Does sensitization occur to prepulse inhibition of the startle reflex effects of repeated apomorphine treatments in rats?

There are conflicting reports as to whether or not the effects of dopamine agonist effects at reducing prepulse inhibition of the acoustic startle reflex develop sensitization with repeated treatments. In this experiment, rats (12 per each dose group) were treated for 10 days prior to startle-testing on each day with 0 (vehicle), 50, 200 or 800 mu g/kg of apomorphine. Startle testing was conducted with each rat receiving no stimulus trials (null trials), startle pulse only trials (40 ms 105 dB white noise), prepulse only trials (20 ms 72 dB 5 kHz tone) and prepulse+pulse trials with a 100 ms stimulus onset asynchrony (SOA, i.e. the lead time from onset of prepulse to onset of pulse). The rats were then challenged after 5-7 days of withdrawal from the treatment regimen with a vehicle and apomorphine (200 mu g/kg) injection with the order of injection counterbalanced. A range of SOAs and two different prepulse intensities (68 and 70 dB) were presented to every rat on the challenge tests. Sensitization developed during treatment to the increase in motor activity produced by the two higher doses, and to the increase in an orienting response produced by the prepulse stimulus in the highest dose group, but not to the prepulse inhibition effect of the drugs. The 50 mu g/kg inhibitory autoreceptor selective dose decreased responses on the first of three blocks of both null trials and prepulse only trials. The two higher doses dose-dependently increased startle reflex amplitudes on the prepulse + pulse trials (reduced prepulse inhibition), but this effect did not exhibit sensitization during treatment. The lowest dose significantly increased prepulse inhibition relative to the vehicle-treated group on the first block of trials over days. After apomorphine challenge, sensitization to the effects of apomorphine on reducing prepulse inhibition was apparent for some dose groups at some SOAs. Sensitization to the effects of apomorphine on prepulse inhibition can be demonstrated upon a subsequent drug challenge if pretreatments are associated exclusively with the startle testing environment.