Periaqueductal Gray Of Rats Research Articles

The chemokine CX3CL1/fractalkine interferes with the antinociceptive effect induced by opioid agonists in the periaqueductal grey of rats

We have reported that there is heterologous interaction between the mu, delta or kappa opioid receptors and the receptors for the chemokines CCL5/RANTES or CXCL12/SDF-1 in the regulation of antinociception in rats. CX3CL1/fractalkine, a chemokine that exclusively binds to CX3CR1, has been found to affect morphine analgesia and tolerance in the spinal cord. The purpose of the present study was to see if the interaction between the chemokine CX3CL1/fractalkine receptor and mu, delta or kappa opioid receptors occurs in the periaqueductal grey (PAG) of adult male S-D rats. The cold-water tail-flick (CWT) test was used to measure antinociception. The results showed that intra-PAG injection of 100 ng CX3CL1/fractalkine 30 min before administration of 400 ng DAMGO, 100 ng DPDPE or 20 μg dynorphin significantly reduced the antinociception induced by each of these peptides. These results demonstrate that activation of the CX3CL1 receptor diminishes the effect of mu, delta and kappa opioid agonists on their receptors in the PAG of rats.

Social defeat, a paradigm of depression in rats that elicits 22-kHz vocalizations, preferentially activates the cholinergic signaling pathway in the periaqueductal gray

Gene expression profiles in the periaqueductal gray (PAG) of adult Long-Evans rats as a function of a stressful social defeat in inter-male fighting encounters were examined. This social subordination model mimics prototypical behavioral changes that parallel aspects of clinical depression, has been postulated to simulate early changes in the onset of depression in the losers, and has been successfully utilized for the evaluation of antidepressant activity. The 22-kHz ultrasonic vocalizations (USVs) have been shown to reflect negative emotional states akin to anxiety and depression. Social defeat is the most robust and reliable method of eliciting these calls. The PAG has been shown to be a key brain region for the generation of 22-kHz ultrasonic vocalizations, and 22-kHz USVs have been shown to be controlled by the mesolimbic cholinergic system. In this present study, we examined gene expression changes in the PAG of social subordinate rats compared to dominant rats that do not Exhibit 22-kHz USVs. We found that social defeat significantly altered the genes associated with cholinergic synaptic transmission in the PAG. The most robust of these were the increased expression of the β2 subunit of the nicotinic acetylcholine receptor (CHRNB2) and the T subunit of acetylcholinesterase (ACHE) in the subordinate animals. These changes were corroborated by quantitative real-time polymerase chain reaction (qRT-PCR) and found to be exclusive to the PAG compared to seven other brain regions examined. These data suggest that cholinergic transmission in the PAG is involved in the generation of 22-kHz USVs and provide potential therapeutic targets for the treatment of affective disorders.

Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V 2 rather than V 1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine–enkephalin (L–Ek), methionine–enkephalin (M–Ek) and β-endorphin (β-Ep), but did not change the concentrations of dynorphinA 1–13 (DynA 1–13), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), γ-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L–EK, M–Ek, β-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA 1–13, OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone — an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.

Cholinergic modulation of periaqueductal grey neurons: Does it contribute to epileptogenesis after organophosphorus nerve agent intoxication?

Previous work has shown that a single focal microinjection of the unselective cholinergic agonist, carbachol, into the periaqueductal grey (PAG) of the midbrain is sufficient to induce forebrain seizures in rats. In order to determine the cholinergic mechanisms underlying epileptogenesis at the cellular and network level of the PAG, we performed whole-cell recordings from rat PAG neurons in vitro and examined how the activation of muscarinic and nicotinic receptors modulates cellular excitability and synaptic responses. Stimulation of muscarinic receptors produced either a pirenzepine-sensitive depolarization (40% of PAG neurons), or a gallamine-sensitive hyperpolarization (20%), suggesting the involvement of M1 and M2 receptors, respectively. In the remaining neurons (40%), no change was observed. Voltage-clamp recordings showed that muscarinic depolarization resulted from the inhibition of a resting K + current, in part accompanied by simultaneous activation of a presumed non-selective cation current. Muscarinic hyperpolarization was caused by the activation of a G protein-coupled, inwardly rectifying K + current. Stimulation of muscarinic receptors enhanced the frequency of spontaneous inhibitory postsynaptic currents (IPSCs), but strongly suppressed evoked IPSCs. In addition, nicotine almost doubled the frequency of miniature IPSCs. Based on our findings and the network properties of the PAG, we advance a scenario in which excessive stimulation of cholinergic receptors would substantially contribute to generalized seizures after organophosphorus nerve agent poisoning.

