Noradrenaline effects and pH

Rat pineal exhibits two electrophysiological patterns of response to microiontophoretic norepinephrine application

The spontaneous activity of 117 pineal units was recorded in urethane-anesthetized rats. The pineal units exhibited a wide range of firing rates of which 50% were on average slower than 14 spikes per second. Superior cervical ganglion (SCG) stimulation was studied in 76 pineal units; this stimulation caused excitation in 55% of the units. Microiontophoretic application of norepinephrine (NE) induced changes of firing rates in 61% of the pineal units tested. Two patterns of activity following NE microiontophoresis was observed: increase in firing rate (64%) and decrease in firing rate (36%). NE-induced excitation was observed only in those units excited by SCG stimulation. When NE and SCG stimulation were applied together, partial summation of the excitation induced by each one alone was observed. None of the units in which NE depressed the firing rate responded to SCG stimulation. Local application of propranolol blocked the excitation initiated by SCG stimulation as well as the excitation and the depression induced by NE microiontophoresis.

Effects of ionophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex

Changes in spontaneous activity of rat S1 cortical neurones with identified receptive fields were investigated in reply to ionophoresed noradrenaline (NA). Extracellular levels of NA were maintained constant by continuous electrochemical analysis at the carbon fibre recording tip of the multibarrel micro-electrode. In the absence of NA there were clear differences in spike amplitude, firing rate and pattern of firing of deep (800-1400 micron) and superficial (0-800 micron) cells. Superficial cells responded to low (5 X 10(-8) to 5 X 10(-7) M) NA concentrations with simple inhibition. Recovery occurred within a minute or so of extracellular NA concentrations falling below detectable (10(-8) M) levels. Increases in local concentration merely stopped cells firing. In contrast, cells located in the deep zone could often be excited by very low NA concentrations (less than 10(-8) M), with inhibition occurring at levels 10-100 times greater. Most cells, however, were inhibited, with threshold doses for a 50% change in firing rate much higher than for superficial cells. Some cells in the deep zone showed sustained increases in firing rate following an ionophoretic trial. This could occur for periods of up to 1 h after ceasing a trial. Such effects could be produced by levels as low as 10(-7) M-NA. Interspike interval analysis for deep cells suggested that their spontaneous activity resembled that established for slow-wave sleep. During and after excitation by NA the pattern of firing of small groups of these cells changed to that established for the waking state. The effect could persist for up to 1 h following a short (2-5 min) ionophoretic trial.

The effects of sex hormones, prolactin, and chorionic gonadotropin on pineal electrical activity in guinea pigs

Microelectrophoretic application of sex hormones onto pineal cells in guinea pigs has shown different responses in pregnant females as compared to males. In pregnant females estrone caused excitation in 74% of the cells tested, while progesterone and testosterone, prolactin, and HCG were inhibitory in a majority of the cells tested, while progesterone and testosterone, prolactin, and HCG were inhibitory in a majority of the cells. In contrast, in males estrone caused excitation of only 19% but inhibition of 37%. A smaller percentage of cells was inhibited by progesterone, while the predominant response to testosterone was excitation. These results suggest that the pineal gland may be under a feedback control.

On the mechanism of action of clonidine: effects on single central neurones

1 Noradrenaline and clonidine were applied by microiontophoresis to single neurones in the cerebral cortex and medullary reticular formation of anaesthetized rats.2 Of a total of 247 neurones studied, 79% of medullary units and 60% of cortical units responded in the same manner to both noradrenaline and clonidine. The usual response was a depression of neuronal firing rate.3 It proved possible to antagonize some responses to both substances by the microiontophoresis of bulbocapnine, whilst leaving unaffected similar responses to 5-hydroxytryptamine.4 On 13% of the cells, clonidine produced an increase of firing rate. This effect could not be attributed to a post-synaptic antagonism of tonically released endogenous noradrenaline, but may indicate a presynaptic action of clonidine, reducing noradrenaline release.5 These observations are thought to support the idea that clonidine may have an agonist action on noradrenaline receptors in the brain.

The excitation of neurons by noradrenaline

Responses of single cortical neurons to microelectrophoretically applied noradrenaline at pH 3.1 and 5.0 and to hydrogen ions were compared in the halothane-anaesthetized cat. Of 16 neurons sensitive to noradrenaline, 13 were excited and 3 were depressed by noradrenaline at both pH values, whereas hydrogen ions released from an HCl solution did not affect the firing rate. Dose-response studies showed that noradrenaline at pH 3.1 was more potent than at pH 5.0. It is concluded that excitatory responses to noradrenaline are not artifacts and that the adjustment of the pH of noradrenaline solutions with NaOH should be avoided.

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.