The physiological operation of presynaptic inhibitory autoreceptors

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Autoregulation of neurotransmitter release at autonomic nerve terminals: a questionable theory

1. The evidence for feedback regulation of neurotransmitter release by means of autoreceptors is questioned. 2. Autoreceptor function must meet the expectations for feedback loops. However, this concordance has not been observed in most neuroeffector systems. 3. The characteristics of per pulse transmitter release with changes in the parameters of stimulation in several autonomic systems do not support the ongoing operation of negative feedback loop mediated by locally released transmitter. Also, the effects of antagonists and agonists often do not comply with feedback expectations. 4. Evidence is provided that agonists and antagonists act at different loci to achieve their inhibiting and potentiating effects on transmitter release. 5. Future efforts should be directed to exploring the mechanism(s) of antagonist action and to a system-by-system analysis of the evidence for and against autoreceptor operation.

Pharmacological modulation of endogenous dopamine and DOPAC outflow from nucleus accumbens

The release of endogenous DA and DOPAC from nucleus accumbens slices were studied measuring net outflow of DA and DOPAC in the superfusate of static chambers, to analyze the correlation between DA and DOPAC outflows and identify which DA stores may serve as possible sources for DOPAC formation. Under resting conditions, or following stimulation with low (< 15 mM) KCl concentration, DOPAC outflow was greater than DA. When DA release was stimulated by higher (> 25 mM) KCl concentrations, DA outflow increased, proportionally more than DOPAC. In the virtual absence of Ca2+ in the Krebs solution DA outflow, induced by 25 mM KCl, was reduced to about 10%, while DOPAC outflow was only reduced to 45%. When the synthesis of DA was inhibited with alpha-MPT, DA and DOPAC outflow were unchanged during the first stimulation period. During a second stimulation period, however, their outflow were significantly reduced. Nomifensine, a DA uptake inhibitor, increased the basal DA outflow by about 100%, but only blocked DOPAC basal outflow by about 25%. The 25 mM KCl stimulated DA outflow was not affected by Nomifensine, while the stimulated DOPAC outflow was reduced by about 50%. These results demonstrate that there is a weak correlation between the outflows of DA and DOPAC, suggesting a complex relationship between the mobilization of the different DA pools and DOPAC outflow. The formation of DOPAC from some of these pools, appear to be dependent on the stimulation levels and on the pharmacological manipulation of the tissue.

In vivo regulation of extracellular dopamine in the neostriatum: influence of impulse activity and local excitatory amino acids

It has been suggested that dopamine release can be evoked by excitatory amino acids acting on dopaminergic terminals, as well as by the classical process of impulse-evoked exocytosis. We used in vivo microdialysis to examine whether endogenous excitatory amino acids locally evoked dopamine efflux under basal conditions. Infusion of N-methyl-d-aspartate (NMDA) or kainate into the neostriatum increased extracellular dopamine, and this effect was blocked by co-infusion of 2-amino-5-phosphonovalerate (APV) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively. However, neither these antagonists nor kynurenate decreased extracellular dopamine when administered alone. In contrast, infusion of tetrodotoxin into the medial forebrain bundle reduced extracellular dopamine to below the limit of detection of our assay. These and other findings reviewed in this report suggest to us that extracellular dopamine in the neostriatum is not stimulated locally by endogenous excitatory amino acids.

TTX-sensitive Na+ channels and Ca2+ channels of the L- and N-type underlie the inward current in acutely dispersed coeliac-mesenteric ganglia neurons of adult rats

Inward membrane currents of sympathetic neurons acutely dispersed from coeliac-superior mesenteric ganglia (C-SMG) of adult rats were characterized using the whole-cell variant of the patch-clamp technique. Current-clamp studies indicated that C-SMG neurons retained electrical properties similar to intact ganglia. Voltage-clamp studies designed to isolate Na+ currents revealed that tetrodotoxin (TTX, 1 microM) completely inhibited the large transient inward current. Half activation potential (Vh) and slope factor (K) were -26.8 mV and 6.1 mV, respectively. Inactivation parameters for Vh and K were -65 mV and 8.2 mV, respectively. Voltage-clamp studies also revealed a high-voltage-activated sustained inward Ca2+ current which was blocked by the removal of external Ca2+ or the presence of Cd2+ (0.1 mM). The dihydropyridine agonist, (+)202-791 (1 microM), caused a small increase (20%) in the amplitude of the Ca2+ current at more negative potentials and markedly prolonged the tail currents. omega-Conotoxin GIVA (omega, CgTX, 15 microM) caused a 66% inhibition of the high-voltage-activated Ca2+ current amplitude. Norepinephrine (1 microM) caused a 49% reduction in the peak Ca2+ current. This study is the first demonstration that dispersed C-SMG neurons from adult rats retain electrical characteristics similar to intact ganglia. A TTX-sensitive Na+ current as well as a high voltage-activated sustained Ca2+ current underlie the inward current in C-SMG neurons. The macroscopic Ca2+ current is composed of a small dihydropyridine-sensitive (L-type current) and a large omega-CgTx-sensitive (N-type current) component.(ABSTRACT TRUNCATED AT 250 WORDS)