812

Serotonin, social status and aggression Donald H Edwards* and Edward A Kravitzt

Serotonin, social status and aggression appear to be linked

in many animal species, including humans. The linkages are

complex, and, for the most part, details relating the amine to

the behavior remain obscure. During the past year, important

advances have been made in a crustacean model system

relating serotonin and aggression. The findings include the

demonstration that serotonin injections will cause transient

reversals in the unwillingness of subordinate animals to engage in agonistic encounters, and that at specific synaptic

sites involved in activation of escape behavior, the direction of

the modulation by serotonin depends on the social status of

the animal.

Addresses *Department of Biology, PO Box 4010, Georgia State University, Atlanta, Georgia 30302-4010, USA; e-mail: biodhe@panther.Gsu.edu +Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, Massachusetts 02 115, USA; e-mail: ekravitz@warren.med.harvard.edu

Current Opinion in Neurobiology 1997, 7:812-819

http://biomednet.com/elecref/0959438800700812

0 Current Biology Ltd ISSN 0959-4388

Abbreviations 5.7-DHT 5,7_dihydroxytryptamine GABA y-aminobutyric acid BHIAA 5-hydroxyindole acetic acid 5HT 5-hydroxytryptamine; serotonin 5HTP 5-hydroxytryptophan LG lateral giant interneuron MAO monoamine oxidase MG medial giant interneuron PCPA para-chlorophenylalanine TRYP tryptophan

Introduction A large and often contradictory literature describes the linkages between serotonin, social status and aggression in animals and humans. Much of the confusion, particularly in earlier studies, comes from the limited measures used to define serotonergic function and from the many different ways that investigators measure social status and ‘aggression’. Despite this confusion, most investigators will agree that serotonin is involved in aggression and in the establishment or maintenance of status. As a result, several broad generalizations have emerged: first, in mammals, lowered serotonergic function is believed to be associated with enhanced aggression and lower status, whereas higher serotonergic function is associated with reduced aggression, affiliative behavior, and good social skills, all of which contribute to higher status (cf. [l-3]); second, in fish, enhanced serotonergic function is seen in subordinate animals, which show reduced fighting behavior [4,5]; third, in crustaceans, enhanced aggression

and dominant status appear to be associated with increased serotonergic function, a result directly opposite to that seen in most vertebrates [6,7**,8”].

In this review, experimental approaches used in this field will be critiqued, and several model systems will be described. Peptides (such as arginine vasopressin and gonadotropin-releasing hormone [9**,10]), steroid hormones (such as glucocorticoids and androgens [ 1 l-13]), and other amines (such as octopamine [14]) also appear co play important roles in aggression, but descriptions of these substances and their possible interactions with serotonergic systems are beyond the scope of this review.

Relating serotonergic function to aggression Serotonergic function is inferred to be important in a behavior if serotonin-neuron-specific pharmacological manipulations alter the behavior, or if the levels of the amine, its precursor or its metabolites correlate in a consistent way with a behavioral state. Various pharma- cological manipulations are possible: injecting tryptophan (TRYP) or 5hydroxytryptophan (SHTP) to enhance synthesis of serotonin; inhibiting the synthesis of the amine, using drugs such as para-chlorophenylalanine (PCPA); depleting or destroying amine nerve terminals using reserpine, fenfluramine or 5,7_dihydroxytryptamine (5,7-DHT); blocking receptor function using a variety of reagents that are more or less specific for the large number of subtypes of serotonin receptors; using agonists of the same large family of receptors; using selective uptake blockers such as fluoxetine (Prozac); and using inhibitors of either of the two forms (type A and B) of the enzyme monoamine oxidase (MAO), which oxidizes amines to aldehydes.

A serious limitation of the pharmacological methods is that the reagents used have global actions influencing serotonergic functioning throughout the body in the case of systemic injections, or in all neurons, neuronal systems, and glia in the immediate vicinity in the case of more selective CNS injections. Investigators have attempted to circumvent these difficulties by using multiple drugs that are capable of influencing different aspects of serotonin neuron function, and, in such cases, clearer patterns emerge (see examples below). Even here, however, the same pharmacological intervention can sometimes produce exactly opposite behavioral results depending on the social situation. A very clear example of this comes from studies of vervet monkeys showing that TRYP injection can increase or decrease aggression, depending on whether the animals were raised in a social group or in isolation [15].

