The emergence of kisspeptin as a crucial regulator of the hypothalamo-pituitary-gonadal (HPG) axis over the last 14 years has answered many questions as to the control of reproductive hormone secretion from the hypothalamus. More recently, the role of kisspeptin outside the HPG axis has received increasing attention in the hope of delineating the pathways linking various sensory and social behaviours to reproduction. These studies, in a range of species from zebrafish to humans, have identified a role for kisspeptin in behavioural networks related to reproduction including olfaction, audition, fear, anxiety, mood, and sexual arousal. The available evidence suggests that extrahypothalamic kisspeptin signalling encourages positive aspects of emotional and sexual brain processing in a presumed drive towards reproduction and ultimately maintenance of the species at a population level. In this review, we examine these studies, which collectively propose that kisspeptin may integrate sexual and emotional brain processing with the control of the HPG axis.
Reproduction is required for survival of all species. The success of reproduction, however, depends on a multitude of ingredients encompassing anatomical, metabolic, hormonal, and psychological factors. These factors interplay to achieve reproductive success, but in some species such as humans, reproduction is not always the desired outcome. In humans, sexual and emotional brain processing can provide sought-after reward and satisfaction without the need for reproduction.
While we classically think of testosterone as the main reproductive hormone governing sexual brain processing to maintain libido, significant evidence suggests that other factors are also involved. Kisspeptin describes a family of peptide hormones of varying amino acid length cleaved from the product of the KISS1 gene in primates (including humans) and the Kiss1 gene in non-primates. The kisspeptin peptides share a common carboxy-terminal sequence necessary for their action on kisspeptin receptors (encoded by KISS1R/Kiss1r). Although initially identified for their role in metastasis suppression in 1996 , in 2003, two seminal papers identified the critical importance of kisspeptin signalling in reproduction, demonstrating that absence of KISS1R results in failure of puberty and subsequent infertility [2,3]. A similar failure of puberty has more recently been observed in the absence of KISS1 . Conversely, activating mutations of KISS1 and KISS1R lead to central precocious puberty [5,6]. Therefore, kisspeptin and its cognate receptor are obligate for successful reproduction. These findings have spawned a plethora of studies that place kisspeptin signalling at the apex of the hypothalamo-pituitary-gonadal (HPG) axis, and examined the various factors governing these signals and their resultant effects on the downstream gonadotrophins and gonadal sex steroids .
Kisspeptin is secreted by kisspeptin neurones within the hypothalamus and activates kisspeptin receptors upon gonadotrophin-releasing hormone (GnRH) neurones resulting in GnRH release into the local hypophyseal-portal circulation. This GnRH arrives at the gonadotrophs of the anterior pituitary, and stimulates the release of the gonadotrophins (luteinising hormone [LH] and follicle stimulating hormone [FSH]) into the systemic circulation. The gonadotrophins then stimulate the gonads to release testosterone in males, and oestradiol as well as progesterone in females. However, kisspeptin signalling is not limited to the hypothalamus but also occurs in other extrahypothalamic brain regions. It is these locations for kisspeptin signalling that gave the first clues for kisspeptin's role in sexual and emotional processing.
In this review, we examine the literature to date supporting a key role for kisspeptin in sexual and emotional brain processing in various species. We identified the relevant publications by a series of PubMed searches, employing relevant keywords either alone or in combination: Kisspeptin, Kiss1, Kiss1r, Gpr54, Reproduction, Sex, Emotion, Behaviour, Olfaction, Audition, Fear, Mood, Anxiety. Relevant data were subsequently extracted from the identified publications, and secondary data sources identified therein. To ensure the inclusion of the most current data available, searches were performed up until the 8th of August 2017.
Kisspeptin Signalling outside the Hypothalamus
Extensive work has identified Kiss1 and its cognate receptor expression within the rodent hypothalamus, predominantly within the arcuate nucleus (ARC), anteroventral periventricular nucleus (AVPV), preoptic periventricular nucleus, and preoptic area, which are all areas implicated in the modulation of GnRH secretion as part of the HPG axis [8,9,10,11,12,13,14,15]. In primates (including humans), hypothalamic KISS1 and KISS1R mRNA is predominantly located within the infundibular nucleus, preoptic area, and the rostral periventricular zone (RP3V) [16,17,18,19,20].
