Oxytocin reduces exploratory motor behaviour and shifts the activity towards the centre of the arena in male rats

Water deprivation induces hypoactivity in rats independently of oxytocin receptor signaling at the central amygdala

INTRODUCTION

Vasopressin (AVP) and oxytocin (OXT) are neuropeptides produced by magnocellular neurons (MCNs) of the hypothalamus and secreted through neurohypophysis to defend mammals against dehydration. It was recently demonstrated that MCNs also project to limbic structures, modulating several behavioral responses.

METHODS AND RESULTS

We found that 24 h of water deprivation (WD) or salt loading (SL) did not change exploration or anxiety-like behaviors in the elevated plus maze (EPM) test. However, rats deprived of water for 48 h showed reduced exploration of open field and the closed arms of EPM, indicating hypoactivity during night time. We evaluated mRNA expression of glutamate decarboxylase 1 (Gad1), vesicular glutamate transporter 2 (Slc17a6), AVP (Avpr1a) and OXT (Oxtr) receptors in the lateral habenula (LHb), basolateral (BLA) and central (CeA) amygdala after 48 h of WD or SL. WD, but not SL, increased Oxtr mRNA expression in the CeA. Bilateral pharmacological inhibition of OXTR function in the CeA with the OXTR antagonist L-371,257 was performed to evaluate its possible role in regulating the EPM exploration or water intake induced by WD. The blockade of OXTR in the CeA did not reverse the hypoactivity response in the EPM, nor did it change water intake induced in 48-h water-deprived rats.

DISCUSSION

We found that WD modulates exploratory activity in rats, but this response is not mediated by oxytocin receptor signaling to the CeA, despite the upregulated Oxtr mRNA expression in that structure after WD for 48 h.

Oxytocin-conjugated saporin injected into the substantia nigra of male rats alters the activity of the nigrostriatal dopaminergic system: A behavioral and neurochemical study

Saporin conjugated to oxytocin (OXY-SAP) destroys neurons expressing oxytocinergic receptors. When injected unilaterally in the substantia nigra of male rats, OXY-SAP causes a dose-dependent decrease up to 55 % in nigral Tyrosine Hydroxylase (TH)-immunoreactivity compared to control mock peptide BLANK-SAP- and PBS-treated rats or the contralateral substantia nigra. TH decrease was parallel to a dopamine content decrease in the ipsilateral striatum compared to BLANK-SAP- or PBS-treated rats or the contralateral striatum. OXY-SAP-treated rats showed a small but significant increase of locomotor activity 28 days after intranigral injection in the Open field test compared to BLANK-SAP- or PBS-treated rats, in line with an inhibitory role of nigral oxytocin on locomotor activity. OXY-SAP-, but not BLANK-SAP- or PBS-treated rats, also showed marked dose-dependent rotational turning ipsilateral to the injected substantia nigra when challenged with d-amphetamine, but not with apomorphine. Under isoflurane anesthesia OXY-SAP-treated rats showed levels of extracellular dopamine in the dialysate from the ipsilateral striatum only half those of BLANK-SAP- or PBS-treated rats or the contralateral striatum. When treated with d-amphetamine, OXY-SAP_60/120 rats showed increased extracellular dopamine levels in the dialysate from the ipsilateral striatum two third/one third only of those found in BLANK-SAP- or PBS-treated rats or the contralateral striatum, respectively. These results show that OXY-SAP destroys nigrostriatal dopaminergic neurons expressing oxytocin receptors leading to a reduced striatal dopamine function.

Molecular, biochemical and behavioural evidence for a novel oxytocin receptor and serotonin 2C receptor heterocomplex

The complexity of oxytocin-mediated functions is strongly associated with its modulatory effects on other neurotransmission systems, including the serotonin (5-hydroxytryptamine, 5-HT) system. Signalling between oxytocin (OT) and 5-HT has been demonstrated during neurodevelopment and in the regulation of specific emotion-based behaviours. It is suggested that crosstalk between neurotransmitters is driven by interaction between their specific receptors, particularly the oxytocin receptor (OTR) and the 5-hydroxytryptamine 2C receptor (5-HTR_2C), but evidence for this and the downstream signalling consequences that follow are lacking. Considering the overlapping central expression profiles and shared involvement of OTR and 5-HTR_2C in certain endocrine functions and behaviours, including eating behaviour, social interaction and locomotor activity, we investigated the existence of functionally active OTR/5-HTR_2C heterocomplexes. Here, we demonstrate evidence for a potential physical interaction between OTR and 5-HTR_2Cin vitro in a cellular expression system using flow cytometry-based FRET (fcFRET). We could recapitulate this finding under endogenous expression levels of both receptors via in silico analysis of single cell transcriptomic data and ex vivo proximity ligation assay (PLA). Next, we show that co-expression of the OTR/5-HTR_2C pair resulted in a significant depletion of OTR-mediated Gαq-signalling and significant changes in receptor trafficking. Of note, attenuation of OTR-mediated downstream signalling was restored following pharmacological blockade of the 5-HTR_2C. Finally, we demonstrated a functional relevance of this novel heterocomplex, in vivo, as 5-HTR_2C antagonism increased OT-mediated hypoactivity in mice. Overall, we provide compelling evidence for the formation of functionally active OTR/5-HTR_2C heterocomplexes, adding another level of complexity to OTR and 5-HTR_2C signalling functionality. This article is part of the special issue on Neuropeptides.

Oxytocin has sex-specific effects on social behaviour and hypothalamic oxytocin immunoreactive cells but not hippocampal neurogenesis in adult rats

Oxytocin regulates social behaviours, pair bonding and hippocampal neurogenesis but most studies have used adult males. Our study investigated the effects of oxytocin on social investigation and adult hippocampal neurogenesis in male and female rats. Oxytocin has poor penetration of the blood-brain barrier, therefore we tested a nanoparticle drug, TRIOZAN™ (Ovensa Inc.), which permits greater blood-brain-barrier penetration. Adult male and female rats were injected daily (i.p.) for 10 days with either: oxytocin in PBS (0.5 or 1.0 mg/kg), oxytocin in TRIOZAN™ (0.5 or 1.0 mg/kg), or vehicle (PBS) and tested for social investigation. Oxytocin decreased body mass and increased social investigation and number of oxytocin-immunoreactive cells in the supraoptic nucleus (SON) of the hypothalamus in male rats only. In both sexes, oxytocin decreased the number of immature neurons (doublecortin+ cells) in the ventral hippocampus and reduced plasma 17β-estradiol levels in a dose- and delivery-dependent way. Oxytocin in TRIOZAN™ reduced "sedation" observed post-injection and increased certain central effects (oxytocin levels in the hypothalamus and neurogenesis in the ventral hippocampus) relative to oxytocin in PBS, indicating that the nanoparticle may be used as an alternative brain delivery system. We showed that oxytocin has sex-specific effects on social investigation, body mass, "sedation", and the oxytocin system. In contrast, similar effects were observed in both sexes in neurogenesis and plasma 17β-estradiol. Our work suggests that sex differences in oxytocin regulation of brain endpoints is region-specific (hypothalamus versus hippocampus) and that oxytocin does not promote social investigation in females.

