Behavioral responses during the initial exposures to a low dose of cocaine in late preweanling and adult rats

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

Importance of dopaminergic neurotransmission for the RU 24969-induced locomotor activity of male and female rats during the preweanling period

There is disagreement about whether the locomotor activity produced by serotonin (5-HT) _1A/1B receptor agonists is ultimately mediated through a dopaminergic mechanism or is independent of dopamine (DA) system functioning. Using a developing rat model, we examined whether DA neurotransmission is necessary for the locomotor activity produced by 5-HT_1A/1B receptor stimulation. Depending on experiment, male and female preweanling rats were pretreated with vehicle, the monoamine-depleting agent reserpine, the 5-HT synthesis inhibitor 4-chloro-DL-phenylalanine methyl ester hydrochloride (PCPA), the DA synthesis inhibitor ∝-methyl-DL-p-tyrosine (AMPT), or the D1 and D2 receptor antagonists SCH 23390 and raclopride, respectively. After completing the pretreatment regimen, the behavioral effects of saline and the 5-HT_1A/1B receptor agonist RU 24969 were assessed during a 2-h test session. Locomotor activity in the center and margin of the testing chamber was recorded. RU 24969's locomotor activating effects were sensitive to blockade of the D2 receptor, but not the D1 receptor. The DA synthesis inhibitor (AMPT) significantly attenuated the RU 24969-induced locomotor activity of preweanling rats, as did the 5-HT synthesis inhibitor PCPA. The latter result suggests that presynaptic 5-HT_1A/1B receptors may have a role in mediating RU 24969-induced locomotion during the preweanling period. DA neurotransmission, especially involving D2 receptors, is necessary for the 5-HT_1A/1B-mediated locomotor activity of preweanling rats. The actions of PCPA, reserpine, and SCH 23390 differ substantially between preweanling and adult rats, suggesting that the neural mechanisms underlying these DA/5-HT interactions vary across ontogeny.

Effects of repeated RU 24969 treatment on the locomotor activity, motoric capacity, and axillary temperatures of male and female preweanling rats

Serotonin (5-HT) _1A and _1B receptors have been implicated in behavioral sensitization, but adult rats appear to develop tolerance to RU 24969 (a 5-HT_1A/1B receptor agonist) rather than a sensitized response. The purpose of the present study was to determine whether a one- or four-day pretreatment regimen of RU 24969 would cause sensitization or tolerance in male and female preweanling rats. Depending on experiment, rats were pretreated with RU 24969 (0, 2.5, or 5 mg/kg) for 1 or 4 days (PD 17-20), while testing with lower or higher doses of RU 24969 occurred on PD 22. Locomotor activity, motoric capacity, and axillary temperatures were recorded. The role of Pavlovian contextual conditioning was assessed by administering RU 24969 to rats in either the home cage or a novel environment. On the first pretreatment day, RU 24969 caused both an increase in forward locomotion and motoric impairment, along with a substantial decrease in axillary temperatures. Repeated treatment with the same dose of RU 24969 caused all three dependent measures to show a tolerance response. When given a higher dose of RU 24969 on the test day, the responses lost due to repeated drug treatment were fully (locomotor activity) or partially (motoric capacity and axillary temperatures) reinstated. There was no evidence of behavioral tolerance. Results are consistent with the hypothesis that a subsensitivity of 5-HT_1A/1B receptors is at least partially responsible for the tolerance caused by RU 24969, but dispositional tolerance cannot be excluded as a contributing factor.

Importance of D1 and D2 receptor stimulation for the induction and expression of cocaine-induced behavioral sensitization in preweanling rats