Endogenous levels of TRYP, serotonin, and its major metabolite, 5hydroxyindole acetic acid (SHIAA), can be

Serotonin. social status and aggression Edwards and Kravitz 813

measured in different brain regions or in cerebrospinal fluid, blood, or urine. The ratio of SHIAA/serotonin has been used widely to indicate release and metabolism of the amine. There are, however, major problems with this approach. For example, when measuring the levels of urinary metabolites, one must be aware that they derive mostly from peripheral tissues, such as the gut and blood platelets, in which over 95% of the body levels of serotonin are found. An important problem with the brain studies is that one measures total amounts of these substances in large brain areas in which possibly only a small portion of the serotonergic functioning is concerned with the behavior under examination. When measuring TRYP levels, a serious problem is that TRYP is an essential amino acid necessary for protein synthesis in all cells of the body. Therefore, only a very small portion of the measured TRYP is likely to have anything to do with serotonin neuron functioning. One can anticipate further that other behaviors in which serotonin neuron function plays a role, such as feeding or affective state, might also be influenced by changes in status. What will be measured, therefore, when entire brain regions (such as the cortex) are used as a tissue source, will average the influence of the change in status on all behaviors modulated by serotonin in those brain regions. The direction of the influence of serotonin on various behaviors may not be the same in different species of animals and may be very dependent on the social context, even within single species. Therefore, extracting or even detecting the component specifically concerned with aggression using this approach may prove difficult.

Finally, important changes in serotonergic functioning, produced, for example, by changes in social status, could be attributable to different actions of serotonin on target tissues rather than to any change whatsoever in the amount of serotonin released or metabolized. The effects of serotonin are mediated by many different receptors with diverse and even opposing actions. Short- or long-term changes in the relative proportions of receptors, or in their efficacy, could easily transform the effects of unchanging amounts of serotonin on target tissues in either dominant or subordinate animals.

New molecular methods, which are under development but not yet available, in which specific genes having to do with serotonin neuron function could be removed at specific sites in the nervous system and at precise times in development should help in this context [16*]. One important molecular method that is available offers the potential ability to kill specific neurons of a particular transmitter phenotype at whatever time in development one desires, but this method has not yet seen widespread use [17]. The popular ‘knock-out’ and other transgenic methods, in their present iteration, have confused as much as they have shed light on the role of amine neurons in aggression. For example, four distinct ‘knock- out’ mutants (calcium/calmodulin kinase II, nitric oxide

synthase, 5HT,u receptor and RIAO-A) and one transgenic overexpressing transforming growth factor ~1 all show enhanced aggression [16*,18-221. Direct linkages between most of these proteins and amine neurons are remote at best.

To illustrate further, a large literature suggests that connections exist between levels of serotonin in the brain, serotonin neuron function and aggression. Evidence comes from studies in which animals are treated with sub- stances that raise serotonin levels and decrease aggression (e.g. TRYP, 5HTP, or inhibitors of MAO) or that lower serotonin levels and enhance aggression (e.g. 5,7-DHT, PCPA). Yet, in recent much publicized experiments [23], the complete inactivation of hIAO-A by mutation in humans and by knock-outs in mice, both of which should raise amine levels, also raised aggression [22,24,25]. Dramatic increases in urinary levels of amine metabolites were found in affected individuals in the human studies [24], and important increases were seen in brain amine levels in the rodent knock-outs [ZZ], particularly in younger animals. Thus, the anticipated increases in amine levels were observed, but the behavioral manifestations were exactly opposite to those seen in the pharmacological studies. One serious problem demonstrated by these results is that the chronic loss of an amine-metabolizing enzyme (or indeed of any protein) induced by mutation leaves that substance missing throughout embryonic development and the entire life of an organism. The consequences of such losses are difficult to predict, and a phenotype obsened in older animals may relate to developmental compensations or consequences of the loss of the protein, and may not be directly related to the behavior being looked at in the older animals. Indeed, in the mouse hlAO-.4 knock-out studies, serious cortical damage seems to have taken place, as the whisker barrel fields appear to be missing in young animals [z?].

With most existing technology, therefore, the inability to selectively target the neurons believed involved in a behavior seriously compromises the ability to directly implicate substances such as serotonin in the behavior under examination. In more recent studies, investigators have used multifaceted and multidimensional approaches to explore the role of serotonin in aggression. In these approaches, arrays of pharmacological reagents are used to change serotonin neuron function in different ways. In addition, careful control is made of the behavioral situation- the environments in which animals are housed and reared, as well as the arenas in which experiments are performed, are under tight control, and the behaviors (in addition to aggression) are monitored and quantified. The behavioral experiments are carried out in the wild, in naturalistic settings, or in experimental situations that model the normal environment in which the animals live. The results of several of these types of studies follow here, after which a description of a crustacean model system for aggression that we have been examining will be presented.

