Characterization of McDonald's intermediate part of the Central nucleus of the amygdala in the rat

Abstract

The actual organization of the central nucleus of the amygdala (CEA) in the rat is mostly based on cytoarchitecture and the distribution of several cell types, as described by McDonald in 1982. Four divisions were identified by this author. However, since this original work, one of these divisions, the intermediate part, has not been consistently recognized based on Nissl-stained material. In the present study, we observed that a compact condensation of retrogradely labeled cells is found in the CEA after fluorogold injection in the anterior region of the tuberal lateral hypothalamic area (LHA) in the rat. We then searched for neurochemical markers of this cell condensation and found that it is quite specifically labeled for calbindin (Cb), but also contains calretinin (Cr), tyrosine hydroxylase (TH) and methionine-enkephalin (Met-Enk) immunohistochemical signals. These neurochemical features are specific to this cell group which, therefore, is distinct from the other parts of the CEA. We then performed cholera toxin injections in the mouse LHA to identify this cell group in this species. We found that neurons exist in the medial and rostral CEAl that project into the LHA but they have a less tight organization than in the rat.

Morphofunctional Organization of the Connections From the Medial and Intermediate Parts of the Central Nucleus of the Amygdala Into Distinct Divisions of the Lateral Hypothalamic Area in the Rat

“…In a recent study about the characterization of the perifornical projections from the CEA (27), we noted that retrograde tracer injections into the caudal lateral hypothalamus labeled many more neurons in the CEAm than in the CEAi, while rostral perifornical injections labeled a dense cluster in the CEAi. The study of Han and Swanson (34) is also interesting as these authors injected retrograde and anterograde tracers into the LHAs.…”

“…In particular projections into the caudal PVH are similar. This region is also connected to the parastrial nucleus and to the DMH which are also targeted by the CEAm and/or CEAi [see (27)]. Interestingly, this region does not project into the PSTN, and our retrograde injections also failed to label perikarya in this part of the LHA.…”

“…In the course of our study, we obtained many injections within the borders of the CEA. Only experiments with injection sites restricted to the CEAm/CEAi are reported here as only these parts of the CEA project into the LHA (22, 27). The CEAl and the CEAc do not send significant projections into the LHA (22, 27).…”

“…The study of Han and Swanson (34) is also interesting as these authors injected retrograde and anterograde tracers into the LHAs. They illustrated that the rostral CEAm contained no retrogradely labeled cells, while the rostral CEAl [corresponding to the CEAi, see (27)], and adjacent caudal CEAm contained such retogradely labeled cells. The paper of Reppucci and Petrovich (35) also noted intense labeling in the rostral CEAl after retrograde tracer injections into the perifornical region.…”

“…However, the CEA also projects into the perifornical LHA. In a recent study (27), we identified that these projections originate from the medial part of the CEA (CEAm), but also from the intermediate part (CEAi), while the capsular and lateral parts of the CEA send only very sparse projections into the hypothalamus. We noted that mostly the anterior perifornical region of the LHA was innervated by the CEAm/i, but this term is quite vague and this region can be sub-divided as in the Rat Brain Maps (28).…”