Our results indicate that hcrtr1 and 2 signaling oppose one another in the regulation of depression-like behaviors. More specifically, these data suggest that hcrtr1 and hcrtr2 signaling may act to counterbalance each other in regions of the brain that regulate antidepressant-like responses such as in the hippocampus, ventral tegmental area, or prefrontal cortex [23
] We postulate that states that favor activation of orexin neurons projecting to regions that express hcrtr1, such as occurs during highly stressful situations [24
], may create a negative affective state that worsens depression-like symptoms. The finding that orexin receptor signaling may promote a negative emotional state is consistent with previous reports demonstrating that orexin-A peptide promotes the development of anhedonia, increasing intracranial self-stimulation thresholds [8
], while re-instatement of drug seeking is blocked by the hcrtr1 antagonist SB-334867 [8
]. Our data is also in agreement with a significant number of studies demonstrating a role for hcrtr1 in the modulation of reward dependent behavior. The enhancement of cocaine [25
] and morphine place preference [26
] is dependent on hcrtr1 activation, while operant responding for palatable food also requires functional hcrtr1 receptor signaling [28
]. We contend that a negative emotional state may be produced by hcrtr1 signaling, acting to enhance both the conditioning to and responding for a particular reward.
Furthermore, our data also illustrate how a negative emotional state along with the development of anhedonia (based on other reports [8
]) may be enhanced, through disruption of hcrtr2 signaling, leaving hcrtr1 transmission intact. For example, states that favor activation of hcrtr2 expressing neurons, such as calorie restriction, may promote antidepressant-like responses [13
]. An important prediction of this hypothesis is that unique hcrtr1 and hcrtr2 neural circuits exist with potentially distinct patterns of innervation and activation by different stimuli. The existence of unique circuits would allow for the orexin system to integrate various inputs into coordinated behavioral and physiological responses. Interestingly, recent data highlight the differential regulation of orexin receptor expression in the medial prefrontal cortex. In this particular example, the authors describe a shift in orexin receptor expression, with enhanced hcrtr1 expression accompanying a trend in the reduction of hcrtr2 expression [29
]. In this case, the change in orexin receptor expression is expected to drive operant responding following exposure to social stress. Our data compliment these findings, as we demonstrate that changes in orexin receptor expression modulate depression like behavior.
Testing this hypothesis, that selective activation of orexin receptor subtypes can produce distinct effects on behavior, can subsequently be examined using the mouse model systems described for the first time in this report. Using cre recombinase delivered either virally or expressed from a transgene, restoration of either hcrtr1 or 2 may be accomplished in select brain nuclei. These receptor-restored mice could then be analyzed for depression and anxiety-like behavior, permitting a mapping of the sites of orexin receptor expression that are responsible for the modulation of affect.
The transcriptionally blocked orexin receptor mice described here are a valuable tool in the investigation of orexinergic modulation of behavior. While developmental compensation, due to embryonic gene deletion, is often a concern when analyzing genetic mutations, there is no evidence, at least in terms of the effect of orexin on depression like behavior, that deletion of orexin receptors has produced such compensation in the reactivatable hcrtr1 and 2 lines. While deficits in sleep state transitioning are clearly evident in the transcriptionally blocked hcrtr2 null [11
], data presented in this report demonstrates that the hcrtr1 selective antagonist SB-334867 mimics the effect of the hcrtr1 deletion on depression like behavior.
The studies presented here are the first to describe the effect of hcrtr1 and 2 receptor-specific modulation of depression like behavior. Interestingly, in limited literature describing orexin action in modulation of depression like behavior, chronic hcrtr1 receptor antagonism was shown to enhance the presentation of depression-like behavior [30
]. Our data is then highly relevant to the discussion of the role of orexin receptor-acting compounds in the treatment of depression; that there may be significant differences in the modulation of affect between the acute and chronic actions of orexin receptor targeted therapeutics. Alternately, the use of the non-selective hcrtr receptor agonist, orexin-A, along with the hcrtr1 selective antagonist at potentially non-selective doses in vivo, may result in the non-specific targeting of both hcrtr1 and 2. This would make it difficult to discern any differences in the modulation of depression like behavior by either hcrtr1 or 2.
Our data suggests that modulation of depression like behavior depends on the balance between hcrtr1 and 2 receptor signaling, thus the use of non-selective hcrtr1 and 2 agonists and antagonists at non-selective doses would not be expected to be able to discern the true roles of both receptors in the modulation of behavior.
Finally, considering that the orexin system has become an attractive target for the development of novel pharmacologic treatments for a variety of neuropsychiatric disorders including narcolepsy, insomnia, and drug addiction, careful monitoring of behavioral responses to these medications will be required to determine the potential for both novel applications as well as potentially unwanted side effects, based on the conclusions drawn from our studies.