Centrally administered corticotropin-releasing hormone and peripheral injections of strychnine hydrochloride potentiate the acoustic startle response in preweanling rats.

Abstract

Attempts to condition fear potentiation of startle (FPS) in rats younger than 23 days of age have not been successful, regardless of the type of aversively conditioned stimulus used (P. S. Hunt, R. Richardson, & B. A. Campbell, 1994; R. Richardson, G. Paxinos, & J. Lee, 2000; R. Richardson & A. Vishney, 2000). In the present study, the authors report that peripheral injections of strychnine hydrochloride, a glycine receptor antagonist, and intracerebroventricular infusions of corticotropin releasing hormone (CRH) both potentiated the acoustic startle response (ASR) in 16-18-day-old rats. Because strychnine and CRH have distinct sites of activation in the primary startle pathway, it can be concluded that this pathway is functional and modifiable in rats younger than 23 days of age. This finding suggests that the failure to observe conditioned FPS in preweanling rats is due to an immaturity of the secondary fear circuit responsible for enhancing the ASR during a fear state.

Behavioral expression of learned fear in rats is appropriate to their age at training, not their age at testing

“…First, given that rats youngerthan 23 days of age reliably exhibit learned fear when measures such as freezing are recorded (see , for a review), there is no doubt that the amygdala undergoes the necessary neural plasticity for exhibiting learned fear. Second, recent evidence has shown that neural activity in the primary startle pathway can be pharmacologically increased, thus leading to larger startle responses in rats as young as 16 days of age (the youngest age tested; Weber & Richardson, 2001). Taken together, this shows that all the pieces required for conditioned fear potentiation of startle being observed are present in rats younger than 23 days of age; however, they simply do not exhibit the effect.…”

Behavioral and Anatomical Interactions between Dopamine and Corticotropin-Releasing Factor in the Rat

“…In rats, an increase in startle amplitude by intracerebroventricular CRF has been demonstrated reliably in many laboratories (Weber and Richardson, 2001;Conti et al, 2002;Servatius et al, 2005), is predominantly mediated by CRF1 receptors (Jones et al, 1998), and has been shown to be mostly independent of hypothalamicpituitary-adrenal axis activity (Lee et al, 1994) and autonomic (sympathetic) nervous system activation (Liang et al, 1992b). Here, we report for the first time that dopaminergic neurotransmission plays an important role in CRF-enhanced startle.…”

Updating memories: Changing the involvement of the prelimbic cortex in the expression of an infant fear memory

“…After surgery, rats were subcutaneously injected with 0.03-0.05 ml each of cephazolin (100 mg/ml) and benacillin (300 mg/ml). Previous studies have demonstrated that the P23 rat recovers from surgery well within 24 h (Weber and Richardson, 2001;Kim and Richardson, 2008;Kim et al, 2009;Li et al, 2012). P35 rats were weighed to monitor their recovery, and all rats showed comparable weight gain compared to their unoperated littermates.…”