Role of cholinergic, opioidergic and GABAergic neurotransmission of the dorsal hippocampus in the modulation of nociception in guinea pigs

AIMS: Several physiological, pharmacological and behavioral lines of evidence suggest that the hippocampal formation is involved in nociception. The hippocampus is also believed to play an important role in the affective and motivational components of pain perception. Thus, our aim was to investigate the participation of cholinergic, opioidergic and GABAergic systems of the dorsal hippocampus (DH) in the modulation of nociception in guinea pigs. MAIN METHODS: The test used consisted of the application of a peripheral noxious stimulus (electric shock) that provokes the emission of a vocalization response by the animal. KEY FINDINGS: Our results showed that, in guinea pigs, microinjection of carbachol, morphine and bicuculline into the DH promoted antinociception, while muscimol promoted pronociception. These results were verified by a decrease and an increase, respectively, in the vocalization index in the vocalization test. This antinociceptive effect of carbachol (2.7 nmol) was blocked by previous administration of atropine (0.7 nmol) or naloxone (1.3 nmol) into the same site. In addition, the decrease in the vocalization index induced by the microinjection of morphine (2.2 nmol) into the DH was prevented by pretreatment with naloxone (1.3 nmol) or muscimol (0.5 nmol). At doses of 1.0 nmol, muscimol microinjection caused pronociception, while bicuculline promoted antinociception. SIGNIFICANCE: These results indicate the involvement of the cholinergic, opioidergic and GABAergic systems of the DH in the modulation of antinociception in guinea pigs. In addition, the present study suggests that cholinergic transmission may activate the release of endorphins/enkephalin from interneurons of the DH, which would inhibit GABAergic neurons, resulting in antinociception.     

Propranolol and methylatropine antagonize the cardiovascular effects produced by microinjection of the TRH analog MK-771 into the preoptic suprachiasmatic nucleus

Thyrotropin-releasing hormone (TRH) has been shown to increase heart rate as well as blood pressure when administered into rat brain. The present study investigated the mechanism by which the TRH analog MK-771 produces these effects when injected into the preoptic suprachiasmatic nucleus (POSC). MK-771, at a dose of 125 pmol (50 ng), produced significant increases in both heart rate and blood pressure. These effects occurred within 5 minutes of microinjection and lasted approximately 20-30 minutes. Pretreatment with either the beta-adrenergic antagonist propranolol or the muscarinic antagonist methylatropine, administered into the POSC, significantly altered the response produced by MK-771. Propranolol, at a dose of 7 nmol, and methylatropine at a dose of 0.5 nmol, significantly inhibited the tachycardia produced by MK-771. In addition, methylatropine, at a dose of 0.5 nmol, significantly reduced the increase in diastolic pressure produced by the TRH agonist. These results are consistent with the idea that TRH agonists, when administered centrally, produce cardiovascular alterations through the autonomic nervous system.     

Cardiovascular responses to microinjection of ATP into the nucleus tractus solitarii of awake rats

Microinjection of increasing doses of ATP (0.31, 0.62, 1.25, and 2.5 nmol/50 nl) into the nucleus tractus solitarii (NTS) produced a dose-dependent pressor response. Prazosin abolished the pressor response and produced no change in the bradycardic response to ATP. Microinjection of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (0.25 nmol/50 nl), a nonselective P2 receptor antagonist into the NTS, reduced the bradycardic response but had no effect on the pressor response to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Microinjection of suramin (2 nmol/50 nl), another nonselective P2 receptor antagonist, had no effect on the pressor and bradycardic responses to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Antagonism of A1 receptors of adenosine with 1,3-dipropyl-8-cyclopentylxanthine also produced no changes in the cardiovascular responses to microinjection of ATP into the NTS. The involvement of excitatory amino acid (EAA) receptors in the pressor and bradycardic responses to microinjection of ATP into the NTS was also evaluated. Microinjection of kynurenic acid, a nonselective EAA receptor antagonist (10 nmol/50 nl), into the NTS reduced the bradycardic response and had no effect on the pressor response to microinjection of ATP into the NTS. The data show that 1) microinjection of ATP into the NTS of awake rats produced pressor and bradycardic responses by independent mechanisms, 2) the activation of parasympathetic component may involve an interaction of P2 and EAA receptors in the NTS, and 3) the sympathoexcitatory response to microinjection of ATP into the NTS was not affected by the blockade of P2, A1, or EAA receptors.     

