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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., Methods: The experiments were performed on anesthetized rats. The bioelectrical activities of the somatosensory cortex and the CA1 region of the hippocampus of these rats were recorded. Pharmacologic inhibition of NO synthase (NOS) through the nonselective and brain-selective inhibitors, N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), was performed., Results: The treatments caused the appearance of an interictal discharge activity in both the structures. The latency of induction and the duration of the interictal discharge activity were strictly related to the dose of NOS inhibitor used. In some cases, after L-NAME treatment at high doses, it was possible to note spike and wave afterdischarge activity in the hippocampus. All the NOS inhibitor-mediated excitatory effects were abolished by intraperitoneal (i.p.) pretreatment with the N-methyl-D-aspartic acid (NMDA) receptor antagonists (DL-2-amino-5-phosphonovaleric acid, 2-APV; dizolcipine, MK-801) and partly suppressed after the i.p. injection of the non-NMDA antagonist (6-cyano-7-nitroquinoxaline-2,3-dione; CNQX)., Conclusions: All data showed that the reduction of NO levels in the nervous system causes the functional prevalence of the excitatory neurotransmission, which is probably due to an NMDA overactivity caused by the absence of the NO-mediated modulatory action. Thus, it is possible to hypothesize a neuroprotective role for NO, probably through a selective desensitization of the NMDA receptors.

The study has shown an excitatory influence exerted by lateral habenula (LH) on hippocampal pyramidal cells. The modulatory influence is paradoxically serotonine-mediated; in fact all LH stimulation effects were abolished by intrahippocampal iontophoretic methysergide application. The data suggest the involvement of dorsal raphe nucleus. In fact, the dorsal raphe nucleus stimulation caused on hippocampus an expected inhibitory effect antagonized by intrahippocampal iontophoretic methysergide application. In the context of this neural structure we have highlighted a disinhibitory relation between two types of cells: slow serotonergic efferent neurones and fast GABAergic interneurones. The disinhibitory hypothesis is also supported by the following experimental tests performed on both slow and fast raphe cells: a) LH stimulation at low and high frequencies; b) iontophoretic administration of NMDA and GABA; c) LH stimulation during intraraphe iontophoretic injection of 2-APV (NMDA antagonist) and bicuculline (GABA antagonist).

Previously, we have demonstrated that lateral habenula (LH) modulates the bioelectric activity of the hippocampus through the dorsal raphe nucleus functional involvement. In this study we have, preliminarily, electrophysiologically identified two types of raphe neurons: "slow" (S cells, serotonergic in nature); and "fast" (F cells, presumably GABAergic in nature). Then, we have shown that LH electrical stimulation at lower frequency induced an excitation of S and F neurons. LH stimulation at higher frequency inhibited only S neurons. Furthermore, iontophoretic NMDA excited S and F neurons. The excitatory effects of LH stimulation were antagonized by the iontophoretic 2-APV (NMDA antagonist). Iontophoretic GABA inhibited only S neurons. Iontophoretic bicuculline antagonized the LH-induced inhibition os S neurons. The data suggested a direct (NMDA-mediated) and indirect (through the F GABAergic inhibitory interneuron) influence of the LH on the serotonergic efferent neuron.

In previous experimental studies, carried out on cats, we demonstrated that electrical stimulation of lateral habenula (LH) at 0.5-3.0 Hz or 5-20 Hz had a double effect (low frequency-excitation; high frequency-inhibition) on the spontaneous firing rate of single hippocampal neurones. Our results, in agreement with similar case studies, allowed us to hypothesise that in the habenular modulation of the hippocampus the raphe nucleus is probably involved. In fact, all the effects of LH stimulation were antagonised by the iontophoretic intrahippocampal application of methysergide. In the present series of experiments, performed on rats, it was possible to demonstrate that LH stimulation at 1-10 Hz causes an excitation of a progressively major number of hippocampal neurones depending upon the increase of frequency stimulation. The absence of habenulo-induced effects after a iontophoretic application of methysergide on single hippocampal units suggests the involvement of the raphe nucleus. Furthermore, in consideration of recent anatomical evidences demonstrating an excitatory projection between LH and raphe nucleus, intraraphal N-methyl-D-aspartate (NMDA) application, performed through a Hamilton microsyringe, induces an inhibitory effect. All the results suggest that in the raphe context it is possible to hypothesise the presence of an intrinsic interneurone, directly activated by the excitatory projection arising from the LH; this interneurone is likely inhibitory on the serotonergic raphe-hippocampus efferent neurone. This functional organization is responsible for the effect of LH stimulation at different frequencies as well as for the effects of intraraphal NMDA application.

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.

In the cat, the effects of lateral habenula stimulation, at different ranges of frequency, on hippocampal units were studied. Habenular stimulation at low frequency excited, while at high frequency inhibited the greater part of hippocampal units. Moreover, in order to clarify the possible pathway involved in the habenulo-hippocampal circuit, the effects of iontophoretic acetylcholine and serotonin on hippocampal units were compared with those of habenular stimulation. Iontophoretic acetylcholine induced both excitatory and inhibitory responses while serotonin induced only inhibitory responses. Iontophoretic atropine blocked the effects of acetylcholine ejection but did not antagonize stimulation effects; ion-tophoretic methysergide induced an increase of basal firing of hippocampal units and antagonized both serotonin and habenular stimulation inhibition. The results suggest an influence of lateral habenula to the hippocampus which does not appear to be cholinergically-mediated. A possible involvement of the raphe as a relay station in the habenulo-hippocampal pathway is discussed.

An investigation was made into the links between electric activity of antral and of duodeno-jejunal musculature in different functional conditions. The function of the gastroduodenal junction in this linking mechanism was analysed. The following observations were made: (a) in the absence of gastric stimulation, the slow electric activities of stomach and duodenum appear to be completely independent; (b) the gastroduodenal junction evidences no electric activity of its own but is affected by that of the two adjacent structures; (c) chemical stimulation of the gastric mucosa causes activation of the electric and mechanical activity of the stomach and analogous activation of duodenal musculature; this effect is mediated by the gastroduodenal junction; (d) very probably, the transmission of gastric activation to the duodenum is myogenic for it ceases after surgical transection but not after cooling or after ligature. The possible functional role of the pyloric junction in the complex gastroduodenal mechanism is discussed.

A putative integrative function of the striatum was evaluated through the study of the electrical activity of sensory responsive caudate neurones. Both nervous (radial nerve) and auditory stimulations were delivered in order to characterize populations of neurones affected by peripheral stimuli; the units were previously activated by iontophoretic glutamate. On these units the iontophoretic ejection of ACh and DA was tested. Experimental results demonstrated a prevalent excitatory effect of ACh, while DA appeared to exert a drastic decrease on firing rate. A comparison between peripheral stimuli and chemical substances was made. The result of such study showed a most important action of the neurotransmitters employed. The activity of caudate units following single shock activation was also explored. This investigation underlined a certain degree of facilitatory influence of ACh; DA, on the contrary, had the tendency to exert a marked inhibitory action. The results are interpreted in view of the striatal peculiar position on cortico-striato-thalamo-cortical circuit. An integrative function of basal ganglia on sensori-motor activity of the cortex is postulated and the importance of ACh and DA is emphasized.