They're synthesized, they're packaged into vesicles, and then they're just waiting.

神经递质不断合成 包装进小泡中,时刻准备着

耶鲁公开课 - 生物医学工程探索课程节选

In some cells they're recycled, that is the cell is able to take up the neurotransmitter after it's released and restore it, but most often there are enzyme systems inside the pre-synaptic membrane where those neurotransmitters are synthesized.

在一些细胞中神经递质可以循环利用,这些细胞可以吸收,已经释放出的神经递质并重新储存起来,但更常见的是突触前膜中含有酶系统,在此合成神经递质

耶鲁公开课 - 生物医学工程探索课程节选