Seventy-two male Wistar rats from the Animal Resource Center of Western Australia served as subjects. The rats, weighing between 258-355 g on arrival, were housed in the colony room, a temperature-controlled animal-housing facility on a 12 hour light/dark cycle (lights on 0700-1900), for eight days before beginning the experiment. Testing was conducted in eight standard Med Associates startle apparatus in a fan-ventilated and sound-attenuated chamber lit with a dim red light. All drugs were dissolved in a saline vehicle and injected subcutaneously with an injection volume of 1 ml/kg. Doses were 3 mg/kg (+)amphetamine sulfate, 0.05 mg/kg raclopride, 2.0 mg/kg ketanserin; all doses are expressed as the weight of the salt.

Procedures

Animals were randomly assigned to one of six groups: vehicle/vehicle (n=12), vehicle/amphetamine (n=13), ketanserin/vehicle (n=11), ketanserin/amphetamine (n=11), raclopride/vehicle (n=13), and raclopride/amphetamine (n=12). Each animal was tested three times, with three to four days between each session. The first test was a baseline non-drug test in which the rats were injected with a saline solution (1 ml/kg). The second two tests were drug tests. Animals were then placed in the startle chambers for a 10-minute acclimatisation period with a background noise of 65 dB. Fourteen trials were presented in random order in each block: (i) a null trial, with no startle or prepulse stimulus (only the background noise), (ii) two pulse-only trials, and (iii) 11 prepulse + pulse trials. The pulse-only stimulus was a 40 ms broad spectrum white noise at 105 dB with a rise/fall time of 0 ms. The prepulse stimulus was an 8 ms, 70 dB white noise with a rise/fall time of 1 ms. The interval between the onset of the prepulse stimulus and the onset of the pulse stimulus (SOA) varied in 10 ms intervals from 10 ms to 100 ms, with an additional 120 ms SOA trial included. The inter-trial interval varied randomly between 10-20 sec. In all, 12 blocks were conducted, resulting in 168 trials per animal.

There were no significant differences in startle responses or in PPI among the groups on the baseline (vehicle only) test (data not shown). Responses on the two drug test day were averaged to provide a single measure under each trial condition. As can be seen in Figure 1, amphetamine increased motor activity on nonstimulus (null) trials, and this effect was blocked by raclopride but not ketanserin.

The present study examined the effects of blockade of D2-like and 5-HT2 receptors on rats treated with amphetamine, by measuring maximal PPI and the SOA of maximal PPI. It was predicted that the indirect dopamine agonist, amphetamine, would decrease the amount of PPI, as well as reduce the SOA at which maximal inhibition occurred. In line with this prediction, amphetamine was found to have a significant attenuating effect on PPI. This finding replicates the well-established literature on amphetamine-induced deficits in PPI (Druhan et al., 1998; Mansbach et al., 1988; Swerdlow et al., 1994). In further support of the experimental hypothesis, amphetamine was also found to significantly reduce the SOA at which maximum inhibition occurred, replicating the unpublished findings of this laboratory.

When administered alone, raclopride was found to reduce startle on startle-only and prepulse + pulse trials. This finding replicates the findings of Johansson and colleagues (1995) who found that PPI was facilitated by treatment with the D2 antagonists, haloperidol and raclopride. Raclopride blocked the effect of amphetamine on general motor activity (measured on null trials), as would be expected. Ketanserin was without effect on amphetamine-induced motor activity. Both raclopride and ketanserin reduced startle, but neither blocked the effect of amphetamine at increasing startle. Therefore, amphetamine's effects on increasing startle on startle-stimulus only trials does not appear to depend on either dopamine D2 or 5-HT2 receptors. Raclopride, but not ketanserin, blocked the effects of amphetamine on reducing maximum PPI. However, neither raclopride nor ketanserin blocked the effects of amphetamine on shifting the SOA of maximum PPI. This was somewhat surprising, as unpublished data from this laboratory indicate that two selective D2 agonists (+)PHNO and quinpirole shift the SOA of maximum PPI without reducing maximum PPI across SOAs, when PPI is defined as a proportion of the startle amplitudes on the startle-stimulus only trials (as opposed to difference scores).

Thus, amphetamine produces two independent effects on PPI. D2 receptors blocked by raclopride are necessary for the effect of amphetamine on maximum PPI across SOAs. As selective D2 agonists do not reduce maximum PPI when defined as a proportion of the startle amplitudes on the startle-stimulus only trials, it appears that D2 receptors are necessary but not sufficient for this effect. However, D2 receptors are not necessary for the shift to a shorter SOA at which maximum PPI occurs. As selective D2 receptor agonists can shift the SOA at which maximum PPI occurs, it appears that D2 receptors are sufficient, but not necessary, for this action. 5-HT2 receptors are not necessary for either effect of amphetamine.

It can be concluded that dopamine D2 receptors are involved in both effects on PPI, but via separate mechanisms.

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