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Sexual attraction and the selective social attachments that often
follow are two of the most powerful driving forces of human behav-
ior, profoundly influencing art, music, literature and politics
throughout history. The presence of strong, enduring relationships
between sexual partners is widespread in nearly all societies, particu-
larly in societies where monogamy is a predominant feature of the
social organization. Whether humans have a biological propensity to
practice monogamy (or perhaps more correctly, serial monogamy) is
debatable; however, there is little doubt that the ability to form
intense social attachments—or pair bonds—with a mate has a biolog-
ical architecture with definable molecular and neural mechanisms.
Studies using monogamous rodents as models for social attachment
are providing insights into the biology of pair-bond formation.
The term ‘monogamy’ implies a social organization in which a
male and female mate exclusively with each other, although extra-
pair copulations are not unusual in monogamous species
1
.For this
reason, the term ‘monogamy’ is used here to refer to a social organi-
zation in which each member of a mating pair displays selective (but
not exclusive) affiliation and copulation, as well as nest sharing, with
the partner; it also typically implies biparental care of offspring.
Only 3–5% of mammals exhibit a monogamous social structure as
defined by these criteria
2
. One group of species in particular, voles in
the genus Microtus, has emerged as a valuable tool for investigating
the neurobiological mechanisms of pair-bond formation
3,4
.Here we
review recent discoveries concerning the molecular, cellular and neu-
robiological pathways that result in the development of a pair bond
in the monogamous prairie vole (Microtus ochrogaster). These stud-
ies provide a framework for understanding the regulation and evolu-
tion of complex social behavior and may provide insights into the
human social brain.
Peptidergic regulation of the pair bond
Like humans, voles display a remarkable diversity in social organiza-
tion. For example, prairie voles form enduring pair bonds and are
biparental, but montane (Microtus montanus) and meadow (Microtus
pennsylvanicus) voles are nonmonogamous and typically do not dis-
play biparental care
5–7
.In nature, the majority of prairie voles that
lose a mate never take on another partner
8
.
In the laboratory, researchers study pair-bond formation using a
partner-preference test. The testing apparatus consists of three cham-
bers connected by tubes. The ‘partner’ and a novel ‘stranger’ are teth-
ered in their own chambers, whereas the subject is free to move
throughout the apparatus during a 3-h test. Pair bonding is inferred
when subjects spend significantly more time in close proximity to the
partner compared to the stranger (partner preference). In prairie
voles, mating facilitates formation of partner preference, although
cohabitation without mating may also result in partner-preference
formation under some circumstances
9
.
Two neuropeptides emerged initially as critical mediators of
partner-preference formation in prairie voles: oxytocin and arginine
vasopressin (AVP). Oxytocin also regulates mother-infant bonding
in sheep
10
,whereas AVP has been implicated in several male-typical
social behaviors, including aggression, scent marking and
courtship
11,12
.Infusion of oxytocin into the cerebral ventricles of
female prairie voles accelerates pair bonding, as these females require
only a brief cohabitation with a male, without mating, to form a
partner preference
13
. Likewise, central AVP infusion facilitates
pair-bond formation in male prairie voles without mating
14
.
Administration of selective oxytocin receptor (OTR) and AVP recep-
tor 1a (V1aR) antagonists prevents pair-bond formation in female
and male prairie voles, respectively
13–16
.Although both peptides may
facilitate pair-bond formation in either sex
16
,oxytocin seems to be
more important in females, whereas AVP is more critical in
males
14,15,17
.The mechanism underlying this sex difference in
behavioral response to oxytocin and AVP is unclear, because receptor
densities in the brain are similar in males and females. Early social
experience may also influence adult social behavior, as developmen-
tal studies suggest that neonatal oxytocin exposure enhances the like-
lihood of partner preference formation in adult male prairie voles
18
.
