Susan Pyner

Identification of branching paraventricular neurons of the hypothalamus that project to the rostroventrolateral medulla and spinal cord

The paraventricular nucleus of the hypothalamus has efferent connections to autonomic nuclei known to ultimately regulate cardiovascular function. Studies have revealed projections to the sympathetic preganglionic neurons of the spinal cord and presympathetic motor neurons of the rostral ventrolateral medulla. This study set out to establish whether the same neurons in the paraventricular nucleus innervate both these regions. In rats the fluorescent neuroanatomical tracers FluoroGold, Fast Blue or Dextran tetramethyl rhodamine were injected into either the rostral ventrolateral medulla or T2 region of the spinal cord. After a suitable survival period (five to seven days) three populations of neurons could be identified in the paraventricular nucleus, double-labelled neurons and single-labelled neurons resulting from the injections into the spinal cord or injections into the rostral ventrolateral medulla. The neurons were of similar size regardless of the dye content. Most neurons were found in the parvocellular subdivision of the mid rostral paraventricular nucleus. The number of labelled neurons decreased in the caudal sections. This study provides an anatomical basis for three means of influence that the paraventricular nucleus can have on sympathetic activity; a hierarchical in series projection via the rostral ventrolateral medulla; a projection running in parallel with this but bypassing the rostroventrolateral medulla; and a branching population innervating neurons in both the rostral ventrolateral medulla and spinal cord. The paraventricular nucleus of the hypothalamus is an important brain area concerned with maintaining cardiovascular homeostasis. This anatomical study has not only provided confirmatory evidence that direct projections arising from the paraventricular hypothalamic nucleus do project to the rostral ventrolateral medulla and spinal cord, regions known to influence cardiovascular regulation. The study has identified a branching projection originating in the paraventricular nucleus of the hypothalamus that projects to both the rostral ventrolateral medulla and the spinal cord. Thus the paraventricular nucleus of the hypothalamus has three pathways in which to influence cardiovascular homeostasis.

The paraventricular nucleus of the hypothalamus sends efferents to the spinal cord of the rat that closely appose sympathetic preganglionic neurones projecting to the stellate ganglion

Abstract Using a combination of anterograde and retrograde neuronal tract-tracing techniques, the descending projections from the paraventricular nucleus of the hypothalamus (PVN) to the brain/spinal cord and in particular those axonal projections that appear to be contiguous with sympathetic preganglionic neurones (SPN) projecting to the stellate ganglion have been studied. Descending PVN pathways were located by the anterograde transport of biotinylated dextran amine (BDA), whilst SPN were retrogradely labelled with cholera B toxin subunit conjugated to horseradish peroxidase (CB-HRP). BDA-labelled PVN axons terminated in both hypothalamic and extrahypothalamic (including the midbrain, medulla and spinal cord) brain nuclei, with dense terminal labelling observed particularly in the arcuate hypothalamic nucleus and adjacent median eminence, in the solitary tract, vagal nuclei and in the intermediolateral region of the spinal cord (IML). Varicose descending PVN fibres in the IML were often observed to closely appose both the cell soma and dendrites of retrogradely labelled SPN (projecting to the stellate ganglion) in the spinal cord. In addition, it was shown that PVN descending axons crossing to the contralateral side of the spinal cord were closely associated with retrogradely labelled SPN projecting to the superior cervical ganglion. Such findings suggest that descending pathways from the PVN may exhibit a direct influence on cardiac sympathetic outflow and may also influence the behaviour of the contralateral population of SPN projecting to the superior cervical ganglion.

CONTROL OF SYMPATHETIC OUTFLOWS BY THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

1. The functional role of the paraventricular nucleus (PVN) has been examined by studying its connections with cardiovascular neurons in the medulla and spinal cord and its influence on activity in several sympathetic nerves.

