Glucagon-like peptide 1 receptor activation: anti-inflammatory effects in the brain

The glucagon-like peptide 1 is a pleiotropic hormone that has potent insulinotropic effects and is key in treating metabolic diseases such as diabetes and obesity. Glucagon-like peptide 1 exerts its effects by activating a membrane receptor identified in many tissues, including different brain regions. Glucagon-like peptide 1 activates several signaling pathways related to neuroprotection, like the support of cell growth/survival, enhancement promotion of synapse formation, autophagy, and inhibition of the secretion of proinflammatory cytokines, microglial activation, and apoptosis during neural morphogenesis. The glial cells, including astrocytes and microglia, maintain metabolic homeostasis and defense against pathogens in the central nervous system. After brain insult, microglia are the first cells to respond, followed by reactive astrocytosis. These activated cells produce proinflammatory mediators like cytokines or chemokines to react to the insult. Furthermore, under these circumstances, microglia can become chronically inflammatory by losing their homeostatic molecular signature and, consequently, their functions during many diseases. Several processes promote the development of neurological disorders and influence their pathological evolution: like the formation of protein aggregates, the accumulation of abnormally modified cellular constituents, the formation and release by injured neurons or synapses of molecules that can dampen neural function, and, of critical importance, the dysregulation of inflammatory control mechanisms. The glucagon-like peptide 1 receptor agonist emerges as a critical tool in treating brain-related inflammatory pathologies, restoring brain cell homeostasis under inflammatory conditions, modulating microglia activity, and decreasing the inflammatory response. This review summarizes recent advances linked to the anti-inflammatory properties of glucagon-like peptide 1 receptor activation in the brain related to multiple sclerosis, Alzheimer's disease, Parkinson's disease, vascular dementia, or chronic migraine.

TREK channels in Mechanotransduction: a Focus on the Cardiovascular System

Mechano-electric feedback is one of the most important subsystems operating in the cardiovascular system, but the underlying molecular mechanism remains rather unknown. Several proteins have been proposed to explain the molecular mechanism of mechano-transduction. Transient receptor potential (TRP) and Piezo channels appear to be the most important candidates to constitute the molecular mechanism behind of the inward current in response to a mechanical stimulus. However, the inhibitory/regulatory processes involving potassium channels that operate on the cardiac system are less well known. TWIK-Related potassium (TREK) channels have emerged as strong candidates due to their capacity for the regulation of the flow of potassium in response to mechanical stimuli. Current data strongly suggest that TREK channels play a role as mechano-transducers in different components of the cardiovascular system, not only at central (heart) but also at peripheral (vascular) level. In this context, this review summarizes and highlights the main existing evidence connecting this important subfamily of potassium channels with the cardiac mechano-transduction process, discussing molecular and biophysical aspects of such a connection.

The Relevance of GIRK Channels in Heart Function

Among the large number of potassium-channel families implicated in the control of neuronal excitability, G-protein-gated inwardly rectifying potassium channels (GIRK/Kir3) have been found to be a main factor in heart control. These channels are activated following the modulation of G-protein-coupled receptors and, although they have been implicated in different neurological diseases in both human and animal studies of the central nervous system, the therapeutic potential of different subtypes of these channel families in cardiac conditions has remained untapped. As they have emerged as a promising potential tool to treat a variety of conditions that disrupt neuronal homeostasis, many studies have started to focus on these channels as mediators of cardiac dynamics, thus leading to research into their implication in cardiovascular conditions. Our aim is to review the latest advances in GIRK modulation in the heart and their role in the cardiovascular system.

Are TREK Channels Temperature Sensors?

