Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: I. Effects of stimulation of A1, A2, A6 and C2 cell groups

State-dependent activity dynamics of hypothalamic stress effector neurons

The stress response necessitates an immediate boost in vital physiological functions from their homeostatic operation to an elevated emergency response. However, the neural mechanisms underlying this state-dependent change remain largely unknown. Using a combination of in vivo and ex vivo electrophysiology with computational modeling, we report that corticotropin releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN), the effector neurons of hormonal stress response, rapidly transition between distinct activity states through recurrent inhibition. Specifically, in vivo optrode recording shows that under non-stress conditions, CRH_PVN neurons often fire with rhythmic brief bursts (RB), which, somewhat counterintuitively, constrains firing rate due to long (~2 s) interburst intervals. Stressful stimuli rapidly switch RB to continuous single spiking (SS), permitting a large increase in firing rate. A spiking network model shows that recurrent inhibition can control this activity-state switch, and more broadly the gain of spiking responses to excitatory inputs. In biological CRH_PVN neurons ex vivo, the injection of whole-cell currents derived from our computational model recreates the in vivo-like switch between RB and SS, providing direct evidence that physiologically relevant network inputs enable state-dependent computation in single neurons. Together, we present a novel mechanism for state-dependent activity dynamics in CRH_PVN neurons.

Hypertension in Prenatally Undernourished Young-Adult Rats Is Maintained by Tonic Reciprocal Paraventricular-Coerulear Excitatory Interactions

Prenatally malnourished rats develop hypertension in adulthood, in part through increased α₁-adrenoceptor-mediated outflow from the paraventricular nucleus (PVN) to the sympathetic system. We studied whether both α₁-adrenoceptor-mediated noradrenergic excitatory pathways from the locus coeruleus (LC) to the PVN and their reciprocal excitatory CRFergic connections contribute to prenatal undernutrition-induced hypertension. For that purpose, we microinjected either α₁-adrenoceptor or CRH receptor agonists and/or antagonists in the PVN or the LC, respectively. We also determined the α₁-adrenoceptor density in whole hypothalamus and the expression levels of α_1A-adrenoceptor mRNA in the PVN. The results showed that: (i) agonists microinjection increased systolic blood pressure and heart rate in normotensive eutrophic rats, but not in prenatally malnourished subjects; (ii) antagonists microinjection reduced hypertension and tachycardia in undernourished rats, but not in eutrophic controls; (iii) in undernourished animals, antagonist administration to one nuclei allowed the agonists recover full efficacy in the complementary nucleus, inducing hypertension and tachycardia; (iv) early undernutrition did not modify the number of α₁-adrenoceptor binding sites in hypothalamus, but reduced the number of cells expressing α_1A-adrenoceptor mRNA in the PVN. These results support the hypothesis that systolic pressure and heart rate are increased by tonic reciprocal paraventricular-coerulear excitatory interactions in prenatally undernourished young-adult rats.

Stress-Induced Alterations of Norepinephrine Release in the Bed Nucleus of the Stria Terminalis of Mice

Stress can drive adaptive changes to maintain survival during threatening stimuli. Chronic stress exposure, however, may result in pathological adaptations. A key neurotransmitter involved in stress signaling is norepinephrine. Previous studies show that acute stress elevates norepinephrine levels in the bed nucleus of the stria terminalis (BNST), a critical node regulating anxiety and upstream of stress responses. Here, we use mice expressing channelrhodopsin in norepinephrine neurons to selectively activate terminals in the BNST, and measure norepinephrine release with optogenetics-assisted fast-scan cyclic voltammetry (FSCV). We demonstrate that while corticosterone habituates to chronic restraint stress, cFos activation of medullary norepinephrine neurons shows equivalent activation under both acute and chronic stress conditions. Mice exposed to a single restraint session show an identical optically stimulated norepinephrine release profile compared to that of unexposed mice. Mice experiencing 5 days of restraint stress, however, show elevated norepinephrine release across multiple stimulation parameters, and reduced sensitivity to the α₂-adrenergic receptor (AR) antagonist idazoxan. These data are the first to examine norepinephrine release in the BNST to tonic and phasic stimulation frequencies, and confirm that repeated stress alters autoreceptor sensitivity.