However, it is interesting to consider the sites of kisspeptin signalling expression outside the hypothalamus to glean clues as to functional roles for kisspeptin outside the HPG axis. Kisspeptin has been identified in the stria terminalis, amygdala, and thalamus using a polyclonal rabbit antibody to identify the terminal amino acid sequence of kisspeptin . Kiss1 mRNA has also been identified in the rodent amygdala where its expression is positively modulated by gonadal sex steroids [21,22]. Other brain regions demonstrating Kiss1r expression include the habenula, thalamus, hippocampus, and the olfactory system [15,23,24]. In humans, KISS1 and KISS1R mRNA has been identified outside the hypothalamus in the amygdala, caudate, cingulate, globus pallidus, hippocampus, medial and superior frontal gyrus, nucleus accumbens, parahippocampal gyrus, substantia nigra, putamen, and thalamus in varying degrees [19,20].
Functional studies in rodents have shown that extrahypothalamic kisspeptin signalling can influence the HPG axis to modulate reproductive hormone secretion. Direct intra-amygdala injection of kisspeptin results in increased LH secretion, while conversely blocking endogenous amygdala kisspeptin signalling with a kisspeptin antagonist decreases both LH secretion and LH pulsatility , clearly demonstrating the influence of kisspeptin signalling outside the hypothalamus on the HPG axis.
Collectively, these data demonstrate an anatomical brain framework for kisspeptin signalling outside the hypothalamus extending to areas involved in emotions including sexual behaviour, fear, anxiety, mood, and olfaction. This is highlighted by the identification of kisspeptin and its cognate receptor in key limbic structures that orchestrate emotions including the amygdala, hippocampus, cingulate, and thalamus as well as paralimbic structures that feed into this system such as the accumbens, stria terminalis, putamen, and globus pallidus .
Kisspeptin and Olfaction
Olfactory stimuli initiate a wide range of social behaviours and emotions and are well established to play a crucial role in mammalian reproduction . Recently, an anatomical framework for kisspeptin's role in olfaction has been identified in rodents involving the amygdala, which is a central structure in the olfactory system . Using microinjections of retro- and anterograde tracers, neuronal connections were observed between the accessory olfactory bulb and kisspeptin neurones within the amygdala with these latter neurones also projecting via the stria terminalis to GnRH neurones in the preoptic area . Furthermore, fluorescence immunochemistry revealed close appositions between these amygdala kisspeptin neurones and vasopressinergic and (tyrosine hydroxylase-positive) dopaminergic neurones , which both have established roles in social behaviours including affiliation, motivation, and reward [28,29]. Interestingly, half of the olfactory mitral cells were inhibited by kisspeptin administration (as determined by electrophysiological studies), but the precise significance of this and the interactions with other cells of the olfactory system such as inhibitory granule cells remains to be determined . It is feasible that the changes in kisspeptin expression in the amygdala during the oestrous cycle (peaking around the time of mating ) may contribute to the filtering of odour cues by the olfactory system at this crucial time, but this requires further study.
The earliest evidence for a role for kisspeptin signalling in olfactory reproductive behaviour came from a study examining the consequences of global Kiss1r knock-out in male mice . Wild-type males spent >70% of their time investigating (i.e., sniffing) female mice, but testosterone-replaced Kiss1r knock-out male mice did not investigate females preferentially over males, despite normal olfactory function confirmed by a hidden cookie test . These findings provided an important indication for the role of the kisspeptin receptor in opposite-sex olfactory reproductive behaviour. Along these lines, selective activation of posterodorsal medial amygdala kisspeptin neurones (using DREADD technology [Designer Receptors Exclusively Activated by Designer Drugs]) increased the time spent by male mice investigating oestrous females . However, this activation also resulted in increased duration spent socialising with a juvenile mouse, and so suggests that amygdala kisspeptin signalling may modulate not only opposite-sex preference but also juvenile odour preference. Whether the effects in this latter study are via kisspeptin secretion or other neurotransmitters released from kisspeptin neurones remains to be determined.
