Peptides in the limbic system: neurochemical codes for co-ordinated adaptive responses to behavioural and physiological demand

Studying the central actions of angiotensin using the expression of immediate-early genes: expectations and limitations

Intracerebral infusions of angiotensin II (Ang-II) elicit a widespread but discrete expression of c-fos (and other immediate-early genes: IEGs) in the basal forebrain and brainstem. This has given us a new approach to the study of the central actions of Ang-II (and other peptides). It is unlikely that the dipsogenic response to Ang-II is directly related to c-fos expression in the AV3V, since the onset of behaviour occurs much before that of gene expression, and suppression of drinking by preloading rats with water does not alter Ang-II-induced c-fos in this part of the brain. Whether endocrine or cardiovascular actions are directly related to c-fos needs to be established. Water intake inhibits c-fos in the SON and PVN; whether this is a direct interaction between Ang-II and altered osmolality on magnocellular neurons or a secondary event of an action elsewhere (e.g., in the AV3V) remains uncertain. AV3V (median preoptic nucleus) lesions also inhibit Ang-II-evoked c-fos in the SON and PVN as well as in brainstem Ang-II sensitive sites such as the NTS and parabrachial nucleus, suggesting that the AV3V may be a nodal site for the distributed action of Ang-II. The rapid dipsogenic effects of Ang-II may involve glutamate receptors, since i.c.v. dizocilpine (an NMDA open channel antagonist) reduces both drinking and c-fos expression after i.c.v. Ang-II. This relatively new IEG technique acts as an anatomical and temporally specific marker of neuronal response to Ang-II, and has already added to knowledge about the central actions of Ang-II at the level of the neuron. Combining this approach with other methods allows cellular events in the brain to be related to the functional effects of Ang-II and its adaptive role in regulating water and salt metabolism.

Wiring and volume transmission in the central nervous system: the concept of closed and open synapses

During the past two decades, several revisions of the concepts underlying interneuronal communication in the central nervous system (CNS) have been advanced. Our group has proposed to classify intercellular communication in the CNS under two general frames: 'wiring' (WT) and 'volume' transmission (VT). WT is characterized by a single 'transmission channel' made by cellular (neuronal or glial) structures and with a region of discontinuity not larger than a synaptic cleft. VT is characterized by the diffusion from a cell source (neuronal or glial) of chemical and electrical signals in the extracellular fluid (ECF) for a distance larger than the synaptic cleft Based on morphological and functional characteristics, and in light of the distinction proposed, six main modes of intercellular communication can be recognized in the CNS: gap-junction, membrane juxtaposition, and closed synapse (which represent WT-type modes of communication); open synapse, paracrine transmission and endocrine-like transmission (which represent VT-type modes of communication). Closed and open synapses are distinguished on the basis of the sealing of the signal within or the leakage of the signal outside the synapse Intra-synaptic restriction or extra-synaptic diffusion of transmitters are insured by a number of anatomical arrangements (e.g. glial ensheathment of synapse, size of the synaptic cleft) and functional mechanisms (e.g. density and location of transmitter re-uptake sites and metabolic enzymes). Some central synapses can switch from closed to open state and vice versa, e.g. by changing the amount of transmitter released. Finally, a synapse containing several transmitters can work as an open synapse for one transmitter and as a closed synapse for another.

Behavioural, autonomic and endocrine responses associated with C-fos expression in the forebrain and brainstem after intracerebroventricular infusions of endothelins

Endothelins are a range of peptides (endothelin-1, endothelin-2, and endothelin-3) well known to act peripherally as powerful cardiovascular-regulating agents. Recently, they have been shown to be localized in CSN, where they may act as central neurotransmitters. A variety of putative roles has been ascribed to them in the CNS. To identify those regions of the brain capable of responding to these peptides, the expression of c-fos (an immediate-early gene), has been used to map patterns of activation following intracerebroventricular (i.c.v.) infusions of endothelins in Lister-hooded rats. This has been correlated with changes in heart rate, core temperature and plasma corticosterone levels. Endothelin-3 i.c.v. (50 pmol) decreased both heart rate and core temperature (both recorded by telemetry). This effect lasted for about 30-45 min. Endothelin-1 (10 pmol) or endothelin-3 (50 pmol) i.c.v. induced c-fos expression in the specific regions in the forebrain and brainstem. Strong expression was found in the septum, bed nucleus of the stria terminalis, parvicellular paraventricular nucleus, the central nucleus of the amygdala, dorsal motor nucleus of the vagus and solitary nucleus. There was less marked c-fos expression in other areas of the basal forebrain, such as the organum vasculosum of the lamina terminals, median preoptic nucleus, supraoptic nucleus and the magnocellular. There are two classes of endothelin receptor (A and B). An endothelin-A receptor antagonist, BQ-123, abolished c-fos expression in all structures in the forebrain and brainstem following endothelin-1 infusions. However, an endothelin-B agonist (TetraAla endothelin-1) did not induce discernible c-fos expression in the forebrain or brainstem. These results suggest that the endothelin-A receptor is responsible for endothelin-dependent c-fos induction in the brain. Interactions between endothelins and angiotensin II were also studied. The pattern of c-fos induced by endothelin-3 and angiotensin II was different (particularly in the anteroventral region of the third ventricle). Furthermore, prior infusions of endothelin-3 interfered with the expression of c-fos induced by subsequent angiotensin II, and also suppressed the latter's dipsogenic effect. These results show that endothelin-3 and angiotensin II interact at both behavioural and cellular levels, and that endothelins may play significant roles in the central control of fluid balance and autonomic activity.

Behavioral consequences of intracerebral vasopressin and oxytocin: focus on learning and memory

Since the pioneering work of David de Wied and his colleagues, the neuropeptides arginine vasopressin and oxytocin have been thought to play a pivotal role in behavioral regulation in general, and in learning and memory in particular. The present review focuses on the behavioral effects of intracerebral arginine vasopressin and oxytocin, with particular emphasis on the role of these neuropeptides as signals in interneuronal communication. We also discuss several methodological approaches that have been used to reveal the importance of these intracerebral neuropeptides as signals within signaling cascades. The literature suggests that arginine vasopressin improves, and oxytocin impairs, learning and memory. However, a critical analysis of the subject indicates the necessity for a revision of this generalized concept. We suggest that, depending on the behavioral test and the brain area under study, these endogenous neuropeptides are differentially involved in behavioral regulation; thus, generalizations derived from a single behavioral task should be avoided. In particular, recent studies on rodents indicate that socially relevant behaviors triggered by olfactory stimuli and paradigms in which the animals have to cope with an intense stressor (e.g., foot-shock motivated active or passive avoidance) are controlled by both arginine vasopressin and oxytocin released intracerebrally.

Intercellular communication in the brain: wiring versus volume transmission

During the past two decades several revisions of the concepts underlying interneuronal communication in the central nervous system have been advanced. We propose here to classify communicational phenomena between cells of the central neural tissue under two general frames: "wiring" and "volume" transmission. "Wiring" transmission is defined as intercellular communication occurring through a well-defined connecting structure. Thus, wiring transmission is characterized by the presence of physically identifiable communication channels within the neuronal and/or glial cell network. It includes synaptic transmission but also other types of intercellular communication through a connecting structure (e.g., gap junctions). "Volume" transmission is characterized by signal diffusion in a three-dimensional fashion within the brain extracellular fluid. Thus, multiple, structurally often not well characterized extracellular pathways connect intercommunicating cells. Volume transmission includes short- (but larger than synaptic cleft, i.e. about 20 nm) and long-distance diffusion of signals through the extracellular and cerebrospinal fluid. It must be underlined that the definitions of wiring and volume transmission focus on the modality of transmission and are neutral with respect to the source and target of the transmission, as well as type of informational substance transmitted. Therefore, any cell present in the neural tissue (neurons, astroglia, microglia, ependyma, tanycytes, etc.) can be a source or a target of wiring and volume transmission. In this paper we discuss the basic definitions and some distinctive characteristics of the two types of transmission. In addition, we review the evidence for different types of intercellular communication besides synaptic transmission in the central nervous system during phylogeny, and in vertebrates in physiological and pathological conditions.

