Functional Implications of Neurotransmitter Segregation

Abstract

Here, we present and discuss the characteristics and properties of neurotransmitter segregation, a subtype of neurotransmitter cotransmission. We review early evidence of segregation and discuss its properties, such as plasticity, while placing special emphasis on its probable functional implications, either in the central nervous system (CNS) or the autonomic nervous system. Neurotransmitter segregation is a process by which neurons separately route transmitters to independent and distant or to neighboring neuronal processes; it is a plastic phenomenon that changes according to synaptic transmission requirements and is regulated by target-derived signals. Distant neurotransmitter segregation in the CNS has been shown to be related to an autocrine/paracrine function of some neurotransmitters. In retinal amacrine cells, segregation of acetylcholine (ACh) and GABA, and glycine and glutamate to neighboring terminals has been related to the regulation of the firing rate of direction-selective ganglion cells. In the rat superior cervical ganglion, segregation of ACh and GABA to neighboring varicosities shows a heterogeneous regional distribution, which is correlated to a similar regional distribution in transmission strength. We propose that greater segregation of ACh and GABA produces less GABAergic inhibition, strengthening ganglionic transmission. Segregation of ACh and GABA varies in different physiopathological conditions; specifically, segregation increases in acute sympathetic hyperactivity that occurs in cold stress, does not vary in chronic hyperactivity that occurs in hypertension, and rises in early ages of normotensive and hypertensive rats. Given this, we propose that variations in the extent of transmitter segregation may contribute to the alteration of neural activity that occurs in some physiopathological conditions and with age.

Keywords: classical transmitters; co-release; cotransmission; cotransmitters; plasticity; segregation.

FIGURE 1

Axotomy increases the segregation of VAChT from m-Enk-containing varicosities in the SCG. Exogenous NGF prevents this increase. (A) Micrographs showing SCG sections double-immunostained for VAChT (blue) and m-Enk (green) in control (ctl), after axotomy (axt) and after axotomy and NGF restitution (rt). (B) Bar graph showing the degree of segregation in the three experimental conditions. Axotomy significantly increased the segregation of VAChT from m-Enk-containing varicosities, while after axotomy and NGF restitution the segregation of VAChT from m-Enk was significantly different from the axotomy group, but not from control, indicating that exogenous NGF prevents this increase. *p < 0.05; calibration bar 10 μm (reproduced with permission from Vega et al., 2016).

FIGURE 2

Segregation of acetylcholine (ACh) and GABA varies in different physiopathological conditions. (A) Merged images showing the immunolabeling of GAD67 (marker for GABA; red), vesicular ACh transporter (VAChT; marker for ACh; green), and co-localization of both labels (yellow) in superior cervical ganglion (SCG) from control and cold-stressed rats. (B) Bar graph showing that cold stress increased the percentage of segregation of VAChT/GAD67 compared with control rats. Scale bar = 10 μm, *p < 0.05. (C) Merged images showing the co-localization (yellow) of GAD67 (red) and VAChT (green) in SCG from 12-week-old (wo; left panels) and 6-wo (right panels) SHR and WKy rats. (D) Bar graphs showing that segregation is greater in the SCG of 6-wo than in the 12-wo of both strains of rats; segregation is similar between ganglia from SHR and WKy rats. (E) Immunostaining for GAD67 in SCG from SHR and WKy rats at 12-wo and 6-wo. (F) Bar graphs showing that the percentage of GAD67-containing varicose fibers increases significantly in SHR at 6-wo and 12-wo, and that it is larger in 6-wo than in 12-wo of both strains. Scale bar = 10 μm (modified from Merino-Jiménez et al., 2018).

FIGURE 3

Sympathetic ganglion neurons exhibit different activation and expression of long-term potentiation (LTP) according to their intraganglionic regional location. (A) Input-output curve of ganglionic transmission recorded in the external carotid nerve (ECN; ∘) and the internal carotid nerve (ICN; •), which contain axons of caudal and rostral neurons, respectively. Stimuli of similar amplitude evoked a greater response in the ECN than in the ICN. Insets show a set of compound action potentials (CAPs) evoked by each input intensity tested, recorded in the ICN (a) and in the ECN (b). (B) Time course of ganglionic LTP showed as ΔR/R0 (mean ± SEM), evoked in the caudal region, recorded in the ECN (∘) and in rostral region, recorded in ECN (•) (reproduced with permission from Elinos et al., 2016 and Martínez et al., 2020).

FIGURE 4

Hypothetical scheme postulating that segregation and independent release of ACh and GABA result in reduced GABA inhibition of cholinergic effects. Drawing depicts presynaptic boutons either co-releasing ACh (green circles) and GABA (red circles) from the same or different vesicles or releasing these neurotransmitters independently from separate endings. In the first case, the excitatory action of ACh coincides with the inhibitory action of GABA, resulting in a reduced cholinergic effect, while in the independent release each transmitter exerts its effects separately, avoiding the inhibitory action of GABA on the effect of ACh.