Studies employing opposite-sex urinary odours also provide evidence for the importance of kisspeptin signalling in olfactory-reproductive pathways in male as well as female rodents. Male wild-type mice exposed to oestrous female urine demonstrated significantly increased Fos expression in kisspeptin neurones in the RP3V (consisting of the AVPV and periventricular nucleus) of the hypothalamus compared to males exposed to male urine or control water . Moreover, an even greater increase in RP3V kisspeptin neuronal Fos expression was observed in female wild-type mice exposed to male urinary odours (compared to female oestrous urine or control water) . Consistent with this, a recent study in female rats (ovariectomised and implanted with preovulatory oestradiol levels) demonstrated increased AVPV and limbic (but not ARC) kisspeptin neuronal Fos expression following exposure to male-soiled bedding . Furthermore, these female rats showed enhanced LH surges compared to female rats exposed to clean or female-soiled bedding, therefore providing functional relevance .
Taken together, these studies (summarised in Fig. 1) provide an anatomical and physiological framework for kisspeptins role in the integration of olfactory cues and social behaviours (via vasopressin and dopamine) with the HPG axis in both male and female rodents.
Kisspeptin and Audition
Another sensory modality associated with reproduction is audition. A recent study demonstrated that male mice typically emit song-like “ultrasonic vocalisations” (USVs) in response to females, which results in their subsequent approach. Furthermore, the presence of males USVs increased the number of subsequent offspring compared to the absence of USVs (by devocalisation) . Interestingly, male USVs increased female kisspeptin neuronal activity (determined by pCREB expression [phosphorylation of Cyclic AMP Response Element Binding]) in the ARC (but not the AVPV), which correlated with duration of female searching behaviour for the speaker (i.e., the male) . Together with data demonstrating kisspeptin receptor expression in the dorsal cochlear nucleus , this suggests a role for kisspeptin in auditory reproductive processing.
Kisspeptin and Fear
Fear is a cardinal emotion that can impair reproductive performance and therefore influence onward behaviours to protect against threats to the species. In several fish species, alarm substance (AS) is released by specialised epidermal cells when their skin is damaged by a predator. This AS release results in a fear reaction (such as erratic movement and freezing) in neighbouring fish which detect the AS using their sensitive chemoreceptors . In this way, AS can also be used to evoke fear in fish behavioural studies . In zebrafish, AS-evoked fear stimuli significantly reduces Kiss1 and serotonin-related gene transcription (pet1 and slc6a4a), while intracranial kisspeptin administration suppresses AS-evoked fear responses (erratic movement and freezing) . In subsequent studies, the authors observed that pharmacological blockade of serotonin receptors (5-HT1A and 5-HT2) removed this anxiolytic effect of kisspeptin  and that kisspeptin modulated the serotonergic system here primarily via glutamatergic neurotransmission . Furthermore, selective inactivation of Kiss1 neurones (using kisspeptin conjugated to saporin) resulted in reduced Kiss1 immunoreactivity and Fos expression within the habenula and raphe (key structures in the fear response), which resulted in attenuated AS-evoked fear responses . The role of kisspeptin in fear has as yet not been explored in other species, but these studies in zebrafish provide evidence that kisspeptin can attenuate fear responses via serotonergic pathways, a finding with implications for successful reproduction and conservation of the species.
Kisspeptin and Mood
The hypothalmo-pituitary-adrenal axis regulates adrenocorticotropin hormone and corticosteroid release in response to anxiety, an effect that does not appear to be influenced by peripheral kisspeptin administration in rodents  or humans . However, stress-induced plasma corticosterone decreases kisspeptin signalling in the hypothalamus  providing evidence for an interplay between hormonal aspects of anxiety, stress, and the kisspeptin system.
The functional data regarding kisspeptin's role in anxiety is perhaps less clear. Intracerebroventricular kisspeptin injection in male rodents decreased time spent in the open arms of a maze, suggesting that kisspeptin administration increases anxiety . By contrast, a more recent study observed that DREADD activation of medial amygdala kisspeptin neurones in male rodents increased time in the open arms of a maze suggesting a decrease in anxiety . This latter study is in agreement with studies in zebrafish demonstrating anxiolytic effects of kisspeptin administration in a novel tank diving test designed to assess anxiety (increased top-to-bottom transitions), associated with enhanced 5-HT-related gene expression [36,43]. Therefore, these contrasting data propose a differential effect between global activation of central kisspeptin receptors via central kisspeptin administration in rodents and zebrafish, versus pure activation of medial amygdala kisspeptin neurones which may release kisspeptin as well as additional neurotransmitters. Further work is therefore required to clarify the role of kisspeptin signalling in anxiety across species.
