Are noradrenaline excitations artefacts?

Arginine analogs modify signal detection by neurons in the visual cortex

Nitric oxide (NO) modulates neurotransmitter release, induction of long-term synaptic potentiation and depression, and activity levels of neurons. However, it is not known whether NO contributes to the ability of the CNS to distinguish sensory signals from background noise and/or extract sensory information with greater reliability. We addressed these questions in the visual cortex, in vivo, using electrophysiological recording and analysis of signal detection from individual neurons. This was combined with microiontophoretic application of arginine analogs that either upregulate or downregulate the brain's endogenous NO-generating pathways or compounds that produce exogenous NO. Protocols that enhance NO levels generally increased the number of action potentials per trial evoked by visual stimuli, improved signal detection, and decreased the coefficient of variation of visually evoked responses, whereas NO-reducing protocols predominantly had complementary effects. Control experiments demonstrate that these effects are likely attributable to the specific ability of these arginine compounds to modify NO levels versus other nonspecific effects. Differential effects between neighboring cells and between single-cell receptive subfields suggest that these actions have a significant direct neural component versus exclusively operating indirectly on neurons through the central vascular actions of NO.

Laminar distributions of neurons sensitive to acetylcholine, noradrenaline and dopamine in the dorsolateral prefrontal cortex of the monkey

Sensitivities of neurons to acetylcholine (ACh), noradrenaline (NA) and dopamine (DA) were investigated at different depths of the dorsolateral prefrontal cortex (PFC) in awake or halothane-anesthetized macaque monkeys, using microiontophoretic techniques with multi-barreled electrodes. The laminar locations of tested neurons (n = 403) were estimated by reconstructing electrode tracks based on the microlesion made by passing a current through the recording barrel, which contained a carbon fiber. Iontophoretically applied drugs induced excitatory or inhibitory responses. Neurons excited by ACh (n = 105) were located mainly in layers III and V, and those inhibited by ACh (n = 126) were in layers III and IV. The majority of the NA-sensitive neurons (n = 123) were NA-inhibited neurons (n = 100), and were found most often in layers III and IV. The ratio of DA-sensitive neurons (excited, n = 74; inhibited, n = 63) to tested neurons was higher in the deep layers than in the superficial ones. These results indicate that sensitivities of the PFC neurons to ACh, NA and DA are not uniform between cortical layers, suggesting that each of these substances may predominantly influence the neuronal activity of particular layers of the monkey PFC.

Iontophoretic studies of histamine and histamine antagonists in the feline vestibular nuclei

The activity of single neurons in the vestibular neuronal complex of midcollicular decerebrate, decerebellectomized cats were recorded and their responsiveness to iontophoretically applied histamine and other agents determined. The majority of the cells tested were inhibited by iontophoresis of histamine while 24% were excited by this agent. Neurons exhibiting inhibitory responses were widely distributed throughout the four vestibular nuclei and adjacent reticular formation whereas excitatory responses to histamine were obtained mainly in the region of the lateral vestibular nucleus. The H2-receptor blocking agents metiamide and cimetidine were examined as to their actions on spontaneously firing cells and cells affected by histamine. Metiamide was selective in blocking histamine-induced inhibition but not excitation while cimetidine was ineffective in blocking either response. These results suggest that histamine has both inhibitory and excitatory actions on brain stem neurons and metiamide is an effective antagonist of histamine-induced inhibition.

Adrenoceptors in intracerebral resistance vessels

1. The effects of tyramine and isoprenaline on hypothalamic blood flow (HBF) were measured in conscious rabbits. 2. Injections of small doses of tyramine caused an increase in HBF while larger doses caused a decrease in HBF. 3. Isoprenaline injections also produced an increase in HBF. 4. The vasodilatation induced by isoprenaline and the small dose of tyramine was blocked by propranolol. 5. The vasoconstriction induced by the larger doses of tyramine was abolished by phenoxybenzamine. 6. Chemical sympathectomy of the hypothalamus with 6-hydroxydopamine and depletion of biogenic amines by reserpine also abolished tyramine-induced vasocoonstriction. 7. These results suggest the presence of alpha- and beta-adrenoceptors in cerebral resistance vessels, and that these receptors may be activated by released (endogenous) noradrenaline.

The reflection of ejecting and retaining currents in the time-course of neuronal responses to microelectrophoretically applied drugs

The role of ejecting and retaining currents in determining the time-course of neuronal responses to microelectrophoretically applied drugs (acetylcholine, glutamate, noradrenaline, 5-hydroxytryptamine, and mescaline) was investigated. Comparing the parameters of excitatory responses to ejecting currents of successively increasing intensity, the following changes were observed: the response latency became progressively shorter, the plateau became higher, and the recovery time was prolonged. An increase in the intensity or duration of the pre-ejection retaining current resulted in the prolongation of the response latency and the latency to plateau, but did not alter the plateau itself. An increase in the intensity of the post-ejection retaining current reduced the recovery time of the response.

Cholinergic mechanisms in the rat somatosensory cerebral cortex

1. The responses of identified cells in the rat cerebral cortex to cholinomimetic and anticholinergic substances has been investigated.2. Acetylcholine and muscarinic agonists have an excitatory action on 80% of pyramidal tract cells. This response is found especially on cells responding to specific thalamic stimulation and the burst of spikes evoked from this site can sometimes be blocked by the iontophoresis of atropine. This strongly suggests an excitatory transmitter function for acetylcholine in a specific thalamocortical pathway.3. Experiments on non-pyramidal tract cells have detected a muscarinic depression of some cells, and a nicotinic excitation of some cells above a depth of 600 mu in the cortex.4. It is suggested that the increased release of acetylcholine from the cortex produced by atropine administration may be due to an excess of muscarinic inhibitory over excitatory synapses in the cortex.

Responses of blood vessels to various amines applied by microiontophoresis

Various amines have been applied to arterioles of the rat intestine and mesentery by microiontophoresis. Noradrenaline produced a profound constriction with a long latency and time course. This effect was prevented by phentolamine but not by propranolol. Histamine caused a partial relaxation of noradrenaline-constricted vessels, but had no apparent action on “normal” vessels. Acetylcholine and methacholine had no apparent effect on the vessels. The results are discussed in relation to previous findings. It is suggested that the effects of microiontophoretically applied amines on blood vessels may be of importance in studies of the actions of these substances in the central nervous system using this technique.