Functional characterization of the lateral septal calbindin neurons in maternal care

Abstract 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 Statement Maternal 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. Competing Interest Statement The authors have declared no competing interest.