Armstrong W E, Stern J E
Department of Anatomy and Neurobiology, University of Tennessee, Memphis, College of Medicine 38163, USA.
Prog Brain Res. 1998;119:101-13. doi: 10.1016/s0079-6123(08)61564-2.
Oxytocin and vasopressin secreting neurones of the hypothalamic supraoptic nucleus share many membrane characteristics and a roughly similar morphology. However, these two neurone types differ in the relative expression of some intrinsic and synaptic currents, and in the extent of their respective dendritic arbors. Spike depolarizing afterpotentials are present in both types, but more frequently give rise to prolonged burst discharges in vasopressin neurones. Oxytocin, but not vasopressin neurones, are characterized by a depolarization-activated, sustained outward rectifier which turns on near spike threshold, and which can produce prolonged spike frequency adaptation. When this sustained current is deactivated by small hyperpolarizing pulses, a rebound depolarization sufficient to evoke short spike trains follows the offset of these pulses. Both oxytocin and vasopressin neurones exhibit a transient outward rectification underlain by an Ia-type current. This transient rectifier delays spiking to depolarizing stimuli from a relatively hyperpolarized baseline, and is more prominent in vasopressin neurones. As a result, oxytocin neurones may be more reactive to depolarizing inputs. Both cell types receive glutamatergic, excitatory synaptic inputs and both possess R,S- alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subtypes. The AMPA receptor channel on both cell types is characterized by a relatively high calcium permeability and voltage-dependent rectification, characteristic of a diminished presence of the GluR2 AMPA subunit. However, AMPA-mediated synaptic transients are larger, and decay faster, in oxytocin compared with vasopressin neurones, suggesting a potential difference for synaptic integration. The characteristics of NMDA-mediated synaptic transients are similar in oxytocin and vasopressin neurones, but some data suggest NMDA receptors may be less involved in the glutamatergic activation of oxytocin neurones. In both cell types, synaptic release of glutamate often coactivates AMPA and NMDA receptors. The dendritic morphology of oxytocin and vasopressin neurones in female rats differs from one another and exhibits considerable plasticity as a function of endocrine state. In virgin rats, oxytocin neurones have more dendritic branches and a greater total dendritic length compared with lactation, when the arbor is much less extensive. A complementary change occurs in vasopressin dendrites, which are more extensive during lactation. This reorganization suggests that oxytocin neurones may be more electronically compact during lactation. In addition, such dramatic shifts in overall dendritic length imply that significant gains and losses in either the total number of synapses, or in synaptic density, are incurred by both cell types as a function of reproductive state.
下丘脑视上核分泌催产素和血管加压素的神经元具有许多共同的膜特性和大致相似的形态。然而，这两种神经元类型在一些内在电流和突触电流的相对表达以及各自树突分支的范围上存在差异。两种类型的神经元都存在动作电位去极化后电位，但在血管加压素神经元中更频繁地引发长时间的爆发性放电。催产素神经元（而非血管加压素神经元）的特征是具有一种去极化激活的持续外向整流电流，该电流在动作电位阈值附近开启，并可产生长时间的动作电位频率适应性变化。当这种持续电流被小的超极化脉冲失活时，这些脉冲结束后会跟随一个足以诱发短动作电位序列的反弹去极化。催产素和血管加压素神经元都表现出由Ia型电流介导的瞬时外向整流。这种瞬时整流将动作电位从相对超极化的基线延迟到去极化刺激，并且在血管加压素神经元中更为显著。因此，催产素神经元可能对去极化输入更具反应性。两种细胞类型都接受谷氨酸能兴奋性突触输入，并且都拥有R,S-α-氨基-3-羟基-5-甲基异恶唑-4-丙酸（AMPA）和N-甲基-D-天冬氨酸（NMDA）受体亚型。两种细胞类型上的AMPA受体通道的特征是具有相对较高的钙通透性和电压依赖性整流，这是GluR2 AMPA亚基存在减少的特征。然而，与血管加压素神经元相比，催产素神经元中AMPA介导的突触瞬变更大且衰减更快，这表明在突触整合方面存在潜在差异。催产素和血管加压素神经元中NMDA介导的突触瞬变特征相似，但一些数据表明NMDA受体可能较少参与催产素神经元的谷氨酸能激活。在两种细胞类型中，谷氨酸的突触释放通常会共同激活AMPA和NMDA受体。雌性大鼠中催产素和血管加压素神经元的树突形态彼此不同，并且表现出作为内分泌状态函数的显著可塑性。在未孕大鼠中，与哺乳期相比，催产素神经元具有更多的树突分支和更长的总树突长度，哺乳期时树突分支则少得多。血管加压素树突发生互补性变化，哺乳期时更为广泛。这种重组表明哺乳期时催产素神经元在电方面可能更为紧密。此外，整体树突长度的这种显著变化意味着两种细胞类型都会随着生殖状态的变化在突触总数或突触密度上产生显著的增减。
