The present findings may be the first to report that injected NGF has an antidepressant-like effect; a preliminary report of these findings has been presented (Overstreet et al., 2004a
). The reduction in immobility after chronic NGF treatment was seen under a variety of conditions. Several different doses of NGF reduced the exaggerated immobility (increased swimming) of the FSL rat and they tended not to counteract the low social interaction behavior and had mixed effects on the reduced locomotor activity. Furthermore, chronic treatment with NGF reduced immobility in outbred Sprague-Dawley rats to an extent similar to fluoxetine but not as strong as DMI. These findings are consistent with the suggestions of others that NGF may play a role in depression (Angelucci et al., 2000
). It is quite remarkable that these antidepressant effects can be observed with such low doses. The standard concentration of NGF, which was used for most of these studies, was 40 ng/ml. With the rats in these studies weighing an average of 400 grams, a dose of 100 ng/kg was used. Most other antidepressant compounds require higher doses; for example, DMI and fluoxetine are typically administered in a dose of 5 mg/kg (Overstreet et al., 2005
) and others may require 10 mg/kg or more (Overstreet and Griebel, 2004
; Overstreet et al., 2004b
The fact that NGF treatment also increases locomotor activity under some conditions raises the possibility that the reduction in immobility is a simple consequence of the stimulant effects of NGF. However, that cannot be the case, as there were several instances where immobility was reduced without an effect on locomotor activity. Data from other studies reinforce the view that antidepressant effects on locomotor activity are independent of those on immobility. In a recent study, for example, both citalopram and DMI reduced swim test immobility in the FSL rats, but only citalopram increased locomotor activity (Overstreet et al., 2004b
). Earlier on, it was reported that the psychomotor stimulants like amphetamine and scopolamine do not decrease immobility (Overstreet et al., 1995
). A key point about the present data is that NFG treatment consistently reduced immobility under a variety of conditions but the effects on locomotor activity are only seen occasionally.
To integrate these results we can describe the profile for DMI and then determine which, if any, of the NGF treatments resemble this profile. As indicated earlier, DMI increases swimming to the level of the FRL rats () and also increases social interaction (), but does not affect locomotor activity (). 2.96 pg and 14.8 pg of NGF normalize swimming (); but do not increase social interaction () or locomotor activity (). Drugs that increase activity are not looked upon favorably as potential antidepressants because of possible nonspecific stimulant effects. Therefore, the robust effects on swimming of 8 and 40 ng of NGF are problematic. However, this increase is still below the activity exhibited by the FRL rats (), so it may be a partial normalization. Under this view, 2.96 pg and 8 and 40 ng of NGF would be the best because they tend to normalize each of the abnormal behavioral responses of the FSL rats.
In an attempt to elucidate the basis of the antidepressant effects of NGF, we examined three paradigms that have been commonly used to investigate the mechanisms of action of antidepressants. The first was regional brain c-fos expression. This measure was recorded both after acute treatment with NGF and after chronic treatment in which the animals were sacrificed two hours after exposure to the swim tank. Neither of these procedures provided any evidence that NGF was acting like typical antidepressants. Both Beck (1995)
and Dahmen et al. (1997)
reported that a variety of antidepressants increased c-fos expression in the central amygdala following acute administration. Yet there was no such change after acute treatment with NGF ().
The present study included groups that were chronically treated with NGF at a dosage schedule that reduced swim test immobility. Despite the significant behavioral difference between the NGF- and vehicle-treated rats, they exhibited similar c-fos expression (). In contrast, other antidepressants alter the c-fos response to swim stress following chronic treatment (Duncan et al., 1996
). Thus, NGF appears to be an antidepressant without the biochemical properties associated with other compounds. Although the rats were handled prior to the acute injections, there is still the possibility that the effects of injection stress obscured any effects of NGF itself on c-fos expression. All we can conclude is that the effects of NGF were not different from the effects of saline vehicle. However, other studies have concluded that the effects of other antidepressants on c-fos expression do differ from saline vehicle (Beck, 1995
; Dahmen et al., 1997
; Duncan et al., 1996
Another paradigm that has been utilized in an attempt to clarify the mechanism of action of antidepressants is fluoxetine-induced elevation of corticosterone. Duncan et al. (1998)
demonstrated that the corticosterone response to acute fluoxetine treatment was blunted following chronic treatment with a variety of antidepressants. The present study confirmed that chronic treatment with DMI would blunt the corticosterone response to fluoxetine (). However, chronic treatment with NGF had no such effect. At the same time, this study confirmed the antidepressant-like effects of NGF (). Thus, other antidepressants are behaving differently from NGF.
The last paradigm to be considered is the functional down-regulation of 5-HT1A
receptors following chronic treatment with antidepressants (Albert et al., 1996
; Gray and Roth, 2001
; Overstreet et al., 2005
; Stahl, 1994
; Toth, 1996
). The present report confirmed previous observations regarding fluoxetine: chronic treatment with this SSRI blunted the hypothermic response to 8-OH-DPAT, a 5-HT1A
receptor agonist, as well as a reduced behavioral response to DOI, a 5-HT2A/C
receptor agonist (). However, the responses to these two agonists were unchanged in rats chronically treated with NGF (), despite both compounds reducing swim test immobility. Thus, although NGF clearly has antidepressant-like properties, it does not induce a biochemical profile similar to fluoxetine.
The data presented in this paper allow us to suggest that NGF is a novel antidepressant because it does not alter c-fos expression in a way similar to other antidepressants or induce a blunting of the corticosterone response to fluoxetine or a down-regulation of 5-HT1A of 5-HT2A receptors. What about other mechanisms that might be involved.?
An important consideration is whether NGF is having its antidepressant properties via actions in the brain or in the periphery. The conventional wisdom is that NGF does not readily penetrate the blood-brain barrier, so its effects must be related to actions at some peripheral site. However, there is some evidence that NGF does indeed cross the blood-brain barrier (Kastin et al., 1999
; Poduslo and Curran, 1996
), so we cannot determine whether peripheral or central actions are responsible for it effects. However, it is relevant that stimulation of the vagus nerve, essentially a peripheral treatment but with central consequences, has antidepressant-like effects (Nemeroff et al., 2006
; Park et al., 2007
Another consideration is whether the well-known trophic effects of NGF on cholinergic neurons (e.g., Huy et al., 2008
; Williams et al., 1991
) could be responsible for its antidepressant-like effects. The evidence suggests that this mechanism is more likely to increase immobility, rather than decrease it. Rats with increased cholinergic function tend to have elevated swim test immobility (Overstreet, 2002
; Overstreet et al., 1998
). Furthermore, there is little evidence that cholinergic antagonists have antidepressant-like effects even though they may increase activity (Overstreet et al., 1995
). It is unlikely, therefore, that the trophic effects of NGF on cholinergic neurons contributes to its antidepressant-like effects.
In conclusion, the antidepressant potential for NGF has been established by demonstrating that it reduces swim test immobility in both the FSL rat model of depression and in older outbred Sprague-Dawley rats. However, no effects were seen on c-fos expression, 5-HT1A-mediated hypothermia, fluoxetine-induced increases in corticosterone, and 5-HT2A-mediated head shakes. Thus, NGF has actions predictive of antidepressant efficacy with a novel, as yet undetermined, mechanism of action.