1.6.1 Learned H elplessness.
Learned helplessness (O v e rm ier & Seligm an, 1967) is a stress m odel o f depression in which a subset o f anim als following exposure to uncontrollable stress exhibit a deficit in learning to escape controllable stress. The behavioural condition is induced through application o f uncontrollable and unpredictable aversive stimuli. The condition can be induced in 10-20% o f rats with footshock, in a single training session o f betw'een 40 and 60 min depending on the sensitivity o f the rat strain and intensity o f the footshock. H elpless anim als have face validity as a m odel o f depression. Helpless a nim als have w eight loss, agitated locom otor behaviour, sleep changes, decreased libido, decreased learning in som e tests but not in spatial learning tasks and alterations in the HPA axis with elevated corticosterone which is not suppressible by dexam ethasone (Adrien et al. 1991; Dess et al. 1988; Dess et ai. 1989; G reenberg et al. 1989) Helpless anim als also show less preference for sucrose than control anim als indicative o f an anhedoniac state ( G a m b a ra n a et al. 2001; M inor et al. 1994; V ollm a yr & Henn 2003). H elpless anim als respond to antidepressant treatment. Intra-peritoneal a dm inistration o f antidepressants w'as found to reverse helplessness (i.e. induced escape behaviours) at day 5 o f treatment (S h e rm a n et al. 1982). Interestingly this replicates the lag p hase in a n tid e p re ssan t treatm ent seen in h u m an s and lends predictive validity to helplessness as a m odel o f depression. Furtherm ore B D N F a m olecule thought to m ediate anti-depressant activity (C o y le & D um an 200 3 ) reverses learned h e lp le ssn e ss b e h a v io u r (S h ira y a m a et al. 20 0 2 ; S iu c ia k et al. 1997). 5 - H Ti b r e c e p to r s a re u p - re g u la te d in the corte x , hippocam pus and septum o f learned helpless anim als (E dw ards et al. 1991). C om pared to nLH (non-learned helpless) hippocam pal slices, slices from LH (learned helpless) rats s h o w a s ig n ific a n t in cre ase in e n d o g e n o u s a n d K ^ -stim ulated 5-H T release (E dw a rds & Henn 1992). M oreover PC PA lesions that depleted 5-HT protect against the developm ent o f helplessness (B row n et al. 1982; E dw ards et al. 1986). Both beta- adrenergic and mu opioid receptors in the h ip p o ca m p u s and cortex are upregulated in
LH anim als (Henn 1993; Martin et al. 1990). H elplessness behaviour is also associated with dysregulation o f the HPA-axis. Follow ing inescapable shock corticosterone levels are ra is e d but are not s u p p re s s e d by d e x a m e th a s o n e ( G r e e n b e r g et al. 1989). F u r th e r m o r e a d r e n a l e c t o m i s e d rats a p p e a r m o re s e n s i t iv e to th e e f f e c ts o f uncontrollable shock (E dw ards et al. 1990).
