to record the frequency and duration of standing, moving, sniffing, rearing, grooming, scratching, sniffing air, freezing, head-swinging and licking. A series of factor analyses was also performed in order to further characterize treatment-induced changes in the structure of behaviour. Compounds studied were d-amphetamine (0.5, 1.5, 3, 6 mg/kg), cocaine (5, 10, 20, 50 mg/kg) and caffeine (5, 10, 20, 40 mg/kg). Although all three psychostimulants increased the automated measure of general locomotor activity, cocaine (which produced the largest effects) monotonically increased general activity over the dose range tested, whereas the stimulant effects of the other two compounds were either reduced (d-amphetamine) or eliminated (caffeine) at higher doses. More detailed observations provided confirmation of the differences in effect produced by these compounds. For example, the frequency and duration of 'moving' dose-dependently increased after cocaine, while d-amphetamine and caffeine again produced bell-shaped dose-response curves. However, whereas low-intermediate doses of d-amphetamine reduced the mean duration of moving, sniffing and rearing, no such effect was observed at the highest dose tested. This finding, together with the appearance of licking in the behavioural repertoire, suggests a stereotyped character to responses seen at high doses of this compound, though neither cocaine nor caffeine induced stereotypy. As factor analyses also revealed quite different behavioural structures associated with these three drugs, present findings demonstrate that detailed observation of behaviour represents a useful approach to research on the behavioural pharmacology of psychostimulants.

Differential alterations in basal and D-amphetamine-induced behavioural pattern following 6-OHDA or ibotenic acid lesions into the dorsal striatum

It is well known that the corpus striatum is related to the sterotyped activation induced by several psychostimulants. In this study we analyzed the effects of 6-OHDA, in comparison with those of ibotenic acid lesions, into the dorsal striatum, on the behavioural pattern induced by saline or D-amphetamine treatment. A computerized technique for recording the animal motor activity was developed in order to define in a detailed way the behavioural profile in lesioned and sham-operated rats induced by the saline or D-amphetamine treatment. A 6-OHDA lesion into the dorsal striatum modified the basal behavioural pattern which was mainly characterized by reduced motor activation while ibotenic acid lesion affected the structure of the basal behavioral pattern. D-Amphetamine administration in 6-OHDA lesioned rats induced a behavioural stimulation, but a decreased motor and stereotyped activation was observed compared to the sham-operated animals treated with D-amphetamine. In contrast, D-amphetamine administration in the ibotenic acid-lesioned rats induced a motor and stereotyped activity which was not reduced compared to that seen after D-amphetamine treatment in sham-operated rats. These results suggest that these two types of lesion induced differential effects on the behavioural pattern either after saline or after D-amphetamine administration. Dopaminergic neurotransmission in the dorsal striatum plays a permissive role on the emergence of the behavioural responses, while the dorsal striatum circuitry plays a crucial role on the organization of the behavioural pattern. In addition, dopaminergic activity in this structure serves a primary control in the D-amphetamine-elicited motor activation or stereotypy, while the striatal structure is involved in the shaping of the D-amphetamine behavioural pattern.

Birth insult increases amphetamine-induced behavioral responses in the adult rat

We have previously reported that an apparently uncomplicated Caesarean section birth produces long-term alterations in steady-state levels of dopamine in the central nervous system of the rat. In addition, adult rats that had been born by Caesarean section, either with or without acute global anoxia, showed markedly greater dopamine release from the nucleus accumbens in response to repeated stress, in comparison to vaginally born controls. The aim of the present study was to test whether these birth complications also result in long-term changes in behavior mediated by dopamine systems. For this, we investigated effects of a low dose (0.5 mg/kg) of amphetamine on activity levels in three-month-old rats that had been born vaginally (control), by rapid Caesarean section, or by Caesarean section with 15 min of global anoxia. Amphetamine induced a significantly greater increase in locomotor activity in animals born by Caesarean section or by Caesarean section+ 15 min anoxia, in comparison to the drug's effects in vaginally born controls. Behavioral responses were further analysed from video recordings of the animals' behavior. In confirmation of automated activity counts, both animals born by Caesarean section and by Caesarean section + 15 min anoxia showed a significant increase in the duration and frequency of moving and a decrease in the duration and frequency of standing, in comparison to vaginally born controls. Animals delivered by Caesarean section showed a significant increase in the duration of sniffing and a decrease in the duration and frequency of grooming when compared to vaginally born controls. Animals delivered by Caesarean section + 15 min anoxia showed a significant increase in the duration and frequency of rearing, in comparison to controls. The pattern of behavioral changes observed indicates that, as adults, animals born by Caesarean section and by Caesarean section with added global anoxia both show heightened behavioral responses to amphetamine, in comparison to vaginally born animals. These findings highlight the sensitivity of dopamine pathways to variations in birth procedure and add experimental support to epidemiological evidence implicating birth complications in the pathophysiology of disorders involving central dopaminergic neurons, such as schizophrenia.