The role of the dorsal noradrenergic pathway of the brain (locus coeruleus) in the regulation of liver cytochrome P450 activity

Our previous study conducted after intracerebroventricular DSP-4 injection showed an important stimulating role of a brain noradrenergic system in the neuroendocrine regulation of liver cytochrome P450 (CYP) expression. The aim of the present research was to study involvement of the dorsal noradrenergic pathway of the brain (originating from the locus coeruleus) in the expression of liver cytochrome P450. The experiment was carried out on male Wistar rats. Local injection of 6-hydroxydopamine to the locus coeruleus selectively decreased noradrenaline level in the brain (e.g. in the hypothalamus). The serum concentration of the growth hormone rose, while that of the thyroid hormones or corticosterone remained unchanged. A comparative study into cytochrome P450 isoform activity revealed significant increases in the activity of liver CYP2C11 and CYP3A after administration of 6-hydroxydopamine. The observed increase in the activity of CYP2C11 positively correlated with that in CYP protein level, while the enhanced activity of CYP3A was not accompanied with a simultaneous change in the enzyme protein. A 5-day-injection of noradrenaline into the lateral ventricles produced opposite effects on the CYP isoforms. It is concluded that damage to or activation of the dorsal noradrenergic innervation of the periventricular nucleus of the hypothalamus containing somatostatin (a growth hormone release-inhibiting factor) may be responsible for the changes observed in the activity of isoforms CYP2C11 and CYP3A that are regulated by the growth hormone. The obtained results indicate that the dorsal noradrenergic pathway plays an inhibitory (but not a crucial) role in the neuroendocrine regulation of cytochrome P450.

Psychiatric symptoms of noradrenergic dysfunction: a pathophysiological view

What psychiatric symptoms are caused by central noradrenergic dysfunction? The hypothesis considered in this review is that noradrenergic dysfunction causes the abnormalities in arousal level observed in functional psychoses. In this review, the psychiatric symptoms of noradrenergic dysfunction were inferred pathophysiologically from the neuroscience literature. This inference was examined based on the literature on the biology of psychiatric disorders and psychotropics. Additionally, hypotheses were generated as to the cause of the noradrenergic dysfunction. The central noradrenaline system, like the peripheral system, mediates the alarm reaction during stress. Overactivity of the system increases the arousal level and amplifies the emotional reaction to stress, which could manifest as a cluster of symptoms, such as insomnia, anxiety, irritability, emotional instability and exaggerated fear or aggressiveness (hyperarousal symptoms). Underactivity of the system lowers the arousal level and attenuates the alarm reaction, which could result in hypersomnia and insensitivity to stress (hypoarousal symptoms). Clinical data support the hypothesis that, in functional psychoses, the noradrenergic dysfunction is in fact associated with the arousal symptoms described above. The anti-noradrenergic action of anxiolytics and antipsychotics can explain their sedative effects on the hyperarousal symptoms of these disorders. The results of animal experiments suggest that excessive stress can be a cause of long-term noradrenergic dysfunction.

Hypothalamo-pituitary-adrenocortical axis, glucocorticoids, and neurologic disease

Neurologic diseases are often accompanied by significant life stress and consequent increases in stress hormone levels. Glucocorticoid stress hormones are known to have deleterious interactions with neurodegenerative processes, and are hypersecreted in neurologic disorders as well as in comorbid psychiatric conditions, such as depression. This article highlights the state of our knowledge on mechanisms controlling activation and inhibition of glucocorticoid secretion, outlines signaling mechanisms used by these hormones in neural tissue, and describes how endogenous glucocorticoids can mediate neuronal damage in various models of neurologic disease. The article highlights the importance of controlling stress and consequent stress hormone secretion in the context of neurologic disease states.

Exercise training-induced remodeling of paraventricular nucleus (nor)adrenergic innervation in normotensive and hypertensive rats

Activation of oxytocin (OT)ergic projections from the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarii contributes to cardiovascular adjustments during exercise training (EXT). Moreover, a deficit in this central OTergic pathway is associated with altered cardiovascular function in hypertension. Since PVN catecholaminergic inputs, known to be activated during EXT, modulate PVN cardiovascular-related functions, we aimed here to determine whether remodeling of PVN (nor)adrenergic innervation occurs during EXT and whether this phenomenon is affected by hypertension. Confocal immunofluorescence microscopy and tract tracing were used to quantify changes in (nor)adrenergic innervation density in PVN subnuclei and in identified dorsal vagal complex (DVC) projecting neurons (PVN-DVC) in EXT normotensive [Wistar-Kyoto rat (WKY)] and hypertensive [spontaneously hypertensive rat (SHR)] rats. In WKY, EXT increased the density of PVN dopamine β-hydroxylase immunoreactivity (DBHir) (160%). Furthermore, the number and density of DBHir boutons overlapping PVN-DVC OTergic neurons were also increased during EXT (130%), effects that were blunted in SHR. Conversely, while DBHir in the medial parvocellular subnucleus (an area enriched in corticotropin-releasing hormone neurons) was not changed by EXT in WKY, a diminished DBHir was observed in trained SHR. Overall, these data support the concept that the PVN (nor)adrenergic innervation undergoes plastic remodeling during EXT, an effect that is differentially affected during hypertension. The functional implications of PVN (nor)adrenergic remodeling in relation to the central peptidergic control of cardiovascular function during EXT are discussed.

