Behavioral Neuroscience, lecture on BNDF in Fear Conditioning
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Behavioral Neuroscience
Summers
Fear
Afferent Path for Fear
Amygdala
Fear Conditioning
Pavlovian potentiated startle Afferent Shock (US) Pathway
Efferent Output
Neuromuscular Production of Fear Potentiated Startle
Integration of Fear Conditioning
BDNF
Substance P
ACh
Fear figures
Fear Conditioning Circuitry
end     Acronyms/Abbreviations
Fear Conditioning
VI.V Brain Derived Neurotrophic Factor (BDNF) 		

	A. a neuropeptide trophic factor made in brain and muscle
	
		1. Also found in the circulation

	B. Synthesis and Release of BDNF

		1. BDNF mRNA translated into preproBDNF protein

		2. proBDNF is cleaved to mature BDNF by plasmin or BMPs

			a. plasminogen  tPA   plasmin

		3. Neural mechanisms for stimulation of BDNF synthesis

			a. Synaptic activity, NMDA, L-VSCC, Ca++   BDNF

		4. Environmental/Social stimulation of BDNF synthesis

			a. Exercise stimulates increased BDNF

			b. Learning stimulates increased BDNF

		5. Pathological/Disorder related stimulation of BDNF synthesis

		6. BDNF is released in 2 ways

			a. Constituitive secretory pathway

				i. Smaller amounts, constant release

			b. Regulated secretory pathway

				i. majority of release 

				ii. intracellular cleavage
				
	C. BDNF receptors - TrKB
	
		1. Receptors are the tropomyosin related kinase type - specifically TrKB
		
		2. proBDNF binds more to P75 neurotrophin receptors (P75NTR)
		
		3. mBDNF binds more readily to TrKB
		
			a. BDNF bound TrKB receptors are internalized (endocytosis)
			
			b. Internalized vesicles containing BDNF/ TrKB can be transported
			
				i. moved along cytoskeletal elements
			
				ii. by anterograde or retrograde transport
				
		4. 2nd Messenger Systems
		
			a. TrKB has tyrosine kinase elements - part of intracellular domain
			
				i. TrKB  PI3K  Akt
				
				ii. TrKB  PLC IP3Ca++PKC  NFkB
				
					1) NFkB induces BDNF transcription
					
				iii. TrKB  RAS MEK  ErK/MAPK RSK2 
				    CREB Ser133 phosphorylation
				
					1) pCREB binds to CRE on DNA
					
					2) CRE are present in BDNF promoters
					
					3) CREB also activated by CaMK2&4 and PKA 
					
			b. P75NTR   NF- kB
			
				i. P75NTR  RhoA-GTP
				
				ii. P75NTR  JnK
				
	D. BDNF gene is found on human chromosome 11
	
		1. BDNF in the human brain - 11 exons  (Mice 9)

			a. 10 promoters (1-VIIIh), 1 open reading frame (IX; ORF)
	
				i. Promoters = I, II, III, IV, V, Vh, VI, VII, VIII, VIIIh (h = human)

					1) In the 5'UTR

						a) 5' = upstream or beginning
						   UTR = untranslated region

				ii. hBDNF gene also has 5 transcriptional start sites (ATG)

					1) similar to TATA box: RNA Polymerase II binding sites

				iii. Promoters I and IV are activity dependent
				
					1)CRE present in BDNF promoters
				
				iv. ORF is translated into preproBDNF protein

					1) only coding portion is found in exon 9
					
					2) ORF ends with a 3'UTR 
					   (3' = tail or downstream DNA)

			b. BDNF gene can have a mutation at locus point 66

				i. Valine (Val) replaced by methionine (Met) = Val66Met

					1) Valine in position 66 is normal (Val/Val)

