Noradrenaline (NA), in the USA also called norepinephrine, belongs to the group of monoamines. Its structure contains 1) a benzene ring with two attached hydroxyl groups (-OH) located adjacently and 2) an ethylamine side chain with one amine group (-NH2). NA is synthesized in the brain from the amino acid tyrosine. The first step of its biosynthesis, which is also its rate-limiting step (Fitzpatrick, 1991), converts tyrosine via tyrosine hydroxylase into 3,4-dihydroxy-l-phenylalamine (L-DOPA). L-DOPA is then converted in the presence of coenzyme pyridoxal phosphate into dopamine by the aromatic amino acid decarboxylase or DOPA decarboxylase. In noradrenergic neurons (specifically in its synaptic vesicles), dopamine is then converted further into NA by dopamine-β-hydroxylase (DBH), also called dopamine-β-monooxygenase. This step involves a hydroxylation and requires ascorbic acid as electron donor.
NA is released from synaptic vesicles via exocytosis from specific nerve terminals (see below), the axons of which originate in the locus coeruleus. Its action is then terminated primarily by reuptake into respective nerve terminals via the Na+ and Cl--dependent norepinephrine transporter (NET;; Blakely et al., 1994;; Bonisch & Bruss, 2006).
NA undergoes degradation by two enzymes, namely monoamine oxidase (MAO) and cathecol-O-methyltransferase (COMT). MAO has two different isozymes called as MAO A and B (Shih, 1991). MAO A is found concentrated in adrenergic (noradrenaline) cells in the locus coeruleus (Shih et al., 1999a), and this isozyme catalyses NA oxidative deamination to yield 3,4-dihydroxyphenylglicol and 3,4-dihydroxymandelic acid. COMT then catalyses the chemical reaction in which the methyl group of S-adenosylmethionine is transferred to the 3-position hydroxyl group of both 3,4-dihydroxyphenylglicol and 3,4- dihydroxymandelic acid to yield 3-methoxy-4-hydroxy-phenylglycol (MHPG) and 3- methoxy-4-hydroxy-mandelic acid (vanillylmandelic acid;; VMA) respectively. The degradation by these two enzymes can also happen in the reverse order. In this case, NA is initially converted into normetanephrine by COMT, which then undergoes oxidative deamination catalyzed by MAO A to end up with the same final two products of MHPG and VMA.
This cycle of biosynthesis and degradation primarily occurs in noradrenergic cells. In the mammalian brain, noradrenergic cell bodies can be grouped into the locus coeruleus, the dorsal medullary group (located close to the dorsal motor nucleus of vagus nerve in the medulla), and the lateral tegmental system (located in the pontine tegmentum and reticular formation of the pons). Among these three groups of nuclei, the locus coeruleus is the most important one because its axons widely and diffusely innervate brain. The axons project rostrally (dorsal bundle) to innervate thalamic nuclei and different regions of cortex including the neocortex, hippocampus, the basolateral amygdala, and the septum (Loughlin et al., 1986). Other axons project dorsally to innervate cerebellum (Olson & Fuxe, 1971), and caudally to innervate the spinal cord (Hancock & Fougerousse, 1976;; Nygren & Olson, 1977).
NA signals at synapses via AR. Based on their pharmacological profiles and the associated signalling pathways, AR are classified into three different groups. Note that all groups are G-protein-coupled receptors (GPCR). The first group consists of α1-AR, which are coupled to the Gq-protein. There are three subtypes of α1-AR: α1A, α1B, and α1D-AR (Bylund et al., 1994;; Hieble et al., 1995). In the brain, α1A-AR are expressed in the cerebral cortex (Jones et al., 1986;; Parkinson et al., 1988;; Papay et al., 2006;; Santana et al., 2013), hippocampus (Zilles et al., 1991;; Papay et al., 2006), dorsal thalamus, hypothalamus, midbrain, pontine olivary nuclei, trigeminal nuclei, cerebellum
(Jones et al., 1986;; Papay et al., 2006), and the nucleus accumbens (Mitrano et al., 2012). α1B-AR are also expressed in cerebral cortex, thalamus, dorsal and medullary raphe nuclei (Nicholas et al., 1991). α1D-AR have been found in cerebral cortex, hippocampus, olfactory bulb, the dorsal geniculate and ventral posterolateral nuclei of thalamus (Sadalge et al., 2003). Binding of NA to α1-AR activates the Gq-protein to cause the activation of PLCβ, which then hydrolyses PIP2 to produce IP3 and DAG. The former increases Ca2+ release from intracellular stores, whilst the latter may activate protein kinase C (PKC).
