Rostroventrolateral medullary neurons modulate glucose homeostasis in the rat.

Several lines of evidence support the view that the premotor sympathetic input to the adrenal gland arises from the rostroventrolateral medulla (RVLM). The aim of this study was to determine whether RVLM neurons play a role in glucose homeostasis. We identified RVLM neurons that control epinephrine secretion by searching for medullospinal neurons that responded to neuroglucoprivation induced by systemic 2-deoxyglucose (2-DG) administration. We tested the effect of disinhibition of the RVLM on arterial blood pressure and plasma glucose concentration. RVLM medullospinal barosensitive neurons (n = 17) were either unaffected or slightly inhibited by 2-DG. In contrast, we found a group (n = 6) of spinally projecting neurons that were excited by 2-DG administration. These neurons were not barosensitive and had spinal conduction velocities in the unmyelinated range (<1 m/s). These neurons may mediate epinephrine secretion and participate in the counterregulatory responses to neuroglucoprivation. To test the hypothesis that activation of the RVLM leads to adrenomedullary activation and subsequent hyperglycemia, we applied the GABA(A) antagonist bicuculline to the RVLM and measured blood pressure, heart rate, and blood glucose in rats with intact adrenals or after bilateral adrenalectomy. Disinhibition of the RVLM resulted in hypertension, tachycardia, and hyperglycemia (4.9 ± 0.3 to 14.7 ± 0.9 mM, n = 5, P < 0.05). Adrenalectomy significantly reduced the hyperglycemic response but did not alter the cardiovascular responses. These data suggest that the RVLM is a key component of the neurocircuitry that is recruited in the counterregulatory response to hypoglycemia.

Gastric leptin: a novel role in cardiovascular regulation.

Gastric-derived leptin affects satiety and gastrointestinal function via vagal mechanisms and has been shown to interact with the gut hormone cholecystokinin (CCK). CCK selectively inhibits splanchnic sympathetic nerve discharge (SND) and the activity of a subset of presympathetic vasomotor neurons in the rostroventrolateral medulla (RVLM). The present study sought to examine the effects of gastric leptin on arterial pressure (AP), heart rate (HR), SND, and RVLM neuronal activity to determine whether its effects on cardiovascular regulation are dependent on CCK(1) receptors and vagal afferent transmission. To mimic gastric leptin, leptin (15-30 microg/kg) was administered close to the coeliac artery in anesthetized, artificially ventilated Sprague-Dawley rats. Within 5 min, leptin selectively decreased the activity of RVLM neurons also inhibited by CCK (-27 +/- 4%; P < 0.001; n = 15); these inhibitory effects were abolished following administration of the CCK(1) receptor antagonist lorglumide. Leptin significantly decreased AP and HR (-10 +/- 2 mmHg, P < 0.001; and -8 +/- 2 beats/min, P < 0.01; n = 35) compared with saline (-1 +/- 2 mmHg, 3 +/- 2 beats/min; n = 30). In separate experiments, leptin inhibited splanchnic SND compared with saline (-9 +/- 2% vs. 2 +/- 3%, P < 0.01; n = 8). Bilateral cervical vagotomy abolished the sympathoinhibitory, hypotensive, and bradycardic effects of leptin (P < 0.05; n = 6). Our results suggest that gastric leptin may exert acute sympathoinhibitory and cardiovascular effects via vagal transmission and CCK(1) receptor activation and may play a separate role to adipose leptin in short-term cardiovascular regulation.

Insulin-responsive autonomic neurons in rat medulla oblongata.

Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin-responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 U/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase-stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N-methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9 ± 0.3 mmol/L to 1.7 ± 0.2 mmol/L (mean ± SEM; n = 6); saline injection had no effect. In insulin-treated rats, many PNMT-immunoreactive C1 neurons had Fos-immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3% ± 1.4% (n = 8) of C1 neurons were Fos-positive. Insulin also induced Fos in 47.2% ± 2.0% (n = 5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT-positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2, or C3 neurons and no DMV neurons were Fos-positive. The high numbers of PNMT-immunoreactive and ChAT-immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.

