Species differences in the distribution of central 5-HT1 binding sites: a comparative autoradiographic study between rat and guinea pig

Neuronal chemo-architecture of the entorhinal cortex: A comparative review

The identification of neuronal markers, that is, molecules selectively present in subsets of neurons, contributes to our understanding of brain areas and the networks within them. Specifically, recognizing the distribution of different neuronal markers facilitates the identification of borders between functionally distinct brain areas. Detailed knowledge about the localization and physiological significance of neuronal markers may also provide clues to generate new hypotheses concerning aspects of normal and abnormal brain functioning. Here, we provide a comprehensive review on the distribution within the entorhinal cortex of neuronal markers and the morphology of the neurons they reveal. Emphasis is on the comparative distribution of several markers, with a focus on, but not restricted to rodent, monkey and human data, allowing to infer connectional features, across species, associated with these markers, based on what is revealed by mainly rodent data. The overall conclusion from this review is that there is an emerging pattern in the distribution of neuronal markers in the entorhinal cortex when aligning data along a comparable coordinate system in various species.

The relationship between the claustrum and endopiriform nucleus: A perspective towards consensus on cross-species homology

With the emergence of interest in studying the claustrum, a recent special issue of the Journal of Comparative Neurology dedicated to the claustrum (Volume 525, Issue 6, pp. 1313-1513) brought to light questions concerning the relationship between the claustrum (CLA) and a region immediately ventral known as the endopiriform nucleus (En). These structures have been identified as separate entities in rodents but appear as a single continuous structure in primates. During the recent Society for Claustrum Research meeting, a panel of experts presented data pertaining to the relationship of these regions and held a discussion on whether the CLA and En should be considered (a) separate unrelated structures, (b) separate nuclei within the same formation, or (c) subregions of a continuous structure. This review article summarizes that discussion, presenting comparisons of the cytoarchitecture, neurochemical profiles, genetic markers, and anatomical connectivity of the CLA and En across several mammalian species. In rodents, we conclude that the CLA and the dorsal endopiriform nucleus (DEn) are subregions of a larger complex, which likely performs analogous computations and exert similar effects on their respective cortical targets (e.g., sensorimotor versus limbic). Moving forward, we recommend that the field retain the nomenclature currently employed for this region but should continue to examine the delineation of these structures across different species. Using thorough descriptions of a variety of anatomical features, this review offers a clear definition of the CLA and En in rodents, which provides a framework for identifying homologous structures in primates.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Non-additive modulation of synaptic transmission by serotonin, adenosine, and cholinergic modulators in the sensory thalamus

The thalamus relays sensory information to the cortex. Oscillatory activities of the thalamocortical network are modulated by monoamines, acetylcholine, and adenosine, and could be the key features characteristic of different vigilance states. Although the thalamus is almost always subject to the actions of more than just one neuromodulators, reports on the modulatory effect of coexisting neuromodulators on thalamic synaptic transmission are unexpectedly scarce. We found that, if present alone, monoamine or adenosine decreases retinothalamic synaptic strength and short-term depression, whereas cholinergic modulators generally enhance postsynaptic response to presynaptic activity. However, coexistence of different modulators tends to produce non-additive effect, not predictable based on the action of individual modulators. Acetylcholine, acting via nicotinic receptors, can interact with either serotonin or adenosine to abolish most short-term synaptic depression. Moreover, the coexistence of adenosine and monoamine, with or without acetylcholine, results in robustly decreased synaptic strength and transforms short-term synaptic depression to facilitation. These findings are consistent with a view that acetylcholine is essential for an "enriched" sensory flow through the thalamus, and the flow is trimmed down by concomitant monoamine or adenosine (presumably for the wakefulness and rapid-eye movement, or REM, sleep states, respectively). In contrast, concomitant adenosine and monoamine would lead to a markedly "deprived" (and high-pass filtered) sensory flow, and thus the dramatic decrease of monoamine may constitute the basic demarcation between non-REM and REM sleep. The collective actions of different neuromodulators on thalamic synaptic transmission thus could be indispensable for the understanding of network responsiveness in different vigilance states.

