Neurotransmitter Switching? No Surprise

Abstract

Among the many forms of brain plasticity, changes in synaptic strength and changes in synapse number are particularly prominent. However, evidence for neurotransmitter respecification or switching has been accumulating steadily, both in the developing nervous system and in the adult brain, with observations of transmitter addition, loss, or replacement of one transmitter with another. Natural stimuli can drive these changes in transmitter identity, with matching changes in postsynaptic transmitter receptors. Strikingly, they often convert the synapse from excitatory to inhibitory or vice versa, providing a basis for changes in behavior in those cases in which it has been examined. Progress has been made in identifying the factors that induce transmitter switching and in understanding the molecular mechanisms by which it is achieved. There are many intriguing questions to be addressed.

Figure 1. Microcultures Enabled Serial Assays of Single Neurons during Transition from Adrenergic to Cholinergic Status

Top: a solitary neuron from the superior cervical ganglion of a newborn rat embryo; 19 days in vitro. The arrow at H points to a cluster of cardiac myocytes. Inset shows an impulse in this neuron; scales are 20 ms and 50 mV. After Furshpan et al. (1976). Bottom: assay of a solitary neonate-derived rat sympathetic ganglion neuron that underwent a transition from adrenergic to cholinergic phenotype. (A–F) At 17 days in vitro, intracellular recording revealed no autaptic effect of a single action potential (A), and 2 s of 20 Hz stimulation exerted an excitatory effect on cardiac myocytes (B) that was blocked by 1 μM propranolol (C). At 62 days in vitro, a single action potential in the same neuron generated a pronounced autaptic effect (D), and the effect of the same stimulus train on cardiac myocytes was inhibitory (E) and blocked by 0.2 μM atropine (F). Vertical scale: 80 mV for (A), (B), and (F); 40 mV for other traces. Horizontal scale: 40 ms for (A) and (D); 20 s for other traces. After Potter et al. (1986).

Figure 2

Ultrastructural evidence for transmitter switching provided by electron micrographs of axonal synapses (autapses, arrows; top two panels) and varicosities (bottom two panels) of neonatal rat superior cervical ganglion neurons grown in microcultures after transmitter properties had been electrophysiologically identified. Synaptic vesicles are larger and rounded in cholinergic neurons (two left panels) and smaller and more pleomorphic with dense cores in adrenergic neurons (two right panels). 14, 10, 19, and 21 days in vitro. After Landis (1976).

Figure 3. Demonstrating the Adrenergic to Cholinergic Switch In Vivo, the Number of Small Granular Vesicles in Axons of Sympathetic Neurons Associated with Developing Sweat Glands Decreased with Rat Postnatal Age

(A–E) The percentage declined sharply from 52% at 7 days to 0% at 21 days (A). Small granular vesicles (arrows) are evident at 7 days (B) and 10 days (C), rare at 14 days (D), and absent at 21 days (E). After Landis and Keefe (1983).

Figure 4. Spontaneous Calcium Transients Are Both Necessary and Sufficient to Determine Normal Differentiation of the GABAergic Phenotype

Top: development of GABA immunoreactivity of cultured Xenopus spinal neurons in the presence and in the absence of extracellular calcium. 40% of neurons in the cultures develop GABA immunoreactivity during the first day in culture. Acquisition of immunoreactivity is suppressed by growth in calcium-free medium in comparison with controls. Bottom: tuning curve reveals stimulation of GABA expression by imposition of natural frequencies of calcium transients during growth in calcium-free medium. Normal maturation of GABA expression is stimulated by calcium transients at a frequency of 3/hr. After Spitzer et al. (1993) and Gu and Spitzer (1995).

Figure 5. Suppression or Enhancement of Spike Activity In Vivo Causes Homeostatic Superposition or Replacement of One Transmitter with Another in the Xenopus Larval Neural Tube

Left: controls doubly stained for VGluT (red, excitatory) plus glycine or GABA (purple, inhibitory) illustrate the normal distribution of immunoreactivity. Center: embryos in which spike activity was bilaterally suppressed by expression of hKir2.1, stained as at left. Pink indicates coexpression of excitatory and inhibitory transmitters. Right: embryos in which spike activity was bilaterally enhanced by expression of rNav2aαβ, stained for glutamate and a marker of sensory Rohon-Beard cell identity (HNK-1), plus glycine or GABA. Light blue denotes coexpression of marker and inhibitory transmitter; white shows coexpression of marker and both excitatory and inhibitory transmitters. After Borodinsky et al. (2004).

Figure 6. Activity-Dependent Transmitter Specification Drives Selection of Receptors at the Embryonic Xenopus Neuromuscular Junction

Top: expression of nAChR, NMDAR, AMPAR, GABA_AR, and GlyR transcripts in skeletal muscle during normal development. RT-PCR was used for detection of subunit transcripts of five neurotransmitter receptors in muscle, notochord, and neural tube at three stages of development. Tissue-specific RNA was analyzed from embryos at 1 day and at 1.3 days and from larvae at 3 days. Bottom: whole-cell recordings from a larval Xenopus muscle cell of the axial musculature of 3-day spike-suppressed larva in which motor neurons express glutamate. Example of pharmacological identification of glutamatergic mpscs. A single mpsc is shown on an expanded time base to illustrate its kinetics. Performed in 2 mM Ca²⁺, Mg²⁺-free saline and 3 μM TTX; V_h = −80 mV. After Borodinsky and Spitzer (2007).

Figure 7. Illumination Changes the Number of Dopaminergic Neurons in the Ventral Suprachiasmatic Nucleus of the Larval Xenopus Hypothalamus

Top: transverse sections show the core (dashed inner circle) and annular neurons (between dashed circles) in 2-hr black- and white-adapted larvae triply labeled with anti-sense to tyrosine hydroxylase (TH) transcripts and antibodies to TH and NPY. Bottom: quantification of changes in TH immunoreactivity in response to either background illumination or incident light. Scale bar, 60 μm. **p < 0.001. After Dulcis and Spitzer (2008).

Figure 8. Deep Brain Stimulation of Rat Anterior Thalamus Leads to Increased Numbers of Tyrosine Hydroxylase-Stained Neurons in the Ventral Tegmental Area

(A–D) Top: there is no change following stimulation in the mammillothalamic tract (MTT) (A and B) in contrast to the increase after stimulation in the ATT (C and D). Bottom: stereological quantification of the numbers of TH-stained neurons in the VTA and substantia nigra pars compacta (SNc) following stimulation in different brain regions. EC, entorhinal cortex. Scale bar: (A) and (C), 500 μm; (B) and (D), 50 μm. *p < 0.05. After Dela Cruz et al. (2014).

Figure 9. Changes in Postsynaptic Receptor Populations Are Associated with Changes in Presynaptic Transmitter Expression in the Adult Rat Brain

Changing exposure from long-day (19 hr light, 5 hr dark, 19L:5D) to balanced-day (12L:12D) or short-day (5L:19D) photoperiods switches transmitter expression in the PaVN and PeVN from somatostatin (SST) to dopamine (DA). Top: colocalization of D2R and SST2/4R on corticotrophin releasing factor (CRF) neurons lining the walls of the third ventricle (dotted lines), following exposure of rats to each of the photoperiods. Bottom: the number of labeled CRF neurons was averaged for 10 30-μm sections of the rostral hypothalamus. Scale bar, 40 μm. **p < 0.01. After Dulcis et al. (2013).