Perifornical hypothalamic pathway to the adrenal gland: Role for glutamatergic transmission in the glucose counter-regulatory response.

Adrenaline is an important counter-regulatory hormone that helps restore glucose homeostasis during hypoglycaemia. However, the neurocircuitry that connects the brain glucose sensors and the adrenal sympathetic outflow to the chromaffin cells is poorly understood. We used electrical microstimulation of the perifornical hypothalamus (PeH) and the rostral ventrolateral medulla (RVLM) combined with adrenal sympathetic nerve activity (ASNA) recording to examine the relationship between the RVLM, the PeH and ASNA. In urethane-anaesthetised male Sprague-Dawley rats, intermittent single pulse electrical stimulation of the rostroventrolateral medulla (RVLM) elicited an evoked ASNA response that consisted of early (60±3ms) and late peaks (135±4ms) of preganglionic and postganglionic activity. In contrast, RVLM stimulation evoked responses in lumbar sympathetic nerve activity that were almost entirely postganglionic. PeH stimulation also produced an evoked excitatory response consisting of both preganglionic and postganglionic excitatory peaks in ASNA. Both peaks in ASNA following RVLM stimulation were reduced by intrathecal kynurenic acid (KYN) injection. In addition, the ASNA response to systemic neuroglucoprivation induced by 2-deoxy-d-glucose was abolished by bilateral microinjection of KYN into the RVLM. This suggests that a glutamatergic pathway from the perifornical hypothalamus (PeH) relays in the RVLM to activate the adrenal SPN and so modulate ASNA. The main findings of this study are that (i) adrenal premotor neurons in the RVLM may be, at least in part, glutamatergic and (ii) that the input to these neurons that is activated during neuroglucoprivation is also glutamatergic.

Cortical modulation of the cardiovascular system.

Cortical modulation of central cardiovascular control mechanisms has been recognized for many decades. However, it is only recently that the mechanisms underlying cortical influences on circulatory function have been systematically examined. This review considers the view that certain regions of the cerebral cortex, including the medial prefrontal cortex (MPFC) and insular cortex (IC), participate in specific aspects of central circulatory control. Anatomical investigations indicate that these cortical areas are connected with hypothalamic, midbrain, pontine and medullary brain regions involved in cardiovascular control. Lesions of the MPFC and IC have demonstrated modulation of the activity of cardiovascular reflexes such as the baroreceptor heart rate reflex and involvement in conditioned cardiovascular responses. Electrophysiological studies have provided evidence that cortical regions are able to influence premotor sympathoexcitatory vasomotor neurons within the rostral ventrolateral medulla and subsequently alter sympathetic vasomotor tone. Cortical regions such as the IC receive visceral sensory information arising from baroreceptors and chemoreceptors within the cardiovascular system. In contrast, the MPFC receives afferents predominantly from limbic sources, although its outputs include structures associated with central sympathetic vasomotor control. Cortical modulation of circulatory function has been demonstrated in man and may underlie the cardiovascular components of a number of conditions. It is suggested that cortical areas involved in visceral sensory or visceral motor processes associated with circulatory function may be involved in generation of patterns of cardiovascular responses specific for certain behaviours.

Adrenaline: insights into its metabolic roles in hypoglycaemia and diabetes.

Adrenaline is a hormone that has profound actions on the cardiovascular system and is also a mediator of the fight-or-flight response. Adrenaline is now increasingly recognized as an important metabolic hormone that helps mobilize energy stores in the form of glucose and free fatty acids in preparation for physical activity or for recovery from hypoglycaemia. Recovery from hypoglycaemia is termed counter-regulation and involves the suppression of endogenous insulin secretion, activation of glucagon secretion from pancreatic α-cells and activation of adrenaline secretion. Secretion of adrenaline is controlled by presympathetic neurons in the rostroventrolateral medulla, which are, in turn, under the control of central and/or peripheral glucose-sensing neurons. Adrenaline is particularly important for counter-regulation in individuals with type 1 (insulin-dependent) diabetes because these patients do not produce endogenous insulin and also lose their ability to secrete glucagon soon after diagnosis. Type 1 diabetic patients are therefore critically dependent on adrenaline for restoration of normoglycaemia and attenuation or loss of this response in the hypoglycaemia unawareness condition can have serious, sometimes fatal, consequences. Understanding the neural control of hypoglycaemia-induced adrenaline secretion is likely to identify new therapeutic targets for treating this potentially life-threatening condition.

