Role of adrenoceptors in vasopressin, oxytocin and prolactin responses to conditioned fear stimuli in the rat

The medial amygdala-medullary PrRP-synthesizing neuron pathway mediates neuroendocrine responses to contextual conditioned fear in male rodents

Fear responses play evolutionarily beneficial roles, although excessive fear memory can induce inappropriate fear expression observed in posttraumatic stress disorder, panic disorder, and phobia. To understand the neural machineries that underlie these disorders, it is important to clarify the neural pathways of fear responses. Contextual conditioned fear induces freezing behavior and neuroendocrine responses. Considerable evidence indicates that the central amygdala plays an essential role in expression of freezing behavior after contextual conditioned fear. On the other hand, mechanisms of neuroendocrine responses remain to be clarified. The medial amygdala (MeA), which is activated after contextual conditioned fear, was lesioned bilaterally by infusion of N-methyl-d-aspartate after training of fear conditioning. Plasma oxytocin, ACTH, and prolactin concentrations were significantly increased after contextual conditioned fear in sham-lesioned rats. In MeA-lesioned rats, these neuroendocrine responses but not freezing behavior were significantly impaired compared with those in sham-lesioned rats. In contrast, the magnitudes of neuroendocrine responses after exposure to novel environmental stimuli were not significantly different in MeA-lesioned rats and sham-lesioned rats. Contextual conditioned fear activated prolactin-releasing peptide (PrRP)-synthesizing neurons in the medulla oblongata. In MeA-lesioned rats, the percentage of PrRP-synthesizing neurons activated after contextual conditioned fear was significantly decreased. Furthermore, neuroendocrine responses after contextual conditioned fear disappeared in PrRP-deficient mice. Our findings suggest that the MeA-medullary PrRP-synthesizing neuron pathway plays an important role in neuroendocrine responses to contextual conditioned fear.

Oxytocin-messages via the cerebrospinal fluid: behavioral effects; a review

The cerebrospinal fluid (CSF) usually is considered as a protective 'nutrient and waste control' system for the brain. Recent findings suggest, however, that the composition of CSF is actively controlled and may play an influential role in the changes in brain activity, underlying different behavioral states. In the present review, we present an overview of available data concerning the release of oxytocin into the CSF, the location of the oxytocin-receptive brain areas and the behavioral effects of intracerebroventricular oxytocin. About 80% of the oxytocin-receptive areas are located close to the ventricular or subarachnoid CSF, including the hypothalamic 'Behavior Control Column' (L.W.Swanson, 2003). As a conclusion we suggest that 'CSF-oxytocin' contributes considerably to the non-synaptic communication processes involved in hypothalamic-, brainstem- and olfactory brain areas and behavioral states and that the flowing CSF is used as a 'broadcasting system' to send coordinated messages to a wide variety of nearby and distant brain areas.

Salt loading reduces hypothalamic noradrenaline release after noxious stimuli

Salt loading reduces neuroendocrine responses to stressful stimuli. Noxious stimuli facilitate noradrenaline release in the hypothalamus and, as a result, activate oxytocin neurones. Here, we examined effects of salt loading upon plasma oxytocin concentrations and noradrenaline release in the hypothalamus after footshocks. Male rats were allowed to drink 2% NaCl for 7 days. Salt loading reduced the footshock-induced increase in plasma oxytocin concentrations and noradrenaline release in the supraoptic nucleus (SON). Acute administration of hypertonic saline also attenuated the footshock-induced noradrenaline increase in the supraoptic nucleus. In contrast, salt loading did not significantly change activation of A1 catecholaminergic neurones in the medulla oblongata, as measured by expression of Fos protein. These data suggest that salt loading presynaptically suppresses noradrenaline release in the hypothalamus and oxytocin release into the blood after footshocks.