1.6.2 C ongenital Learned H elplessness.
In an effort to increase the yield o f helpless anim als following shock training research groups have dev e lo p e d a congenital learned helplessness m odel in Sprague-D aw ley rats. In this model selective breeding takes place, m ating anim als susceptible to learned helplessness following shock exposure, in this w ay genes are selected which predispose for a helpless phenotype. As a control, anim als resistant to the learned helplessness p h e n o ty p e fo llo w in g tra in in g are m ated. A fte r m u ltip le g e n e ra tio n s tw o distinct p h e n o ty p e s have e volve d. Firstly a congenital L earned H elpless line (cL H ) which exhibit a helpless p h e n o ty p e in the absence o f uncontrollable shock. A second group term ed congenital n o n -L e a m e d Helpless (cN L H ) are a strain o f anim als resistant to the effects o f u n c o n tro lla b le shock exposure. U nlike learned helpless outbred S prague Dawley rats that becom e learned helpless following uncontrollable shock exposure cLH rats do not respond to antidepressant treatment. cLH and cN LH anim als show similar a cquisition o f the M orris w a te r m aze task (V o llm a y r et al. 2004) T h e y also show sim ilar m em o ry retrieval in the probe trial. Both cLH and cN L H a nim als acquire the FR l response for sucrose. S E R T is upregulated in the raphe nuclei o f cLH rats and 5 H Tib n iR N A is up reg u la te d in the raphe dorsalis o f c N L H rats (N e u m a ie r et al. 2002). Mu opioid receptors o f cLH rats are up regulated in the hippocam pus and cortex but dow n regulated in the h y pothalam us (Henn et al. 1993). There are no differences in the levels o f c o rtico s tero n e be tw e en n o n stre sse d and stressed cLH and cN L H rats (V o llm a y r et al. 2001). cLH rats have an altered responsiveness to early stress which result in c h anges to the HPA axis and the renin-angiotensin system (E dw a rds 1999; King et al. 1993; K ing & E dw ards 1999). M o re o v e r cLH a nim als have an im paired BD N F response to stress (V o llm a y r et al. 2001). Interestingly cN LH anim als show a
d e c re a s e in h ip p o c a m p a l n e u r o p e p tid e Y ( N P Y ) m R N A o f th e o r d e r o f 3 0 - 3 5 % w h e n c o m p a r e d to S D o r c L H a n im a ls ( L a c h m a n et al. 1992). In a d d itio n , c L H ra ts s h o w a ltered g e n e e x p r e s s io n in c lu d in g a d e c r e a s e in h i p p o c a m p a l C R E B , P K A , P K C a an d G S K - 3 p m R N A ( K o h e n et al. 2 0 0 3 ) . F in a lly , in th e h y p o t h a l a m u s o f c L H rats e x p re ssio n o f b cl-2 m R N A is increased.
1.6.3 Flinders Sensitive Line.
T h e F lin d e rs S e n s itiv e L ine ( F S L ) is a w i d e l y a c c e p t e d a n im a l m o d e l o f d e p r e s s i o n d e v e lo p e d o r ig in a lly at F lin d e rs U n iv e rsity , A u stra lia . It is a S p r a g u e - D a w le y line b re d s e le c tiv e ly for in d iv id u a ls s u p e r s e n s itiv e to c h o lin e r g ic a g e n ts ( O v e r s tr e e t & R u ssell 1982; O v e r s tr e e t 1986; O v e r s tr e e t et al. 1986). T h e log ic b e h in d th e d e v e lo p m e n t o f su ch a m o d e l c a m e fro m th e c h o l i n e r g ic h y p o t h e s i s o f d e p r e s s i o n ( J a n o w s k i et al.
1972). D e p r e s s e d h u m a n s are k n o w n to s h o w e x a g g e r a te d r e s p o n s e s to c h o l i n e r g ic a g o n is ts (J a n o w 's k y & R isc h 1984; J a n o w s k y & R isc h 1987). A ls o t r e a t m e n t w ith c h o l i n o m im e ti c s re s u lts in d e p r e s s e d m o o d , s h o r te n e d R E M la te n c y a n d e le v a tio n o f H P A fu n c tio n all o f w h ic h are s y m p t o m s o f d e p r e s s i o n (Ja n o w 'sk y et al. 1980). T h e F lin d e r s s e n s i t i v e lin e h a s fa ce, c o n s t r u c t a n d p r e d i c t i v e v a l i d i ty as a m o d e l o f d e p r e s s io n . F a c e v a lid ity is c o n f ir m e d b y th e o b s e r v a t io n s th a t w h e n c o m p a r e d to co n tro l a n im a ls F S L a n im a ls w e ig h less ( O v e r s tr e e t 1993) are less a c tiv e in an o p e n