Angiotensin II receptors and angiotensin converting enzyme in the medulla oblongata.

Quantitative in vitro autoradiography was used to map angiotensin II (ANG II) receptors and angiotensin converting enzyme (ACE) in sections from rat, rabbit, sheep, and human medulla oblongata and to follow changes in receptor and ACE density after disruption of vagal projections by nodose ganglionectomy in the rat. ANG II receptors and ACE are both concentrated in the nucleus of the solitary tract and dorsal motor nucleus of vagus of the rat, rabbit, sheep, and human. An ANG II receptor-containing band connecting the nucleus of the solitary tract with the dorsolateral medulla was seen in rabbit and human tissue, providing evidence for association of ANG II receptors with vagal afferent fibers. ANG II receptors were found to be concentrated in the rostral and caudal ventrolateral medulla, which corresponded to the region of C1 and A1 catecholamine-containing cell groups in the rabbit. This localization was also evident in rat and human tissue. In all four species, a prominent, ANG II receptor-rich band in the intermediate reticular nucleus was found to connect the ventrolateral medulla and the dorsal vagal complex. In humans and sheep, this band contains puncta that overlie cell bodies. One week after nodose ganglionectomy in the rat, the density of ANG II receptors in the ipsilateral dorsal vagal complex fell markedly. This fall was most prominent in the rostral dorsal motor nucleus of vagus (to 46% of control density) and in the nucleus of the solitary tract (to 56% of control). ACE levels and calcitonin gene-related peptide receptor density were unchanged in both nuclei after ganglionectomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentrations of histamine in the hypothalamus of the rat: effect of extraction volume and interpretation of the effects of acutely-administered morphine.

The effect of varying the ratio of extraction volume to tissue weight (EVR) on the apparent concentration of histamine (HA) in the hypothalamus of the rat was examined. Increasing the weight of tissue (by pooling 1, 2 or 3 hypothalami), in a constant extraction volume, resulted in progressive decreases in apparent concentration of histamine in the hypothalamus. These concentrations were 642, 450 and 282 ng/g, respectively. Morphine (10 mg/kg, i.p.) significantly reduced the concentration of histamine in the hypothalamus. Expressed as percentages of the saline-control values (obtained for the extraction volume to tissue weight of 35.7, 57.2 and 118.4 ml/g), treatment with morphine resulted in 24, 17 and 11% reductions in the concentration of histamine in the hypothalamus, respectively. However, expressed in terms of ng/g, the reductions in histamine induced by morphine were 68, 75 and 69 ng/g, respectively. It is concluded that morphine may consistently affect a single pool of histamine. The possibility that de novo histamine is formed in the homogenate during the extraction process is discussed.

The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, attenuates the Bezold-Jarisch reflex in the anaesthetized rat.

The effects of MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist, on the Bezold-Jarisch reflex elicited by i.v. doses of 5-hydroxytryptamine (5-HT) was investigated in urethane-anaesthetized rats. Activation of the Bezold-Jarisch reflex with 5-HT (0.5-16 microgram/kg) produced pronounced dose-dependent hypotensive and bradycardic responses which were attenuated by MK-801 (1 mg/kg, i.v.) but not by saline. The data suggests that activation of the Bezold-Jarisch reflex by 5-HT involves a glutamatergic synapse presumably located within the brainstem vagal reflex arc.

Impaired arterial baroreceptor reflex and cardiopulmonary vagal reflex in conscious spontaneously hypertensive rats.