Selective and non-selective OT receptor agonists induce different locomotor behaviors in male rats via central OT receptors and peripheral V1a receptors

Oxytocin (OT) continues to inspire much research due to its diverse physiological effects. While the best-understood actions of OT are uterine contraction and milk ejection, OT is also implicated in maternal and bonding behaviors, and potentially in CNS disorders such as autism, schizophrenia, and pain. The dissection of the mechanism of action of OT is complicated by the fact that this peptide activates not only its cognate receptor but also vasopressin type 1a (V1a) receptors. In this study, we evaluated OT and a selective OT receptor (OTR) agonist, FE 204409, in an automated assay that measures rat locomotor activity. The results showed: 1) Subcutaneous (sc) administration of OT decreased locomotor behavior (distance traveled, stereotypy, and rearing). This effect was reversed by a V1a receptor (V1aR) antagonist ([Pmp1,Tyr(ME)2]AVP, sc), suggesting that OT acts through peripheral V1aR to inhibit locomotor activity. 2) A selective OTR agonist (FE 204409, sc) increased stereotypy. This effect was reversed by an OTR antagonist dosed icv, suggesting a central OTR site of action. Our findings identify distinct behavioral effects for OT and the selective agonist FE 204409, adding to the growing body of evidence that the V1aR mediates many effects attributed to OT and that peptides administered systemically at supra-physiological doses may activate receptors in the brain. Our studies further emphasize the importance of utilizing selective agonists and antagonists to assess therapeutic indications.

Oxytocin facilitates adaptive fear and attenuates anxiety responses in animal models and human studies-potential interaction with the corticotropin-releasing factor (CRF) system in the bed nucleus of the stria terminalis (BNST)

Despite its relatively well-understood role as a reproductive and pro-social peptide, oxytocin (OT) tells a more convoluted story in terms of its modulation of fear and anxiety. This nuanced story has been obscured by a great deal of research into the therapeutic applications of exogenous OT, driving more than 400 ongoing clinical trials. Drawing from animal models and human studies, we review the complex evidence concerning OT's role in fear learning and anxiety, clarifying the existing confusion about modulation of fear versus anxiety. We discuss animal models and human studies demonstrating the prevailing role of OT in strengthening fear memory to a discrete signal or cue, which allows accurate and rapid threat detection that facilitates survival. We also review ostensibly contrasting behavioral studies that nonetheless provide compelling evidence of OT attenuating sustained contextual fear and anxiety-like behavior, arguing that these OT effects on the modulation of fear vs. anxiety are not mutually exclusive. To disambiguate how endogenous OT modulates fear and anxiety, an understudied area compared to exogenous OT, we survey behavioral studies utilizing OT receptor (OTR) antagonists. Based on emerging evidence about the role of OTR in rat dorsolateral bed nucleus of stria terminalis (BNST) and elsewhere, we postulate that OT plays a critical role in facilitating accurate discrimination between stimuli representing threat and safety. Supported by human studies, we demonstrate that OT uniquely facilitates adaptive fear but reduces maladaptive anxiety. Last, we explore the limited literature on endogenous OT and its interaction with corticotropin-releasing factor (CRF) with a special emphasis on the dorsolateral BNST, which may hold the key to the neurobiology of phasic fear and sustained anxiety.

C-tactile afferents: Cutaneous mediators of oxytocin release during affiliative tactile interactions?

Low intensity, non-noxious, stimulation of cutaneous somatosensory nerves has been shown to trigger oxytocin release and is associated with increased social motivation, plus reduced physiological and behavioural reactivity to stressors. However, to date, little attention has been paid to the specific nature of the mechanosensory nerves which mediate these effects. In recent years, the neuroscientific study of human skin nerves (microneurography studies on single peripheral nerve fibres) has led to the identification and characterisation of a class of touch sensitive nerve fibres named C-tactile afferents. Neither itch nor pain receptive, these unmyelinated, low threshold mechanoreceptors, found only in hairy skin, respond optimally to low force/velocity stroking touch. Notably, the speed of stroking which C-tactile afferents fire most strongly to is also that which people perceive to be most pleasant. The social touch hypothesis posits that this system of nerves has evolved in mammals to signal the rewarding value of physical contact in nurturing and social interactions. In support of this hypothesis, we review the evidence that cutaneous stimulation directly targeted to optimally activate C-tactile afferents reduces physiological arousal, carries a positive affective value and, under healthy conditions, inhibits responses to painful stimuli. These effects mirror those, we also review, which have been reported following endogenous release and exogenous administration of oxytocin. Taken together this suggests C-tactile afferent stimulation may mediate oxytocin release during affiliative tactile interactions.

Regional c-Fos expression induced by peripheral oxytocin administration is prevented by the vasopressin 1A receptor antagonist SR49059

Peripherally administered oxytocin induces a wide range of behavioural and physiological effects that are thought to be mediated by the oxytocin receptor (OTR). However, oxytocin also has considerable affinity for the vasopressin 1A receptor (V_1AR), such that various oxytocinergic effects may in fact be mediated by the V_1AR rather than the OTR. Here we used c-Fos immunohistochemistry to determine the extent to which the regional pattern of neuronal activation produced by peripheral oxytocin involves the V_1AR. Male Wistar rats were administered oxytocin (1mg/kg, IP) alone, or following pre-treatment with the V_1AR antagonist SR49059 (1mg/kg, IP), and were assessed for locomotor activity changes and for c-Fos expression across a number of brain regions. Oxytocin reduced the distance travelled by rats during a 70min test session, and this inhibitory behavioural effect was prevented by SR49059. Consistent with previous reports, oxytocin increased c-Fos expression in a number of brain regions. In several of these regions-the supraoptic and paraventricular (PVN) nuclei of the hypothalamus, locus coeruleus and nucleus of the solitary tract-the c-Fos response was prevented by SR49059 pre-treatment. Notably, SR49059 inhibited the c-Fos activation in oxytocin-synthesising magnocellular neurons in the PVN. However, c-Fos expression in the central amygdala to oxytocin was unaffected by SR49059. The current findings add to an increasing body of research suggesting that many of the functional effects of oxytocin may be V_1AR mediated.

