Differential regulation of neurotrophin and trk receptor mRNAs in catecholaminergic nuclei during chronic opiate treatment and withdrawal

Abstract

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their receptors trkB and trkC, respectively, are expressed in the locus coeruleus (LC) and ventral tegmental area (VTA), brain regions known to be involved in opiate addiction. Previously, administration of exogenous neurotrophins has been shown to oppose effects of chronic morphine treatment on LC and VTA neurons. However, the response of endogenous neurotrophins in LC and VTA to opiate treatment is unknown. In this study, BDNF, NT-3, trkB, and trkC mRNAs were analyzed in these regions after chronic morphine treatment and during antagonist precipitated withdrawal. Although chronic morphine exposure resulted in only modest increases in BDNF and NT-3 mRNA expression in LC, precipitated withdrawal led to a marked, rapid, and prolonged increase in BDNF mRNA and a delayed decrease in NT-3 mRNA. Levels of trkB and trkC mRNAs, which were unchanged by chronic morphine treatment, were elevated in LC at 2 and 6 hr of withdrawal. By 20 hr, trkB mRNA levels in LC had returned to control, whereas trkC mRNA levels fell below control values. In contrast to the substantial alterations observed in LC, there was no regulation of the neurotrophins or trk mRNAs within the VTA during chronic opiate treatment or withdrawal, with the exception of an increase in trkB mRNA at 6 hr of withdrawal. These results suggest that neurotrophins and their receptors per se may be involved in opiate-induced plasticity of the LC, whereas other mechanisms would appear to be involved in the VTA.

Fig. 1.

Analysis of BDNF (A), NT-3 (B), trkB (C), and trkC (D) mRNA expression in LC after chronic opiate treatment. Note the significant increase in BDNF (A) and NT-3 (B) mRNA levels (asterisks) in LC after chronic opiate exposure. Although there was a trend toward a decrease in trkC mRNA levels (D; p = 0.062), no significant alterations in trkB (C) or trkC (D) mRNA expression were observed in LC with chronic morphine treatment. Results are expressed as a percentage of control values. Values are the mean ± SEM.

Fig. 2.

Effect of precipitated opiate withdrawal on BDNF mRNA expression in LC. A, Film autoradiograms demonstrating BDNF cRNA hybridization in LC of control rats and rats exposed to 2 or 6 hr of opiate withdrawal. B, Film autoradiograms of BDNF mRNA expression in LC in control rats and rats exposed to 20 or 70 hr of opiate withdrawal. C, Densitometric analysis of film autoradiograms reveals robust increases in BDNF mRNA levels at 2, 6, 20, and 70 hr of opiate withdrawal as compared with control (p < 0.05). Results are expressed as a percentage of control values. Values are the mean ± SEM (SEM for 2 and 6 hr control group = ±6; SEM for 20 and 70 hr control group = ±6.5.) *^,†Significantly increased from control. ** Significantly increased from control and 2 hr. ^††Significantly different from control and 20 hr. Scale bar, 500 μm.

Fig. 3.

Effect of precipitated opiate withdrawal on NT-3 mRNA expression in LC. A, Film autoradiograms of NT-3 mRNA expression in LC of control rats and of rats after 2 or 6 hr of opiate withdrawal. B, Film autoradiograms demonstrating the expression of NT-3 mRNA in LC of control rats and rats exposed to 20 or 70 hr of opiate withdrawal. C, Results from analysis of film autoradiograms show that there are no alterations in NT-3 mRNA levels at 2 and 6 hr of opiate withdrawal as compared with control. In contrast, there was a decrease in NT-3 mRNA levels at 20 and 70 hr of opiate withdrawal as compared with control (p < 0.05). Results are expressed as a percentage of control values. Values are the mean ± SEM (SEM for 2 and 6 hr control group = ±4.4; SEM for 20 and 70 hr control group = ±4.) *Significantly decreased from control. **Significantly different from control and 70 hr. Scale bar, 500 μm.

Fig. 4.

Effect of precipitated opiate withdrawal ontrkB mRNA expression in LC. A, Film autoradiograms demonstrating trkB mRNA expression in LC of control rats and rats exposed to 2 or 6 hr of opiate withdrawal.B, Film autoradiograms of trkB cRNA hybridization in LC in control rats and rats exposed to 20 or 70 hr of opiate withdrawal. C, Densitometric analysis of film autoradiograms reveals an increase in trkB mRNA levels at 2 and 6 hr of opiate withdrawal as compared with control (p < 0.05), and no changes intrkB mRNA expression at 20 and 70 hr of opiate withdrawal as compared with control. Results are expressed as a percentage of control values. Values are the mean ± SEM (SEM for 2 and 6 hr control group = ±1.1; SEM for 20 and 70 hr control group = ±1.7.) *Significantly increased from control. Scale bar, 500 μm.

Fig. 5.

Effect of precipitated opiate withdrawal ontrkC mRNA expression in LC. A, Film autoradiograms showing the expression of trkC mRNA in LC of control rats and rats exposed to 2 or 6 hr of opiate withdrawal.B, Film autoradiograms of trkC mRNA expression in LC in control rats and rats exposed to 20 or 70 hr of opiate withdrawal. C, Densitometric analysis of film autoradiograms demonstrates an increase in trkC mRNA levels at 2 and 6 hr of opiate withdrawal as compared with control (p < 0.05), followed by a decrease intrkC mRNA levels at 20 and 70 hr of opiate withdrawal as compared with control (p < 0.05). Results are expressed as a percentage of control values. Values are the mean ± SEM (SEM for 2 and 6 hr control group = ±2.7; SEM for 20 and 70 hr control group = ±3.7.) *Significantly increased from control. ^†Significantly decreased from control. Scale bar, 500 μm.

Fig. 6.

Film autoradiograms of BDNF (A, B), NT-3 (C, D), trkB(E, F), and trkC (G, H) cRNA hybridization in the ventral midbrain of control (A, C, E, G) and morphine-treated (B, D, F, H) rats. Note that no significant alterations in neurotrophin or neurotrophin receptor mRNA levels were observed in VTA after chronic opiate exposure. SNpc, Substantia nigra pars compacta. Scale bar, 1710 μm.

Fig. 7.

Analysis of BDNF (A), NT-3 (B), trkB (C), and trkC (D) mRNA expression in VTA after chronic opiate treatment. No significant alterations in neurotrophin (A, B) or neurotrophin receptor (C, D) mRNA levels were observed in VTA with chronic morphine treatment. Results are expressed as a percentage of control values. Values are the mean ± SEM.

Fig. 8.

Densitometric analysis of neurotrophin (A, B) and neurotrophin receptor (C, D) mRNA expression in VTA during precipitated opiate withdrawal. No significant alterations in BDNF (A), NT-3 (B), or trkC (D) mRNA levels were observed in VTA during withdrawal from opiates. C, trkB mRNA levels were significantly increased as compared with control at 6 hr of morphine withdrawal (p < 0.05). Results are expressed as a percentage of control values. Values are the mean ± SEM. *Significantly increased from control.