or cell death is induced. These mechanism are vital to maintain homeostasis of the cell and ensure proper function of the mitochondria. In plants One of the most studied retrograde signaling molecules in plants are reactive oxygen species (ROS). These compounds, previously believed to be damaging to the cell, have since been discovered to act as a signaling molecule. Reactive oxygen species are created as a by-product of aerobic respiration and act on genes involved in the stress response. Depending on the stress, reactive oxygen species can act on neighboring cells to initiate a local signal. By doing this, surrounding cells are "primed" to react to the stress because genes involved in stress response are initiated prior to encountering the stress. The chloroplast can also act as a sensor for pathogen response and drought. Detection of these stresses in the cell will induce the formation of compounds that can then act on the nucleus to produce pathogen resistance genes or drought tolerance. In neuroscience The primary purpose of retrograde neurotransmission is regulation of chemical neurotransmission. For this reason, retrograde neurotransmission allows neural circuits to create feedback loops. In the sense that retrograde neurotransmission mainly serves to regulate typical, anterograde neurotransmission, rather than to actually distribute any information, it is similar to electrical neurotransmission. In contrast to conventional (anterograde) neurotransmitters, retrograde neurotransmitters are synthesized in the postsynaptic neuron, and bind to receptors on the axon terminal of the presynaptic neuron. Additionally, retrograde signaling initiates a signaling cascade that focuses on the presynaptic neuron. Once retrograde signaling is initiated, there is an increase in action potentials that begin in the presynaptic neuron, which directly impacts the postsynaptic neuron by increasing the number of its receptors. Endocannabinoids like anandamide are known to act as retrograde messengers, as is nitric oxide. Retrograde signaling may also play a role in long-term potentiation (LTP), a proposed mechanism of learning and memory, although this is controversial. Formal definition of a retrograde neurotransmitter In 2009, Regehr et al. proposed criteria for defining retrograde neurotransmitters. According to their work, a signaling molecule can be considered a retrograde neurotransmitter if it satisfies all of the following criteria: The appropriate machinery for synthesizing and releasing the retrograde messenger must be located in the postsynaptic neuron Disrupting the synthesis and/or release of the messenger from the postsynaptic neuron must prevent retrograde signaling The appropriate targets for the retrograde messenger must be located