Paraventricular-coerulear interactions: role in hypertension induced by prenatal undernutrition in the rat

Rats submitted to fetal growth retardation by in utero malnutrition develop hypertension when adult, showing increased hypothalamic mRNA expression for corticotropin-releasing hormone (CRH) and increased central noradrenergic activity. As hypothalamic CRH serves as an excitatory neurotransmitter within the locus coeruleus (LC) and coerulear norepinephrine plays a similar role within the paraventricular nucleus (PVN) of the hypothalamus, we studied, in both normal and prenatally undernourished 40-day-old anesthetized rats, the effects of intra-LC microinjection of CRH and intra-PVN microinjection of the alpha(1)-adrenoceptor antagonist prazosin on multiunit neuronal activity recorded simultaneously from the two nuclei, as well as the effects on systolic pressure. Undernutrition was induced during fetal life by restricting the diet of pregnant mothers to 10 g daily, whereas mothers of control rats received the same diet ad libitum. At day 40 of postnatal life: (i) undernourished rats showed increased neuronal activity in the PVN and LC, as well as increased systolic pressure; (ii) intra-LC CRH stimulated LC and PVN neurons and increased systolic pressure only in normal rats; (iii) intra-PVN prazosin decreased LC and PVN neuronal activity and systolic pressure only in undernourished rats; and (iv) in normal rats, prazosin prevented the stimulatory effect of CRH only in PVN activity; in undernourished rats, prazosin allowed CRH to regain its stimulatory effects. The results point to the existence of an excitatory PVN-LC closed loop, which seems to be hyperactive in prenatally undernourished rats as a consequence of fetal programming; this loop could be responsible, in part, for the hypertension developed by these animals.

Hypothalamic-Pituitary-Adrenal Axis, Glucocorticoids, and Neurologic Disease

Neurologic diseases often are accompanied by significant life stress and consequent increases in stress hormone levels. Glucocorticoid stress hormones are known to have deleterious interactions with neurodegenerative processes and are hypersecreted in neurologic disorders and comorbid psychiatric conditions. This review highlights the current state of knowledge of mechanisms controlling activation and inhibition of glucocorticoid secretion, outlines signalling mechanisms used by these hormones in neural tissue, and describes how endogenous glucocorticoids can mitigate neuronal damage in models of neurologic disease. This review highlights the importance of controlling stress and consequent stress hormone secretion in the context of neurologic disease states.

Prenatal stress alters Fos protein expression in hippocampus and locus coeruleus stress-related brain structures

Prenatal stress (PS) durably influences responses of rats from birth throughout life by inducing deficits of the hypothalamo-pituitary-adrenal (HPA) axis feedback. The neuronal mechanisms sustaining such alterations are still unknown. The purpose of the present study was to determine whether in PS and control rats, the exposure to a mild stressor differentially induces Fos protein in hippocampus and locus coeruleus, brain areas involved in the feedback control of the HPA axis. Moreover, Fos protein expression was also evaluated in the hypothalamic paraventricular nucleus (PVN) that reflect the magnitude of the hormonal response to stress. Basal plasma corticosterone levels were not different between the groups, while, PS rats exhibited higher number of Fos-immunoreactive neurons than controls, in the hippocampus and locus coeruleus in basal condition. A higher basal expression of a marker of GABAergic synapses, the vGAT, was also observed in the hypothalamus of PS rats. Fifteen minutes after the end of the exposure to the open arm of the elevated plus-maze (mild stress) a similar increased plasma corticosterone levels was observed in both groups in parallel with an increased number of Fos-immunoreactive neurons in the PVN. Return to basal plasma corticosterone values was delayed only in the PS rats. On the contrary, after stress, no changes in Fos-immunoreactivity were observed in the hippocampus and locus coeruleus of PS rats compared to basal condition. After stress, only PS rats presented an elevation of the number of activated catecholaminergic neurons in the locus coeruleus. In conclusion, these results suggest for the first time that PS alters the neuronal activation of hippocampus and locus coeruleus implicated in the feedback mechanism of the HPA axis. These data give anatomical substrates to sustain the HPA axis hyperactivity classically described in PS rats after stress exposure.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Noradrenergic regulation of parvocellular neurons in the rat hypothalamic paraventricular nucleus

