Behavioral Neuroscience, lecture on Leech and its behavior
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 àsensitization of shortening reflex eliminated
i. 5-HT depletion à also eliminates sensitization
(1) ability to shorten unchanged
b. S cell - 5-HT interaction
C. Sensitization
1. Sensitizing stimuli È2 - 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 Ü 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 Ü 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 ÈS activity È R È releases 5-HT È 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 È S-cell Èmyomodulin release?
6. Positive Feedback loop
D. Maintenance of sensitization
1. After sensitizing stimuli
a. S-cell activity È during shortening
i. +correlation S activity: intensity of shortening response
(1) No correlation in unsensitized S cells
2. D sensitization response due to á activity of S-cell
a. 5-HT È S-cell excitability*
i. 5-HT7 receptor
(1) GsÈACÈcAMPÈPKA
(2) & modulates ion channels
ii. È firing rate + Ê stimulus threshold needed
iii. ò S-cell afterhyperpolarization
(1) *smaller AHP Ü quicker 2nd action potential
(2) mediated by Ca++ dependent K+ channels
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 È S-cell activity
i. ÈL 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 È 5-HT release
i. È heightened arousal
(1) Èsensitized 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 GsÈcAMP & 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