Perhaps the observed chemical changes are a result of volume transmission, an intracellular mode of communication that potentially affects and modulates the activity of entire brain regions.
If parameters need to be changed globally into a certain direction, then volume transmission through the cerebrospinal fluid (CSF-VT) is an option (see Table 1).
Cryo fixation therefore appears to suggest a reduction in the influence of volume transmission revealing an ultrastructure that might favour wiring transmission (Kullmann, 2000).
The fact that α7 nAChRs respond to low concentrations of Ch open up the possibility that they might be activated by volume transmission in a non synaptic manner by the diffusing Ch, which derives from acetylcholine hydrolysis [46], [47].
Our results suggest that i) peptidergic neurons in the Drosophila ventral nerve cord have separated in- and output compartments in specific areas, and ii) volume transmission is a prevailing way of peptidergic communication within the CNS.
It is through this volume that a second type of signaling (volume transmission, VT) takes place, which contrasts with signaling due to 'wiring transmission' (WT) by neurons.
In conclusion, β-END plays a complicated but major role in the mechanisms controlling pain, and transport via the CSF (volume transmission) forms an essential part in these effects [ 14].
In the neurotransmission process, dopamine acts as an extrasynaptic messenger molecule via volume transmission across the synaptic cleft to bind to extrasynaptic receptors (G protein-coupled receptors) and transporters [ 12– 16] (Fig. 2).
Although a clear role for ACh in volume transmission and for nAChRs in nicotine dependence has been established, several observations suggest that ACh may have additional functions in cholinergic neurons.
When released within the CNS, neuropeptides might act as "local hormones" via volume transmission (signal substance diffusion in a three-dimensional fashion within the extracellular space also termed paracrine signaling), or as co-transmitters that act at or nearby synapses.