Facilitative role of endogenous oxytocin in noradrenaline release in the rat supraoptic nucleus

Oxytocin is released not only from the axon terminals in the neurothypophysis but also from the dendrites in the hypothalamus. In the present study, we examined the role of dendritic oxytocin release in regulating presynaptic noradrenaline release within the hypothalamus. In vivo microdialysis experiments showed that local application of oxytocin augmented high-K+-induced noradrenaline release in the hypothalamic supraoptic nucleus. Oxytocin application to the hypothalamic synaptosomal preparation in vitro also potentiated high-K+-induced noradrenaline release. The effect of oxytocin was dose-dependent and was blocked by an oxytocin receptor antagonist. We then examined roles of oxytocin released from the dendrites using in vivo microdialysis. Local application of an oxytocin receptor antagonist impaired noradrenaline release in the supraoptic nucleus in response to high-K+ solution or noxious stimuli. An i.c.v. injection of an oxytocin receptor antagonist also impaired oxytocin release from the pituitary after noxious stimuli. These data suggest that dendritic oxytocin facilitates activation of oxytocin neurons, at least in part by augmentation of noradrenaline release via a presynaptic action.

Neuromedin U facilitates oxytocin release from the pituitary via β adrenoceptors

Neuromedin U activates noradrenergic neurones in the medulla oblongata and oxytocin neurones in the hypothalamus. Here we examined roles of noradrenergic transmission in oxytocin release from the pituitary after intracerebroventricular administration of neuromedin U in rats. Neuromedin U administration facilitated noradrenaline release in the supraoptic nucleus. Administration of a beta1 adrenoceptor antagonist, metoprolol, or a beta2 antagonist, ICI 118551 but not an alpha1 antagonist, benoxathian, reduced increases in plasma oxytocin concentrations observed after administration of neuromedin U, but plasma ACTH concentrations were not significantly changed. All theses data suggest that neuromedin U stimulates oxytocin release from the pituitary, at least in part, via activation of beta adrenoceptors.

Facilitative role of prolactin-releasing peptide neurons in oxytocin cell activation after conditioned-fear stimuli

Emotional stress activates oxytocin neurons in the hypothalamic supraoptic and paraventricular nuclei and stimulates oxytocin release from the posterior pituitary. Oxytocin neurons in the hypothalamus have synaptic contact with prolactin-releasing peptide (PrRP) neurons. Intracerebroventricular administration of PrRP stimulates oxytocin release from the pituitary. These observations raise the possibility that PrRP neurons play a role in oxytocin response to emotional stress. To test this hypothesis, we first examined expression of Fos protein, an immediate early gene product, in the PrRP neurons in the medulla oblongata after conditioned-fear stimuli. Conditioned-fear stimuli increased the number of PrRP cells expressing Fos protein especially in the dorsomedial medulla. In order to determine whether PrRP cells projecting to the supraoptic nucleus are activated after conditioned-fear stimuli, we injected retrograde tracers into the supraoptic nucleus. Conditioned-fear stimuli induced expression of Fos protein in retrogradely labeled PrRP cells in the dorsomedial medulla. Finally we investigated whether immunoneutralization of endogenous PrRP impairs oxytocin release after emotional stimuli. An i.c.v. injection of a mouse monoclonal anti-PrRP antibody impaired release of oxytocin but not of adrenocorticotrophic hormone or prolactin and did not significantly change freezing behavior in response to conditioned-fear stimuli. From these data, we conclude that PrRP neurons in the dorsomedial medulla that project to the hypothalamus play a facilitative role in oxytocin release after emotional stimuli in rats.

Involvement of medullary A2 noradrenergic neurons in the activation of oxytocin neurons after conditioned fear stimuli