5-HT excites globus pallidus neurons by multiple receptor mechanisms

Anatomical and neurochemical studies indicated that the globus pallidus receives serotonergic innervation from raphe nuclei but the membrane effects of 5-HT on globus pallidus neurons are not entirely clear. We address this question by applying whole-cell patch-clamp recordings on globus pallidus neurons in immature rat brain slices. Under current-clamp recording, 5-HT depolarized globus pallidus neurons and increased their firing rate, an action blocked by both 5-HT(4) and 5-HT(7) receptor antagonists and attributable to an increase in cation conductance(s). Further experiments indicated that 5-HT enhanced the hyperpolarization-activated inward conductance which is blocked by 5-HT(7) receptor antagonist. To determine if 5-HT exerts any presynaptic effects on GABAergic and glutamatergic inputs, the actions of 5-HT on synaptic currents were studied. At 10 microM, 5-HT increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) but had no effect on both the frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs). However, 5-HT at a higher concentration (50 microM) decreased the frequency but not the amplitude of the mIPSCs, indicating an inhibition of GABA release from the presynaptic terminals. This effect was sensitive to 5-HT(1B) receptor antagonist. In addition to the presynaptic effects on GABAergic neurotransmission, 5-HT at 50 microM had no consistent effects on glutamatergic neurotransmission, significantly increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in 4 of 11 neurons and decreased the frequency of mEPSCs in 3 of 11 neurons. In conclusion, we found that 5-HT could modulate the excitability of globus pallidus neurons by both pre- and post-synaptic mechanisms. In view of the extensive innervation by globus pallidus neurons on other basal ganglia nuclei, this action of 5-HT originated from the raphe may have a profound effect on the operation of the entire basal ganglia network.

Chronic lithium administration to rats selectively modifies 5-HT2A/2C receptor-mediated brain signaling via arachidonic acid

The effects of chronic lithium administration on regional brain incorporation coefficients k* of arachidonic acid (AA), a marker of phospholipase A2 (PLA2) activation, were determined in unanesthetized rats administered i.p. saline or 1 mg/kg i.p. (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), a 5-HT2A/2C receptor agonist. After injecting [1-(14)C]AA intravenously, k* (brain radioactivity/integrated plasma radioactivity) was measured in each of 94 brain regions by quantitative autoradiography. Studies were performed in rats fed a LiCl or a control diet for 6 weeks. In the control diet rats, DOI significantly increased k* in widespread brain areas containing 5-HT2A/2C receptors. In the LiCl-fed rats, the significant positive k* response to DOI did not differ from that in control diet rats in most brain regions, except in auditory and visual areas, where the response was absent. LiCl did not change the head turning response to DOI seen in control rats. In summary, LiCl feeding blocked PLA2-mediated signal involving AA in response to DOI in visual and auditory regions, but not generally elsewhere. These selective effects may be related to lithium's therapeutic efficacy in patients with bipolar disorder, particularly its ability to ameliorate hallucinations in that disease.

Developmental regulation of 5-HT1A receptor mRNA in the fetal limbic system: response to antenatal glucocorticoid

The developmental changes in 5-HT1A receptor mRNA expression associated with advancing gestational age were examined in the fetal guinea pig hippocampus and dentate gyrus (DG) by in situ hybridization. We found that 5-HT1A receptor mRNA was present in the hippocampal CA1 subfield and dentate gyrus (DG), and was significantly (P < 0.05) elevated in the DG during the period of rapid brain growth [gestational day (gd) 50; term = 70 days]. Glucocorticoids have been shown to alter 5-HT1A receptor mRNA expression in the adult, but nothing is known about their impact on the developing fetal brain. Expression of 5-HT1A receptor mRNA in the fetal hippocampus was measured following repeated maternal administration (gd40, 41, 50, 51, 60 and 61) of synthetic glucocorticoid (dexamethasone; 1 and 10 mg/kg). Levels of 5-HT1A receptor mRNA were significantly (P < 0.005) elevated in CA1 and DG following repeated exposure to high-dose glucocorticoid (10 mg/kg) in male, but not in female fetuses. Because fetal exposure to glucocorticoids programs hypothalamo-pituitary-adrenal (HPA) function, and hippocampal serotonin is known to influence glucocorticoid receptor (GR) expression, the glucocorticoid-mediated changes in 5-HT1A receptor mRNA may play a role in the programming of HPA function.

Effects of early environment on field CA3a pyramidal neuron morphology in the guinea-pig

There is evidence that early environmental conditions have profound effects on the morphology of the dentate granule cells. The aim of the present study was to obtain information about the effects of early environment on neuron morphology in the hippocampal field CA3, a structure closely linked to the dentate gyrus. The dendritic trees and the somata of field CA3a pyramidal neurons were quantified in Golgi-stained brains of guinea-pigs of both sexes raised in either a social or an isolated environment. Two pyramidal neuron types were found in CA3a, characterized by either a long or a short shaft. Environment affected the apical tree of the long-shaft neurons only in males and that of the short-shaft neurons in both sexes. In isolated males the long-shaft neurons had a decrease in the number of dendritic intersections (62-82%), branching points (76%) and length (71%) in the middle third of the apical tree. The short-shaft neurons had a decrease in the number of intersections at two distal levels only in both isolated males (26, 83%) and females (77, 82%). The shaft spine density was affected by environment in the long-shaft neurons of males only, with a density increase (110%) in isolated males. In both sexes the basal tree of only the long-shaft neurons was affected by environment. Isolated males had a decrease in the number of dendritic intersections (65-88%), primary dendrites (80%) and dendritic length (88%) and isolated females had a decrease in the number of intersections (51-89%), branching points (77%) and dendritic length (85%). The soma major axis of only the long-shaft neurons was affected by environment with a reduction in isolated males (90%) but an increase in isolated females (111%). These results demonstrate dendritic atrophy of CA3a pyramidal neurons following early isolation and a different reactivity to environment of the two CA3a pyramidal neuron types, their apical and basal trees and the two sexes. The dendritic atrophy of CA3a neurons caused by isolation is likely to be associated with an impairment in the physiology of the hippocampal formation and in the forms of memory in which the hippocampal formation plays a major role.