Stereotyped behavior: effects of d-amphetamine and methylphenidate in the young rat

The proclivity of d-amphetamine and methylphenidate to induce perseverative motoric and vocal side effects detracts from the clinical efficacy of these stimulants in the treatment of Attention-Deficit Hyperactivity Disorder (ADHD). In an attempt to develop a model for these deleterious treatment effects, this study explored the behavioral influences exerted by d-amphetamine and methylphenidate in the young laboratory rat. This experiment revealed that doses of these stimulants that typically induce stereotypy provoke diverging behavioral profiles: while animals given 5 mg/kg d-amphetamine exhibited repetitive sniffing activity, rats treated with 30 mg/kg methyl-phenidate displayed perseverative gnawing behaviors. Although pretreatment with the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) significantly attenuated both stimulant-induced stereotypies, the effect of PCPA on d-amphetamine-induced sniffing was more profound than on methylphenidate-induced gnawing. High-performance liquid chromatography (HPLC) analysis of monoamine levels in the striatum, frontal cortex, and thalamus indicated that PCPA induced an overall 89% depletion of serotonin across all conditions. These findings shed some light on the neurochemical mechanisms that underlie the differential effects of d-amphetamine and methylphenidate on stereotyped motor activity in the rat, and suggest future experiments for understanding the role of serotonin in such effects. Further, these results have implications for the differential side effects observed from each of these stimulants when used clinically in children with ADHD.

6-Hydroxydopamine lesion of locus coeruleus and the antiparkinsonian potential of NMDA-receptor antagonists in rats

Behavioral and neurochemical effects after bilateral 6-hydroxy-dopamine locus coeruleus- (LC) lesion were examined in rats and compared to sham-lesioned controls. Behavior after treatment with the antiakinetic drugs dizocilpine, amantadine, memantine or L-DOPA as well as joint treatment of these drugs with haloperidol were tested in an open field with holeboard and in an experimental chamber. Under saline spontaneous activity (open field with holeboard) and sniffing (experimental chamber) were reduced after lesion. Injection of the proparkinsonian drug haloperidol decreased sniffing in all rats but to a greater extent in LC-lesioned rats. In combination with haloperidol none of the tested drugs could completely compensate for the motor deficits induced by the lesion. Neurochemical data revealed a reduced content of noradrenaline in the prefrontal cortex and in the posterior striatum of LC-lesioned rats. These results indicate that loss of LC neurons intensifies parkinsonian symptoms induced by blockade of dopamine D2-receptors, and lowers the antiakinetic potential of dizocilpine, amantadine, memantine or L-DOPA.

Changes in the structure of the agonistic behavior of mice produced by D-amphetamine

The effects of three acute doses of D-amphetamine (0.25, 1.5 and 3 mg/kg) were studied in a model of isolation-induced aggression in male mice. An ethopharmacological analysis of the encounters was carried out, which studied the frequency, total and mean duration of different behavioral categories, including the temporal distribution of attacks and the duration of inter-attack intervals. The results show a reduction in the total and mean duration of the Attack category and an increase in motor activity manifested by longer durations, both total and mean, of Non Social Exploration and shorter Immobility. The temporal analysis of Attack revealed an increase in the number of very short (< 15 s) inter-attack intervals and a temporal redistribution of the attacks to later in the course of the social encounters. These results confirm for a complex behavior such as aggression, that D-amphetamine, even at low doses, favors a fragmentation and repetition of motor routines with a simultaneous reduction in the influence of environmental cues on the control of behavior.

