Reduced isolation-induced pup ultrasonic communication in mouse pups lacking brain serotonin

Abstract

Background: Serotonin (5-hydroxytryptamine, 5-HT) is a key modulatory neurotransmitter in the mammalian central nervous system (CNS) that plays an important role as a developmental signal. Several lines of evidence associate altered 5-HT signaling with psychopathology in humans, particularly neurodevelopmental disorders such as autism spectrum disorders (ASD). ASD are characterized by persistent social and communication deficits along with stereotyped and repetitive patterns of behavior, with all symptoms emerging early during development.

Methods: Here, we employed a mouse model devoid of brain 5-HT due to the lack of the gene encoding tryptophan hydroxylase 2 (Tph2), the initial and rate-limiting enzyme of 5-HT synthesis in the CNS. Tph2 null mutant (Tph2 (-/-) ) mice show normal prenatal development; however, they display for yet unknown reasons severe growth retardation during the first postnatal weeks. We investigated, therefore, whether Tph2 (-/-) mice display deficits in isolation-induced ultrasonic vocalizations (USV) as pups during early life. Isolation-induced USV are the most commonly studied behavioral measure to assess developmental delays and communication deficits in rodent models for ASD, particularly as they serve an important communicative function in coordinating mother-pup interactions.

Results: Tph2 (-/-) mouse pups displayed a clear deficit in the emission of isolation-induced USV, as compared to heterozygous and wildtype littermates, exactly during growth retardation onset, including reduced call numbers and deficits in call clustering and temporal organization.

Conclusions: The ultrasonic communication impairment displayed by Tph2 (-/-) mouse pups is likely to result in a deficient mother-infant interaction, presumably contributing to their growth retardation phenotype, and represents a prominent feature relevant to ASD.

Keywords: Animal models; Autism; Communication; Neurodevelopmental disorders; Serotonin; Ultrasonic vocalizations.

Figure 1

Isolation-induced ultrasonic vocalizations (USV) in Tph2 ^-/- mouse pups emitted at postnatal days (PND) 3, 6, and 9. (A) Total number (n), (B) average call duration in milliseconds (ms), (C) average call peak frequency in kilohertz (kHz), and (D) average call peak amplitude in decibel (dB) of isolation-induced USV emitted during the 10-min isolation from mother and littermates. Black circles: Tph2 wildtype (Tph2 ^+/+) control mice; grey circles: Tph2 heterozygous (Tph2 ^+/-) mice; white circles: Tph2 null mutant (Tph2 ^-/-) mice. Data are presented as means ± standard errors of the mean. *P < .050 Tph2 ^+/+ vs. Tph2 ^-/-.

Figure 2

Distribution of individual isolation-induced ultrasonic vocalizations (USV) in Tph2 ^-/- mouse pups emitted at postnatal day (PND) 6. Density plots depicting the distribution of individual isolation-induced USV depending on call peak frequency in kilohertz (kHz) and call peak amplitude in decibel (dB) in Tph2 wildtype (Tph2 ^+/+) control mice (A) and Tph2 null mutant (Tph2 ^-/-) mice (B), with color coding reflecting frequencies as percentages. Frequency histograms depicting the distribution of individual USV depending on call peak frequency in kilohertz (kHz) (C) and call peak amplitude in decibel (dB) (D) in percentages, with isolation-induced USV emitted by Tph2 wildtype (Tph2 ^+/+) control mice (black area) and Tph2 null mutant (Tph2 ^-/-) mice (grey area).

Figure 3

Sequential analysis of the durations of subsequent isolation-induced ultrasonic vocalizations (USV) indicating a non-random call emission pattern in Tph2 ^-/- mouse pups at postnatal day (PND) 6. Correlations between the call durations of given isolation-induced USV and the call durations of the previous ones (N − 1), the call durations of the ones two before (N − 2), or the call durations of the ones three before (N − 3) for Tph2 wildtype (Tph2 ^+/+) control mice (black circles) and Tph2 null mutant (Tph2 ^-/-) mice (white circles). *P < .100 Tph2 ^+/+ vs. Tph2 ^-/-; **P < .005 Tph2 ^+/+ vs. Tph2 ^-/-.

Figure 4

Developmental profile of Tph2 ^-/- mouse pups at postnatal days (PND) 3, 6, and 9. (A) Body temperature in degrees Celsius (°C) and (B) body weight in grams (g) in pups tested for isolation-induced ultrasonic vocalizations. Black circles: Tph2 wildtype (Tph2 ^+/+) control mice; grey circles: Tph2 heterozygous (Tph2 ^+/-) mice; white circles: Tph2 null mutant (Tph2 ^-/-) mice. Data are presented as means ± standard errors of the mean. **P < .005 Tph2 ^+/+ vs. Tph2 ^-/-; ***P < .001 Tph2 ^+/+ vs. Tph2 ^-/-; ^## P < .005 Tph2 ^+/- vs. Tph2 ^-/-; ^### P < .001 Tph2 ^+/- vs. Tph2 ^-/-. Please note that the error bars in B are too small to be visible.