Effect of feeding before, during and after milking on dairy cow behaviour and the hormone cortisol

The present study was performed to find out how dairy cow behaviour and the level of the adrenocortical hormone cortisol were influenced by feeding in relation to milking The experiment was designed as a Latin square and performed on 24 primiparous and multiparous Swedish Red and White dairy cows. Each treatment period lasted for 21 days, including 11 days when the cows became accustomed to the new routines. The cows were exposed to three treatments: feeding 1·5 h before milking (FBM), feeding exactly at the same time as milking (FDM) and feeding 1·5 h after milking (FAM). Recordings of individual behaviour were made with instantaneous direct observation of each cow. For the statistical analyses the recordings were divided into two groups: 0 to 1·5 h after feeding and 0 to 1·5 h after milking. Results with a P-value below 0·05 were regarded as statistically significant. The interesting findings in this study were: (a) 0 to 1·5 h after feeding, the percentage of lying was higher in FDM and the percentage of lying while ruminating was higher in FDM; (b) 0 to 1·5 h after milking, the cows in FAM had a significantly lower percentage of social interactions and a significantly higher percentage of oral activity; and (c) feeding-related level of cortisol stayed higher 30 to 60 min after feeding in FAM compared with FBM and FDM, whereas milking-related cortisol secretion tended to be lower in FDM than in FBM and FAM These results demonstrate that feeding during milking may be an important management routine, also for dairy cow behaviour.

Involvement of nigral oxytocin in locomotor activity: A behavioral, immunohistochemical and lesion study in male rats

Oxytocin is involved in the control of different behaviors, from sexual behavior and food consumption to empathy, social and affective behaviors. An imbalance of central oxytocinergic neurotransmission has been also associated with different mental pathologies, from depression, anxiety and anorexia/bulimia to schizophrenia, autism and drug dependence. This study shows that oxytocin may also play a role in the control of locomotor activity. Accordingly, intraperitoneal oxytocin (0.5-2000μg/kg) reduced locomotor activity of adult male rats. This effect was abolished by d(CH2)5Tyr(Me)(2)-Orn(8)-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles at the dose of 2μg/rat, which was ineffective on locomotor activity. Oxytocin (50-200ng/site) also reduced and d(CH2)5Tyr(Me)(2)-Orn(8)-vasotocin (2μg/site) increased locomotor activity when injected bilaterally into the substantia nigra, a key area in the control of locomotor activity. Conversely, the destruction of nigral neurons bearing oxytocin receptors by the recently characterized neurotoxin oxytocin-saporin injected into the substantia nigra, increased basal locomotor activity. Since oxytocin-saporin injected into the substantia nigra caused a marked reduction of neurons immunoreactive for tyrosine hydroxylase (e.g., nigrostriatal dopaminergic neurons) and for vesicular glutamate transporters VGluT1, VGluT2 and VGluT3 (e.g., glutamatergic neurons), but not for glutamic acid decarboxylase (e.g., GABAergic neurons), together these findings suggest that oxytocin influences locomotor activity by acting on receptors localized presynaptically in nigral glutamatergic nerve terminals (which control the activity of nigral GABAergic efferent neurons projecting to brain stem nuclei controlling locomotor activity), rather than on receptors localized in the cell bodies/dendrites of nigrostriatal dopaminergic neurons.

Oxytocin decreases diurnal and nocturnal arterial blood pressure in the conscious unrestrained spontaneously hypertensive rat

In this study, we assessed the effects of oxytocin (OT) on mean arterial blood pressure (MAP), heart rate (HR), and locomotor activity (LA) in male spontaneous hypertensive rats (SHR) and Sprague-Dawley (SDR) controls using telemetry. OT was given by intravenous injections of 0.1, 0.2 or 0.4mg/kg to assess short term acute effects or by daily subcutaneous injections of 0.5 or 1.0mg/kg for 5 days. Compared to the saline infusion, (i) intravenous OT, regardless of concentration, increased MAP in SHR and SDR, (ii) HR increased, but was periodically lower in both strains with 0.2 or 0.4mg/kg, and (iii) no effects of OT on LA were observed. Subcutaneous injections demonstrated that (i) 1.0mg/kg for 5days lowered diurnal MAP and HR in SDR and SHR, persisting for 6 days, (ii) 1.0mg/kg decreased nocturnal HR in SDR, (iii) 0.5 and 1.0mg/kg decreased MAP with minor effects on HR in the SHR, and lastly (iv) OT decreased LA mainly during the diurnal cycle in both strains. Our main results show that OT induces significant beneficial effects on cardiovascular function over several diurnal and nocturnal cycles in the SHR, with the most prominent effect being a robust decrease in MAP.

Body temperature and cardiac changes induced by peripherally administered oxytocin, vasopressin and the non-peptide oxytocin receptor agonist WAY 267,464: a biotelemetry study in rats

BACKGROUND AND PURPOSE

There is current interest in oxytocin (OT) as a possible therapeutic in psychiatric disorders. However, the usefulness of OT may be constrained by peripheral autonomic effects, which may involve an action at both OT and vasopressin V1A receptors. Here, we characterized the cardiovascular and thermoregulatory effects of OT, vasopressin (AVP) and the non-peptide OT receptor agonist WAY 267,464 in rats, and assessed the relative involvement of the OT and V1A receptors in these effects.

EXPERIMENTAL APPROACH

Biotelemetry in freely moving male Wistar rats was used to examine body temperature and heart rate after OT (0.01 - 1 mg kg(-1); i.p.), AVP (0.001 - 0.1 mg kg(-1); i.p.) or WAY 267,464 (10 and 100 mg kg(-1); i.p.). The actions of the OT receptor antagonist Compound 25 (C25, 5 and 10 mg kg(-1)) and V1A receptor antagonist SR49059 (1 and 10 mg kg(-1)) were studied, as well as possible V1A receptor antagonist effects of WAY 267,464.

