Postsynaptic structures of axon terminals of cortical double bouquet cells

皮质GABA能非锥体细胞(NP)调节皮质锥体细胞的活动。我们研究了双束(DB)细胞的突触连接，双束(DB)细胞是NP细胞的一个亚型。用双重免疫组织化学方法，在第II/III层DB细胞亚群中发现钙维A(CR)和促肾上腺皮质激素释放因子(CRF)。因此，CR和CRF阳性DB细胞可被认为是不同的细胞亚型。我们进一步研究了这两种亚型DB细胞的突触后结构，以了解它们的靶结构的差异。用全细胞、电流钳记录和细胞内注射生物细胞素的方法，在幼年大鼠额叶皮质分离的脑片中鉴定出NP细胞。用CR和CRF抗血清免疫染色后，用DAB染色。用连续超薄切片三维重建突触后结构：1个未鉴定的DB细胞、2个CR-DB细胞和2个CRF-DB细胞的细胞内染色轴突终末。未鉴定的细胞和两个CR-DB细胞显示出类似的结果。它们支配树突(50-68%)、棘突(19-47%)和少见的体细胞(2-9%)。目标脊椎通常具有不对称的输入。这可能表明DB细胞特异性地抑制了这种兴奋性输入。一些靶树突(30-37%)在其轴突上可见许多非对称性突触，表明为NP细胞的树突。其余的靶树突(17-31%)既有锥体细胞，也有NP细胞。CRF-DB细胞的神经支配靶点略有不同。树突(63%~73%)和棘突(27%~37%)为主要靶点，但未见胞体。有许多不对称输入的树枝较少(10-18%)，垂直细长的树枝最多(20-30%)。这些观察结果表明，CR-和CRF-DB细胞亚型可能参与了大脑皮层微环路的不同功能。

Cortical GABAergic nonpyramidal (NP) cells regulate activities of cortical pyramidal cells. We investigated synaptic connections made by double bouquet (DB) cells, a subtype of NP cells. Using the double immunohistochemistry, calretinin (CR) and corticotrophin releasing factor (CRF) were found in largely separate subgroups of DB cells in layers II/III. Thus, CR-and CRF-positive DB cells may be considered as different cell subtypes. We further studied postsynaptic structures of these two subtypes of DB cells to see differences in their target structures. NP cells were identified in isolated slices of young rat frontal cortex by whole cell, current-clamp recording, followed by intracellular injection of biocytin. After immunostaining with antisera against CR and CRF, the cells were stained with DAB. The postsynaptic structures were reconstructed three-dimensionally from serial ultra-thin sections of the intracellulary-stained axon terminals of one chemically unidentified DB cell, two CR-DB cells and two CRF-DB cells. The unidentified cell and two CR-DB cells showed similar results. They innervated dendritic shafts (50-68%), spines (19-47%) and rarely somata (2-9%). The target spines usually had an asymmetrical input. This may indicate the DB cell specifically inhibits this excitatory input. Some target dendrites (30-37%) received many asymmetrical synapses on their shafts, which indicated dendrites of NP cells. The rest of the target dendrites (17-31%) were of both pyramidal and NP cells. The innervation targets of CRF-DB cells were slightly different. The dendritic shafts (63-73%) and spines (27-37%) were the major targets, but no somata were observed. The dendrites with many asymmetrical inputs were observed less (10-18%) and vertically elongated dendrites were identified frequently (20-30%). These observations indicated CR-and CRF-DB cell subtypes may be involved in distinct functions in cortical microcircuit.