Firing patterns of maternal rat prelimbic neurons during spontaneous contact with pups

Abstract

Extracellular single unit activity was recorded from medial prefrontal cortex (mPFC) of postpartum dams over the course of 3 days while they engaged in spontaneous pup-directed behaviors and non-specific exploratory behavior. Out of 109 units identified over the course of the experiment, 15 units were observed to be pup-responsive and 15 increased their discharge rates non-specifically while not attending to pups. An association between neuronal activity and typical maternal behaviors (e.g., retrieval, pup-grooming, nursing) was not observed. Instead, brief bouts of snout contact with pups were accompanied by phasic increases and decreases in spike rates. The observed pup contact responsive cells might play a role in processing of sensory feedback from pups or the transmission of modulatory output to other subcortical maternal brain areas.

Figure 1

In vivo electrophysiology in awake behaving postpartum rats. A) Digital still frames collected during an actual experiment. Sequence of behaviors observed as follows: a – baseline exploration and activities before pups, b– pups placed in corner of cage opposite to area where dam is huddled, c – approach towards pups that can result in simple mouthing, sniffing or, d – picking up and retrieving a pup. e - shows an actual pup being retrieved (white circle and arrow) and one already in the nest (red circle and arrow). In f all pups are in the nest and the last pup is being retrieved. g, lactation. B) Clustering in PCA feature space was used to detect units on single wires. First and second principal components were used. Shown are 2 units and noise cluster. C) waveform amplitudes exceeded the cut-off threshold of 60 μV. D) Anatomical MRI scans were used to locate microwire placement sites. Arrow indicates the site where electrode tips were observed. E–F) Single unit firing of a representative neuron in the medial prefrontal cortex during pup contact. Images show examples of upright movement and brief pup contact. Perievent time histogram are for a neuron during pup contact and upright movement. Spike counts binned at 100 msec.

Figure 2

Maternal behavior in the test cage environment. Top, animals were grouped according to the expression of retrieval behavior. Four out of 5 rats did not retrieve pups and therefore this behavior could not be used to generate timestamps for the neuronal analysis. Middle, rats showed significant amounts of spontaneous pup contact regardless of whether or not they expressed retrieval behavior or other pup-directed behaviors. This pup directed exploratory behavior was used for the neuronal analysis. Bottom, upright movement (upward ‘rearing’ activity) was consistently expressed by animals and was used as a spontaneously expressed control behavior.

Figure 3

Mean single unit firing (mean Z scores ± standard error) in medial prefrontal cortex of maternal rats during up contact and upright movement. Perievent time histograms for each unit was analyzed around spontaneous pup contact and the non-specific exploratory behavior. Spike counts are binned at 500ms. A) Units that were pup responsive, as indicated by increased discharges during pup contact but not rearing. Bar graphs on right summarize peak firing averaged over 2 seconds (in Z scores ± standard error). B) Units that were not pup responsive, as indicated by increased discharges during upright movement and not pup contact. Bar graphs on right summarize peak firing averaged over 2 seconds (in Z scores ± standard error). Symbols at each time point of line graphs denote significantly different from control behavior (two way analysis of variance with repeated measures; circle p<0.001; asterisk p < 0.05). Asterisk over bars denotes significant difference (Wilcoxon’s matched paried t-test *p = 0.02, **p = 0.001).

Figure 4

Average changes in firing rate (mean z score ± standard error) during pup contact. Pup responsive units are classified as occuring during pup contact and following contact. Arrow indicates time of observation of the behavior event (pup contact). N’s indicate number of units and not number of animals. Symbols denote significant differences from * post-contact responsive and φ contact responsive (Wilcoxon’s matched paried t-test p < 0.05).

Figure 5

Mean single unit firing (mean Z scores ± standard error) in medial prefrontal cortex of maternal rats during up contact and upright movement. Datasets were subdivided according to whether or not animals displayed retrieval behavior. Perievent time histograms for each unit was analyzed around spontaneous pup contact and the non-specific exploratory behavior. Spike counts are binned at 500ms. Data presented as normalized Z scores ± standard error.

Figure 6

Changes in single unit firing during and following pup contact. Top) Proportion of units showing increased and suppressed firing responses (FR) during pup contact and rearing activity. Middle) Proportion of units showing increased firing during the expression of either pup contact or rearing activity (timed) and units showing changes following the expression of these behaviors (prolonged). A contingency test showed significant differences between pup contact and control units (χ² = 9.7, p = 0.002). Bottom) Proportion of single units that are responsive during pup contact and upright movement, subdivided into animals showing retrieval or that failed to do so. A significantly greater proportion of units responsive during pup contact belonged to the non retrieving maternal animals (χ² = 4.5, p = 0.03).