Held ( B u s h n e ll et al. 1995), h a v e e l e v a te d a m o u n t s o f R E M sle e p ( S h ir o m a n i et al. 1988; B e n c a et al. 1996) and h a v e a r e d u c e d in ta k e o f sa c c h a r in fo llo w in g e x p o s u r e to a c h r o n ic m ild s tre s s p a r a d ig m . ( P u c ilo w s k i et al. 1993). C o n s tr u c t v a lid ity h a s also b e e n s a tis f ie d . N u m e r o u s b i o c h e m i c a l / p h a r m a c o l o g i c a l h y p o t h e s e s h a v e b e e n p u t f o r w a r d to e x p l a i n d e p r e s s i o n . A m o n g s t th e m o s t i m p o r t a n t a m o n g t h e m is th e m o n o a m i n e h y p o t h e s i s o f d e p r e s s i o n in w h i c h d e p r e s s i o n in h u m a n s is lin k e d to a b e rra n t m o n o a m i n e a c tiv ity ( S c h ild k r a u te t 1995; M a e s & M e l t z e r 1995; J a n o w s k y et al. 1995). F S L a n im a ls h a v e a ltered m o n o a m i n e r g i c m e ta b o lis m c o n s is te n t w ith su ch a h y p o th e s is s u g g e s tin g c o n s tru c t valid ity o f th e m o d e l ( Z a n g e n et al. 1997; S e r o v a et al. 1998., Z a n g e n et al. 1999a; Z a n g e n et al. 1 999b). F in a lly p r e d ic t iv e v a lid ity w a s c o n f ir m e d w h e n it w a s s h o w n that, as in h u m a n s , c h r o n ic b ut no t a c u te tre a tm e n t w ith
know n anti-depressants was required to alleviate sym ptom s o f depression in this model (Overstreet et al. 1995).
1.6.4 M aternal Separation
Maternal separation is an animal model for early life stress. It has been suggested that e xposure to a dverse early life events m ay precipitate the dev e lo p m en t o f psychiatric disorders in adulthood (Heim & Nem eroff, 1999). M aternal separation has been shown to result in changes in neurochem ical, neuroendocrine and behavioural changes in the adult anim al (Cirulii et al. 2003). As these changes were reported to be long-lasting it was hypothesised that maternal separation m ay alter LTP in the Flinders depression model.
1.7 Aims.
Stress/depression is a debilitating disorder which affects ~ 10% o f the population at any giv en time. It is a disorde r w hich has serious im p lic a tio n s for both the individual concerned as well as for the econom y o f the W e ste rn W orld. M uch effort has been spent on de veloping stress/depression m odels. It is hoped that from the study o f these m odels effective pharm aceutical interventions for the treatm ent o f stress/depression m ay be developed. Currently, most pharm acological agents used for the treatm ent o f stress/depression and anxiety involves manipulation o f the serotonergic system.
It is widely accepted that stress affects synaptic plasticity. O ur laboratory has developed an acute mild stress model which is o f use in the study o f how synaptic plasticity m ay be affected by stress exposure. Xu et al. (1997) show ed that exposure to a mild elevated platform stress prevented the induction o f LTP in C A l . Further work by Shakesby et al. (2 0 0 2 ) has sh o w n that p h a rm a c o lo g ic a l m a n ip u la tio n o f the serotonergic system s ubseque nt to stress e xposure m ay rec o v e r LTP. T h e w ork p rese nted in this thesis develops further the experimental w ork presented by Shakesby et al. (2002).
The specific aim s o f this thesis were as follows;
( 1) To exam ine the effect o f elevated platform stress on the induction o f LTP in vivo.
(2) To investigate how' drugs that alter e xtracellular 5-H T c o n centration m ay affect the induction o f LTP in the acutely stressed animal.
(3) To investigate which m em bers o f the 5-H T receptor family are important in m ediating recovery o f LTP, i f any, following stress exposure.
(4) To investigate LTP induction and other electrophysiological param eters in 2 w idely recognised and accepted depression m odels: Learned helplessness and Flinders Sensitive Line.
It is hop e d that this w ork will c o n trib u te to our u n d e rs ta n d in g o f th e effects o f s tr e s s / d e p r e s s io n on s y n a p tic p l a s tic ity , a n d m a y in tu rn lead to i m p r o v e d pharm acological interventions to treat these psychiatric illnesses.