Kisspeptin's modulation of the serotonergic system has also been explored in relation to mood (summarised in Fig. 1). During a forced swimming test in rodents designed to assess depressive behaviour, intracerebroventricular administration of kisspeptin dose-dependently reversed immobility, climbing, and swimming times suggesting antidepressant-like effects which were attenuated by pretreatment with 5-HT2 and α2-adrenergic antagonists .
In healthy young men, peripheral kisspeptin administration enhances prefrontal activity (as determined by functional magnetic resonance imaging [fMRI]) in response to negative-evoked visual stimuli such as images of car crashes or terminal patients . This is in keeping with the role of the prefrontal area in the internalisation of safety to reduce fear and anxiety to negative stimuli  and is an area known to express kisspeptin receptor mRNA in humans . Furthermore, peripheral kisspeptin administration to healthy men reduces negative mood in psychometric tests compared to vehicle  consistent with the aforementioned study in rodents .
These data from rodent, zebrafish, and human studies therefore implicate kisspeptin signalling in the modulation of mood and anxiety with antidepressant-like effects which may have clinical implications.
Kisspeptin and Sexual Processing
Kisspeptin is a reproductive hormone and so it seems logical that it may have effects on sexual processing beyond the HPG axis. Sexual processing is a fundamental driver of behaviour and subsequent reproduction, safeguarding the survival of most mammalian species . The established involvement of several limbic and paralimbic structures including cognitive (cingulate, thalamus), emotional (amygdala, insula), motivational (putamen, precentral gyrus), and physiological (thalamus) centres in sexual processing permit the processing of a stimulus as sexual through to the autonomic activation required for sexual behaviour . In healthy young heterosexual men, kisspeptin administration enhances brain activity (as determined by fMRI) in several of these limbic and paralimbic structures implicated in sexual processing, in response to visual-evoked sexual stimuli including the anterior and posterior cingulate and amygdala , and consistent with structures expressing kisspeptin and kisspeptin receptors in humans [19,20] and rodents [10,15,21,23,24]. Furthermore in this human study, kisspeptins enhancement of activity in several of these structures of the sexual-processing network (including the cingulate, putamen, and globus pallidus) correlated with decreased aversion to sex as assessed by psychometric questionnaires.
Drive and reward are key components that govern behaviour  with neural substrates belonging to the mesolimbic reward and fronto-striatal-amygdala-midbrain networks [49,50]. Kisspeptin administration activates key components of these networks (including the hippocampus, amygdala, and cingulate) to a greater extent in men with lower baseline drive and reward traits in response to visual-evoked sexual stimuli . This suggests that kisspeptin signalling could enhance reward-system activity during sexual arousal (particularly in those generally less responsive to reward), thereby triggering a desire for sexual activity and possibly subsequent reproduction.
Along these lines, a recent study in rodents demonstrated that direct bilateral injection of kisspeptin into the posterodorsal subnucleus of the medial amygdala dose-dependently triggered multiple erections in male rats, an effect that was blocked by pretreatment with a kisspeptin antagonist (pep-234) . Importantly, erections were not observed when kisspeptin was injected into the lateral cerebral ventricle suggesting a direct effect of kisspeptin at the site of the amygdala. In contrast, downstream androgen injection into the posterodorsal subnucleus of the medial amygdala did not result in spontaneous erections unless an oestrous female was present, demonstrating that kisspeptin can uniquely trigger erections without the presence of the oestrous female and its associated olfactory cues . Collectively, these data therefore reveal a specific role for kisspeptin signalling in sexual brain processing encompassing limbic and paralimbic brain activity, sexual appetite, and erections.
In addition, the aforementioned study in heterosexual young men also identified an enhancement of limbic and paralimbic brain activity by kisspeptin administration, on viewing non-sexual couple-bonding (i.e., romantic) images . This included the anterior and posterior cingulate, amygdala, thalamus, and globus pallidus, which are all implicated in romantic, maternal, and unconditional love [53,54,55,56] and consistent with structures known to be involved in kisspeptin signalling in humans [19,20] and rodents [10,15,21,23,24]. Interestingly, kisspeptins enhanced activation of the amygdala in response to bonding images correlated with improved positive mood , a potential mechanism that may contribute to the desire to bond with a partner.
