Restricted expression between parvalbumin and substance P receptor NK₁ in interneurones of the lateral amygdala

Increasing evidence suggests that substance P (SP) and its receptor, namely the neurokinin 1 receptor (NK₁-R), play an important role in the modulation of stress-related, affective and/or anxious behaviours. Both SP and NK₁-R are expressed in brain regions critically involved in stress, fear and affective responses such as the amygdala, hippocampus and frontal cortex. In this study we aimed at identifying the types of NK₁-R immunoreactive neurones in the basolateral complex of the amygdala according to their content of other neurochemical markers by dual or triple labelling immunofluorescence. The basolateral amygdaloid complex consists of the lateral (LA) and basolateral (BL) nuclei, which are believed to be cytoarchitectonically similar. Our study reveals that in the rat LA, 38.7 ± 6.7% of NK₁-R immunopositive interneurones (124/331) co-express the calcium binding protein parvalbumin (PV), representing 15.2 ± 3.4% (124/820) of PV⁺ neurones. Conversely, in the BL no coexistence between NK₁-R (293 neurones counted) and parvalbumin (2385 neurones counted) expressing interneurones was detected. These results suggest that interneurones in the LA and BL differentially express molecules involved in cell signalling and indicate a distinct organization in local interneurones. The BL resemble the hippocampal region CA1, in which NK₁-R-expressing neurones do not coexist with PV[1].