Behavioral Neuroscience, lecture on Afferent Path and SCN
USD Department of Biology
Behavioral Neuroscience
Summers
Rhythmicity & Behavior
Sensory input for Rhythmicity
Afferent path to the SCN
Circadian Rhythms
rhythms Efferent SCN output
Integration of Rhythms into Behavior
Glu
VIP
AVP
GABA
5-HT
Figures of Rhythmicity
Retina-RGC-SCN
Molecular SCN
end  Acronyms/Abbreviations   Syllabus
BIOLOGICAL RHYTHMS
IV. Afferent Path to the SCN 			

	A. Rods and Cones of the Retina become hyperpolarized by light
	    
		1. light ® unbending (11th C cis®trans) retinal in rhodopsin
	   
		2. transrhodopsin PDE  cGMP
		
		3. 5'-GMP cannot bind to cyclic nucleotide-gated Na+ channels
		
		4.  normal Na+ influx  hyperpolarization
		
		5. hyperpolar action potential stimulates retinal ganglia cells
	
	   
	B. Photosensitive Melanopsin and Crypotchrome in RGC also
	   stimulated by light 	    

		1. both retinal and RGC signals required
	
	
	C. a small bundle of fibers from RGC in the optic nerve innervates
	     ventrolateral region (core) of SCN

		1. retinohypothalamic tract
		
			a. first branch from optic nerve
		
		2. RHT terminals release Glu and PACAP
		
			a. Ca++ CAM Kinase
			
			b. PKA pCREB

				i. kinases stimulate mPER1&2

				
	D. optic nerve also projects to the Intergeniculate Leaflet (IGL) of the
	   Lateral Geniculate Nucleus of the thalamus and superior colliculus
	   or optic tectum (homologues)

	   			
V. Suprachiasmatic Nucleus (SCN)

	A. 10,000 neurons in rat; 50,000 in humans
	
		1. small cells (8-12 mm)
		
		2. sparse dendritic arbors - remain mostly within SCN


	B. anterior border of the hypothalamus (just above the optic chiasm)

		1. core - ventrolateral - larger cells
		
			a. located directly above the optic chiasm

			b. receives 1o and 2o visual afferents

			c. contains neurons producing VIP and GRP
			
				i. colocalized with GABA

			d. optic nerve to IGL to SCN

				i. geniculohypothalamic tract (GHT)
				   returns information to SCN

					(1) GABA/NPY neurons

				ii. IGL/GHT integrates photic & nonphotic information

				iii. LGN also innevates the median/dorsal raph
			  	     (d raph innervates LGN)
				
				iv. paraventricular thalamus (PVT; also to shell)
				
			e. median raph innervates SCN core

				i. secretes 5-HT binds 5-HT7

		2. shell- dorsomedial - smaller cells surround core
		
			a. receives input from nonvisual sources
			
				i. hypothalamus
				
				ii. retrochiasmatic area
				
				iii. limbic structures
				
				iv. brainstem and PVT

			b. SCN shell contains neurons secreting AVP and calretinin
			
				i. colocalized with GABA


		3. SCN efferent projections are topographically
		   organized by core and shell


	C. SCN cells are autogenic - spontaneous neural firing in vitro

		1. very low resting potentials (near threshold)

		2. cyclic firing activity

			a. single cell fires at 24h periodicity

			b. high (8-10 Hz) during the day, low (2-4 Hz) at night

				i. intracellular [Cl-]  during the day, ¯ at night

				ii. inhibitory and excitatory GABA

VI. Vasoactive Intestinal Peptide (VIP)

VII. Arginine Vasopressin (AVP)

VIII. g-Aminobutyric Acid (GABA)

VIII.V Serotonin (5-HT)