Internal human body normal temperature fluctuates between 36.5 and 37.5°C and it is generally measured in the oral cavity. Interestingly, most electrophysiological studies on the functioning of ion channels and their role in neuronal behavior are carried out at room temperature, which usually oscillates between 22 and 24°C, even when thermosensitive channels are studied. We very often forget that if the core of the body reached that temperature, the probability of death from cardiorespiratory arrest would be extremely high. Does this mean that we are studying ion channels in dying neurons? Thousands of electrophysiological experiments carried out at these low temperatures suggest that most neurons tolerate this aggression quite well, at least for the duration of the experiments. This also seems to happen with ion channels, although studies at different temperatures indicate large changes in both, neuron and channel behavior. It is known that many chemical, physical and therefore physiological processes, depend to a great extent on body temperature. Temperature clearly affects the kinetics of numerous events such as chemical reactions or conformational changes in proteins but, what if these proteins constitute ion channels and these channels are specifically designed to detect changes in temperature? In this review, we discuss the importance of the potassium channels of the TREK subfamily, belonging to the recently discovered family of two-pore domain channels, in the transduction of thermal sensitivity in different cell types.

Holdemanella biformis improves glucose tolerance and regulates GLP-1 signaling in obese mice

Impaired glucose homeostasis in obesity is mitigated by enhancing the glucoregulatory actions of glucagon-like peptide 1 (GLP-1), and thus, strategies that improve GLP-1 sensitivity and secretion have therapeutic potential for the treatment of type 2 diabetes. This study shows that Holdemanella biformis, isolated from the feces of a metabolically healthy volunteer, ameliorates hyperglycemia, improves oral glucose tolerance and restores gluconeogenesis and insulin signaling in the liver of obese mice. These effects were associated with the ability of H. biformis to restore GLP-1 levels, enhancing GLP-1 neural signaling in the proximal and distal small intestine and GLP-1 sensitivity of vagal sensory neurons, and to modify the cecal abundance of unsaturated fatty acids and the bacterial species associated with metabolic health. Our findings overall suggest the potential use of H biformis in the management of type 2 diabetes in obesity to optimize the sensitivity and function of the GLP-1 system, through direct and indirect mechanisms.

Contribution of K2P Potassium Channels to Cardiac Physiology and Pathophysiology

Years before the first two-pore domain potassium channel (K2P) was cloned, certain ion channels had already been demonstrated to be present in the heart with characteristics and properties usually attributed to the TREK channels (a subfamily of K2P channels). K2P channels were later detected in cardiac tissue by RT-PCR, although the distribution of the different K2P subfamilies in the heart seems to depend on the species analyzed. In order to collect relevant information in this regard, we focus here on the TWIK, TASK and TREK cardiac channels, their putative roles in cardiac physiology and their implication in coronary pathologies. Most of the RNA expression data and electrophysiological recordings available to date support the presence of these different K2P subfamilies in distinct cardiac cells. Likewise, we show how these channels may be involved in certain pathologies, such as atrial fibrillation, long QT syndrome and Brugada syndrome.

Glucagon-Like Peptide-1 (GLP-1) in the Integration of Neural and Endocrine Responses to Stress

Glucagon like-peptide 1 (GLP-1) within the brain is produced by a population of preproglucagon neurons located in the caudal nucleus of the solitary tract. These neurons project to the hypothalamus and another forebrain, hindbrain, and mesolimbic brain areas control the autonomic function, feeding, and the motivation to feed or regulate the stress response and the hypothalamic-pituitary-adrenal axis. GLP-1 receptor (GLP-1R) controls both food intake and feeding behavior (hunger-driven feeding, the hedonic value of food, and food motivation). The activation of GLP-1 receptors involves second messenger pathways and ionic events in the autonomic nervous system, which are very relevant to explain the essential central actions of GLP-1 as neuromodulator coordinating food intake in response to a physiological and stress-related stimulus to maintain homeostasis. Alterations in GLP-1 signaling associated with obesity or chronic stress induce the dysregulation of eating behavior. This review summarized the experimental shreds of evidence from studies using GLP-1R agonists to describe the neural and endocrine integration of stress responses and feeding behavior.