Effects of acute treatment with dopaminergic drugs on open field behavior of adult monkeys treated with lead during the first year postpartum

A pharmacological challenge of nonhuman primate open field behavior, similar to that which previously assessed the cholinergic system (10), was used here to measure potential lead-induced alterations in the dopaminergic system. Monkeys that had been treated with lead during the first year postpartum were assessed at 7 years of age in the open field after acute intramuscular injection of apomorphine (0.0-0.3 mg/kg) or haloperidol (0.0-30 micrograms/kg). Duration of environmental exploration indicated a possible greater responsivity to 0.2 mg/kg apomorphine in the lead-treated monkeys; however, in all other behaviors, lead-treated monkeys responded to both drugs similarly to controls. Regardless of lead treatment, apomorphine administration decreased duration of inactivity and increased environmental exploration; the latter possibly included an increase in stereotypical behavior that might have been recorded as environmental exploration. Haloperidol had no significant effects on open field behavior over the dose range tested. Open field behavioral alterations previously reported for these monkeys at 4-6 years of age were no longer strongly exhibited by the lead-treated monkeys in any portion of the current study. Latency to enter the open field was marginally increased in the lead-treated group but levels of environmental exploration were comparable to controls. These results indicate an attenuation of lead-related effects with maturity and/or familiarity with the open field.

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: effects on spontaneous release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on the extracellular levels of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10-11, 21-22, and 35-36 days old and adults and quantified by HPLC with electrochemical detection. Apomorphine attenuated extracellular levels of dopamine in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine on extracellular dopamine was observed in all age groups except at 10-11 days of age. Extracellular levels of 3,4-dihydroxyphenylacetic acid were also significantly decreased in all age groups, consistent with the hypothesis that synthesis-modulating dopamine autoreceptors in the neostriatum are functional by 5 days of age. Apomorphine had a significantly greater effect on extracellular 3,4-dihydroxyphenylacetic acid levels at the 0.05 and 0.1 mg/kg doses in the 5- and 10-11-day-old age groups compared with the other ages. Absolute levels of extracellular dopamine were significantly attenuated at 5 days of age compared with the other ages, and absolute levels of extracellular 3,4-dihydroxyphenylacetic acid monotonically increased with age.

Intrinsic and extrinsic influences on play fighting in rats: effects of dominance, partner's playfulness, temperament and neonatal exposure to testosterone propionate

Play fighting is a frequent activity of juvenile rats and appears to show marked variability amongst individuals in that some rats play a great deal and others very little. This study attempted to identify some of the factors involved in producing this individual variability. The major influence over an individual's frequency of play as a juvenile was found to be the frequency of play by the partner. That is, play appears to be contagious, in that a high playing animal stimulates its partner to play frequently as well. In male juveniles, but seemingly not in female juveniles, the subsequent adult status of one partner as dominant influences the subordinate-to-be to initiate more playful contacts. In addition to these extrinsic influences, however, there appear to be intrinsic factors that influence whether an individual is a high or low playing animal. One intrinsic factor appears to be 'boldness', so that bolder animals tend to initiate more playful contacts. Higher players tend to be more susceptible to the stereotypy-inducing effects of the dopamine agonist, apomorphine, and tend to be more dependent upon the playful activity of the partner to maintain their own high levels of play. Both of these characteristics are consistent with other studies comparing bold and timid rats. Boldness, however, only seems to influence how much play a rat will exhibit, not how much play it is capable of exhibiting. Neonatal testosterone augmentation increases juvenile play fighting but not apomorphine susceptibility, suggesting that a high player need not be a bold animal. The total frequency of play an individual is capable of initiating appears to depend upon perinatal exposure to androgens. Boldness and the playfulness of the partner appear to modulate the expression of this hormonally set value.