Hypothalamic norepinephrine mediates limbic effects on adrenocortical secretion

The purpose of this study was to elucidate the role of norepinephrine (NE) in the mediation of adrenocortical responses following limbic stimuli. The effects of stimulation of the dorsal and ventral hippocampus and the midbrain reticular formation on the plasma corticosterone (CS) levels was studied in rats with vehicle or 6-hydroxydopamine (6-OHDA) injected bilaterally into the paraventricular nucleus of the hypothalamus (PVN). The injection of 6-OHDA caused a very significant reduction in the concentration of PVN NE and blocked the rise in plasma CS following the stimulation of the above three limbic structures. The basal CS level and the response to ether stress were not affected. The present study supports previous observations on the stimulatory role of NE on CS secretion and that the modulatory effects of extrahypothalamic limbic structures on the adrenocortical activity depend on the presence of NE in the PVN.     

Paraventricular nucleus anandamide signaling alters eating and substrate oxidation

In the central nervous system, the endocannabinoid anandamide [N-arachidonoylethanolamine (AEA)] is believed to increase food intake through on-demand activation of hypothalamic circuits. The present study examined the effects of hypothalamic paraventricular nucleus (PVN) injections of AEA (25-400 pmol) on food intake and energy substrate oxidation [respiratory quotient (RQ)]. PVN administration of AEA increased eating behavior and RQ, indicating enhanced carbohydrate oxidation. Further, PVN administration of the cannabinoid type 1 receptor inverse agonist AM251 (5-10 μg) attenuated both the eating and the RQ responses elicited by AEA (100 pmol). AM251 administered alone did not alter food intake or RQ. Overall, these findings are consistent with a role for PVN cannabinoid type 1 receptors in the regulation of eating and energy homeostasis.     

Paraventricular nucleus ablation disrupts daily torpor in Siberian hamsters

Siberian hamsters were maintained in a short-day photoperiod (8 h light/day) at 15°C; body temperature (T_b) was telemstrically monitored at 10-min intervals over the course of 4 months. Animals manifesting repeated torpor bouts (T_b < 30°C) were subjected to lesions of the paraventricular nucleus (PVN) or a sham operation. In the 8 weeks after surgery, 67% of the animals with complete bilateral ablation of the PVN failed to express torpor, circadian T_b rhythms, as determined by periodogram analysis, were normal in all operated and sham-operated animals. Body mass did not change in PVN-ablated animals that continued to express torpor; in contrast, rapid and sustained increases in body mass were manifested by all hamsters that terminated expression of torpor. Expression of torpor is attenuated in the absence of the PVN; it is suggested that the PVN influences torpor Indirectly by regulating body mass or the availability of metabolic fuels.     

Paraventricular hypothalamic nucleus: Axonal projections to the brainstem

The paraventricular hypothalamic nucleus (PVH) contains many neurons that innervate the brainstem, but information regarding their target sites remains incomplete. Here we labeled neurons in the rat PVH with an anterograde axonal tracer, Phaseolus vulgaris leucoagglutinin (PHAL), and studied their descending projections in reference to specific neuronal subpopulations throughout the brainstem. While many of their target sites were identified previously, numerous new observations were made. Major findings include: 1) In the midbrain, the PVH projects lightly to the ventral tegmental area, Edinger‐Westphal nucleus, ventrolateral periaqueductal gray matter, reticular formation, pedunculopontine tegmental nucleus, and dorsal raphe nucleus. 2) In the dorsal pons, the PVH projects heavily to the pre‐locus coeruleus, yet very little to the catecholamine neurons in the locus coeruleus, and selectively targets the viscerosensory subregions of the parabrachial nucleus. 3) In the ventral medulla, the superior salivatory nucleus, retrotrapezoid nucleus, compact and external formations of the nucleus ambiguus, A1 and caudal C1 catecholamine neurons, and caudal pressor area receive dense axonal projections, generally exceeding the PVH projection to the rostral C1 region. 4) The medial nucleus of the solitary tract (including A2 noradrenergic and aldosterone‐sensitive neurons) receives the most extensive projections of the PVH, substantially more than the dorsal vagal nucleus or area postrema. Our findings suggest that the PVH may modulate a range of homeostatic functions, including cerebral and ocular blood flow, corneal and nasal hydration, ingestive behavior, sodium intake, and glucose metabolism, as well as cardiovascular, gastrointestinal, and respiratory activities. J. Comp. Neurol. 518:1460–1499, 2010. © 2009 Wiley‐Liss, Inc.     

