The role of D2-autoreceptors in regulating dopamine neuron activity and transmission.

Dopamine D2-autoreceptors play a key role in regulating the activity of dopamine neurons and control the synthesis, release and uptake of dopamine. These Gi/o-coupled inhibitory receptors play a major part in shaping dopamine transmission. Found at both somatodendritic and axonal sites, autoreceptors regulate the firing patterns of dopamine neurons and control the timing and amount of dopamine released from their terminals in target regions. Alterations in the expression and activity of autoreceptors are thought to contribute to Parkinson's disease as well as schizophrenia, drug addiction and attention-deficit hyperactivity disorder (ADHD), which emphasizes the importance of D2-autoreceptors in regulating the dopamine system. This review will summarize the cellular actions of dopamine autoreceptors and discuss recent advances that have furthered our understanding of the mechanisms by which D2-receptors control dopamine transmission.

Normalizing dopamine D2 receptor-mediated responses in D2 null mutant mice by virus-mediated receptor restoration: comparing D2L and D2S.

D2 receptor null mutant (Drd2(-/-)) mice have altered responses to the rewarding and locomotor effects of psychostimulant drugs, which is evidence of a necessary role for D2 receptors in these behaviors. Furthermore, work with mice that constitutively express only the D2 receptor short form (D2S), as a result of genetic deletion of the long form (D2L), provides the basis for a current model in which D2L is thought to be the postsynaptic D2 receptor on medium spiny neurons in the basal forebrain, and D2S the autoreceptor that regulates the activity of dopamine neurons and dopamine synthesis and release. Because constitutive genetic deletion of the D2 or D2L receptor may cause compensatory changes that influence functional outcomes, our approach is to identify aspects of the abnormal phenotype of a Drd2(-/-) mouse that can be normalized by virus-mediated D2 receptor expression. Drd2(-/-) mice are deficient in basal and methamphetamine-induced locomotor activation and lack D2 receptor agonist-induced activation of G protein-regulated inward rectifying potassium channels (GIRKs) in dopaminergic neurons. Here we show that virus-mediated expression of D2L in the nucleus accumbens significantly restored methamphetamine-induced locomotor activation, but not basal locomotor activity, compared to mice receiving the control virus. It also restored the effect of methamphetamine to decrease time spent in the center of the activity chamber in female but not male Drd2(-/-) mice. Furthermore, the effect of expression of D2S was indistinguishable from D2L. Similarly, virus-mediated expression of either D2S or D2L in substantia nigra neurons restored D2 agonist-induced activation of GIRKs. In this acute expression system, the alternatively spliced forms of the D2 receptor appear to be equally capable of acting as postsynaptic receptors and autoreceptors.

Interaction of vasomotor and exocrine neurons in bullfrog paravertebral sympathetic ganglia.

A 2 min sample of an intracellular recording of in vivo synaptic activity from a vasomotor C-neuron in a bullfrog sympathetic ganglion was converted to a series of stimulus pulses. This physiologically derived activity was used to stimulate preganglionic C-fibres of similar ganglia studied in vitro. Intracellular recordings were made from exocrine B-cells within the ganglia. Although they do not receive fast, nicotinic synaptic input from preganglionic C-fibres, B-cell excitability was profoundly increased by stimulation of C-fibres with physiologically derived activity. Also, subthreshold depolarizing current pulses that failed to generate action potentials in B-cells under control conditions almost always generated action potentials whilst C-fibres were activated. These effects were attenuated or prevented by the luteinizing hormone releasing hormone antagonist, [D-pyro-Glu1,D-Phe2,D-Trp3,6]-LHRH (70 microM). The physiological release of luteinizing hormone releasing hormone from C-fibres therefore causes an interaction between vasomotor and exocrine outflow within a paravertebral sympathetic ganglion.

Development of resistance to the growth inhibitory effects of transforming growth factor beta 1 during the spontaneous transformation of rat liver epithelial cells.