Although other factors, including stress and stress hormones, also
The neurobiology of pair bonding
Larry J Young
1
& Zuoxin Wang
2
A neurobiological model for pair-bond formation has emerged from studies in monogamous rodents. The neuropeptides oxytocin
and vasopressin contribute to the processing of social cues necessary for individual recognition. Mesolimbic dopamine is involved
in reinforcement and reward learning. Concurrent activation of neuropeptide and dopamine receptors in the reward centers of the
brain during mating results in a conditioned partner preference, observed as a pair bond. Differential regulation of neuropeptide
receptor expression may explain species differences in the ability to form pair bonds. These and other studies discussed here have
intriguing implications for the neurobiology of social attachment in our own species.
1
Center for Behavioral Neuroscience, Department of Psychiatry and
Behavioral Sciences, and Yerkes National Primate Research Center, Emory
University, Atlanta, Georgia 30322, USA.
2
Department of Psychology and
Program in Neuroscience, Florida State University, Tallahassee, Florida
32306, USA.
Correspondence should be addressed to L.J.Y. (lyoun03@emory.edu).
Published online 26 September 2004; doi:10.1038/nn1327
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T HE
S EXUAL B RAIN
© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience
modulate partner-preference formation
19–21
,for the sake of clarity
we will restrict our focus to oxytocin and AVP, as well as their inter-
actions with other neurotransmitter systems.
The first hypotheses about the neuroanatomical basis of pair-
bond formation came from comparisons of OTR and V1aR distri-
butions in the brains of monogamous and nonmonogamous vole
species. Compared to nonmonogamous species, monogamous
prairie voles have higher densities of OTR in the caudate putamen
and nucleus accumbens (Fig. 1a,b)
22
and higher densities of V1aR
in the ventral pallidum, medial amygdala and mediodorsal thala-
mus (Fig. 1c,d)
23
.Some of these regions are involved in pair-bond
formation. For example, infusion of an OTR antagonist into the
prefrontal cortex and nucleus accumbens of females, but not the
caudate putamen, blocks mating-induced partner-preference
formation (Fig. 1e)
24
.In males before mating, blocking V1aR neu-
rotransmission in the ventral pallidum, but not the medial amyg-
dala or mediodorsal thalamus, also inhibits partner-preference
formation (Fig. 1f)
25
.Infusion of V1aR antagonist into the lateral
septum, which expresses some V1aR in both species, also prevents
mating-induced pair-bond formation in males
26
.
Sex, reward and pair bonding
Results from anatomical and pharmacological studies indicate that
the prefrontal cortex, nucleus accumbens and ventral pallidum are
all critical brain regions in pair-bond formation. These regions are
also involved in the mesolimbic dopamine reward system, suggest-
ing that pair-bond formation uses the same neural circuitry as
reward. Reward processing depends on the mesocorticolimbic
dopaminergic system consisting of dopamine neurons in the ventral
tegmental area and their projections to the nucleus accumbens, pre-
frontal cortex and other brain areas
27
.The ventral pallidum is a
major target of the nucleus accumbens
28,29
, and it further processes
and relays stimuli from the nucleus accumbens to mediate locomo-
tor responses to rewarding stimuli
30,31
.Dopamine release within
this circuit is critically involved in natural reward (food intake and
mating) as well as maladaptive (drug) reward
27,32,33
.Studies also
implicate this circuit in conditioned reward learning, such as drug-
induced place preferences
34
, in which neutral stimuli become asso-
ciated with rewarding stimuli.
Given that mating is rewarding in rodents
34–36
and facilitates pair-
bond formation in voles, some researchers have hypothesized that
pair bonding may be the result of conditioned reward learning, in
which an association forms between the reinforcing properties of sex
(unconditioned stimulus) and the specific olfactory signature of the
partner (conditioned stimulus)
3,37
.For example, both male and
female rats prefer to spend time in the chamber in which they copu-
lated (a conditioned place preference)
38,39
, and this sexual condition-
ing depends on D1-type and D2-type dopamine receptor activation
in the nucleus accumbens
40
.
