Evidence against serotonin involvement in the tonic component of electrically induced convulsions and in carbamazepine anticonvulsant activity

Sensitization and kindling phenomena in mood, anxiety, and obsessive-compulsive disorders: the role of serotonergic mechanisms in illness progression

A number of untreated or inadequately treated psychiatric illnesses often demonstrate syndrome progression manifested by either increasing frequency, severity, or spontaneity of episodes. Behavioral sensitization to psychomotor stimulants (and its cross sensitization to stress) and electrophysiological kindling provide two very different models for conceptualizing physiological and behavioral abnormalities that progress in severity in response to the same inducing stimulation over time. These models are highly indirect, and the behaviors induced and specific pharmacologic interventions do not directly parallel those in many of these psychiatric syndromes. Nonetheless, these preclinical models help us conceptualize potential mechanisms involved in syndrome progression based on experience-dependent modifications of the genome at the level of transcriptional regulation. In both preclinical models, agents that are effective in the earlier developmental phase of sensitization or kindling are not necessarily effective in amelioration of the full-blown syndromes, and vice versa. Thus these models also suggest a variety of intervention principles that can be directly tested in the clinic, such as differential efficacy of treatment as a function of stage of evolution of the given syndrome. Although serotonergic mechanisms do not appear central to the basic phenomena of sensitization and kindling, they appear capable of modulating their development and severity. As such, it becomes of considerable importance to assess whether serotonergic mechanisms that have been implicated in acute treatment of mood and anxiety syndromes are also involved in the longitudinal course and prevention of syndrome progression or occurrence. Identification of the more precise molecular mechanisms involved might provide a target for new therapeutic approaches to these recurrent and potentially disabling major psychiatric illnesses.

Alterations in monoamine levels in discrete regions of rat brain after chronic administration of carbamazepine

Carbamazepine (25 mg/kg body weight) was administered intraperitoneally to adult male Wistar rats for 45 days and norepinephrine (NE), dopamine (DA) and serotonin (5-HT) levels were simultaneously assayed in discrete brain regions by high performance liquid chromatographic (HPLC) method. Experimental rats displayed no behavioral abnormalities. Body and brain weights were not significantly different from control group of rats. After exposure it was observed that norepinephrine levels were elevated in motor cortex (P < 0.01) and cerebellum (P < 0.05), while dopamine levels were decreased in these two regions (P < 0.001, P < 0.05). However, dopamine levels were increased in hippocampus (P < 0.01). Serotonin levels were significantly decreased in motor cortex (P < 0.001) and hypothalamus (P < 0.001) but increased in striatum-accumbens (P < 0.001) and brainstem (P < 0.001). These results suggest that carbamazepine may mediate its anticonvulsant effect by differential alterations of monoamine levels in discrete brain regions particularly in motor cortex and cerebellum.

Evidence that serotonin receptors are involved in the anticonvulsant effect of yohimbine in mice

The threshold of seizures induced by electroconvulsive shock (ECS) was determined in mice and the effects of alpha 2-adrenoceptor antagonists (yohimbine, rauwolscine, idazoxan), alpha 2-adrenoceptor agonists (clonidine, B-HT 920), serotonin antagonists (methysergide, metergoline) and a serotonin agonist (quipazine) were studied. The interaction of yohimbine with clonidine, methysergide, metergoline and quipazine was also examined. It was found that yohimbine and rauwolscine elevated the seizure threshold, while idazoxan was ineffective. Clonidine and B-HT 920 were also ineffective, but quipazine elevated the seizure threshold. Methysergide and metergoline did not change the seizure threshold alone, but both of them antagonized the anticonvulsant effect of yohimbine. Clonidine failed to antagonize yohimbine's effects. Quipazine potentiated yohimbine's action. These results suggest that, in mice, the protective effect of yohimbine against ECS-induced seizures may be mediated via serotonin receptors and not via alpha 2-adrenoceptors.

Variation in threshold and pattern of electroshock-induced seizures in rats depending on site of stimulation

Although most laboratories employ transcorneal stimulation as a means of producing electroshock seizures, transauricular stimulation is also used by many investigators. The present study shows that seizures produced with transcorneal electroshock differ from those produced by transauricular electroshock in several ways: transauricular stimulation is more effective at eliciting tonic convulsions; the threshold for clonus is lower when transcorneal electrodes are used; and the face and forelimb clonus produced by transcorneal stimulation cannot be produced with transauricular stimulation at any current. The present findings are consistent with the hypothesis that tonic seizures are more easily triggered with transauricular stimulation because they originate in the brainstem and because this brain region is preferentially activated when ear-clip electrodes are used.

