Similarities in the behavior and molecular deficits in the frontal cortex between the neurotensin receptor subtype 1 knockout mice and chronic phencyclidine-treated mice: relevance to schizophrenia

Abstract

Much evidence suggests that targeting the neurotensin (NT) system may provide a novel and promising treatment for schizophrenia. Our recent work shows that: NTS1 knockout (NTS1(-/-)) mice may provide a potential animal model for studying schizophrenia by investigating the effect of deletion NTS1 receptor on amphetamine-induced hyperactivity and neurochemical changes. The data indicate a hyper-dopaminergic state similar to the excessive striatal DA activity reported in schizophrenia. The present study was done to determine if NTS1(-/-) mice also have similar changes in behavior, in prefrontal neurotransmitters, and in protein expression, as observed in wild type (WT) mice treated with the psychotomimetic phencylclidine (PCP), an animal model for schizophrenia. Our results showed many similarities between untreated NTS1(-/-) mice and WT mice chronically treated with PCP (as compared with untreated WT mice): 1) lower PCP-induced locomotor activity; 2) similar avolition-like behavior in forced-swim test and tail suspension test; 3) lower prefrontal glutamate levels; 4) less PCP-induced dopamine release in medial prefrontal cortex (mPFC); and 5) down-regulation of mRNA and protein for DA D(1), DA D(2), and NMDAR2A in mPFC. Therefore, these data strengthen the hypothesis that the NTS1(-/-) mouse is an animal model of schizophrenia, particularly for the dysfunction of the prefrontal cortex. In addition, after chronic PCP administration, the DA D(1) receptor was up-regulated in NTS1(-/-) mice, results which suggest a possible interaction of NTS1/DA D(1) in mPFC contributing to chronic PCP-induced schizophrenia-like signs.

Figure 1

Coronal sections showing microdialysis probe placement within mPFC for all animals. Panel A: the placement of the probe in mPFC. Numbers below the figure represent the position of the slice relative to bregma. The figure was adapted from Paxinos and Franklin (Paxinos and Franklin, 2001). Panel B: A picture of real brain coronal section. After each microdialysis, the brains were removed and sectioned to verify the correct position of the probe by injecting trypan blue through probe. Black arrow points out probe location.

Figure 2

Effect of acute and chronic PCP or saline administration on locomotor activity in WT and NTS1^−/− mice. Panel A. Effect of acute PCP or saline administration on locomotor activity in both WT and NTS1^−/− mice. Panel B. Effect of chronic PCP or saline administration on locomotor activity in both WT and NTS1^−/− mice. All data are shown as mean ± S.E.M (N=5–7 for each group). *, P<0.05.

Figure 3

Effect of chronic PCP or saline administration on forced swim-induced immobility in WT and NTS1^−/− mice. Results are expressed as the mean±S.E.M. (N=5–7 rats). *, P < 0.05 versus WT mice with chronic saline administration.

Figure 4

Effect of chronic PCP or saline administration on the amount of immobility in WT and NTS1^−/− mice during the tail-suspension test. Behavior was observed every 5 s during the 6-min test period and scored as mobile or immobile. Results are expressed as the mean number of counts over the 6-min period (±S.E.M.) (N=5 mice for each group). *, P < 0.05 versus WT mice with chronic saline administration.

Figure 5

The basal concentration of neurotransmitters in mPFC in both WT and NTS1^−/− mice in the acute and chronic PCP studies. The samples were measured by microdialysis coupled with CE. Panel A and B: The basal level of amino acids and DA in mPFC of WT and NTS1^−/− mice. Panel C and D: Effect of chronic PCP or saline administration on basal level of amino acids and dopamine in mPFC. All data are shown as mean ± S.E.M (N=4–7 for each group). *, P<0.05.

Figure 6

Effect of acute PCP or saline injection on DA and glutamate release in mPFC in both WT and NTS1^−/− mice. Panel A and C, Effect of acute PCP or saline injection on DA release in mPFC. The data are shown as % of baseline (± S.E.M) (N=4–7 for each group). Panel B and D, Effect of acute PCP or saline injection on glutamate release in mPFC. The data are shown as AUC (± S.E.M) (N=4–7 for each group). $, P<0.05, significantly different from saline injection within the group; *, P<0.05, increase vs. AS- NTS1^−/−, AP-WT and AP- NTS1^−/−.

Figure 7

Effect of chronic PCP or saline injection on DA and glutamate release in mPFC in both WT and NTS1^−/− mice. Panel A and C, Effect of chronic PCP or saline injection on DA release in mPFC. The data are shown as real concentration (nM) (± S.E.M) (N=4–7 for each group). Panel B and D, Effect of chronic PCP or saline injection on glutamate release in mPFC. The data are shown as AUC (± S.E.M) (N=4–7 for each group). $, P<0.05, significantly different from saline injection within the group; *, P<0.05, increase vs CS- NTS1^−/−, CP-WT and CP- NTS1^−/−. #, P<0.05, decrease vs. CP-WT.

Figure 8

DA, glutamate glycine, GABA, and aspartate levels in mPFC homogenates of CS-WT, CS-NTS1^−/−, CP-WT and CP- NTS1^−/− mice as measured by CE-LIFD. Mouse brains were harvested, dissected on ice, and frozen at −80°C until assayed. Brain tissue was homogenized as described in Materials and Methods and applied to CE-LIFD for detection of neurotransmitter levels (pg/mg tissue). The data were shown as mean (pg/mg wet tissue) ± S.E.M (N=5~7 for each group). P<0.05 is considered significant. *, significantly different from WT.

Figure 9

Average R.Q. ratio of DA D₁ and DA D₂ receptors, and TH (Panel A); NMDAR1 and NMDAR2A (Panel B) for WT and NTS1^−/− mice following chronic PCP administration relative to saline treated mice. Mouse brains were harvested, dissected on ice, and frozen at −80°C until assayed. Brain tissue was homogenized and assayed as described in Materials and Methods. Results are presented as mean ± S.E.M (N=5~7 for each group). P<0.05 is considered significant. *, P<0.05.