Catecholamines 9/5/2018

Catecholamines Neurotransmitters Distribution Synthesis Vesicular Storage Inactivation Regulation Release

Chemical structure of biogenic amines First identified in the 1950’s in the peripheral NS, later in the brain.

NE in the PNS

Functional Roles of DA in the CNS Mesolimbic: Role in addiction, reward, mood, dysfunction associated with mental disorders Mesocortical: Role in attention, mood, decision making, dysfunction associated with mental disorders Nigrostriatal: Role in movement initiation, dysfunction associated with Parkinson’s disease

Functional roles for NE in CNS

LC neurons firing rate Copyright © 2004 Allyn and Bacon

Remember 201 lab—analogs of tyrosine inhibit TH (alpha-methyl-tyrosine)

Regulatory elements to control expression levels and spatial distribution. For example, DBH must be suppressed in neurons that release DA.

Vesicular storage of NE Vesicle storage: protect from degradation, provide releasable pool of NT. SDCV release NE under physiologic conditions, mostly newly synthesized or imported NT in SDCV. LDCV contain DBH and are the site of NE synthesis; they may be released under demanding/stressful conditions. Then DBH also released. LDCV are more often found in chromaffin cells and the turnover of CA in them is very low (>1 day half life). In SDCV, NE half life is 3-43 minutes—requires rapid refilling of vesicles to maintain supply of releasable NT.

Effects of VMAT2 loss of function mutation % of control catecholamine levels in VMAT2 KO mice Again from 201 lab, inhibiting VMAT with reserpine depleted CA levels. Behavioral effects?

Role of VMAT2 in DA neurons VMAT2 deficient mice have lower striatal DA, a-synuclein accumulation, and SN DA neuron loss. VMAT2 loss of function also affects NE and probably 5-HT uptake into vesicles.

Similarities?

Inactivation: Reuptake 12 transmembrane regions, phosphorylation sites on inside of cell regulate activity (decrease) Drugs that act here: ritalin, MPP+, 6-OHDA, amphetamines, cocaine

Inactivation: Extracellular degradation Degradation pathways- may take several routes, using MAO and COMT. Storage in vesicles protects from intracellular MAO-A

COMT polymorphisms COMT-15% striatal DA -60% pre-frontal DA How common COMT polymorphisms affect levels of the protein soluble>membrane bound Met form, soluble, lower activity=dopamine in synapses longer An excerpt from Diamond review: Might it be possible that females have higher (more nearly optimal) baseline levels of dopamine whereas males have slightly too low baseline levels of dopamine? Hence, males’ performance improves when dopamine levels are slightly increased but females’ performance does not. This would be consistent with males being more susceptible to disorders of too little dopamine (e.g., attention deficit hyperactivity disorder [ADHD]) and females being more susceptible to disorders of too much dopamine (e.g., depression). Certainly, there is evidence that COMT activity is reduced epigenetically by estrogen and that COMT enzyme activity is roughly 30% lower in females than males (Cohn and Axelrod 1971; Boudikova et al. 1990). If this line of reasoning were correct, then having a still more sluggish COMT enzyme would not be beneficial to women. Indeed, whereas the more sluggish methionine-containing COMT enzyme might be more beneficial to men; the faster-acting valine version of the COMT enzyme might be more beneficial to women. I contacted Art and Paige, “Quick. Analyze your results separately for men and women.” Everything fell into place. Males homozygous for the methionine version of the COMT gene performed much better on the Wisconsin Card Sort; whereas females homozygous for the valine version of the COMT gene showed superior performance on the task (Scaile et al., in preparation). We would predict that this gender difference in the effects of COMT genotype on prefrontally-dependent cognitive functions should be even greater in young adults, because young women have higher circulating levels of estrogen, and estrogen facilitates dopamine tone and downregulates COMT enzymatic activity (Ho et al. 2006). COMT-15% striatal DA -60% pre-frontal DA Chen et al, 2004 AJHG

COMT polymorphisms Sex differences in effect of polymorphism Females: high DA, lower levels of COMT activity, no effect of met isoform Males: low DA, higher COMT activity, improve with met isoform Idea of optimal DA concentrations, especially in PFC Animals lacking COMT have higher DA in prefrontal cortex Explain differential susceptibility to DA related illness ADHD too little DA (men) Depression too much DA (women)

Regulation of CA synthesis Important to maintain supply of CA over range of activity End product inhibition works best for NE neurons: Two types: local-catechol binds cofactor BH4, further limiting its availability to act as a cofactor to TH and make more dopamine Long-term-catechol binds Fe3+ site of TH almost irreversibly inhibiting its activity Both types are relieved by phosphorylation of N terminal serines (see next slides)

Phosphorylation affects kinetic properties of TH Phosphorylation of TH changes affinity for the cofactor, can increase activity for PTH4 (BH4) and decrease affinity for NT Long term or drug induced regulation involves new mRNA and protein synthesis

Regulation by neural stimulation Neural stimulation increases TH activity (top) Stimulation increases the affinity for the cofactor BH4 (decreases the Km) (bottom left) Stimulation decreases the affinity for the transmitter NE (increases the Ki) (bottom right) These changes may be mediated by phosphorylation events

Release of CA Ca2+ dependent exocytosis (DBH release w LDCV) Release at varicosities

Feedback: phentolamine is a reversible a-adrenergic receptor antagonist. Blocking receptors leads to greater evoked release of NE. Possibly through autoreceptors?

Role of pre-synaptic autoreceptors 1. Inhibit release by blocking Ca entry 2. Blocks release by inhibiting synthesis and vesicle uptake Heteromers of dopamine D2 autoreceptor (short isoform of the D2 receptor or D2SR) and non-α7 nicotinic acetylcholine receptor (non-α7 nAChR) represent an important mechanism by which D2SR modulates dopamine (DA) release, in addition to D2SR-mediated modulation of dopamine synthesis and dopamine transport to the vesicles and a D2SR-mediated stimulation of GIRKs. In the D2SR-non-α7 nAChR heteromeric complex, D2SR stimulation decreases the effects of non-α7 nAChR activation, which induces dopamine release by directly acting on the protein machinery involved in vesicular fusion (vesicle- and plasma-membrane proteins: v-prot. and p-prot., respectively), through Ca2+ influx, or by activating N- and P/Q-type voltage-dependent calcium channels (VDCCs). In black and red, stimulatory and inhibitory effects, respectively. Tyr, tyrosine; VMAT, vesicular monoamine transporter.

Tonic vs bursting activity Tonic firing: single spikes, all DA typically cleared after the spike Burst firing: released DA can accumulate-amount depends on regional variations in DAT levels D2 autoreceptor antagonists increase overall firing rate and amount of bursting

Tonic-single spike vs burst in a spiking VTA DA neuron Extracellular recordings. Note time scale difference-1 sec vs 50 ms.