Metabotropic Modulation of Potassium Channels During Synaptic Plasticity

Besides their primary function mediating the repolarization phase of action potentials, potassium channels exquisitely and ubiquitously regulate the resting membrane potential of neurons and therefore have a key role establishing their intrinsic excitability. This group of proteins is composed of a very diverse collection of voltage-dependent and -independent ion channels, whose specific distribution is finely tuned at the level of the synapse. Both at the presynaptic and postsynaptic membranes, different types of potassium channels are subjected to modulation by second messenger signaling cascades triggered by metabotropic receptors, which in this way serve as a link between neurotransmitter actions and changes in the neuron membrane excitability. On the one hand, by regulating the resting membrane potential of the postsynaptic membrane, potassium channels appear to be critical towards setting the threshold for the induction of long-term potentiation and depression. On the other hand, these channels maintain the presynaptic membrane potential under control, therefore influencing the probability of neurotransmitter release underlying different forms of short-term plasticity. In the present review, we examine in detail the role of metabotropic receptors translating their activation by different neurotransmitters into a final effect modulating several types of potassium channels. Furthermore, we evaluate the consequences that this interplay has on the induction and maintenance of different forms of synaptic plasticity.

SGK1.1 Reduces Kainic Acid-Induced Seizure Severity and Leads to Rapid Termination of Seizures

Approaches to control epilepsy, one of the most important idiopathic brain disorders, are of great importance for public health. We have previously shown that in sympathetic neurons the neuronal isoform of the serum and glucocorticoid-regulated kinase (SGK1.1) increases the M-current, a well-known target for seizure control. The effect of SGK1.1 activation on kainate-induced seizures and neuronal excitability was studied in transgenic mice that express a permanently active form of the kinase, using electroencephalogram recordings and electrophysiological measurements in hippocampal brain slices. Our results demonstrate that SGK1.1 activation leads to reduced seizure severity and lower mortality rates following status epilepticus, in an M-current–dependent manner. EEG is characterized by reduced number, shorter duration, and early termination of kainate-induced seizures in the hippocampus and cortex. Hippocampal neurons show decreased excitability associated to increased M-current, without altering basal synaptic transmission or other neuronal properties. Altogether, our results reveal a novel and selective anticonvulsant pathway that promptly terminates seizures, suggesting that SGK1.1 activation can be a potent factor to secure the brain against permanent neuronal damage associated to epilepsy.

Muscarinic modulation of TREK currents in mouse sympathetic superior cervical ganglion neurons

Muscarinic receptors play a key role in the control of neurotransmission in the autonomic ganglia, which has mainly been ascribed to the regulation of potassium M-currents and voltage-dependent calcium currents. Muscarinic agonists provoke depolarization of the membrane potential and a reduction in spike frequency adaptation in postganglionic neurons, effects that may be explained by M-current inhibition. Here, we report the presence of a riluzole-activated current (IRIL ) that flows through the TREK-2 channels, and that is also inhibited by muscarinic agonists in neurons of the mouse superior cervical ganglion (mSCG). The muscarinic agonist oxotremorine-M (Oxo-M) inhibited the IRIL by 50%, an effect that was abolished by pretreatment with atropine or pirenzepine, but was unaffected in the presence of himbacine. Moreover, these antagonists had similar effects on single-channel TREK-2 currents. IRIL inhibition was unaffected by pretreatment with pertussis toxin. The protein kinase C blocker bisindolylmaleimide did not have an effect, and neither did the inositol triphosphate antagonist 2-aminoethoxydiphenylborane. Nevertheless, the IRIL was markedly attenuated by the phospholipase C (PLC) inhibitor ET-18-OCH3. Finally, the phosphatidylinositol-3-kinase/phosphatidylinositol-4-kinase inhibitor wortmannin strongly attenuated the IRIL , whereas blocking phosphatidylinositol 4,5-bisphosphate (PIP2 ) depletion consistently prevented IRIL inhibition by Oxo-M. These results demonstrate that TREK-2 currents in mSCG neurons are inhibited by muscarinic agonists that activate M1 muscarinic receptors, reducing PIP2 levels via a PLC-dependent pathway. The similarities between the signaling pathways regulating the IRIL and the M-current in the same neurons reflect an important role of this new pathway in the control of autonomic ganglia excitability.

