Inactivation or inhibition of neuronal activity in the medial prefrontal cortex largely reduces pup retrieval and grouping in maternal rats

Abstract

Previous research suggests that the maternal medial prefrontal cortex (mPFC) may play a role in maternal care and that cocaine sensitization before pregnancy can affect neuronal activity within this region. The present work was carried out to test whether the mPFC does actually play a role in the expression of maternal behaviors in the rats and to understand what specific behaviors this cortical area may modulate. In the first experiment, tetrodotoxin (TTX) was used to chemically inactivate the mPFC during tests for maternal behavior latencies. Lactating rats were tested on postpartum days 7-9. The results of this first experiment indicate that there is a large effect of TTX-induced inactivation on retrieval behavior latencies. TTX nearly abolished the expression of maternal retrieval of pups without significantly impairing locomotor activity. In the second experiment, GABA-mediated inhibition was used to test maternal behavior latencies and durations of maternal and other behaviors in postpartum dams. In agreement with experiment 1, it was observed that dams capable of retrieving are rendered incapable by inhibition in the mPFC. GABA-mediated inhibition in the mPFC largely reduced retrieval without altering other indices of maternal care and non-specific behavior such as ambulation time, self-grooming, and inactivity. Moreover, in both experiments, dams were able to establish contact with pups within seconds. The overall results indicate that the mPFC may play an active role in modulating maternal care, particularly retrieval behavior. External factors that affect the function of the frontal cortical site may result in significant impairments in maternal goal-directed behavior as reported in our earlier work.

Figure 1

Latency to retrieve 6 pups to nest site during a 15- minute test. Only data for the first and last pup is shown. Groups correspond to PD 7–9 dams microinjected with TTX (n = 13) or aCSF vehicle (n = 9). All data are presented as average seconds ± standard error of the mean. Asterisks denote significant differences at p < 0.05. Above the graphs are Paxinos and Watson atlas coronal maps showing rostral Bregma coordinates 2.70–3.70 mm where microinjections were given.

Figure 2

A) Latency to express pup licking, grouping into the nest site and crouching over pups during a 15- minute test. Groups correspond to PD 7–9 dams microinjected with TTX (n = 13) or aCSF vehicle (n = 9). All data are presented as average seconds ± standard error of the mean. Asterisks denote significant differences at *p < 0.05 or **p < 0.01. B) Measurement of locomotor activity in response to a novel environment after testing for maternal behaviors. Groups correspond to PD 7–9 dams microinjected with TTX (n = 13) or aCSF vehicle (n = 9). Injections were given into the mPFC approximately 90 minutes before testing. All data are presented as mean ± standard error of the mean. Asterisks denote significant differences at p < 0.05.

Figure 3

Latency to first contact pups during a 15-minute test on two separate days of testing. Shown on the right is the latency to inactivity (sleep). Rats were tested on day 1 following a single microinjection into the mPFC of aCSF vehicle and then tested on day 2 following administration within the same area of a GABA agonist cocktail mixture (n = 9). Controls received vehicle injections into the mPFC on both days (n = 7). On the right to the graphs are Paxinos and Watson atlas coronal maps showing rostral Bregma coordinates 2.70–4.20 mm where microinjections were given.

Figure 4

Latency to retrieve 12 pups during 15-minute tests on two separate days of testing. Only data for the first and last pup is shown. A) Rats received aCSF vehicle microinjections on day 1 as a baseline measurement of their retrieval behavior and then received a GABA agonist cocktail mixture on day 2 (n = 9). B) Rats received aCSF vehicle microinjections on both test days (n = 7). All data are presented as average seconds ± standard error of the mean. Asterisks denote significant differences at p < 0.05.

Figure 5

Percentage of pups retrieved on two separate test days. Groups and number of animals are the same as Fig. 4–5. All data presented as average percent ± standard error of the mean. Asterisks denote significant differences at p < 0.05. *Compared to Day 1 aCSF group, **compared to Day 2 GABA Agonists group.

Figure 6

Duration of maternal behaviors on two separate days of testing. Groups and number of animals are the same as Fig. 4–5. All data are presented as average seconds ± standard error of the mean. Asterisk and symbol denotes significant differences at p < 0.05.

Figure 7

Duration of other home cage behaviors on two separate days of testing. Groups and number of animals are the same as Fig. 4–5. All data are presented as average seconds ± standard error of the mean. Asterisk and symbol denotes significant differences at p < 0.05