A new model of Pde4d deficiency: genetic knock-down of PDE4D enzyme in rats produces an antidepressant phenotype without spatial cognitive effects

Abstract

Phosphodiesterases (PDEs) are a superfamily of intracellular second messenger cyclic nucleotide hydrolyzing enzymes composed of 12 families. The Pde4 family has been implicated in depression and cognition, and PDE4 inhibitors have been evaluated as antidepressants and possible cognitive enhancers. Pde4d(-/-) mice show an antidepressant phenotype and learning enhancement on some tests, but not others as do mice treated with PDE4 inhibitors. Here, we report for the first time the behavioral phenotype of a new Pde4d knock-down (KD) rat model of PDE4D deficiency. Consistent with other data on PDE4D deficiency, Pde4d KD rats showed depression resistance in the Porsolt forced swim test and hyperreactivity of the acoustic startle response with no differential response on prepulse inhibition, suggesting no sensorimotor gating defect. Pde4d KD rats also exhibited a small exploratory activity reduction but no difference following habituation, and no enhanced spatial learning or reference memory in the Morris water maze. A selective improvement in route-based learning in the Cincinnati water maze was seen as well as enhanced contextual and cued fear conditioning and a more rapid rate of cued extinction from their higher freezing level that declined to wild-type (WT) levels only after ∼20 extinction trials. The rat model confirms Pde4d's role in depression but not in spatial learning or memory enhancement and shows for the first time higher fear conditioning and altered extinction compared with controls. The new model provides a tool by which to better understand the role of PDE4D in neuropsychiatric disorders and for the development of alternate treatment approaches.

Fig. 1

CWM and Fear Extinction. A, Total errors committed during CWM testing for 18 consecutive days, 2 trials per day. There was a significant genotype effect during the middle portion of maze learning with KD rats making fewer errors than WT rats. Group sizes were = Total (males/females): WT = 32 (16/16), KD = 26 (13/13); sex did not interact with genotype. B, Fear extinction. Following conditioned fear training, rats were tested for contextual fear 24 h after training (see Fig. 2) and cued fear 48 h after training, including with repeated cue presentations (extinction), i.e., rats were given 30 trials consisting of alternate 30 s periods of tone and 30 s of no-tone (no foot-shock). From extinction trials 1–22 Pde4d KD rats were more immobile than WT controls (i.e., they showed less extinction than controls). Group sizes are as in Fig. 2. *P < 0.05; **P < 0.01 vs. WT.

Fig. 2

Conditioned Fear Learning. Conditioned fear consisted of training (no differences in time spent moving vs. not moving were seen during training. 24 h after training, rats were tested for conditioned fear, assessed as percent time immobile/freezing to the same context or in response to the conditioning tone. (A) contextual fear (percent time spent freezing when placed back in the original test chamber), (B) cued fear (percent time spent freezing in test chamber with new floor after presentation of the tone), and (C) cued extinction (percent time spent freezing as in (B) with repeated tone presentations). There were no genotype differences in immobility during training or cued retention. There were significant genotype differences during contextual retention and cued extinction. Panel C showed extinction averaged across trials (see Fig. 1 for the extinction curve by trial). Group sizes were = Total (males/females): WT = 26 (14/12), KD = 23 (14/9); sex did not interact with genotype. *P < 0.05; ***P < 0.001 vs. WT.

Fig. 3

Morris water maze (MWM) hidden platform learning. Data are mean ± SEM of 4 trials per day for path length (m) to reach the goal during 6 days of training during (A) acquisition, (B) reversal, and (C) shift trials with progressively smaller platforms (10, 7, and 5 cm in diameter). No differences in hidden platform learning were observed. Group sizes were = Total (males/females): WT = 30 (15/15), KD = 27 (15/12); sex did not interact with genotype.

Fig. 4

MWM Probe trials. 24 h after the last training trial for each phase shown in Fig. 5, rats were given a single 30 s probe trial with the platform removed. Top, mean ± SEM platform site crossovers; Bottom, mean ± SEM swim speed (cm/s). (A) Acquisition-probe, (B) reversal-probe, and (C) shift-probe. KD rats swam significantly slower on acquisition-probe compared with WT rats. Group sizes are as in Fig. 5. *P < 0.05 vs. WT.

Fig. 5

Locomotor Activity. Locomotor activity was tested for 60 min and analyzed for (A) exploration (first 5 min), (B) habituation (first 30 min), (C) baseline (last 30 min). KD males and females were significantly less active during the first 30 min compared with same sex controls. Females were less active during the first 5 min compared with female WT rats. Top: males; Bottom, females. Group sizes were = Total (males/females): WT = 26 (13/13), KD = 20 (11/9). **P < 0.01 vs. WT.

Fig. 6

Acoustic startle response with prepulse inhibition (ASR/PPI) amplitude (V_max: measured in mV). Data are mean ± SEM of 10 trials of each type averaged together. Trial order was by Latin square design. Trials of no stimulus were included as an internal control to ensure system integrity. (A) Average startle response for each prepulse intensity level (0, 70, and 76 db). No effects were seen on the no stimulus trials (not shown). KD rats showed significant hyperreactivity to the basic startle signal and in the presence of a 70 dB prepulse, but at 76 db prepulse the effect was marginal compared with WT rats. (B) Percent prepulse inhibition as a function of response on no-prepulse trials. Group sizes were = Total (males/females): WT = 25 (12/13), KD = 19 (10/9). Sex did not interact with genotype. †P<0.10; *P < 0.05 vs. WT; ***P < 0.001 vs. WT.