814 Motor systems

Rodent studies: 5HTlNB receptors and aggression Recent studies by Olivier, Mos and coworkers [26] have focused on two models of aggression in rats: a resident-intruder model, which involves the introduction of a strange male into an established colony of one or two males and a female, and a maternal aggression model, which involves the introduction of an intruder animal into the cage of an early nursing female rat.

These experimental situations are designed to resemble behavioral patterns that have been observed in studies of wild rats. In addition to measuring the aggression directed against the intruder in each case, other behav- ioral parameters are measured, including social interest (e.g. sniffing and grooming), exploration (e.g. locomotion, digging and sniffing), and inactivity (e.g. sitting and lying). Together, these four parameters account for most of the ways that animals spend their time during 15 minute agonistic encounters.

Olivier et al. [26] tested the effects of injecting subtype- specific and nonspecific agonists of different rodent serotonin receptors into aggressing animals and measured the percentage of time the animals spent displaying each of the four behavioral parameters. 5HTrA-specific agonists decreased aggression in both behavioral model systems, but also significantly increased inactivity and appeared to have sedative effects on the animals. SHT,, and 5HTzc agonists also decreased aggression, but increased inactivity and induced ‘wet-dog shaking’. Mixed SHT~A/R and SHT~B/~C agonists were the only drugs tested that reduced aggression without significantly inducing sedation or other undesirable side effects, at least at the dosages tested.

Highly specific agonists or antagonists do not yet exist for most of the serotonin receptor subtypes. This limits the interpretation of even these carefully executed experiments. Nonetheless, by linking their results with the work of other investigators, Olivier et a/. [26] tentatively conclude that activation of postsynaptic SHT~B receptors appears to play an important role in the modulation of aggression. They add that studies with 5HTrn receptor knock-outs showing enhanced aggressiveness in mice [ 191 supports their suggestion, but the latter studies suffer from the criticism raised above regarding the absence of this receptor subtype throughout development.

Modifying amine function in primate colonies: effects on social status In another carefully controlled set of experiments, Raleigh and coworkers [Z] examined the effects of manipulating amine neuron function on aggression and social status in vervet monkey social groups. The monkeys were housed in outdoor enclosures in 12 social groups consisting of 3 males, at least 3 females and their offspring. The experi- ment was divided into five distinct 8 week intervals: first,

a normal dominance relationship was established; second, the dominant male was removed from the group, and one of the other males (treated male) was injected with an agent that either enhanced (e.g. tryptophan or fluoxetine) or reduced (e.g. fenfluramine or cyproheptadine) serotonin neuron function for half of the treatment period, while the third male was injected with vehicle; third, the original dominant male was returned to the group; fourth, the dominant male was removed from the group again, and the treated males were injected with reagents that had the opposite effect to that of the first treatment; fifth, the original dominant male was again returned to the group.

The results obtained in the treatment periods were clear and consistent. In the unstable situation, in which a dominant male was removed from the group (the second and fourth intervals), animals whose treatments enhanced serotonergic function became dominant by virtue of their enhanced affiliative behavior and social skills, enlisting females in defense of their status. In contrast, animals with lowered serotonergic function invariably showed higher levels of initiating aggression and lower subordinate social rank. In all cases, the original untreated dominant males regained their social status when returned to their social group (the third and fifth intervals). Raleigh et al. [Z] draw the cautious conclusion “that serotonergic systems promote the acquisition of male dominance in unstable social conditions”. These results also strongly support those of other investigators, making very clear the distinction between aggression and dominance.

Crustaceans: dominance hierarchies and serotonin neuron function As in earlier studies that addressed fundamental questions in neuronal function, so in the study of complex behaviors such as aggression, the ability to bring questions to the level of single large identified nerve cells remains a great virtue of invertebrate model systems. Here, one has the ability to ask whether particular neurons or their targets are modified as a consequence of social interactions, and, ultimately, to define those modifications at the level of the precise changes taking place. Crustacean species in particular exhibit well-defined behaviors, such as aggression, that can be quantified [27-291, and serotonin and octopamine (the phenol analogue of norepinephrine) appear to play important roles in agonistic behavior in these animals [6].