The second group consists of α2-AR, which are coupled to the Gi/o-protein and inhibit adenylyl cyclase, resulting in a lowering of intracellular cAMP. Activated Gβγ subunits of the Gi/o-trimer are also capable of activating K+ channels (Williams et al., 1985) and inhibiting voltage-dependent Ca2+ channels (Boehm, 1999;; Timmons et al., 2004). Three genes have been found to code for α2-AR, namely α2A, α2B, and α2C. In brain, the expression of these three subtypes are as follows. α2A-AR are found in the locus
coeruleus, cerebral cortex, hippocampus, hypothalamus, amygdala, brain stem and septum, α2B-AR in the thalamus and α2C-AR in the cerebral cortex, hippocampus, basal ganglia, olfactory tubercle, and striatum (Scheinin et al., 1994;; King et al., 1995;; Holmberg et al., 1999).
The third and last group contains the β-AR, which are coupled to the Gs-protein. When NA binds to β-AR, the Gs-protein activates adenylyl cyclase, causing an increase in the intracellular cAMP concentration. There are three different subtypes: β1, β2, and β3. In the brain, β1-AR are highly expressed in layer I and II of cerebral cortex, cingulate cortex, hippocampus, ventral striatum (the islands of Calleja), and the mediodorsal and ventral nuclei of thalamus. β2-AR are highly expressed in the molecular layer of cerebellum and the central, paraventricular, and caudal lateral posterior nuclei of thalamus. Both β1 and β2-AR are co-expressed to a similar extent in layer IV of cerebral cortex, the substantia
nigra, olfactory tubercle, medial preoptic nucleus, and the nuclei in the medulla (Rainbow et al., 1984). β3-AR are expressed in cerebral cortex and hypothalamus (Evans et al., 1999) and the subgranular zone of hippocampus (Jhaveri et al., 2010).
One of the outcomes of AR activation is a change in neuronal excitability and an associated de- or hyperpolarization. As all AR are metabotropic receptors, these outcomes are generated downstream of GPCR activation, mostly with specific channel(s) as targets or effectors. For example, NA can activate α1-AR to suppress a leak K+ conductance, and/or β-AR to enhance the hyperpolarization-activated cation current (Ih), resulting in a switch from rhythmic bursting activity to sparse spike firing in the thalamus (Pape & McCormick, 1989;; McCormick et al., 1991). NA, via α2-AR
activation that relieves the tonic inhibition of TREK-2 channels by protein kinase A (PKA), can also cause a hyperpolarization to reduce the excitability of layer II/III pyramidal neurons of entorhinal cortex (Xiao et al., 2009). In addition, the Gβγ subunits of activated α2-AR can open a G-protein-activated inwardly rectifying K+ (GIRK) conductance, inducing a hyperpolarization in locus coeruleus neurons (Williams et al., 1985). In cortical and hippocampal pyramidal neurons, NA enhances the excitability by suppressing a slow Ca2+-activated K+ current (IAHP) downstream of β-AR activation (Madison & Nicoll, 1982;; Haas & Konnerth, 1983;; Foehring et al., 1989;; McCormick et al., 1991;; Satake et al., 2008).
Another outcome of noradrenergic activity is the modulation in transmitter release. There is ample of evidence in the literature showing that NA modulates both spontaneous (mEPSP/Cs) and evoked transmitter release (EPSP/Cs). Some examples include the following: In layer II/III pyramidal cells of the medial prefrontal cortex (mPFC), NA can increase the mEPSC frequency via activation of presynaptic α1-AR (Zhang et al., 2013). However, in layer II/III and V pyramidal cells of mPFC and temporal cortex, NA depresses evoked EPSCs (Roychowdhury et al., 2014) via α1-AR activation (Salgado et
al., 2016). In CA3 pyramidal cells of organotypic slice cultures from rat, NA depresses the evoked EPSP amplitude via presynaptic α1-AR activation (Scanziani et al., 1993). In addition, a recent study in layer II of rat barrel cortex in this laboratory found that NA increases the mEPSC frequency in a subset of pyramidal cells (Choy et al., 2017a), but depresses the EPSC amplitude in all pyramidal pairs recorded from (Choy et al., 2017b;; for details see part 4).
By modulating neuronal excitability and/or neurotransmitter release in different regions of brain, NA plays an important role in many physiological brain functions, such as attention and arousal (Mitchell & Weinshenker, 2010). It also affects the consolidation of long-term memory in the amygdala (Clem & Huganir, 2013) and hippocampus (O'Dell et al., 2015;; Walling et al., 2016). Furthermore, α1-AR antagonists are widely used in the treatment of psychotic episodes (Wadenberg et al., 2000;; Ma et al., 2006;; Lopez-Gil et al., 2010).