Elevation of guinea pig brain preprodynorphin mRNA expression and hypothalamic-pituitary-adrenal axis activity by "binge" pattern cocaine administration

The endogenous opioid system and the hypothalamic-pituitary-adrenal (HPA) axis have been implicated in many of the neurobiological effects of cocaine. Previous studies in our laboratory showed that "binge" pattern cocaine administration increases preprodynorphin (ppDyn) mRNA levels in the caudate putamen and circulating levels of corticosterone in the rat. The present study extended these findings to guinea pigs, a species known to have a kappa opioid receptor profile similar to that of humans. Male guinea pigs were treated with: (a) "binge" pattern cocaine for 7 days (subchronic) (3 x 15 mg/kg/day, hourly, intraperitoneal); (b) "binge" pattern saline for 5 days followed by "binge" pattern cocaine for 2 days (subacute); or (c) "binge" pattern saline for 7 days. Thirty minutes after the final injection, levels of ppDyn mRNA were quantitated in the nucleus accumbens, caudate putamen, frontal cortex, amygdala, hippocampus, and hypothalamus using a solution hybridization RNase protection assay. Regional distribution of ppDyn mRNA levels in the guinea pig brain was similar to that found in rat, with highest levels in the nucleus accumbens and caudate putamen. In the caudate putamen, ppDyn mRNA was significantly increased following either 2 days (38% increase) or 7 days (32% increase) of "binge" pattern cocaine administration as compared to saline-treated controls. No significant changes in ppDyn mRNA levels were found in any other brain region. Both subacute and subchronic "binge" cocaine administration significantly elevated plasma levels of adrenocorticotropin hormone (ACTH) and cortisol. However, the ACTH and cortisol increases were significantly blunted following 7 days of "binge" cocaine administration as compared to 2 days of drug treatment, reflecting the development of HPA tolerance or adaptation to repeated cocaine administration. Thus, the ppDyn mRNA and HPA responses to cocaine in guinea pigs are similar to those observed in rats.

5-HT(1A) receptor agonist-antagonist binding affinity difference as a measure of intrinsic activity in recombinant and native tissue systems

1. It has been reported that radiolabelled agonist : antagonist binding affinity ratios can predict functional efficacy at several different receptors. This study investigates whether this prediction is true for recombinant and native tissue 5-HT(1A) receptors. 2. Saturation studies using [(3)H]-8-OH-DPAT and [(3)H]-MPPF revealed a single, high affinity site (K(D)approximately 1 nM) in HEK293 cells expressing human 5-HT(1A) receptors and rat cortex. In recombinant cells, [(3)H]-MPPF labelled 3 - 4 fold more sites than [(3)H]-8-OH-DPAT suggesting the presence of more than one affinity state of the receptor. [(3)H]-Spiperone labelled a single, lower affinity site in HEK293 cells expressing h5-HT(1A) receptors but did not bind to native tissue 5-HT(1A) receptors. These data suggest that, in transfected HEK293 cells, human 5-HT(1A) receptors exist in different affinity states but in native rat cortical tissue the majority of receptors appear to exist in the high agonist affinity state. 3. Receptor agonists inhibited [(3)H]-MPPF binding from recombinant 5-HT(1A) receptors in a biphasic manner, whereas antagonists and partial agonists gave monophasic inhibition curves. All compounds displaced [(3)H]-8-OH-DPAT and [(3)H]-spiperone binding in a monophasic manner. In rat cortex, all compounds displaced [(3)H]-MPPF and [(3)H]-8-OH-DPAT in a monophasic manner. 4. Functional evaluation of compounds, using [(35)S]-GTPgammaS binding, produced a range of intrinsic activities from full agonism, displayed by 5-HT and 5-CT to inverse agonism displayed by spiperone. 5. [(3)H]-8-OH-DPAT : [(3)H]-MPPF pK(i) difference correlated well with functional intrinsic activity (r=0.86) as did [(3)H]-8-OH-DPAT : [(3)H]-spiperone pK(i) difference with functional intrinsic activity (r=0.96). 6. Thus agonist : antagonist binding affinity differences may be used to predict functional efficacy at human 5-HT(1A) receptors expressed in HEK293 cells where both high and low agonist affinity states are present but not at native rat cortical 5-HT(1A) receptors in which only the high agonist affinity state was detectable.