Noradrenergic projections to the hypothalamic paraventricular nucleus have been implicated in the secretory regulation of several anterior pituitary hormones, including adrenocorticotropin, thyroid-stimulating hormone, growth hormone and prolactin. In an attempt to elucidate the effects of norepinephrine on the central control of pituitary hormone secretion, we looked at the actions of norepinephrine on the electrical properties of putative parvocellular neurons of the paraventricular nucleus using whole-cell current-clamp recordings in hypothalamic slices. About half (51%) of the putative parvocellular neurons recorded responded to norepinephrine with either a synaptic excitation or a direct inhibition. Norepinephrine (30-300microM) caused a marked increase in the frequency of excitatory postsynaptic potentials in about 36% of the parvocellular neurons recorded. The increase in excitatory postsynaptic potentials was blocked by prazosin (10microM), but not by propranolol (10microM) or timolol (20microM), indicating that it was mediated by alpha(1)-adrenoreceptor activation. It was also blocked by ionotropic glutamate receptor antagonists, suggesting that the excitatory postsynaptic potentials were caused by glutamate release. The increase in excitatory postsynaptic potentials was completely abolished by tetrodotoxin, indicating the spike dependence of the norepinephrine-induced glutamate release. In a separate group comprising 14% of the parvocellular neurons recorded, norepinephrine elicited a hyperpolarization (6.2+/-0.69mV) that was blocked by the beta-adrenoreceptor antagonists, propranolol (10microM) and timolol (20microM), but not by the alpha(1)-receptor antagonist, prazosin (10microM). This response was not blocked by tetrodotoxin (1.5-3microM), suggesting that it was caused by a direct postsynaptic action of norepinephrine. The topographic distribution within the paraventricular nucleus of the norepinephrine-responsive and non-responsive parvocellular neurons was mapped based on intracellular biocytin labeling and neurophysin immunohistochemistry. These data indicate that one parvocellular subpopulation, consisting of about 36% of the paraventricular parvocellular neurons, receives an excitatory input from norepinephrine-sensitive local glutamatergic interneurons, while a second, separate subpopulation, representing about 14% of the parvocellular neurons in the paraventricular nucleus, responds directly to norepinephrine with a beta-adrenoreceptor-mediated inhibition. This suggests that excitatory inputs to parvocellular neurons of the paraventricular nucleus are mediated mainly by an intrahypothalamic glutamatergic relay, and that only a relatively small subset of paraventricular parvocellular neurons receives direct noradrenergic inputs, which are primarily inhibitory.

Noradrenergic neurotransmission at PVN in locus ceruleus-induced baroreflex suppression in rats

We investigated the role of ascending noradrenergic projections from the locus ceruleus (LC) to the paraventricular nucleus (PVN) of the hypothalamus in LC-induced suppression of the baroreceptor reflex (BRR) response in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. On the basis of in vivo microdialysis and high-performance liquid chromatography-electrochemical detection, microinjection of L-glutamate (5 nmol) into the LC resulted in a site-specific increase in norepinephrine (NE) concentration in the dialysate collected from the parvocellular subnucleus of the PVN. The temporal course of this increase in extracellular NE concentration in the PVN coincided with the time course of inhibition elicited by the LC on the BRR response. Microinfusion of NE (10, 50, or 100 nM) into the parvocellular subnucleus of the PVN by reverse microdialysis also promoted a parallel increase in NE at the PVN and a reduction in the BRR response. Inhibition of the BRR response induced by microinjection into the PVN of the alpha 1-adrenoceptor agonist phenylephrine (10 nmol) or chemical activation of the LC was reversed by bilateral PVN microinjection of prazosin (100 pmol). However, local application to the PVN of the alpha 2- or beta-adrenoceptor agonist guanabenz (10 nmol) or isoproterenol (10 nmol) was ineffective. Our results suggest that NE released from the LC-PVN noradrenergic projection may participate in LC-induced suppression of the BRR response by activating the alpha 1-adrenoceptors at the parvocellular subnucleus of the PVN.

