Synaptic connectivity and electrophysiological properties of the nucleus of the lateral olfactory tract.

Abstract

The sense of smell is tightly linked to emotions, a link that is thought to rely on the direct synaptic connections between the olfactory bulb and nuclei of the amygdala. However, there are multiple pathways projecting olfactory information to the amygdala and their unique functions are unknown. Among them, is the pathway via the nucleus of the lateral olfactory tract (NLOT) that receives input from olfactory regions and projects to the basolateral amygdala. NLOT has been very little studied and consequentially its function is unknown. Furthermore, formulation of informed hypotheses about NLOT function is at this stage limited by the lack of knowledge about its connectivity and physiological properties. Here, we used virus-based tracing methods to systematically reveal inputs into NLOT, as well as NLOT projection targets in mice of both sexes. We found that the NLOT is interconnected with several olfactory brain regions and with the basolateral amygdala. Some of these connections were reciprocal, and some showed unique interhemispheric patterns. We tested the excitable properties of NLOT neurons and the properties of each of the major synaptic inputs. We found that the NLOT receives powerful input from piriform cortex, tenia tecta, and the basolateral amygdala, but only very weak input from the olfactory bulb. When input crosses threshold, NLOT neurons respond with calcium-dependent bursts of action potentials. We hypothesize that this integration of olfactory and amygdalar inputs serves behaviors that combine smell and emotion.Significance statement Despite the well-known anatomical connections between olfactory and amygdalar brain regions, their physiological properties remain largely understudied. One major pathway by which olfactory and amygdalar signals interact, is via the nucleus of the lateral olfactory tract (NLOT). NLOT has been little studied and its function is yet unclear. The lack of physiological information hinders informed hypotheses. Here, we characterize the synaptic and intrinsic properties of NLOT neurons. We show that the NLOT receives converging olfactory and amygdalar inputs, and that NLOT neurons respond to input with high-rate bursts of action potentials. This suggests that the NLOT, that harbors ∼2500 cells, encodes a low-dimensional signal that is of high importance. We hypothesize that the NLOT assigns emotional value to odors.