The temporary maintenance of a rat liver epithelial cell population at confluence before passaging followed by periods of rapid proliferation resulted in the generation of spontaneous transformants after about 108 population doublings. The appearance of morphologically aberrant transformants correlated directly with an increased resistance of the population to the growth inhibitory effects of transforming growth factor beta 1 (TGF-beta 1). Clonal cell lines derived from the transformants were resistant to TGF-beta 1 dependent inhibition of DNA synthesis. These cell lines were also highly tumorigenic and aneuploid, with characteristic gross chromosomal abnormalities, and they expressed a number of phenotypic markers common to rat liver epithelial cells transformed by oncogenes or chemicals. In contrast, apparently normal looking cell lines cloned from the same population were nontumorigenic and near diploid, with few chromosomal abnormalities, and they were as sensitive to TGF-beta 1 as early passage normal rat liver epithelial cells. Morphologically normal late passage rat liver epithelial cells were sensitive to transformation by the DNA hypomethylating agent 5-aza-2-deoxycytidine, in contrast to earlier passage cells, and this transformation was accompanied by the development of resistance to the growth inhibitory effects of TGF-beta 1. These findings suggest that acquisition of resistance to the effects of growth inhibitors such as TGF-beta 1 is an important and possibly essential stage in the spontaneous transformation of rat liver epithelial cells.

Altered responsiveness of rat liver epithelial cells to transforming growth factor beta 1 following their transformation with v-raf.

The effects of transforming growth factor beta (type 1) (TGF-beta 1) on DNA synthesis, cell proliferation, and protein synthesis were examined in a series of v-raf-transformed rat liver epithelial (RLE) cells, which exhibit a range of transformed phenotypes. All of the transformed cells were relatively resistant to the growth-inhibitory effects of TGF-beta 1, compared to normal RLE cells and control cells infected with a helper virus. The more tumorigenic cell lines had very few surface receptors for TGF-beta 1 and showed no increase in the secretion of a number of specific proteins, including fibronectin, following TGF-beta 1 treatment. In contrast, the more normal-looking, less tumorigenic v-raf-transformed cells bound similar amounts of TGF-beta 1 as normal RLE and control cells and showed a similar pattern of TGF-beta 1-stimulated protein secretion. These findings suggest that the effects of TGF-beta 1 on cell proliferation and on the expression of certain secreted proteins are mediated through different mechanisms. Following transformation of RLE cells with v-raf, the signalling pathways controlling TGF-beta 1 growth inhibition are perturbed, while those involved in regulating the synthesis of certain proteins may remain intact. Thus, the escape from the various distinct biological effects of TGF-beta 1 may be an important stage in the progression of neoplastic transformation of RLE cells in vitro.

Effects of interleukin-6 on the growth of normal and transformed rat liver cells in culture.

Recombinant human interleukin-6 produced a dose-dependent inhibition of DNA synthesis in both growing and mitogen-stimulated cultures of normal rat liver epithelial cells and also in primary rat hepatocytes. A significant inhibition of DNA synthesis (P less than 0.001) was obtained with 1 ng/ml (10 Units/ml) interleukin-6 in normal rat liver epithelial cells. The ID50 for inhibition of DNA synthesis in primary rat hepatocytes was 1 ng/ml. In contrast to the effects of transforming growth factor beta (Type I), where an almost complete inhibition of DNA synthesis could be achieved with either cell type, the maximal inhibition observed with interleukin-6 for both of these cell types was about 45%. Thus distinct mechanisms are involved in the inhibition of liver cell growth by these growth modulators. Transformed liver-derived cell lines were relatively resistant to the growth inhibitory effects of both interleukin-6 and TGF-beta 1 compared with the normal cells. However, human Hep G2 cells, which were completely resistant to the growth inhibitory effects of TGF-beta 1, were moderately inhibited by interleukin-6, indicating that the mechanisms responsible for the acquired resistance to growth inhibition is different for these growth inhibitors. The ability of interleukin-6 to function as a growth inhibitor in vitro was confirmed using normal rat liver epithelial cells. Interleukin-6 at a concentration of 10 ng/ml produced a significant decrease (P less than 0.05) in the proliferation of these cells. These data demonstrate that interleukin-6 may have the capability of functioning as a growth regulatory polypeptide for liver cells in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)