Cellular action of cholecystokinin-8S-mediated excitatory effects in the rat periaqueductal gray

The peptide cholecystokinin (CCK) is one of the major neurotransmitters modulating satiety, nociception, and anxiety behavior. Although many behavioral studies showing anti-analgesic and anxiogenic actions of CCK have been reported, less is known about its cellular action in the central nervous system (CNS). Therefore, we examined the action of CCK in rat dorsolateral periaqueductal gray (PAG) neurons using slice preparations and whole-cell patch–clamp recordings. Application of CCK-8S produced an inward current accompanied by increased spontaneous synaptic activities. The CCK-8S-induced inward current ( I CCK) was recovered after washout and reproduced by multiple exposures. Current–voltage plots revealed that I CCK reversed near the equilibrium potential for K + ions with a decreased membrane conductance. When several K + channel blockers were used, application of CdCl 2, TEA, or apamin significantly reduced I CCK. I CCK was also significantly reduced by the CCK 2 receptor antagonist, L-365,260, while it was not affected by the CCK 1 receptor antagonist, L-364,718. Furthermore, we examined the effects of CCK-8S on miniature excitatory postsynaptic currents (mEPSCs) in order to determine the mechanism of CCK-mediated increase on synaptic activities. We found that CCK-8S increased the frequency of mEPSCs, but had no effect on mEPSC amplitude. This presynaptic effect persisted in the presence of CdCl 2 or Ca 2+-free bath solution, but was completely abolished by pre-treatment with BAPTA-AM, thapsigargin or L-365,260. Taken together, our results indicate that CCK can excite PAG neurons at both pre- and postsynaptic loci via the activation of CCK 2 receptors. These effects may be important for the effects of CCK on behavior and autonomic function that are mediated via PAG neurons.

Decreased morphine analgesia in rat overexpressing β-arrestin 2 at periaqueductal gray

β-arrestin 2 plays important physiological roles in regulating the function of the μ opioid receptor (μOR) in vivo. Periaqueductal gray (PAG) is a potential structure where morphine produces its antinociception, but it is unclear whether β-arrestin 2 plays its regulatory effect on morphine at PAG. In the present study β-arrestin 2 overexpression was induced by adenovirus at PAG of rats. Morphine was administrated in these rats through PAG microinjection and systemic administration. The antinociceptive effects of morphine were abolished in rats received microinjection of morphine at PAG and partially attenuated in rats received systemic administration of morphine. These findings support the notion that PAG is an important site where β-arrestin 2 plays its regulatory effects on morphine analgesia.

Co-localization of midbrain projections, progestin receptors, and mating-induced fos in the hypothalamic ventromedial nucleus of the female rat

In female rats, sexual behavior requires the convergence of ovarian hormone signals, namely estradiol and progesterone, and sensory cues from the male on a motor output pathway. Estrogen and progestin receptors (ER and PR) are found in neurons in the hypothalamic ventromedial nucleus (VMH), a brain region necessary for lordosis, the stereotypic female copulatory posture. A subset of VMH neurons sends axonal projections to the periaqueductal gray (PAG) to initiate a motor output relay, and some of these projection neurons express PR. Previous studies showed that VMH neurons are activated during mating, based on the expression of the immediate early gene Fos. Many of the activated neurons expressed ER; however, it is not known if such activated neurons co-express PR. Fluorogold, a retrograde tracer, was injected into the PAG of ovariectomized rats to label neurons projecting from the VMH. Hormone-treated animals then were mated, and their brains were immunohistochemically stained for PR and Fos. Of the Fos-positive neurons, 33% were double-labeled for PR, 19% were double-labeled with Fluorogold, and 5% were triple-labeled for Fos, PR, and the retrograde tracer. The majority of triple-labeled neurons were found in the rostral, rather than caudal, portion of the VMH. These results show that PR-containing neurons are engaged during sexual behavior, which suggests that these neurons are the loci of hormonal–sensory convergence and hormonal–motor integration.

