Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Abstract

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

Figure 1. The cortical cholinergic input system

a | Basal forebrain (BF) efferent cholinergic projections to the entire cortical mantle, and the main telencephalic afferent projection systems of the BF (view at a sagittal section). Cholinergic neurons originate from the nucleus basalis of Meynert, the substantia innominata and the vertical and horizontal nuclei of the diagonal band of Broca (collectively termed the BF) and innervate all cortical areas and layers. The prefrontal cortex (PFC) is the only cortical region, in rodents and primates, that is known to project back to the BF both directly and indirectly (through the nucleus accumbens (NAc)). The BF, PFC and NAc are also all innervated by dopaminergic neurons from the ventral tegmental area (VTA), and these dopaminergic neurons in turn are contacted by PFC projections. This organization suggests a profound control of the BF by the PFC. Not shown are brainstem projections to the BF. b | A composite map showing the three-dimensional distribution of cholinergic cells projecting to four arbitrarily defined mediolateral sectors of the neocortex. Cells projecting to different regions are colour-coded (medial: red; intermediary sector: blue and yellow; lateral parts of the neocortex: green). Note the relatively ordered rostromedial to caudolateral distribution of cells projecting to mediolaterally located cortical areas.c | A surface density-based render of the major organizational features in the BF (unit space: 400 × 400 × 50 μm; density threshold > 2 cells per voxel; the numbers along the z axis are the layers (sections) and the x and y values correspond to the voxel indices; for details see REF. 14). The colours of the units represent the brain regions that the cholinergic cells in those areas project to (blue: posteromedial cortex; yellow: medial prefrontal cortex; red: barrel cortex; green: posterior insular-perirhinal cortex; light blue: agranular insular-lateral orbital cortex; magenta: lateral frontal (motor) cortex). ACh, acetylcholine; GABA, γ-aminobutyric acid. Parts b and c are reproduced, with permission, from REF. 14 © (2002) Springer.

Figure 2. Cholinergic fibre distribution in the cortex

Coronal sections of the medial prefrontal cortex of the rat, visualized using choline acetyl-transferase (CHAT) immunohistochemistry (a) or a histochemical method for revealing acetylcholinesterase (ACHE)-positive fibres (b), are shown to illustrate the distribution of cholinergic fibres in the cortex. The low resolution sections in parts a and b show the anterior cingulate cortex (AC), the prelimbic cortex (Prel) and the infralimbic cortex (Infral); the expansions show photomicrographs of the stippled areas, with the cortical layers indicated for part b (note that in the rat the Prel is agranular (there is no layer IV)). CHAT immunoreactivity reveals fine varicose fibres and darkly stained bipolar interneurons with axons and dendrites that are organized perpendicularly to the pial surface. The phenotype of these neurons remains elusive: they do not express p75 receptors and thus are unaffected by local infusions of the cholinotoxin 192 immunoglobulin G–saporin. Similarly, visualization of ACHE-positive fibres reveals dense cholinergic input in all layers. Except for some minor layer-specific organizational differences, the two methods reveal essentially similar patterns of cholinergic input (see also REF. 18). The density of cholinergic inputs is similar throughout the cortex, except that there are higher densities of cholinergic input to entorhinal and olfactory regions. Part b is modified, with permission, from REF. 72 © (2008) Elsevier.

Figure 3. Major steps in the synthesis, release and metabolism of ACh, and the main characteristics of wired and volume transmission

Except for localized increases in choline resulting from acetylcholine (ACh) hydrolysis by acetylcholinesterase (ACHE), extracellular concentrations of choline are stable at ∼4.85 μM. To synthesize ACh, choline is transported into the terminal through choline transporter (CHT). In the terminal, choline acetyltransferase (CHAT) catalyses the synthesis of ACh from choline and acetyl CoA (AcCoA). The capacity of CHT is the most significant determinant of the rate of ACh synthesis. ACh is packed into vesicles by vesicular acetylcholine transporter (VACHT) and released on depolarization of the terminal. Following release, ACh can bind to nicotinic (n) and muscarinic (m) ACh receptors (AChRs) and is rapidly hydrolysed by ACHE to yield choline and acetate. In the wired model of cholinergic neurotransmission (a), the presence and high catalytic activity of ACHE restricts the neurotransmission to classic synapses or junctional complexes. By contrast, in the volume model of cholinergic neurotransmission (b), most presynaptic cholinergic terminals in the cortex do not form junctional complexes and so neurotransmission is mediated by ACh that escapes hydrolysis because of insufficient or regulated availability and/or activity of ACHE. This ACh reaches the extracellular space and can stimulate non-junctional nAChRs and mAChRs. As discussed in the main text, the generation of second-scale cholinergic transients seems to represent a more important characteristic of cholinergic neurotransmission than either mode of neurotransmission.