Heterologous sensitization of adenylate cyclase activity by serotonin in the rat cerebral cortex

Developmental neurotoxicity of low dose diazinon exposure of neonatal rats: effects on serotonin systems in adolescence and adulthood

The developmental neurotoxicity of organophosphate pesticides targets serotonin (5HT) systems, which are involved in emotional and appetitive behaviors. We exposed neonatal rats to daily doses of diazinon on postnatal days 1-4, using doses (0.5 or 2mg/kg) spanning the threshold for barely-detectable cholinesterase inhibition. We then evaluated the effects on 5HT(1A) and 5HT(2) receptors, and on the 5HT transporter in cerebral cortical regions and the brainstem in adolescence through adulthood. Diazinon evoked a lasting deficit in 5HT(1A) receptors in males only, whereas it caused a small but significant increase in 5HT transporters in females; neither effect showed a significant regional selectivity. This pattern differed substantially from that seen in earlier work with another organophosphate, chlorpyrifos, which at pharmacodynamically similar doses spanning the threshold for cholinesterase inhibition, evoked a much more substantial, global upregulation of 5HT receptor expression; with chlorpyrifos, effects on receptors were seen in females, albeit to a lesser extent than in males, and were also regionally distinct. The effects of diazinon were nonmonotonic, showing larger alterations at the lower dose, likely reflecting positive trophic effects of cholinergic stimulation once the threshold for cholinesterase inhibition is exceeded. Our results reinforce the idea that different organophosphates have fundamentally distinct effects on the developmental trajectories of specific neurotransmitter systems, unrelated to their shared action as cholinesterase inhibitors. The effects on 5HT circuits expand the scope of behavioral endpoints that need to be considered in evaluating the developmental neurotoxicity of organophosphates.

Permanent, sex-selective effects of prenatal or adolescent nicotine exposure, separately or sequentially, in rat brain regions: indices of cholinergic and serotonergic synaptic function, cell signaling, and neural cell number and size at 6 months of age

Nicotine is a neuroteratogen that disrupts neurodevelopment and synaptic function, with vulnerability extending into adolescence. We assessed the permanence of effects in rats on indices of neural cell number and size, and on acetylcholine and serotonin (5HT) systems, conducting assessments at 6 months of age, after prenatal nicotine exposure, adolescent exposure, or sequential exposure in both periods. For prenatal nicotine, indices of cell number and size showed few abnormalities by 6 months, but there were persistent deficits in cerebrocortical choline acetyltransferase activity and hemicholinium-3 binding to the presynaptic choline transporter, a pattern consistent with cholinergic hypoactivity; these effects were more prominent in males than females. The expression of 5HT receptors also showed permanent effects in males, with suppression of the 5HT(1A) subtype and upregulation of 5HT(2) receptors. In addition, cell signaling through adenylyl cyclase showed heterologous uncoupling of neurotransmitter responses. Nicotine exposure in adolescence produced lasting effects that were similar to those of prenatal nicotine. However, when animals were exposed to prenatal nicotine and received nicotine subsequently in adolescence, the adverse effects then extended to females, whereas the net effect in males was similar to that of prenatal nicotine by itself. Our results indicate that prenatal or adolescent nicotine exposure evoke permanent changes in synaptic function that transcend the recovery of less-sensitive indices of structural damage; further, prenatal exposure sensitizes females to the subsequent adverse effects of adolescent nicotine, thus creating a population that may be especially vulnerable to the lasting behavioral consequences of nicotine intake in adolescence.

Lasting effects of nicotine treatment and withdrawal on serotonergic systems and cell signaling in rat brain regions: separate or sequential exposure during fetal development and adulthood