					2) Val/Met = heterozygote, Met/Met = mutant

			c. Produce multiple (18) mRNAs

		2. many mRNAs - but only one protein

		3. Epigenetics 

			a. heritable Ds in genome function without D in DNA sequence
			
			b. histones on DNA
			
				i. acetyl groups added = acetylation activates genes
			
					1) HAT = Histone Acetyltransferase
					
					2) HDAC = Histone Deacetylase
			
				ii. methyl groups = methylation = repress gene activity
			
					1) HMT = Histone Methyltransferase
					
					2) HDM = Histone Demethylase
				
				iii. MeCP2 - anchors repressor enzymes to histone
			
			c. DNA can also be methylated at the cpg (cytosine phosphate guanine) site
			
			d. Chronic Defeat Stress & Maternal Separation Stress can result  
			   in histone and/or DNA methylation
			
				i. methylation and deacetylation of IV promoter
				   silences BDNF gene
				
					1) methylation is reversible
					   via environmental interactions

	E. BDNF Regional localization

		1. Found in a range of tissue/cell types: CNS (brain and spinal cord), retina,
		   motor neurons, muscle, kidneys, prostate, and saliva

			a. high concentrations in hippocampus and cerebral cortex 

			b. e.g. made in  ventral tegmental area (VTA) & prefrontal cortex (PFC)

				i. anterograde transport: VTA  nucleus accumbens (NAc)

		2. Receptors in CNS, heart, lungs, kidney

		3. Differential sub-cellular localization

			a. Differential mRNA stability

			b. BDNF mRNA with long 3'UTR are localized in dendrites

				i. Long 3'UTR carries signal for transport to dendrite

			c. BDNF mRNA with short 3'UTR is restricted to the soma

				i. UTR transcription regulated separately

	F. BDNF Function

		1. Mature BDNF protein stimulates neurogenesis, growth and differentiation
		   of neurons and synapses, LTP, learning, pain sensitization, and addiction

		2. BDNF activates genes

			a. BDNF stimulates tPA, GAP-43, Ta1-tubulin genes 

				i. proBDNF  mBDNF cleavage
				
				ii. promotes axonal regeneration

			b. stimulates genes that promote LTP, learning & neural plasticity

		3. BDNF stimulated neuronal activity

			a. BDNF stimulates long-term potentiation (LTP)

				i. Early phase LTP (eLTP)

					1) Independent of protein synthesis

					2) enhance transmitter release and sensitization

					3) receptor trafficking, un-silencing synapses

						a) AMPA receptor subunit
						
						b) cytoskeletal structure

					4)  branching and spine density
					
					5) lasts up to 3h

						a) eLTP duration is variable

				ii. Late phase LTP (lLTP)

					1) Protein synthesis involved

					2) Associated with new synapse formation

					3) Lasts over 3h

			b. proBDNF  binds P75NTR   stimulates LTD

			
		4. BDNF and Learning
		
			a. BDNF expression is increased by learning & memory tasks
						
				i. passive avoidance, spatial, instrumental  
			
			b. BDNF or TrKB gene ablation results in learning impairments
			
			c. but... mBDNF over-expression learning & memory

				i. Increases anxiety behavior
		
			d. see dysfunction

	G. BDNF: Dysfunction and pathophysiology

		1. Affective Disorders
		
			a. G196A (Val66Met) mutation is associated with anxiety,
			    depression, feeding disorders, OCD, biopolar disorder,
			    asthma, alterations in episodic memory
			
				i. BDNF delivered in a small single dose  depression
			
			b. Depressed patients have  hippocampal, PFC, & plasma BDNF
			
			c. Depressed patients have higher BDNF in NAc and VTA
			
			d. Antidepressants  BDNF, TrKB, and neurogenesis
			
				i. Antidepressant effects: blocked by inhibiting BDNF
				   or neurogenesis

		2. Neuropathic Pain
		
			a. Pain stimulates elevated BDNF protein levels in spinal cord
			
				i. BDNF stimulates LTP in spinal cord dorsal horn C fibers
			
			b. BDNF in spinal cord can enhance pain reception & sensitivity
			
				i. potentially produces hyperalgesia and allodynia
				
		3. Addiction
		
			a. Cocaine  BDNF + phosphorylation of TrKB in NAc 
			
			b. BDNF/TrKB action in NAc enhances addiction and relapse
			
			c. stress and addiction
			
				i. Cocaine stimulates increased BDNF + TrKB in PFC
				
				ii. stress cocaine-induced BDNF, TrKB, ERK1&2 in PFC  
				
				iii. Stress  drug seeking, use and relapse due to
				      BDNF, TrKB and ERK in PFC following cocaine use

VI. Afferent Pathways for Fear; US: Shock