Consistent with the hypothesis that pair bonding involves condi-
tioned learning, dopamine within the nucleus accumbens is critical
for partner preference formation in prairie voles (Fig. 2). The
nucleus accumbens in voles contains dopamine terminals and
receptors (Fig. 2b)
41,42
and mating results in an increase (51%) in
extracellular dopamine in the nucleus accumbens of females
(Fig. 2c)
43
.Mating also tends to increase dopamine turnover in the
nucleus accumbens of males
42
.Systemic administration, or local
injection into the nucleus accumbens, of haloperidol (a nonselective
dopamine receptor antagonist) blocks mating-induced partner
preferences, whereas apomorphine (a nonselective dopamine recep-
tor agonist) facilitates partner preference without mating in both
male and female prairie voles (Fig. 2a)
42,43
.
The dopaminergic regulation of pair-bond formation in the
nucleus accumbens is receptor subtype–specific: activation of D2, but
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Figure 1 OTR and V1aR regulation of pair bonding in prairie voles. (a,b) Monogamous prairie voles (a) have higher densities of OTR in the nucleus
accumbens (NAcc) and caudate putamen (CP) than do nonmonogamous montane voles (b). Both species have OTR in the prefrontal cortex (PFC).
(c,d) Male prairie voles (c) have higher densities of V1aR in the ventral pallidum (VP) than do montane voles (d). (e) A selective OTR antagonist (OTA)
infused bilaterally into the NAcc or PFC, but not the CP, blocks partner-preference formation in female prairie voles
24
. (f) Infusion of a selective V1aR
antagonist (V1aRA) into the VP, but not into the mediodorsal thalamus (MDthal) or medial amygdala (MeA), prevents mating-induced partner-preference
formation in male prairie voles
25
. Scale bar, 1 mm.
© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience
the density of D1, but not D2, receptors in the nucleus accumbens. No
changes in dopamine receptor binding occur in other dopaminergic
brain areas, including the caudate putamen
41
.As D1 activation in the
nucleus accumbens prevents pair bonding in males
41
, this increase in
D1 receptor density may serve to prevent the formation of new pair
bonds, thereby maintaining the current pair bond and stabilizing the
monogamous social organization.
not D1, receptors in the nucleus accumbens of female prairie voles
accelerates partner preferences without mating, whereas blockade of
D2 receptors antagonizes this behavior (Fig. 2d)
43,44
.In males, D2
receptor activation also facilitates partner preference, but D1 receptor
activation blocks partner preferences induced by mating or by D2
receptor activation
41
.Male prairie voles that have mated and pair
bonded with a female for two weeks also show a significant increase in
Figure 3 Sagittal view of a prairie vole brain illustrating a proposed neural
circuit model for pair bonding. In this model, mating activates the VTA,
resulting in increased dopamine activity in the prefrontal cortex (PFC) and
nucleus accumbens (NAcc). Concurrently, olfactory signals from the mate
are transmitted via the olfactory bulb (OB) to the medial nucleus of the
amygdala (MeA). Oxytocin acts in the MeA, and AVP acts in the lateral
septum (LS) to facilitate olfactory learning and memory. Mating also
stimulates increased extracellular concentrations of oxytocin in the PFC and
NAcc of females, and of vasopressin in the ventral pallidum (VP) of males.
AVP fibers in the LS and VP originate from cell bodies in the MeA and
bed nucleus of the stria terminalis (not shown). The source of oxytocin
projections to the NAcc, MeA and PFC has not been defined (hence the
dotted line), but they most likely originate from some population of cell
bodies in the preoptic or hypothalamic area (POA/Hyp). Glutamatergic
projections from the PFC to the NAcc are thought to be important in
reinforcement and therefore potentially in pair bonding. The concurrent activation of the dopaminergic system and the oxytocin or AVP system in the NAcc
or VP potentially results in the development of a conditioned partner preference. The VP is a major output relay of the NAcc and modulates motor output in
response to reinforcing stimuli via projections to the mediodorsal thalamus (MdThal) and cortical and mesencephalic motor nuclei.