Region-specific changes in 5-HT(1A) receptor-activated G-proteins in rat brain following chronic buspirone

5-Hydroxytryptamine(1A) (5-HT(1A)) receptors, which activate inhibitory G-proteins, are implicated in psychiatric disorders including anxiety and depression. Studies suggest that chronic 5-HT(1A) receptor agonist administration alters 5-HT(1A) receptor function, but the effect of chronic treatment on 5-HT(1A) receptor-activated G-proteins is unclear. In this study, agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate (GTPgammaS) binding was examined following chronic administration of buspirone. Brains were processed for [35S]GTPgammaS autoradiography using R(+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) for 5-HT(1A) receptors or baclofen for GABA(B) receptors. Net 8-OH-DPAT-stimulated [35S]GTPgammaS binding was decreased by 25-30% in the septum and dorsal raphe nucleus of buspirone-treated animals. No significant changes in 8-OH-DPAT-stimulated [35S]GTPgammaS binding were found in the prefrontal, entorhinal or cingulate cortices or hippocampus in buspirone-treated rats. GABA(B) receptor-stimulated [35S]GTPgammaS binding was increased by 25% in the hippocampus, with no significant changes in any other region examined. These results demonstrate region-specific alterations in 5-HT(1A) and GABA(B) receptor-activated G-proteins following chronic buspirone treatment, which may contribute to the clinical effects of this drug.

The 5HT(1A) receptor ligand, S15535, antagonises G-protein activation: a [35S]GTPgammaS and [3H]S15535 autoradiography study

4-(Benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) is a highly selective ligand at 5-HT(1A) receptors. The present study compared its autoradiographic labelling of rat brain sections with its functional actions, visualised by guanylyl-5'-[gamma-thio]-triphosphate ([35S]GTPgammaS) autoradiography, which affords a measure of G-protein activation. [3H]S15535 binding was highest in hippocampus, frontal cortex, entorhinal cortex, lateral septum, interpeduncular nucleus and dorsal raphe, consistent with specific labelling of 5-HT(1A) receptors. In functional studies, S15535 (10 microM) did not markedly stimulate G-protein activation in any brain region, but abolished the activation induced by the selective 5-HT(1A) agonist, (+)-8-hydroxy-dipropyl-aminotetralin ((+)-8-OH-DPAT, 1 microM), in structures enriched in [3H]S15535 labelling. S15535 did not block 5-HT-stimulated activation in caudate nucleus or substantia nigra, regions where (+)-8-OH-DPAT was ineffective and [3H]S15535 binding was absent. Interestingly, S15535 attenuated (+)-8-OH-DPAT and 5-HT-stimulated G-protein activation in dorsal raphe, a region in which S15535 is known to exhibit agonist properties in vivo [Lejeune, F., Millan, M.J., 1998. Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)(1A) receptors: WAY100,635-reversible actions of the highly selective ligands, flesinoxan and S15535. Synapse 30, 172-180.]. The present data show that (i) [3H]S15535 labels pre- and post-synaptic populations of 5-HT(1A) sites in rat brain sections, (ii) S15535 exhibits antagonist properties at post-synaptic 5-HT(1A) receptors in corticolimbic regions, and (iii) S15535 also attenuates agonist-stimulated G-protein activation at raphe-localised 5-HT(1A) receptors.

Magnitude of 5-HT1B and 5-HT1A receptor activation in guinea-pig and rat brain: evidence from sumatriptan dimer-mediated [35S]GTPgammaS binding responses

The present study reports on G-protein activation by recombinant 5-HT receptors and by native 5-HT1A and 5-HT1B receptors in guinea-pig and rat brain using agonist-stimulated [35S]GTPgammaS binding responses mediated by a new 5-HT ligand, a dimer of sumatriptan. Dimerization of sumatriptan increased the binding affinity for h 5-HT1B (pKi: 9.22 vs. 7.79 for sumatriptan), h 5-HT1D (9.07 vs. 8.08) and also h 5-HT1A receptors (7.80 vs. 6.40), while the binding affinity for h 5-ht1E (6.67 vs. 6.19) and h 5-ht1F (7.37 vs. 7.78) receptors was not affected. Sumatriptan dimer (10 microM) stimulated [35S]GTPgammaS binding mainly in the superficial gray layer of the superior colliculi, hippocampus and substantia nigra of guinea-pig and rat coronal brain sections. This fits with the labelling by the 5-HT1B/1D receptor antagonist [3H] GR 125743. The observed [35S]GTPgammaS binding responses in the substantia nigra are likely to be mediated by stimulation of the 5-HT1B receptor subtype, since they were antagonized by the 5-HT1B inverse agonist SB 224289 (10 microM), and not by the 5-HT2A/1D antagonist ketanserin (10 microM). Quantitative assessment of the [35S]GTPgammaS binding responses in the substantia nigra of rat showed highly efficacious responses for both sumatriptan dimer and its monomer. In contrast, less efficacious agonist responses (51+/-10% and 35+/-13%, respectively) were measured in the guinea-pig substantia nigra. This may suggest that the G-protein coupling efficacy of 5-HT1B receptors is different between the substantia nigra of both species. In addition, the sumatriptan dimer also activated guinea-pig and rat hippocampal 5-HT1A receptors with high efficacy in contrast to sumatriptan. Therefore, dimerization of sumatriptan can be considered as a new approach to transform a partial 5-HT1A agonist into a more efficacious agonist. In conclusion, the sumatriptan dimer stimulates G-protein activation via 5-HT1B receptors besides 5-HT1A receptors in guinea-pig and rat brain. The magnitude of the 5-HT1B receptor responses is superior for sumatriptan and its dimer in rat compared to guinea-pig substantia nigra.