Induction of Fos-like immunoreactivity in the lower brainstem and the spinal cord of the rat by intraperitoneal administration of an endogenous satiety substance, 2-buten-4-olide

Induction of Fos in neurons by intraperitoneal injection of 2-buten-4-olide (2-B40), an endogenous satiety substance, was studied immunohistochemically in the brainstem and spinal cord of the rat. Rats injected intraperitoneally with 2-B40 (100 mg/kg) were allowed to survive for 2 h before perfusion. Fos-like immunoreactivity was observed in neurons of the intermediolateral nucleus, ventral reticular formation, lateral reticular nucleus, nucleus of the solitary tract, locus coeruleus, lateral parabrachial nucleus and dorsal raphe nucleus, as well as in tyrosine hydroxylase-immunoreactive neurons of the cell groups A1, A2, A5, A6, A7, C1, C2 and C3.

State-dependent cellular activity patterns of the cat paraventricular hypothalamus measured by reflectance imaging

Activity within the cat paraventricular hypothalamus (PVH) during sleep and waking states was measured by quantifying intrinsic tissue reflectivity. A fiber optic probe consisting of a 1.0 mm coherent image conduit, surrounded by plastic fibers which conducted 660 nm source light, was attached to a charge-coupled device camera, and positioned over the PVH in five cats. Electrodes for assessing state variables, including electroencephalographic activity, eye movement, and somatic muscle tone were also placed. After surgical recovery, reflected light intensity was measured continuously at 2.5 Hz during spontaneously varying sleep/waking states. Sequential state transitions from active waking to quiet waking, quiet sleep and active sleep were accompanied by progressively increased levels of PVH activity. Overall activity was highest during active sleep, and decreased markedly upon awakening. Moment-to-moment activity oscillated in the 0-0.1 Hz range, especially during active sleep and active waking; this oscillation diminished during quiet sleep. Distinct sub-regions of enhanced or diminished activity emerged within the imaged area in a state-dependent manner. We conclude that PVH activity changes with behavioral state in a regionally specific manner, and that overall activity increases during quiet sleep, and is even more enhanced in active sleep. PVH activation could be expected to stimulate pituitary release of adrenocorticotropic hormone (ACTH) and affect input to autonomic regulatory sites. Since ACTH and corticotropin releasing factor elicit arousal, and since the PVH projects to other brain areas which modulate state, we speculate that the PVH plays a role in shaping characteristics of sleep/waking states.

Sleep increase after immobilization stress: role of the noradrenergic locus coeruleus system in the rat

In a preliminary study we showed that the sleep rebound occurring after sleep deprivation is decreased in rats treated with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a neurotoxic agent specific for the noradrenergic cells of the locus coeruleus (LC). Sleep deprivation methods not only involve sleep loss, but also stress, which per se may induce an increase in sleep duration. Extensive research showed that the locus coeruleus is involved in stress. To evaluate the participation of LC in this mechanism, the effect of DSP-4 treatment was studied on sleep duration following a short intense stress in the absence of sleep loss. The results showed that the augmentation of sleep after 1 h of immobilization stress is lower in DSP-4-treated rats (slow-wave sleep duration, -24%; paradoxical sleep duration, -52%). These findings suggest that the increase in sleep induced by such a stressor is mediated, at least in part, by the noradrenergic LC.

The hypothalamic paraventricular nucleus in two types of Wistar rats with different stress responses. II. Differential Fos-expression

The present study investigates the role of corticotropin-releasing hormone (CRH) neurons in stress regulation by a comparison of stress induced Fos-immunoreactivity and CRH-immunoreactivity in the hypothalamic paraventricular nucleus (PVH) of APO-SUS (apomorphine-susceptible), APO-UNSUS (apomorphine-unsusceptible), normal Wistar and adrenalectomized Wistar (ADX) rats. The first two types represent a good model to study the role of the PVH in stress regulation, since they show different stress responses and a differential synaptic organization of the PVH. After placement on an open field for 15 min all rats showed an increase in the number of Fos-immunoreactive nuclei compared to control handling. Interestingly, open field stress, but not control handling, induces significantly fewer Fos-immunoreactive nuclei in the PVH of APO-SUS rats (1255 +/- 49) compared to APO-UNSUS rats (1832 +/- 201). Experiments with ADX rats revealed that 93% of the CRH-immunoreactive neurons contained a Fos-immunoreactive nucleus, which suggests that the differential Fos-expression in APO-SUS and APO-UNSUS rats represents a differential activation of the CRH neurons. This hypothesis is discussed in relation to reported differences in stress responses, stress-induced ACTH levels and synaptic organization of the PVH.

c-fos expression in hypothalamic neurosecretory and brainstem catecholamine cells following noxious somatic stimuli