Paraventricular nucleus, stress response, and cardiovascular disease

Abstract The paraventricular nucleus of the hypothalamus (PVN) is a complex effector structure that initiates endocrine and autonomic responses to stress. It receives inputs from visceral receptors, circulating hormones such as angiotensin II, and limbic circuits and contains neurons that release vasopressin, activate the adrenocortical axis, and activate preganglionic sympathetic or parasympathetic outflows. The neurochemical control of the different subgroups of PVN neurons is complex. The PVN has been implicated in the pathophysiology of congestive heart failure and the metabolic syndrome.     

Depletion of hypothalamic norepinephrine and serotonin enhances the dexamethasone negative feedback effect on adrenocortical secretion.

The role of norepinephrine (NE) and serotonin (5-HT) in the negative feedback effect of dexamethasone (DEX) on the adrenocortical response to ether stress was investigated. Injection of the catecholamine neurotoxin, 6-hydroxydopamine, into the ventral noradrenergic bundle or the paraventricular nucleus of the hypothalamus (PVN) which produced a very significant depletion in hypothalamic NE content enhanced the negative feedback effect of DEX. Injection of the 5-HT neurotoxin, 5,7-dihydroxytryptamine, into the raphé nuclei or PVN, which caused a depletion of hypothalamic 5-HT, produced a similar effect on the adrenocortical response to DEX. The degree of negative feedback may be viewed as a balance of neural stimulatory and glucocorticoid influences of the hypothalamus. Thus the removal of the stimulatory effects of NE and 5-HT on adrenocortical secretion, by the neurotoxic lesions, enhanced the inhibitory influence of DEX.     

GABAergic modulation mediates antinociception produced by serotonin applied into thalamic nucleus submedius of the rat

Our previous studies have indicated that the thalamic nucleus submedius (Sm) is involved in modulation of nociception as part of an ascending component of an endogenous analgesic system consisting of spinal cord-Sm-ventrolateral orbital cortex (VLO)-periaqueductal gray (PAG)-spinal cord loop and that microinjection of 5-hydroxytryptamine (5-HT) into Sm produces antinociception. The aim of the present study was to examine whether the gamma-aminobutyric acid (GABA)ergic modulation is involved in the Sm 5-HT-evoked antinociception. Experiments were carried out on lightly anesthetized rats with an implanted cannula targeting the Sm nucleus. The microinjection of GABA(A) receptor antagonist bicuculline dose-dependently depressed the tail flick (TF) reflex. A smaller dose (100 ng) of bicuculline enhanced the inhibition of TF reflex produced by 5-HT application into Sm, whereas application of GABA (2.5 microg) did not influence the TF reflex but significantly attenuated the 5-HT-evoked inhibition. These results indicate that GABA(A) receptor may be involved in mediating the 5-HT-induced antinociception in Sm possibly through a disinhibition mechanism.     

Interactions between serotonin, thyrotropin-releasing hormone, and substance P in the CNS regulation of adrenocortical secretion