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Figure 2 Dopamine regulates pair bonding in female prairie voles. (a) Females injected intraperitoneally with saline or saline containing a D1 receptor
antagonist (D1-ant), but not a D2 antagonist (D2-ant), showed partner preferences after 24 h of cohabitation with mating, indicating the importance of D2
receptors in pair-bond formation
76
. (b) Photomicrographs of immunoreactive staining for tyrosine hydroxylase (TH) and dopamine transporter (DAT) as well
as autoradiographic binding for D1 and D2 dopamine receptors in the nucleus accumbens (NAcc) and caudate-putamen (CP) of the prairie vole brain.
Scale bar, 500 µm. (c) Estrus female voles that mated with a male showed a significant (51%) increase above the baseline in extracellular dopamine
concentration in NAcc
43
. (d) Intra-NAcc administration of a D2 antagonist (D2-ant) blocked mating-induced partner preferences
43
. (e) Intra-NAcc
administration of a D2 agonist (D2-ago) induced partner-preference formation after 6 h of cohabitation with a male in the absence of mating
44
. This
behavior was blocked by coadministration of either the D2 antagonist (D2-ant) or the oxytocin receptor antagonist (OTA), suggesting that concurrent
activation of both D2 and oxytocin receptors in NAcc is essential for pair-bond formation
44
.
Dopamine
Oxytocin
Vasopressin
Olfactory signals
VTA
MeA
NAcc
VP
LS
CP
PFC
OB
Motor cortex
MdThal
Motor nuclei
POA/Hyp
© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience
Although mating-induced dopamine release in the nucleus accum-
bens is important for pair-bond formation in prairie voles, mating
also induces dopamine release in the nucleus accumbens of other
species of rodents, such as rats, which do not form pair bonds
35,36
.
Why then does dopamine induce pair bonding only in monogamous
prairie voles? The answer may lie in the interaction of the oxytocin,
AVP and dopamine systems within the reward circuitry.
In female prairie voles, administration of an OTR antagonist into
the nucleus accumbens blocks partner preferences induced by D2
receptor activation, whereas blockade of D2 receptors in the nucleus
accumbens prevents partner preference formation induced by oxy-
tocin
44
(Fig. 2e). These data indicate that dopamine and oxytocin are
not acting sequentially, but rather that concurrent activation of both
oxytocin and dopamine D2 receptors, and the interaction between
these two systems in the nucleus accumbens, are necessary for pair-
bond formation in females. Similar studies have not been done in
males, but given that the D2 receptors in nucleus accumbens
42
and
V1aR in the ventral pallidum
25,45
are important for pair bonding in
males, and that these two areas are interconnected
28,29
, it is likely that
the dopamine and AVP systems also interact in the nucleus accum-
bens–ventral pallidum circuitry to influence pair-bond formation in
males. The nature of the interaction of these
systems in voles is not clear. Studies on drug
tolerance and addiction suggest that these
neuropeptides may modulate the role of
dopamine in the reward circuitry
46
.
Furthermore, dopamine administration
induces central oxytocin release, whereas
oxytocin administration increases central
dopamine levels in the rat
47,48
.The interac-
tion may also be indirect, with concurrent
activation modulating downstream circuits
involved in olfactory learning and condition-
ing, for example.
A neurobiological model for pair bonding
How might the oxytocin, AVP and dopamine
systems interact to facilitate pair-bond for-
mation? There are now several studies sug-
gesting that both oxytocin and AVP are
involved in the neural processing of sensory
cues involved in social learning. In rodents,
both neuropeptides are implicated in the
processes required to identify the olfactory
signatures of conspecifics (social recogni-
tion)
49
.Oxytocin knockout mice fail to rec-
ognize individuals to which they have been
previously exposed
50
, and infusions of oxy-
tocin in the medial amygdala completely
restore social recognition in these mice
51
.