Fear-related stimuli activate oxytocin neurons in the hypothalamus and facilitate oxytocin release from the pituitary. Oxytocin neurons in the supraoptic nucleus receive direct noradrenergic innervations from the A1 and A2 cell groups in the medulla oblongata. In the present study, we investigated the role of hypothalamic-projecting noradrenergic neurons in controlling oxytocin cell activity following fear-related stimuli in rats. An unconditioned fear stimulus (intermittently applied footshock) or conditioned fear stimulus induced expression of Fos protein, a protein product of an immediate-early gene, in magnocellular oxytocin neurons in the supraoptic or paraventricular nucleus. A neurotoxin, 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine, microinjected into the vicinity of the supraoptic nucleus, selectively depleted the noradrenaline contents of the nucleus and blocked the Fos expression in the supraoptic nucleus after the unconditioned or conditioned fear stimulus. In the medulla oblongata, the unconditioned fear stimulus induced expression of Fos protein in both A2/C2 and A1/C1 catecholaminergic neurons. On the other hand, the conditioned fear stimulus induced expression of Fos protein preferentially in the A2/C2 neurons. Furthermore, the unconditioned fear stimulus induced Fos expression in the A1/C1 and A2/C2 catecholaminergic neurons labelled with retrograde tracers previously injected into the supraoptic nucleus. The conditioned fear stimulus induced Fos expression preferentially in the A2/C2 catecholaminergic neurons labelled with the retrograde tracers. These data suggest that the conditioned fear-induced oxytocin cell activity is mediated by the A2 noradrenergic neurons projecting to oxytocin neurons, while the unconditioned fear response is mediated by both A2 and A1 noradrenergic neurons.

Learning and memory in agmatine-treated rats

Agmatine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist, was examined for its role in water maze place learning, contextual and auditory-cued (discrete) fear learning and conditioned taste aversion learning, when administered systemically. Male Wistar rats were given saline or 1, 5, 10 or 50 mg/kg agmatine ip 20 min prior to or 30 min following daily training sessions in a hidden-platform (place learning) water maze task. Agmatine did not affect latencies to find the hidden platform or preference for the training quadrant during probe trials. When administered 20 min prior to contextual or auditory-cued fear-conditioning sessions, these doses of agmatine evoked a linear dose-dependent impairment in the magnitude of learned fear to the contextual stimuli when assessed during extinction trials 24 h later, but had no effect on the magnitude of learned fear to the auditory stimulus. Inferences of baseline motor activity and ability to respond to the presentation of footshock stimuli were not affected by the treatment. Injections of 50 mg/kg agmatine concurrently with a malaise-evoking agent following presentations to a novel sucrose solution abolished learned taste aversions; this agent did not evoke conditioned taste aversions alone. These studies indicate that systemically administered agmatine selectively impairs behavioral inferences of specific types of learning and memory.

Roles of central interleukin-1 on stress-induced-hypertension and footshock-induced-analgesia in rats

Central interleukin-1 (IL-1) plays an important role in mediating the neural, endocrine, and behavioral responses to stressors. Here we tested whether central IL-1 is involved in stress-induced hypertension or footshock (FS)-induced-analgesia. We observed that: (1) intracerebral ventricular injection of (ICV) IL-1beta induced pressor responses; (2) hypertension induced by IL-1beta was blocked by ICV an IL-1 antagonist, IL-1ra; (3) ICV IL-1ra attenuated the pressor response induced by FS but intravenous injection of IL-1ra did not significantly reduce this response; (4) the hypertensive response to conditioned fear stimuli was reversed by ICV IL-1ra; (5) FS-induced-analgesia was attenuated by ICV IL-1ra and this effect disappeared 15 min after ICV IL-1ra. These results suggest that both the pressor response to FS or conditioned fear stimuli and short lasting analgesia induced by FS are mediated by central IL-1.

Nociceptive stimulation increases NO synthase mRNA and vasopressin heteronuclearRNA in the rat paraventricular nucleus

Nociceptive stimulation causes neuroendocrine responses such as arginine vasopressin (AVP) release and activation of the hypothalamo-pituitary-adrenal (HPA) axis. We examined the effects of nociceptive stimulation on the expression levels of neuronal nitric oxide synthase (nNOS) mRNA, heteronuclear (hn)RNA for AVP and AVP mRNA in the rat paraventricular nucleus (PVN) and supraoptic nucleus (SON), using in situ hybridization histochemistry. For nociceptive stimulation, formalin (5%) or saline was injected subcutaneously (s.c.) into the bilateral hind paws of rats. The expression of the nNOS gene in the PVN was significantly increased 2 and 6 h after s.c. injection of formalin in comparison with that in untreated and saline injected rats. The expression of the nNOS gene in the SON did not change in the untreated, saline- and formalin-injected rats. The AVP hnRNA in the PVN and SON was also significantly increased 15, 30 min and 2 h after s.c. injection of formalin, though AVP mRNA did not change at any time points that we studied. Plasma concentration of AVP was significantly increased 15 min after s.c. injection of formalin. These results suggest that NO in the PVN may be involved in nociceptive stimulation-induced neuroendocrine responses.

