Valproate Exposure in ovo Attenuates the Acquisition of Social Preferences of Young Post-hatch Domestic Chicks

Abstract

Embryonic exposure to valproic acid (VPA) is known to produce sociability deficits, resembling human autistic phenotypes, in several vertebrate species. Animals living in groups prefer the proximity of peers and have the ability to perceive and to respond to social signals for modifying behavior. Chicks of Galliform birds, known to display early preference behaviors, have been used extensively for adaptive learning studies. Young precocial birds seem to be useful models also for studying the effect of embryonic VPA treatment. Here, domestic chicken eggs were injected with sodium valproate (200 μl of 35 μmol/L solution) or with vehicle (distilled water) on the 14th day of incubation. After hatching, the chicks were tested for one-trial passive avoidance learning at day 1, vocalization due to isolation as a measure of stress level (day 2), approach preference to large versus small groups of age-matched conspecifics (day 5), and to those with normal versus blurred head features (day 7). In addition, we tested the preference of birds to conspecifics reared in group versus those reared in isolation (day 9), as well as the preference of chicks to familiar versus non-familiar conspecifics (day 21). Our findings confirm previous reports concerning an adverse effect of VPA on embryonic development, including a tendency for aborted or delayed hatching and, occasionally, for locomotor disorders in a small percentage of birds (eliminated from later studies). Otherwise, VPA treatment did not impair motor activity or distress level. Memory formation for the aversive stimulus and discrimination of colors were not impaired by VPA treatment either. Innate social predispositions manifested in approach preferences for the larger target group or for the birds with natural facial features remained unaffected by VPA exposure. The most prominent finding was attenuation of social exploration in VPA-exposed birds (expressed as the frequency of positional switches between two stimulus chicks after the first choice), followed by a deficit in the recognition of familiar conspecifics, unfolding at the end of the third week. Social exploration and recognition of familiar individuals are the key elements impaired at this stage. The results underline the importance of early social exploration in ASD.

Keywords: autism spectrum disorder; avian; developmental disorder; embryonic development; social brain network; social cohesion.

Figure 1

Experimental setup for the runway tests (A) and the Y-maze test (B).

Figure 2

Number of pecks on target beads in taste aversion learning (mean ± s.e.m.). Asterisks denote significant differences between the VPA and control groups.

Figure 3

(A) Vocalization intensity during the isolation test in 10-s intervals (mean ± s.e.m.). Asterisks represent significant difference between VPA and control groups. Hash mark denotes significant difference compared to the previous 10-s intervals in both experimental groups. (B) Frequency of distress calls (DCs) during the isolation test (mean ± s.e.m.). Asterisks represent significant difference between VPA and control groups.

Figure 4

(A) Percentage of chicks approaching one end of the runway as their first choice between video recordings of a large group vs. a small group of chicks. Asterisks denote a choice significantly different from random choice. (B) Latency of the first choice (physical contact with the screen, mean ± s.e.m). (C) Time spent at each end of the runway (mean ± s.e.m). Asterisk denotes significant difference between the times spent in the proximity of the two screens.

Figure 5

Percentage of chicks approaching one end of the runway as their first choice between video recordings of normal individuals vs. digitally distorted ones (head blurred). Asterisks denote a choice significantly different from random choice.

Figure 6

(A) Percentage of chicks approaching one end of the Y-maze as their first choice between a socially reared (socialized) chick and another one reared in isolation (isolated). The first choice of the VPA and control chicks did not differ significantly. (B) Latency of the first choice (physical contact with goal box, mean ± s.e.m.). (C) Number of positional switches performed by the experimental chicks between the two goal boxes after the first choice (mean ± s.e.m.). Asterisk denotes significant difference between VPA and control chicks.

Figure 7

(A) Percentage of chicks approaching one end of the runway as their first choice between a familiar and an unfamiliar chick. Asterisk denotes a choice significantly different from random choice. (B) Time spent at each end of the runway (mean ± s.e.m.). Asterisk denotes significant difference between the two goal boxes. (C) Number of positional switches between the two goal boxes performed by the experimental chicks after their first choice (mean ± s.e.m.). Asterisk denotes significant difference between VPA and control chicks.