KEY RESULTS

OT and AVP dose-dependently reduced body temperature and heart rate. WAY 267,464 had similar, but more modest, effects. SR49059, but not C25, prevented the hypothermia and bradycardia induced by OT and AVP. WAY 267,464 (100 mg·kg(-1)) prevented the effects of OT, and to some extent AVP.

CONCLUSIONS AND IMPLICATIONS

Peripherally administered OT and AVP have profound cardiovascular and thermoregulatory effects that appear to principally involve the V1A receptor rather than the OT receptor. Additionally, WAY 267,464 is not a simple OT receptor agonist, as it has functionally relevant V1A antagonist actions.

Oxytocinergic manipulations in corticolimbic circuit differentially affect fear acquisition and extinction

The oxytocinergic system promotes social behavior and reduces anxiety. The significant roles and functional interactions of the medial prefrontal cortex and the amygdala in the regulation of fear provide a unique experimental setting to examine the effects of oxytocin on extinction of fear. In this study we manipulated the oxytocin system at different time points in either the infralimbic region of the medial prefrontal cortex (IL-mPFC), the basolateral amygadala (BLA) or in the central amygdala (CeA). Manipulations of the oxytocin following retrieval of fear in the IL-mPFC resulted in facilitation of subsequent extinction. In contrast, in the BLA, manipulating the oxytocinergic system after the retrieval of fear was associated with contrasting effects; whereas the microinjection of exogenous synthetic oxytocin was associated with impaired extinction, the microinfusion of WAY-267474 facilitated extinction. In contrast, intra-BLA microinfusion of the selective agonist TGOT did not affect freezing. Oxytocin manipulations in the CeA had no effect on subsequent extinction. Contrasting effects were also found when the drugs were injected before conditioning. Whereas oxytocin manipulations in the BLA enhanced fear and impaired extinction, in the CeA the microinfusion of the selective agonists (WAY-267474 and TGOT), but not synthetic oxytocin, resulted in reduced freezing levels. These results show that in the rat, the oxytocinergic system differentially regulated fear and extinction in region and temporal-dependent manners and further join data to show that contrary to the prevailing belief that oxytocin is solely involved in reducing fear, oxytocin can also act as an enhancer of fear responses.

The nonpeptide oxytocin receptor agonist WAY 267,464: receptor-binding profile, prosocial effects and distribution of c-Fos expression in adolescent rats

Previous research suggests that the nonpeptide oxytocin receptor (OTR) agonist WAY 267,464 may only partly mimic the effects of oxytocin in rodents. The present study further explored these differences and related them to OTR and vasopressin 1a receptor (V(1a) R) pharmacology and regional patterns of c-Fos expression. Binding data for WAY 267,464 and oxytocin were obtained by displacement binding assays on cellular membranes, while functional receptor data were generated by luciferase reporter assays. For behavioural testing, adolescent rats were tested in a social preference paradigm, the elevated plus-maze (EPM) and for locomotor activity changes following WAY 267,464 (10 and 100 mg/kg, i.p.) or oxytocin (0.1 and 1 mg/kg, i.p.). The higher doses were also examined for their effects on regional c-Fos expression. Results showed that WAY 267,464 had higher affinity (K(i) ) at the V(1a) R than the OTR (113 versus 978 nm). However, it had no functional response at the V(1a) R and only a weak functional effect (EC(50) ) at the OTR (881 nm). This suggests WAY 267,464 is an OTR agonist with weak affinity and a possible V(1a) R antagonist. Oxytocin showed high binding at the OTR (1.0 nm) and V(1a) R (503 nm), with a functional EC(50) of 9.0 and 59.7 nm, respectively, indicating it is a potent OTR agonist and full V(1a) R agonist. WAY 267,464 (100 mg/kg), but not oxytocin, significantly increased the proportion of time spent with a live rat, over a dummy rat, in the social preference test. Neither compound affected EPM behaviour, whereas the higher doses of WAY 267,464 and oxytocin suppressed locomotor activity. WAY 267,464 and oxytocin produced similar c-Fos expression in the paraventricular hypothalamic nucleus, central amygdala, lateral parabrachial nucleus and nucleus of the solitary tract, suggesting a commonality of action at the OTR with the differential doses employed. However, WAY 267,464 caused greater c-Fos expression in the medial amygdala and the supraoptic nucleus than oxytocin, and lesser effects in the locus coeruleus. Overall, our results confirm the differential effects of WAY 267,464 and oxytocin and suggest that this may reflect contrasting actions of WAY 267,464 and oxytocin at the V(1a) R. Antagonism of the V(1a) R by WAY 267,464 could underlie some of the prosocial effects of this drug either through a direct action or through disinhibition of oxytocin circuitry that is subject to vasopressin inhibitory influences.

Psychosomatic pain in children: A psychomuscular tension reaction?

Although recurrent psychosomatic pain is a common paediatric condition, our understanding of it is still limited. In this article, the clinical picture is presented and the concept of psychosomatic pain is discussed. Some recent scientific data are presented including a controlled blind study of pain pressure thresholds showing significantly reduced mean values over common pain areas in a group of children with psychosomatic pain. According to another study, these children also have significantly reduced concentrations of the hypothalamic hormone oxytocin and cortisol. With these data as a point of departure, a model for psychosomatic pain reaction is outlined in which stress is assumed to induce muscular tension, in turn triggering the nociceptive process. In this psychosomatic reaction, the hypothalamus is believed to have a central regulatory function.

Anxiolytic-like activity of oxytocin in male mice: behavioral and autonomic evidence, therapeutic implications

RATIONALE

Oxytocin (OT) acts as a neuromodulator/neurotransmitter within the central nervous system (CNS) and regulates a diverse range of CNS functions. Notably, evidence from studies in females has revealed an important role for OT in regulating anxiety behavior.

OBJECTIVES

The objective of this study was to examine the effects of OT on both behavioral and autonomic parameters of the anxiety response in male mice using three pharmacologically validated preclinical models of anxiety: the four-plate test (FPT), elevated zero maze (EZM), and stress-induced hyperthermia (SIH).