Noxious somatic stimuli elicit vasopressin secretion, an effect thought to result from activation of a facilitatory input from A1 catecholamine cells of the medulla oblongata. To better characterize the A1 cell response and effects on other neuroendocrine A1 projection targets, particularly within the paraventricular nucleus, we have now mapped c-fos expression in neurochemically identified catecholamine and neurosecretory cells following a noxious somatic stimulus. Unilateral hind paw pinch significantly increased c-fos expression in contralateral A1 cells whereas other brainstem catecholamine cell groups were unaffected. Expression of c-fos was also increased in the supraoptic nucleus, this effect being more pronounced for vasopressin than oxytocin neurosecretory cells and, as with A1 cells, primarily on the side contralateral to the stimulated paw. In contrast, the increase in the paraventricular nucleus was greater in oxytocin rather than in vasopressin cells. Additionally there was a significant rise in c-fos expression in medial parvocellular paraventricular nucleus cells of noxiously stimulated animals. Notably, the majority of tuberoinfundibular corticotropin-releasing factor cells are located in this medial parvocellular zone. These results are consistent with and expand on those previously reported from electrophysiological and anatomical studies. The finding of differing neurosecretory cell responses between supraoptic and paraventricular nuclei has interesting implications with regard to the afferent control of neurosecretory cell activity. For example, the substantially greater activation of supraoptic versus paraventricular nucleus vasopressin cells, despite being innervated by the same medullary noradrenergic cell group, raises the possibility of a differential input or differences in responsiveness. Furthermore, the activation of paraventricular nucleus parvocellular cells is consistent with suggestions that the A1 cell group provides an excitatory input to this population.

Stress, antidepressant drugs, and the locus coeruleus

This review presents a synthesis of a large body of seemingly inconsistent literature on the role of the locus coeruleus-norepinephrine (LC-NE) system and the corticotropin-releasing hormone (CRH)-median eminence system in mediating the CNS effects of stress and the therapeutic effects of antidepressant drugs. The clinical implications of these findings for the etiology and treatment of stress-related psychiatric disorders such as depression will be discussed.

Medullary synaptic inputs to thyrotropin-releasing hormone (TRH)-containing neurons in the hypothalamus: an ultrastructural study combining WGA-HRP anterograde tracing with TRH immunocytochemistry

Ascending projections from the A1/C1 cell group and from the A2 cell group in the medulla oblongata was studied in the light microscope by anterograde tracing of Phaseolus vulgaris leucoagglutinin and in the electron microscope by anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) combined with thyrotropin-releasing hormone (TRH) immunocytochemistry in the hypothalamic paraventricular nucleus (PVN). WGA-HRP-labeled axon terminals originating from neurons in the A1/C1 or the A2 cell group were found to make synaptic contacts with TRH-containing cell bodies and dendrites in the medial parvocellular part of the PVN, usually forming axo-dendritic synapses. Of all the afferent synapses on TRH neurons in the PVN, 9.8-20.9% of the presynaptic axon terminals were WGA-HRP-positive. This indicates that each brain stem catecholaminergic cell group that contribute to innervation of the PVN is in a position to modulate the activity of TRH neurons.

Stress update Adaptation of the hypothalamic-pituitary-adrenal axis to chronic stress

The hypothalamic-pituitary-adrenal (HPA) axis exhibits a circadian rhythm, activation by stress, and inhibition by corticosteroids. Activity in the HPA axis is very sensitive to inhibition by corticosteroids when they are administered exogenously. When stress-induced corticosteroid secretion occurs, however, normal activity in the HPA is not inhibited and may even be augmented. Experiments in rats have shown that stress also induces facilitation of subsequent activity in the HPA axis that appears to balance the inhibitory effects of corticosterone and thus maintains responsiveness to new, acute stresses in chronically stressed rats. Stress-induced facilitation of HPA axis activity may be mediated by a parallel stress-induced (CRH-dependent) increase in the capacity of brain noradrenergic cell groups to respond to acute stress. A continually responsive HPA axis, even under conditions of chronic stress, appears to be important for survival. Stress-induced increases in glucocorticoid secretion to levels sufficient to occupy glucocorticoid receptors enable appropriate thermoregulatory and cardiovascular responses to acute stress. There is, however, an overall metabolic cost to the animal of maintaining continued activity in the HPA axis during chronic stress.