The activity of baroreceptor reflexes and cardiopulmonary reflexes was examined in conscious spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) rats. The baroreceptor heart rate reflex, elicited by phenylephrine- and nitroprusside-induced changes in blood pressure, had a reduced range and lower heart rate plateau in SHR than in WKY rats, which suggests impaired vagal control of the heart rate in SHR. Cardiopulmonary receptor reflex activity was assessed by intravenous injections of phenyldiguanide which evoke the Bezold-Jarisch reflex. Phenyldiguanide elicited dose-dependent bradycardic and hypotensive responses in WKY rats, but these were significantly attenuated in SHR. This is the first demonstration of impaired Bezold-Jarisch responses in conscious SHR and provides evidence of both impaired vagally mediated arterial baroreceptor activity and impaired cardiopulmonary receptor activity in this rat strain.

Medial prefrontal cortical lesions and baroreceptor heart rate reflex sensitivity in the spontaneously hypertensive rat.

Previous investigations have shown that an excitotoxic lesion of the medial prefrontal cortex (MPFC) in normotensive Sprague-Dawley rats results in a reduction in the sensitivity of the baroreceptor heart rate reflex. The aim of this study was to examine the importance of the MPFC in regulation of the heart rate reflex in conscious, unrestrained spontaneously hypertensive rats (SHR). The MPFC was lesioned by bilateral microinfusions of the excitotoxin N-methyl-D-aspartic acid (NMDA). Baroreceptor heart rate reflex testing was performed by measuring reflex heart rate changes in response to blood pressure alterations induced by nitroprusside and phenylephrine with subsequent computerized sigmoidal curve-fitting of the data. Lesion of the MPFC did not significantly alter resting, systolic and diastolic blood pressure, heart rate or baro-reflex parameters (gain, thresholds, range or plateaus). These observations suggest that the putative descending facilitatory influence from the MPFC to brainstem areas, involved in baroreceptor reflex regulation observed in normotensive rats, may be defective in SHR.

Increased nerve growth factor inducible-A gene and c-fos messenger RNA levels in the rat midbrain and hindbrain associated with the cardiovascular response to electrical stimulation of the mesencephalic cuneiform nucleus.

Functional neuronal connections associated with the cardiovascular response to unilateral low-intensity electrical stimulation of the mesencephalic cuneiform nucleus were examined in the halothane-anaesthetized and paralysed rat by in situ hybridization histochemistry using specific 35S-labelled oligonucleotides for detection of nerve growth factor inducible-A gene (NGFI-A) and c-fos messenger RNAs. Stimulation of the cuneiform nucleus increased mean arterial pressure and heart rate by 20 +/- 0.5 mmHg and 35 +/- 3 b.p.m., respectively, while no significant cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals. Cuneiform nucleus stimulation produced increased NGFI-A and c-fos messenger RNA levels in the Kölliker-Fuse and parabrachial nuclei ipsilaterally, and the cuneiform nucleus, dorsal periaqueductal gray and caudal ventrolateral medulla bilaterally at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. NGFI-A, but not c-fos, messenger RNA expression was increased bilaterally in the caudal portion of the nucleus of the solitary tract and inferior olive. These results are consistent with previous neuroanatomical tract-tracing studies of afferent and efferent pathways from the cuneiform nucleus and indicate that these midbrain and hindbrain structures may be involved in the pressor and tachycardic responses associated with stimulation of the cuneiform nucleus. The ipsilateral nature of responses in certain brain areas may be explained by the absence of decussating pathways and/or the presence of multisynaptic connections which attenuate bilateral signal transmission. Characterization of these activated neuronal structures using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the cardiovascular responses to stimulation of the cuneiform nucleus.

Neuronal activation in the forebrain following electrical stimulation of the cuneiform nucleus in the rat: hypothalamic expression of c-fos and NGFI-A messenger RNA.

