Somatostatin 2 Receptors in the Spinal Cord Tonically Restrain Thermogenic, Cardiac and Other Sympathetic Outflows.

The anatomical and functional characterization of somatostatin (SST) and somatostatin receptors (SSTRs) within the spinal cord have been focused in the dorsal horn, specifically in relation to sensory afferent processing. However, SST is also present within the intermediolateral cell column (IML), which contains sympathetic preganglionic neurons (SPN). We investigated the distribution of SSTR2 within the thoracic spinal cord and show that SSTR2A and SSTR2B are expressed in the dorsal horn and on SPN and non-SPN in or near the IML. The effects of activating spinal SSTR and SSTR2 were sympathoinhibition, hypotension, bradycardia, as well as decreases in interscapular brown adipose tissue temperature and expired CO2, in keeping with the well-described inhibitory effects of activating SSTR receptors. These data indicate that spinal SST can decrease sympathetic, cardiovascular and thermogenic activities. Unexpectedly blockade of SSTR2 revealed that SST tonically mantains sympathetic, cardiovascular and thermogenic functions, as activity in all measured parameters increased. In addition, high doses of two antagonists evoked biphasic responses in sympathetic and cardiovascular outflows where the initial excitatory effects were followed by profound but transient falls in sympathetic nerve activity, heart rate and blood pressure. These latter effects, together with our findings that SSTR2A are expressed on GABAergic, presumed interneurons, are consistent with the idea that SST2R tonically influence a diffuse spinal GABAergic network that regulates the sympathetic cardiovascular outflow. As described here and elsewhere the source of tonically released spinal SST may be of intra- and/or supra-spinal origin.

Somatostatin in the rat rostral ventrolateral medulla: Origins and mechanism of action.

Somatostatin (SST) or agonists of the SST-2 receptor (sst2 ) in the rostral ventrolateral medulla (RVLM) lower sympathetic nerve activity, arterial pressure, and heart rate, or when administered within the Bötzinger region, evoke apneusis. Our aims were to describe the mechanisms responsible for the sympathoinhibitory effects of SST on bulbospinal neurons and to identify likely sources of RVLM SST release. Patch clamp recordings were made from bulbospinal RVLM neurons (n = 31) in brainstem slices prepared from juvenile rat pups. Overall, 58% of neurons responded to SST, displaying an increase in conductance that reversed at -93 mV, indicative of an inwardly rectifying potassium channel (GIRK) mechanism. Blockade of sst2 abolished this effect, but application of tetrodotoxin did not, indicating that the SST effect is independent of presynaptic activity. Fourteen bulbospinal RVLM neurons were recovered for immunohistochemistry; nine were SST-insensitive and did not express sst2a . Three out of five responsive neurons were sst2a -immunoreactive. Neurons that contained preprosomatostatin mRNA and cholera-toxin-B retrogradely transported from the RVLM were detected in: paratrigeminal nucleus, lateral parabrachial nucleus, Kölliker-Fuse nucleus, ventrolateral periaqueductal gray area, central nucleus of the amygdala, sublenticular extended amygdala, interstitial nucleus of the posterior limb of the anterior commissure nucleus, and bed nucleus of the stria terminalis. Thus, those brain regions are putative sources of endogenous SST release that, when activated, may evoke sympathoinhibitory effects via interactions with subsets of sympathetic premotor neurons that express sst2 .

GABA and enkephalin tonically alter sympathetic outflows in the rat spinal cord.

GABA and enkephalin provide significant innervation of sympathetic preganglionic neurons. Despite some investigation as to the identity of premotor sources of these innervations no comprehensive analyses have been conducted. Similarly, although data describing the cardiovascular effects of blockade of GABAA receptors in the spinal cord is available, the effects at other sympathetic outflows are unknown. In contrast the sympathetic effects of opioid blockade in the spinal cord are unclear. The aims of this study were to identify potential sympathetic premotor sources of GABAergic and enkephalinergic input to the spinal cord and to describe the sympathetic and cardiovascular effects of spinal GABAA receptor and delta/mu opioid receptor blockade in urethane anaesthetised rats. Glutamic acid decarboxylase (GAD67) and preproenkephalin (PPE) mRNA were found in all regions containing sympathetic premotor neurons, with the medullary raphe and RVMM providing the major GABAergic projections, while the PVN, RVMM and medullary raphe provided the major enkephalinergic projections. Intrathecal injection of bicuculline, a GABAA antagonist, elicited large and prolonged increases in all outflows measured, confirming previous work describing a tonic GABAergic influence on vasomotor tone, and revealing a tonic GABAergic inhibition of interscapular brown adipose tissue temperature. Intrathecal naloxone elicited transient small inhibitory effects only on MAP and HR. Thus GABA acting in the spinal cord plays an important role in the tonic suppression of sympathetic outflows while enkephalin appears to play only a minor role.

Brain sources of inhibitory input to the rat rostral ventrolateral medulla.

The rostral ventrolateral medulla (RVLM) contains neurons critical for cardiovascular, respiratory, metabolic, and motor control. The activity of these neurons is controlled by inputs from multiple identified brain regions; however, the neurochemistry of these inputs is largely unknown. Gamma-aminobutyric acid (GABA) and enkephalin tonically inhibit neurons within the RVLM. The aim of this study was to identify all brain regions that provide GABAergic or enkephalinergic input to the rat RVLM. Neurons immunoreactive for cholera toxin B (CTB-ir), retrogradely transported from the RVLM, were assessed for expression of glutamic acid decarboxylase (GAD67) or preproenkephalin (PPE) mRNA using in situ hybridization. GAD67 mRNA was expressed in CTB-ir neurons in the following regions: the nucleus of the solitary tract (NTS, 6% of CTB-ir neurons), area postrema (AP, 8%), caudal ventrolateral medulla (17%), midline raphe (40%), ventrolateral periaqueductal gray (VLPAG, 15%), lateral hypothalamic area (LHA, 25%), central nucleus of the amygdala (CeA, 77%), sublenticular extended amygdala (SLEA, 86%), interstitial nucleus of the posterior limb of the anterior commissure (IPAC, 56%), bed nucleus of the stria terminals (BNST, 59%), and medial preoptic area (MPA, 53%). PPE mRNA was expressed in CTB-ir neurons in the following regions: the NTS (14% of CTB-ir neurons), midline raphe (26%), LHA (22%), zona incerta (ZI, 15%), CeA (5%), paraventricular nucleus (PVN, 13%), SLEA (66%), and MPA (26%). Thus, limited brain regions contribute GABAergic and/or enkephalinergic input to the RVLM. Multiple neurochemically distinct pathways originate from these brain regions projecting to the RVLM.