a predictor. Low CSF 5-HIAA predicts a higher rate of future suicidal acts, as well as the maximal seriousness of suicidal acts in the lifetime of the individual. More lethal suicide attempts are associated with low CSF 5-HIAA (Malone et al., 1996).
The evidence suggests that serotonin mediates inhibition of impulsive action. Low function of the serotonergic systems may predispose individuals to suicidal and other potentially harmful impulsive acts. Animal and human studies link low serotonin function to impulsive aggression. Impulsive aggression but not planned or predatory aggression correlates with low CSF 5-HIAA (Lidberg et al., 1985; Linnoila et al., 1983; Virkkunen et al., 1989a; Virkkunen et al., 1989b; Virkkunen et al., 1987) suggesting that impulsivity plays a role in suicide attempters and predicting a negative correlation of impulsivity and CSF 5-HIAA. Non-human primate studies find such a relationship between impulsivity and CSF 5-HIAA (Higley et al., 1996). On the other hand, no link has been consistently demonstrated between CSF 5-HIAA and depressed mood or hopelessness.
To assess the role of the serotonergic system in suicide attempters, researchers can measure the release of prolactin following administration of fenfluramine (see previous section in this chapter). This works because fenfluramine causes the release of serotonin and inhibits its reuptake. Serotonin in turn evokes the release of prolactin into the blood stream. One caveat with this measurement is that endogenous dopaminergic activity may also modulate the prolactin responses to fenfluramine and dopamine has been associated with depression (Kapur and Mann, 1992).
A blunted prolactin response appears to be associated with a history of suicide attempt (Coccaro et al., 1989; Correa et al., 2000; Malone et al., 1996; Mann et al., 1995; see Newman et al., 1998 for review). Coccaro et al. (1989) found a blunted prolactin response in patients with a personality disorder or major depression characterized by suicidal acts compared to similar patients without a history of suicide attempt. Lopez-lbor et al. (1988), however, did not find this correlation of reduced prolactin response with suicide attempts in patients with major depression, though they did find a relationship with severity of the diagnosis. Similarly Mann et al. (1995) found that significantly more (78 percent) of the younger depression cases had a blunted prolactin response compared to only 29 percent of the older group. These younger depressed people were also distinguished from the older group in clinical characteristics including higher frequency of comorbid borderline personality disorder, younger age at onset of the depression, greater lethal intentions for recent suicide