Enhanced brain-derived neurotrophic factor signaling in the nucleus accumbens of juvenile rats

Abstract

Brain-derived neurotrophic factor (BDNF) signaling through its receptor, tropomyosin receptor kinase B (TrkB), plays a critical role in neural plasticity and its dysregulation in striatum and prefrontal cortex (PFC) has been implicated in the etiology of mental health disorders such schizophrenia and drug addiction. In the present study, we characterized age-dependent differences in BDNF signaling and TrkB expression within the nucleus accumbens (NAc), caudate putamen (CP) and PFC in rats and determined the effects of administration of the dopamine agonist, SKF 83959, which activates the Gq-coupled dopamine receptors, the dopamine D5 receptor and the D1-D2 receptor heteromer. As proBDNF binds with high affinity to the p75 neurotrophin receptor (p75NTR), expression levels of these proteins were also assessed. The present findings showed that juvenile rats (aged 26-28 days) exhibited significantly elevated basal BDNF expression and activation of full-length TrkB (TrkBfull) in NAc compared to their adult counterparts, as evidenced by increased TrkBfull phosphorylation. These changes were concomitant with an increase in the relative expression of TrkBfull compared to the truncated isoform, TrkB.T1, in NAc and CP. Conversely, in PFC the basal expression of BDNF in juvenile rats was significantly lower than in adult rats with an elevated relative expression of TrkBfull. Acute administration of SKF 83959 to juvenile rats abolished the age-dependent differences in BDNF expression in NAc and PFC, and in the relative expression of TrkBfull in NAc and CP. Together these findings indicate that the expression and/or signaling of BDNF and TrkB in striatum and PFC of juvenile rats is fundamentally different from that of adult rats, a finding that may have implications in neuropsychiatric disorders that exhibit age-dependent susceptibility such as schizophrenia and drug addiction.

Fig. 1

Age-specific and SKF 83959-dependent effects on BDNF expression in NAc, CP and PFC of adult and juvenile rats. (a) Animals exhibited an age-dependent difference in basal BDNF expression in NAc, with juvenile rats exhibiting significantly higher expression than their adult counterparts. SKF 83959 (1.5 mg/kg s.c.) administration resulted in a modest but insignificant increase in BDNF levels in juvenile animals, however a substantial increase in BDNF expression was induced in adult rats (b) Adult and juvenile rats showed similar basal BDNF levels in CP. SKF 83959 treatment had no effect on BDNF expression in this region. (c) Juvenile rats exhibited significantly lower basal BDNF expression in PFC than adult rats. SKF 83959 significantly elevated BDNF expression in both age groups. Representative blots for each region are also shown. GAPDH was used as a loading control. Data are represented as mean ± SEM, and are expressed as a percent of adult/saline controls. {NAc: Drug, F(1,22)=13.0, P=0.002; Age, F(1,22)=3.8, P=0.06, n.s.; Drug × Age, F(1,22)=0.7, P=0.40, n.s.; CP: Drug, F(1,22)=0.01, P=0.94, n.s.; Age, F(1,22)=0.3, P=0.61, n.s.; Drug × Age, F(1,22)=0.4, P=0.54, n.s.; PFC: Drug, F(1,23)=22.3, P<0.001; Age, F(1,23)=0.02, P=0.88, n.s; Drug × Age, F(1,23)=3.1, P=0.09, n.s.}. N=6-7/group per experiment. *P<0.05 compared to basal expression of adult rats; ^##P<0.01 compared to saline controls of the same age.

Fig. 2

Juvenile rats exhibit enhanced TrkBfull activation in NAc. (a) Juvenile rats exhibited increased basal phosphorylation of TrkBfull compared to their adult counterparts. SKF 83959 (1.5 mg/kg s.c.) had no effect on TrkBfull phosphorylation 90 minutes following administration. (b-c) Adult and juvenile rats showed similar phosphorylation levels of TrkBfull in both CP and PFC. SKF 83959 did not alter the phosphorylation status of TrkB in CP or PFC of either age group. Representative blots for each region are also shown. GAPDH was used as a loading control. Data are represented as mean ± SEM, and are expressed as a percent of adult/saline controls. {NAc: Drug, F(1,21)=1.3, P=0.27, n.s.; Age, F(1,22)=9.0, P=0.007; Drug × Age, F(1,22)=0.16, P=0.69, n.s.; CP: Drug, F(1,22)=3.8, P=0.06, n.s.; Age, F(1,22)=6.5, P=0.02; Drug × Age, F(1,22)=1.4, P=0.25, n.s.; PFC: Drug, F(1,21)=0.04, P=0.85; Age, F(1,21)=0.03, P=0.86, n.s; Drug × Age, F(1,21)=.68, P=0.42, n.s.}. N=6-7/group per experiment. *P<0.05 compared to basal expression of adult rats.