Spontaneous bursting and rhythmic activity in the cuneate nucleus of anaesthetized rats

Spontaneous and rhythmic neuronal activity in dorsal column nuclei has long been identified in anesthetized cats. Here, we have studied the spontaneous behavior of cuneate cells in anesthetized rats through extracellular recording, showing that most cuneate neurones recorded (155 of 185) fired spontaneously. Overall, 74% of these spontaneously firing neurones were single-spiking and 26% were bursting. Cells were considered "bursting" when more than 50% of the spontaneous spikes belonged to bursts. Nevertheless, occasional bursts were seen in 33% of spontaneous cuneate cells which were classified as single-spiking. Rhythmic firing was observed in about 14% of both spontaneously bursting and single-spiking cells, and these cells were located close to the obex (+/-0.5 mm). Although the spike-frequency was mostly in the range 0-15 spikes/s, spontaneous rhythmic activity was circumscribed mainly to the alpha/beta-like range, both in single-spiking (26.1+/-3.6 Hz, n=16) and bursting cells (19.5+/-4.1 Hz, n=6). Lemniscal stimulation often activated several antidromic units with the same latency. About 65% of cuneolemniscal cells were spontaneously active and of these, 83% were single-spiking and 11% rhythmic (all single-spiking). In cells that were not antidromically activated from the medial lemniscus, short latency orthodromic responses consistent with excitation by recurrent lemniscal collaterals were often observed following lemniscal activation. Interestingly, only cells completely unresponsive to lemniscal stimulation showed rhythmic bursting. Most spontaneous cells responded with a burst to natural receptive field stimulation, while rhythmic cells became temporally arrhythmic. These results demonstrate, for the first time, that rat cuneate neurones can fire bursts spontaneously. Besides, this bursting activity can be rhythmic. These two properties, and the fact that groups of cuneolemniscal cells share the same conduction velocity, probably imply the reinforcement of temporal and spatial summation at their targets when they are synchronously recruited by the stimulation of overlapping receptive fields.

[The development of the concept of neuronal resting potential. Fundamental and clinical aspects]

INTRODUCTION AND AIMS

Since the classical works carried out on the squid giant axon far less attention has been directed towards the study of the resting membrane potential than to the study of changes in potential (action potential, synaptic potentials, and so forth). It is often assumed that the resting potential depends on an independent current of the voltage called the leakage current, although, except on rare occasions, it has not been possible to characterise such a current either pharmacologically or molecularly. In this work our aim is to review and update the concept of resting potential.

DEVELOPMENT

The outlook at present offers a complex situation in which several factors, in addition to leakage currents, play a role in maintaining resting potentials. These factors include ionic currents across non-inactivating voltage-dependent channels, the sodium/potassium pump, and certain currents with characteristics that are similar to those of the classical leakage current. The interaction of all these components gives rise to complex sub-threshold behaviours in the neurons, such as intrinsic rhythmic oscillations, which leads us away from the passive concept of the resting potential.

CONCLUSIONS

An ever-increasing number of descriptions of intrinsic sub-threshold, rhythmic activity are being reported, which could suggest that they are a generalised phenomenon in the nervous system. Further studies need to be conducted into the complex mechanisms that determine the resting potential in order to gain an understanding of the phenomena of neuronal excitability and to find an explanation for some of the many pathological conditions that affect the nervous system.