Selective V1aR antagonist or antisense V1aR
administered into the lateral septum of rats
also inhibits social recognition
52,53
,whereas
infusion of AVP or overexpression of the
V1aR in this region enhances social recogni-
tion abilities
54
. V1aR knockout mice also
exhibit a complete loss of social recogni-
tion
55
.However, both oxytocin and V1aR
knockout mice perform normally in other
olfactory and cognitive tasks, suggesting that
this deficit is specific for social discrimina-
tion
50,55
.Given this role for oxytocin and AVP in social recognition
and the interaction of these peptides with mesolimbic dopamine, a
reasonable hypothesis is that pair bonding results from the conver-
gence of social discrimination circuits and the reinforcing properties
of the mesolimbic dopamine reward circuit (Fig. 3).
Mating in both males and females correlates with neural activity in
several brain regions, including the ventral tegmental area
(VTA), medial amygdala, preoptic area and hypothalamus
25,56
.
Dopaminergic projections from the VTA release dopamine in the
nucleus accumbens and prefrontal cortex, which has strong gluta-
matergic projections back to the nucleus accumbens
27
.Concurrently,
olfactory cues from the partner are processed by the main and acces-
sory olfactory bulbs, and subsequently by the medial amygdala and
lateral septum, which are critical for social recognition. The medial
amygdala and bed nucleus of the stria terminalis are major sources of
AVP fibers projecting to the ventral pallidum and lateral septum
57
,
whereas oxytocin fibers in the nucleus accumbens most likely origi-
nate from neurons in the preoptic area or hypothalamus
58
.Activation
of these areas during mating may result in local release of these pep-
tides. Indeed, in vivo microdialysis shows that AVP is released in the
male prairie vole ventral pallidum during mating (J.C. Morales and
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Figure 4 The molecular genetics of pair bonding. (a) Male meadow voles overexpressing the V1aR
receptor in the ventral pallidum (V1aR-vp) showed enhanced mating-induced partner preferences
compared to control animals (Ctrl all). Infusion of a D2 receptor antagonist (D2-ant) before mating
abolished the partner preference in these males (V1aR-vp + D2-ant)
6
. (b) V1aR binding autoradiograms
illustrating the increased expression of V1aR in the ventral pallidum (vp) in experimental males
compared to control animals. (c) The structure of the Avpr1a gene of prairie and montane voles. The
gene is highly homologous except for an expanded microsatellite sequence in the 5′ flanking region of
the prairie vole gene. Yellow boxes indicate coding regions with the black bars representing the seven
transmembrane domains. Gray indicates untranslated regions. The green and red boxes indicate
the relative length and position of the microsatellite sequences in the montane and prairie vole
genes, respectively. (d) The effect of the microsatellite sequence on expression as determined by
a transcription reporter assay. The prairie vole promoter was spliced upstream of firefly luciferase.
Exchanging only the prairie vole microsatellite sequence (prairie MS) with the montane vole sequence
(montane MS) resulted in a significant alteration in luciferase expression in a rat A7r5 cell line.
© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience
L.J.Y., unpublished data), and vaginocervical stimulation increases
central oxytocin release in sheep
59
.Thus, mating ultimately results in
the concurrent activation of D2 receptors in the nucleus accumbens
of both sexes, OTR in the prefrontal cortex and nucleus accumbens of
females and V1aR in the ventral pallidum of males. As a result, the
reinforcing, hedonic properties of mating may become coupled with
the olfactory signatures of the mate, resulting in a conditioned part-
ner preference, much in the same way as drugs of abuse result in con-
ditioned place preferences. In this model, the basic mechanism of
bonding is similar in males and females; the neuropeptides are simply
modulating two different nodes of the same circuits. In nonmonoga-
mous species, sexual activity can also result in conditioned prefer-
ences for nonsocial stimuli, including neutral odors placed on
the sexual partner
60
.However, in nonmonogamous species, the
dopamine system and the oxytocin and AVP systems are uncoupled
because of the low densities of OTR and V1aR in this pathway.