Behavioural and physiological consequences of acute social defeat in growing gilts: effects of the social environment

Endocrine, behavioural and immunologic processes, together with body growth, were evaluated in gilts that were defeated at 10 weeks of age in resident-intruder tests. Immediately after defeat, gilts were either separated from or reunited with a familiar conspecific (litter-mate; always a barrow). Gilts were assigned to one of four treatments: (a) DI: defeat, followed by isolation (separation from original litter-mate; n=8); (b) I: no defeat, isolation (control group; n=9); (c) DP; defeat, followed by pair-housing (reunion with original litter-mate; n=8); and (d) P: no defeat, pair-housing (control group; n=8). The following general conclusions were derived: (1) social defeat caused pronounced short-term elevations in hypothalamic-pituitary-adrenal (HPA) and sympathetic-adrenal medullary activities, and of prolactin levels. Moreover, as soon as 1h after defeat, percentages of blood lymphocytes and neutrophilic granulocytes were, respectively, decreased and increased; (2) social defeat had some long-lasting influence on behaviour and physiology, but isolation predominantly determined responses in the longer term. Defeat, as well as isolation, resulted in increased cardiovascular activities compared to P controls, as observed in a novel object test (NOT: +7 days) and an aversion test (AVT: +14 days). Moreover, defeated as well as isolated gilts did not habituate to a repeated novel environment test (NET: -7, +2 and +7 days) in terms of frequencies of vocalising, whereas P controls did. Isolation, through the separation from any other pig, was responsible for the other observed long-term characteristics, which developed progressively. Isolated gilts showed high mobilities and high cortisol responses in the repeated NET (+7 days), not being habituated. This contrasted the reactions of pair-housed gilts, which were much reduced. In addition to their high cardiovascular activities in the NOT and the AVT, isolated gilts also displayed higher heart rates in the repeated NET and during human presence following the NOT, compared to pair-housed gilts. Finally, isolated gilts were more inhibited to approach a novel object (in the NOT) than pair-housed pigs; and (3) stress responses of defeated gilts were modulated by the subsequent social environment. Stimulation of the HPA-axis (plasma- and salivary cortisol) was prolonged in those defeated gilts which were isolated (observed in the first hour). Changes in leucocyte subsets were still observed after 3 days in DI, but were 'normalised' within 1 day in DP gilts. Two days after defeat, habituation to the repeated NET in terms of mobility and salivary cortisol responses occurred in control and DP gilts, but not in DI gilts. We argue that these effects of the social environment shortly after defeat were related to a stress-reducing effect of a stable social relationship, i.e. social support.

Catecholaminergic mechanisms underlying neurohypophysial hormone responses to unconditioned or conditioned aversive stimuli in rats

Oxytocin release from the neurohypophysis is facilitated by systemic cholecystokinin octapeptide (CCK) administration and noxious stimuli. Oxytocin release after CCK administration is mediated by A2 noradrenergic neurones while the release after noxious stimuli appears to be mediated by A1 noradrenergic neurones. On the other hand, facilitation of vasopressin release after noxious stimuli is not dependent upon noradrenergic neurones but on dopamine receptors. Environmental stimuli previously paired with noxious stimuli (conditioned fear stimuli) or novel environmental stimuli facilitate oxytocin release and suppress vasopressin release. These neuroendocrine responses to conditioned fear stimuli, but not to novel stimuli, are impaired by central noradrenaline depletion or i.c.v. adrenoceptor antagonists. These data suggest that there are at least two types of stress responses in neuroendocrine systems, one noradrenaline dependent, and one noradrenaline independent. It is also suggested that noradrenergic neurones are functionally heterogeneous in the control of oxytocin release.