Neurochemistry of the paraventricular nucleus of the hypothalamus: Implications for cardiovascular regulation

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and endocrine homeostasis. The PVN integrates specific afferent stimuli to produce an appropriate differential sympathetic output. The neural circuitry and some of the neurochemical substrates within this circuitry are discussed. The PVN has at least three neural circuits to alter sympathetic activity and cardiovascular regulation. These pathways innervate the vasculature and organs such as the heart, kidney and adrenal medulla. The basal level of sympathetic tone at any given time is dependent upon excitatory and inhibitory inputs. Under normal circumstances the sympathetic nervous system is tonically inhibited. This inhibition is dependent upon GABA and nitric oxide such that nitric oxide potentiates local GABAergic synaptic inputs onto the neurones in the PVN. Excitatory neurotransmitters such as glutamate and angiotensin II modify the tonic inhibitory activity. The neurotransmitters oxytocin, vasopressin and dopamine have been shown to affect cardiovascular function. These neurotransmitters are found in neurones of the PVN and within the spinal cord. Oxytocin and vasopressin terminal fibres are closely associated with sympathetic preganglionic neurones (SPNs). Sympathetic preganglionic neurones have been shown to express receptors for oxytocin, vasopressin and dopamine. Oxytocin causes cardioacceleratory and pressor effects that are greatest in the upper thoracic cord while vasopressin cause these effects but more significant in the lower thoracic cord. Dopaminergic effects on the cardiovascular system include inhibitory or excitatory actions attributed to a direct PVN influence or via interneuronal connections to sympathetic preganglionic neurones.

Identification of an efferent projection from the paraventricular nucleus of the hypothalamus terminating close to spinally projecting rostral ventrolateral medullary neurons.

The paraventricular nucleus of the hypothalamus is increasingly being viewed as an important site for cardiovascular integration because of its connections to regions in the brain and spinal cord which are known to be important in cardiovascular control. Like the vasomotor neurons of the rostral ventrolateral medulla, descending axons from paraventricular neurons can be identified that form synapses on sympathetic preganglionic neurons in the thoracic spinal cord. The purpose of this study was to determine whether paraventricular axons project to the rostral ventrolateral medulla and whether they are closely apposed to reticulospinal neurons in this region. Descending paraventricular axons were labelled with biotin dextran amine, while rostral ventrolateral medullary neurons were retrogradely labelled from the spinal cord with wheatgerm agglutinin conjugated to horseradish peroxidase. This revealed, within the rostral ventrolateral medulla, paraventricular axon and terminal varicosities closely apposed to and apparently contiguous with retrogradely labelled spinally projecting neurons. Thus our study at the light microscopical level has shown the potential for the paraventricular nucleus to directly influence rostral ventrolateral reticulospinal neurons. We suggest these connections, if confirmed by electron microscopy, could be one means by which activation of paraventricular neurons elicits alterations in blood pressure.

Terminals of paraventricular spinal neurones are closely associated with adrenal medullary sympathetic preganglionic neurones: immunocytochemical evidence for vasopressin as a possible neurotransmitter in this pathway

Abstract A recent study using transsynaptically transported pseudorabies virus, injected into the adrenal gland, showed labelled neurones in the paraventricular nucleus (PVN) of the hypothalamus, indicating that these neurones send projections to sympathoadrenal preganglionic neurones (SPNs). However, this technique cannot conclusively demonstrate that the pathway is monosynaptic. In order to investigate the possibility of a direct projection from the PVN to SPNs, the present study used the anterograde tracer biotin dextran amine to label paraventricular spinal projections and the retrograde tracer cholera toxin B conjugated to horseradish peroxidase to label SPNs. In addition, because electrophysiological evidence suggests vasopressin to be a neurotransmitter candidate in this pathway, immunocytochemical identification of the peptide and retrograde labelling of SPNs to the adrenal medulla were used to investigate this. The results of these studies show spinally projecting paraventricular axons with terminal varicosities closely associated with SPNs. Therefore some of these associations may represent boutons forming synaptic contact on SPNs. Similarly, vasopressin fibres were found close to the dendrites and soma of SPNs. It is suggested that spinal axons originating from paraventricular neurones can provide a direct influence on adrenal medullary function, that vasopressin is a possible neurotransmitter involved in some of these connections and this is one means by which the paraventricular nucleus can generate a defence to stressful stimuli.

The projection and synaptic organisation of NTS afferent connections with presympathetic neurons, GABA and nNOS neurons in the paraventricular nucleus of the hypothalamus.

Highlights ► Ascending NTS fibres target PVN-associated neurons. ► Suggest glutamate as the neurotransmitter. ► Anatomical basis for the different functions of cardiovascular receptors.

