Site and mechanism of behavioral tolerance to cocaine: a study of dopamine release in Wistar-Kyoto and spontaneously hypertensive rats

DIFFERENTIAL LOCOMOTOR RESPONSES IN MALE RATS FROM THREE STRAINS TO ACUTE METHYLPHENIDATE

Genetic variability is an important consideration in the study of the effect of drugs on humans and animals. Specifically, the effect of drugs on behavior varies from one rat strain to another. Methylphenidate (MPI), commonly known as Ritalin, is a psychostimulant with a pharmacological profile similar to amphetamine and cocaine, and is widely used in the treatment of attention deficit/hyperactivity disorder (ADHD). The purpose of this study was to examine the acute dose-response characteristics of MPD on three different male rat strains: spontaneously hypertensive/hyperactive rats (SHR), Wistar-Kyoto (WKY) rats, and Sprague-Dawley (SD) rats. Results showed clear differences in the pattern of behavioral response to the various MPD doses among the three rat strains. The 0.6 mg/kg MPD dose had no effect on any of the strains. At the dose of 2.5 mg/kg MPD, WKY rats reached peak locomotor activity within the initial 10 min, whereas SHR and SD rats reached peak activity in approximately 30 min. Furthermore, the effects on locomotor activity of SHR were longer in duration and higher in locomotor activity, whereas the effects on SD rats were shorter in duration and amplitude. Following the 10.0 mg/kg MPD, WKY, and SD mats reached their peak locomotor activity within 80 min post-injection, whereas SHR reached their peak locomotor activity within the initial 10 min post-injection. In addition, the SHR strain exhibited increased behavioral sensitivity to MPD as expressed by the latency, duration, and amplitude of their response. In conclusion, the dose-response experiment on the acute effects of MPD demonstrated that there are genetic differences in the responses to MPD.

Comparison of the behavioural pharmacology of the Lister-Hooded with 2 commonly utilised albino rat strains

The use of animal models (particularly rats) in research for developing drugs for central nervous system diseases is well validated. However a range of strains are often utilised in these models. The Lister-Hooded (LH) strain is beginning to be increasingly used in preclinical investigations. Thus, the objective of the present study was to investigate the comparative behavioural pharmacology of this strain, with the two most widely used rat strains, namely the Sprague-Dawley (SD), and Wistar (W) strains. The tests used were the forced swim test (FST) for antidepressants, the amphetamine-locomotor activity test for antipsychotics, the elevated plus maze (EPM) for anxiolytics, as well as tests of general locomotor activity using home cage monitoring (HCM) and the open field test. Continuous HCM revealed a significantly higher daily activity and lower nocturnal activity for LH compared to the other strains; there were no strain-related differences in the open field test. In the FST, there were no strain differences in immobility time and a similar magnitude of desipramine-induced reduction in immobility across strains. In the locomotor activity test, control LH rats showed significantly higher activity whilst significant amphetamine-induced hyperactivity was seen only with the LH and W strains. In the EPM, control LH rats had a significantly larger percentage of open arm entries, whilst only the SD strain displayed a significant diazepam-induced increase in this parameter. These findings suggest that strain variation can cause markedly different results in behavioural pharmacological tests where locomotor activity plays a significant role, and should be taken into account when selecting a strain for evaluating the behavioural effects of psychotropic drugs. Such differences in locomotor activity in the LH strain could be accounted for by an altered diurnal pattern in this strain.

In vivo basal and amphetamine-induced striatal dopamine and metabolite levels are similar in the spontaneously hypertensive, Wistar–Kyoto and Sprague–Dawley male rats

Nigrostriatal alterations are proposed to partially underlie the hypertension and hyperactivity exhibited by the spontaneously hypertensive rat (SHR). Here, in vivo microdialysis was used to measure baseline and d-amphetamine (AMPH)-stimulated striatal dopamine (DA) and metabolite levels in adult male SHR, Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats. At approximately 19 weeks of age, baseline levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were measured after which time, each rat was injected intraperitoneally with 2 mg/kg AMPH and samples were collected for the subsequent 200 min. There were no significant strain differences in baseline levels of DA, HVA, and 5-HIAA. The baseline level of DOPAC was decreased in the WKY relative to the SD. AMPH treatment altered DA, DOPAC, HVA, and 5-HIAA to a similar extent in all strains; thus, there were no significant strain differences, nor did the area under the curve (AUC) for DA levels differ between strains. AUC for DOPAC was significantly smaller for the WKY relative to the SD strain, likely due to the lower baseline level. At the single dose of amphetamine used here, the results indicate that in vivo DA levels in the SHR are similar to the WKY and SD strains.

Cocaine preexposure fails to sensitize the acquisition of cocaine-induced taste aversions

In two separate experiments, rats were given either an intraperitoneal (IP) injection of 10 mg/kg cocaine once a day for 10 consecutive days (Experiment 1) or a single IP injection of 40 mg/kg of cocaine (Experiment 2) prior to receiving repeated pairings of a novel saccharin solution with cocaine (32 mg/ kg; subcutaneous; SC). Although vehicle-preexposed subjects given saccharin-cocaine pairings readily acquired an aversion to the cocaine-associated saccharin solution, subjects preexposed to cocaine (whether 10 times or only once) displayed a retarded acquisition of the aversion. That is, cocaine preexposure attenuated the acquisition of cocaine-induced taste aversions. There was no difference in the degree of attenuation between the two preexposure conditions. Thus, under conditions that are effective in inducing sensitization within other behavioral preparations there was no evidence of sensitized cocaine-induced taste aversions. The results from the present investigation are similar to reports from this laboratory and others demonstrating that preexposure to cocaine, as with a range of other psychoactive drugs, results in weaker taste aversions. The basis for the attenuating effects of cocaine preexposure was discussed in terms of an adaptation to the aversive effects of cocaine.