Newly developed blockers of the M-current do not reduce spike frequency adaptation in cultured mouse sympathetic neurons

The M-current (I(K(M))) is believed to modulate neuronal excitability by producing spike frequency adaptation (SFA). Inhibitors of M-channels, such as linopirdine and 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE991), enhance depolarization-induced transmitter release and improve learning performance in animal models. As such, they are currently being tested for their therapeutic potential for treating Alzheimer's disease. The activity of these blockers has been associated with the reduction of SFA and the depolarization of the membrane observed when I(K(M)) is inhibited. To test whether this is the case, the perforated patch technique was used to investigate the capacity of I(K(M)) inhibitors to alter the resting membrane potential and to reduce SFA in mouse superior cervical ganglion neurons in culture. Linopirdine and XE991 both proved to be potent blockers of I(K(M)) when the membrane potential was held at -30 mV (IC(50) 2.56 and 0.26 microM, respectively). However, their potency gradually declined upon membrane hyperpolarization and was almost null when the membrane potential was kept at -70 mV, indicating that their blocking activity was voltage dependent. Nevertheless, I(K(M)) could be inhibited at these hyperpolarized voltages by other inhibitors such as oxotremorine-methiodide and barium. Under current-clamp conditions, neither linopirdine (10 microM) nor XE991 (3 microM) was effective in reducing the SFA and both provoked only a small slowly developed depolarization of the membrane (2.27 and 3.0 mV, respectively). In contrast, both barium (1 mM) and oxotremorine-methiodide (10 microM) depolarized mouse superior cervical ganglion neurons by about 10 mV and reduced the SFA. In contrast to classical I(K(M)) inhibitors, the activity of linopirdine and XE991 on the I(K(M)) is voltage dependent and, thus, these newly developed I(K(M)) blockers do not reduce the SFA. These results may shed light on the mode of action of these putative cognition enhancers in vivo.

Calcium current components in intact and dissociated adult mouse sympathetic neurons

We examined which types of high threshold Ca(2+) channels are activated by depolarization in intact and dissociated sympathetic neurons from adult mouse superior cervical ganglia (SCG). Ba(2+) currents were recorded with microelectrodes and discontinuous voltage clamp from neurons in intact ganglia, and using the perforated patch clamp technique in dissociated cells. Peak current was larger in intact neurons, although the voltage dependence was similar. Successive application of omega-conotoxin GVIA, omega-conotoxin MVIIC and nifedipine revealed that the total current in intact cells was composed by 29% N-type, 13% P/Q-type, 32% L-type and 26% resistant to blockade (R-type). In dissociated cells, the N component was larger and the L component smaller, whereas P/Q-type and R-type were similar. Peak currents evoked with an action potential waveform instead of a square pulse were larger in both preparations but the proportions of each component were similar. We conclude that dissociating and culturing somata results in data that only partially reflect the situation in intact neurons. Assuming that the main effect of dissociation is the removal of mature dendritic membrane, the data suggest that L channels are more abundant on dendrites and N channels on the soma of intact sympathetic neurons, whereas P/Q and R channels may be uniformly distributed over the cell surface. Finally, in intact SCG neurons from rats, the proportions of current evoked by a pulse were: 49% N-type, 11% P/Q-type, 21% L-type and 20% R-type when nifedipine was applied last, suggesting that there are species differences in the expression of L and N channels.

The role of calcium in M-current inhibition by muscarinic agonists in rat sympathetic neurons

I have investigated the role of Ca2+ on M-current (IK(M)) inhibition by the muscarinic agonist oxo-M using the perforated patch voltage clamp technique. Oxo-M inhibited IK(M) in cultured SCG cells with an IC50 of 1.2 microM in 2 mM [Ca2+]o, and 13.1 microM in nominally Ca(2+)-free external solution. BAPTA-AM, ryanodine and thapsigargin (substances which modulate [Ca2+]i) did not affect IK(M) or the inhibitory action of oxo-M in either 2 or 0 mM extracellular Ca2+. Caffeine (10 mM) inhibited M current by approximately 30% in both 2 and 0 mM [Ca2+]o; this inhibition was not affected by [Ca2+]i modulators. Unexpectedly, the effect of oxo-M (10 microM) was enhanced after application of caffeine (10 mM) in either 2 or 0 mM [Ca2+]o. Thus, the effect of muscarinic agonists on IK(M) was blunted in Ca(2+)-free extracellular solutions, but neither oxo-M nor caffeine appeared to inhibit IK(M) through an elevation of [Ca2+]i. I suggest that resting levels of [Ca2+]i are necessary for a normal inhibition, with lower levels inducing an impairment of the inhibition of IK(M) by muscarinic agonists.