G-protein activation at 5-HT1A receptors by the 5-ht1F ligand LY334370 in guinea-pig brain sections and recombinant cell lines

1. G-protein activation by the 5-ht1F receptor agonist 5-(4-fluorobenzoyl)amino-3-(1-methylpiperidin-4-yl)-1H-indole fumarate (LY334370) was investigated by use of autoradiography of receptor-activated G-proteins in guinea-pig brain sections and [35S]-GTPgammaS binding responses in cell lines stably expressing human 5-HT1A (h 5-HT1A) receptors. 2. LY334370 (10 microM) caused little or no stimulation of [35S]-GTPgammaS binding in guinea-pig brain regions enriched in 5-ht1F binding sites (e.g., claustrum, caudate/putamen and thalamic nuclei), as identified by labelling with 10 nM [3H]-sumatriptan plus 10 nM 5-carboxamidotryptamine (5-CT). 3. Application of LY334370 (10 microM) to guinea-pig brain sections resulted in an increase of [35S]-GTPgammaS binding in hippocampus (123+/-17%), lateral septum (58+/-14%), dorsal raphe (57+/-10%), entorhinal (37+/-11%) and cingulate cortex (28+/-10%). This distribution fits with the G-protein activation mediated by 5-HT1A receptors as found with lisuride (10 microM), and labelling of 5-HT1A receptors by 140 pM [125I]-4-(2'-methoxy-phenyl)- -[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI). 4. The LY334370-mediated [35S]-GTPgammaS response was antagonized by the selective, silent 5-HT1A receptor antagonist N-[2-[4-(2-methoxyphenyl)1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohex anecarboxa-mide (WAY100635, 1 microM) in each of the brain structures investigated. The distribution pattern of the [35S]-GTPgammaS binding response and the antagonist profile suggest that the LY334370-induced response in guinea-pig brain is mediated by 5-HT1A receptors. 5. The maximal LY334370-induced [35S]-GTPgammaS binding response (83 to 94%) in membranes of recombinant C6-glial/h 5-HT1A and HeLa/h 5-HT1A cells was close to that of 5-HT, suggesting LY334370 to exert high intrinsic activity at h 5-HT1A receptors. 6. In conclusion, in guinea-pig brain sections and recombinant cell lines the 5-ht1F receptor agonist LY334370 causes G-protein activation that is mediated by 5-HT1A receptors. Caution should be taken when employing this ligand as a putative selective 5-ht1F agonist.

Imipramine increases the 5-HT1A receptor-mediated inhibition of hippocampal neurons without changing the 5-HT1A receptor binding

The effect of repeated treatment with imipramine on the 5-HT1A receptor-mediated inhibition of a population spike was studied in the rat CA1 hippocampal region ex vivo. Serotonin (5-hydroxytryptamine, 5-HT) and the selective 5-HT1A receptor agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) decreased dose-dependently the amplitude of population spikes; this effect was blocked by the selective 5-HT1A receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpro panamide dihydrochloride [(S)-WAY 100135]. Repeated (14 days, twice daily), but not single, administration of imipramine (10 mg/kg) shifted the dose-response curves for serotonin and 8-OH-DPAT to the left. Repeated treatment with imipramine did not change the density of 5-HT1A receptors in the hippocampus as measured by autoradiography using [3H]8-OH-DPAT as a ligand. The latter findings indicate that the imipramine-induced increase in the responsiveness of hippocampal neurons to stimulation of 5-HT1A receptors may not involve an increase in the density of this receptor subtype. To find out whether the efficacy of the postreceptor transduction mechanism is changed by repeated treatment with imipramine, we examined the effect of baclofen. The baclofen-induced inhibition of the population spike was not changed by imipramine. Our results suggest that repeated treatment with imipramine induces sensitization to the inhibitory effects of 5-HT1A receptor agonists in the hippocampus.

Autoradiographic localization of 5-CT-insensitive 5-HT1-like recognition sites in guinea pig and rat brain

Quantitative autoradiographic studies, with [3H]5-HT, were used to investigate the distribution of 5-CT-insensitive 5-HT1-like (5-HT1E/1F) recognition sites in rat and guinea pig brain. For comparison and control purposes the distribution of the closely related 5-HT1D binding site, which is abundant in the guinea pig but not the rat, was also investigated, as well as total specific [3H]5-HT binding. Results from this study confirm the previously described regional distribution of the 5-HT1D binding site and also revealed a predominance of 5-CT-insensitive 5-HT1-like 5-HT1E/1F) recognition sites in the olfactory tubercle, caudate putamen, nucleus accumbens and substantia nigra of both species. Interestingly 5-CT-insensitive 5-HT1-like (5-HT1E/1F) recognition sites were particularly dense in the claustrum of the guinea pig, but not the rat.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

In vivo binding of [123I]4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)- p-iodobenzamido-]ethyl-piperazine, p-MPPI, to 5-HT1A receptors in rat brain

The in vivo regional distribution and pharmacological profile of a novel iodinated phenylpiperazine derivative, [123I]p-MPPI (4-(2'-methoxy-)phenyl-1-[2'-(N-2"pyridinyl)-p-iodobenzamido-]ethy l- piperazine), in the rat brain were evaluated for use as a potential in vivo imaging agent for 5-HT1A receptors. The new ligand penetrated the blood-brain barrier quickly and efficiently, with 1.2% of the injected dose found in the whole brain at 2 min post i.v. injection. The rate of radioactivity washout was slowest from the hippocampal region, followed by the hypothalamus, cortex, striatum, and cerebellum. The maximum ratio of hippocampus/cerebellum was 3.3 at 30 min postinjection. The specific binding of the radioligand in the hippocampal region, an area rich in 5-HT1A receptor density, was blocked by pretreatment with a dose of (+/-) 8-OH-DPAT (2 mg/kg, i.v.) or WAY 100635 (1 mg/kg, i.v.), whereas the regional distribution of [123I]p-MPPI was unaffected by pretreatment with non-5-HT1A agents such as ketanserin or haloperidol. Ex vivo autoradiographic studies further confirmed that the specific binding of [125I]p-MPPI is associated with 5-HT1A receptor sites. These results indicate that [123I]p-MPPI may be a useful candidate for noninvasive single photon emission computed tomography (SPECT) imaging of 5-HT1A receptor sites in the living human brain.

The effect of 5-HT1A receptor agonists on locomotor activity in the guinea-pig

1. The present study examined the effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, ipsapirone and buspirone, all agonists at the 5-HT1A receptor, on the locomotor activity of guinea-pigs. The effects of these drugs were contrasted with those of the non-selective 5-HT agonist, 5-methoxy-N,N-dimethyl tryptamine (5-MeO-DMT) and the dopamine D2 antagonist, raclopride. 2. 8-OH-DPAT, flesinoxan and 5-MeO-DMT markedly increased the locomotor activity of naive, unhabituated guinea-pigs in a dose-dependent manner. Buspirone also did so, although to a lesser extent and for a shorter time. The doses at which this effect was seen were higher than those normally employed in rats. Ipsapirone and raclopride had no significant effects on locomotor activity. 3. The locomotor activity increasing effect of 1.0 mg kg-1 8-OH-DPAT was blocked by the selective 5-HT1A antagonist (S)-UH-301 (3.0 and 10.0 mg kg-1), but not by (-)-alprenolol (15.0 mg kg-1). Ipsapirone (30.0 mg kg-1) and raclopride (3.0 mg kg-1) antagonized 8-OH-DPAT-induced locomotor activity but only to a small extent. The 5-HT reuptake inhibitor, zimelidine (10.0 mg kg-1) had no effect. 4. The effect of the 5-HT1A agonists in the guinea-pig contrasts with the effects of 8-OH-DPAT on the locomotor activity of unhabituated rats and mice tested in the same apparatus, but are similar to the effects of 8-OH-DPAT on habituated rats, which show a low baseline of activity. 5. These results support the suggestion that 5-HTIA agonists may have an intrinsic activating effect which may be masked by other effects of the drug (e.g. hypothermia, 5-HT syndrome). The rank ordering of the 5-HTIA agonists also suggests that the degree to which the drugs increase locomotor activity is related to their agonist efficacy at the postsynaptic 5-HTIA receptor.

Serotonin and serotonin receptors in the central auditory system

Immunohistochemical and ligand-binding techniques were used to visualize the neurotransmitter serotonin and one of its receptors, the 5-HT1A subtype, in auditory nuclei of the brainstem. Serotonergic fibers and terminal endings were found in all auditory nuclei extending from the cochlear nucleus to the inferior colliculus, including the superior olivary complex and the nuclei of the lateral lemniscus. The density of the innervation varied between and within each nucleus. All serotonergic cell bodies were located outside the auditory nuclei. The 5-HT1A receptor subtype was found in the cochlear nucleus as well as in the inferior colliculus. With no serotonergic cell bodies present in the auditory nuclei, the present neuroanatomic and neurochemical findings support behavioral and neurophysiologic findings that the serotonergic system may modulate central auditory processing.

A comparative autoradiographic study of 5-HT1D binding sites in human and guinea-pig brain using different radioligands

Quantitative receptor autoradiography was used to examine the 5-hydroxytryptamine (5-HT, serotonin) binding sites labelled with serotonin-5-O-carboxymethyl-glycyl-[125I]tyrosinamide ([125I]GTI) in human and guinea-pig brain. Competition experiments using 5-carboxamidotryptamine (5-CT), 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129) and sumatriptan revealed monophasic displacement curves in various brain regions, suggesting that a homogeneous population of 5-HT1D binding sites was labelled. Displacement of [3H]5-HT (in the presence of 100 nM 8-hydroxy-2(N-dipropylamino)tetralin (8-OH-DPAT) and 100 nM mesulergine) with unlabelled GTI resulted in monophasic competition curves in substantia nigra, globus pallidus and central gray. In contrast, biphasic displacement was observed in hippocampus, nucleus accumbens, claustrum, caudate-putamen and frontal cortex. The distribution of [125I]GTI sites was compared to that of [3H]5-HT binding sites (under so-called '5-HT1D conditions', i.e. in the presence of 100 nM 8-OH-DPAT and 100 nM mesulergine, in order to block 5-HT1A and 5-HT1C sites, respectively) in human and guinea-pig brain. Qualitative analysis revealed differences in the distributions of [125I]GTI and [3H]5-HT binding sites. Regions such as CA3 and CA4 of the hippocampus, claustrum and putamen showed [3H]5-HT binding (under '5-HT1D conditions') but no [125I]GTI binding sites, indicating that [3H]5-HT labels besides a GTI sensitive (5-HT1D) receptor population, a non-5-HT1A/1B/1C/1D [3H]5-HT binding site in human and guinea-pig brain. The distribution of these non-5-HT1A/1B/1C/1D [3H]5-HT binding sites was studied with [3H]5-HT under conditions where 5-HT1A, 5-HT1C and 5-HT1D [3H]5-HT binding sites were saturated by the presence of 100 nM 8-OH-DPAT, 100 nM mesulergine and 1 microM GTI. Significant densities of these non-5-HT1A/1B/1C/1D sites were observed in cortical areas, hippocampal structures, nucleus accumbens, amygdala, caudate-putamen and claustrum. It is concluded that [125I]GTI does not label the 5-HT1E binding site, since all competition curves obtained with this radioligand were monophasic. By contrast, [3H]5-HT labels non-5-HT1A/1B/1C/1D [3H]5-HT binding sites, but it remains to be established whether these sites represent a single receptor population.

Autoradiographic characterisation and localisation of 5-HT1D compared to 5-HT1B binding sites in rat brain

The regional distribution and the pharmacology of the binding sites labelled with the novel 5-hydroxytryptamine (serotonin) 5-HT1B/1D selective radioligand serotonin-O-carboxy-methyl-glycyl-[125I]tyrosinamide (abbreviated [125I]GTI for the sake of simplicity) was determined using quantitative autoradiography in rat brain. The distribution of [125I]GTI binding sites was largely comparable to that of [125I]iodocyanopindolol ([125I] ICYP) which labels 5-HT1B binding sites (in the presence of 8-OH-DPAT (8-hydroxy-[2N-dipropylamino]tetralin) and isoprenaline, to prevent binding to 5-HT1A and beta-adrenoceptor binding sites), although a detailed analysis revealed differences. The pharmacology of the [125I]GTI binding sites was analysed using compounds known to display high affinity for and/or distinguish between 5-HT1B and 5-HT1D sites: 5-carboxamidotryptamine (5-CT), sumatriptan, CP 93129 (5-hydroxy-3(4-1,2,5,6-tetrahydropyridyl)-4-azaindole), (-)pindolol, PAPP (4[2-[4-[3-(trifluoromethyl)phenyl]-1- piperazinyl]ethyl]benzeneamine), rauwolscine, and 8-OH-DPAT. The displacement of [125I]GTI by 5-CT was monophasic. By contrast, the selective 5-HT1B compound CP 93129 and (-)pindolol produced biphasic curves showing a majority of high affinity sites in the globus pallidus and the substantia nigra, whereas PAPP and sumatriptan (which are somewhat 5-HT1D selective) produced biphasic curves indicating a minority of high affinity sites in these areas. In addition, by blocking the 5-HT1B sites with 100 nM CP 93129, the remaining population of [125I]GTI binding sites could be studied and was found to have high affinity for PAPP, rauwolscine and 8-OH-DPAT. The pharmacological profile of the major binding component was typical of the 5-HT1B type: 5-CT > CP 93129 > or = (-)pindolol > sumatriptan > or = PAPP > rauwolscine. The profile of the minor component of [125I]GTI binding is best characterised as that of a 5-HT1D site: 5-CT > PAPP > or = sumatriptan > rauwolscine > (-)pindolol > or = CP 93129. The localisation of the non 5-HT1B [125I]GTI binding sites was characterised by blocking the 5-HT1B receptors with 100 nM CP 93129. Low densities of the 5-HT1D recognition sites were found to be present in globus pallidus, ventral pallidum, caudate-putamen, subthalamic nucleus, entopeduncular nucleus, substantia nigra (reticular part), nuclei of the (normal and accessory) optic tract, different nuclei of the geniculate body and frontoparietal cortex, although higher densities of 5-HT1B sites were always observed in the same structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Epileptiform activity induced by 4-aminopyridine in guinea-pig and rat neocortices

