5,7-Dihydroxytryptamine as a tool to study the functional role of central 5-hydroxytryptamine neurons

Perifornical hypothalamic orexin and serotonin modulate the counterregulatory response to hypoglycemic and glucoprivic stimuli

Previous reports suggested an important role for serotonin (5-hydroxytryptamine [5-HT]) in enhancing the counterregulatory response (CRR) to hypoglycemia. To elucidate the sites of action mediating this effect, we initially found that insulin-induced hypoglycemia stimulates 5-HT release in widespread forebrain regions, including the perifornical hypothalamus (PFH; 30%), ventromedial hypothalamus (34%), paraventricular hypothalamus (34%), paraventricular thalamic nucleus (64%), and cerebral cortex (63%). Of these, we focused on the PFH because of its known modulation of diverse neurohumoral and behavioral responses. In awake, behaving rats, bilateral PFH glucoprivation with 5-thioglucose stimulated adrenal medullary epinephrine (Epi) release (3,153%) and feeding (400%), while clamping PFH glucose at postprandial brain levels blunted the Epi response to hypoglycemia by 30%. The PFH contained both glucose-excited (GE) and glucose-inhibited (GI) neurons; GE neurons were primarily excited, while GI neurons were equally excited or inhibited by 5-HT at hypoglycemic glucose levels in vitro. Also, 5-HT stimulated lactate production by cultured hypothalamic astrocytes. Depleting PFH 5-HT blunted the Epi (but not feeding) response to focal PFH (69%) and systemic glucoprivation (39%), while increasing PFH 5-HT levels amplified the Epi response to hypoglycemia by 32%. Finally, the orexin 1 receptor antagonist SB334867A attenuated both the Epi (65%) and feeding (47%) responses to focal PFH glucoprivation. Thus we have identified the PFH as a glucoregulatory region where both 5-HT and orexin modulate the CRR and feeding responses to glucoprivation.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

REM sleep is impaired by a small amount of alcohol in young women sensitive to alcohol

OBJECTIVE

The effect of a small amount of alcohol on the sleep structure in relation to alcohol sensitivity was examined using polysomnography (PSG).

METHODS

Alcohol sensitivity was evaluated using alcohol patch test for all subjects. PSGs were performed on three nights after one night for acclimation, and subjects consumed no alcohol, 0.28 or 0.69 g ethanol/kg body weight, respectively, before going to bed. The percentages of sleep time in each sleep stage of 1, 2, 3+4 and rapid eye movement (REM), REM latency, and REM cycle were calculated.

SUBJECTS

Thirteen healthy female students (age 21.1 +/- 0.7 years) were enrolled in this study.

RESULTS

In all subjects, there were no significant differences in any of the sleep parameters between baseline night and alcohol nights. Six of the 13 subjects were sensitive to alcohol, in whom %stage REM was significantly decreased by alcohol consumption (baseline night: 18.3 +/- 6.2%, alcohol night I: 9.8 +/- 5.1% and alcohol night II: 11.0 +/- 2.8%), and the REM latency was significantly prolonged. The standard deviation of REM cycle was significantly greater on alcohol nights I and II than baseline night. There were no significant differences in other sleep parameters. In the other seven subjects who were insensitive to alcohol, none of the sleep parameters were significantly affected by alcohol consumption.

CONCLUSION

REM sleep was adversely affected by a small amount of alcohol in alcohol-sensitive healthy young women. Alcohol sensitivity might play some important role in impaired REM sleep by an ingestion of a small amount of alcohol.

Serotonin reuptake inhibitors do not prevent 5,7-dihydroxytryptamine-induced depletion of serotonin in rat brain

Although the selective toxicity of 5,7-dihydroxytryptamine (5,7-DHT) is thought to depend on the drug's transport into serotonin (5HT) neurons via the 5HT transporter, few studies have critically examined this postulation. We therefore evaluated if 5,7-DHT-induced reductions in 5HT concentrations and synthesis rate in rat brain are blocked by pretreatment with 5HT-selective reuptake inhibitors. Rats pretreated with desipramine (DMI) (to prevent norepinephrine depletion) received intracerebroventricular injections of 5,7-DHT (5, 50, 100, 200 microg/rat) 30 min after fluoxetine (20 mg/kg ip). Forty-eight hours later, they received m-hydroxybenzylhydrazine 30 min before sacrifice. The concentrations of 5HT and 5-hydroxytryptophan (5HTP, an index of 5HT synthesis) were measured in hypothalamus, cortex and brainstem. Each 5,7-DHT dose produced significant reductions in 5HT and 5HTP concentrations in all regions examined (5 microg reduced 5HT but not 5HTP), effects that were not blocked by fluoxetine. Two other 5HT reuptake blockers (chlorimipramine, alaproclate) also failed to block the 5HT and 5HTP depleting actions of 5,7-DHT. Desipramine blocked 5,7-DHT-induced norepinephrine (NE) depletion. Pretreatment with the 5HT receptor antagonist metergoline, or the 5HT(1A) agonist 8-hydroxy-(di-n-propylamino)tetralin (to slow 5HT neuronal firing rate) also failed to antagonize the 5HT depleting action of 5,7-DHT. Together, the data strongly suggest that the mechanism by which 5,7-DHT depletes the brain of serotonin does not involve 5HT-transporter-mediated concentration of neurotoxin in 5HT neurons, may not involve 5HT receptor interaction, and does not depend on the firing rate of the 5HT neuron.

Neural and hormonal consequences of neonatal 5,7-dihydroxytryptamine may not be associated with serotonin depletion

The neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) is often used in neonatal rats to induce specific, rapid, and permanent depletion of brain serotonin (5-HT). One assumed benefit of using this drug in neonates is that it is well-tolerated, with pups exhibiting few side effects normally attributed to 5-HT depletion. Here, we present evidence that 5,7-DHT administered neonatally induces seizure-like behavior, decreases weight gain, and increases plasma corticosterone without depletion of brain 5-HT.

A neurotoxic lesion of serotonergic neurones using 5,7-dihydroxytryptamine does not disrupt latent inhibition in paradigms sensitive to low doses of amphetamine

Testing the effects of low doses of d-amphetamine on latent inhibition (LI) in two different conditioning paradigms, passive avoidance and conditioned taste aversion, provided evidence of their pharmacological equivalence. For passive avoidance, LI was expressed by the decreased latency to enter a shock compartment in preexposed rats placed 5 min in the compartment during 3 consecutive days before conditioning. In the conditioned taste aversion paradigm, a group of rats was preexposed to a solution of sucrose also for 3 consecutive days prior to the establishment of an association between sucrose and sickness elicited by an injection of LiCl. On the following day, the preexposed rats drunk more sucrose when allowed to choose between one tube containing water and an other containing sucrose. In both paradigms, 0.25 mg/kg d-amphetamine, injected daily on the 3 preexposure days and on the conditioning day, decreased LI. A dose of 0.5 mg/kg suppressed LI in the passive avoidance paradigm. The effect of a serotonergic lesion induced by i.c.v. injection of 5,7-dihydroxytryptamine (5,7-DHT) was evaluated in the same paradigms. The lesion procedure that lowered hippocampal serotonin and 5 HIAA levels by more than 80% did not affect LI. Taken together, the present results lessens the hypothesis that LI is prone to an opposing influence of the two monoaminergic systems considered in this work.

Doubly dissociable effects of median- and dorsal-raphé lesions on the performance of the five-choice serial reaction time test of attention in rats

Six experiments examined the effects of selective median (MRN)- and dorsal (DRN)-raphé nucleus lesions on the performance of the five-choice serial reaction time task. In this test rats are required to localize brief visual stimuli presented randomly in one of five locations in approximately 30 min sessions of 100 trials. Both accuracy and latency to respond are measured, as well as the incidence of premature and perseverative responding. Selective 5-HT lesions were induced by intra-raphé infusions of 5,7-dihydroxytryptamine following pretreatment with both a noradrenergic and a dopaminergic re-uptake inhibitor. Analysis of tissue monoamine content demonstrated that the MRN lesion profoundly depleted hippocampal 5-HT (by about 90%) without affecting noradrenaline and dopamine, whereas the DRN lesion primarily depleted (by about 80%) nucleus accumbens and caudate-putamen 5-HT. Rats with 5-HT lesions of the MRN performed the task with a similar degree of accuracy to that exhibited by sham-operated controls. Although the MRN lesion did not affect the latency to respond correctly to the visual targets the lesioned animals collected the food reward significantly faster than the controls. A transient increase in the number of premature responses also resulted from this lesion. In contrast the DRN lesion produced a transient but significant increase in the accuracy of performance, and increased both the speed and the probability of responding. The similarity of the effects following global forebrain 5-HT depletion and the selective DRN lesion suggests that the 5-HT projections of the DRN rather than the MRN may play an important role in impulsive behaviour following 5-HT depletion.

I. Serotonin (5-HT) within dopamine reward circuits signals open-field behavior. II. Basis for 5-HT--DA interaction in cocaine dysfunctional behavior

Light microscopic immunocytochemical studies, using a sensitive silver intensification procedure, show that dopamine (DA) and serotonin (5-HT) axons terminate on neurons in the nucleus accumbens (NAcc) (A10) terminals and also in dorsal striatum (DSTr) (A9) terminals. The data demonstrate a prominent endogenous anatomic interaction at these distal presynaptic sites between the neurotransmitters 5-HT and DA; the pattern of the 5-HT-DA interaction differs between A10 and A9 terminals. Moreover, in distinction to the variance shown anatomically between 5-HT--DA interactions at distal A9 and A10 sites, the 5-HT--DA interactions at the level of DA somatodendrites, the proximal site, are similar, i.e. 5-HT terminals in the midbrain tegmentum are profuse and have a massive overlap with DA neurons in both ventral tegmental area (VTA) and substantia nigra pars compacta (SNpc). We suggest with reference to the DA neurons of A10 and A9 pathways, inclusive of somatodendrites (sites of proximal presynaptic interactions in the midbrain) and axons (sites of distal presynaptic interactions), that 5-HT--DA interactions in A10 terminals are more likely to exceed those in the DStr arrangement. Furthermore, our neuroanatomic data show that axonally released DA at A10 terminals may originate from proximal 5-HT somatodendrites, i.e. dorsal raphe (DR) or the proximal DA somatodendrites, VTA. In vivo microvoltammetric studies were done with highly sensitive temporal and spatial resolution; the studies demonstrate basal (endogenous) real time 5-HT release at distal A10 and distal A9 terminal fields and real time 5-HT release at proximal A10 VTA somatodendrites. In vivo microvoltammetric studies were performed concurrently and on line with studies of DA release, also at distal A10 and distal A9 terminal fields and at proximal A10 somatodendrites. Serotonin release was detected in a separate voltammetric peak from the DA voltammetric peak. The electrochemical signal for 5-HT release was detected within 10-12 s and that for DA release within 12-15 s, after each biogenic amine diffused through the synaptic environment onto the microelectrode surface. The electrochemical signal for 5-HT and a separate electrochemical signal for DA are detected on the same voltammogram within 22-27 s; each electrochemical signal represents current changes in picoamperes, within seconds of detection time. The amplitude of each electrochemical signal reflects the changes in diffusion of each biogenic amine to the microelectrode surface. Each neurotransmitter has a distinct potential at which oxidation occurs; this results in a recording which has a distinct peak for a specific neurotransmitter. The concentration of each neurotransmitter within the synaptic environment is directly related to the electrochemical signal detected via the Cottrell equation. Voltammograms were recorded every 5 min. At the time that basal 5-HT release and basal DA release were recorded within same animal control, open-field behavioral studies were performed, also concurrently, by infrared photocell beams. The frequency of each behavioral parameter was monitored every 100 ms; the number of behavioral events, were summated every 5 min during the time course of study. Thus, the detection of neurotransmitters occurs in real time, while simultaneously monitoring the animal's behavior by infrared photocell beams. The results from the in vivo microvoltammetric and behavioral data from this study show that basal 5-HT release at distal A10 and A9 terminals dramatically increased with DA release. Moreover, each increase in basal 5-HT release, at both A10 and at A9 terminal fields occurred consistently and at the same time as each increase in open-field locomotion and stereotypy occurred naturally during the animal's exploration in a novel chamber. Thus, the terminology 'synchronous and simultaneous' describes aptly the correlation between 5-HT release at distal A10 and A9 terminal fields and open-field locomo

Acquisition of conditioned inhibition in rats is impaired by ablation of serotoninergic pathways

The effects of chronically ablating the serotoninergic inputs to various regions of the rat brain on the ability to solve a feature-negative discrimination was measured. After intracerebroventricular administration of the specific neurotoxin 5,7-dihydroxytryptamine, the rats exhibited an impaired capacity to solve such a discrimination, irrespective of whether auditory or visual stimuli were used. Further behavioural analysis revealed that this effect was not due to a reduced capacity to form excitatory associations, since both groups responded equally to reinforced stimuli. By contrast, the lesion more likely resulted in a failure to endow the non-reinforced stimuli with inhibitory properties. This suggestion was supported by the observation that, in a retardation test, the conditioned inhibitor aroused less inhibition in the lesioned group than in vehicle-injected controls. Furthermore, the conditioned inhibitor failed to pass a summation test in lesioned animals, again indicating that their hampered ability to master the discrimination was the result of an impairment in the formation of inhibitory associations. It is concluded that destruction of central 5-hydroxytryptamine-containing pathways impairs the functioning of brain areas underlying inhibitory associative learning.

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of hallucinogenic drugs. III: The mechanistic basis for supersensitivity to the LSD stimulus following serotonin depletion

The present study was designed to determine the effects of p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) administration on (1) the levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat brain, (2) the sensitivity of LSD-trained rats to the stimulus effects of LSD, and (3) the maximal levels of 5-HT2A and 5-HT2C receptor mediated phosphoinositide (PI) hydrolysis in rat brain. PCA and PCPA both produced a significant depletion of whole brain 5-HT and 5-HIAA concentrations. The depletion of serotonin with PCPA, but not PCA, resulted in supersensitivity of LSD-trained subjects to the stimulus effects of LSD. Neither PCPA nor PCA treatment altered the maximal level of 5-HT2A receptor-mediated PI hydrolysis. However, PCPA, but not PCA, treatment resulted in a significant upregulation (46%, P < 0.05) of the maximal level of 5-HT2C receptor mediated PI hydrolysis. These data suggest that upregulation of the 5-HT2C receptor mediates the supersensitivity to LSD discriminative stimulus which follows the depletion of central nervous system serotonin by PCPA.

Plasticity and ontogeny of the central 5-HT transporter: effect of neonatal 5,7-dihydroxytryptamine lesions in the rat

5,7-Dihydroxytryptamine (5,7-DHT) is unique as a serotonin (5-HT) neurotoxin in that i.p. injection of neonatal rats increases concentrations of 5-HT in brainstem while depleting 5-HT in cortex, hippocampus and spinal cord. To study the mechanism of this effect we measured the 5-HT transporter or uptake site, a presynaptic marker, using [3H]paroxetine binding. There were significant regional differences in Bmax of vehicle-injected rats: brainstem, diencephalon > striatum, cortex, spinal cord > hippocampus, cerebellum. There were also regional differences in the ontogeny of bindings sites: at postnatal day 7, [3H]paroxetine sites were 39% of adult levels in cortex compared to 63% in brainstem. Thirty days after 100 mg/kg 5,7-DHT i.p., Bmax of [3H]paroxetine binding was significantly increased in brainstem (+67%) and diencephalon (+136%), whereas it decreased in cortex (-59%), hippocampus (-94%) and spinal cord (-99%), striatum (-41%) and cerebellum (-37%). KD remained unaltered. In dose-response studies (0-200 mg/kg), 50 mg/kg was the threshold dose for Bmax effects and 200 mg/kg was lethal. In weekly time-course studies, changes were apparent 1 week after 5,7-DHT lesions. Binding site increases in diencephalon and brainstem were not maximal until 3 weeks after injection, whereas percent decreases in cortical sites remained unchanged at each week studied. Lesion effects on the ontogeny of [3H]paroxetine binding sites were region-dependent: cortical sites continued to increase with age but spinal sites did not. There was no significant recovery in spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

The functional significance of neonatal 5,7-dihydroxytryptamine lesions in the rat: response to selective 5-HT1A and 5-HT2,1C agonists

To study the involvement of serotonin (5-HT) receptor subtypes in behavioral supersensitivity following neonatal 5,7-dihydroxytryptamine (5,7-DHT) lesions, we measured acute behavioral responses to a single dose of selective 5-HT1A (8-OH-DPAT) or 5-HT2,1C (DOI) agonist compared to 5-hydroxytryptophan (5-HTP) in rats injected with 5,7-DHT intraperitoneally or intracisternally 14 weeks earlier. Only intraperitoneal 5,7-DHT injection resulted in brainstem 5-HT hyperinnervation, but cortical 5-HT depletions were also less. Effects of DOI, such as shaking behavior and forepaw myoclonus, were enhanced by 5,7-DHT lesions made intracisternally not intraperitoneally, whereas 8-OH-DPAT-evoked behaviors, such as forepaw myoclonus and head weaving, were enhanced more by the intraperitoneal route. The main consequence of intraperitoneal compared to intracisternal 5,7-DHT injection on supersensitivity to 5-HT agonists was increased presynaptic 5-HT1A responses and decreased 5-HT2,1C responses. In contrast, 5-HTP evoked more shaking behavior and less of the serotonin syndrome with the intraperitoneal compared to the intracisternal route of 5,7-DHT injection. Behavioral supersensitivity to 5-HTP, which was attributable to 5-HT1A, 5-HT2,1C, and possibly to other 5-HT receptors, was orders of magnitude greater than that elicited by direct receptor agonists and more clearly differentiated between rats with 5,7-DHT lesions and their controls, and between routes of 5,7-DHT injections, than responses to 5-HT agonists at the dose studied. 5,7-DHT induced dysregulation of 5-HT receptors, including both presynaptic and postsynaptic changes and altered interactions between receptor subtypes, better explains these data than postsynaptic changes alone.

A general role for serotonin in the control of behavior: studies with intracerebral 5,7-dihydroxytryptamine

Multiple injections of 5,7-dihydroxytryptamine (5,7-DHT) into the rat brainstem reduced forebrain levels of serotonin and 5-hydroxyindoleacetic acid to 3-10% of the levels observed in control rats that had received intrabrainstem injection of a Locke's solution vehicle. This treatment reduced or abolished atropine-resistant cerebral activation (ARCA) in most cases. In rats in which ARCA was impaired or lost, a number of behavioral abnormalities were observed. These included: high levels of locomotion in an open field test; a deficiency in swimming to, and climbing upon, a visible platform in a water-filled tank; deficient social behavior; and impaired performance in a simple test of active avoidance. These deficits were not due to low level motor impairment. The 5,7-DHT-treated rats displayed a circadian rhythm of activity in running wheels. It is proposed that ascending serotonergic projections are an important component in the cerebral control of the Type 1 behavior with which the occurrence of ARCA is closely linked. Since Type 1 behavior includes such motor patterns as walking and manipulation of objects with the limbs, which are essential components of a great variety of behavioral performances, it is to be expected that a loss of ascending serotonergic function will result in a generalized deficit in behavior.

Lack of effect of hyperprolactinemia on serotonin turnover in ovariectomized and ovariectomized estrogen-treated rats

Hyperprolactinemia suppresses luteinizing hormone (LH) and prolactin (PRL) secretion under a variety of experimental conditions. The secretion of both of these hormones is regulated at the hypothalamic level by several neurotransmitters, including serotonin (5-HT). Therefore, we examined the effect of hyperprolactinemia on 5-HT neuronal activity in key hypothalamic areas that are rich in 5-HT terminals and which are known to regulate the release of LH and PRL. Young cycling virgin rats were ovariectomized (day 0). From days 11-16, animals were injected with ovine prolactin (oPRL, 4 mg/kg, s.c.) or vehicle every 8 h. On day 14, one-half of the oPRL- and vehicle-treated rats were implanted with 20-mm long Silastic capsules containing estradiol (180 micrograms/ml). On day 16, animals were killed at 08.00, 12.00 or 18.00 h or treated with pargyline (75 mg/kg) and killed 10 min later. Trunk blood was collected and serum was radioimmunoassayed for LH and endogenous rat PRL (rPRL). Brains were removed, frozen, sectioned and the medial preoptic, suprachiasmatic, and arcuate nuclei, median eminence and globus pallidus were microdissected. Serotonin was measured using high pressure liquid chromatographic methodology. We were unable to detect any feedback effect of hyperprolactinemia on 5-HT turnover in any brain area of ovariectomized or ovariectomized estradiol-treated rats at any time of day that we examined. Several potential reasons for the absence of an effect of hyperprolactinemia on serotonergic function are discussed.

Endogenous levels of serotonin and 5-hydroxyindoleacetic acid in specific areas of the pigeon CNS: effects of serotonin neurotoxins

Endogenous levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined by high-performance liquid chromatography (HPLC) in specific regions of the pigeon central nervous system (CNS). High endogenous 5-HT levels in the visual wulst and brainstem and medium 5-HT content in the optic lobes were found. The cerebellum and retina showed low endogenous 5-HT levels. Similar endogenous 5-HIAA levels were measured in the visual wulst, optic lobes and brainstem, whereas the 5-HIAA content of the cerebellum and retina was significantly lower. The effects of para-chloroamphetamine (p-CA) and 5,7-dihydroxytryptamine (5,7-DHT) on the 5-HT and 5-HIAA content of the same regions were studied. Six days after p-CA treatment, the 5-HT content of the visual wulst, optic lobes, brainstem and the 5-HIAA content of the optic lobes and cerebellum markedly decreased. Nine days after 5,7-DHT administration, the 5-HT and 5-HIAA content of the visual wulst and optic lobes was significantly reduced. At longer survival times, serotonergic systems were differentially affected depending on both the neurotoxin treatment and the specific brain regions examined. The 5-HT content of the pigeon retina was not modified by p-CA treatment, whereas 5,7-DHT intravitreally injected caused a pronounced 5-HT depletion. Our results demonstrate that selective neurotoxins for serotonergic systems can provide a useful denervation tool for the study of serotonergic function in the pigeon CNS.

