Long-term effects of a single adult methamphetamine challenge: minor impact on dopamine fibre density in limbic brain areas of gerbils

Abstract

Background: The aim of the study was to test long-term effects of (+)-methamphetamine (MA) on the dopamine (DA) innervation in limbo-cortical regions of adult gerbils, in order to understand better the repair and neuroplasticity in disturbed limbic networks.

Methods: Male gerbils received a single high dose of either MA (25 mg/kg i.p.) or saline on postnatal day 180. On postnatal day 340 the density of immunoreactive DA fibres and calbindin and parvalbumin cells was quantified in the right hemisphere.

Results: No effects were found in the prefrontal cortex, olfactory tubercle and amygdala, whereas the pharmacological impact induced a slight but significant DA hyperinnervation in the nucleus accumbens. The cell densities of calbindin (CB) and parvalbumin (PV) positive neurons were additionally tested in the nucleus accumbens, but no significant effects were found. The present results contrast with the previously published long-term effects of early postnatal MA treatment that lead to a restraint of the maturation of DA fibres in the nucleus accumbens and prefrontal cortex and a concomitant overshoot innervation in the amygdala.

Conclusion: We conclude that the morphogenetic properties of MA change during maturation and aging of gerbils, which may be due to physiological alterations of maturing vs. mature DA neurons innervating subcortical and cortical limbic areas. Our findings, together with results from other long-term studies, suggest that immature limbic structures are more vulnerable to persistent effects of a single MA intoxication; this might be relevant for the assessment of drug experience in adults vs. adolescents, and drug prevention programs.

Figure 1

Dopamine immunoreactive fibres in each of the quantified regions. Representative photomicrographs, taken from a saline control, of dopamine (DA) immunoreactive fibres of each of the quantified regions. A.1: Prefrontal cortex; A.2: Layer VI of the prelimbic area; A.3: Layer IV of the lateral orbital and agranular insular areas. B.1: Nucleus accumbens (NAC); B.2: Medial shell of NAC; B.3: Lateral core of NAC; B.4: Olfactory tubercle. C.1: Amygdala (AMY); C.2: Central nucleus of AMY; C.3: Basolateral nucleus of AMY. Note the differential innervation pattern and density of DA fibres in the respective regions. Scale bars: 1000 μm (A.1, B.1, C.1); 50 μm (A.2-3, B.2-4, C.2-3).

Figure 3

Dopamine innervation density in four regions of the gerbil brain. Dopamine (DA) innervation density ± S.E.M. is presented in four regions of the gerbil brain, namely agranular insular and lateral orbital as well as prelimbic areas of the prefrontal cortex, the olfactory tubercle, core and shell areas of the nucleus accumbens (NAC), and the central and basolateral nuclei of the amygdala complex. Methamphetamine treatment generally tends to increase the DA innervation. However, a significant region-specific change in response to a single adult methamphetamine treatment exclusively occurs in the shell of the NAC (+11%; p = 0.0332). The difference in the core appears somewhat more pronounced but is not significant due to higher variance (+21%; p = 0.1011). Student's t-Test, significance value: * p < 0.05. Following methamphetamine treatment, ANOVA detected a significant overall increase of DA innervation in core and shell of the NAC (F(1,16) = 4.7316; p = 0.0472).

Figure 2

Photomicrographs of Calbindin and Parvalbumin immunoreactive neurons in the nucleus accumbens. Overview (A) and higher magnifications (A1, A2) of the Calbindin innervation of the NAC. The majority of CB+ cells is located in the core, which border to the shell is detectable (black arrows). PV+ cells are almost exclusively located in the shell (B1, B2), however, the overall density is much lower compared to CB+ cells. Scale bars: 2000 μm (A, B); 200 μm (A1, B1); 100 μm (A2, B2).

Figure 4

Calbindin and parvalbumin cell densities and cell areas in the nucleus accumbens. Calbindin (CB) and parvalbumin (PV) cell densities and cell areas ± S.E.M. are presented for the nucleus accumbens (NAC). PV-positive cells and CB-positive cells are predominantly located in the shell and in the core of the NAC, respectively, where they were quantified. No statistically significant effect of a single adult methamphetamine challenge could be detected for either number (cell density) or cumulative size (cell area) of both PV and CB cells. Generally, the number of CB cells is considerably higher and their average size is doubled compared to PV cells.