Neurodevelopmental vulnerability to nicotine extends from fetal stages through adolescence. The recently proposed "sensitization-homeostasis" model postulates that, even in adulthood, nicotine treatment permanently reprograms synaptic activity. We administered nicotine to rats throughout gestation or in adulthood (postnatal days PN90-107), using regimens that reproduce plasma levels in smokers, assessing effects on serotonin (5HT) receptors, the 5HT transporter and responses mediated through adenylyl cyclase (AC). Evaluations were then made on PN105, PN110, PN120 and PN180. Prenatal nicotine exposure elicited persistent suppression of 5HT1A receptors and upregulation of 5HT2 receptors, effects that were selective for males and that first emerged in young adulthood. In addition, AC activity was reduced and there was uncoupling of receptor-mediated responses. With nicotine exposure restricted to adulthood, there were few changes in 5HT synaptic proteins during treatment or in the first 2 weeks post-treatment, distinctly different from the robust alterations seen earlier with similar nicotine regimens given in adolescence. Nevertheless, there was long-term upregulation of the proteins in males at 6 months of age; females were unaffected. Exposure to prenatal nicotine followed by adult nicotine overcame the protection of females, so that they, too showed long-term effects not seen with either treatment alone; the effects in males were exacerbated in an additive manner. Our results indicate that the effects of nicotine during prenatal or adolescent stages are indeed distinct from the effects in adults, but that even adults show persistent changes after nicotine exposure, commensurate with the sensitization-homeostasis model. These effects may contribute to lifelong vulnerability to readdiction.

Prenatal nicotine exposure alters the responses to subsequent nicotine administration and withdrawal in adolescence: Serotonin receptors and cell signaling

Offspring of women who smoke during pregnancy are themselves more likely to take up smoking in adolescence, effects that are associated with a high rate of depression and increased sensitivity to withdrawal symptoms. To evaluate the biological basis for this relationship, we assessed effects on serotonin (5-hydroxytryptamine, 5HT) receptors and 5HT-mediated cellular responses in rats exposed to nicotine throughout prenatal development and then given nicotine in adolescence (postnatal days PN30-47.5), using regimens that reproduce plasma nicotine levels found in smokers. Evaluations were then made during the period of adolescent nicotine treatment and for up to one month after the end of treatment. Prenatal nicotine exposure, which elicits damage to 5HT projections in the cerebral cortex and striatum, produced sex-selective changes in the expression of 5HT(1A) and 5HT2 receptors, along with induction of adenylyl cyclase (AC), leading to sensitization of heterologous inputs operating through this signaling pathway. Superimposed on these effects, the AC response to 5HT was shifted toward inhibition. By itself, adolescent nicotine administration, which damages the same pathways, produced similar effects on receptors and the 5HT-mediated response, but a smaller overall induction of AC. Animals exposed to prenatal nicotine showed a reduced response to nicotine administered in adolescence, results in keeping with earlier findings of persistent desensitization. Our results indicate that prenatal nicotine exposure alters parameters of 5HT synaptic communication lasting into adolescence and changes the response to nicotine administration and withdrawal in adolescence, actions which may contribute to a subpopulation especially vulnerable to nicotine dependence.

Organophosphate insecticides target the serotonergic system in developing rat brain regions: disparate effects of diazinon and parathion at doses spanning the threshold for cholinesterase inhibition

BACKGROUND

In the developing brain, serotonin (5HT) systems are among the most sensitive to disruption by organophosphates.

OBJECTIVES

We exposed neonatal rats to daily doses of diazinon or parathion on postnatal days (PND)1-4 and evaluated 5HT receptors and the 5HT transporter in brainstem and forebrain on PND5, focusing on doses of each agent below the maximum tolerated dose and spanning the threshold for cholinesterase inhibition: 0.5, 1, or 2 mg/kg for diazinon, and 0.02, 0.05, and 0.1 mg/kg for parathion.

RESULTS

Diazinon evoked up-regulation of 5HT1A and 5HT2 receptor expression even at doses devoid of effects on cholinesterase activity, a pattern similar to that seen earlier for another organophosphate, chlorpyrifos. In contrast, parathion decreased 5HT1A receptors, again at doses below those required for effects on cholinesterase. The two agents also differed in their effects on the 5HT transporter. Diazinon evoked a decrease in the brainstem and an increase in the forebrain, again similar to that seen for chlorpyrifos; this pattern is typical of damage of nerve terminals and reactive sprouting. Parathion had smaller, nonsignificant effects.

CONCLUSIONS

Our results buttress the idea that, in the developing brain, the various organophosphates target specific neurotransmitter systems differently from each other and without the requirement for cholinesterase inhibition, their supposed common mechanism of action.

