Strain differences in the gating-disruptive effects of apomorphine: relationship to gene expression in nucleus accumbens signaling pathways

Abstract

Background: Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is deficient in certain psychiatric disorders, including schizophrenia. Sprague Dawley (SD) rats are more sensitive to PPI-disruptive effects of apomorphine (APO) at long interstimulus intervals (ISIs) (60-120 msec) and less sensitive to PPI-enhancing effects of APO at short ISIs (10-30 msec) compared with Long Evans (LE) rats.

Methods: Prepulse inhibition was tested in SD and LE rats after APO (.5 mg/kg) or vehicle in a within- subject design and sacrificed 14 days later. Total RNA was extracted from the nucleus accumbens (NAC). Approximately 700 dopamine-relevant transcripts on the Affymetrix 230 2.0 microarray were analyzed.

Results: As previously reported, SD rats exhibited greater APO-induced PPI deficits at long intervals and less APO-induced PPI enhancement at short intervals compared with LE rats. One hundred four genes exhibited significantly different NAC expression levels in these two strains. Pathway analysis revealed that many of these genes contribute to dopamine receptor signaling, synaptic long-term potentiation, or inositol phosphate metabolism. The expression of some genes significantly correlated with measures of APO-induced PPI sensitivity in either SD or LE rats. The expression of select genes was validated by real-time reverse transcription polymerase chain reaction (RT-PCR).

Conclusions: Differences in PPI APO sensitivity in SD versus LE rats are robust and reproducible and may be related to strain differences in the expression of genes that regulate signal transduction in the NAC. These genes could facilitate the identification of targets for ameliorating heritable gating deficits in brain disorders such as schizophrenia.

Figures

Figure 1

The effects of APO on PPI at prepulse intervals of 10 - 120 ms in LE and SD rats. Inset shows APO effects on startle magnitude. Significantly different from VEH (same strain) = * p p < 0.001.

Figure 2

Hierarchical Clustering Analysis of significant gene expression differences (p

Figure 3

Dopamine Receptor Signaling: Genes differentially expressed in the NAC of LE vs. SD rats are significantly over represented in the dopamine-receptor signaling pathway. In LE rats, COMT, DRD1a and ADCY5 mRNA levels in the NAC (red-filled shapes) are higher than in SD rats. In contrast, MAO(b), ADCY4, PDE10A, PKA (Prkag2) and PP1 (Ppp1r3c) (green-filled shapes) exhibit reduced expression in LE rats compared to SD rats. Adapted from Ingenuity ® System's database of canonical pathways for Dopamine Receptor Signaling. Where the biochemical annotation in the Ingenuity canonical pathway differs from the gene symbol used by Affymetrix (see Table 1), the Affymetrix gene symbol is included in parentheses.

Figure 4

Synaptic Long-Term Potentiation: Genes differentially expressed in the NAC of LE vs. SD rats are significantly over represented in the synaptic long-term potentiation pathway. In LE rats, genes that represent the ionotropic AMPA (Gria1) and NMDA (Grin2b) classes of glutamate receptors exhibit significantly higher mRNA expression levels (red-filled shapes) compared to SD rats, as do CAMK2 and CaN (Ppp3ca). Conversely, genes coding for the metabotropic glutamate receptor mGluR (Grm5) are expressed at higher levels in SD compared to LE rats (green-filled shapes), as are several genes for enzymes in this pathway, e.g., PKA (Prkag2), PP1 (Ppp1r3c), CAMK2(b), P(r)KC (delta, epsilon, iota and theta subunits), and MAPK1. Adapted from Ingenuity® System's database of canonical pathways for Synaptic Long-Term Potentiation. Where the biochemical annotation in the Ingenuity canonical pathway differs from the gene symbol used by Affymetrix (see Tables 1 & 2), the Affymetrix gene symbol is included in parentheses.

Figure 5

Real time RT-PCR for COMT and PIK3R1. Expression values were normalized to GAPDH. Values represent the mean percentage difference from SD rats ± SEM. **** (p < 0.001), and (p < 10 -7).