The Timing of Synapse Formation During Development Drives Social Behavior in Rats

Post by Shireen Parimoo

What's the science?

In the first week after birth, hippocampal neurons first exhibit GABA-mediated (inhibitory neurotransmitter) signaling followed by glutamatergic (excitatory neurotransmitter) signaling. Cortical neuronal populations exhibit glutamatergic activity in the first postnatal week, followed later by GABAergic activity. These developmental sequences of neurotransmitter activity correspond to the maturation of neurons in the brain. Network-level (groups of neurons)patterns of activity are important for complex behaviors like social interaction. However, it is not known whether a particular developmental sequence initiates a shift toward network-level activity that enables the development of more complex behaviors. This week in Nature Communications, Naskar and colleagues used electrophysiology and chemogenetics to investigate the relationship between the development of functional synapses and complex social behavior in newborn rats.

How did they do it?

The authors first recorded synaptic activity from pyramidal cells (neurons) in layers II/III and V of somatosensory cortex slices from rat pups between 2 and 15 days old (i.e. ‘postnatal days’ 2 and 15). They used whole-cell patch clamp electrophysiology to record postsynaptic currents to determine the relative timing of glutamate and GABA synaptogenesis (i.e. the creation of synapses). They also examined whether synaptogenesis was related to structural changes in these somatosensory cortex layers by staining the slices and counting the number of dendritic spines while mice were between 7 and 15 days old. They assessed the development of 1) complex sensory and motor behavior in pups by observing huddling behavior, and 2) more basic sensory and motor behavior by measuring performance on the modified SHIRPA test.development. Behavioral results from the SHIRPA test can be clustered in revealed three main categories of behaviors: (i) primitive behaviors like reflexes, and (ii) motor and (iii) sensory behaviors. Huddling behavior (more complex) included the time that pups spent together, the number of different clusters they formed, and the number of times they switched from one cluster to another.

To investigate the relationship between synaptogenesis and huddling behavior, the authors transfected somatosensory layer II/III precursor cells with inhibitory Designer Receptors Exclusively Activated by a Designer Drug (iDREADDs; synthetic receptors) or a control green fluorescent protein (GFP). The authors then recorded huddling behavior in pups before and after CNO application; clozapine N-oxide (CNO) is a synthetic drug that inhibits activity in cells expressing iDREADDs but has no effect on GFP-expressing cells. Finally, they investigated whether serotonergic transmission affects the relationship between cortical activity and huddling behavior. This was done by measuring serotonin transporter (SERT) levels in cortical slices of projections to the somatosensory cortex and assessing huddling behavior after increasing serotonergic activity with citalopram (inhibits serotonin reuptake) in the pups.

What did they find?

In layer II/III neurons of the somatosensory cortex, glutamatergic currents were observed at postnatal day 5, before GABAergic currents at postnatal day 7, and the frequency of both currents increased between postnatal days 8 and 9. This increase in the frequency of currents was associated with an increase in the number of dendritic spines on somatosensory neurons. By contrast, glutamatergic and GABAergic synapse formation occurred at the same time in layer V somatosensory neurons, whereas GABAergic currents preceded glutamatergic currents in hippocampal neurons. This indicates that developmental sequences are not the same across different cortical layers within a brain region or across different brain regions entirely.

The motor and sensory behaviors assessed in the SHIRPA were observed before the development of functional cortical synapses, suggesting that these basic behaviors are not dependent on the somatosensory cortex. On the other hand, the emergence of the more complex huddling behavior was aligned with the timeline of cortical synaptogenesis. Specifically, pups exhibited greater huddling behavior between postnatal days 8 and 9, consistent with the increase in synaptic current frequency in the somatosensory cortex. In fact, inhibiting somatosensory neurons by CNO in iDREADD-expressing pups reduced huddling behavior compared to control pups. This relationship between somatosensory cortical activity and huddling behavior was modulated by serotonergic transmission. Serotonin neurons project to layers I-IV of the somatosensory cortex, and there was a larger concentration of SERT on axons projecting to the somatosensory cortex on postnatal day 9 compared to postnatal day 7. Moreover, enhancing serotonergic activity with citalopram had no effect on basic behaviors as measured by the SHIRPA, but led to earlier emergence of huddling behavior compared to control pups.

What's the impact?

This study demonstrates that functional synapses follow a non-uniform pattern of development both within and across different regions of the brain. It further shows that early social behaviors occur concurrently with the development of glutamatergic and GABAergic synapses in the rodent somatosensory cortex, and that this is modulated by serotonergic transmission. Overall, this research provides deeper insight into the relationship between the precise timing of cellular processes during development and the emergence of complex, social behaviors.

Naskar et al. The development of synaptic transmission is time-locked to early social behaviors in rats. Nature Communications (2019). Access the original scientific publication here.