RESULTS

In the FPT, both peripherally (3-30 mg/kg i.p.) and centrally (1-10 microg i.c.v.) administered OT produced dose-dependent increases in punished crossings, indicating an anxiolytic-like effect. The effects of centrally administered OT in the FPT were blocked with peripheral administration of a brain-penetrant OT receptor (OTR) antagonist WAY-162720 (30 mg/kg i.p.), and the effects of peripherally administered OT were blocked with central administration of a non-penetrant OTR antagonist L-371,257, suggesting OT acts centrally. In the EZM, centrally administered OT (0.1-1.0 microg, i.c.v.) produced significant increases in the percentage time spent in the open quadrants of the maze, comparable to alprazolam (0.5-1.0 microg, i.c.v.). In SIH, OT (1-10 mg/kg i.p.) dose-dependently attenuated stress-induced increases in core body temperature, comparable to the reference anxiolytic chlordiazepoxide (CDP) (10 mg/kg i.p.).

CONCLUSIONS

These results provide specific behavioral and autonomic evidence of anxiolytic-like effects for oxytocin in males and, together with previously reported observations in females, suggest the potential utility of OTR agonism as a therapeutically relevant mechanism of action for novel anxiolytics in both sexes.

The psychobiology of emotion: the role of the oxytocinergic system

A necessary condition for the individual's survival is the capacity for mental, behavioral, and physiological adaptation to external and internal conditions. Consequently, the integrated organism strives to maintain a dynamic, functional balance and integrity under varying conditions. Effective individual adaptation processes are basically dependent on the functioning of the integrated psychophysiological system. In humans, the brain plays a fundamental role in these processes. It serves the adaptation of individuals to current and anticipated conditions by selecting, interpreting, and transforming information into mental, behavioral, and physiological responses. In doing so, the incoming information is linked to existing structures of emotions, values, and goals. Consequently, the interpretation of external information may vary and become subjective depending on an individual's present and past experiences (see e.g., Magnusson, 2003). Hitherto, empirical research has been mainly concerned with the aspect of the psychophysiological system, which is activated in situations that are perceived by the individual as threatening, harmful, or demanding and in which the fight-flight and stress responses described by Cannon (1929) and Selye (1976) play an important role. The aim of this article is to draw attention to a component of the psychophysiological system, the calm and connection system, underlying well-being and socialization. By including this new system, the model of the integrated individual becomes more complete and it enriches the understanding of emotional aspects of brain functioning.

Breast-feeding and maternal health in online mothers

Self-reports of mothers currently breast-feeding (n = 561) and mothers who had breast-fed in the past (n = 452) were compared for perceived stress, self-reports of upper respiratory infection symptoms, and physician visits for psychological illnesses. Possible demographic confounds were controlled statistically. In analyses examining breast-feeding status as a dichotomous variable (current vs. past), breast-feeding was negatively associated with perceived stress and upper respiratory symptoms (the latter association dissolved when controlling for perceived stress), but not with physician visits for psychological illnesses. However, analyses of the continuous variables of frequency of breast-feeding and cumulative amount of breast-feeding revealed negative associations, and analyses of times since last nursing revealed positive associations with likelihood for physician visits for psychological illnesses. Frequency of bottle-feeding was positively associated with perceived stress. The results support the interrelatedness of breast-feeding and maternal health in online mothers.

Acupuncture mechanisms for clinically relevant long-term effects--reconsideration and a hypothesis

From the author's direct involvement in clinical research, the conclusion has been drawn that clinically relevant long-term pain relieving effects of acupuncture (>6 months) can be seen in a proportion of patients with nociceptive pain. The mechanisms behind such effects are considered in this paper. From the existing experimental data some important conclusions can be drawn: 1. Much of the animal research only represents short-term hypoalgesia probably induced by the mechanisms behind stress-induced analgesia (SIA) and the activation of diffuse noxious inhibitory control (DNIC). 2. Almost all experimental acupuncture research has been performed with electro-acupuncture (EA) even though therapeutic acupuncture is mostly gentle manual acupuncture (MA). 3. Most of the experimental human acupuncture pain threshold (PT) research shows only fast and very short-term hypoalgesia, and, importantly, PT elevation in humans does not predict the clinical outcome. 4. The effects of acupuncture may be divided into two main components--acupuncture analgesia and therapeutic acupuncture. A hypothesis on the mechanisms of therapeutic acupuncture will include: 1. Peripheral events that might improve tissue healing and give rise to local pain relief through axon reflexes, the release of neuropeptides with trophic effects, dichotomising nerve fibres and local endorphins. 2. Spinal mechanisms, for example, gate-control, long-term depression, propriospinal inhibition and the balance between long-term depression and long-term potentiation. 3. Supraspinal mechanisms through the descending pain inhibitory system, DNIC, the sympathetic nervous system and the HPA-axis. Is oxytocin also involved in the long-term effects? 4. Cortical, psychological, "placebo" mechanisms from counselling, reassurance and anxiety reduction.

Oxytocin facilitates behavioural, metabolic and physiological adaptations during lactation

The aim of this article is to propose that oxytocin not only stimulates milk let down, but also adapts behaviour and physiology to facilitate lactation in mammals including dairy cattle. Circulating oxytocin as well as neurogenic oxytocin participates in these regulatory processes. In short, oxytocin stimulates maternal interaction and attachment between mother and young. It also participates in the metabolic prerequisites for milk production by e.g. stimulating glucagon release and thereby, mobilisation of glucose. Digestive and anabolic aspects of metabolism are also stimulated, e.g. by increased vagal nerve activity. Adaptations consistent with an antistress like pattern are also induced. Cortisol levels are decreased as well as blood pressure, and behaviours characterised by calm, reduced levels of anxiety and more social activity are promoted. These effects seem to be present in monogastric animals as well as in ruminants. The expression of various aspects of these adaptations vary according to the special needs and living environmental circumstances of different species. The mechanisms behind the effect spectrum of oxytocin are being explored in other experimental models. A second aim of this paper is to suggest that efficiency of lactation can be promoted by facilitating oxytocin release in connection with milking by enhancing the amount of sensory stimulation.

Systemic oxytocin treatment modulates alpha 2-adrenoceptors in telencephalic and diencephalic regions of the rat

Systemic subchronic oxytocin treatment significantly and substantially increased the B(max) values of the alpha 2 agonist [(3)H]UK14.304 binding sites in the hypothalamus, the amygdala and the paraventricular thalamic nucleus of the rat as shown by quantitative receptor autoradiography. These results suggest that long-term modulation of autonomic and neuroendocrine functions and emotional behaviours elicited by brain oxytocin may involve enhancement of central alpha 2-adrenoceptor function.