Rostroventrolateral medulla neurons preferentially project to target-specified sympathetic preganglionic neurons

The rostroventrolateral medulla is a key site for the regulation of vasomotor tone. Sympatho-excitatory neurons project from this region to contact sympathetic preganglionic neurons located in the intermediolateral nucleus of the thoracic and lumbat spinal cord. Functional studies show that stimulation of specific sites in the ventral medulla lead to selective activation of different vascular effectors. The present study was designed to determine the anatomical basis for this selectivity in vasomotor control. Anterograde and retrograde tracing methods were utilized to determine if the descending rostral ventrolateral projection is topographically organized such that neurons in particular locations within the nucleus project preferentially and contact a specific group of sympathetic preganglionic neurons. For this purpose spinally-projecting neurons at 15 sites from three separate rostrocaudal locations within the rostroventrolateral medulla in nine rats were anterogradely labelled with biotin dextran amine. The spinal cord was examined for axon terminals having close apposition to two groups of sympathetic preganglionic neurons, those projecting to the superior cervical ganglion and those to the adrenal medulla which were retrogradely labelled with cholera B chain-conjugated horseradish peroxidase. Areas of close apposition between retrogradely-labelled dendrites, cell bodies and anterogradely-labelled axons were found. Axons descending from the more rostral part of the rostroventrolateral medulla produced the highest density of close appositions to sympathetic preganglionic neurons in both target-specific populations. Caudal rostroventrolateral medulla injection sites gave rise to a less dense distribution of axons and terminals around the spinal sympathetic nuclei. This study has demonstrated that spinally-projecting neurons in the rostroventrolateral medulla are both topographically and viscerotopically organized. It is suggested that such an arrangement provides the means for selective and differential control of autonomic effectors and in particular those involved in cardiovascular regulation.

Right Atrial Stretch Induces Renal Nerve Inhibition and c‐fos Expression in Parvocellular Neurones of the Paraventricular Nucleus in Rats

The paraventricular nucleus of the hypothalamus plays a pivotol role in the regulation of plasma volume. Part of the response to an increase in volume load is an inhibition of renal sympathetic nerve activity. The present experiments were designed to determine which subnuclei of the paraventricular nucleus are involved in this sympatho‐inhibitory response. Experiments were performed on anaesthetised rats. Activated neurones were recognised by the expression of the early gene c‐fos, identified by immunohistochemical labelling of its protein product Fos. Plasma volume loading with 4% Ficoll 70, using an infusion‐withdrawal procedure (2 ml over 1 min) repeated 15 times over 1 h revealed a total of 775 ± 101 (n = 6) Fos‐positive neurones scattered throughout both the magnocellular and parvocellular subnuclei. In comparison, sustained hypertension resulted in 452 ± 56 (n = 3) Fos‐positive neurones similarly distributed, whereas a normotensive control group (n = 3) displayed 115 ± 18 Fos‐positive neurones. Because of this lack of a specific effect we used a more selective stimulation of right atrial receptors via a balloon placed at the junction of the superior vena cava and the right atrium so it did not impede venous return. Inflation of the balloon inhibited renal sympathetic nerve activity (36 ± 5%, n = 7) and repetitive inflation over 1 h resulted in c‐fos activation of a small number of neurones (54 ± 14) located only in the parvocellular subnuclei. Whether these are inhibitory interneurones acting within the paraventricular nucleus, or spinally projecting neurones which inhibit or excite renal sympathetic activity by an action in the spinal cord remains to be determined.

The paraventricular nucleus and heart failure

What is the topic of this review? This review gives an update on the cellular and molecular mechanisms within the autonomic nervous system involved in non‐pathological and pathological cardiovascular regulation. What advances does it highlight? For cardiovascular homeostasis in non‐pathological conditions to be maintained, discrete neural networks using specified signalling mechanisms at both cellular and molecular levels are required. In heart failure, the cell signalling protein partners CAPON and PIN decrease the bioavailability of nitric oxide by inhibiting neuronal nitric oxide synthase activity, leading to the removal of tonic neuronal inhibition. Following a myocardial infarction, pro‐inflammatory cytokines in the paraventricular nucleus and the subsequent generation of reactive oxygen species, via angiotensin II activation of the angiotensin II type 1 receptor, increase neuronal excitability further, leading to sympathetic excitation.