Responses to GABA-A receptor blockade in the hypothalamic PVN are attenuated by local AT1 receptor antagonism

Blockade of GABA-A receptors in the hypothalamic paraventricular nucleus (PVN) has been repeatedly shown to increase arterial blood pressure (ABP), heart rate (HR), and sympathetic nerve activity (SNA), but the mechanism(s) that underlies this response has not been determined. Here, we tested whether full expression of the response requires activation of local ANG II AT1 receptors. ABP, HR, and renal SNA responses to PVN microinjection of bicuculline methobromide (BIC; 0.1 nmol) were recorded before and after microinjection of vehicle (saline); losartan (or L-158809), to block local AT1 receptors; or PD123319, to block AT2 receptors. After PVN microinjection of vehicle or PD123319 (10 nmol), BIC significantly (P < 0.05) increased mean arterial pressure (MAP), HR, and renal SNA. However, PVN microinjection of 2 and 20 nmol of losartan dose dependently reduced responses to PVN-injected BIC, with the 20-nmol dose nearly abolishing MAP (P < 0.005), HR (P < 0.05), and renal SNA (P < 0.005) responses. Another AT1 receptor antagonist, L-158809 (10 nmol), produced similar effects. Neither losartan nor L-158809 altered baseline parameters. Responses to PVN injection of BIC were unchanged by losartan (20 nmol) given intravenously or into the PVN on the opposite side. MAP, HR, and renal SNA responses to PVN microinjection of l-glutamate (10 nmol) were unaffected by PVN injection of losartan (20 nmol), indicating that effects of losartan were not due to nonspecific depression of neuronal excitability. We conclude that pressor, tachycardic, and renal sympathoexcitatory responses to acute blockade of GABA-A receptors in the PVN depend on activation of local AT1 receptors.     

The effect of ACTH on the plasma concentrations of the "hypertensinogenic" steroids, 17 alpha-hydroxyprogesterone and 17 alpha,20 alpha-dihydroxy-4-pregnen-3-one in sheep

The plasma concentrations of 17 alpha-hydroxyprogesterone (17 alpha OHP) and 17 a'20 alpha-dihydroxy-4-pregnen-3-one (17 alpha 20 alpha OHP) have been measured in sheep during 5 days of ACTH administration at 20 micrograms/kg/day a rate of infusion known to produce hypertension. Five days of ACTH administration produced a progressive increase in plasma 17OHP from 0.45 +/- 0.12 to 128.9 +/- 28.4 nmol/l and in 17 alpha 20 alpha OHP from 0.54 +/- 0.15 to 73.1 +/- 7.2 nmol/l. Calculation of the blood production rate of both steroids during ACTH treatment confirms that the rates of infusion of 17OHP (3.0 mumol/h) and 17 alpha 20 alpha OHP (1.5 mumol/h) used to produce hypertension, when infused together with the other major ovine adrenocortical steroids, produced plasma concentrations in the range as found following administration at a rate to increase blood pressure.     

Naloxone-precipitated morphine withdrawal increases pontine glutamate levels in the rat

Extracellular fluid (ECF) levels of glutamate (Glu) and aspartate (Asp) were measured in the locus coeruleus (LC) during morphine withdrawal by using microdialysis in conscious morphine-dependent Sprague-Dawley rats. Guide cannulae were implanted chronically and rats were given intracerebroventricular (i.c.v.) infussions of morphine (26 nmol/1 μl/ht) of saline (1 μl/hr) for 3 days. Microdialysis probes (2 mm tip) were inserted into the LC 24 hr before precipitation of withdrawal by i.c.v. injection of naloxone (12 or 48 nmol/5 μl). Behavioral evidence of withdrawal (teeth-chattering, wet-dog shakes, etc.) was detected following naloxone challenge in morphine, but not in saline-infused rats. Increases (P<0.01) in ECF levels of Glu (and Asp, to a lesser degree) were noted after naloxone-precipitated withdrawal only in the morphine group. The ECF Glu levels in the LC increased from 9.6 ± 2.7 to 15.5 ± 5.0 μM following 12 nmol/5 μl naloxone, and from 9.5 ± 1.9 to 20.5 ± 3.3 μM following 48 nmol/5 μl naloxone, before and in the first 15 min sample after the precipitation of withdrawal in the morphine-dependent rats, respectively. These results provide direct evidence to support the role of excitatory amino acids within the LC in morphine withdrawal.     