Induction of c-fos immunoreactivity in tyrosine hydroxylase and phenylethanolamine-N-methyltransferase immunoreactive neurons of the medulla oblongata of the rat after phosphate-buffered saline load in the urethane-anaesthetized rat

We have studied the induction of c-fos immunoreactivity (c-fos IR) in catecholaminergic and vasopressinergic immunoreactive neurons after repeated phosphate-buffered saline (PBS) loading or after repeated elicitation of the baroreceptor reflex via repeated infusion of the vasoconstrictor agent L-phenylephrine. About 75% and 30%, respectively, of the tyrosine-hydroxylase immunoreactive (IR) cell bodies of the ventral noradrenaline (NA) A1/adrenaline (A) C1 and dorsal NA A2/A C2 areas and 60% and 30%, respectively, of the phenylethanolamine N-methyltransferase IR nerve cells of the adrenaline C1 and C2 areas and 25% of the vasopressin (VP) IR neurons of the supraoptic (SO) nucleus developed nuclear c-fos IR after repeated PBS loading. This phenomenon remained unaltered by the repeated elicitation of the baroreceptor reflex. These results suggest that the activation of volume receptors promotes homeostatic responses via activation of early genes in subsets of central medullary noradrenaline and adrenaline neurons and SO VP neurons of the urethane-anaesthetized rat.

Noradrenergic innervation of the hypothalamus of rhesus monkeys: distribution of dopamine-beta-hydroxylase immunoreactive fibers and quantitative analysis of varicosities in the paraventricular nucleus

The distribution of noradrenergic processes within the hypothalamus of rhesus monkeys (Macaca mulatta) was examined by immunohistochemistry with an antibody against dopamine-beta-hydroxylase. The results revealed that the pattern of dopamine-beta-hydroxylase immunoreactivity varied systematically throughout the rhesus monkey hypothalamus. Extremely high densities of dopamine-beta-hydroxylase-immunoreactive processes were observed in the paraventricular and supraoptic nuclei, while relatively lower levels were found in the arcuate and dorsomedial nuclei and in the medial preoptic, perifornical, and suprachiasmatic areas. Moderate levels of dopamine-beta-hydroxylase immunoreactivity were found throughout the lateral hypothalamic area and in the internal lamina of the median eminence. Very few immunoreactive processes were found in the ventromedial nucleus or in the mammillary complex. Other midline diencephalic structures were found to have high densities of dopamine-beta-hydroxylase immunoreactivity, including the paraventricular nucleus of the thalamus and a discrete subregion of nucleus reuniens, the magnocellular subfascicular nucleus. A moderate density of dopamine-beta-hydroxylase immunoreactive processes were found in the rhomboid nucleus and zona incerta whereas little dopamine-beta-hydroxylase immunoreactivity was found in the fields of Forel, nucleus reuniens, or subthalamic nucleus. The differential distribution of dopamine-beta-hydroxylase-immunoreactive processes may reflect a potential role of norepinephrine as a regulator of a variety of functions associated with the nuclei that are most heavily innervated, e.g., neuroendocrine release from the paraventricular and supraoptic nuclei, and gonadotropin release from the medial preoptic area and mediobasal hypothalamus. Additionally, quantitative analysis of dopamine-beta-hydroxylase-immunoreactive varicosities was performed on a laser scanning microscope in both magnocellular and parvicellular regions of the paraventricular nucleus of the hypothalamus. The methodology employed in this study allowed for the high resolution of immunoreactive profiles through the volume of tissue being analyzed, and was more accurate than conventional light microscopy in terms of varicosity quantification. Quantitatively, a significant difference in the density of dopamine-beta-hydroxylase-immunoreactive varicosities was found between magnocellular and parvicellular regions, suggesting that parvicellular neurons received a denser noradrenergic input. These differential patterns may reflect an important functional role for norepinephrine in the regulation of anterior pituitary secretion through the hypothalamic-pituitary-adrenal stress axis.

Selective effects on CRF neurons and catecholamine terminals in two stress-responsive regions of adult rat brain after prenatal exposure to diazepam

Earlier work demonstrated that prenatal exposure to diazepam (DZ) selectively affected the noradrenergic (NE) terminals in the hypothalamus, leading to decreased basal NE levels, turnover rate, and release in adult offspring as well as altered responses to stressors in these NE projections. The exposure also affected plasma hormonal responses to stressors. In the present work, we used immunocytochemistry to study the effects of prenatal DZ exposure on NE terminals and on corticotropin-releasing factor (CRF)-containing neurons in the paraventricular nucleus (PVN) of the hypothalamus. DZ exposure (2.5 or 10 mg/kg over gestational days 14-20) led to a decrease in dopamine-beta-hydroxylase (DBH)-immunoreactivity (-ir) and a decrease in CRF-ir containing cells within the PVN of adult rats. The exposure also decreased DBH-ir in the ventral portion of the bed nucleus of the stria terminalis (BNST) but did not affect CRF-ir in the oval nucleus of BNST. Therefore, this study provides anatomic evidence that targeting benzodiazepine binding sites prenatally affects two neurotransmitter systems involved in responses to stressors.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: III. Effects of adrenoceptor agonists and antagonists