Forebrain neuronal connections associated with the cardiovascular response to unilateral, low-intensity, electrical stimulation of the mesencephalic cuneiform nucleus were examined in the halothane-anesthetized and paralysed rat by in situ hybridization histochemistry using specific 35S-labelled oligonucleotides for detection of c-fos and nerve growth factor inducible-A gene (NGFI-A) messenger RNAs. Stimulation of the cuneiform nucleus led to increases in mean arterial pressure and heart rate, whereas no cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals [see concurrent mid- and hindbrain study [Lam W. et al. (1996) Neuroscience 71, 193-211]. Cuneiform nucleus stimulation was associated with increased c-fos and NGFI-A messenger RNA levels bilaterally in the ventromedial, dorsomedial and lateroanterior hypothalamic nuclei, lateral and anterior hypothalamic areas, and ipsilaterally in the medial amygdaloid nucleus, at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. C-fos, but not NGFI-A, messenger RNA expression was increased bilaterally in the piriform cortex and subparafascicular thalamic nucleus. These results are consistent with the existence of direct and indirect projections between the cuneiform nucleus and the aforementioned activated areas, the functions of which may include the control of reproduction and metabolism, as well as cardiovascular regulation. The ipsilateral nature of responses in certain brain areas may be explained by the absence of decussating pathways and/or the presence of multisynaptic connections which attenuate bilateral signal transmission. The existence of structures that are known to receive afferent projections from the cuneiform nucleus, but that were not activated, may be explained by synaptic depolarization not reaching the threshold for immediate early gene expression or by a net inhibitory effect on innervated neurons. Characterization of these activated forebrain regions using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the response to stimulation of the cuneiform nucleus.

Medial prefrontal cortex depressor response: role of the solitary tract nucleus in the rat.

The depressor response elicited by unilateral low intensity electrical stimulation of the rat ventral medial prefrontal cortex may be mediated by a connection with the solitary tract nucleus. We tested this hypothesis by (i) examining the influence of medial prefrontal cortex stimulation on the induction of Fos-like immunoreactivity in neurons in the medulla oblongata, and (ii) by testing the effect of inhibition of solitary tract nucleus neurons on the medial prefrontal cortex stimulation-evoked depressor response. Depressor responses (>10 mmHg) were elicited by electrical stimulation of the medial prefrontal cortex every minute for 1 h ('Stimulated' group). Control animals were treated identically but did not receive electrical stimulation ('Unstimulated' group). Neurons exhibiting Fos-like immunoreactivity were abundant at the stimulation site which included the infralimbic area, and dorsal peduncular cortex. Medullary Fos-like immunoreactivity observed in the 'Stimulated' and 'Unstimulated' groups exceeded levels observed in untreated rats and was detected in the rostral, caudal and intermediate areas of the ventrolateral medulla, and the commissural, intermediate, medial and lateral regions of the solitary tract nucleus, as well as the medial vestibular nucleus, and the dorsal motor nucleus of the vagus. The number of neurons displaying Fos-like immunoreactivity in the ipsilateral solitary tract nucleus and caudal ventrolateral medulla of the 'Stimulated' group was found to be significantly elevated compared to the contralateral side (P<0.05), and the 'Unstimulated' group bilaterally. Inhibition of solitary tract nucleus neurons using bilateral injections of the GABA(A) receptor agonist muscimol (44 pmol/25 nl) inhibited the sympathetic vasomotor baroreflex and attenuated the depressor and sympathoinhibitory response to medial prefrontal cortex stimulation by 62% and 65%, respectively. These findings suggest that the projection from the medial prefrontal cortex to the solitary tract nucleus is excitatory and support the hypothesis that the depressor response elicited by medial prefrontal cortex stimulation is mediated, in part, by a cortico-solitary projection which activates the intramedullary baroreflex pathway.

Excitotoxin-induced degeneration of rat vagal afferent neurons.

The inferior vagal or nodose ganglion contains the perikarya of vagal afferent neurons that function as cardiopulmonary and abdominal visceral receptors as well as aortic arch baroreceptors. In this study we have sought to utilize the axon-sparing properties of the excitotoxins kainic acid, N-methyl-D-aspartic acid and alpha-amino-3-hydroxy-4-isoxazolepropionic acid to destroy the perikarya of these sensory neurons and thus selectively de-afferent the vagus in the rat. Kainic acid (0.5 nmol/microliters, 2 X 2 microliters) was applied topically to both nodose ganglia and the rats were allowed to recover for 7-8 days. Baroreceptor heart rate reflex activity was assessed in these conscious rats. