Vittorio La Grutta

Nitric Oxide and Glutamate Interaction in the Control of Cortical and Hippocampal Excitability

Summary: Purpose: We investigated the role of nitric oxide (NO) as a new neurotransmitter in the control of excitability of the hippocampus and the cerebral cortex, as well as the possible functional interaction between NO and the glutamate systems.

Substantia nigra-mediated anticonvulsant action: a possible role of a dopaminergic component.

A number of neural pathways may mediate nigral control of epilepsy. According to the literature, a GABAergic nigrotectal pathway may be responsible for the control exerted by the substantia nigra on the diffusion of discharges toward spinal targets, while the nigrothalamic projection may transfer nigral influence on premotor neocortical epilepsy. Since there is probably an anatomical nigrohippocampal pathway arising from dopaminergic cells in the substantia nigra, we tested the effects of stimulating the substantia nigra pars compacta (SNpc) on focal hippocampal epilepsy induced by penicillin injection in the cat. The possibility of dopamine involvement was further tested by studying the effects of intraperitoneal injection of haloperidol, a dopamine receptor blocking agent on nigrohippocampal influences, while to verify the precise site of action, in other groups of cats, sulpiride and apomorphine (D-receptor antagonist and agonist, respectively) were locally administered in the dorsal hippocampus. Furthermore, modifications of hippocampal epileptiform EEG were studied in control conditions and following SNpc electrolytic lesions. Results showed a strong nigral suppressive effect on focal hippocampal epilepsy. Nigral stimulation induced a significant decrease in both frequency and amplitude of hippocampal spikes, which disappeared either about 10 min after i.p. injection of haloperidol 1 mg/kg or about 5 min after intrahippocampal administration of sulpiride, and did not return during a further hour or more of experimental observation. It should be emphasized that in the absence of nigral stimulation, both haloperidol and sulpiride did not modify hippocampal spike frequency. Apomorphine application to dorsal hippocampus induced a marked reduction of hippocampal epileptiform activity parallel to the effect observed during SNpc stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of nitric oxide synthase influences the activity of striatal neurons in the rat

The activity of single units in the striatum of urethane-anesthetized rats was recorded before and after the systemic administration of 7-nitro-indazole (7-NI; 50 mg/kg intraperitoneally), a selective inhibitor of neuronal nitric oxide (NO) synthase. Two neuronal types were clearly distinguishable electrophysiologically, on the basis of either discharge frequency pattern or features of the individual spike waveform (spike duration, negative phase/total duration ratio, and negative phase/total amplitude ratio). Only sporadically discharging neurons (basal firing rate, <0.1 spikes/s) were influenced by 7-NI, which caused a statistically significant increase in their firing rate. In contrast, the activity of continuously discharging neurons (basal firing rate, 4-6 spikes/s) was not affected. We hypothesize that NO neurotransmission could exert a tonic inhibitory influence upon sporadically discharging striatal neurons, which are presumably striatal output neurons.

Relay stations and neurotransmitters between the pallidal region and the hippocampus

The effects of internal pallidum and lateral habenula stimulation on epileptiform activity of cats hippocampus were studied. A steady interictal activity was induced by locally applied sodium penicillin (PCN) solution. Both pallidal and habenular electrical stimulation caused an increase in spike frequency and amplitude. Intraperitoneally injected atropine sulphate failed to modify pallidal and habenular influences. Intraperitoneal methysergide bimaleate (5-HT antagonist) suppressed the effects of habenular stimulation. In contrast to the effects of pallidal and habenular stimulation, raphe electrical stimulation inhibited hippocampal spiking and intra-raphal muscimol (a GABA receptor agonist) enhanced hippocampal-based epilepsy. After muscimol, raphe stimulation at the same threshold parameters failed to affect hippocampal activity. In cats with habenular lesions hippocampal spike frequency and amplitude were reduced and intra-raphal muscimol did not affect the hippocampus. The results are discussed in the light of a complex interrelationship between basal ganglia and hippocampus. The role of the lateral habenula and of the medial raphe as relay stations between the two regions is emphasized.

Nitric oxide-induced inhibition on striatal cells and excitation on globus pallidus neurons: a microiontophoretic study in the rat

Single units were recorded in the striatum and in the globus pallidus (GP) of urethane-anesthetized rats under microiontophoretic administration of either Nomega-nitro-L-arginine methyl ester (L-NAME, inhibitor of nitric oxide synthase), or 3-morpholino-sydnonimin-hydrocloride (SIN-1, nitric oxide, NO donor). A steady baseline firing of sporadically discharging striatal neurons (basal firing rate <0.1 spikes/s) was evoked by a pulsed microiontophoretic ejection of glutamate. On striatal neurons, microiontophoretic application of SIN-1 induced a current-dependent inhibition (11/13), whereas L-NAME administration produced a clear excitation (9/9). On GP cells, the administration of SIN-1 had excitatory effects (10/15), whereas the administration of L-NAME reduced the neuronal activity (6/6). We hypothesize that NO could exert an intrinsic regulatory action on the activity of both striatal and GP cells.

Lateral habenula and hippocampal units: electrophysiological and iontophoretic study

In previous works we studied, on cats, the effects of lateral habenula (LH) stimulation on hippocampal units. In particular, the results showed an excitation or an inhibition in relation to the stimulation frequency (0.5-3.0 Hz or 5.0-20 Hz, respectively). All the LH stimulation effects were antagonised by iontophoretic intrahippocampal application of methysergide (MS). In this series of experiments it was possible to demonstrate, on rats, that LH stimulation causes an excitatory effect in a major number of hippocampal units in relation to the frequency increase. The inhibitory effect by iontophoretic serotonine application and the reversible blockade of habenular modulation after iontophoretic methysergide administration on hippocampal units suggest, on rats, the involvement of raphe. Such hypothesis, with anatomical evidences demonstrating an excitatory projection between LH and raphe, was confirmed by data concerning the effects of intraraphal NMDA iontophoretic application on hippocampal units (NMDA application for 30 s = excitation; NMDA administration for 10-15 min = inhibition). All the results suggest an habenular modulation of hippocampus through the involvement of the raphe in the context of which an interneurone is inhibitory on the efferent serotonergic raphe-hippocampus projection. This hypothesis finds further support from MS blockade effect during intraraphal NMDA iontophoretic administration.