Improved conditioned avoidance learning by oxytocin administration in high-emotional male Sprague-Dawley rats

OBJECTIVE

To examine anti-stress-like properties of oxytocin as a means to improve conditioned avoidance learning in a low-performing, high-emotional, stock of Sprague-Dawley male rats.

METHODS

Adult male rats of two stocks of the Sprague-Dawley strain, designated Stock A and Stock B, were treated daily with oxytocin (1 mg kg(-1) s. c.) for 5 days preceding four daily conditioned avoidance acquisition sessions (approximately 20 trials per 15 min session). The Stock B animals were previously characterized as high-emotional based on [1] elevated plasma corticosterone, and lowered plasma oxytocin, levels and [2] decreased reaction time and an increased startle amplitude to an acoustic stimulation. Finally, [3] these animals were unable to acquire a conditioned avoidance response within 5 days of training.

RESULTS

The Stock A animals rapidly and statistically significantly acquired the avoidance behaviour within 4 days of daily training, whereas Stock B animals did not improve over this time period. The avoidance performance of Stock B animals was markedly and statistically significantly improved by the oxytocin pre-treatment, whereas the performance of Stock A animals was not affected by the same oxytocin treatment.

CONCLUSIONS

Pre-treatment with oxytocin markedly improved avoidance learning in the Stock B high-emotional animals. It is suggested that the improvement is due to previously demonstrated anti-stress-like properties of oxytocin, rendering the animals able to successfully cope with the demands of the conditioned avoidance situation.

Endocrine and behavioral traits in low-avoidance Sprague-Dawley rats

In the present series of experiments, we have examined the endocrine profile of two stable colonies of Sprague-Dawley rats, here labeled Stock A, and Stock B, differing markedly in their ability to acquire a conditioned avoidance response. On separate occasions, the animals were subjected to five daily sessions (approximately 20 trials per 15 min session) of conditioned avoidance training, measurements of startle reactivity to an auditory stimulation and open-field spontaneous locomotor activity observations. The experiments were concluded by taking blood samples for later analysis of plasma glucose and plasma levels of the following hormones: insulin, gastrin, CCK, glucagon, somatostatin, oxytocin and corticosterone. The low-performing Stock B animals were characterized by [1] being more reactive to sensory stimulation: higher startle amplitude and shorter startle latency; [2] having higher plasma insulin and corticosterone levels, whereas plasma gastrin and oxytocin were significantly lowered and a strong tendency for a decrease also in plasma CCK. There were no differences in spontaneous locomotor activity between the two substrains. Taking total variability in avoidance performance into account, there was a statistically significant positive correlation between plasma oxytocin, as well as gastrin, levels and avoidance performance. The evidence obtained here, and in other laboratories, suggests that the Stock B animals display hormonal changes indicative of a submissive-defensive reaction pattern. Thus, the avoidance acquisition deficits displayed by the present Sprague-Dawley stocks A and B, are in all probability caused by emotional reactions when challenged with external stimuli requiring active responding.

Steroid dependent effects of oxytocin on spontaneous motor activity in female rats

In the present study, dose relationships for effects of oxytocin (OXY) on spontaneous motor activity in female rats were investigated. Ovariectomized (OVX) and cycling female Sprague-Dawley rats were given OXY 10-1000 microg/kg s.c. or saline, 10 min before registration of motor activity in an open-field arena. In the OVX rats, 100 microg/kg of OXY increased the activity in the center of the arena, whereas 1000 microg/kg decreased locomotor activity (LA). In the cycling rats, OXY 100-1000 microg/kg decreased LA during diestrus, while 1000 microg/kg also decreased LA during metestrus. The latter dose also reduced the exploratory behavior during estrus. In a second experiment, OVX rats were pretreated with estradiol benzoate (EB) and progesterone (P). When P levels were predominant, OXY 10-1000 microg/kg decreased LA. Oxytocin 10-100 microg/kg given after pretreatment with EB increased the activity in the center of the arena, whereas 1000 micro/kg given in the presence of both EB and P increased peripheral activity (PA). These results show that the effects of OXY on motor activity in female rats are modified by female sex steroid hormones.

Effects of central oxytocin administration on intraoral intake of glucose in deprived and nondeprived rats

We evaluated the effects of lateral intracerebroventricular administration of oxytocin (OT) and/or a selective oxytocin-receptor antagonist (OTX), 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-OT, on ingestion of intraorally delivered 12.5% glucose in rats that were either nondeprived or deprived of food for 20 h. In deprived rats, OT delivered 30 min before an initial intake test yielded a dose-related reduction of intraoral glucose intake. The highest dose tested, 20 nmol, reduced intraoral glucose intake by 45%. The effect was short-lived, however. Intraoral intake for a second test, initiated 60 min after the termination of the first, increased as a function of OT dose so that total session intake was unaffected by OT treatment. The suppression of intraoral intake by 20 nmol OT was reversed by pretreatment (45 min before testing) with OTX. In nondeprived rats, by contrast, OT yielded no effect on first-test, second-test, or total session intakes. Significant increases in first-test and total session intakes were obtained when OTX (20 nmol) was administered alone both in deprived (32% increase in first-test intake) and nondeprived (31% increase) rats. In general, the results obtained are consistent with the suggestion that OT contributes to the control of meal size and, in particular, to the process of satiation, which is the aspect of ingestive control highlighted by the specialized intake test used in the present study.

Influences of fat and carbohydrate on postprandial sleepiness, mood, and hormones

Paired studies were conducted in 18 healthy volunteers (9 men, 9 women) to investigate whether differences in mood and daytime sleepiness induced by high-fat-low-carbohydrate (CHO) and low-fat-high-CHO morning meals were associated with specific hormonal responses. Plasma insulin concentrations were significantly higher after low-fat-high-CHO meals, and cholecystokinin (CCK) concentrations were significantly higher after high-fat-low-CHO meals. Subjects tended to feel more sleepy and less awake 2-3 h after the high-fat-low-CHO meal, and ratings of fatigue were significantly greater 3 h after the high-fat-low-CHO meal than after the low-fat-high-CHO meal. The results of the present study are consistent with the hypothesis that there is an association between the lassitude experienced after a meal and the release of CCK.

