Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

NO-mediated cGMP synthesis in cultured cholinergic neurons from the basal forebrain of the fetal rat

Previously, using brain slices, we reported NO-mediated cGMP synthesis in all cholinergic fibers in the rat neocortex. In order to answer the question whether this property of cholinergic fibers was present before or developed after birth, we investigated properties of NO-responsiveness of cultured cholinergic forebrain neurons. Basal forebrain neurons of E16 fetal rat were cultured. Under the conditions chosen and after one day of culturing, all cells had attained a cholinergic phenotype using choline acetyltransferase or the vesicular acetylcholine transporter molecule as markers. Between 95-99% of the cells also expressed neuronal NOS. In the presence of 1 mM IBMX, a non-selective phosphodiesterase (PDE) inhibitor, 10 microM of the NO donor diethylamine-NONOate (DEANO) increased cGMP synthesis in 80% of the cells. cGMP levels in the cultured forebrain neurons were also increased when cells were stimulated with DEANO in the presence of the selective PDE inhibitors BAY 60-7550 (PDE2), sildenafil (PDE5), or the mixed type inhibitor papaverine (PDE2,5,10). Subpopulations of cells from the basal forebrain expressed mRNA for PDE2, PDE5, and PDE9. Atropine increased cGMP levels in an NO-dependent manner in a small population of cultured forebrain cells in the presence of IBMX. In conclusion, cultured cholinergic basal forebrain neurons present a heterogeneous cell population in the magnitude of their response to NO. NO-responsiveness of the cultured cholinergic neurons is already detectable after one day of culturing and indicates that NO-sensitivity of the cholinergic neurons of the rat basal forebrain is present well before birth.

cGMP: a second messenger for acetylcholine in the brain?

Already 30 years ago, it became apparent that there exists a relationship between acetylcholine and cGMP in the brain. Acetylcholine plays a role in a great number of processes in the brain, however, the role of cGMP in these processes is not known. A review of the data shows that, although the connection between NO-mediated cGMP synthesis and acetylcholine is firmly established, the complexities of the heterosynaptic pathways and the oligosynaptic structures involved preclude a clear definition of the role of cGMP in the functioning of acetylcholine presently.

Endogenous expression of nNOS protein in several neuronal cell lines

Several neuronal cell lines were screened for endogenous expression of neuronal nitric oxide synthase (nNOS) protein using Western blot analysis. Detectable levels of the nNOS protein were evident in the SK-N-SH, SH-SY5Y, and N1E-115 neuroblastoma cell lines, as well as the NG108-15 neuroblastoma x glioma hybrid. Only trace amounts were visible in Neuro2A human neuroblastoma cells. The presence of endogenously expressed nNOS in these cells may allow for the study of the interaction between nNOS and the endogenous receptor systems expressed in the same cells.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

The generation of nitric oxide by G protein-coupled receptors

The highly reactive free radical gas, nitric oxide, serves a variety of biomodulatory functions and has been implicated in a growing array of physiological and pathophysiological states. The striking differences between this labile substance and other, more conventional, signaling molecules highlight the tight degree of nitric oxide regulation that is required in order to maintain appropriate cellular homeostasis. The generation of nitric oxide represents a common component of the signal transduction pathways of a number of chemical signaling molecules that act via binding to G protein-coupled receptors. This review focuses on the relationship between this receptor superfamily, the generation of nitric oxide via the actions of the nitric oxide synthases and some of the inter- and intracellular roles of nitric oxide.