Cockerham Renee, Liu Shaolin, Cachope Roger, Kiyokage Emi, Cheer Joseph F, Shipley Michael T, Puche Adam C
University of Maryland School of Medicine, Baltimore, Maryland 21201.
CHDI Foundation/CHDI Management, New York, New York 10001, and.
J Neurosci. 2016 Jul 20;36(29):7779-85. doi: 10.1523/JNEUROSCI.0658-16.2016.
The efficacy of neurotransmission depends on multiple factors, including presynaptic vesicular release of transmitter, postsynaptic receptor populations and clearance/inactivation of the transmitter. In the olfactory bulb (OB), short axon cells (SACs) form an interglomerular circuit that uses GABA and dopamine (DA) as cotransmitters. Selective optical activation of SACs causes GABA and DA co-release, resulting in a fast, postsynaptic GABA inhibitory response and a slower G-protein-coupled DA rebound excitation. In most systems, vesicular release of DA is cleared by the dopamine transporter (DAT). However, in the OB, high levels of specific DA metabolites suggest that enzymatic catalysis by catechol-O-methyl-transferase (COMT) predominates over DAT re-uptake. To assess this possibility we measured the amount of the DA breakdown enzyme, COMT, present in the OB. Compared with the striatum, the brain structure richest in DA terminals, the OB contains 50% more COMT per unit of tissue. Furthermore, the OB has dramatically less DAT compared with striatum, supporting the idea that COMT enzymatic breakdown, rather than DAT recycling, is the predominant mechanism for DA clearance. To functionally assess COMT inactivation of vesicular release of DA we used fast-scan cyclic voltammetry and pharmacological blockade of COMT. In mice expressing ChR2 in tyrosine hydroxylase-containing neurons, optical activation of SACs evoked robust DA release in the glomerular layer. The COMT inhibitor, tolcapone, increased the DA signal ∼2-fold, whereas the DAT inhibitor GBR12909 had no effect. Together, these data indicate that the OB preferentially employs COMT enzymatic inactivation of vesicular release of DA.
In the olfactory bulb (OB), odors are encoded by glomerular activation patterns. Dopaminergic short axon neurons (SACs) form an extensive network of lateral connections that mediate cross talk among glomeruli, releasing GABA and DA onto sensory nerve terminals and postsynaptic neurons. DA neurons are ∼10-fold more numerous in OB than in ventral tegmental areas that innervate the striatum. We show that OB has abundant expression of the DA catalytic enzyme catechol-O-methyl-transferase (COMT), but negligible expression of the dopamine transporter. Using optogenetics and fast-scan cyclic voltammetry, we show that inhibition of COMT increases DA signals ∼2-fold. Thus, in contrast to the striatum, which has the brain's highest proportion of DAergic synapses, the DA catalytic pathway involving COMT predominates over re-uptake in OB.
神经传递的效能取决于多种因素，包括突触前递质的囊泡释放、突触后受体群体以及递质的清除/失活。在嗅球（OB）中，短轴突细胞（SACs）形成一个球间回路，该回路使用GABA和多巴胺（DA）作为共递质。对SACs进行选择性光激活会导致GABA和DA共同释放，从而产生快速的突触后GABA抑制反应和较慢的G蛋白偶联DA反弹兴奋。在大多数系统中，DA的囊泡释放通过多巴胺转运体（DAT）清除。然而，在OB中，高水平的特定DA代谢产物表明儿茶酚-O-甲基转移酶（COMT）的酶催化作用比DAT再摄取更为重要。为了评估这种可能性，我们测量了OB中存在的DA分解酶COMT的量。与纹状体（DA终末最丰富的脑结构）相比，OB每单位组织中的COMT含量多50%。此外，与纹状体相比，OB中的DAT显著更少，这支持了COMT酶促分解而非DAT再循环是DA清除的主要机制这一观点。为了从功能上评估COMT对DA囊泡释放的失活作用，我们使用了快速扫描循环伏安法和COMT的药理学阻断。在含酪氨酸羟化酶的神经元中表达ChR2的小鼠中，对SACs进行光激活会在小球层诱发强烈的DA释放。COMT抑制剂托卡朋使DA信号增加了约2倍，而DAT抑制剂GBR12909则没有效果。总之，这些数据表明OB优先采用COMT对DA囊泡释放进行酶促失活。
在嗅球（OB）中，气味由小球激活模式编码。多巴胺能短轴突神经元（SACs）形成广泛的侧向连接网络，介导小球之间的相互作用，将GABA和DA释放到感觉神经终末和突触后神经元上。OB中的DA神经元数量比支配纹状体的腹侧被盖区多约10倍。我们表明OB中DA催化酶儿茶酚-O-甲基转移酶（COMT）表达丰富，但多巴胺转运体的表达可忽略不计。使用光遗传学和快速扫描循环伏安法，我们表明抑制COMT可使DA信号增加约2倍。因此，与脑中DA能突触比例最高的纹状体不同，在OB中涉及COMT的DA催化途径比再摄取更为重要。