We must stress that the study of the neurobiology of social bonding
in voles is in its infancy, and the model described here draws heavily
on neuroanatomical, behavioral and pharmacological data obtained
from rat studies, particularly those focusing on conditioned learning,
reinforcement and addiction. Our model is clearly limited in scope,
and raises many more questions than it answers. For example, what is
the nature of the interactions between the peptide and dopamine sys-
tems? How are other neurotransmitters such as glutamate and GABA
involved in pair bonding? How do the circuits involved in olfactory
processing converge on the reward circuits, and how does this result
in olfactory conditioning? Is there a role for other structures such as
the hippocampus and cortex? And finally, what types of molecular
and synaptic changes take place to lead to the development of a per-
manent pair bond. Thus, although this admittedly is an oversimpli-
fied model, it provides a neurobiological framework in which to
generate and test hypotheses regarding pair bonding.
The molecular basis of the pair bond
If our current model is correct, one would predict that pair-bond-
ing behavior could be potentially induced in a nonmonogamous
species by expressing OTR or V1aR in the nucleus accumbens or
ventral pallidum. We tested this prediction using viral
vector–mediated gene transfer to overexpress Avpr1a, the gene
encoding V1aR, in the ventral pallidum of the nonmonogamous
male meadow vole
6
(Fig. 4a,b). After cohabitation with a receptive
female during which copulation occurred, these transgenic animals
showed enhanced partner preference compared to controls.
Pretreating virus-treated voles with a D2 receptor antagonist pre-
vented partner preferences (Fig. 4a). This study has remarkable
implications for the evolution of complex behavior, suggesting that
mutations altering the expression pattern of a single gene can have
a profound impact on complex social behaviors.
How did the differential patterns of V1aR and OTR expression
emerge between monogamous and nonmonogamous species?
Because researchers have studied this question most extensively
with respect to Avpr1a,we will limit our discussion to this gene. The
Avpr1a genes in the prairie vole and nonmonogamous montane vole
are highly homologous
61
.However, approximately 660 base pairs
upstream of the transcription start site, the prairie vole Avpr1a gene
contains 500 base pairs of highly repetitive sequence, known as a
microsatellite; in montane and meadow voles, this repetitive
sequence is much shorter (Fig. 4c). Microsatellite sequences are
highly unstable
62
, and there are several examples of genes for which
polymorphic microsatellites in the regulatory region result in differ-
ential levels of expression
63,64
.It is clear that the sequences proximal
to the Avpr1a coding region determine the pattern of expression, as
a transgenic mouse expressing a prairie vole Avpr1a gene that
included 2.4 kb of upstream sequence expressed the Avpr1a gene in
a pattern similar to that of the prairie vole
61
.The species-specific
microsatellite also modulates gene expression in a cell type–specific
manner. Replacing the prairie vole microsatellite with the shorter
montane vole sequence increases reporter gene expression in a tran-
scription reporter assay (Fig. 4d)
65
.Although the results, obtained
in a cultured rat A7r5 cell line, are in the opposite direction from
what one would predict from the species differences in ventral palli-
dal receptor binding, the effect is consistent with the higher level of
V1aR expression in the septum of montane voles compared to
prairie voles. As the effect of the microsatellite on expression is cell
type–specific, it is likely that in other cell lines, and indeed in the
brain itself, the prairie vole microsatellite might yield higher levels
of transcription than the montane vole sequence. Together, these
data suggest that expansion or contraction of this microsatellite in
the 5′ flanking regulatory region of this gene could have been the
molecular event that resulted in the altered expression of the V1aR
gene in the preexisting reward circuit, resulting in the biological
potential to develop conditioned partner preferences. We do not
know, however, which selective pressures influenced the frequency
of the microsatellite alleles, and consequently the monogamous
social structure, in voles. The human V1aR gene (AVPR1A) has
three highly polymorphic microsatellite sequences in the 5′flanking
region, and it has been suggested that variation in one of these
sequences may be associated with autism
66,67
.
