Functional characterization of the lateral septal calbindin neurons in maternal care
The ventral subdivision of the lateral septum (LSv) is a forebrain region linked to maternal care with a high density of calbindin-containing (Cb+) neurons whose properties, connections and functions remain unknown. In the present study, it was established that the majority of pup-activated neurons are inhibitory Cb+ in the LSv and the density of activated neurons in the LSv is close to that of the medial preoptic area (MPOA), a central brain area in maternal care regulation. Electrophysiological recordings of LSvCb+ and LSvCb- neurons revealed that LSvCb+ neurons exhibited higher membrane resistance, lower spike amplitude and rise slope compared to LSvCb- neurons. The chemogenetic inhibition of LSvCb+ neurons led to reduction in pup-licking behavior without affecting other maternal behavior or eliciting anxiety- or depression-like behavior. To regulate licking behavior, LSvCb+ neurons connect with other maternally involved brain areas. Anterograde and retrograde tract-tracing revealed that two-thirds of the MPOA-projecting LSv neurons are Cb+. A subset of neurons within the posterior intralaminar nucleus (PIL) in the lateral thalamus, a proposed relay nucleus of tactile and auditory information from the pups, express a maternally induced neuropeptide, the parathyroid hormone 2 (PTH2). Their fibers closely apposed Cb+ neurons in the LSv. Using double labeling, we also identified PTH2 receptors on LSvCb+ neurons, which are presumably activated by PTH2 released from nearby terminals. Using electron microscopy, we confirmed synaptic connection between PTH2+ fibers and inhibitory neurons in the LSv. The results provide evidence that Cb+ neurons of the LSv are components of the maternal circuitry. Significance StatementMaternal care depends on pup cues, but many responsible circuit elements remain uncharacterized. We identify inhibitory calbindin-expressing neurons in the ventral subdivision of the lateral septum (LSv) containing a major pup-activated neuronal population with distinctive electrical properties. Their selective inhibition reduced pup-licking without affecting other maternal, anxiety- or depression-like behaviors, suggesting a specific role in caregiving. Detailed circuit tracing data revealed that a significant proportion of LSv calbindin-positive neurons extend their projections to the medial preoptic area, a well-established core maternal regulatory region. We further demonstrate synaptic input from maternally relevant parathyroid hormone 2 (PTH2)-expressing thalamic afferents onto LSv inhibitory neurons, possibly conveying pup stimuli. The data suggest that LSv calbindin neurons are key nodes in maternal circuitry.