Dominance relationships Two lobsters placed together in a confined space will fight (engage in agonistic encounters), and if the animals cannot escape each other, the fights will escalate in intensity through a series of highly ritualized steps [27-291. The entire encounter can be quantified, which allows definition of the parts of the encounters altered by pharmacological manipulation (7”,27,30]. The behavior in the laboratory differs from that observed in the wild, where encounters are of brief duration, usually decided by size disparity. The

Serotonin, social status and aggression Edwards and Kravitz 815

Figure 1

I Al -5HT neuron

Phasic system (escape I fighting)

Tonic system (posture)

Tail

/

“, ,“,, ,, 1 Motor neurons 1 <a{ &

Schematic diagram of the connections of a serotomn-containing neurosecretoiy cell in the Al ganglion (Al -5HT cell) and an artist’s drawing of the cell (inset). The axonal extension of these cells runs from the Al ganglion, through all the anterior thoracic ganglia to the subesophageal ganglion. Serotonin is released from these cells from two sets of endings in each of the anterior ganglia: one set of endings lies wlthln central neuropil regions, and the other set runs along the second thoraclc nerve roots. These cells influence both tonic (postural) and phasic (escape/fighting) muscle systems, and function as gain-setters (see text for details). The inset is reprinted with permission from [421, and the schematic is reprinted with permission from [351.

consequence of winning an encounter is that animals are more likely to win their next encounters, whereas losing animals are unwilling to fight against either winners or losers of previous encounters for periods of up to several days. Such animals appear to develop a ‘loser’ menralicy. which can be reversed for short periods of time by injections of serotonin [7**,30]. Statistical analyses of fights allows us to suggest chat serotonin injections increase the willingness of animals to fight (enhance their ‘aggressive motivation’) and does not result from, for example, simply turning on motor programs directing animals to walk in a forward direction. The reversal of status induced by serotonin can be prevented if fluoxetine (also known as Prozac, a major antidepressant medication and an inhibitor of serotonin uptake) is injected along with the amine. This

suggests rhat uptake of the injected amine by serotonergic nerve terminals and subsequent release of elevated levels of serotonin might be key parts of the behavioral reversals we observe [7**,30].

How do crustacean serotonin neurons work? Studies exploring the role of amines in aggression in crustaceans began with the observations that amine injection into animals triggered the appearance of postures resembling those seen in dominant (serotonin) and subordinate (octopamine) animals ([31]; for a review of earlier studies, see [6]). The recent focus is on a single pair of large serotonin neurosecretory neurons, whose cell bodies are in the first abdominal ganglion (Al). These cells (called Al-5HT cells) appear to play important

816 Motor systems

roles in both tonic (postural) and phasic (escape and fighting) motor systems through the release of serotonin from two distinct sets of endings. One set of endings in peripheral neurosecretory plexuses is the source of circulating serotonin that influences exoskeletal muscles. A second set of endings within each anterior ganglion in the ventral nerve cord enables the cells to interact with and influence central motor circuitries [32] (inset Figure 1). The cells are spontaneously active, and their rate of firing is strongly influenced by synaptic input. Thus far, we know that these cells receive both strong inhibitory input-probably from GABAergic neurons in the A3 ganglion [33] and extensor postural command neurons-and strong excitatory input-from flexor commands [34], medial (MS) and lateral (LG) giant interneurons, and activating sensory nerve roots of the last abdominal ganglion [35]. Bath-applied serotonin and octopamine inhibit the firing of Al-5HT cells, whereas the pentapeptide proctolin, which colocalizes with serotonin in these neurons, enhances their firing [36].

Effects of serotonin on tonic (postural) circuitries The effects of activation of the Al-5HT neurosecretory cells on postural command circuitries are different from the actions of bath-applied amines (see right side of Figure 1): when the cells fire, they do not direct the readout of motor programs from the ventral nerve cord as do bath-applied or injected amines. Instead, the cells function as ‘gain-setters’, being activated by and capable of enhancing motor output from CNS circuitries that generate postural flexion (the postural stance characteristic of dominant animals). Extensor commands (which trigger stances resembling those seen in subordinate animals) inhibit the firing of the cells. However, if Al-5HT cells are forced to fire with an intracellular electrode, they serve as gain-setters for extensor commands as well. Thus, these neurons appear to function as general gain setters, with the specificity being determined by the circuitry that either increases or decreases their rate of firing [34].