Extracellular field recordings were performed in guinea-pig and rat neocortical slice preparations maintained in vitro. Bath application of the convulsant drug 4-aminopyridine (4-AP, 100 microM) induced spontaneous epileptiform potentials in 80% of the guinea-pig neocortical slices and only in 6% of the neocortical slices from rat. In both species spontaneous epileptiform activity consisted of a 4-16 s long ictal-like discharge that recurred with a frequency range of 0.01-0.02 Hz. In rat neocortical slices stimulus-induced responses resembled the spontaneous occurring epileptiform events. Ictal-like discharges in guinea-pig neocortical slices were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist 3-((+/-)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (5 microM), while those in the rat disappeared during perfusion with the non-NMDA excitatory amino acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (1-3 microM). These results indicate that the neocortex of guinea-pig has a higher propensity to generate 4-AP-induced spontaneous epileptiform activity than that of rat. Furthermore the epileptiform activity in these two species requires a different involvement of excitatory amino acid receptors.

5-Carboxamidotryptamine-insensitive 5-HT1-like receptors are concentrated in guinea pig but not rat, claustrum

5-CT (5-carboxamidotryptamine)-insensitive (5-HT1E/5-HT1F) 5-HT1-like recognition sites have been mapped autoradiographically in rat and guinea pig brain using [3H]5-HT in the presence of 5-CT and mesulergine to mask 5-HT1A, 5-HT1B, 5-HT1D and 5-HT2C binding sites. Binding was more dense in the guinea pig but in both species 5-CT-insensitive 5-HT1-like sites were located in the olfactory tubercle, interpeduncular nucleus, caudate putamen, nucleus accumbens, substantia nigra, frontal cortex and hippocampus. These receptors were particularly marked in the claustrum of the guinea pig but not the rat.

Autoradiographic mapping of 5-HT1 receptors in the guinea-pig brain with particular reference to the 5-HT1D receptor sites

The anatomical distribution of 5-HT1 receptors in the guinea-pig brain was studied by means of in vitro quantitative autoradiography using [3H]-5-HT as ligand. The relative presence of the subtypes of the 5-HT1 binding site was investigated by adding selective concentrations of 8-OH-DPAT, (-)21,009, mesulergine and 5-CT. In addition, differentiation of 5-HT1D receptors was achieved by incubation of the tissues with [3H]-5-HT in the presence of 100 nmol/l 8-OH-DPAT together with 100 nmol/l mesulergine. Areas presenting high densities of 5-HT1A receptors included the neocortex (internal layers), hippocampal formation (dentate gyrus, CA1 field), septum and raphe nuclei, while 5-HT1C sites accounted for most of the [3H]-5-HT binding to the choroid plexus. Non 5-HT1A-non 5-HT1C sites (mainly 5-HT1D and, also probably, 5-HT1E receptors) were clearly predominant in the guinea-pig brain. These sites were mainly present in the neocortex (external layers), basal ganglia, hypothalamus and midbrain (substantia nigra, superior colliculus). As previously described, sites with the properties of 5-HT1B receptors could not be clearly identified in the guinea-pig brain. The present results, in addition to providing a detailed map of the 5-HT1 receptors in the guinea-pig brain, indicate that the guinea-pig is a useful laboratory animal for the study of 5-HT1D receptors.

Serotonin 1B receptors form a transient vibrissa-related pattern in the primary somatosensory cortex of the developing rat

Serotonin (5-HT)-immunoreactive axons have a patterned distribution in the primary sensory cortices of developing rodents. This distribution becomes apparent shortly after birth and disappears around the end of the second postnatal week. We employed binding of [125I]cyanopindolol in the presence of isoproterenol to determine whether 5-HT1B receptors have a similar transient spatiotemporal distribution. In rats killed on postnatal day (P-) 8, 5-HT1B receptors have a distribution closely matching that of 5-HT-immunoreactivity. The receptors are very dense in lamina IV of both the primary visual and somatosensory cortices and, like 5-HT immunoreactive axons within the somatosensory cortex, form patches matching the distribution of the mystacial vibrissae. In adult animals, the density of these receptors in lamina IV and the supragranular layers of the visual and somatosensory cortices is reduced relative to that in the surrounding cortex. Autoradiograms of the flattened cortices of adult rats yield a 'negative image' of the pattern observed in perinatal animals. Thus, one subclass of 5-HT receptors, the 5-HT1B receptor, has a spatial distribution in cortex which changes in development much like that for serotoninergic axons.