Is the muscarinic receptor that mediates potentiation of dopamine release negatively coupled to the cyclic GMP system?

Twenty years of dopamine research: individual differences in the response of accumbal dopamine to environmental and pharmacological challenges

Individual differences in the dopaminergic system of the nucleus accumbens of rats have extensively been reported. These individual differences have frequently been used to explain individual differences in response to environmental and pharmacological challenges. Remarkably, only little attention is paid to the factors that underlie these individual differences. This review gives an overview of the studies that have been performed in our institute during the last 20 years to investigate individual differences in accumbal dopamine release. Data are summarised demonstrating that individual differences in accumbal dopamine release are due to individual differences in: the functional reactivity of the noradrenergic system, the accumbal concentration of vesicular monoamine transporters and tyrosine hydroxylase as well as in the quantal size of the presynaptic pools of dopamine. Our data are embedded in the available literature to create a model that illustrates the putative hardware giving rise to the individual-specific release of accumbal dopamine. An important role is contributed to individual differences in the reactivity of the: hypothalamic-pituitary-adrenal axes, the reactivity of second messenger systems as well in the aminergic reactivity of the accumbens shell and core. The consequences of the individual-specific make-up and reactivity of the nucleus accumbens on the regulation of behaviour and the response to drugs of abuse will also be discussed. Apart from agents that interact with dopaminergic receptors, re-uptake or breakdown, noradrenergic agents as well as agents that interact with vesicular monoamine transporters or tyrosine hydroxylase are suggested to have therapeutic effects in subjects that are suffering from diseases in which the dopaminergic system is disturbed.

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.

Discriminatory roles for D1 and D2 dopamine receptor subtypes in the in vivo control of neostriatal cyclic GMP

The D1 and D2 subtypes of the dopamine receptor have been distinguished by their opposing effects on levels of neostriatal cyclic adenosine monophosphate (cAMP). The studies reported here show that the content of cyclic guanosine monophosphate (cGMP) in the mouse neostriatum is modulated by dopaminergic drugs in a manner which also discriminates D1 and D2 receptors. D1 receptor stimulation with SKF 38393 produced up to 90%, dose-related increases in neostriatal cGMP, whereas D1 antagonism with SCH 23390 decreased cGMP by 30% and blocked the increase induced by SKF 38393. D2 receptor stimulation with quinpirole did not alter cGMP levels whereas D2 antagonism increased cGMP by 40-60% after haloperidol and by up to 100% after sulpiride. The increases in neostriatal cGMP levels following D1 agonism were potentiated in an additive manner by haloperidol. Thus, neostriatal cGMP content is positively controlled by D1 agonism and negatively controlled by or unlinked to the D2 receptor. The reciprocal control of neostriatal cGMP levels by D1- and D2-selective compounds may contribute to the separate as well as combined actions of D1 and D2 ligands.

Interaction of reversible and irreversible cholinesterase inhibitors on the monosynaptic reflex in neonatal rats

The ability of physostigmine (PHY) and pyridostigmine (PYR) to protect against the segmental synaptic depression caused by sarin was examined in isolated spinal cords from neonatal rats. The monosynaptic reflex was unaffected at concentrations up to 0.1 microM PHY or 0.3 microM PYR but raising the concentrations of either drug produced a concentration-dependent depression of the monosynaptic reflex which could be completely antagonized by atropine. The monosynaptic reflex was depressed by 50% at 0.45 microM PHY and 2 microM PYR with maximal depression occurring at 1 microM PHY (to about 10% of control) and 10 microM PYR (to about 35% of control). Pretreating the cords with 0.1 microM PHY and PYR for 30 min failed to protect against the depressant effects of sarin even though they inhibited total cholinesterase (ChE) by 27 and 21%, respectively. Both PHY and PYR depressed total ChE activity of the spinal cord in a concentration-dependent manner with 50% inhibition of ChE occurring at 0.8 microM. These results suggest that the carbamates affect segmental transmission by activation of a muscarinic receptor, that protective carbamylation of ChE is ineffective against organophosphorus-induced segmental depression, and that inhibition of ChE is unrelated to both carbamate- and organophosphorus-induced depression of the monosynaptic reflex.

Distribution of cholinergic receptors in the rat and human neocortex

Autoradiographic labelling of muscarinic (M1, M2, NMS binding sites) and nicotinic receptors shows an inhomogeneous distribution over architectonically identified cortical areas of the rat brain with highest concentrations in the medial prefrontal and frontal areas. Beside this general trend the areal patterns of different receptors are slightly varying. The laminar distribution of these receptors in the rat and human neocortex is characterized by two different patterns, one with highest receptor densities in the supragranular layers (M1 receptors, NMS binding sites), the other with a preferential labelling of layer IV and (with a lower intensity) layer V (M2 and nicotinic receptors). M1 receptors and NMS binding sites are codistributed at the laminar level with each other and with GABAA, D1, 5-HT1 and glutamate receptors; M2 receptors are codistributed only with nicotinic receptors. Immuno-histochemical studies with antibodies against muscarinic and nicotinic receptors demonstrate that these structures occur mainly in pyramidal and spiny stellate cells and to a lesser extent (13%) in a variety of interneurons. The immunoreactivity is visible in the perikaryon, dendrites and postsynaptic membranes. Neurons are found in the human neocortex, which react exclusively with one of the two antibodies, but a fraction of the neurons (about 30%) contains antigenic sites reacting with both antibodies. This is interpreted as colocalization of nicotinic and muscarinic receptors in some cortical neurons.