Effects of serotonin on phasic (escape and fighting) circuitries Recently, we have demonstrated that these same serotonin cells function in the phasic motor circuitries used in escape and possibly in fighting behavior [35]. Thus, firing LG or MG axons elicits a depolarizing synaptic response in Al-5HT neurons, which usually fires the cell. MG cell bodies are located in the supraesophageal ganglion (brain) of the lobster. When activated, their large rapidly conducting axons trigger a tailflip that propels the animals backwards. LG cell bodies are arranged segmentally, and their fused axons ascend the ventral nerve cord. Activation of LG axons causes an upwards-directed tailflip, which is seen in both escape and fighting behaviors-although in crayfish, the tailflips elicited in fighting behavior are not mediated via firing of the LG axons ([37”]; see below). Pre-firing the Al-5HT cells reduces the effectiveness of the input to the cells from the LG axons, and reduces

their spontaneous firing. In crayfish, sensory input to the LG from the telson is modulated by serotonin in opposite ways in dominant and subordinate animals ([8”,38]; see below).

These are interesting neurons as they do not appear to be involved in the point-to-point wiring that typifies most neuronal circuits. Instead, they are capable of reaching and, by release of serotonin, influencing the broad areas of the nervous system (i.e. sensory neurons, CNS neurons, muscles) concerned with postural regulation and motor systems (i.e. they are system-specific, rather than circuit-specific neurons). Intrinsic changes in the functioning of such cells or changes in the responsiveness to serotonin of their targets, therefore, become potentially interesting ways to modify the effectiveness of neuronal circuitries without the necessity of changing the core wiring diagram. Social experience in crayfish does cause such changes to occur, as is illustrated below [8”,38].

Figure 2

(a) (b) (d

Current Op,n,on I” Neurholog)

Patring of isolate (I), subordinate (S) or dominant (D) crayfish (labeled circles) leads to changes in social status and delayed changes in the modulatory effect of serotonin on the LG neuron. (a) A pair of isolate crayfish quickly become dominant and subordinate, but the changes in the modulatory effect of serotonin (filled pattern -see key) take 12 days to develop and 8 days to revert when the subordinate IS re-isolated. (b) A pair of subordinate crayfish fight and one becomes dominant; the change in serotonin’s effect takes 12 days. (c) A pair of dominant crayfish fight, and one becomes subordinate. LIttIe change occurs in the modulatory effect of serotonin in the subordinate animal, even after 38 days.

Social experience changes serotonin responsiveness at specific synaptic sites Synaptic input to the LG in the tailflip escape circuit of crayfish has been known to be modulated by serotonin for some time [39]; recently, however, it was shown that the modulation depends on the social status of the animal [8**,38]. In socially isolated crayfish (isolates), serotonin enhances the response of the LG to stimulation of a sensory nerve, a response that is not readily reversed by superfusion with saline. In subordinate animals, serotonin

Serotonin, social status and aggression Edwards and Kravitz 817

inhibits the LG response to the same stimulation, whereas in dominant crayfish, as in isolates, serotonin facilitates the response. Unlike the response facilitation in isolates, however, the enhancement in dominants and the inhibition in subordinates are readily reversed by washing with saline.

Dominant and subordinate crayfish pairs can be generated by pairing long-term isolates. Although subordinates can be identified by their behavior within an hour of pairing, the inhibitory effect of serotonin on the LG synaptic input takes two weeks to develop, and then can be reversed by re-isolating animals for 8 days (Figure Za).

Social promotion from subordinate to dominant status is seen in one of the animals when subordinates are paired (Figure Zb). The inhibitory effect of serotonin on the LG response in subordinates changes to a facilitation in new dominants over a 12-day period following the pairing. When two dominant animals are paired, a social demotion of one of the animals to subordinate is seen, but the effect of serotonin on the LG response remains facilitatory in the new subordinate animal for at least 38 days after the demotion [So*,381 (Figure 2). Thus, the change in receptor responsiveness associated with acquiring dominant status in crayfish, once formed, is more stable than the change associated with subordinate status. A similar pattern is seen in cichlid fish [40], in which the growth of a particular group of gonadotropin-releasing hormone (GnRH)-containing neurons that accompanies dominant status is not readily reversed on demotion of the animals to subordinate.

The different effects of serotonin on the synaptic responses of LG in dominant, subordinate and isolate crayfish appear to result from differences in the pop- ulations of serotonin receptors present in LG neurons in the three animal groups. As in vertebrate systems, multiple serotonin receptors are present in crustaceans [41]. Experiments with two vertebrate agonists suggest that serotonin receptors (or their downstream effecters) are either transformed or replaced following a change in social status. The transition from isolate status, whereby serotonin-mediated facilitation decays very slowly co either dominant or subordinate status, induces the appearance of receptors showing rapidly decaying inhibitory and facilitatory responses. The changes appear balanced in such a way as to enable inhibitory mechanisms to govern the response in subordinate crayfish, and facilitatory mechanisms to govern in dominant animals [8**,32].