Alterations of serotonin synaptic proteins in brain regions of neonatal Rhesus monkeys exposed to perinatal environmental tobacco smoke

Serotonin (5HT) systems play important roles in brain development, and early perturbations of 5HT receptor expression produce permanent changes in 5HT synaptic function and associated behaviors. We exposed pregnant Rhesus monkeys to environmental tobacco smoke (ETS) during gestation and for up to 3 months postnatally and examined the expression of 5HT(1A) and 5HT(2) receptors, and of the presynaptic 5HT transporter in brain regions containing 5HT projections (frontal, temporal and occipital cortex) and cell bodies (midbrain). Perinatal ETS exposure elicited upregulation of 5HT(1A) receptor expression without parallel changes in the other two proteins, a pattern consistent with specific 5HT receptor dysregulation, rather than universal disruption of 5HT synaptic development. The effects seen here for ETS in a primate model are virtually identical in direction, magnitude and regional selectivity to those obtained previously for prenatal nicotine administration in rats. Specifically, early 5HT(1A) overexpression alters the program for future synaptic and behavioral 5HT responses, thus providing a mechanistic link for the shared effects of ETS and nicotine on a specific pathway responsible for behavioral anomalies associated with perinatal tobacco exposure. These results reinforce the need to reduce ETS exposure of pregnant women and young children.

Serotonergic cell signaling in an animal model of aging and depression: olfactory bulbectomy elicits different adaptations in brain regions of young adult vs aging rats

Aging involves neuronal and synaptic loss, and maintenance of function depends on adaptations in cellular responsiveness. We studied olfactory bulbectomy (OBX), a model that recapitulates monoaminergic dysfunction in depression, in 10-week vs 19-month-old rats, and evaluated 5HT (5-hydroxytryptamine, serotonin) mechanisms. OBX elicited little change in 5HT1A receptors in the cerebral cortex or striatum of either age group. In contrast, 5HT2 receptors showed disparate effects, with a decrease in the cerebral cortex of young OBX but not aging OBX rats, whereas the latter group showed a selective decrease in striatal 5HT2 receptors. Greater differences were apparent for 5HT-mediated cell signaling, assessed for the adenylyl cyclase (AC) cascade. In young animals, 5HT had a stimulatory effect on AC that was unaltered by OBX. However, in aging animals, the pattern of 5HT responses showed marked alterations in response to OBX: under basal conditions, stimulatory effects were enhanced but when AC was activated with forskolin, 5HT became markedly inhibitory in the striatum of aged OBX animals. Assessment of the relative AC responses to two direct stimulants that act on different epitopes of the enzyme, forskolin and Mn2+, pointed to a shift in the AC isoform and/or its ability to associate with G-proteins as the mechanism underlying the age-related differences for OBX effects. These data indicate that there are biological distinctions in the response of 5HT systems to OBX in young adult vs aging animals, which, if present in geriatric depression, could provide a mechanistic basis for differences in responses to antidepressants that act on 5HT.

5-hydroxytryptamine action in the rat olfactory bulb: In vitro electrophysiological patch-clamp recordings of juxtaglomerular and mitral cells

The olfactory bulb, first relay of olfactory pathways, is densely innervated by serotoninergic centrifugal fibers originating from the raphe nuclei. Although serotonin innervation was reported to be involved in olfactory learning in mammals, the action of this neurotransmitter on its putative cellular targets has been never described through unitary recordings. This lack of data initiated the present study where the effects of 5HT on juxtaglomerular and mitral cells are analyzed using whole-cell recordings on olfactory bulb slices. Serotonin depolarizes 34% of 525 JG cells. A multivariate statistical analysis of juxtaglomerular cells characteristics shows that the serotonin responsive cell group can be individualized regarding their tonic discharge-mode in response to a direct current injection, their lower expression of hyperpolarization-activated cation current and their low membrane capacities. The use of ion channel blockers and ramp voltage protocol indicate that serotoninergic depolarization of juxtaglomerular cells may be due to a nonselective cation current with a reversal potential of -44 mV. Pharmacological tests with serotonin receptor antagonists and agonists reveal that 5HT action on juxtaglomerular cells would be mainly mediated by 5HT2C receptors. In mitral cells, serotonin acts on 49.1% of the 242 tested cells, inducing two types of responses. A first subset of mitral cells (26.8%, n=65) were hyperpolarized by serotonin. This response would be indirect and mediated by action of GABA on GABAA receptors since it was antagonized by bicuculline. The involved GABAergic neurons are hypothesized to be juxtaglomerular and granular cells, on which serotonin would act mainly via 5HT2C and via 5HT2A receptors respectively. The second subset of mitral cells (22.3%, n=54) were directly depolarized by serotonin acting through 5HT2A receptors. Our data on serotonin action on juxtaglomerular cells and mitral cells reveal a part of functional mechanisms whereby serotonin can act on olfactory bulb network. This is expected to enrich the understanding of its determining role in olfactory learning.