Stroking of the abdomen causes decreased locomotor activity in conscious male rats

The specific aim of the present study was to determine if stroking in conscious rats can influence spontaneous locomotor behavior in an open-field arena. For this purpose, conscious rats were held across the scapula and the ventral side of the abdomen was stroked at a pressure of 100-150 mm H2O and at a speed of approximately 20 cm/s. The stimulation frequency was approximately 40 strokes/min and the duration 2, 5, and 10 min. Animals held for 10 min served as controls. There was a significant decrease in rearing and locomotion and a significant increase in peripheral activity in the open-field arena after the treatment. Maximal effects were obtained after 5 min of stroking. These effects were consistent with a stroking-induced sedative effect similar to that seen in this open-field arena model following neuroleptics or large doses of oxytocin.

An anxiolytic action of oxytocin is enhanced by estrogen in the mouse

The established role of oxytocin (OT) in facilitation of steroid-modulated reproductive and affiliative behaviors led to the speculation that it may have anxiolytic actions under certain hormonal conditions. NIH-Swiss mice were tested for responsiveness to OT in two behavioral tests of anxiety, the holeboard apparatus and elevated plus-maze. Dose-response assessment indicated that 3 mg/kg was the optimal dose for peripherally administered (IP) OT on the elevated plus-maze. There were no consistent effects at any dose on the holeboard apparatus. In ovariectomized mice pretreated with estradiol (E2), peripherally administered OT increased the number of entrances onto the open arms, as well as the amount of time on the open arms compared to other groups (ANOVA; p < 0.05). There was little to no effect of OT in ovariectomized animals not pretreated with E2. When OT was administered intracerebroventricularly (ICV), there was an increase in entrances and time on the open arms compared to that of females infused with arginine vasopressin (AVP). This increase was apparent in ovariectomized females, but was further enhanced in those pretreated with E2 (ANOVA; p < 0.05). In contrast, the combination of E2 pretreatment and ICV AVP decreased the number of entrances and time spent on the open arms of the elevated plus-maze compared to those receiving OT, suggesting an estrogen-modulated anxiogenic action of AVP. Analyses of [125]I-OVTA binding density indicated a significant increase in binding density in the lateral septum of E2-treated females compared to OIL-treated controls (ANOVA; p < 0.05). There was no effect of E2 treatment on [125]I-OVTA binding density in the amygdala or ventromedial nucleus of the hypothalamus. Taken together, these data indicate that OT exerts an anxiolytic action that is enhanced in the presence of circulating estrogen. This behavioral effect may be mediated by estrogen-induced increases in OT binding density in the lateral septum and may be important to the facilitation of social interactions.

Effects of 5-HT agonists, selective for different receptor subtypes, on oxytocin, CCK, gastrin and somatostatin plasma levels in the rat

Adult male Sprague-Dawley rats were administered the 5-HT subtype selective receptor agonists 8-OH-DPAT (0.5-2.0 mg/kg), buspirone (2-8 mg/kg) (5-HT1A), TFMPP (0.125-2.0 mg/kg) (5-HT1B), DOI (0.125-2.0 mg/kg) (5-HT2A) and m-CPBG (1.25-20.0 mg/kg) (5-HT3), subcutaneously. Oxytocin, cholecystokinin (CCK), somatostatin and gastrin plasma levels were determined by standard RIA techniques 30 and 120 min after injection of the respective 5-HT receptor agonist. It was found that the 5-HT1A and the 5-HT2A/C, but not the 5-HT2B or the 5-HT3 receptor agonists produced an increase in plasma oxytocin levels and these effects were, at least partially, antagonized by the corresponding subtype selective antagonists (-)pindolol (2 mg/kg) and ritanserin (2 mg/kg), respectively, administered 10 min before 8-OH-DPAT (0.5 mg/kg) or DOI (0.5 mg/kg). The maximal response to the 5-HT1A receptor agonists (approx. 120 nmol/l) was from 8 to 5 times the maximal response to the 5-HT2A C receptor agonist. In addition, 8-OH-DPAT and DOI caused a decrease in plasma CCK levels, whereas the 5-HT1B receptor agonist TFMPP gave rise to an increase in plasma CCK levels. There were no statistically significant effects by any of the 5-HT receptor agonists on plasma somatostatin or gastrin levels under the present conditions. It is suggested that the clinical effects of new anxiolytic 5-HT1A receptor agonists, such as buspirone, to an extent may be mediated via an increased release of oxytocin.

Central oxytocin increases food intake and daily weight gain in rats

The present study was performed to investigate the effects of centrally administered oxytocin on weight gain and food intake in rats. Two substrains of Sprague-Dawley rats (A and B) differing in average daily weight gain were used. Female rats of substrain A gained 2 g per day and males gained 7 g. Female rats of substrain B gained 5 g per day and males gained 8 g. Animals were implanted with a stainless steel guide cannula, allowing ICV injections into the lateral ventricle. ICV injections of 1, 5, or 10 micrograms of oxytocin or isotonic saline in a volume of 5 microliters were given. In females, ICV treatment with either saline or 5 micrograms of oxytocin caused a transient loss of weight within 24 h of treatment. However, in the more slowly growing females of substrain A depression in body weight was observed after a single treatment with saline, whereas the body weight of oxytocin-treated females showed less marked depression and rapidly returned to the pretreatment weight. After a 3-day treatment period an even greater difference in daily weight gain was seen between oxytocin-treated and saline-treated female rats of substrain A. In contrast, no difference in daily weight gain or food intake was observed between oxytocin- and saline-treated male rats of substrain A, nor in females or males of the more rapidly growing substrain B. Intraperitoneal injections of 5 micrograms of oxytocin did not influence food intake or daily weight gain in female rats of substrain A. These data suggest that oxytocin may act centrally to influence food intake and daily weight gain in slowly growing female Sprague-Dawley rats.

The oxytocin antagonist 1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin reverses the increase in the withdrawal response latency to thermal, but not mechanical nociceptive stimuli following oxytocin administration or massage-like stroking in rats

In this study the effect of exogenous oxytocin and of massage-like stroking on the withdrawal latency responses to heat and mechanical nociceptive stimulation were investigated in rats. A hot-plate test and the Randall-Selitto test were used to assess the withdrawal responses. Exogenous oxytocin (0.1-1 mg/kg) and stroking (a low frequency mechanical stimulation) significantly increased the withdrawal latencies in response to mechanical and to thermal nociceptive stimuli. The effect of oxytocin and of stroking in the hot-plate test was reversed by the oxytocin antagonist (1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin) directed against the uterine receptor. In contrast, the antagonist did not affect the prolonged response latency in the mechanical nociceptive stimulation test following either exogenous oxytocin or stroking. These results support the view that (1) oxytocin administration affects directly or indirectly nociceptive related behaviour in response to heat stimulation, and (2) massage-like stroking may have an anti-nociceptive effect via activation of oxytocinergic mechanisms. Since the response to mechanical stimulation was not blocked by the antagonist the mechanisms mediating the withdrawal latency to heat and mechanical stimulation could be different.