中缝大核在臂旁内侧核区注射吗啡所致呼吸抑制效应中的作用

实验在33只浅麻醉、肌肉麻痹、人工呼吸及切断双侧颈迷走神经的家兔上进行。观察中缝大核区电解损毁或微量注射利多卡因对呼吸活动及臂旁内侧核区微量注射吗啡所致呼吸抑制效应的影响。结果是:电解损毀中缝大核区,使呼吸频率增加,膈神经放电的幅度和频率均无明显变化,而臂旁内侧核区微量注射吗啡抑制呼吸的程度减轻;中缝大核区微量注射利多卡因,则部分消除臂旁内侧核区微量注射吗啡的呼吸抑制效应。中缝大核旁网状结构电解损毁或微量注射利多卡因,不影响吗啡的呼吸抑制效应。上述结果提示,中缝大核区可能在脑桥臂旁内侧核区微量注射吗啡抑制呼吸的机制中起一定作用。     

Dynorphin (1-13): analgesia, hypothermia, cross-tolerance with morphine and beta-endorphin

Intracerebroventricular administration of 20, 40 and 60 nmol of dynorphin (1-13) produced analgesia, as assessed by flinch/jump response to footshock, and hypothermia in the rat. Rats developed tolerance to both the analgesic and thermic effects of the 20 nmol dose of dynorphin. Dynorphin and beta-endorphin showed cross-tolerance with respect to their analgesic but not their thermic effects. Dynorphin and morphine also produced cross-tolerant analgesic effects. Naloxone (10 mg/kg, IP) completely blocked the barrel rolling produced by 20 nmol dynorphin but did not alter its analgesic or thermic effects.     

Dynorphin-A-(1–13) antagonizes morphine analgesia in the brain and potentiates morphine analgesia in the spinal cord

Intrathecal injection of subanalgesic doses of morphine (7.5 nmol) and dynorphin-A-(1–13) (1.25 nmol) in combination resulted in a marked analgesic effect as assessed by tail flick latency in the rat. The analgesic effect of the composite dynorphin/morphine was dose-dependent in serial dilutions so that a composition of 1/8 of the analgesic dose of dynorphin and 1/3 that of morphine produced an analgesic effect equipotent to full dose of either drug applied separately. The analgesic effect induced by dynorphin/morphine mixture was not accompanied by motor dysfunction and was easily reversed by a small dose (0.5 mg/kg) of naloxone. Contrary to the augmentatory effect of dynorphin on morphine analgesia in the spinal cord, intracerevroventricular (ICV) injection of 20 nmol of dynorphin-A-(1–13) exhibited a marked antagonistic effect on the analgesia produced by morphine (120 nmol, ICV). The theoretical considerations and practical implications of the differential interactions between dynorphin-A-(1–13) and morphine in the brain versus spinal cord are discussed.     

Nitric oxide at the CVLM is involved in the attenuation of the reflex bradycardia in renovascular hypertensive rats

