An investigation of neurones that possess the alpha 2C-adrenergic receptor in the rat dorsal horn.

The function of the alpha(2C) subclass of adrenergic receptor in the spinal cord is unclear at present. Immunoreactivity for this receptor is found predominantly on axon terminals of the superficial dorsal horn but limited information is available about the properties and origin of these axons. The aim of this study was to determine which classes of neurone give rise to axons that possess this receptor and to investigate the synaptic organisation of these terminals. A series of double-labelling experiments was performed to investigate the relationship between the alpha(2C) receptor and each one of 14 chemical markers that label various types of axon terminal in the dorsal horn. Tissue was examined with two-colour confocal laser scanning microscopy. Quantitative analysis revealed that alpha(2C)-adrenergic receptors are not present on terminals of unmyelinated or peptidergic primary afferents and descending noradrenergic or serotoninergic axons. They were found on a proportion of terminals belonging to a mixed population of excitatory and inhibitory spinal interneurones, including those that contain neurotensin, somatostatin, enkephalin, GABA and neuropeptide Y. However, a greater proportion of terminals originating from excitatory interneurones were found to possess the receptor. Electron microscopic analysis revealed that alpha(2C)-adrenergic receptor immunoreactivity is predominantly associated with axon terminals that are presynaptic to dendrites but a small proportion of immunoreactive terminals formed axo-axonic synaptic arrangements. These studies indicate that noradrenaline can modulate transmission in the dorsal horn by acting through alpha(2C)-adrenergic receptors on terminals of spinal interneurones.

Alpha(2A)-adrenergic receptors are present in mu-opioid receptor containing neurons in rat medial nucleus tractus solitarius.

Agonists of the alpha-2A-adrenergic- (alpha(2A)-AR) and the mu-opioid-receptor (muOR) jointly affect autonomic functions that are also disregulated in animals undergoing withdrawal from chronic administration of the muOR agonist morphine. Cardiovascular and gastrointestinal reflexes are mediated, in part, by the medial nucleus of the solitary tract (mNTS) at caudal (cNTS) and intermediate (iNTS) subregions. Together, this evidence suggests that alpha(2A)-AR and muOR may be colocalized within many of the same neuronal profiles in both the intermediate and caudal mNTS. In order to examine whether alpha(2A)-AR and muOR are present within common somata, dendrites, or axon terminals in the mNTS, we used electron microscopic immunocytochemistry for the detection of antisera against each receptor at intermediate and caudal levels of this brain region. Most of the dually labeled profiles were somata and dendrites. Of all dual-labeled profiles in the iNTS 49% were somata and were 47% dendrites, whereas in the cNTS 61% were somata and 32% were dendrites. Within dual-labeled profiles, the intracellular distribution of alpha(2A)-AR and muOR differed. MuOR was more frequently associated with the plasmalemma, whereas alpha(2A)-AR was often affiliated with vesicular organelles. Few axon terminals, and even fewer glia, contained both markers. We also frequently observed single-labeled alpha(2A)-AR glia that apposed exclusively muOR-containing dendrites or axon terminals. These findings indicate that somata and dendrites contain functional sites for convergent muOR and alpha(2A)-AR activation. In addition, each receptor is positioned for involvement in intercellular signaling between apposed neurons and glia. Activation of alpha(2A)-AR on muOR-containing somata or dendrites, or on glia apposed to muOR-containing neurons, may help to account for the efficacy of alpha(2A)-AR agonists in relieving some of the autonomic symptoms of opiate withdrawal.

Cellular and subcellular distribution of alpha 2A-adrenergic receptors in the visual cortex of neonatal and adult rats.

Activation of alpha 2-adrenergic receptors (alpha 2AR) in the cerebral cortex has been shown to modulate visually guided delayed response tasks as well as anxiety and depression. We used an antiserum directed specifically against the A subtype of alpha 2AR (alpha 2AAR) to determine the cell types and subcellular sites for noradrenergic reception mediated by this receptor in the adult and the developing rat visual cortices. Light microscopic examination of adult tissue revealed numerous labeled perikarya in layers II-VI, many of which appeared distinctly pyramidal. A few perikarya in layer I also were immunoreactive. In all layers, alpha 2AAR immunoreactivity (alpha 2AAR-ir) was present within proximal dendrites and fine processes. In neonatal tissue, there was an intense, distinct band of immunoreactivity spanning the layer composed of tightly packed immature cell bodies, i.e., the cortical plate. The band dissipated as this tier differentiated postnatally into the supragranular layers. Electron microscopy showed that the supragranular layers, which contain the highest density of noradrenergic fibers, also contain the highest areal density of labeled postsynaptic junctions beyond 2 weeks of age. Throughout the ages, the majority of immunoreactivity occurred at sites which, in single ultrathin sections, appeared to be nonjunctional sites of axons, dendrites, and in glial processes. Our observations indicate that (1) both pyramidal and nonpyramidal neurons are receptive to norepinephrine via alpha 2AAR, (2) alpha 2AAR synthesis is robust prior to synaptogenesis, and (3) alpha 2AAR operates both pre- and postsynaptically.

alpha 2-, alpha 2A-, alpha 2B/2C-Adrenoceptor subtype antagonists prevent lipopolysaccharide-induced fever response in rabbits.

Exogenous pyrogens, e.g., bacterial lipopolysaccharides (LPS), are thought to stimulate macrophages to release endogenous pyrogens, e.g., TNF alpha, IL-1 beta, and IL-6, which act in the hypothalamus to produce fever. We studied the effect of different alpha 1- and alpha 2-adrenoceptor subtype antagonists, applied intraperitoneally, on the febrile response induced by LPS in rabbits. Evidence was obtained that prazosin, an alpha 1- and alpha 2B/2C-adrenoceptor antagonist; WB-4101, an alpha 1- and alpha 2A-adrenoceptor antagonist; CH-38083, a highly selective alpha 2-adrenoceptor antagonist (alpha 2:alpha 1 > 2000); BRL-44408, an alpha 2A-adrenoceptor antagonist; and ARC-239, an alpha 2B/2C- and also alpha 1-adrenoceptor antagonist, blocked the increase of colonic temperature of the rabbit produced by 2 micrograms/kg LPS administered intravenously without being able in themselves to affect colonic temperature. In addition, prazosin, WB-4101 and CH-38083 antagonized the fall in skin temperature that occurred at the time when the colonic temperature was rising in control animals injected with LPS. All these results suggest that norepinephrine, through stimulation of both alpha 1- and alpha 2- (alpha 2A- and alpha 2B/2C-) adrenoceptor subtypes, is involved in producing fever in response to bacterial LPS.