An Opiate Cocktail that Reduces Morphine Tolerance and Dependence

Morphine is an exceptionally effective analgesic whose utility is compromised by the development of tolerance and dependence to the drug. Morphine analgesia and dependence are mediated by its activity at the mu opioid peptide (MOP) receptor [1]. The MOP receptor is activated not only by morphine, but also by other opiate drugs such as methadone and endogenous opioids such as endorphins. Morphine, however, is a unique opioid agonist ligand because it fails to induce endocytic trafficking of the MOP receptor [2], whereas the endogenous ligands and methadone do facilitate endocytosis [3]. Using the unique pharmacology of the MOP receptor and its proposed existence as an oligomeric structure [4], we designed a pharmacological cocktail that facilitates endocytosis of the MOP receptor in response to morphine. This cocktail consists of morphine and a small dose of methadone. Importantly, this cocktail, while retaining full analgesic potency, does not promote morphine dependence. We further demonstrate that dependence is reduced, at least in part, because endocytosis of the MOP receptor in response to morphine prevents the upregulation of N-methyl-D-aspartate (NMDA) receptors.

Developmental change of GABAergic postsynaptic current in rat periaqueductal gray

The present study was designed to examine developmental changes of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in periaqueductal gray (PAG) neurons mechanically isolated from young (12- to 18-day) and adult (8- to 12-week) rats. While the frequency of mIPSCs was similar, the current amplitude in adult rats was significantly smaller than in young rats. In the study of mIPSC kinetics, all kinetic parameters except for the fast decay time in adult rats were smaller or shorter than in the case of young rats. The present study demonstrates that a decrease in the amplitude of GABAergic mIPSC during development may result from a decrease in the GABA contents of synaptic vesicles and from changes in the kinetics of postsynaptic GABA-activated Cl − channels.

Activation of mu opioid receptors in the ventrolateral periaqueductal gray inhibits reflex micturition in anesthetized rats

This study tested the hypothesis that morphine and other opiates cause urinary retention by activating mu opioid receptors in the midbrain periaqueductal gray (PAG) region. Selective mu, delta and kappa receptor agonists were microinjected into the PAG of urethane-anesthetized rats and the amplitude and incidence of bladder contractions were recorded during continuous saline infusion. Arterial pressure was monitored through a femoral artery catheter. Microinjection of the mu receptor agonist DAMGO into the ventrolateral PAG (vlPAG) suppressed volume-evoked bladder contractions completely. Bladder contractions ceased within 5 min of DAMGO injection and remained essentially undetectable for the rest of the 20 min recording period. Microinjection of the delta receptor agonist DPDPE into the vlPAG did not significantly affect either the amplitude of bladder contractions or the time interval separating contractions. The kappa receptor agonist U-69593 caused no discernible change in amplitude but increased the interval between bladder contractions significantly. Microinjection of DAMGO, DPDPE or U-69593 into the lateral or dorsolateral PAG columns was ineffective. DAMGO injection into the vlPAG increased arterial pressure whereas DPDPE and U-69593 produced a small but significant depressor response. These data support the hypothesis that mu and kappa receptors in the vlPAG participate in the micturition reflex.

P.3.022 Role of GABA/benzodiazepine receptors of the dorsal periaqueductal grey of rats in the mediation of innate and conditioned anxiety responses

P.3.022 Role of GABA/benzodiazepine receptors of the dorsal periaqueductal grey of rats in the mediation of innate and conditioned anxiety responses

Involvement of calcitonin gene-related peptide and its receptor in anti-nociception in the periaqueductal grey of rats

The present study investigated the role of calcitonin gene-related peptide (CGRP) in the modulation of nociception in periaqueductal grey (PAG) of rats. Hindpaw withdrawal latencies (HWLs) were tested by hot-plate and Randall Selitto tests. The HWLs to thermal and mechanical stimulation increased significantly after intra-PAG administration of 0.26 or 0.13 nmol of CGRP, but not 0.026 nmol of CGRP. The anti-nociceptive effects induced by CGRP were significantly blocked by intra-PAG administration of 0.026 or 0.26 nmol of the CGRP1 receptor antagonist CGRP8-37. Furthermore, administration of CGRP into the decussation of superior cerebellar peduncle, out of PAG, did not elicit anti-nociceptive effects during 60 min after the injection. The results demonstrated that CGRP plays an important role in anti-nociception in PAG of rats, and CGRP1 receptor is involved in the CGRP-induced anti-nociception.

Cellular actions of opioids on periaqueductal grey neurons from C57B16/J mice and mutant mice lacking MOR-1.

1 Patch clamp recordings were made from periaqueductal grey (PAG) neurons in vitro to investigate the cellular actions of opioids in wild-type C57B16/J mice and mutant mice lacking the first exon of the micro -opioid (MOP) receptor. 2 In wild-type mice, the kappa-(KOP) agonist U-69593 (300 nM) and the mixed micro /delta-opioid agonist met-enkephalin (10 micro M), but not the delta-(DOP) agonist deltorphin (300 nM), reduced the amplitude of evoked GABA(A)-mediated inhibitory postsynaptic currents (IPSCs). Met-enkephalin and U-69593 also reduced the rate of spontaneous miniature IPSCs, but had no effect on their amplitude and kinetics. In micro -receptor-deleted mice, only U-69593 (300 nM) reduced the amplitude of evoked IPSCs. 3 In wild-type mice, the MOP agonist DAMGO (3 micro M) produced an outward current in 76% of the neurons. Deltorphin and U-69593 produced outward currents in 24 and 32% of the neurons, respectively. In micro -receptor-deleted mice, deltorphin and U-69593 produced similar outward currents in 32 and 27% of the neurons, respectively, while DAMGO was without effect. All neurons in both the wild-type and micro -receptor-deleted mice responded with similar outward currents to either the GABA(B) receptor agonist baclofen (10 micro M), or the opioid-like receptor ORL1 (NOP) agonist nociceptin (300 nM). 4 The DAMGO-, deltorphin-, U-69593-, baclofen- and nociceptin-induced currents displayed inward rectification and reversed polarity at -109 to -116 mV. 5 These findings indicate that micro -, delta- and kappa-opioid receptor activation has complex pre- and postsynaptic actions within the mouse PAG. This differs to the rat PAG where only micro -opioid receptor actions have been observed.

Group I metabotropic glutamate receptors modulate glutamate and γ-aminobutyric acid release in the periaqueductal grey of rats

In this study, we investigated the effects of group I metabotropic glutamate (mglu) receptor ligands on glutamate and γ-aminobutyric acid (GABA) extracellular concentrations at the periaqueductal grey level by using in vivo microdialysis. An agonist of group I mglu receptors, ( S)-3,5-dihydroxyphenylglycine [( S)-3,5-DHPG, 1 and 2 mM], as well as a selective agonist of mglu 5 receptors, ( RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 2 and 4 mM), both increased dialysate glutamate and GABA concentrations. 7-(Hydroxyimino)cyclopropa-[ b]-chromen-1α-carboxylate ethyl ester (CPCCOEt, 1 mM), a selective mglu 1 receptor antagonist, and 2-methyl-6-(phenylethynyl)pyridine (MPEP, 0.5 mM), a selective mglu 5 receptor antagonist, perfused in combination with DHPG, antagonized the effect induced by DHPG on the extracellular glutamate and GABA concentrations. MPEP (0.5 mM), perfused in combination with CHPG, antagonized the increased glutamate and GABA extracellular levels induced by CHPG. MPEP (1 mM) decreased the extracellular concentrations of glutamate but did not modify the dialysate GABA concentrations. Moreover, as the intra-periaqueductal grey perfusion of ( RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [( RS)-CPP, 100 μM], a selective N-methyl- d-aspartate (NMDA) glutamate receptor antagonist, did not change the extracellular concentrations of glutamate, this suggests that the MPEP-induced decrease in glutamate is not a consequence of NMDA receptor blockade. These data show that group I mglu receptors in the periaqueductal grey may modulate the release of glutamate and GABA in awake, freely moving rats. In particular, mglu 5, but not mglu 1, receptors seem to be functionally active on glutamate terminals.

Involvement of local cholecystokinin in the tolerance induced by morphine microinjections into the periaqueductal gray of rats