Hypertension is associated to an increase in central oxidative stress and an attenuation of the baroreflex control of arterial pressure. The present study evaluated the effect of alterations in the levels of nitric oxide (NO) and superoxide anion in the caudal ventrolateral medulla (CVLM), a key area of the brainstem for the baroreflex control of arterial pressure, in renovascular hypertensive rats (2K1C). Baseline mean arterial pressure (MAP), heart rate (HR), and reflex bradycardia were evaluated 30 days after renal artery occlusion in anesthetized (urethane, 1.2 g/kg, i.p.) 2K1C or normotensive (SHAM) rats. The MAP, HR, and baroreflex control of HR were evaluated before and after CVLM microinjections of the non-selective NOS inhibitor L-NAME (10 nmol), the NO precursor L-ARG (50 nmol), or the antioxidant ascorbic acid, Vit C (10 nmol). In both 2K1C and SHAM animals, CVLM microinjection of L-NAME produced a decrease in MAP, whereas L-ARG induced a significant increase in MAP. However, microinjection of Vit C into the CVLM produced a decrease in MAP and HR only in 2K1C and not in SHAM rats. Cardiovascular effects produced by microinjection of l-ARG into the CVLM were abolished by prior microinjection of L-NAME in the CVLM of 2K1C and SHAM rats. Microinjection of L-NAME into the CVLM increased the sensitivity of reflex bradycardia in 2K1C animals. In contrast, the CVLM microinjection of L-ARG reduced reflex bradycardia only in SHAM rats. Vit C in the CVLM did not change reflex bradycardia in either 2K1C or in SHAM rats. These results suggest that increased oxidative stress in the CVLM during hypertension contributes to the reduced baroreflex sensitivity and to maintain hypertension in the 2K1C model.     

Cholera and pertussis toxins inhibit differently hypothermic and anti-opioid effects of neuropeptide FF

In mice pretreated intracerebroventricularly (i.c.v.) with pertussis or cholera toxins, effects of neuropeptide FF (NPFF), on hypothermia and morphine-induced analgesia, were assessed. NPFF and a potent NPFF agonist, 1DMe (0.005-22 nmol) injected into the lateral ventricle decreased morphine analgesia and produced naloxone (2.5 mg x kg(-1), s.c.)-resistant hypothermia after administration into the third ventricle. Cholera toxin (CTX 1 microg, i.c.v.) pretreatment (24 or 96 h before) inhibited the effect of 1DMe on body temperature, but failed to reverse its anti-opioid activity in the tail-flick test. CTX reduced hypothermia induced by a high dose of morphine (8 nmol, i.c.v.) but not the analgesic effect due to 3 nmol morphine. Pertussis toxin (PTX) pretreatment inhibited both morphine-hypothermia and -analgesia but did not modify hypothermia induced by 1DMe. The present results suggest that NPFF-induced hypothermia depends on the stimulation of Gs (but not Gi) proteins. In contrast, anti-opioid effects resulting from NPFF-receptor stimulation do not involve a cholera toxin-sensitive transducer protein.     

中脑导水管周围灰质在侧脑室注入微量吗啡或纳洛酮所引起的镇痛或拮抗镇痛中的作用

本工作进一步探索中脑导水管周围灰质(PAG)在吗啡镇痛与纳洛酮拮抗吗啡镇痛中的作用。实验在清醒受限制的大鼠上进行,以电刺激鼠尾出现的甩尾和嘶叫为痛反应指标。结果表明:(1)侧脑室注射微量纳洛酮后,可使电刺激 PAG 或注射微量吗啡于 PAG 所引起的镇痛效应受到明显拮抗;(2)损毀 PAG 或注射微量纳洛酮于 PAG 后,可使由侧脑室注入微量吗啡所引起的镇痛效应显著减弱。由此可见 PAG 既是侧脑室注射吗啡镇痛作用的重要中枢部位,又是侧脑室注射纳洛酮拮抗吗啡镇痛的重要中枢部位。     

Stimulation of food intake following opioid microinjection into the nucleus accumbens septi in rats

The involvement of opioid peptides in the regulation of food intake has been postulated. However, it is not known how they are involved in this regulation and which brain region is responsible for the mediation of their effects. We studied the effect of a microinjection of opioid agonists and antagonists into the nucleus accumbens septi (NAS) on the food intake in rats, as this area is known to be important for motivation. Male Wistar rats were implanted stereotaxically with guide cannulae. Rats were not allowed food prior to drug treatment and solutions (1 microliter) were microinjected bilaterally. Food intake was measured throughout a 2 hr period after the drug injection. Infusions into the NAS of 2, 5 and 10 nmol of morphine, D-ala2, D-Leu5-enkephalin (DADLE), and beta-endorphin (beta E), or of 5 and 10 nmol of alpha-neoendorphin (ANEO) induced a dose-dependent increase in the food intake. Dynorphin (DYN) also increased the food intake, but only at a 10 nmol dose. The new, highly selective delta agonist D-Pen2,5-enkephalin (DPDPE) induced a dose-dependent increase in the food intake. Naloxone in doses of 2 and 10 nmol antagonized the increased food intake induced by morphine, beta E, ANEO and DYN in a dose-dependent manner, but only partly antagonized the effect of DADLE on the food intake. The selective mu-receptor antagonist beta-funaltrexamine (beta-FNA), in a dose of 5 nmol completely blocked the increase in the food intake induced by morphine but not by DADLE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of neuropeptide Y on the hypothalamic-pituitary-adrenal axis in the dog

There is increasing evidence that neuropeptide Y (NPY) affects the release of pituitary hormones, including adrenocorticotropic hormone (ACTH). The present study was designed to clarify the mechanism by which NPY activates the hypothalamic-pituitary-adrenal (HPA) axis in the dog. Mongrel dogs were equipped with a chronic cannula allowing intra-third (i.t.v.) or intra-lateral (i.l.v.) cerebroventricular administration. A 1.19 nmol, i.t.v. dose of NPY produced as great an ACTH and cortisol response as did equimolar ovine corticotropin releasing factor (CRF). This action of NPY was dose-dependent and shared by peptide YY (PYY) and pancreatic polypeptide (PP), other members of the PP family peptide. Intravenously (i.v.) administered NPY (1.19-11.9 nmol) was much less potent than i.v. CRF in stimulating ACTH and cortisol secretion. However, i.v. NPY significantly increased plasma ACTH and cortisol concentrations, raising the possibility that NPY may modulate the activity of corticotrophs. We have next investigated the possible relationship between NPY and CRF on the HPA axis. Pretreatment with a novel CRF antagonist, alpha-helical CRF9-41 (130.9 nmol i.t.v. or 261.8 nmol i.v.), partly but significantly attenuated the ACTH and cortisol responses to i.t.v. NPY (1.19 nmol). Furthermore, adding a subthreshold dose of i.t.v. NPY (0.119 nmol) to i.t.v. CRF (1.19 nmol) or i.v. NPY (2.38 nmol) to i.v. CRF (0.595 nmol) resulted in the potentiation of CRF-induced ACTH secretion. These results indicate that NPY may activate the HPA axis in concert with CRF probably at hypothalamic and/or pituitary levels. The present findings that NPY evokes ACTH secretion and potentiates the effectiveness of CRF as a secretagogue, together with high concentrations of NPY in the hypothalamus and pituitary portal blood, suggest that NPY is involved in the multihormonal control of ACTH release.     

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.     

尖核和杏仁核在脑啡肽和β—内啡肽的释放中的相互作用

The functional relations between nucleus accumbens and amygdala were investigated with intracranial microinjection, push-pull perfusion and radioimmunoassay in the rabbit. Microinjection of morphine 20 micrograms into nucleus accumbens increased the immunoreactive (ir) enkephalin content in amygdala perfusate from a control level of 0.43 +/- 0.43 fmol/0.5 ml (normal saline group) to 61.6 +/- 16.3 fmol/0.5 ml (P less than 0.01); and ir-beta-endorphin content from 1.88 +/- 0.98 fmol/0.5 ml to 4.80 +/- 1.12 fmol/0.5 ml (P less than 0.05). On the other hand, microinjection of morphine into amygdala increased the release of ir-enkephalins (2.41 +/- 1.41 vs 34.6 +/- 8.4, P less than 0.01) and ir-beta-endorphin (1.79 +/- 0.64 vs 5.58 +/- 1.39 P less than 0.05) in the perfusates of N. accumbens. The results indicate the existence of reciprocal reinforcement of opioid release between the two nuclei, which may take part in a putative positive feedback mechanism in the cerebral analgesic system.     

