Patterns of neural activity associated with differential acute locomotor stimulation to cocaine and methamphetamine in adolescent versus adult male C57BL/6J mice

Abstract

Adolescence is a time period when major changes occur in the brain with long-term consequences for behavior. One ramification is altered responses to drugs of abuse, but the specific brain mechanisms and implications for mental health are poorly understood. Here, we used a mouse model in which adolescents display dramatically reduced sensitivity to the acute locomotor stimulating effects of cocaine and methamphetamine. The goal was to identify key brain regions or circuits involved in the differential behavior. Male adolescent (postnatal day (PN), 30-35) and young adult (PN, 69-74) C57BL/6J mice were administered an i.p. injection of cocaine (0, 15, 30 mg/kg) or methamphetamine (0, 2, 4 mg/kg) and euthanized 90 min later. Locomotor activity was monitored continuously in the home cage by video tracking. Immunohistochemical detection of Fos protein was used to quantify neuronal activation in 16 different brain regions. As expected, adolescents were less sensitive to the locomotor stimulating effects of cocaine and methamphetamine as indicated by a rightward shift in the dose response relationship. After a saline injection, adolescents showed similar levels of Fos as adults in all regions except the dorsal caudate (CPuD) and lateral caudate (CPuL) where levels were lower in adolescents. Cocaine and methamphetamine dose dependently increased Fos in all brain regions sampled in both adolescents and adults, but Fos levels were similar in both age groups for a majority of regions and doses. Locomotor activity was correlated with Fos in several brain areas within adolescent and adult groups, and adolescents had a significantly greater induction of Fos for a given amount of locomotor activity in key brain regions including the caudate where they showed reduced Fos under baseline conditions. Future research will identify the molecular and cellular events that are responsible for the differential psychostimulant-induced patterns of brain activation and behavior observed in adolescent versus adult mice.

Figure 1

Locations where Fos positive cells were counted (boxes, shown roughly to scale, were 1 × 0.63 mm). Reprinted from The Mouse Brain in Stereotaxic coordinates, 2nd edition, Paxinos G and Franklin K, Figures 17, 22, 25, 30, 33, 42, 52, Copyright 2001, with permission from Elsevier. As noted, for the piriform cortex and the dentate gyrus, the nucleus was outlined by hand and particles were counted only within the outlined structures. Legend: PFC=prefrontal cortex, M1=motor cortex, Cg=cingulate cortex, NACC=nucleus accumbens core, NACS=nucleus accumbens shell, Pir=piriform cortex, CPuD=dorsal caudate, CPuL=lateral caudate, CPuV=ventral caudate, LS=lateral septum, BNST=bed nucleus of the stria terminalis, VP=ventral pallidum, GP=globus palidus, SX=somatosensory cortex, DG=dentate gyrus, V1=visual cortex.

Figure 2

Reduced locomotor response to cocaine and methamphetamine in adolescent male C57BL/6J mice as compared to adults. Average distance traveled in 5 min bins (± SE) is plotted against time separately for adults (filled symbols) and adolescents (open symbols). Animals were given a saline injection at 60 min, and either saline or drug injection at 120 min. Data for the cocaine trials are shown on top and methamphetamine on the bottom. Each data point represents the average of 8 individuals. All graphs share the same x- and y-axis labels.

Figure 3

Acute cocaine increases Fos in a dose-dependent fashion in adolescents and adults. Representative sections stained for Fos showing the dorsal caudate of adolescents and adults 90 min after an intraperitoneal injection of saline, 15, or 30 mg/kg cocaine. The dots represent Fos-positive nuclei, total magnification was 100×.

Figure 4

Increased Fos response from cocaine and methamphetamine in the dorsal caudate for a given level of locomotor activity in adolescents as compared to adults. Number of Fos positive cells in the dorsal caudate is plotted against distance traveled in the 90 minute period following an injection of either 30 mg/kg cocaine (top) or 2 mg/kg methamphetamine (bottom). Adolescents (open symbols) are shown separately from adults (filled symbols). The simple linear regression lines are shown separately for each age group. Both graphs share the same x-axis label.