Prednisolone blocks extreme intermale social aggression in seizure-induced, brain-damaged rats: implications for the amygdaloid central nucleus, corticotrophin-releasing factor, and electrical seizures

In two separate blocks of experiments, the extreme within-group aggression which is typically associated with limbic seizure-induced brain injury in male rats was attenuated or abolished within two days by the administration of prednisolone in the water supply. The effect was specific to the aggression and was not simulated by dexamethasone. The results support the hypothesis that interference with inhibitory inputs to the central nucleus of the amygdala and the enhanced stimulation by corticotrophin-releasing factor facilitates physical aggression within groups of male rats. Potential relevance to curbing aggression ("conflict") between groups of male humans is discussed.

Regional changes in c-fos expression in the basal forebrain and brainstem during adaptation to repeated stress: correlations with cardiovascular, hypothermic and endocrine responses

Acute stress is known to evoke a discrete pattern of c-fos expression in the brain. The work reported here shows that this pattern is modified in regionally specific ways following repeated stress, and that this can be correlated with changes in telemetered heart rate, core temperature and corticosterone output that occur during adaptation. Intact male rats were restrained for 60 min daily for one or 10 days. Stress-induced tachycardia was maximal 10 min following the onset of restraint, and decreased thereafter. The peak value was not altered by repeated restraint, but levels fell towards baseline values more rapidly with increasing bouts of stress. Core temperature showed marked reduction during the first 10 min of the initial stress, followed by a minor (and not very consistent) overshoot during the remainder of the stress period. In contrast to heart rate, stress-induced hypothermia did not alter during repeated restraint. Corticosterone was raised dramatically immediately following the first 60-min session of restraint, and this was attenuated by repeated stress. Sixty minutes after the end of the first stress session, there was pronounced c-fos expression in the lateral septum, lateral preoptic area, lateral hypothalamic area, all divisions of the hypothalamic paraventricular nucleus, the medial (but not central) amygdala, the locus ceruleus and a brainstem structure (thought to be Barrington's nucleus), compared to rats transferred to the testing room but not restrained. Sixty minutes after the 10th stress session, c-fos expression was markedly decreased in some of these areas compared with the pattern observed after the first stress, especially in the paraventricular nucleus (dorsal and medial parvicellular regions) and in medial amygdala. However, all other areas measured demonstrated a sustained response even after repeated stress. There were no significant differences in c-fos expression in rats repeatedly transferred to the testing room (but not stressed) compared to singly transferred counterparts. These results show that both neuronal and physiological responses adapt to a repeated stress, but that in both cases this has highly specific components. It seems likely that adaptive changes in c-fos expression are associated with those in some features of autonomic and endocrine reactions. It is noteworthy that there is evidence that the lateral septum, in which c-fos expression did not diminish after repeated stress, may be involved in temperature control, whereas the paraventricular nucleus, in which c-fos did alter, has been linked with both cardiac and corticoid regulation.

c-fos expression in the paraventricular nucleus of the hypothalamus following intracerebroventricular infusions of neuropeptide Y

Intracerebroventricular (i.c.v.) infusions of neuropeptide Y (NPY) (2500 pmol) induced c-fos protein in the paraventricular nucleus (PVN) of intact male rats 60 min later. The greatest expression was observed in the dorsal (parvicellular) region of the PVN; there were intermediate levels in the lateral (magnocellular) and lowest ones in the medial (parvicellular) regions. Allowing rats to eat during the post-infusion interval did not modify this pattern of c-fos expression. Depriving rats of food for either 24 or 48 h did not induce recognisable expression of c-fos in the PVN, and allowing 24 h-deprived rats to eat also had no effect on PVN c-fos. Plasma insulin was increased by i.c.v. NPY, and raised still further in rats that were allowed to eat following NPY infusions. However, plasma glucose was not altered by either treatment. Food-deprived rats had low levels of insulin, but unaltered blood glucose, compared to controls. These results show that NPY can induce c-fos expression in both parvicellular and magnocellular areas of the PVN. The pattern of expression within the PVN seems to differ from that induced by other peptides, such as angiotensin II, vasopressin and corticotropin-releasing factor, suggesting that distinct populations of neurons are activated by different peptides within the complex structure of the PVN. Food deprivation does not induce c-fos expression within the PVN, though other studies have shown that NPY levels and release are both increased, so there is no simple relation between current energy state, blood levels of either glucose or insulin and c-fos expression within the PVN.