An electrophysiological study of neurones in the rat median raphe and their projections to septum and hippocampus

Extracellular single unit recordings were made in the median raphe nucleus from rats anaesthetized with urethane. Spontaneous firing as well as orthodromic and antidromic responses to stimulation of the fornix and the medial septum were studied. One hundred and twelve units (out of a total of 355) with a regular spontaneous firing rate of 0.2-3 spikes/s were classified as serotonin-containing neurons. Fifty nine of them were antidromically invaded from either the fornix or the medial septum (conduction velocity, 0.8 m/s) and 7 additional neurones from both the fornix and the medial septum. Antidromic action potentials were followed by a period of decreased probability of firing, that was already present below threshold for antidromic invasion, were proportional to the stimulation intensity and had a latency similar to orthodromic inhibition. No preferential topographical distribution within the median raphe nucleus was observed for the serotonin neurones, even those invaded antidromically. Twenty six neurones with a clear-cut anatomical location around the borders of the median raphe nucleus showed a spontaneous rhythmic activity (4-20 spikes/s) characterized by the presence of extremely prolonged silent periods (up to 5 min). Only one of these neurones was invaded antidromically from the medial septum and none from the fornix. Of the remaining non-serotonin neurones, 28 showed a very low firing rate consisting of single action potentials every 10-60 s while 189 had a spontaneous activity of 6-30 spikes/s. Regardless of their firing rate they were all antidromically invaded from the fornix and/or the medial septum and had a conduction velocity of 5 m/s. These experiments demonstrate the electrophysiological heterogeneity of the neuronal population of the median raphe nucleus, the presence of strong projections of both putative serotonin and non-serotonin neurones to the medial septum and, via the fornix, to the hippocampus, and the existence of axonal branching in both types of neurones.

Stimulation of serotonin synthesis in rat brain after antiepilepsirine, an antiepileptic piperine derivative

Piperine and two of its derivatives, antiepilepsirine (AE or 3,4-methylendioxycynnamoylpiperine) and compound 7448 (N-isopropyl 3 (4 chloro-phenyl) propenoylamide) are very effective in stimulating serotonin (5HT) synthesis. AE raises the ratio of free-to-bound tryptophan (TP) in plasma and induces a long-lasting increase of this aminoacid in brain. At the same time in striatum and limbic area it causes a lasting increase in 5 hydroxyindolacetic acid (5HIAA) a 5HT metabolite and to a lesser extent, an increase in the levels of the monoamine itself. Together with this action on 5HT metabolism we found that AE caused release of 3H-5HT from an in vitro synaptosomal preparation. It thus appears that piperine and its derivatives AE and compound 7148 affect the central serotonergic system.

Serotonin mediation of the protective effect of clonidine against pentylenetetrazol-induced seizures in rats

An intraperitoneal injection of 0.5 mg/kg clonidine significantly increased the latency to the first convulsion and reduced tonic seizures and mortality caused by pentylenetetrazol (PTZ), 90 mg/kg, administered subcutaneously to rats. 1 mg/kg clonidine produced similar effects except that tonic seizures were not significantly affected. No effect was observed with 0.01 or 0.1 mg/kg clonidine. Metergoline (1 mg/kg) and methysergide (10 mg/kg), administered intraperitoneally, completely prevented the effect of 0.5 mg/kg clonidine on PTZ-induced seizures. An intraperitoneal injection of 5 mg/kg of d-fenfluramine, a releaser of 5HT from nerve terminals, significantly reduced tonic seizures and completely blocked mortality caused by PTZ but did not significantly modify the latency to the first convulsion. The results suggest that serotonin plays an important role in the protective effect of 0.5 mg/kg clonidine against PTZ-induced seizures. Possible reasons for the different effects of clonidine on different experimental seizures are discussed.

Biochemical effects of carbamazepine: relationship to its mechanisms of action in affective illness

Carbamazepine has a unique spectrum of clinical efficacy in paroxysmal pain syndromes and epilepsy, as well as in affective illness. Evidence of its therapeutic effects in lithium-resistant manic-depressive illness and schizoaffective illness is reviewed. The biochemical effects of carbamazepine as they might be related to either its anticonvulsant or psychotropic properties are presented and discussed. Carbamazepine alters classical neurotransmitter and second messenger systems, as well as a variety of neuropeptides which could relate to mechanisms of action or side effects. Comparison and contrast of the biological effects of carbamazepine with classical treatment agents, such as lithium carbonate, tricyclic antidepressants, and neuroleptics may help elucidate mechanisms underlying the pathophysiology of the mood disorders.