Fig. 3

Effect of age and SKF 83959 on expression of TrkBfull and TrkB.T1 in NAc, CP and PFC of adult and juvenile rats. (a) Representative blots depicting age-related and SKF 83959-induced changes in full length (TrkBfull) and truncated TrkB (TrkB.T1) are shown. GAPDH was used as a loading control. (b) Adult and juvenile rats exhibited similar basal expression levels of the active isoform TrkBfull in all three regions examined. SKF 83959 (1.5 mg/kg s.c.) did not alter TrkBfull expression in NAc, CP or PFC {NAc: Drug, F(1,21)=4.4, P=0.049; Age, F(1,21)=0.4, P=0.54, n.s.; Drug × Age, F(1,21)=0.3, P=0.59, n.s.; CP: Drug, F(1,21)=6.7, P=0.017; Age, F(1,21)=4.7, P=0.042; Drug × Age, F(1,21)=0.9, P=0.35, n.s.; PFC: Drug, F(1,22)=2.2, P=0.15, n.s.; Age, F(1,22)=13.2, P=0.001; Drug × Age, F(1,22)=1.2, P=0.29, n.s.}. (c) Juvenile animals showed lower basal levels of TrkB.T1 expression compared to adults in NAc and CP, with no differences in PFC. SKF 83959 did not alter TrkB.T1 expression in any region in adult rats, but elevated the expression of this TrkB isoform in NAc and CP, but not PFC, of juvenile rats {NAc: Drug, F(1,21)=3.0, P=0.10; Age, F(1,21)=2.6, P=0.12; Drug × Age, F(1,21)=4.2, P=0.048; CP: Drug, F(1,21)=24.1, P=0.0001; Age, F(1,21)=2.8, P=0.11, n.s.; Drug × Age, F(1,21)=4.6, P=0.044; PFC: Drug, F(1,21)=2.7, P=0.11, n.s.; Age, F(1,21)=1.4, P=0.25, n.s.; Drug × Age, F(1,21)=2.7, P=0.11, n.s.}. (d) The ratio of expression of full to truncated TrkB isoforms. Juvenile rats showed a significantly higher basal TrkB ratio (TrkBfull:TrkB.T1) compared to adults in NAc, CP and PFC. SKF 83959 increased the TrkB ratio in adult rats in NAc and PFC, with no effects in these regions in juvenile rats. In CP, SKF 83959 significantly reduced the TrkB ratio in juvenile rats with no effect in adults. Data are represented as mean ± SEM, and are expressed as a percent of adult/saline controls. {NAc: Drug, F(1,20)=0.01, P=0.92, n.s.; Age, F(1,20)=16.1, P=0.001; Drug × Age, F(1,20)=9.5, P=0.006; CP: Drug, F(1,21)=27.8, P=0.0001; Age, F(1,21)=51.2, P=0.0001; Drug × Age, F(1,21)=26.9, P=0.0001; PFC: Drug, F(1,22)=0.1, P=0.71, n.s.; Age, F(1,22)=22.5, P=0.0001; Drug × Age, F(1,22)=0.002, P=0.97, n.s.}. N=6-7/group per experiment. **P<0.01, ***P<0.001 compared to basal expression of adults rats; ^#P<0.05, ^###P<0.001 compared to saline controls of the same age.

Fig. 4

Effect of age and SKF 83959 on proBDNF expression in NAc, CP and PFC. (a-c) No significant differences in the expression of proBDNF were observed between juvenile and adults rats in any of the regions examined. SKF 83959 had no effects on proBDNF expression in either age group.. Representative blots for each region are also shown. GAPDH was used as a loading control. Data are represented as mean ± SEM, and are expressed as a percent of adult/saline controls. {NAc: Drug, F(1,22)=0.01, P=0.91, n.s.; Age, F(1,22)=4.7, P=0.04; Drug × Age, F(1,22)=1.5, P=0.23, n.s.; CP: Drug, F(1,21)=0.9, P=0.34, n.s.; Age, F(1,21)=0.3, P=0.58; Drug × Age, F(1,21)=2, P=0.7, n.s.; PFC: Drug, F(1,22)=0.9, P=0.36; Age, F(1,22)=0.1, P=0.72, n.s; Drug × Age, F(1,22)=1.1, P=0.32, n.s.}. N=6-7/group per experiment. *P<0.05 compared to basal expression of adult rats.

Fig. 5

Effect of age and SKF 83959 on p75NTR expression in NAc, CP and PFC of adult and juvenile rats. (a, b) There were no age-related or SKF 83959-induced alterations in p75NTR expression in NAc or CP. (c) In PFC, basal expression of p75NTR did not differ between age groups. However, SKF 83959 (1.5 mg/kg) induced a significant increase in p75NTR levels in both adult and juvenile rats. Representative blots for each region are also shown. GAPDH was used as a loading control. Data are represented as mean ± SEM, and are expressed as a percent of adult/saline controls. {NAc: Drug, F(1,22)=2.0, P=0.18, n.s.; Age, F(1,22)=0.5, P=0.48, n.s.; Drug × Age, F(1,22)=0.9, P=0.36, n.s.; CP: Drug, F(1,21)=2.6, P=0.11, n.s.; Age, F(1,21)=0.30, P=0.59, n.s.; Drug × Age, F(1,21)=0.9, P=0.35, n.s.; PFC: Drug, F(1,22)=13.0, P=0.002; Age, F(1,22)=2.7, P=0.11, n.s.; Drug × Age, F(1,22)=0.59, P=0.45, n.s.}. N=6-7/group per experiment. ^#P<0.05 compared to saline controls of the same age.