It may be that a more appropriate model of resilient vs. susceptible individuals Gemcitabine concentration lies in assessment of a complex system of responses, rather than along a spectrum of freezing alone. Importantly, the behavioral characteristics of a susceptible female animal may be distinct from those of a susceptible male. This scenario would be consistent with human studies of PTSD symptomatology, which have found sex differences in the most frequently experienced symptoms. For example, women report more distractibility and emotional distress, while men report more emotional numbness and hypervigilance (King et al., 2013). Interestingly, measures of learned fear other than freezing

produce different outcomes in males and females. In classical eyeblink conditioning, a white noise repeatedly paired with a brief shock to an animal’s eyelid produces an anticipatory eyeblink response to subsequent presentations of the noise. Landmark work by Tracy Shors has consistently shown that female rats acquire the conditioned response more rapidly, and maintain higher levels of responding than male rats (Wood and Shors, 1998, Dalla and Shors, 2009 and Maeng and Shors, 2013). Whether eyeblink conditioning thus better taps into the circuits

and mechanisms that mediate sex differences observed in human populations is not clear, but in the following section, we discuss the sex-specific manner in which stress modulates learning in this model. In check details another paradigm, fear-potentiated startle (FPS), an animal is trained to associate a neutral stimulus with a footshock, as in fear conditioning. When a startling noise is later presented in the presence of the conditioned stimulus, animals have exaggerated, or potentiated, startle responses (Walker and Davis, 2002). Mazor et al. (2009) found that female rats had a greater baseline startle amplitude than males, an effect that has also been observed in mice (Adamec

et al., 2006). Toufexis et al. (2007) did not observe this sex difference; however, this group employed an extended conditioning paradigm which may have normalized the fear levels induced by the conditioned stimulus. The work discussed above demonstrates the serious many need for increased fear research in female animals. In many fear paradigms, consensus on the directionality of baseline sex differences has not been reached, something that can only be achieved with further efforts on the part of researchers to both replicate major findings and converge upon standard protocols. In the case of associative learning paradigms, whether the initial strength of the memory itself or the lasting persistence of that memory is a better marker for resilient and vulnerable phenotypes is still unknown. However, the possibility that these markers are different for males and females must be considered when interpreting experimental results.