Immunization with DAT fragments is associated with long-term striatal impairment, hyperactivity and reduced cognitive flexibility in mice

Abstract

Background: Possible interactions between nervous and immune systems in neuro-psychiatric disorders remain elusive. Levels of brain dopamine transporter (DAT) have been implicated in several impulse-control disorders, like attention deficit / hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD). Here, we assessed the interplay between DAT auto-immunity and behavioural / neurochemical phenotype.

Methods: Male CD-1 mice were immunized with DAT peptide fragments (DAT-i), or vehicle alone (VEH), to generate elevated circulating levels of DAT auto-antibodies (aAbs). Using an operant delay-of-reward task (20 min daily sessions; timeout 25 sec), mice had a choice between either an immediate small amount of food (SS), or a larger amount of food after a delay (LL), which increased progressively across sessions (from 0 to 150 sec).

Results: DAT-i mice exhibited spontaneous hyperactivity (2 h-longer wake-up peak; a wake-up attempt during rest). Two sub-populations differing in behavioural flexibility were identified in the VEH control group: they showed either a clear-cut decision to select LL or clear-cut shifting towards SS, as expected. Compared to VEH controls, choice-behaviour profile of DAT-i mice was markedly disturbed, together with long-lasting alterations of the striatal monoamines. Enhanced levels of DA metabolite HVA in DAT-i mice came along with slower acquisition of basal preferences and with impaired shifting; elevation also in DOPAC levels was associated with incapacity to change a rigid selection strategy. This scarce flexibility of performance is indicative of a poor adaptation to task contingencies.

Conclusions: Hyperactivity and reduced cognitive flexibility are patterns of behaviour consistent with enduring functional impairment of striatal regions. It is yet unclear how anti-DAT antibodies could enter or otherwise affect these brain areas, and which alterations in DAT activity exactly occurred after immunization. Present neuro-behavioural alterations, coming along with an experimentally-induced rise of circulating DAT-directed aAbs, open the issue of a potential role for auto-immunity in vulnerability to impulse-control disorders.

Scheme 1

Experimental design, showing mice in batch I (N = 24) and batch II (N = 24).

Figure 1

Circadian Rhythm of Spontaneous Activity. Mean (±SEM, n = 6 pairs) counts per minute (cpm) by undisturbed mice, housed in pairs per home cage (sensor rate 20 Hz). Nocturnal (upper panel) and diurnal (lower panel) activity was recorded for 3 days (PND 51–53), five days after the boost with DAT fragment (DAT-i) or VEH. The 24-h profile of one-hour points was then obtained by averaging the 3 days of continuous registration. * p < 0.05 between DAT-i and VEH control group; # p < 0.05, compared to the previous or the following 1-h point within the same immunization group.

Figure 2

Choice between Rewards, Reaction to Delay. Mean (±SEM, n = 6 per sub-population) choice (%) for the large but late (LL) reward in mice, immunized with DAT fragment (right panel) or vehicle alone (left panel), in a delay-of-reward task. Two sub-populations were formed, based on the slope of preference curves (see Methods). Among VEH controls, individuals segregated into either a flexible (with a “steep” profile, black points) or an inflexible (with a “flat” profile, white points) subgroup. Among DAT-i mice, individuals segregated into either an inflexible (with a “stuck” profile, black points) or a flexible (with a “slow” profile, white points) subgroup. * p < .05 between DAT-i and VEH controls; # p < 0.05 when comparing sub-populations (low- versus high-slope) within the same immunization group.