Depressed concentrations of oxytocin and cortisol in children with recurrent abdominal pain of non-organic origin

The purpose of the present study was to measure plasma concentrations of oxytocin, cortisol and prolactin in children with recurrent abdominal pain of non-organic origin (RAP). Forty children with RAP and 34 controls, matched for age and sex, participated in the study. A blood sample was collected after an overnight fast in association with clinical examinations. Oxytocin, cortisol and prolactin levels were determined by radioimmunoassay (RIA). Oxytocin and cortisol concentrations in the children with RAP were found to be significantly reduced compared with those of the controls (approximately 24 versus 63 pmol/l for oxytocin and 160 versus 300 nmol/l for cortisol, respectively). The low oxytocin and cortisol levels persisted at a second examination 3 months later. No significant differences in the prolactin levels were observed between RAP and controls.

High doses of oxytocin cause sedation and low doses cause an anxiolytic-like effect in male rats

The aim of the present investigation was to explore dose relationships for effects of oxytocin on spontaneous motor activity in the rat. Oxytocin in doses from 1-1000 micrograms/kg was given SC to male Sprague-Dawley rats, and spontaneous motor behavior was measured by means of photocell-operated open-field observations. In the rats treated with low doses of oxytocin (1-4 micrograms/kg), there was a decrease in peripheral locomotor activity. With increasing doses (250-1000 micrograms/kg), there were clear signs of sedative effects as indicated by a suppression of locomotor activity and rearing. The time course for the effect of oxytocin on peripheral activity (1 microgram/kg) and rearing (1 mg/kg) was tested. A maximal effect was obtained within 1 h and, thereafter, the behavior gradually returned to normal within 24 h. This spectrum of effects caused by oxytocin was similar to that of midazolam but different from that induced by raclopride.

Is systemically administered oxytocin an analgesic in rats?

The effect of systemically administered oxytocin and a specific oxytocin antagonist, 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin, on heat pain sensitivity was examined in rats. Intraperitoneal (i.p.) oxytocin at 1 mg/kg, but not at 0.1 and 0.3 mg/kg, significantly increased response latencies on the hot-plate test. However, the rats displayed clear signs of sedation, motor impairment and vasoconstriction after 1 mg/kg oxytocin. Skin temperature on the plantar surface of the hind paws was also significantly decreased by this dose of oxytocin. The oxytocin antagonist (1 mg/kg i.p.) did not influence response latency. Since increased response latency was not the only behavioral effect of oxytocin, we conducted electrophysiological experiments to examine the effect of systemic oxytocin on the nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats. Oxytocin at 0.1 mg/kg i.p. did not influence flexor reflex magnitude, mean blood pressure or heart rate. Oxytocin at 0.3 and 1 mg/kg caused a gradual increase in blood pressure with stronger effect observed with 1 mg/kg. Neither 0.3 nor 1 mg/kg oxytocin significantly influenced the flexor reflex magnitude and heart rate. We thus conclude that systemic oxytocin did not produce analgesia in rats and the observed increase in response latency in the hot-plate test may result from the sedative and vasoconstrictive effects of this peptide. Furthermore, since the oxytocin antagonist did not significantly alter response latency on the hot-plate test, it is unlikely that endogenous oxytocin exerts a tonic effect on the pain threshold in rats.

Role of efferent and afferent vagal nerve activity during reproduction: integrating function of oxytocin on metabolism and behaviour

The vagal nerves convey information between the brain and the gastrointestinal tract in both afferent and efferent direction. The efferent vagal nerves regulate gastrointestinal secretory and motor function and also the activity in the endocrine system of the gut. Gastrointestinal function and anabolic metabolism is optimized during gestation and lactation as a means of saving energy for this calorie-demanding process. This enhanced activity seems to involve central oxytocinergic transmission which increases efferent vagal nerve activity as reflected by altered levels of gastrointestinal hormones in plasma. In parallel a behaviour pattern characterized by sedation is induced. In addition, vagal afferents transfer information from the gut to the brain, e.g. as to the presence or absence of calories in the gastrointestinal tract. It is well known that satiety and sedation following a meal is in part caused by an activation of vagal afferents in response to food intake. Cholecystokinin released from the intestine is one of the factors that triggers the afferent vagal nerve activity. Also reproductive "interactive behaviours" such as milk ejection and maternal behaviour can be induced by CCK via afferent activation of vagal nerves. In contrast, when no food reaches the gut, milk production and ejection is blocked. Parts of these effects are vagally mediated, since they can be observed in vagotomized rats receiving full amounts of calories.

Amperozide and clozapine but not haloperidol or raclopride increase the secretion of oxytocin in rats

The aim of the present study was to investigate whether amperozide, an antipsychotic drug which possesses anti-aggressive and anxiolytic-like properties, stimulates the secretion of oxytocin and if so, by which receptor mechanism. For this purpose, female or male Sprague Dawley rats were given amperozide (0.5, 2.5 and 5.0 mg/kg IP), ritanserin (5.0 mg/kg), raclopride (2.0 mg/kg) and prazosin (1.0 mg/kg) and were subsequently decapitated for collection of blood (30 and 120 min) after injection. Oxytocin levels were measured with radioimmunoassay. Amperozide 2.5 and 5 mg/kg increased plasma levels of oxytocin significantly (P < 0.05 and < 0.001). The effect appeared maximal about 30 min after injection of the drug and oxytocin levels were almost back to basal within 120 min. Similar effects were obtained in female and male rats as well as in animals that were freely fed or food deprived for 24 h. CSF levels of oxytocin were also increased. Ritanserin, a 5-HT2-receptor antagonist but not the D2 receptor antagonist raclopride or the alpha 1-adrenoceptor antagonist prazosin stimulated oxytocin release. In addition, clozapine, a neuroleptic with potent HT2-antagonistic properties, was a potent releaser of oxytocin, whereas haloperidol was without effect. A possible role for oxytocin in the behavioural effects of amperozide and clozapine remains to be explored.