Double-labeling immunohistochemical studies were performed to discern the morphological relationships between corticotropin-releasing factor-immunoreactive (CRF-ir) perikarya and afferent innervation in the hypothalamic paraventricular nucleus (PVN) of the rat. Attention was focussed on the local innervation by serotonin (5-hydroxytryptamine, 5-HT), thyrotropin-releasing hormone (TRH) and substance P (SP)-ir nerve terminal fibers. 5-HT-ir and SP-ir fibers were present in moderate numbers, in close apposition with CRF-ir perikarya. Sparse TRH-ir fibers were observed, but a population of TRH-ir perikarya was found in proximity with the CRF-ir cell bodies. TRH-ir perikarya in the PVN were surrounded by both 5-HT- and SP-ir fibers. Neuroendocrine studies were performed to investigate the interactions between 5-HT, TRH and SP in the regulation of hypothalamo-pituitary-adrenocortical (HPA) secretion. Male rats were prepared bearing cannulae for intracerebroventricular (ICV) or intra-PVN administration of drugs. 5-HT, at all doses tested (0.1, 40, or 80 nmol, ICV), caused increases in plasma corticosterone (CS) concentrations in tail-vein blood collected 20 min after injection. ICV injections of TRH caused dose-dependent increases in plasma CS, but did not further increase HPA responses when injected together with 5-HT. SP alone had little effect, although a significant reduction in plasma CS concentrations was observed in several individual experiments. However, SP (0.1 nmol) significantly attenuated CS responses following high doses of 5-HT (40 and 80 nmol, ICV), although the response to 0.1 nmol 5-HT was not affected. Combined injection of SP with TRH resulted in HPA responses not different from those following TRH alone. Similarly, SP did not reduce the HPA response observed with TRH and 40 nmol 5-HT in combination. Intra-PVN injections of 5-HT (0.1 or 40 nmol) and TRH also increased plasma CS concentrations. Intra-PVN injections of SP had little effect on plasma CS concentrations although a tendency toward a decrease in plasma CS was observed, as with the ICV injections. Combined intra-PVN injection of 5-HT (0.1 nmol) with TRH (0.1 nmol) did not significantly alter the response compared with that observed following TRH alone, although plasma CS concentrations were greater than with 0.1 nmol 5-HT. Combined intra-PVN injections of SP (0.1 nmol) with 5-HT (0.1 nmol) resulted in a significant decrease in plasma CS concentration compared with that following 5-HT alone, but SP did not prevent the CS response to a higher dose of 5-HT (40 nmol). Similar to the results observed following ICV administration, SP in combination with TRH did not affect the HPA response observed with TRH, or with 5-HT and TRH in combination. The results of these studies demonstrate that 5-HT and TRH serve to stimulate adrenocortical secretion, both at the level of the PVN and at extrahypothalamic structures, but that SP exerts an inhibitory influence that is capable of overcoming some of the stimulatory effects of 5-HT.     

Paraventricular thalamic nucleus: Subcortical connections and innervation by serotonin,orexin, and corticotropin‐releasing hormone in macaque monkeys

The present study examines subcortical connections of paraventricular thalamic nucleus (Pa) following small anterograde and retrograde tracer injections in cynomolgus monkeys (Macaca fascicularis). An anterograde tracer injection into the dorsal midline thalamus revealed strong projections to the accumbens nucleus, basal amygdala, lateral septum, and hypothalamus. Retrograde tracer injections into these areas labeled neurons specifically in Pa. Following a retrograde tracer injection into Pa, labeled neurons were found in the hypothalamus, dorsal raphe, and periaqueductal gray. Pa contained a remarkably high density of axons and axonal varicosities immunoreactive for serotonin (5‐HT) and orexin/hypocretin (ORX), as well as a moderate density of fibers immunoreactive for corticotropin‐releasing hormone (CRH). A retrograde tracer injection into Pa combined with immunohistochemistry demonstrated that ORX and 5‐HT axons originate from neurons in the hypothalamus and midbrain. Pa‐projecting neurons were localized in the same nuclei of the hypothalamus, amygdala, and midbrain as CRH neurons, although no double labeling was found. The connections of Pa and its innervation by 5‐HT, ORX, and CRH suggest that it may relay stress signals between the midbrain and hypothalamus with the accumbens nucleus, basal amygdala, and subgenual cortex as part of a circuit that manages stress and possibly stress‐related psychopathologies. J. Comp. Neurol. 512:825–848, 2009. © 2008 Wiley‐Liss, Inc.     

