Activation of orexin/hypocretin projections to basal forebrain and paraventricular thalamus by acute nicotine

Abstract

Orexin/hypocretin neurons of the lateral hypothalamus/perifornical area project to a diverse array of brain regions and are responsive to a variety of psychostimulant drugs. It has been shown that orexin neurons are activated by systemic nicotine adminstration suggesting a possible orexinergic contribution to the effects of this drug on arousal and cognitive function. The basal forebrain and paraventricular nucleus of the dorsal thalamus (PVT) both receive orexin inputs and have been implicated in arousal, attention and psychostimulant drug responses. However, it is unknown whether orexin inputs to these areas are activated by psychostimulant drugs such as nicotine. Here, we infused the retrograde tract tracer cholera toxin B subunit (CTb) into either the basal forebrain or PVT of adult male rats. Seven to 10 days later, animals received an acute systemic administration of (−) nicotine hydrogen tartrate or vehicle and were euthanized 2 h later. Triple-label immunohistochemistry/immunofluorescence was used to detect Fos expression in retrogradely-labeled orexin neurons. Nicotine increased Fos expression in orexin neurons projecting to both basal forebrain and PVT. The relative activation in lateral and medial banks of retrogradely-labeled orexin neurons was similar following basal forebrain CTb deposits, but was more pronounced in the medial bank following PVT deposits of CTb. Our findings suggest that orexin inputs to the basal forebrain and PVT may contribute to nicotine effects on arousal and cognition and provide further support for the existence of functional heterogeneity across the medial-lateral distribution of orexin neurons.

Introduction

The orexins (orexin A and B; also known as hypocretin 1 and 2) are neuropeptides expressed in neurons of the lateral hypothalamus and contiguous perifornical area (LH/PFA) of the mammalian brain [7], [34]. Consistent with the role of these peptides in arousal and modulation of state-dependent behavior [2], [38], orexin neurons are activated by a variety of different psychostimulant drugs, including d-amphetamine [12], methamphetamine [11], modafinil [38] and caffeine [27]. We have recently shown that acute nicotine administration also activates orexin neurons as measured by expression of the immediate-early gene product, Fos [30]. Nicotine also upregulates expression of orexins and orexin receptors in the rat brain [22]. These studies strongly suggest that the orexin system is a common target of acute psychostimulant treatment and may, in turn, mediate some of the behavioral, reinforcing, rewarding and addictive effects of many drugs of abuse. Indeed, orexin peptides have been strongly implicated in responses to cocaine and morphine [3], [18].

While the rodent brain contains only a few thousand orexin neurons, the target projections of this relatively small group of cells are impressively diverse [32]. One way that the orexin system might accomplish its wide array of divergent functions is through stimulus-dependent activation of different target projections. Some functional and anatomical heterogeneity has already been suggested for this system, with orexin neurons of the lateral hypothalamus implicated in the rewarding properties of natural and pharmacological reinforcers while more medially-located orexin neurons may regulate arousal and stress responses [17]. Functional and connectional differences across the medial-lateral gradient of orexin neurons have also been suggested based on studies investigating Fos expression in orexin neurons as a function of behavioral state [11] and antipsychotic drug responses [12].

Rostral targets that could contribute to the effect of orexins on arousal and attention include the basal forebrain and the paraventricular nucleus of the thalamus (PVT). These areas receive moderate to dense orexin innervation [10], [14], [23], [32] and play important roles in cortical activation in support of arousal and attention. Functionally, intrabasalis administration of OxA promotes wakefulness [9], [40], increases prefrontal cortical ACh release [14] and excites cholinergic neurons [8]. Wakefulness-related release of OxA within the basal forebrain has also been reported [24].

Similarly, recent studies indicate that orexins increase firing in PVT neurons, implicating a role in wakefulness [19], [20]. Collectively, these background data suggest that orexin projections to both the basal forebrain and PVT may be part of the neural substrates underlying nicotine effects on arousal, wakefulness and attention. Here, we combined neuronal tract-tracing using the retrograde tracer cholera toxin b (CTb) with Fos expression to examine the effect of acute nicotine administration on orexin neurons projecting to the basal forebrain or PVT.