Several studies have examined the effects of reproductive hormones on libido and associated markers of sexual brain processing. A recent meta-analysis demonstrated that testosterone replacement improves libido, sexual thoughts, sexual motivation, sexual satisfaction, and nocturnal erections in most but not all studies of hypogonadal men . Furthermore, testosterone supplementation has no significant overall effect on libido in eugonadal men  or in men complaining solely of reduced sexual desire . In addition, testosterone replacement to hypogonadal men is unable to fully restore libido to that of age-matched eugonadal controls .
It is therefore possible, that other factors are also important in modulating libido that may include upstream kisspeptin signalling. Certainly, the expression pattern of kisspeptin and its cognate receptor in limbic and paralimbic structures would point towards this as well as the more recent studies identifying a role in sexual brain processing and erection generation. Further studies are required to determine if kisspeptin signalling could be exploited in future therapies for patients with sexual and emotional disorders.
Collectively, these data (summarised in Fig. 1) reveal a reproductive role for kisspeptin beyond the HPG axis. Kisspeptin may therefore serve to integrate sexual processing with reproductive function from reducing sexual aversion and establishing reward through to the achievement of erections and appropriate reproductive hormone release.
It is interesting to speculate as to the mechanism by which kisspeptin can modulate sexual processing. In the human study above, kisspeptin had no effect on several hormones known to be involved in limbic brain processing including testosterone, oxytocin, and cortisol . Furthermore, the isoform of kisspeptin used in this human study (kisspeptin-54) appears to cross the blood-brain barrier in rodents and so may obtain direct access to the extrahypothalamic kisspeptin signalling network within the brain [40,60].
The studies demonstrating antidepressant-like effects of kisspeptin and effects on fear suggest an interplay with serotonergic and adrenergic systems [37,44], while the studies in olfaction demonstrate close appositions between kisspeptin neurones in the amygdala with vasopressinergic and dopaminergic afferents . Other neuroendocrine systems may also be at work, as kisspeptin has established roles in nitric oxide , neurokinin B , dynorphin , gamma-aminobutyric acid , glutamate , and cocaine- and amphetamine-regulated transcript  signalling. Hence, there is a complex set of pathways that kisspeptin signalling can interact with, to bring about the aforementioned effects on emotions and sexual processing that will no doubt be a subject of future study.
Conclusion and Future Directions
The early findings of the extensive apparatus for kisspeptin signalling outside the hypothalamus [10,15,19,20,23] have triggered a wealth of studies aiming to attach a functional relevance to this apparatus. In this review, we have detailed the state of play so far (summarised in Fig. 1). Kisspeptin signalling is required for male olfactory partner preference , is enhanced by opposite-sex urinary odours [32,33], modulates mate preference , enhances auditory-reproductive behaviour , dampens fear responses , modulates anxiety [31,36,42], has antidepressant-like effects [40,44], and triggers erections . Furthermore, in healthy heterosexual men, kisspeptin enhances limbic and paralimbic brain activity specifically in response to visual-evoked sexual and romantic stimuli and these enhancements correlate with behavioural measures including sexual aversion, reward, and mood . Overall, these studies propel kisspeptin into an emerging field of study, the roles of kisspeptin in sexual and emotional processing; a field that is closely linked to kisspeptin's established role in the regulation of the HPG axis. In this way, kisspeptin may serve to integrate sexual and emotional behaviours with the HPG axis so as to modulate associated and appropriate behaviours to ultimately achieve well-being, successful reproduction, and survival of the species.
Future work will no doubt investigate the plethora of related behaviours in various species and attempt to delineate the precise neuronal pathways involved. To date, most of the data comes from male species, so it will be of great interest in the coming years to investigate if there is a sexual dimorphism in these sexual and emotional roles. Furthermore, with a better understanding of these processes, there may emerge potential therapeutic applications to aid patients with associated disorders of sexual and emotional processing.
The Section of Endocrinology and Investigative Medicine is funded by grants from the MRC and Biotechnology and Biological Sciences Research Council, and is supported by the NIHR Imperial Biomedical Research Centre Funding Scheme. A.N.C. is supported by the NHS and BRC. W.S.D. is funded by an NIHR Research Professorship.
The authors have declared that no conflict of interest exists.