Paraventricular thalamus mediates context-induced reinstatement (renewal) of extinguished reward seeking

Paraventricular thalamus (PvTh) is uniquely placed to contribute to reinstatement of drug and reward seeking. It projects extensively to regions implicated in reinstatement including accumbens shell (AcbSh), prefrontal cortex (PFC) and basolateral amygdala (BLA), and receives afferents from other regions important for reinstatement such as lateral hypothalamus. We used complementary neuroanatomical and functional approaches to study the role of PvTh in context-induced reinstatement (renewal) of extinguished reward-seeking. Rats were trained to respond for a reward in context A, extinguished in context B and tested in context A or B. We applied the neuronal tracer cholera toxin B subunit (CTb) to AcbSh and examined retrograde-labelled neurons, c-Fos immunoreactivity (IR) and dual c-Fos/CTb labelled neurons in PvTh and other AcbSh afferents. In PvTh there was c-Fos IR in CTb-positive neurons associated with renewal showing activation of a PvTh–AcbSh pathway during renewal. In PFC there was little c-Fos IR in CTb-positive or negative neurons associated with renewal. In BLA, two distinct patterns of activation and retrograde labelling were observed. In rostral BLA there was significant c-Fos IR in CTb-negative neurons associated with renewal. In caudal BLA there was significant c-Fos IR in CTb-positive neurons associated with being tested in either the extinction (ABB) or training (ABA) context. We then studied the functional role of PvTh in renewal. Excitotoxic lesions of PvTh prevented renewal. These lesions had no effect on the acquisition of reward seeking. These results show that PvTh mediates context-induced reinstatement and that this renewal is associated with recruitment of a PvTh–AcbSh pathway.     

Glutamate receptors within the nucleus of solitary tract contribute to pancreatic secretion stimulated by intraduodenal hypertonic saline

It is well known that central transmission of vago-vagal reflex within the nucleus of solitary tract (NST) plays an important role in the regulation of gastrointestinal functions. The present study was designed to assess the role of NST glutamate receptor mechanism in pancreatic secretion evoked by intraduodenal hypertonic saline (HS) in anesthetized rats. Intraduodenal infusion of HS significantly (P<0.01) stimulated pancreatic protein output (from 2.60+/-0.09 to 4.18+/-0.24 mg/15 min). Bilaterally microinjected L-glutamate (5 nmol) into the medial nucleus of solitary tract (mNST) produced a significant increase of pancreatic protein secretion (from 2.65+/-0.12 to 4.80+/-0.34 mg/15 min, P<0.01). Bilateral injection of glutamate receptor antagonist kynurenic acid (KYN, 5 nmol) into the mNST completely abolished the increase of pancreatic protein output stimulated by intraduodenal HS (from 4.28+/-0.21 to 2.83+/-0.19 mg/15 min). Either NMDA receptor antagonist dl-2-amino-5-phosphonopentanoic acid (AP5, 1.5 nmol) or AMPA/Kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 1.5 nmol) injected into the mNST markedly attenuated (P<0.05) the pancreatic protein secretion stimulated by intraduodenal HS. In conclusion, these findings showed that blockade of the NST glutamate receptors, including NMDA and AMPA/Kainate receptors antagonized pancreatic secretion evoked by intraduodenal osmolality factor, and suggested that glutamate receptor mechanism within the NST contributed to the central regulation of pancreatic secretion.     

Paraventricular nucleus neurons producing neurotensin after lipopolysaccharide treatment project to the median eminence

Inflammation consists in secretion of cytokines that stimulate the hypothalamo-pituitary-adrenal (HPA) axis to release the anti-inflammatory corticosterone. Upstream in this axis are corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) whose multipeptidergic phenotype changes: both corticotropin-releasing hormone mRNAs and neurotensin mRNAs are up-regulated. Combining in situ hybridization with a retrograde neuronal marker, we demonstrated that neurotensin-containing neurons in the paraventricular nucleus project to the median eminence.     