Central serotonin neurones in avoidance learning: interactions with noradrenaline and dopamine neurones

The effects of p-chloroamphetamine (PCA), a serotonin (5-HT) releaser, on acquisition and retention were examined in male Sprague-Dawley rats using a one-way active avoidance task. PCA was found to impair avoidance acquisition and retention in a time dependent fashion which followed closely the temporal effects of the drug on 5-HT release in the brain. Thus, the avoidance deficit is related to the rate of change and not to the steady-state levels of 5-HT. The 5-HT releasing effect was most pronounced in the forebrain with less effect in the spinal cord. PCA caused time dependent, regional variations in catecholamine content, which was not related to avoidance performance. The avoidance and retention impairment induced by PCA was blocked by the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) but not by depletion of catecholamines with alpha-methyl-p-tyrosine (H44/68) or by the noradrenergic-selective neurotoxin DSP4. Analysis of the time dependent effects of PCA on monoamine content in saline or PCPA-treated rats indicated that the temporal effects of PCA on avoidance performance is not due to a direct or indirect action on catecholamine neurones. The present experiments support the view that the ascending serotonergic pathways play a significant role in aversive learning in the rat.

Effects of neurotoxin-induced brainstem lesions on the respiratory responses of conscious rats

Respiration was recorded in unanaesthetized rats breathing air, or CO2 in air, 4-8 days after injection into the rostral brainstem at the level of the dorsal raphe nucleus of either a catecholaminergic neurotoxin, a serotoninergic neurotoxin, or the vehicle (0.5% ascorbic acid) of these drugs. Some rats were pretreated with the noradrenergic neurone protecting agent, desmethylimipramine (DMI). Vehicle injection resulted in an increase in the frequency (f) sensitivity to CO2. In two of four cases, injection of 6-hydroxydopamine caused a similar response but in the remaining cases injections caused increases in inspiratory duration (TI), expiratory duration (TE) and tidal volume (VT) during air-breathing. The serotonergic neurotoxin, 5,7-dihydroxytryptamine, reduced the frequency sensitivity to CO2 significantly. This reduction was not sustained. In all cases air-breathing f was decreased due to a prolongation of TI; in most cases air-breathing VT was increased. The characteristic effect of midline 5,7-dihydroxytryptamine lesions was an increase in the VT sensitivity to CO2. On the basis of the results, it is proposed that noradrenaline is essential to the processes determining basic respiratory rhythm, whereas dopamine and serotonin are important inhibitors in chemoreflex ventilatory patterns.

The influence of testosterone in the brain of the male rat on levels of serotonin (5-HT) and hydroxyindole-acetic acid (5-HIAA)

There were two groups of rats: one was injected with testosterone propionate (10 mg/kg) every 7 days starting from weaning (23 days old); the other group had gonadectomy on the same day. The levels of 5-HT and 5-HIAA were measured by spectrofluorometry. The concentrations of 5-HT in the diencephalon of the testosterone propionate injected rats decreased significantly at 45 days, tending to become reestablished at 60 days; the rest of the brain followed the same pattern, but was less pronounced. The concentrations of 5-HIAA in the diencephalon and the rest of the brain decrease throughout postnatal development, although the differences are not significant. The castrated rats showed a marked increase at 45 days and later decreased at 60 days without recovering their initial values, in both brain areas. 5-HIAA concentrations were similar to those found in the injected animals. These facts can have various interpretations: early modifications in the brain, feed-back regulation mechanisms at the level of the hypothalamus, decrease in the release of the amine or reduction of its catabolism.

Involvement of spinal serotonergic pathways in nociception but not in avoidance learning

The effects of selective lesions of the descending serotonergic (5-HT) pathways on analgesia and avoidance deficit induced by the 5-HT releasing compound p-chloroamphetamine (PCA, 2.5 mg/kg) were investigated in male rats. Intrathecal injection of 5,6-DHT (20 micrograms/rat) reduced the uptake of labelled 5-HT into spinal synaptosomes by approximately 85% but did not significantly affect the uptake of noradrenaline. The lesions produced a significant hyperalgesia and strongly attenuated the analgesic effect of PCA in the hot-plate test. In the flinch-jump test 5,6-DHT lesioned rats receiving PCA did not differ from the saline control group. Spinal lesioning did not, however, affect one-way active avoidance performance and did not prevent the marked impairment of avoidance performance induced by PCA. Thus, the avoidance deficit caused by PCA is independent of the descending serotonergic pathways and of the analgesia induced by PCA. These results support the view of a differential involvement of the ascending and descending serotonergic projections in behavioural processes controlled by aversive stimuli.