Such slow changes in receptor responsiveness may be a part of a broader pattern of neuronal adaptation to a change in social status that produces corresponding changes in behavior. Indeed, during fights, the stimulus threshold of the LG neuron and the tailflip response it evokes are increased in the subordinate animal of a pair but not in the dominant one [37**]. An increase in the LG

firing threshold in subordinate crayfish is unexpected; one might anticipate that subordinate animals should be more rather than less likely to want to escape. The LG circuit, which is triggered by a tap on the tail, mediates a rapid reflexive movement that pitches the animals upward and forward in a somersault. Other, nongiant neural elements mediate only slightly slower voluntary escape movements that can direct animals in any direction. It might be useful for subordinate animals to suppress tailflips that allow movement in only one direction during fights in favor of more flexible voluntary escape patterns.

It remains uncertain how the changes in serotonin receptor distribution are brought about, but what is clear is that they are linked to changes in status. Moreover, the response is slow compared to the decision-making about whether to give up in an agonistic encounter. The latter decision is made over a time window of minutes to tens of minutes, while the changes in receptor distribution take weeks to manifest themselves. Serotonin may play prominent roles in both the decision-making aspects of the hierarchical relationship formation (as subordinate status can be reversed for short time periods by amine injection), and in the longer-term change in receptor function, but further experiments will be required to establish whether this is so.

We do not yet know the source of endogenous serotonin that modulates the LG response to serotonin. Among the possibilities are serotonin-immunolabelled nerve terminals seen in close apposition to LG axons [PI; on the other hand, the sensitivity of LG axons may be sufficient to respond to the relatively low levels of serotonin found in the haemolymph. Recently, we have found that the LG excites Al-SHT neurons [35], usually leading to the firing of an action potential in the Al-5HT cells (Figure 1). We anticipate, therefore, that the pathway from sensory input, through LG to the Al-5HT cells will function more effectively in the presence of serotonin in dominant animals. Whether the additional serotonin released via this route contributes to the further facilitation of this pathway also remains to be established (Figure 1).

The behavior of animals is strongly influenced by social interactions, and changes in behavior must be accompanied by changes in neuronal function. It would be naive of us to suggest that the changes we report here are the only ways in which changes in social status cause changes in neural function in crustaceans or in any other species. Nor do we know whether these are the most important changes taking place in crustacean nervous systems accompanying changes in status. They are, however, among the first specific changes in neural functioning reported to accompany a maintained change in social status. In that regard, it is interesting that they involve the serotonergic system and concern its role in aggression.

818 Motor systems

Conclusions The study of behavior and the roles served in that behavior by chemical substances or particular sets of neurons has been a frustrating one. Part of the difficulty can be related to the ‘blind men examining an elephant’ phenomenon. Unidimensional approaches -which often were taken in the past and which are resurfacing at present with the availability of powerful molecular methods-just will not do when attempting to understand the neuronal basis for complex behaviors such as aggression. Instead, multidimensional studies will be required to solve these challenging and fascinating problems. In such studies, the same serious attention should be paid to a careful analysis of the behavior of interest, to an examination of how suspected behaviorally relevant substances function at both cellular and systems levels of analysis, and to how, from an understanding of the behavior and the underlying physiological processes, molecular methods can be used to provide important and valuable new tools towards understanding these complex problems.

This paper is one of the first to show that changes in social status are accompanied by selective changes at specific synaptic sites in the nervous system. The results are thoroughly described in the text of this review.

9. . .

In this paper, and a series of related papers, the authors present convincing evidence of the involvement of the peptide arginine vasopressin (AVP) in aggressive behavior in hamsters. When microinjected into the anterior hypo- thalamus, this peptide increases several measures of offensive aggression. Fluoxetine prevents this increase, and this and other data suggest that sero- tonergic presynaptic mechanisms may play a role in modulating the activity of AVP neurons.

10.

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Acknowledgements 14.

\\‘c thank our recent outstanding cnllaborators (R Huber. hl Hixner, \\’ \Vcigrr, F Krasne, S-R Ych and rnan~ nthcrs) for fruitful discussions and for providing the experimental support for the studies wc report in this review. This work was supported by a Rlultidisciplina~ Collaborative Research grant from the National Science Foundation.

15.

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