Effect of prenatal cocaine on early postnatal thermoregulation and ultrasonic vocalization production

Abstract

Prenatal cocaine exposure can alter the postnatal care received by rat pups. Such effects could be caused in part by alterations in pup-produced stimuli that elicit early postnatal maternal care. Pup ultrasonic vocalizations are thought to be a particularly salient stimulus, and when paired with other cues, may elicit maternal attention. Cocaine is known to acutely alter thermoregulatory and cardiac function, thus prenatal cocaine may affect vocalizations through altering these functions. The data presented here determine the impact of full term prenatal cocaine exposure, saline exposure, or no exposure on thermogenic capacity, cardiac function, and the resulting ultrasonic vocalizations across the early postnatal period (days 1-5). Results indicated that while sharing many similar characteristics with saline-exposed and untreated animals, prenatal cocaine exposure was associated with specific alterations in vocalization characteristics on postnatal day 1 (PND 1), including call amplitude. Furthermore, numerous spectral parameters of their vocalizations were found altered on PND 3, including rate, call duration, and frequency, while no alterations were found on PND 5. Additionally, cocaine-exposed pups also showed a reduced thermoregulatory capacity compared to saline animals and reduced cardiac mass compared to untreated animals on PND 5. Together, these findings indicate that prenatal cocaine may be altering the elicitation of maternal care through its impact on vocalizations and thermoregulation, and suggests a potential mechanism for these effects through cocaine's impact on developing stress systems.

Keywords: brown adipose tissue; cardiac; prenatal cocaine; stress; thermogenesis; ultrasonic vocalization.

Figure 1

The thermogenesis testing apparatus. The subject is placed in the subject basket inside the double-walled glass chamber. The walls of the chamber are filled with temperature-controlled water, which actively regulates the internal temperature of the chamber. Forced humidified air is pumped into the chamber throughout testing. Subject and ambient temperatures were measured using thermocouples.

Figure 2

Environmental challenges posed on postnatal days (PND) 3 and 5. Subjects were first exposed to a baseline thermoneutral habituation period for 1 h (PND 3: 36°C, PND 5: 35°C), then a developmentally appropriate moderate thermal challenge for 1 h (PND 3: 32°C, PND 5: 28°C), and lastly an extreme thermal challenge for 1 h (PND 3: 28.5°C, PND 5: 21°C).

Figure 3

Postnatal day 1 ultrasonic vocalizations during a 5 min thermal challenge. Subjects were rapidly chilled by placing them on a 25°C metal plate for 5 min, during which vocalizations were recorded continuously. Pups exposed to prenatal cocaine or saline produced calls with shorter durations than untreated pups (A). Prenatal cocaine exposure also resulted in lower maximum amplitudes (B) and a smaller range of amplitudes (C) compared to both untreated and saline groups. (^*p ≤ 0.05. ^**p ≤ 0.01).

Figure 4

Postnatal day 3 thermoregulation during a thermoneutral baseline period (36°C), 1 h moderate (32°C), and 1 h extreme (28°C) thermal challenge. All data are presented as change from baseline. Prenatal saline exposed animals showed T_IS (A) and T_Back (B), but no change in T_IS-Back (C) compared to both cocaine-exposed and untreated animals. Inset figures show that while cocaine exposed pups did not differ from untreated animals in their change from baseline temperatures, they, along with saline-exposed animals, did statistically differ from untreated animals in their T_IS and T_Back baseline temperatures (Saline vs. Cocaine: ^*p ≤ 0.05, ^**p ≤ 0.01; Saline vs. Untreated: ^†p ≤ 0.05, ^††p ≤ 0.01; Cocaine vs. Untreated: ^‡‡p ≤ 0.01).

Figure 5

Postnatal day 3 ultrasonic vocalization characteristics. Vocalizations were recorded during thermoneutral (35°C), moderate (32°C), and extreme (28°C) thermal challenge periods (data averaged across each period). At this age, pups exposed to prenatal cocaine or saline demonstrated an increased average duration of each call during the thermoneutral period (A) compared to untreated animals. Additionally, cocaine exposed animals showed a reduced average maximum (B) and minimum (C) frequency per call during the extreme thermal challenge period compared to untreated animals (^*p ≤ 0.05).

Figure 6

Raster and histogram depicting the number of ultrasonic vocalizations produced by pups on postnatal day 5 across the 3 h thermal testing period. The first hour was at baseline thermoneutral temperature (35°C), the second hour was at a moderate thermal challenge (28°C), and the third hour was at an extreme thermal challenge (21°C). Each line of the raster represents a single animal's vocalization pattern, with the histogram below summarizing the data across animals. All groups show relatively similar patterns of vocalizing, with the strongest increase in vocalization rates during the extreme thermal challenge. Although the cocaine (B) and saline (C) groups both show increases in vocalizing during the moderate thermal challenge period (ns), these increases appear to be driven by only a few subjects and are not seen in untreated animals (A).

Figure 7

Postnatal day 5 interscapular and back temperatures during a thermoneutral baseline period (34°C), 1 h moderate (28°C), and 1 h extreme (21°C) thermal challenge. Prenatal cocaine exposure was associated with an increase in both interscapular (A) and back (B) temperatures across the later portions of the moderate challenge, and the entirety of the extreme thermal challenge, but no significant difference in T_IS-Back (C). Additionally, there was no difference between group baseline temperatures (inset graphs). (Saline vs. Untreated: ^†p ≤ 0.05, ^††p ≤ 0.01; Cocaine vs. Untreated: ^‡p ≤ 0.05, ^‡‡p ≤ 0.01).

Figure 8

Assessment of cardiac function on postnatal day 5 during a thermoneutral baseline period (35°C), 1 h moderate (28°C), and 1 h extreme (21°C) thermal challenge. Prenatal cocaine exposure was associated with decreases in cardiac mass (A), but no change in cardiac β-Adrenergic receptor concentrations (B) as measured following the thermal challenges. Additionally, prenatal cocaine exposure was not associated with significant change in heart rate (data not shown), R-R Interval (C), or R-R Interval Variability (D) as measured during the thermal challenges. (^*p ≤ 0.05).

Figure 9

The relationship between the cardiac R-R Interval and the interscapular temperature of the pup. Average group temperature and R-R interval for each 10-min bin are presented. An increasing value on the Y-axis indicates a slowing of the cardiac rate. Thus, at colder temperatures, pups tend to have reduced heart rates. Both cocaine-exposed and untreated pups show a similar relationship between these variables, with no significant difference between the two groups.