Mitral cell beta1 and 5-HT2A receptor colocalization and cAMP coregulation: a new model of norepinephrine-induced learning in the olfactory bulb

In the present study we assess a new model for classical conditioning of odor preference learning in rat pups. In preference learning beta(1)-adrenoceptors activated by the locus coeruleus mediate the unconditioned stimulus, whereas olfactory nerve input mediates the conditioned stimulus, odor. Serotonin (5-HT) depletion prevents odor learning, with 5-HT(2A/2C) agonists correcting the deficit. Our new model proposes that the interaction of noradrenergic and serotonergic input with odor occurs in the mitral cells of the olfactory bulb through activation of cyclic adenosine monophosphate (cAMP). Here, using selective antibodies and immunofluorescence examined with confocal microscopy, we demonstrate that beta(1)-adrenoceptors and 5-HT(2A) receptors colocalize primarily on mitral cells. Using a cAMP assay and cAMP immunocytochemistry, we find that beta-adrenoceptor activation by isoproterenol, at learning-effective and higher doses, significantly increases bulbar cAMP, as does stroking. As predicted by our model, the cAMP increases are localized to mitral cells. 5-HT depletion of the olfactory bulb does not affect basal levels of cAMP but prevents isoproterenol-induced cAMP elevation. These results support the model. We suggest the mitral-cell cAMP cascade converges with a Ca(2+) pathway activated by odor to recruit CREB phosphorylation and memory-associated changes in the olfactory bulb. The dose-related increase in cAMP with isoproterenol implies a critical cAMP window because the highest dose of isoproterenol does not produce learning.

Adolescent nicotine administration alters serotonin receptors and cell signaling mediated through adenylyl cyclase

Nicotine is a neuroteratogen that targets synaptic function during critical developmental stages and recent studies indicate that CNS vulnerability extends into adolescence, the age at which smoking typically commences. We administered nicotine to adolescent rats via continuous minipump infusions from PN30 to PN47.5, using 6 mg/kg/day, a dose rate that replicates the plasma nicotine levels found in smokers, and examined 5HT receptors and related cell signaling during nicotine administration (PN45) and in the post-treatment period (PN50, 60, 75). Adolescent nicotine decreased 5HT(2) receptor binding in brain regions containing 5HT projections (hippocampus and cerebral cortex), with selectivity for females in the cerebral cortex; regions containing 5HT cell bodies showed either an increase (midbrain in males) or no change (brainstem). In contrast, there were no significant changes in 5HT(1A) receptors; however, the ability of the receptors to signal through adenylyl cyclase (AC) showed a switch from stimulatory to inhibitory effects in females during the post-treatment period. There were also transient alterations in AC responses to beta-adrenergic receptor stimulation, as well as pronounced induction of the AC response to the non-receptor-mediated stimulant, forskolin. Our results indicate that adolescent nicotine exposure alters the concentrations and functions of postsynaptic 5HT receptors in a manner commensurate with impaired 5HT synaptic function. The direction of change, emergence of defects after the cessation of nicotine administration, and sex-preference for effects in females, all support a relationship of impaired 5HT function to the higher incidence of depression seen in adolescent smokers.

Isoproterenol increases CREB phosphorylation and olfactory nerve-evoked potentials in normal and 5-HT-depleted olfactory bulbs in rat pups only at doses that produce odor preference learning

Norepinephrine (NE) and serotonin (5-HT) are important modulators of early odor preference learning. NE can act as an unconditioned stimulus (UCS), whereas 5-HT facilitates noradrenergic actions. In this study, we examined the phosphorylation of an important transcription factor, cAMP response element binding protein (CREB), which has been implicated in long-term-memory formation (McLean et al. 1999) during NE-induced odor preference learning in normal and olfactory bulb 5-HT-depleted rat pups. We also examined NE modulation of olfactory nerve-evoked field potentials (ON-EFPs) in anesthetized normal and bulbar 5-HT depleted pups. Systemic injection of 2 mg/kg isoproterenol (beta-adrenoceptor agonist) induced odor preference learning, enhanced pCREB expression in the olfactory bulbs at 10 min after odor pairing, and increased ON-EFPs in normal rat pups but not in bulbar 5-HT-depleted rat pups. A dose of 6 mg/kg isoproterenol, which was ineffective in modulating these measures in normal rat pups, induced odor preference learning, enhanced phosphorylated CREB (pCREB) expression, and increased ON-EFPs in bulbar 5-HT-depleted pups. These outcomes suggest that NE and 5-HT promote specific biochemical and electrophysiological changes that may critically underlie odor preference learning.

pCREB in the neonate rat olfactory bulb is selectively and transiently increased by odor preference-conditioned training

Early olfactory preference learning in rat pups occurs when novel odors are paired with tactile stimulation, for example stroking. cAMP-triggered phosphorylation of cAMP response element binding protein (pCREB) has been implicated as a mediator of learning and memory changes in various animals (Frank and Greenberg 1994). In the present study we investigate whether CREB is phosphorylated in response to conditioned olfactory training as might be predicted given the proposed role of the phosphorylated protein in learning. On postnatal day 6, pups were trained for 10 min using a standard conditioned olfactory learning paradigm in which a conditioned stimulus, Odor, was either used alone or paired with an unconditioned stimulus, Stroking (using a fine brush to stroke the pup). In some instances stroking only was used. The pups were sacrificed at 0, 10, 30, or 60 min after the training. Using Western blot analysis, we observed that the majority of olfactory bulbs in conditioned pups (Odor + Stroking) had a greater increase in pCREB activation at 10 min after training than pups given nonlearning training (Odor only or Stroking only). The phosphorylated protein levels were low at 0 min and at 60 min after training. This is in keeping with the slightly delayed and short-lived activation period for this protein. The localization of pCREB increases within the olfactory bulb as seen by immunocytochemistry. Naive pups were not exposed to odor or training. There was a significantly higher level of label in mitral cell nuclei within the dorsolateral quadrant of the bulb of pups undergoing odor-stroke pairing. No significant differences were observed among nonlearning groups (Naive, Odor only, or Stroking only) or among any training groups in the granule or periglomerular cells of the dorsolateral region. The localized changes in the nuclear protein are consistent with studies showing localized changes in the bulb in response to a learned familiar odor. The present study demonstrates that selective increases in pCREB occur as an early step following pairing procedures that normally lead to the development of long-term olfactory memories in rat pups. These results support the hypothesized link between pCREB and memory formation.

Physiological exposure to melatonin supersensitizes the cyclic adenosine 3',5'-monophosphate-dependent signal transduction cascade in Chinese hamster ovary cells expressing the human mt1 melatonin receptor

Here, we report the effects of short exposure to melatonin on the human mt1 (h mt1) melatonin receptor-mediated signaling in Chinese hamster ovary (CHO) cells, and the consequences of an exposure that resembles the physiological pattern of melatonin release on cAMP-mediated signal transduction. Short exposure (10 min) of h mt1 melatonin receptors to melatonin (400 pM) inhibited forskolin-stimulated cAMP formation, cAMP-dependent protein kinase activity, and phosphorylation of the cAMP response element-binding protein. However, treatment of mt1-CHO cells with melatonin in a manner that closely mimics the in vivo activation of melatonin receptors (i.e. 400 pM melatonin for 8 h to mimic darkness) resulted in a supersensitization of the cAMP-dependent signal transduction cascade during the period of withdrawal (i.e. 16 h without melatonin to mimic the light cycle of a diurnal photoperiod). During the period of withdrawal, forskolin induced a time-dependent (1-16 h) increase in cAMP formation (approximately 200% of control cells). This effect of melatonin was dependent on the presence of the h mt1 melatonin receptor, as no potentiation of forskolin-induced cAMP formation was observed in CHO cells transfected only with the neomycin resistance plasmid. The time-dependent increase in forskolin-stimulated cAMP levels resulted in a potentiation of cAMP-dependent protein kinase activity 1 h after withdrawal (approximately 130% of control cells; P < 0.05) and in the number of cells containing the phosphorylated form of cAMP response element-binding protein (approximately 75% of cells at 1 and 16 h compared with 30% in control cells; P < 0.05). An increase in the undissociated state (G alphabetagamma) of Gi proteins may underlie this phenomenon as demonstrated by the increase in pertussis toxin-catalyzed ADP-ribosylation of G proteins (217 +/- 48% of control; P < 0.05) after melatonin withdrawal. This increase in the ribosylation was not due to an up-regulation of Galpha(i) protein, as no significant change in Galpha(i) protein levels occurred at this time. We demonstrated that activation of the h mt1 melatonin receptor in a manner that resembles the physiological pattern of melatonin exposure alters signaling, as potentiation of cAMP-mediated signal transduction events is observed after hormone withdrawal. The CHO cells expressing the human melatonin receptor may provide an in vitro cellular model in which to investigate the putative signaling mechanisms leading to gene regulation by melatonin.