The discharge of subthalamic neurons is modulated by inhibiting the nitric oxide synthase in the rat

The effects induced on the discharge of subthalamic spontaneously active neurons by inhibiting the enzyme nitric oxide synthase was studied in two groups of urethane-anesthetized rats. In the first group of animals (n = 10), the activity of subthalamic single units was recorded before and after the systemic administration of 7-nitro-indazole (7-NI, 50 mg/kg i.p.), a selective inhibitor of neuronal nitric oxide synthase. In the second group of rats (n = 15), Nomega-nitro-L-arginine methyl ester (L-NAME), another inhibitor of nitric oxide synthase, was iontophoretically administered while performing single unit extracellular recordings. The activity of most tested spontaneously discharging neurons (8/10) was influenced by 7-NI administration, which always caused a statistically significant decrease in the firing rate of the responsive cells. In contrast, the iontophoretic administration of L-NAME, although influencing many cells (24/32), did not have univocal effects: in fact, 18 cells were inhibited while 6 neurons were excited in a statistically significant manner. We hypothesize that nitric oxide neurotransmission could exert a tonic modulatory influence upon spontaneously discharging subthalamic neurons, with a prevalent excitatory effect.

Intensity of GABA-Evoked Responses Is Modified by Nitric Oxide-Active Compounds in the Subthalamic Nucleus of the Rat: A Microiontophoretic Study

We have previously described modulatory effects of nitric oxide (NO)–active drugs on subthalamic nucleus (STN) neurons. In this study, the effects of microiontophoretically applied NO‐active compounds on GABA‐evoked responses were investigated in subthalamic neurons extracellularly recorded from anesthetized rats: 45 of 62 cells were excited by S‐nitroso‐glutathione (SNOG), an NO donor, whereas 28 of 43 neurons were inhibited by Nω‐nitro‐L‐arginine methyl ester (L‐NAME), a NOS inhibitor. Nearly all neurons responding to SNOG and/or L‐NAME showed significant inhibitory responses to the administration of iontophoretic GABA. In these cells, the changes induced by NO‐active drugs in the magnitude of GABA‐evoked responses were used as indicators of NO modulation. In fact, when an NO‐active drug was co‐iontophoresed with GABA, significant changes in GABA‐induced responses were observed: generally, decreased magnitudes of GABA‐evoked responses were observed during continuous SNOG ejection, whereas the administration of L‐NAME enhanced GABA responses. In contrast, glutamate‐evoked responses were enhanced by SNOG and dampened by L‐NAME co‐iontophoresis. Furthermore, the iontophoretic administration of bicuculline (a GABAA receptor antagonist) completely abolished the GABA‐evoked inhibitory responses and reduced the magnitude of both the SNOG‐ and L‐NAME‐induced effects. The results suggest that the NO‐mediated modulation of subthalamic neurons could also be a result of an interaction between NO and GABAA neurotransmission. Increased NOS activity has been shown in the hyperactive STN neurons of parkinsonian patients; on the basis of our observations about the influence of NO‐active drugs on the baseline and GABA‐evoked activity of subthalamic cells, such hyperactivity suggests the involvement of increased NO levels and reduced sensitivity to GABA.

Neuropsychology of selective attention and magnetic cortical stimulation

Informed volunteers were asked to perform different neuropsychological tests involving selective attention under control conditions and during transcranial magnetic cortical stimulation. The tests chosen involved the recognition of a specific letter among different letters (verbal test) and the search for three different spatial orientations of an appendage to a square (visuo-spatial test). For each test the total time taken and the error rate were calculated. Results showed that cortical stimulation did not cause a worsening in performance. Moreover, magnetic stimulation of the temporal lobe neither modified completion time in both verbal and visuo-spatial tests nor changed error rate. In contrast, magnetic stimulation of the pre-frontal area induced a significant reduction in the performance time of both the verbal and visuo-spatial tests always without an increase in the number of errors. The experimental findings underline the importance of the pre-frontal area in performing tasks requiring a high level of controlled attention and suggest the need to adopt an interdisciplinary approach towards the study of neurone/mind interface mechanisms.

CCK–nitric oxide interaction in rat cortex, striatum and pallidum

We have chosen to study the effects of both nitric oxide (NO) and cholecystokinin neuromodulatory systems in some motor structures that are frequently involved in excitotoxic phenomena. In particular, 7-nitroindazole, a selective inhibitor of neuronal NO synthase, was administered in control and sulfated cholecystokinin octapeptide-treated rats. Cortical surface, striatal and pallidal depth bioelectric activities were examined through Fast Fourier Transform analysis. Cortical and pallidal recordings revealed an increase of rapid standard rhythms after the inhibition of neuronal NO synthase; in contrast, striatal depth recordings showed a marked increase of slow standard rhythms. All these effects were completely abolished by chronic pre-treatment with sulfated cholecystokinin octapeptide. The results suggest a functional co-operation between cholecystokinin and NO systems in the modulation of the bioelectric activity of all the motor structures examined, and the possibility of preventing excitotoxic damages induced by an anomalous balance between excitatory and inhibitory neurotransmitters in these areas.