Brain Structure and Function

, Volume 219, Issue 5, pp 1603–1613 | Cite as

Ultrastructure and synaptic connectivity of main and accessory olfactory bulb efferent projections terminating in the rat anterior piriform cortex and medial amygdala

  • Sook Kyung Park
  • Jong Ho Kim
  • Eun Sun Yang
  • Dong Kuk Ahn
  • Cheil MoonEmail author
  • Yong Chul BaeEmail author
Original Article


Neurons in the main olfactory bulb relay peripheral odorant signals to the anterior piriform cortex (aPir), whereas neurons of the accessory olfactory bulb relay pheromone signals to the medial amygdala (MeA), suggesting that they belong to two functionally distinct systems. To help understand how odorant and pheromone signals are further processed in the brain, we investigated the synaptic connectivity of identified axon terminals of these neurons in layer Ia of the aPir and posterodorsal part of the MeA, using anterograde tracing with horseradish peroxidase, quantitative ultrastructural analysis of serial thin sections, and immunogold staining. All identified boutons contained round vesicles and some also contained many large dense core vesicles. The number of postsynaptic dendrites per labeled bouton was significantly higher in the aPir than in the MeA, suggesting higher synaptic divergence at a single bouton level. While a large fraction of identified boutons (29 %) in the aPir contacted 2–4 postsynaptic dendrites, only 7 % of the identified boutons in the MeA contacted multiple postsynaptic dendrites. In addition, the majority of the identified boutons in the aPir (95 %) contacted dendritic spines, whereas most identified boutons in the MeA (64 %) contacted dendritic shafts. Identified boutons and many of the postsynaptic dendrites showed glutamate immunoreactivity. These findings suggest that odorant and pheromone signals are processed differently in the brain centers of the main and accessory olfactory systems.


Olfactory bulb neuron  Synaptic connectivity  Ultrastructure  Odor processing  Pheromone processing 



Accessory olfactory bulb


Anterior pirifrom cortex


Glomerular layer




Horseradish peroxidase


Large dense core vesicle


Lateral olfactory tract


Medial amygdala


Posterodorsal part of the medial amygdala


Posteroventral part of the medial amygdala


Main olfactory bulb


Optic tract



This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0009328). The authors sincerely thank Dr. Juli Valtschanoff for his helpful discussion and careful reading of the manuscript.


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© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  1. 1.Department of Anatomy and Neurobiology, School of DentistryKyungpook National UniversityDaeguKorea
  2. 2.Department of Brain Science, Graduate SchoolDaegu Gyeongbuk Institute of Science and TechnologyDaeguKorea

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