Functionally Selective Signaling for Morphine and Fentanyl Antinociception and Tolerance Mediated by the Rat Periaqueductal Gray

Functionally selective signaling appears to contribute to the variability in mechanisms that underlie tolerance to the antinociceptive effects of opioids. The present study tested this hypothesis by examining the contribution of G protein-coupled receptor kinase (GRK)/Protein kinase C (PKC) and C-Jun N-terminal kinase (JNK) activation on both the expression and development of tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray of the rat. Microinjection of morphine or fentanyl into the periaqueductal gray produced a dose-dependent increase in hot plate latency. Microinjection of the non-specific GRK/PKC inhibitor Ro 32-0432 into the periaqueductal gray to block mu-opioid receptor phosphorylation enhanced the antinociceptive effect of morphine but had no effect on fentanyl antinociception. Microinjection of the JNK inhibitor SP600125 had no effect on morphine or fentanyl antinociception, but blocked the expression of tolerance to repeated morphine microinjections. In contrast, a microinjection of Ro 32-0432 blocked the expression of fentanyl, but not morphine tolerance. Repeated microinjections of Ro 32-0432 blocked the development of morphine tolerance and inhibited fentanyl antinociception whether rats were tolerant or not. Repeated microinjections of SP600125 into the periaqueductal gray blocked the development of tolerance to both morphine and fentanyl microinjections. These data demonstrate that the signaling molecules that contribute to tolerance vary depending on the opioid and methodology used to assess tolerance (expression vs. development of tolerance). This signaling difference is especially clear for the expression of tolerance in which JNK contributes to morphine tolerance and GRK/PKC contributes to fentanyl tolerance.     

Further evidence for a role of NMDA receptors in the locus coeruleus in the expression of withdrawal syndrome from opioids.

To examine a role of N-methyl-D-aspartate (NMDA) receptors in the locus coeruleus (LC) in the expression of the withdrawal signs from opioids, rats were continuously infused with morphine (a mu-opioid agonist, 26 nmol/microl per h) or butorphanol (a mu/delta/kappa-mixed opioid agonist, 26 nmol/microl per h) intracerebroventricularly (i.c.v.) through osmotic minipumps for 3 days. An LC injection of NMDA (0.1 and 1 nmol/5 microl) induced withdrawal signs in opioid-dependent animals. However, it did not precipitate any abnormal behaviors in saline-treated control rats. The expression of the withdrawal signs precipitated by NMDA (1 nmol/5 microl), glutamate (10 nmol/5 microl), or naloxone (an opioid antagonist, 24 nmol/5 microl) was completely blocked by pretreatment with a NMDA antagonist, MK-801 (5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptan-5,10-imine), 0.1 mg/kg, i.p. In animals that had been infused with opioids in the same manner, naloxone (48 nmol/5 microl, i.c.v.) precipitated withdrawal signs and increased extracellular glutamate levels in the LC of opioid-dependent rats measured by in vivo microdialysis method. Pretreatment with MK-801, however, did not affect the increases of glutamate levels in the LC. These results further demonstrate that the expression of opioid withdrawal induced by an expeditious release of glutamate in the LC region of opioid-dependent animals might be mainly mediated by the postsynaptic NMDA receptors.     

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.     

Discriminative effects of morphine administered intracerabrally in the rat

Rats were trained in a two-choice discrete trial avoidance paradigm to discriminate between saline and 3.0 mg/kg of morphine administered S.C. The microinjection of 0.3–3.0 μg of morphine into the lateral ventricle produced discriminative effects equivalent to those of the systemic training dose as measured by responding on the morphine-appropriate choice lever. Discriminative effects equivalent to those of the morphine training dose were not consistently produced by administration of morphine into the periaqueductal gray, lateral septum or dorsomedial thalamus in doses as high as 10 μg. However, the discriminative effects of systematically administered morphine were blocked by 10–30 μg of naloxone administered intracerebrally at all of the brain sites tested. Thus, the primary site at which morphine acts to produce discriminative effects in the rat is central, although the specific brain areas mediating these effects remain unidentified. The actions of naloxone could be the result of diffusion of the drug into the ventricular system or into the systemic circulation.