The ventrolateral periaqueductal gray (PAG) is a key structure for the development of opioid tolerance. An increased activity of ‘anti-opioids’ like cholecystokinin (CCK) has been proposed as a possible mechanism for opioid tolerance. The present study evaluates the role of PAG-located CCK in the opioid tolerance induced by repeated microinjections of morphine (MOR) into PAG. Male rats were implanted with chronic guide cannulae aimed at the PAG. Microinjection of MOR (0.5 μg in 0.5 μl) into PAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if each MOR microinjection was preceded (within 15 min) by a microinjection of the non-selective CCK receptor antagonist proglumide (PRO), (0.4 μg in 0.5 μl) into the same PAG site, the microinjections of MOR always produced antinociception and did not induce tolerance. If PRO microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single PRO microinjection into the same PAG site was enough to restore the antinociceptive effect of MOR. On the other hand, if CCK (1 ng in 0.5 μl) was microinjected into PAG, then MOR microinjection administered 15 min later into the same PAG site did not elicit antinociception. These results show that CCK has anti-opioid activity in PAG and that tolerance to MOR in PAG can be prevented or reversed if CCK receptors are blocked with PRO. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg intraperitoneal ) was reversed by a single microinjection of PRO into PAG. This emphasizes the central importance of PAG in the MOR/CCK interactions that lead to opioid tolerance.

Effect of combined treatment with nandrolone and cocaine on the NMDA receptor gene expression in the rat nucleus accumbens and periaqueductal gray.

Anabolic androgenic steroids (AAS) have been suggested to sensitize mechanisms involved in drug dependency and aggressive behaviour. Nucleus accumbens and periaqueductal gray (PAG) are believed to be critical in the functional anatomy of these two phenomena, which are partly mediated by the NMDA receptor. This study was undertaken in order to determine the relationship between the effect of AAS and cocaine (given alone or in combination) on the gene expression of the NMDA receptor subtype NR1 in the rat nucleus accumbens and PAG. Male rats were subjected to 2 weeks of daily doses of cocaine and the anabolic-androgenic steroid nandrolone, administrated separately or in combination, and the effect on the mRNA expression for the NMDA receptor NR1 subunit was studied by Northern blot analysis. In the nucleus accumbens, injection of cocaine alone and the combination of cocaine and nandrolone caused a significant decrease in the NR1 mRNA level compared with that of control rats. The combined treatment significantly down-regulated the transcript in the PAG compared with the groups injected with vehicle, nandrolone or cocaine alone. It was concluded that AAS combined with cocaine may induce additive increases in glutamatergic activity in these brain areas, supporting the notion that these steroids can sensitize mechanisms involved in the reward and the expression of aggression.

Estrogen-induced dendritic spine elimination on female rat ventromedial hypothalamic neurons that project to the periaqueductal gray.

Neurons of the ventromedial hypothalamic nucleus (VMH) that project to the periaqueductal gray (PAG) form a crucial segment of the motor pathway that produces the lordosis posture, the hallmark of female rat sexual behavior. One suggested mechanism through which estrogen facilitates lordosis is by remodeling synaptic connectivity within the VMH. For instance, estrogen alters VMH dendritic spine density. Little is known, however, about the local VMH microcircuitry governing lordosis nor how estrogen alters synaptic connectivity within this local circuit to facilitate sexual behavior. The goal of this study was to define better the neuron types within the VMH microcircuitry and to examine whether estrogen alters synaptic connectivity, as measured by dendritic spine density, on VMH projection neurons. A retrograde tracer was injected into the PAG of ovariectomized rats treated with vehicle or estradiol. Retrogradely labeled VMH neurons were filled with Lucifer yellow, then immunostained for estrogen receptor-alpha (ER alpha). VMH neurons that project to the PAG had more dendrites than functionally unidentified neurons. Additionally, VMH projection neurons could be subdivided into those located within the cluster of ER alpha-containing neurons and those medial to the cluster. Estrogen decreased spine density by 57% on the long primary dendrites of VMH projection neurons located within the ER alpha cluster but not on projection neurons medial to the cluster. Only 4% of the VMH projection neurons expressed ER alpha. These results suggest that estrogen may facilitate sexual behavior by decreasing spines selectively, via an indirect mechanism, on a subset of VMH neurons that project to the PAG.

Synergistic μ-opioid and 5-HT 1A presynaptic inhibition of GABA release in rat periaqueductal gray neurons

The periaqueductal gray (PAG) plays a critical role in descending antinociception. In mechanically dissociated rat PAG neurons, pharmacologically separated spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the nystatin-perforated patch technique. Both DAMGO, a specific μ-opioid receptor agonist, and serotonin inhibited mIPSC frequency in a dose-dependent manner without affecting mIPSC amplitude, respectively, in the same PAG neurons. The presynaptic opioid effect was blocked by a specific μ-opioid receptor antagonist, CTOP. The presynaptic serotonergic effect was mimicked by a specific 5-HT 1A receptor agonist, 8-OH-DPAT, and blocked by the specific antagonist, NAN-190. These opioidergic and serotonergic inhibitions of GABA release employed the similar intracellular mechanism of opening 4-AP-sensitive K + channels via GTP-binding proteins (G-proteins). Subthreshold concentrations of DAMGO (3 nM) significantly decreased mIPSC frequency with subthreshold concentrations of serotonin (3 nM) and this effect was completely blocked by pretreatment with N-ethylmaleimide (NEM), a PTX-sensitive G-protein inhibitor. In contrast, maximum doses of DAMGO (10 μM) did not further inhibit mIPSC frequency with maximum doses of serotonin (10 μM). In conclusion, activation of presynaptic μ-opioid and 5-HT 1A receptors synergistically inhibited GABA release. These results suggest a cellular mechanism within PAG for the analgesic effectiveness of combined therapies using opioids in conjunction with classes of anti-depressants which increase synaptic serotonin levels.

5-HT 1A receptor-mediated activation of G-protein-gated inwardly rectifying K + current in rat periaqueductal gray neurons

5-Hydroxytryptamine (5-HT) has been reported to modulate analgesia produced by opioids or electrical stimulation of the periaqueductal gray (PAG). 5-HT increases K + conductance and inhibits the firing activity of the PAG neurons. We examined the electrophysiological and pharmacological characteristics of the K + current involved in 5-HT-induced hyperpolarization of dissociated rat PAG neurons. Among the neurons tested, 5-HT activated inward K + currents in 30–40%, whilst the remaining 60–70% did not respond to 5-HT. 5-HT activated an inwardly rectifying K + current ( I 5-HT) in a concentration- and voltage-dependent manner. I 5-HT was mimicked by a 5-HT 1A receptor selective agonist, 8-OH-DPAT, and was reversibly blocked by a 5-HT 1A receptor antagonist, piperazine maleate, but not by a 5-HT 2 receptor antagonist, ketanserin. I 5-HT was sensitive to K + channel blockers such as quinine and Ba 2+, but insensitive to 4-aminopyridine, Cs + and tetraethylammonium. I 5-HT was inhibited by GDPβs and was irreversibly activated by GTPγs. I 5-HT was significantly suppressed by N-ethylmaleimide and pertussis toxin, but not by cholera toxin. Second messenger modulators such as staurosporin, forskolin, and phorbol-12-myristate-13-acetate did not alter I 5-HT. The present study indicates that 5-HT-induced hyperpolarization of the PAG neurons results from activation of the pertussis toxin-sensitive G-protein-coupled inwardly rectifying K + currents through 5-HT 1A receptors.

Neurohistological and behavioral evidence for lordosis‐inhibiting tract from lateral septum to periaqueductal gray in male rats

To verify the anatomical and functional connection of the lateral septum (LS) and periaqueductal gray (PAG) in inhibiting female sexual behavior, lordosis, in male rats, retrograde (Fluoro-Gold, FG) or anterograde (Phaseolus vulgaris-leucoagglutinin, PHA-L) tracer was injected into the PAG or LS on the right side, respectively, and FG-labeled cells or PHA-L-labeled axons in the forebrain and mesencephalon were determined in estrogen-treated castrated male rats. A ventral cut (VC) of the septum and a behavioral test were also conducted in some FG-injected rats. Furthermore, lordosis behavior was observed after chemical destruction of the septum by ibotenate. As a result, the lordosis quotient (LQ) in VC males was higher than that in control males without VC. FG-labeled neuronal cell bodies were found in the ipsilateral intermediate part of the LS in the control males but not in this area of the VC males. When neuronal cells in the intermediate part of the bilateral LS were completely destroyed by ibotenate, the LQ was higher than that in sham-lesioned male rats. These results suggest that a direct neural connection of the intermediate LS to the PAG has an inhibitory role in regulating lordosis in male rats. In addition, neuronal cell bodies in the intermediate LS exert an inhibitory influence. In the PHA-L experiment, labeled axons were seen in the ventral part of the LS, the medial forebrain bundle at the chiasmatic level, the lateral hypothalamus, the median region of the mesencephalon, and the rostral PAG in the side ipsilateral to the tracer injection site of the LS. Thus, these areas are thought to be involved in the pathway for lordosis-inhibition from the intermediate part of the LS to the PAG in male rats. J. Comp. Neurol. 431:293–310, 2001. © 2001 Wiley-Liss, Inc.