Brain 5-hydroxytryptamine and learned resistance to punishment

Rats can learn to persist in making a response that is punished. Brain 5-hydroxytryptamine (5-HT) systems are known to mediate acute responding to punishment. This work investigates 5-HT involvement in learned resistance to punishment. Forebrain 5-HT was depleted by intracerebral injection of 5,7-dihydroxytryptamine. Lesioned rats were fully able to learn resistance to punishment, although acute responding to punishment was impaired. Forebrain 5-HT does not mediate learned resistance to punishment.

The pharmacology of zimelidine: a 5-HT selective reuptake inhibitor

Zimelidine (ZIM) and its main active metabolite norzimelidine (NZIM) have been shown to preferentially inhibit 5-hydroxytryptamine (5-HT) neuronal uptake both in vitro and in vivo while having much less effect on noradrenaline (NA) uptake. ZIM in vivo blocked the 5-HT uptake mechanism in the cerebral cortex, hippocampus, striatum, hypothalamus and spinal cord, thus indicating effects on both the ascending and descending 5-HT pathways. ZIM is devoid of a 5-HT releasing action, MAO-inhibitory properties and effects on dopamine (DA) uptake. ZIM failed to reduce NA turnover even in high doses, but markedly reduced 5-HT turnover in very low doses in the rat. ZIM also enhanced 5-HT mediated behaviours in mice in doses related to the inhibition of 5-HT uptake. In contrast to amitriptyline (AMI) and mianserin (MIAN), ZIM only in extremely high doses displayed a 5-HT receptor blocking action in vitro and failed to block 5-HT mediated behaviour. ZIM was practically devoid of action on histamine H1 and H2 receptors, and had also a neglible action on noradrenergic alpha 1- and alpha 2-receptors, and on beta-receptors. Unlike the tricyclic antidepressants (TAD's) ZIM had a negligible action on muscarinic receptors and failed to affect cholinergic induced activity. Long-term treatment with ZIM did not result in any attenuation of the 5-HT uptake blocking potency or the reduction of 5-HT turnover. This long-term treatment slightly reduced the number of beta-receptors in the brain. However, repeated ZIM-treatment induced a new 5-HT receptor binding site characterized by a low affinity and with a high number of binding sites and decreased the number of high affinity 5-HT receptor binding sites. Unlike the TAD's zimelidine failed to block the action of reserpine. Metabolic and behavioural interactions studies in mice showed that ZIM was devoid of any significant interactions with ethanol, barbiturates and benzodiazepines. It is concluded that ZIM markedly differs from both the TAD's and new antidepressants such as mianserin and nomifensine. ZIM seems preferentially to effect the presynaptic 5-HT reuptake mechanism while having a negligible action on noradrenergic, 5-HT, acetylcholine and histamine receptors in the brain.

The effect of serotonin depletion on the discriminability of LSD

Nine groups of rats were trained to discriminate LSD (0.12 mg/kg) from saline in a two-lever, water-reinforced, drug discrimination procedure. After stable discriminative performance was obtained (>95% correct), groups were administered one of several treatments which lower the concentration of serotonin (5-HT) in brain: (1) 12.5, 25, 50, 100 or 200 microgram of 5,7-dihydroxytryptamine (5,7-DHT) intraventricularly (IVT); (2) 3 X 100 mg/kg of p-chlorophenylalanine (PCPA) intraperitoneally (IP); or (3) 20 mg/kg of p-chloroamphetamine (PCA) IP. Control rats received either IVT injections of 5,7-DHT vehicle or IP injections of PCA or PCPA vehicles. Beginning 12 days after treatment, lever preference following various doses of LSD was determined. The results indicated that only the 200 microgram dose of 5,7-DHT and PCPA caused a significant potentiation of LSD-lever responding at the 0.03 mg/kg dose of LSD while all treatments except 12.5 and 25 microgram of 5,7-DHT resulted in significant depletion of 5-HT. Moreover, amount of 5-HT and percent LSD responding following 0.03 mg/kg LSD were not significantly correlated. It was concluded that 5-HT depletion, per se, cannot account for supersensitivity to the behavioral effects of LSD.