Microbubble-induced serotonin secretion in human platelets

The effect of nitrogen (N2) microbubbles on platelets resembles that of common platelet agonists with respect to aggregation (Thorsen T et al., Undersea Biomed Res 1986; 13: 289-303). In the present study we examined the effect of microbubbles on platelet secretion of preloaded 14C-serotonin. We demonstrate that stirring of platelet-rich plasma with N2-microbubbles causes a loss of single platelets that is associated with secretion. However, secretion did not increase above baseline values until after 20 min of microbubble exposure, when platelet aggregation had reached 40%. After that time the secretion rate increased. There was no correlation between secreted serotonin and the degree of platelet aggregation. Although no 14C-serotonin secretion occurred in presence of acetylsalicyclic acid (ASA), microbubble-induced platelet aggregation was only marginally reduced. Epinephrine alone caused significant platelet aggregation but no 14C-serotonin secretion and it enhanced N2-microbubble-induced platelet aggregation and secretion; ASA completely prevented secretion under these circumstances but failed to abolish the enhancement of aggregation compared with microbubbles alone. Earlier studies have shown that platelets adhere to the bubble surfaces (Thorsen T et al., Undersea Biomed Res 1987; 14: 45-59). The results in the present study indicate that non-adhering platelets in the bulk phase are not activated by means of autocrine stimulation through dense granule material.     

Paraventricular nucleus neurons projecting to the dorsomedial medulla are influenced by systemic angiotensin

Activation of subfornical organ (SFO) efferents has been reported to increase the excitability of paraventricular nucleus (PVN) neurons projecting to cardiovascular centres in the medulla. Such findings suggest a role for these neural projections in the cardiovascular effects of systemic angiotensin II (AII) acting at the SFO. The present studies examine the effects of peripheral AII (50-500 ng) on the activity of PVN neurons antidromically identified as projecting to the dorsomedial medulla (DM). A total of 68 neurons were tested, of which 25% were found to be specifically activated by AII, and 25% inhibited as a consequence the increased blood pressure accompanying administration of this peptide. The activity of the remaining neurons tested was unaffected by AII. These studies demonstrate that only a small proportion of PVN neurons projecting to the DM, which have been shown to be influenced by SFO efferents, are also activated by systemic AII. Such data do however, support a significant role for these neurons in the cardiovascular functions of SFO.     

Paraventricular nucleus injections of naloxone methiodide inhibit NPY's effects on energy substrate utilization

Microinjection of neuropeptide Y (NPY) into the paraventricular nucleus (PVN) of the hypothalamus stimulates eating and increases respiratory quotient. In contrast, administration of opioid receptor antagonists reduces food intake and suppresses NPY-induced feeding. The present study examined whether naloxone methiodide, an opioid antagonist, would suppress the potentiation of NPY on energy substrate utilization, when injected into the PVN. Naloxone methiodide was injected at doses of 0.1 and 1.0 g, 10 min prior to NPY treatment. NPY was administered immediately prior to the start of the nocturnal period and RQ was determined using an open-circuit calorimeter. Doses of 50 and 100 pmol NPY alone evoked reliable increases in RQ within 30min of treatment. Following naloxone methiodide pretreatment, the stimulatory action of NPY was significantly attenuated. These data indicate that opioid receptors in the PVN influence the action of NPY on energy substrate utilization.     

Paraventricular nucleus of the hypothalamus: an electrophysiological investigation of neurons projecting directly to intermediolateral nucleus in the cat

Most type I synaptic glomeruli were lacking in adult rats given capsaicin neonatally, while topical cord applications of the drug caused dense degeneration of numerous C_I- and very few C_IIa-terminals. These data give support to the presumable origin of C_I-terminals from unmyelinated primary afferents and suggest that C_IIa-terminals may belong to small myelinated afferents.     

Paraventricular nucleus projections mediating pineal melatonin and gonadal responses to photoperiod in the hamster

Knife cuts were placed around the paraventricular nucleus of the hypothalamus (PVN) in order to identify the pathways mediating photoperiodism and pineal melatonin production in male golden hamsters. Cuts in the coronal plane caudal to the PVN, have no effect on photoperiodic control of the testes unless they actually damage the PVN. Bilateral parasagittal cuts at the medial border of the lateral hypothalamus block short photoperiod-induced gonadal regression. Nighttime levels of pineal melatonin are reduced by these cuts, but unaffected by caudal cuts. Projections from the lateral PVN region descending towards the spinal cord appear to be critical for the control of pineal melatonin production and the control of the testicular function by short photoperiod.     