A hyperpolarization-activated cation current (Ih) contributes to resting membrane potential in rat superior cervical sympathetic neurones

Using perforated-patch voltage-clamp recording, a prominent hyperpolarization-activated inward cation current (Ih) has been identified in dissociated, cultured and replated, superior cervical sympathetic (SCG) neurones from 17-day-old rats. Ih was identified as a slowly activated inward current on hyperpolarizing from -60 mV, with an extrapolated null potential (in 3 mM [K+]out) of -42 mV. The activation range for Ih was -40 to -100 mV, with a half-activation voltage (V0.5) of -63 mV. The current was suppressed by 1 mM Cs+ but not by 1 mM Ba2+. The reversal potential for the current change induced by Cs+ agreed with the null potential for Ih. Ih conferred strong inward rectification to the current-voltage curve negative to -55 mV in both voltage-clamp and current-clamp recording. This inward rectification was reduced by 1 mM Cs+. In a sample of eight cells with initial resting membrane potentials between -51 and -64 mV, Cs+ increased the resting potential of all cells by between 2.5 and 21 mV. These results indicate that Ih contributes a tonic inward (depolarizing) component to the maintenance of the resting membrane potential in SCG neurones.

Effects of a cognition-enhancer, linopirdine (DuP 996), on M-type potassium currents (IK(M)) and some other voltage- and ligand-gated membrane currents in rat sympathetic neurons

Linopirdine is a cognition enhancer which augments depolarization-induced transmitter release in the cortex and which is under consideration for potential treatment of Alzheimer's disease. It has previously been reported to inhibit M-type K+ currents in rat hippocampal neurons. In the present experiments we have tested its effect on whole-cell M-currents and single M-channels, and on a range of other membrane currents, in dissociated rat superior cervical sympathetic ganglion cells. Linopirdine inhibited the whole-cell M-current with an IC50 of 3.4 microM and blocked M-channels recorded in excised outside-out membrane patches but not in inside-out patches. This suggests that linopirdine directly blocks M-channels from the outside. It was much less effective in inhibiting other voltage-gated potassium currents [delayed rectifier (IK(V)), IC50 63 microM; transient (IA) current, IC50 69 microM] and produced no detectable inhibition of the fast and slow Ca(2+)-activated K+ currents IC and IAHP or of a hyperpolarization-activated cation current (IQ/Ih) at 10-30 microM. However, it reduced acetylcholine-activated nicotinic currents and GABA-activated Cl- currents with IC50 values of 7.6 and 26 microM respectively. It is concluded that linopirdine shows some 20-fold selectivity for M-channels among different K+ channels but can also block some transmitter-gated channels. The relationship between M-channel block and the central actions of linopirdine are discussed.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Caprylic acid, a medium chain saturated fatty acid, inhibits the sodium inward current in neuroglioma (NG108-15) cells

The effects of caprylic acid (CA) on ionic currents were investigated, using the whole-cell patch-clamp technique, in differentiated neuroglioma cells. External application of CA reduced the peak amplitude of the inward Na+ current, while outward currents were not affected. CA (1 and 5 mM) reversibly attenuated the peak of the inward Na+ current by 21% (n = 26) and 46% (n = 31), respectively. The inactivation curve in the presence of CA was shifted by 8 mV toward hyperpolarized potentials, with half-inactivation voltage being -47.9 and -55.9 before and after external application of CA (5 mM), respectively. This shift was readily reversed after 5 min wash. The slope remained unchanged (-8.4 and -8.8 mV, respectively, n = 4). The activation process was unaffected (CA 5 mM, n = 8). Under current-clamp conditions, CA 5 mM (but not 1 mM) reversibly reduced the amplitude, and the slope of the rising phase of the action potential. These results agree with the fact that free fatty acids can modulate the activity of ion channels by mechanisms which do not involve enzymatic or membrane disruptive pathways.

