Increased capacity and density of choline transporters situated in synaptic membranes of the right medial prefrontal cortex of attentional task-performing rats

Abstract

Cholinergic neurons innervating the cortex have been conceptualized as a major component of the attention system of the brain. Because of recent evidence indicating plastic mechanisms regulating choline transporter (CHT)-mediated high-affinity choline uptake, which is the rate-limiting step of acetylcholine synthesis, the present experiment determined the capacity of cholinergic terminals to transport choline, and the proportion of choline transporters localized in the membrane of synaptic terminals, in several brain regions of rats performing a cognitive vigilance task (CVT) and a simple reaction time task (SRTT) and nonperforming (NP) rats. Compared with evidence from NP rats, increased choline transporter capacity [as indicated by maximum transporter velocity (Vmax)] and an increased density of CHTs situated in synaptic plasma membrane, relative to intracellular locations, were observed in the medial prefrontal cortex of the right but not left hemisphere of CVT-performing animals. Furthermore, right medial prefrontal Vmax values of CVT-performing rats correlated positively and left medial Vmax values correlated negatively with the animals' performance in signal trials. Measures of CHT function in the brains of SRTT-performing animals did not differ significantly from those in NP rats. The present data support the hypothesis that an increased capacity of choline transporters in the right medial prefrontal cortex, primarily attributable to increased trafficking of transporters from intracellular compartments to the terminal membrane, represents a cellular mechanism contributing to the mediation of attentional performance.

Figure 1.

Main events of the CVT (A) and SRTT (B) and illustration of performance (C). A, After a variable ITI, a signal (illumination of a panel light for 500, 50, or 25 ms; see asterisk) was presented or not (nonsignal). In the CVT, both levers were extended 2 s later, and animals were required to press one lever to report a hit (after signals) and the other one to report a correct rejection (after nonsignals) to receive a water reward (see arrows exemplifying 1 set of rules). Incorrect responses (misses and false alarms) and omissions were not rewarded. Levers were withdrawn after a lever press or after 4 s. B, The SRTT differed from the CVT by making only the correct lever available. C, The ability of CVT-performing animals to detect signals was signal duration dependent (line graph), and they responded correctly in >75% of all nonsignal trials (bar).

Figure 2.

Results from choline uptake assays and correlations with performance. A, Compared with NP controls, the capacity of the CHT to transport choline (as indicated by V_max) was increased in the right but not left mPFC (the asterisk depicts the significant difference indicated by post hoc multiple comparisons using Tukey's honestly significant difference) of CVT-performing rats. B, Results from saturation analyses depicting hemicholinium-sensitive choline uptake by synaptosomes obtained from right mPFC from animals of the three groups. The inset shows the approximate location and extent of the prefrontal and parietal cortical tissue extracted for these experiments (the more ventral portions of the mPFC are not visible). C, Right mPFC V_max values correlated significantly with the animal's ability to detect 500 ms signals (filled circles and positive slope regression line), whereas left PFC V_max scores correlated negatively with this measure (open circles and negative slope regression line; see Results for statistical results).

Figure 3.

A, An increased proportion of CHTs was found in the plasma membrane-enriched fraction obtained from the right mPFC of CVT-performing rats when compared with NP rats (the asterisk depicts the significant difference indicated by post hoc multiple comparisons using Tukey's honestly significant difference). Data from the left mPFC, left or right posterior cortex, or striatum did not differ between groups. B, Left bands, Total CHT protein levels were not affected by performance in either task. β-Actin levels were monitored to verify the amount of samples used for immunoblot analysis. Right bands, CHT activity in LP1 and LP2 fractions, indicating the greater proportion of CHTs in the plasma membranes in the right mPFC of CVT-performing rats when compared with NP rats.