Influence of fluoxetine and p-chloroamphetamine on the somatostatin receptor-adenylyl cyclase system in the rat frontoparietal cortex

There is evidence that suggests a reciprocal functional link between the serotonergic and the somatostatinergic system in the rat frontoparietal cortex. However, to date, the role of endogenous 5-hydroxytryptamine (serotonin) on the regulation of the somatostatin (SS) receptor-adenylyl cyclase (AC) system remains unclear. In the present study, the administration of fluoxetine (10 mg/kg i.p.), a 5-hydroxytryptamine uptake inhibitor in a single dose or administered daily for 14 days increased the number of specific [125I]Tyr11-SS receptors, with no change in the receptor affinity, in rat frontoparietal cortical membranes. However, the capacity of SS to inhibit forskolin (FK)-stimulated AC activity in these membranes was lower than in the control groups. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortical membranes was also decreased in rats acutely and chronically treated with fluoxetine. p-Chloroamphetamine (5 mg/kg i.p.), which leads to a lasting reduction of 5-hydroxytryptamine innervation, administered on days 1, 3 and 5 and the rats sacrificed 1 or 3 weeks after the first injection, decreased the number of SS receptors without changing the receptor affinity. In this experimental group, SS also caused a significantly lower inhibition of FK-stimulated AC activity. p-Chloroamphetamine had no effect on the ability of Gpp(NH)p to inhibit FK-stimulated AC activity in frontoparietal cortical membranes at all the time periods studied. The present results suggest that under normal circumstances some SS receptors are under a tonic stimulatory control through the serotonergic system.

Modulation by 5-hydroxytryptamine of the somatostatin receptor-effector system and somatostatin levels in rat brain

The role of 5-hydroxytryptamine (5-HT) in the acute regulation of the rat brain somatostatin (SS) receptor-effector system and somatostatin-like immunoreactivity (SSLI) content was examined. 5-HT administered i.c.v. in a volume of 10 microliters at a dose of 0.5 microgram (pH 3.4) increased the SSLI concentration at 60 min in the Wistar rat frontoparietal cortex and hippocampus (60%, P < 0.05; 72%, P < 0.01; respectively). These changes were associated with a significant increase in the total number of specific SS receptors in the frontoparietal cortex (24%, P < 0.05) and hippocampus (20%, P < 0.05), without changes in the affinity constant as compared with the control group. No significant differences were seen in the basal and forskolin (FK)-stimulated adenylate cyclase (AC) activities in both brain areas of 5-HT-treated rats when compared to the control group. The capacity of SS to inhibit the FK-stimulated AC activity in the frontoparietal cortex and hippocampus of 5-HT-treated rats was lower than in the control groups. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortical and hippocampal membranes was markedly decreased in 5-HT-treated rats. To determine if the above-mentioned changes were related to the 5-HT activation of central 5-HT1 and 5-HT2 receptors, a non-selective 5-HT1 and 5-HT2 receptor antagonist, methysergide, was administered 60 min before the 5-HT injection. Pretreatment with methysergide (5 mg/kg i.p. in a volume of 400 microliters) prevented the 5-HT-induced changes in the SS receptor-effector system and in SSLI levels in both brain areas. Methysergide alone had no observable effect on the somatostatinergic system. These results suggest that the frontoparietal cortical and hippocampal somatostatinergic system can be regulated by 5-HT receptors.