Behavioral Neuroscience, lecture on Leech and its behavior
USD Department of Biology
Behavioral Neuroscience
Summers
Leech Shortening
Sensory Stimulation of Shortening
S&R Sensitization
Learning
Motor Efferents
Neuromuscular Function
Integration
Glu
5-HT
ACh
Leech figures
Shortening Neurocircuitry
end     Acronyms/Abbreviations
Sensitization - Leech
VI.  S-R Sensitization 			back to  Afferent Sensory Input

	A. S-cell activated during leech body shortening
	
		1. But... shortening NOT triggered by S-cell stimulation
	
			a. S-cell activity by itself does not cause shortening
			
				i. Lesioning S-cell does not abolish shortening
			
			b. Shortening not dependent on intact S-cell chain
				
	B. S cell important for modification of shortening
		
		1. Critical for plasticity/learning (such as sensitization)
					
			a. Lesion S sensitization of shortening reflex eliminated
							
				i. 5-HT depletion  also eliminates sensitization
					
					(1) ability to shorten unchanged

			b. S cell - 5-HT interaction
				
	C. Sensitization
	
		1. Sensitizing stimuli 2 - 3X S-cell activity
		
			a. inluding noxious N stimuli
		
		2. Sensitization initiated
		
			a. S cell necessary
			
		3. S-cell activity propagates down the nerve cord
		
			a. S signals  all of the leech CNS 
			
			b. S cell chain = fast conducting system (FCS) 

				i. S cell axon is largest 
				
				ii. \ fastest conduction velocity 
				
		4. S-cell  synaptic connections with Retzius cells
		
			a. Retzius cells (R cells) are serotonergic (5-HT) neurons
			
				i. R = largest cells in the ganglion
				
				ii. Strong stimuli that induce sensitization strongly activate Retzius cells
			
			b. Stimulus S activity  R  releases 5-HT  sensitization

			c. S cell transmitter colocalization
			
				i. terminals contain large and small vesicles
				
					(1) small vesicles - Glu
					
						(a) bind AMPA-NMDA on R
						
					(2) large vesicles = peptide

		5. S contains Myomodulin
		
			a. Excitatory Neuropeptide 
			
				i. Depolarizes R cells
				
			b. Sensitizing stimuli  S-cell myomodulin release? 
	
		6. Positive Feedback loop
		
	D. Maintenance of sensitization 
	
		1. After sensitizing stimuli
		
			a. S-cell activity  during shortening
		
				i. +correlation S activity: intensity of shortening response
			
					(1) No correlation in unsensitized S cells
				
		2. D sensitization response due to  activity of S-cell
		
			a. 5-HT  S-cell excitability*
			
				i. 5-HT7 receptor
				
					(1)  GsACcAMPPKA
					
					(2) & modulates ion channels
				
				ii.  firing rate +  stimulus threshold needed
				
				iii.   S-cell afterhyperpolarization
				
					(1) *smaller AHP  quicker 2nd action potential
					
					(2) mediated by Ca++ dependent K+ channels
					                                      K leaving the neuron makes the membrane potential fall
	
					
	E. S-cell is likely recruited during sensitization
		
		1. Recruited through its high firing rate
			
			a. S cells occur in all ganglia
			
			b. S-cell synapses onto L motor neurons 
			
				i. contribute to L activity early in shortening reflex
			
					(1) Nonsensitized shortening: 
					    S-cell activity rate = low 
					
						(a) little or no significant influence 
						    on motor neurons

			c. Sensitized shortening  S-cell activity 
			
				i. L motor neuron activity 

		2. S-cell may also be maintaining an increased arousal state
		
			a. through serotonergic signaling with R cells
			
				i. 5-HT released from Retzius cells in a paracrine manner
			
			b. S to R connection  5-HT release
			
				i.  heightened arousal
				
					(1) sensitized shortening response
					
					(2) reticular 5-HT neurons in vertebrates
					    play an important role in arousal
	
			
	F. 5-HT & Sensitization vs Habituation of the Shortening Reflex 
 
		1. large Retzius 5-HT neurons activated by S-cell
		
			a.  5-HT release aids sensitization
			
				i. and dishabituation
				
					(1) Habituation reduces S-cell excitability
					
						(a) similarly to low [5-HT]
				
				ii. via 5-HT7 GscAMP & ion channels
					
			b.  5-HT depletion via 5,7-DHT (5,7-dihydroxytryptamine) toxin
		
				i.  eliminates sensitization
				
				ii. reduces but does not eliminate dishabituation
				
					(1) elimination vs reduction shows different physiological mechanisms
				
				iii. also disrupts classical conditioning of shortening reflex
				
					(1) also but does not eliminate conditioning
				
										
	G.  Positive feedback loop - critical for full sensitization of shortening  reflex

VII. Sensitization and Learning