Search

The distributions of the type I and type II isoforms of cGMP-dependent protein kinase were determined in the rat brain using immunohistochemistry and in situ hybridization, and compared with the localization of NO synthase determined with NADPH-diaphorase histochemistry. The type I cGMP-dependent protein kinase was highly expressed in the Purkinje cells of the cerebellar cortex, where it was closely associated with the NO synthase containing granule and basket cells. This kinase was also found in neurons in the dorsomedial nucleus of the hypothalamus, where it may be regulated by NO or atriopeptides. The type I kinase was not detected in other central neurons. In contrast, the type II kinase was widely distributed in the brain. In particular, it was highly expressed in the olfactory bulb, cortex, septum, thalamus, tectum and various brainstem nuclei. Many regions expressing this kinase also contained, or received innervation from NO synthase positive neurons. These results indicate that type I cGMP-dependent protein kinase may act as a downstream effector for NO only in the cerebellar cortex and the dorsomedial hypothalamus. The type II cGMP-dependent protein kinase appears to be a major mediator of NO actions in the brain.

Whole cell recording techniques were employed to measure whole cell (Gw) and specific membrane (Gm) conductance in turtle and rat pyramidal neurons in slices. Results indicate that rat neurons are 4.2 times more conductive compared with turtle neurons at 25 degrees C, which is accentuated by temperature, so that rat neurons at 37 degrees C are 22 times more conductive than turtle neurons at 15 degrees C. A conductance Q10 of 1.9 was measured for both turtle (15-25 degrees C) and rat (25-35 degrees C) pyramidal neurons. Conductance measurements of turtle pyramidal neurons over 6-9 h of anoxia indicate no statistical difference between Gm or Gw from normoxic control measures. These results indirectly support the concept of low ATP-dependent ion pump activity in the turtle brain as one mechanism for reduced energy expenditure in the normoxic state.

9-Amino-1,2,3,4-tetrahydroacridine (THA), an inhibitor of acetylcholinesterase, has been proposed as a treatment for Alzheimer's disease on the basis of its ability to increase cerebral levels of acetylcholine. THA shares structural features with aminoquinoline compounds known to be inhibitors of histamine-N-methyltransferase (HNMT). THA was found to be a potent competitive inhibitor of rat brain HNMT in vitro, with a Ki of 35 nM with respect to both histamine and S-adenosyl-L-methionine, the co-substrate. Two hours after systemic administration of THA (5 and 10 mg/kg, i.p.), HNMT from rat brain was largely inhibited. The levels of histamine in striatum and cerebral cortex were elevated by this treatment. Thus, THA at moderate doses is able to alter histamine metabolism in the central nervous system.

A family of guanine nucleotide binding proteins (G proteins) is involved in the transduction of information from receptors on the cell surface into cellular responses. Two G proteins, Gi and Gs, were initially defined by their inhibitory or stimulatory actions on adenylyl cyclase, respectively. In addition, brain contains high levels of another G protein, Go. cDNAs for the alpha subunits for these G proteins have been cloned and sequenced. This allowed us to examine the distributions of the mRNAs for the alpha subunits for Gi, Go and Gs in the rat brain using in situ hybridization with radio-labelled, synthetic oligonucleotide probes. Various regions known to contain catecholamine cell groups displayed high levels of G protein mRNA. There is good physiological evidence supporting a role for G proteins in signal transduction in dopaminergic and noradrenergic neurons. Therefore, further experiments were undertaken using in situ hybridization combined with immunohistochemistry to examine G proteins expression in identified catecholamine neurons. The results indicate that the dopaminergic neurons of the substantia nigra and the noradrenergic neurons of the locus ceruleus express the mRNA for the alpha subunits of all three of these G proteins. These data provide evidence for the coexpression of multiple G proteins within identified catecholamine neurons in the brain.

The distributions of catecholamine (CA) neurons in the medulla oblongata of the guinea pig and rat were compared using immunohistochemistry with rabbit antisera against the CA synthesizing enzymes, tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (D beta H) and phenylethanolamine-N-methyltransferase (PNMT). TH and D beta H distributions were similar in the two species. In contrast, the central nervous systems of both normal and colchicine-treated guinea pigs failed to demonstrate immunoreactivity for PNMT, the synthetic enzyme for adrenaline. The concentrations of biogenic amines in the hypothalamus and medulla were determined in guinea pig and rat tissues by high-performance liquid chromatography with electrochemical detection. No adrenaline was detected in the guinea pig brain. Thus it appears that guinea pigs lack central neurons capable of synthesizing adrenaline.

The distribution of neurons displaying choline acetyltransferase (ChAT) immunoreactivity was examined in the feline brain using a monoclonal antibody. Groups of ChAT-immunoreactive neurons were detected that have not been identified previously in the cat or in any other species. These included small, weakly stained cells found in the lateral hypothalamus, distinct from the magnocellular rostral column cholinergic neurons. Other small, lightly stained cells were also detected in the parabrachial nuclei, distinct from the caudal cholinergic column. Many small ChAT-positive cells were also found in the superficial layers of the superior colliculus. Other ChAT-immunoreactive neurons previously detected in rodent and primate, but not in cat, were observed in the present study. These included a dense cluster of cells in the medial habenula, together with outlying cells in the lateral habenula. Essentially all of the cells in the parabigeminal nucleus were found to be ChAT-positive. Additional ChAT-positive neurons were detected in the periolivary portion of the superior olivary complex, and scattered in the medullary reticular formation. In addition to these new observations, many of the cholinergic cell groups that have been previously identified in the cat as well as in rodent and primate brain such as motoneurons, striatal interneurons, the magnocellular rostral cholinergic column in the basal forebrain and the caudal cholinergic column in the midbrain and pontine tegmentum were confirmed. Together, these observations suggest that the feline central cholinergic system may be much more extensive than previous studies have indicated.