Serotonin receptors in hippocampus and frontal cortex

The inhibition by various serotonin agonists and antagonists of the binding of 3 nM 3H-d-LSD, 1.7 nM 3H-serotonin and 0.22 nM 3H-spiperone to homogenates of calf hippocampus and frontal cortex was studied. The 50% inhibitory concentration (IC50) for these drugs versus 3H-d-LSD binding had similar values to and correlated with corresponding IC50 values versus 3H-serotonin binding in the hippocampus, suggesting that 3H-LSD and 3H-serotonin label similar sites in this region. In the calf frontal cortex, serotonin revealed a biphasic inhibition against 3H-d-LSD binding and the tryptamines inhibited over a concentration range of 10 000-fold. The IC25 values of various drugs versus 3H-d-LSD binding correlated with the IC50 values versus 3H-serotonin, but did not correlate with the IC50 values versus 3H-spiperone. These data suggest that 3H-d-LSD bound to more than one serotonin site in the calf frontal cortex and that 3H-spiperone bound to a separate serotonergic site. Scatchard analyses of the binding for these three 3H-ligands indicated that in the calf frontal cortex the density of 3H-d-LSD sites was approximately equal to the sum of the densities for 3H-serotonin (S-1 sites) and 3H-spiperone (S-2 sites). Two weeks after serotonin-depleting radiofrequency heat lesions of the midbrain dorsal and median raphe nuclei in rats, both 3H-serotonin and 3H-LSD showed enhanced binding in the hippocampus. These data support previous suggestions that supersensitivity develops specifically in serotonin receptors following afferent denervation.

Denervation supersensitivity to serotonin in rat forebrain: single cell studies

To investigate the development of denervation supersensitivity to serotonin (5-hydroxytryptamine, 5-HT) in the amygdala (AMYG) and the ventral lateral geniculate nucleus (vLGN), single cell recordings, microiontophoretic, histochemical and biochemical techniques were used in the present study. 5-HT projections to the vLGN and the AMYG were destroyed by 5,7-dihydroxytryptamine (5,7-DHT, a relatively selective toxin for 5-HT neurons) injected directly into the lateral ventricle or the ascending 5-HT pathway in the ventromedial tegmentum area. Enhanced responsiveness of cells to the inhibitory effect of microiontophoretically applied 5-HT (ionto-5-HT) began to develop within 24 h and approached a maximum 7 days after 5,7-DHT pretreatment. In general, the time courses for the reduction in both the density of 5-HT fluorescent varicosities and synaptosomal 5-HT uptake activity paralleled the time course for the development of denervation supersensitivity to 5-HT. During the first 2 days after 5,7-DHT, the enhanced sensitivity was selective for 5-HT; responses to D-lysergic acid diethylamide (LSD), norepinephrine (NE) and gamma-aminobutyric acid (GABA) were unchanged. Seven or more days after 5,7-DHT there was a marked increase of the responsiveness of neurons in the vLGN and the AMYG to both 5-HT and LSD (a 5-HT agonist which is not a substrate for the high affinity 5-HT uptake system). At these later times, the responsiveness of cells in the AMYG to NE and to a lesser extent GABA was also increased. In contrast to the marked supersensitivity seen after 5,7-DHT induced denervation, chronic administration of parachlorophenylalanine, a 5-HT synthesis inhibitor, failed to induce 5-HT supersensitivity.

Reevaluation of the indoleamine hypothesis of depression. Evidence for a reduction of functional activity of central 5-HT systems by antidepressant drugs

The effects of antidepressant drugs on central 5-HT receptor activity were studied in rats and mice. Antidepressant drugs were evaluated for their ability to displace 3H-5-HT and 3H-d-LSD from membrane binding sites in the dorsal neocortex of rats in vitro and for their ability to block 5-HTP and d-LSD induced behavioral effects in mice. The degree of blockade of head-twitches in mice produced by the antidepressants was highly correlated with their affinity for 3H-d-LSD binding sites. A number of antidepressant drugs such as amitriptyline, nortriptyline, mianserine, doxepine, nomifensine and dibenzepine appear to possess marked 5-HT receptor blocking activity at some type of 5-HT receptors in brain. New antidepressant drugs such as zimelidine, which specifically inhibit 5-HT reuptake and do not block 5-HT receptor sites, may after chronic treatment also reduce the functional activity of 5-HT systems by producing adaptive changes in postsynaptic 5-HT mechanisms. Thus, a new indoleamine hypothesis of depression is presented: the therapeutic action of antidepressant drugs may in part be due to a reduced functional acitivity of some central 5-HT systems.