Implications for human bonding
Undoubtedly there are numerous molecular and neurobiological
pathways that could evolve to support pair-bond formation between
mates, and different species may have achieved similar behaviors
through a process of convergent evolution involving different circuits.
We strongly emphasize that there are no hard data demonstrating
common physiological mechanisms for pair-bond formation in voles
and man. In addition, as with many human behaviors, the emergence
of the neocortex and its ability to modify subcortical function cannot
be ignored. Nevertheless, it is intriguing to consider the possibility
that similar mechanisms may underlie the formation of pair bonds in
both humans and rodents. Although it is not known whether human
sexual intercourse results in central oxytocin or AVP release, plasma
oxytocin levels are elevated at the time of orgasm in women, and sim-
ilarly, plasma AVP concentrations increase during sexual arousal in
men
68,69
.These changes may or may not reflect central peptide
release; it is intriguing, however, to consider how aspects of human
sexuality may reflect the influence of intercourse on pair bonding. For
example, human females are ‘hidden ovulators’ and engage in sexual
activity throughout the ovarian cycle. This regular sexual activity may
serve to activate the circuits underlying bonding, thus strengthening
the pair bond. Furthermore, in contrast to other mammalian species,
human females have enlarged mammary tissues independent of lacta-
tion, and breast and nipple stimulation are an integral part of human
sexuality. Nipple stimulation during lactation is one of the most
potent stimuli for oxytocin release
70
.Ifoxytocin is involved in human
social attachment, this aspect of sexual activity may thus serve to rein-
force sexual bonding.
Human imaging studies also provide evidence consistent with the
hypothesis that reward and neuropeptide circuits are involved in pair
bonding in humans. When human subjects viewed photographs of
individuals with whom they claimed to be romantically in love, their
brain activity patterns (as measured by functional magnetic reso-
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nance imaging, fMRI) looked remarkably similar to those observed
after cocaine or µ-opioid infusions, with heavy activation of the VTA
and striatal dopamine regions
71
.Many of the regions activated are
rich in oxytocin, AVP or their respective receptors
72,73
.Similar pat-
terns of activity occur when mothers view images of their own chil-
dren, suggesting some overlap between the neural mechanisms of
maternal attachment and those of romantic love
74
.In addition, the
VTA and striatum show substantial activity (as measured by positron
emission tomography, PET) during ejaculation in men, paralleling
the activation pattern evoked by a heroin rush
75
.
The work reviewed here has focused primarily on the neurobiology
of mating-induced, heterosexual pair bonds. It is also intriguing to
consider whether other types of social bonds, including familial
bonds, close friendships or homosexual relationships might use some
of the same neurobiological mechanisms.
The pair bond is an integral aspect of human sexuality with
important implications for both psychological and physical health.
In the last few years, the neurobiological mechanisms underlying
pair bonding in voles have provided valuable insights into the social
brain. The convergence of mechanisms underlying reward, condi-
tioning and the neural processing of social cues seems to result in the
motivation to maintain selective contact with one’s partner. It is
unclear whether the same mechanisms are involved in rodent and
human pair bonding, but it is likely that in both cases, the social
brain and reward circuits are both involved, perhaps giving credence
to the adage,“love is an addiction.”
ACKNOWLEDGMENTS
The authors acknowledge A.Z. Murphy, E.A.D. Hammock, M.M. Lim, B. Aragona and
T. Curtis for discussion and comments during the writing of this manuscript. The
authors especially thank C.S. Carter and T.R. Insel for their pioneering work, which
laid the foundation for neurobiological studies of social bonding. Much of this work
was supported by National Institute of Mental Health grants to L.J.Y. and Z.X.W.
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Received 23 June; accepted 10 August 2004
Published online at http://www.nature.com/natureneuroscience/
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