Paraventricular nucleus injections of noradrenaline: cardiovascular effects in conscious Long-Evans and Brattleboro rats

The cardiovascular effects of noradrenaline injected into the hypothalamic paraventricular nucleus (PVN) were investigated in conscious Long-Evans (control) rats and homozygous vasopressin (AVP)-deficient Brattleboro rats. Unilateral microinjection of noradrenaline (3-30 nmol) into the PVN produced dose-dependent increases in systolic and diastolic blood pressure of Long-Evans rats, and a concomitant decrease in heart rate. Only the highest dose of noradrenaline tested (30 nmol) caused a significant pressor response in Brattleboro rats (9 +/- 4/9 +/- 4 mm Hg, systolic/diastolic, n = 7) which was significantly smaller than the response produced by the same dose of noradrenaline in Long-Evans rats (32 +/- 8/27 +/- 6 mm Hg, n = 7). Intravenous pretreatment of Long-Evans rats with the V1-receptor antagonist, d(CH2)5Tyr[Et]DAVP, almost completely abolished the pressor effect of noradrenaline (10 nmol) without significantly attenuating the bradycardia. The alpha 2-adrenoceptor antagonist, idazoxan (4 nmol), injected into the PVN abolished the pressor response produced by noradrenaline (10 nmol) in Long-Evans rats but had no significant effect on the bradycardia. Pretreatment with the alpha 1-adrenoceptor antagonist, prazosin (0.7 nmol), significantly attenuated both the pressor and bradycardic effects of noradrenaline in Long-Evans rats. These results suggest that the pressor response produced by microinjection of noradrenaline into the hypothalamic PVN of conscious Long-Evans rats is mediated largely through stimulation of alpha 2-adrenoceptors and is dependent, in part, on release of AVP into the circulation. A component of the bradycardia seen with this intervention may be mediated through stimulation of alpha 1-adrenoceptors in the PVN.     

Effect of central serotonin depletion on adrenocortical responses to neural stimuli

In view of the possible role of serotonin in adrenocortical regulation, basal plasma corticosterone concentrations and the response to ether stress, photic, acoustic, or sciatic nerve stimulation, were studied in rats with 5,7-dihydroxytryptamine or vehicle injected into the raphe nuclei. The neurotoxin inhibited the response to photic stimulation without affecting the other modalities. This may suggest that the depletion of brain serotonin has a differential effect on the transmission of neural impulses which activate the adrenal cortex.     

Basal and stimulated extracellular serotonin concentration in the brain of rats with altered serotonin uptake

We examined the relationship between the density of serotonergic (5-hydroxytryptamine [5-HT]) uptake sites and extracellular 5-HT concentration in the rat brain using microdialysis with two different models, lesions with 5,7-dihydroxytryptamine (50 μg in the dorsal raphe nucleus (DRN) 15 days before) and sublines of rats genetically selected displaying extreme values of platelet 5-HT uptake. Compared to controls, lesioned rats had a reduced cortical concentration of 5-hydroxyindoles (45%), unchanged basal extracellular 5-HT in the DRN and ventral hippocampus (VHPC), and reduced basal 5-hydroxyindoleacetic acid (5-HIAA) concentrations (46%, DRN; 22%, VHPC). Yet the perfusion of 100 mmol/L KCl or 1 μmol/L citalopram elevated dialysate 5-HT significantly more in the DRN and VHPC of controls. In genetically selected rats, platelet 5-HT content and uptake were highly correlated (r² = 0.9145). Baseline dialysate 5-HT (VHPC) was not different between high and low 5-HT rats and from normal Wistar rats. However, KCl or citalopram perfusion increased dialysate 5-HT significantly more in high 5-HT than in low 5-HT rats, and the former displayed a greater in vivo tissue 5-HT recovery. Significant but small differences in the same direction were noted in [³H]citalopram binding in several brain areas, as measured autoradiographically. Thus, basal extracellular 5-HT (but not 5-HIAA) concentrations are largely independent on the density of serotonergic innervation and associated changes in uptake sites. However, marked differences emerge during axonal depolarization or reuptake blockade. The significance of these findings for the treatment of mood disorders in patients with neurological disorders is discussed. Synapse 28:313–321, 1998. © 1998 Wiley-Liss, Inc.