The behavioral manifestations of psychostimulant-induced sensitization vary markedly between young and adult rats, suggesting that the neural mechanisms mediating this phenomenon differ across ontogeny. In this project we examined the importance of D1 and D2 receptors for the induction and expression of cocaine-induced behavioral sensitization during the preweanling period. In the behavioral experiments, rats were injected with reversible D1 and/or D2 antagonists (SCH23390 and/or raclopride) or an irreversible receptor antagonist (EEDQ) either before cocaine administration on the pretreatment day (induction) or before cocaine challenge on the test day (expression). In the EEDQ experiments, receptor specificity was assessed by using selective dopamine antagonists to protect D1 and/or D2 receptors from inactivation. Receptor binding assays showed that EEDQ caused substantial reductions in dorsal striatal D1 and D2 binding sites, while SCH23390 and raclopride fully protected D1 and D2 receptors from EEDQ-induced alkylation. Behavioral results showed that neither D1 nor D2 receptor stimulation was necessary for the induction of cocaine sensitization in preweanling rats. EEDQ disrupted the sensitization process, suggesting that another receptor type sensitive to EEDQ alkylation was necessary for the induction process. Expression of the sensitized response was prevented by an acute injection of a D1 receptor antagonist. The pattern of DA antagonist-induced effects described for preweanling rats is, with few exceptions, similar to what is observed when the same drugs are administered to adult rats. Thus, it appears that maturational changes in D1 and D2 receptor systems are not responsible for ontogenetic differences in the behavioral manifestation of cocaine sensitization.

Effects of D2 or combined D1/D2 receptor antagonism on the methamphetamine-induced one-trial and multi-trial behavioral sensitization of preweanling rats

RATIONALE

There is suggestive evidence that the neural mechanisms mediating one-trial and multi-trial behavioral sensitization differ, especially when the effects of various classes of dopamine (DA) agonists are examined.

OBJECTIVE

The purpose of the present study was to determine the role of the D2 receptor for the induction of one-trial and multi-trial methamphetamine sensitization in preweanling rats.

METHODS

In a series of experiments, rats were injected with saline or raclopride (a selective D2 receptor antagonist), either alone or in combination with SCH23390 (a selective D1 receptor antagonist), 15 min prior to treatment with the indirect DA agonist methamphetamine. Acute control groups were given two injections of saline. This pretreatment regimen occurred on either postnatal days (PD) 13-16 (multi-trial) or PD 16 (one-trial). On PD 17, rats were challenged with methamphetamine and locomotor sensitization was determined.

RESULTS

Blockade of D2 or D1/D2 receptors reduced or prevented, respectively, the induction of multi-trial methamphetamine sensitization in young rats, while the same manipulations had minimal effects on one-trial behavioral sensitization.

CONCLUSIONS

DA antagonist treatment differentially affected the methamphetamine-induced sensitized responding of preweanling rats depending on whether a one-trial or multi-trial procedure was used. The basis for this effect is uncertain, but there was some evidence that repeated DA antagonist treatment caused nonspecific changes that produced a weakened sensitized response. Importantly, DA antagonist treatment did not prevent the one-trial behavioral sensitization of preweanling rats. The latter result brings into question whether DA receptor stimulation is necessary for the induction of psychostimulant-induced behavioral sensitization during early ontogeny.

Role of the D1 receptor for the dopamine agonist-induced one-trial behavioral sensitization of preweanling rats

RATIONALE

The neural mechanisms mediating the ontogeny of behavioral sensitization are poorly understood.

OBJECTIVE

The purpose of the present study was to determine the role of the D1 receptor for the induction of dopamine agonist-induced behavioral sensitization during the preweanling period.

METHODS

In the first experiment, the early ontogeny of R-propylnorapomorphine (NPA)-induced behavioral sensitization was examined by pretreating male and female rats with saline or NPA (0.5, 1, or 2 mg/kg, intraperitoneally (IP)) before placement in activity chambers on postnatal day (PD) 12, 16, 20, or 24. One day later, rats were tested with lower doses of NPA and the occurrence of locomotor sensitization was determined. In subsequent experiments, rats were injected with saline or the D1 receptor antagonist SCH23390 (0.1, 0.5, 1, or 5 mg/kg, IP) 0, 15, 30, or 60 min before cocaine, methamphetamine (METH), or NPA pretreatment. The next day, rats were tested with the same dopamine agonist again and sensitized responding was assessed.

RESULTS

NPA produced one-trial behavioral sensitization at all ages tested. In preweanling rats, SCH23390, regardless of dose, was ineffective at preventing the induction of cocaine-, METH-, or NPA-induced one-trial behavioral sensitization.

CONCLUSIONS

The present results are in partial contrast to adult rodent studies, in which SCH23390 blocks the induction of METH- and apomorphine-induced behavioral sensitization, but not cocaine sensitization. When these findings are considered together, it appears that D1 receptor stimulation is not necessary for the induction of behavioral sensitization during the preweanling period, although D1 receptors may play a more important role in adulthood.

Novelty-induced conditioned place preference, sucrose preference, and elevated plus maze behavior in adult rats after repeated exposure to methylphenidate during the preweanling period

Early treatment with methylphenidate has a persistent effect on the affective (i.e., anxiety- and depressive-like) behaviors of adult rats and mice. Interestingly, age at methylphenidate exposure appears to be a critical determinant influencing the expression of affective behaviors. In the present study, we exposed rats to methylphenidate during the preweanling period (i.e., PD 11-PD 20) because this ontogenetic period is analogous to early childhood in humans (an age associated with increasing methylphenidate usage). Rats were injected with methylphenidate (0, 2, or 5mg/kg) from PD 11 to PD 20 and reactivity to rewarding and aversive stimuli were measured in early adulthood. Specifically, novelty-induced CPP, sucrose preference, and elevated plus maze behavior were assessed on PD 60. Early treatment with 2 or 5mg/kg methylphenidate increased total time spent in the white compartment of the CPP chamber. This methylphenidate-induced effect occurred regardless of exposure condition. Performance on the elevated plus maze was also impacted by early methylphenidate exposure, because rats treated with 5mg/kg methylphenidate spent more time in the closed compartment of the elevated plus maze than vehicle controls. Early methylphenidate exposure did not alter sucrose preference. These data indicate that exposing rats to methylphenidate during the preweanling period differentially affects anxiety-like behavior depending on the type of anxiety-provoking stimulus. Specifically, early methylphenidate exposure decreased aversion to a bright white room when measured on a novelty-induced CPP task, whereas methylphenidate caused a long-term increase in anxiety when measured on the elevated plus maze.

Ontogeny of methamphetamine-induced and cocaine-induced one-trial behavioral sensitization in preweanling and adolescent rats

The ontogenetic profile of psychostimulant-induced one-trial behavioral sensitization has not been determined. The purpose of this study was to systematically assess the ontogeny of methamphetamine-induced and cocaine-induced behavioral sensitization across the preweanling and adolescent periods. To this end, rats were injected with methamphetamine, cocaine, or saline in either an activity chamber or home cage during the preweanling [postnatal day (PD) 12, PD 16, or PD 20], preadolescent (PD 24), or adolescent (PD 34) periods. One day later, rats were challenged with the same psychostimulant and locomotion was measured in an activity chamber. The results showed that methamphetamine produced one-trial locomotor sensitization on PD 13 and PD 17; whereas, cocaine-induced behavioral sensitization was only evident on PD 21. The sensitized responding of preweanling rats was not influenced by environmental context. Interestingly, preadolescent and adolescent rats did not exhibit locomotor sensitization. The latter result is generally consistent with past studies showing that rats from the middle and late adolescent periods do not exhibit cocaine-induced one-trial behavioral sensitization. The present results show that methamphetamine, as well as cocaine, can produce one-trial context-independent behavioral sensitization during early ontogeny, but sensitized responding is only apparent within a narrow developmental window.

One-trial behavioral sensitization in preweanling rats: differential effects of cocaine, methamphetamine, methylphenidate, and D-amphetamine

RATIONALE

Preweanling rats exhibit robust one-trial cocaine-induced behavioral sensitization; however, it is uncertain whether other psychostimulants can also induce sensitization in young rats using the one-trial procedure.

OBJECTIVE

The purpose of this study was to determine whether methamphetamine, methylphenidate, and D: -amphetamine are capable of inducing one-trial locomotor sensitization in preweanling rats.

METHODS

In a series of four experiments, rats were pretreated with cocaine (30 mg/kg), methamphetamine (2-12 mg/kg), methylphenidate (5-20 mg/kg), or amphetamine (5 mg/kg) before being placed in a novel activity chamber or the home cage on PD 19. Rats were then challenged with the same psychostimulant (20 mg/kg cocaine, 1-8 mg/kg methamphetamine, 2.5-7.5 mg/kg methylphenidate, or 1-2 mg/kg amphetamine) on PD 21, with distance traveled being measured for 180 min. In a separate experiment, rats were pretreated with methamphetamine on PD 16-19 and challenged with methamphetamine on PD 21.

RESULTS

Only cocaine, but not various dose combinations of other psychostimulants, was able to produce one-trial behavioral sensitization in preweanling rats. Context-dependent locomotor sensitization was also evident if rats were pretreated with methamphetamine on PD 16-19 and tested on PD 21.

CONCLUSIONS

It is uncertain why only cocaine was able to induce one-trial locomotor sensitization in preweanling rats, but it is possible that: (a) the neural circuitry mediating sensitization differs according to psychostimulant, (b) cocaine is more readily associated with environmental contexts than other psychostimulants, or (c) affinity and pharmacokinetic factors may underlie cocaine's ability to induce one-trial behavioral sensitization in preweanling rats.

Behavioral differences between late preweanling and adult female Sprague-Dawley rat exploration of animate and inanimate stimuli and food

The late preweanling rat has potential as a preclinical model for disorders initially manifested in early childhood that are characterized by dysfunctional interactions with specific stimuli (e.g., obsessive-compulsive disorder and autism). No reports, however, of specific-stimulus exploration in the late preweanling rat are found in the literature. We examined the behavioral responses of normal late preweanling (PND 18-19) and adult rats when presented with exemplars of categorically-varied stimuli, including inanimate objects systematically varied in size and interactive properties, biological stimuli, and food. Preweanlings were faster to initiate specific stimulus exploration and were more interactive with most specific stimuli than adults; the magnitude of these preweanling-adult quantitative differences ranged from fairly small to very large depending upon the stimulus. In contrast, preweanlings were adult-like in their interaction with food and prey. Preweanling response to some stimuli, for example to live pups, was qualitatively different from that of adults; the preweanling behavioral repertoire was characterized by pup-seeking while the adult response was characterized by pup-avoidance. The specific stimulus interactions of preweanlings were less impacted than those of adults by the time of day of testing and placement of a stimulus in an anxiety-provoking location. The impact of novelty was stimulus dependent. The differences in interactions of preweanlings versus adults with specific stimuli suggests that CNS systems underlying these behavior patterns are at different stages of immaturity at PND 18 such that there may be an array of developmental trajectories for various categories of specific stimuli. These data provide a basis for the use of the preweanling as a preclinical model for understanding and medicating human disorders during development that are characterized by dysfunctional interactions with specific stimuli.

Pharmacologic neuroimaging of the ontogeny of dopamine receptor function

Characterization of the ontogeny of the cerebral dopaminergic system is crucial for gaining a greater understanding of normal brain development and its alterations in response to drugs of abuse or conditions such as attention-deficit hyperactivity disorder. Pharmacological MRI (phMRI) was used to determine the response to dopamine transporter (DAT) blockers cocaine and methylphenidate (MPH), the dopamine releaser D-amphetamine (AMPH), the selective D1 agonist dihydrexidine, and the D2/D3 agonist quinpirole in young (<30 days old) and adult (>60 days old) rats. In adult rats, cocaine (0.5 mg/kg i.v.) or MPH (2 mg/kg) induced primarily positive cerebral blood volume (rCBV) changes in the dopaminergic circuitry, but negative rCBV changes in the young animals. Microdialysis measurements in the striatum showed that young rats have a smaller increase in extracellular dopamine in response to cocaine than adults. The young rats showed little rCBV response to the selective D1 agonist dihydrexidine in contrast to robust rCBV increases observed in the adults, whereas there was a similar negative rCBV response in the young and adult rats to the D2 agonist quinpirole. We also performed a meta-analysis of literature data on the development of D1 and D2 receptors and the DAT. These data suggest a predominance of D2-like over D1-like function between 20 and 30 days of age. These combined results suggested that the dopamine D1 receptor is functionally inhibited at young age.

Motivational systems in adolescence: possible implications for age differences in substance abuse and other risk-taking behaviors

Adolescence is an evolutionarily conserved developmental phase characterized by hormonal, physiological, neural and behavioral alterations evident widely across mammalian species. For instance, adolescent rats, like their human counterparts, exhibit elevations in peer-directed social interactions, risk-taking/novelty seeking and drug and alcohol use relative to adults, along with notable changes in motivational and reward-related brain regions. After reviewing these topics, the present paper discusses conditioned preference and aversion data showing adolescents to be more sensitive than adults to positive rewarding properties of various drugs and natural stimuli, while less sensitive to the aversive properties of these stimuli. Additional experiments designed to parse specific components of reward-related processing using natural rewards have yielded more mixed findings, with reports of accentuated positive hedonic sensitivity during adolescence contrasting with studies showing less positive hedonic affect and reduced incentive salience at this age. Implications of these findings for adolescent substance abuse will be discussed.