Stimulation of the ventral noradrenergic ascending bundle (VNAB) at low frequencies (0.5/5 Hz) excited the majority (37/46, 80%) of single paraventricular nucleus (PVN) tuberoinfundibular neurones, with high frequency (50 Hz) trains of stimuli reversing the direction of the response to inhibition for 7/16 (44%) of these excited cells. Iontophoretic application of noradrenaline, or the alpha 1-adrenoceptor agonist 1-phenylephrine, increased the spontaneous electrical activity of most of the cells tested (94% and 72%), whilst application of the alpha 1-antagonist, ergotamine reduced the spontaneous activity of 44% of the cells tested and prevented the excitation following VNAB stimulation for 84% of the cells examined. Application of the beta-adrenoceptor antagonist, propranolol, increased the spontaneous activity of 77% of cells and prevented the inhibitory PVN neuronal responses following high frequency VNAB stimulation of 94% of the cells, often reversing the response to excitation similar to that observed following low frequency VNAB stimulation. The alpha 2-adrenoceptor antagonist, tolazoline, was found to evoke mixed responses from the cells examined but a trend towards a suppression of spontaneous activity and potentiation of VNAB stimulation-evoked responses was observed. The alpha 2-adrenoceptor agonist, clonidine, elicited an initial excitation from the majority of cells tested, with most of the cells then exhibiting an inhibition, either with or without continued application. Excitatory responses following stimulation of the sciatic nerve were recorded from the majority of cells (82.5%) and ergotamine was able to suppress this response for all four cells so tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Iontophoresis of cortisol inhibits responses of identified paraventricular nucleus neurones to sciatic nerve stimulation

Responses of paraventricular nucleus (PVN) neurones were examined following stimulation of the sciatic nerve, and concomitant with iontophoretic application of cortisol. Sciatic nerve stimulation excited the majority of cells (22/24, 92%) and iontophoretic application of cortisol reduced the spontaneous activity of 16 of the cells tested (67%). Cortisol prevented neuronal responses to sciatic nerve stimulation in 11 cases (50%) but some of the cells inhibited by the steroid still responded to the stimulation, whilst some cells unaffected by cortisol alone were found not to respond during exposure to the stimulus. These results indicate an inhibitory role for glucocorticoids in the regulation of PVN neuronal activity and responses to afferent neural stimuli.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: II. Effects of stimulation of the ventral noradrenergic ascending bundle: evidence for cotransmission

In order to further elucidate the neural mechanisms underlying the control of adrenocortical secretion, responses of paraventricular nucleus (PVN) tuberoinfundibular neurones were examined following stimulation of the ventral noradrenergic ascending bundle (VNAB). Stimulation at low frequencies (0.5/5 Hz) excited the majority (52/64, 81%) of cells but only 15 showed a clear-cut, stimulus-locked, activation with onset latency of 44.5 +/- 10.0 msec and offset at 71.9 +/- 11.3 msec: the remaining 37 excited cells showed overall increases in firing after delivery of 5-10 stimuli. High frequency (50 Hz) trains of stimuli reversed the direction of response to inhibition for 14/52 of the excited cells. Inhibition of (nor)adrenaline synthesis by alpha-methylparatyrosine was without effect upon the firing of cells examined or the distribution and latencies of their responses following low frequency stimulation; high frequency trains reversed the response direction of only 4/35 cells, (p less than 0.05 vs. control rats; chi 2-test). Intracerebroventricular administration of 6-hydroxydopamine, a catecholaminergic neurotoxin, reduced the proportion of cells excited by the stimulation (10/47; p less than 0.005; chi 2-test). Unit responses to painful somatosensory stimuli were recorded from the majority of the cells tested (74%), except following 6-hydroxydopamine treatment, when only 38% were excited (p less than 0.005; chi 2-test). The results demonstrate that the VNAB provides an excitatory input to the PVN and that noradrenaline is probably responsible for this effect but a cotransmitter (neuropeptide Y?) may also be responsible for the observed excitatory responses. Inhibitory responses following high frequency stimulation were probably also mediated by (nor)adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)