Pyramidal tract and corticospinal neurons with branching axons to the dorsal column nuclei of the cat

Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 309 neurons were identified antidromically by stimulation of the dorsal column nuclei (229 from the nuneate nucleus and 80 from the gracile nucleus). The study addressed the question whether pericruciate-dorsal column nuclei neurons (corticonuclear cells) sent collaterals to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. Also, the ipsilateral pyramidal tract was stimulated at mid-olivary level, as was the crossed corticospinal tract at C2, Th2 and L2 levels in order to know whether the corticonuclear cells sent their axons to the spinal cord and if so to which level. It was found that more than 95% of the corticonuclear fibres coursed through the pyramidal tract. A significant (28.4%; 88/309) proportion of the the corticonuclear neurons sent collaterals to the red nucleus and/or to the nucleus reticularis gigantocellularis. About 68% (209/309) of the corticonuclear cells did not send their axons to the spinal cord and the remainder were corticospinal neurons. Most of the corticospinal fibres terminated at the cervical level (72/100) and the remaining ended at thoracic (18/100) and lumbar (10/100) segments of the cord. While 63.4% (123/194) of the corticonuclear fibres coursing through the pyramidal tract and ending at supraspinal levels were slow conducting, the great majority of the corticospinal neurons were fast conducting (91/100). The non-corticospinal neurons were significantly slower conducting than the corticospinal cells. The corticogracile neurons were slower conducting than the corticocuneate cells. Of the 88 corticonuclear neurons that sent at least a branch to the sites tested, 50% branched into the red nucleus, 35.2% into the nucleus reticularis gigantocellularis and 14.7% into both nuclei, without significant difference between non-corticospinal and corticospinal cells. Most of the main axons of the corticonuclear cells ended at bulbar and cervical levels (281/309 or 90.9%). The data indicate that pericruciate-dorsal column nuclei neurons form a particular substrate within pyramidal tract cells. They can serve precise functions in motor coordination associated with the selection of their own sensory input. The results are discussed from this point of view.

Pericruciate fibres to the red nucleus and to the medial bulbar reticular formation

Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 455 neurons were classified antidromically according to their sites of termination along the corticospinal tract and whether they sent collateral branches to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. It was found that the majority of the branching fibres that reached the most caudal segments of the cord were fast conducting, while the slower branching axons tended to terminate at more rostral levels of the corticospinal tract. Most of the branching fibres terminated at bulbar and cervical levels (153/182: 84%), and the remaining ended at thoracic (21/182: 11.5%) and at lumbar (8/182: 4.4%) segments of the cord. The non-corticospinal, pyramidal tract fibres branched more (56%) than the corticospinal fibres (26.6%). Within the corticospinal neurons, the degree of branching decreased with distance along the spinal cord. While 57.5% of the pericruciate fibres that projected only as far as the pyramidal tract were slow conducting, the majority of the corticospinal neurons were fast conducting (74.6%). Both pyramidal tract and corticospinal neurons that sent branches to one or to the two sites tested were significantly faster conducting than the neurons which did not branch. A total of 101 corticorubral and corticobulbar neurons which did not respond to pyramidal tract stimulation was also recorded. The data can be of significance in the understanding of co-ordination of different muscles in order to couple movement and posture into a common act. The results are discussed from this point of view.

