Organization of ventrolateral medullary afferents to the hypothalamus

Generalized arousal of mammalian central nervous system

A fundamental capacity of the mammalian CNS is becoming amenable to study with the techniques of functional genomics. Emphasized in this review are ascending connections from the medullary reticular formation and descending connections from the paraventricular nucleus of the hypothalamus. In particular, sex hormone effects on neurons allow us to relate generalized arousal to a specific form of arousal which is required for reproductive behaviors.

Plasma hypernatremia induces c-fos activity in medullary catecholaminergic neurons

Experiments were done in conscious rats to investigate the effect of intravenous infusion of hypertonic saline on the induction of the phosphoprotein Fos in brainstem catecholaminergic neurons. Fos induction, detected immunohistochemically, was used as a marker for neuronal activation. Infusions of 165 mM or 1.4 M NaCl solutions into the jugular vein resulted in Fos-like immunoreactivity in approximately the caudal two thirds of nucleus of the solitary tract (NTS), the caudal and rostral ventrolateral medulla (VLM), and in the lateral aspects of the parabrachial nucleus (PBN). Within caudal NTS a small number (7.9 +/- 1.8%) of Fos labelled neurons were found also to contain tyrosine hydroxylase (TH) or dopamine beta-hydroxylase (DBH) immunoreactivity. In rostral NTS no Fos labelled cells were found to contain phenylethanolamine N-methyltransferase (PNMT) immunoreactivity, although a few (8.5 +/- 2.3%) were immunoreactive to TH. Similarly, in VLM, most of the Fos labelled cells in caudal VLM (65.9 +/- 2.7%) contained either TH or DBH immunoreactivity, whereas in the rostral VLM, 32.2 +/- 4.6% of the Fos labelled cells were also immunoreactive to TH or DBH. However, no Fos cells were found in either the caudal or rostral VLM that were immunoreactive to PNMT. Little or no Fos-like immunoreactive neurons were detected in the brainstem after intravenous infusions of physiological (143 mM) or hypotonic (106 mM) NaCl solutions. These data suggest that noradrenergic neurons of the caudal NTS and VLM are components of central circuits that are involved in osmoregulation and cardiovascular function.

Collateral axonal projections to limbic structures from ventrolateral medullary A1 noradrenergic neurons

Experiments were done to investigate whether catecholaminergic neurons within the ventrolateral medulla (VLM) send collateral axonal projections to the central nucleus of the amygdala (ACe) and the bed nucleus of the stria terminalis (BST). Unilateral microinjections of the fluorescent retrograde tracers fluorogold (FG) or rhodamine labelled latex micro-beads (Rd) were made into either ACe or BST in the rat. Brainstem sections were then processed immunohistochemically for the identification of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase, dopamine beta-hydroxylase (DBH) or phenylethanolamine-N-methyltransferase (PNMT). Retrogradely labelled cell bodies projecting to either ACe or BST were found throughout the rostrocaudal extent of VLM, bilaterally. Approximately 44% of these retrogradely labelled neurons were found to contain both retrograde tracers. In addition, approximately 91% of the VLM neurons that send collateral axonal projections to ACe and BST were also immunoreactive to DBH. None were found to contain PNMT immunoreactivity. These results demonstrate that noradrenergic neurons of the A1 cell group in VLM innervate ACe and BST via collateral axonal projections and suggest that these VLM neurons may be directly involved in relaying cardiovascular afferent and/or visceral afferent information directly to these limbic structures.