Postsynaptic structures of axon terminals of cortical double bouquet cells

皮层gaba能非锥体细胞（NP）调节皮层锥体细胞的活动。我们研究了双花束细胞（DB）的突触连接，它是NP细胞的一个亚型。双重免疫组化发现，calretinin （CR）和促肾上腺皮质激素释放因子（CRF）在II/III层的DB细胞亚群中存在。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DB细胞亚型的突触后结构，以观察其靶结构的差异。采用全细胞、电流钳记录、细胞内注射生物细胞素的方法，在分离的幼年大鼠额叶皮层切片中鉴定NP细胞。用抗血清对CR和CRF进行免疫染色后，用DAB染色。通过对1个化学性质不明的DB细胞、2个CR-DB细胞和2个CRF-DB细胞的细胞内染色轴突末端进行连续超薄切片，三维重建突触后结构。未识别的细胞和两个CR-DB细胞显示相似的结果。它们支配树突轴（50-68%），棘（19-47%），很少支配躯体（2-9%）。目标脊柱通常具有不对称的输入。这可能表明DB细胞特异性地抑制这种兴奋性输入。部分靶树突（30-37%）轴上有许多不对称突触，提示为NP细胞树突。其余的靶树突（17-31%）为锥体细胞和NP细胞。CRF-DB细胞的神经支配目标略有不同。树突轴（63-73%）和棘（27-37%）是主要目标，但未观察到体细胞。具有许多不对称输入的树突较少（10-18%），垂直伸长的树突较多（20-30%）。这些观察结果表明，cr和CRF-DB细胞亚型可能参与皮层微回路的不同功能。

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.