Pyramidal and corticospinal synaptic effects over reticulospinal neurones in the cat

1. The spontaneous activity of 103 precruciate neurones (fifty-eight activated antidromically from the pyramidal tract but not from the corticospinal tract, PTNs; forty-five activated from both sites, CSNs) was used to trigger the average of the intracellularly recorded synaptic noise in 294 reticulospinal neurones (RSNs). These RSNs were recorded in the nucleus reticularis gigantocellularis of the contralateral medial bulbar reticular formation (NRGc) in chloralose-anaesthetized cats. 2. Twelve pyramidal tract neurones (six CSNs) were tested with a single RSN, twenty-six (10 CSNs) with two RSNs each, thirty (13 CSNs) with three RSNs each, and thirty-five (16 CSNs) with four RSNs each. Postsynaptic potentials were observed in the averages generated by twenty PTNs and fifteen CSNs. 3. The only synaptic effect produced by both PTNs and CSNs upon RSNs in our sample was excitatory, and in none of the tested cases (n = 15) were any changes found in the amplitude, shape, or duration of the excitatory postsynaptic potentials (EPSPs) after injection of depolarizing or hyperpolarizing currents. This suggests that the synapses are probably located at the distal dendrites. 4. Recording of the presynaptic spike allowed separation of the conduction time and synaptic delay from the total latency. According to our data there appear to be two different types of excitation of corticofugal neurones over RSNs: a monosynaptic effect produced by both PTNs and CSNs, and a disynaptic effect produced by PTNs but not by CSNs. The disynaptic EPSPs had statistically significant slower rise times and longer widths than the monosynaptic EPSPs.

Effect of retinoic acid deficiency on in vivo and in vitro GH responses to GHRH in male rats

Retinoic acid has recently been shown to increase growth hormone (GH)-gene transcription rate and GH synthesis in vitro. To investigate the role retinoic acid plays in the neuroregulation of GH secretion we have studied GH responses to growth hormone-releasing hormone (GHRH) in retinoic acid-deficient rats. Compared to normally fed male rats, retinoic acid-deficient rats showed a marked impairment in body weight, which was statistically significant after 3 weeks and maximal after 5-6 weeks (p less than 0.001). Yet, in vivo GH responses to different doses of GHRH (1, 5 and 25 micrograms/kg) in pentobarbital-anesthesized rats were similar in both groups. Also, in vitro GH responses to GHRH, forskolin, and KCl were similar in perfused pituitary cells taken from control and retinoic acid-deficient rats. However, further studies carried out in freely-moving rats showed the typical GH secretory pattern usually found in male rats of the control group, while retinoic acid-deficient rats displayed a highly variable GH secretory pattern with GH peaks of much lower amplitude. Finally, after gel electrophoresis of in vitro 35S-labelled proteins, no differences were observed in the molecular forms of GH. Considering these findings on normal pituitary responsiveness and alterations in GH pulsatility, our data suggest that retinoic acid deficiency leads to an alteration in the neuroregulation of GH secretion at the central level.

Rubrospinal tract of the cat: superposition of antidromic responses and changes in axonal excitability following orthodromic activity

Activity was recorded from rubrospinal neurons (RSNs) in anesthetized, paralyzed, artificially ventilated cats. Multiple-unit microelectrodes were used to simultaneously record the activity of neighboring RSNs. When antidromically activated, the RSNs responded forming 'stacks' of superimposed spikes. By using appropriate collision tests, it was found that the spikes forming a stack arose from different neurons. In addition, single extracellular and intracellular recordings were obtained from RSNs. The changes in the axonal excitability of rubrospinal axons were tested following synaptically evoked (by contralateral interpositus (IP) stimulation) and/or directly evoked (by injection of current through the intracellular electrode) action potentials at different postspike delays. Subthreshold stimuli for antidromic activation in absence of orthodromic activity were well suprathreshold for most fibers in a wide range of postspike delays. The supernormal axonal periods were longer-lasting when tested after synaptic spikes (up to an average delay of 100.4 ms; range, 10-500 ms) than after directly evoked spikes (mean delay, 78.8 ms; range, 10-296 ms). If synaptic stimulation fires more RSNs than direct stimulation, then the longer-lasting supernormal periods might be due to the activity of adjacent fibers. An additional increase in external potassium concentration in the vicinity of the axon would explain these results.