Cynthia A. Crawford

Effects of repeated amphetamine treatment on the locomotor activity of the dopamine D1A-deficient mouse

THE role of dopamine D1A receptors in mediating amphetamine-induced sensitization was investigated using the D1A-deficient mouse. During the drug pre-exposure phase, D1A-deficient and control mice were injected for five consecutive days with saline or amphetamine (2 mg/kg, i.p.). Locomotor activity was measured on the first and fifth pre-exposure day. After three abstinence days, mice were given either amphetamine or saline and locomotor activity was again assessed. Mice were then sacrificed and protein kinase A (PKA) activity was measured. In contrast to control mice, D1A-deficient mice did not show a progressive increase in locomotor activity across days. Importantly, both control and mutant mice did exhibit behavioral sensitization, because mice pre-exposed and tested with amphetamine were more active than mice acutely tested with the drug. Even so, the amphetamine-induced locomotor activity of the mutant mice was significantly reduced when compared with similarly treated control mice, indicating that the sensitized response was less pronounced in the D1A-deficient mouse. PKA activity also varied depending on genotype, since amphetamine decreased PKA activity in control but not D1A-deficient mice.

Effects of repeated methylphenidate treatment in the young rat: sensitization of both locomotor activity and stereotyped sniffing.

The behavioral effects of repeated methylphenidate (MPH) treatment were assessed in young rats. In 4 experiments, rats (starting at Postnatal Day 10 or 16) were pretreated on 5 consecutive days with saline or MPH (2.5-20.0 mg/kg i.p.). Sensitization was assessed after 1 or 7 abstinence days, with rats receiving a test day challenge injection of either a low dose of MPH (2.5 mg/kg) or the same dose of MPH as given during pretreatment. Results show that a test day injection of 2.5 mg/kg MPH produced a sensitized locomotor response in rats pretreated with 2.5-20.0 mg/kg MPH. This MPH-induced locomotor sensitization was evident only after 1 abstinence day. Various pretreatment doses of MPH (5, 10, 15, or 20 mg/kg) were capable of sensitizing the stereotyped sniffing of young rats, but only rats pretreated and tested with the highest dose (20 mg/kg) of MPH showed an augmented stereotyped sniffing response that was still robust after 7 abstinence days. Results indicate that young rats are capable of exhibiting sensitization after an extended abstinence period, which contrasts with previous research suggesting that psychostimulant treatment does not produce long-term sensitization in young rats.

Postnatal Development of Glutamate Receptor-Mediated Responses in the Neostriatum

Three experimental approaches were used to examine the maturation of N-methyl-D-aspartate (NMDA) receptors in the neostriatum and compare their developmental profile to that of the non-NMDA receptors [α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA)]. The first, and least conventional approach utilized infrared videomicroscopy to measure NMDA-induced swelling in single cells in a brain slice. The results demonstrated that NMDA receptors display an incremental pattern of postnatal development with no responses at postnatal day (PND) 3, weak responses at PND 7, the largest responses by PND 14 and slight decreases at PNDs 21 and 28. At PNDs 3 and 7, KA-induced cell swelling was proportionately greater than NMDA-induced cell swelling suggesting earlier maturation of this non-NMDA receptor subtype. The second approach used whole-cell patch clamp analysis to examine NMDA currents and compare their maturation to AMPA/KA-induced currents. Though the data are still preliminary, a very similar developmental pattern emerged. NMDA-induced currents were small and developed slowly after PND 7. In contrast, AMPA/KA-induced currents were larger and appeared to develop earlier. Finally, dizocilpine (MK-801) binding was measured in homogenates of neostriatal tissue. The ontogeny of binding resembled a step function with increases between PNDs 3 and 7 and PNDs 14 and 21. Binding peaked at PND 28 and then declined slightly in the adult (PND 60). The affinity of MK-801 for the receptor did not change during postnatal development. These findings demonstrate the pattern of functional development of glutamate receptors in the neostriatum. The NMDA receptor subtype displays minimal functional development until PND 14. In contrast, neostriatal AMPA/KA receptor function appears to precede NMDA receptor function.

Postnatal manganese exposure alters dopamine transporter function in adult rats: Potential impact on nonassociative and associative processes

In the present study, we examined whether exposing rats to a high-dose regimen of manganese chloride (Mn) during the postnatal period would depress presynaptic dopamine functioning and alter nonassociative and associative behaviors. To this end, rats were given oral supplements of Mn (750 microg/day) on postnatal days (PD) 1-21. On PD 90, dopamine transporter (DAT) immunoreactivity and [3H]dopamine uptake were assayed in the striatum and nucleus accumbens, while in vivo microdialysis was used to measure dopamine efflux in the same brain regions. The effects of postnatal Mn exposure on nigrostriatal functioning were evaluated by assessing rotorod performance and amphetamine-induced stereotypy in adulthood. In terms of associative processes, both cocaine-induced conditioned place preference (CPP) and sucrose-reinforced operant responding were examined. Results showed that postnatal Mn exposure caused persistent declines in DAT protein expression and [3H]dopamine uptake in the striatum and nucleus accumbens, as well as long-term reductions in striatal dopamine efflux. Rotorod performance did not differ according to exposure condition, however Mn-exposed rats did exhibit substantially more amphetamine-induced stereotypy than vehicle controls. Mn exposure did not alter performance on any aspect of the CPP task (preference, extinction, or reinstatement testing), nor did Mn affect progressive ratio responding (a measure of motivation). Interestingly, acquisition of a fixed ratio task was impaired in Mn-exposed rats, suggesting a deficit in procedural learning. In sum, these results indicate that postnatal Mn exposure causes persistent declines in various indices of presynaptic dopaminergic functioning. Mn-induced alterations in striatal functioning may have long-term impact on associative and nonassociative behavior.

Electrophysiological and Morphological Analyses of Cortical Neurons Obtained from Children with Catastrophic Epilepsy: Dopamine Receptor Modulation of Glutamatergic Responses

The present study examined the electrophysiological effects produced by activation of specific dopamine (DA) receptors and the distribution of DA receptor subtypes and glutamate receptor subunits [N-methyl-D-aspartate (NMDAR1) and GluR1] in cortical tissue samples obtained from children (ages 3 months to 16 years) undergoing epilepsy surgery. DA receptor activation produced differential effects depending on the receptor subtype that was activated. D1 receptor family agonists generally enhanced cortical excitability and favored the emergence of epileptogenic activity. In contrast, D2 receptor family agonists had more variable effects on cortical excitability and the expression of epileptiform discharges. Activation of D1 or D2 receptors decreased the amplitude of non-NMDA-mediated excitatory postsynaptic potentials. In contrast, DA and D1 agonists increased the amplitude of NMDA-mediated potentials. Immunohistochemical analysis showed that the DA receptor subtypes and glutamate receptor subunits examined were present in all cortical layers and areas throughout development. Whole-cell voltage clamp recordings of pyramidal neurons visualized with differential interference contrast optics and infrared videomicroscopy indicated that these neurons displayed a persistent Na+ current, followed by an outward current. DA reduced the outward current but had little effect on the persistent Na+ current. These results suggest a dual role for DA’s actions in the human cerebral cortex. Activation of D2 receptors or antagonism of D1 receptors may help control seizures in children.

Ontogeny of behavioral sensitization in the rat: effects of direct and indirect dopamine agonists

In the present study, the abilities of NPA (a direct DA receptor agonist) and amphetamine (an indirect DA receptor agonist) to induce short- and long-term behavioral sensitization were assessed in 11- and 17-day-old rats (age at initial injection). Rats were injected on 4 consecutive days with amphetamine (1.0, 2.5, or 5.0 mg/kg), NPA (1.0 mg/kg), or saline. A final test day occurred either 2 days (experiment 1) or 8 days (experiment 2) later. On the test day, rats given successive agonist injections received a single injection of the same agonist again; whereas rats given successive saline injections received either amphetamine or NPA for the first time. Five minutes after injection, locomotor activity (line-crosses), stereotyped sniffing, and vertical activity were measured during a 30-min testing session. The results showed that 11- and 17-day-old rats exhibited behavioral sensitization when tested with NPA or amphetamine after a 2-day interval. In contrast, neither NPA nor amphetamine was able to sensitize the behaviors of preweanling rats when measured 8 days after initial drug treatments. Therefore, these results show that both direct and indirect DA agonists are able to induce short-term behavioral sensitization in preweanling rats, but that the mechanisms responsible for mediating long-term behavioral sensitization have not yet matured.

Cocaine-induced behavioral sensitization in the young rat

Abstract Rationale: Repeated psychostimulant treatment has been shown to sensitize the locomotor activity of young rats, but there is conflicting evidence suggesting that this sensitized response will persist across only a few drug abstinence days. Objective: The purpose of the present study was to determine whether: (a) young rats are capable of expressing a sensitized locomotor response after an extended drug abstinence period, and (b) the longevity of the sensitized response is critically affected by either the number of drug pretreatment days or environmental conditioning factors. Methods: Young rats were pretreated with saline or cocaine (15 mg/kg or 30 mg/kg, i.p.) for either five or ten consecutive days [i.e., on postnatal days (PD) 16–20 or PD 11–20]. After each daily injection, rats were placed in activity chambers, and locomotion was measured for 30 min. To assess environmental conditioning factors, some rats were injected with saline prior to being placed in the activity chambers and then injected with cocaine prior to being returned to the home cage. After one or seven abstinence days (i.e., on PD 22 or PD 28), rats received a challenge injection of saline or cocaine (15 mg/kg) in the activity chamber and locomotion was assessed. Results: Young rats exhibit cocaine-induced locomotor sensitization after either a short (1-day) or long (7-day) drug abstinence period. When a long abstinence period was used, locomotor sensitization was only apparent when cocaine pretreatment lasted for 10 days. Conditioning factors were also important for determining whether locomotor sensitization was expressed, because young rats pretreated with cocaine in the home cage did not show a sensitized locomotor response after seven abstinence days. Conclusions: Young rats are capable of showing cocaine-induced locomotor sensitization after an extended abstinence period. Both the number of drug pretreatment days and the environmental context in which cocaine was given (i.e., the activity chamber or home cage) influenced the longevity of cocaine-induced locomotor sensitization.

Environmental enrichment counters cocaine abstinence-induced stress and brain reactivity to cocaine cues but fails to prevent the incubation effect.

Environmental enrichment (EE) during a period of forced abstinence attenuates incentive motivational effects of cocaine‐paired stimuli. Here we examined whether EE during forced abstinence from cocaine self‐administration would prevent time‐dependent increases in cue‐elicited cocaine‐seeking behavior (i.e. the incubation effect). Rats were trained to self‐administer cocaine, which was paired with light/tone cues, for 15 days while living in isolated conditions (IC). Controls received yoked saline infusions. Subsequently, rats were assigned to live in either continued IC or EE for either 1 or 21 days of forced abstinence prior to a test for cocaine‐seeking behavior. During testing, responding resulted only in presentation of the light/tone cues. Contrary to our prediction, cocaine‐seeking behavior increased over time regardless of living condition during abstinence; however, EE attenuated cocaine‐seeking behavior relative to IC regardless of length of abstinence. Brains were harvested and trunk blood was collected immediately after the 60‐minute test and later assayed. Results indicated that short‐term EE elevated hippocampal brain‐derived neurotrophic factor and reduced plasma corticosterone compared with IC. Furthermore, 21 days of EE during forced abstinence prevented increases in the cue‐elicited amygdala phosphorylated extracellular signal‐regulated kinase expression that was observed in IC rats. These findings suggest that EE attenuates incentive motivational effects of cocaine cues through a mechanism other than preventing the incubation effect, perhaps involving reduction of stress and neural activity in response to cocaine‐paired cues during acute withdrawal.

Dysfunctional play and dopamine physiology in the Fischer 344 rat

Juvenile Fischer 344 rats are known to be less playful than other inbred strains, although the neurobiological substrate(s) responsible for this phenotype is uncertain. In the present study, Fischer 344 rats were compared to the commonly used outbred Sprague-Dawley strain on several behavioral and physiological parameters in order to ascertain whether the lack of play may be related to compromised activity of brain dopamine (DA) systems. As expected, Fischer 344 rats were far less playful than Sprague-Dawley rats, with Fischer 344 rats less likely to initiate playful contacts with a playful partner and less likely to respond playfully to these contacts. We also found that Fischer 344 rats showed less of a startle response and greater pre-pulse inhibition (PPI), especially at higher pre-pulse intensities. The increase in PPI seen in the Fischer 344 rat could be due to reduced DA modulation of sensorimotor gating and neurochemical measures were consistent with Fischer 344 rats releasing less DA than Sprague-Dawley rats. Fast scan cyclic voltammetry (FSCV) revealed Fischer 344 rats had less evoked DA release in dorsal and ventral striatal brain slices and high-performance liquid chromatography revealed Fischer 344 rats to have less DA turnover in the striatum and prefrontal cortex. We also found DA-dependent forms of cortical plasticity were deficient in the striatum and prefrontal cortex of the Fischer 344 rat. Taken together, these data indicate that deficits in play and enhanced PPI of Fischer 344 rats may be due to reduced DA modulation of corticostriatal and mesolimbic/mesocortical circuits critical to the execution of these behaviors.

Dopaminergic function in the neostriatum and nucleus accumbens of young and aged fischer 344 rats

Age-dependent alterations in behavioral and neuronal functioning were assessed in young (2-3 month), middle-aged (12 month), and aged (24 month) Fischer 344 rats treated with the indirect dopamine agonist amphetamine (2.25 or 5 mg/kg), the D1 agonist SKF 38393 (7.5, 15, 30 mg/kg), or the D2 agonist quinpirole (0.3, 1.0, 3.0 mg/kg). Drug-induced changes in activity and stereotypy were measured during a 90-min testing session, with Fos immunohistochemistry being used to assess the neuronal response to dopamine agonist treatment. As expected, aged rats given amphetamine (5 mg/kg) had fewer activity counts and higher stereotypy scores than young rats. Middle-aged rats also had fewer activity counts but were similar in stereotypy scores to young rats. Amphetamine also induced different patterns of Fos immunoreactivity in the neostriatum and nucleus accumbens of young and aged rats, as Fos expression in aged rats exhibited a distinctive dorsal to ventral pattern of decline. In general, SKF 38393 had few age-related actions, although aged rats did show a slight relative increase in stereotypy. In contrast, the D2 agonist quinpirole substantially enhanced the motor activity and Fos expression of young rats, while only modestly affecting aged rats. Hence, these results suggest that the D2 receptor is more vulnerable to the effects of aging than the D1 receptor.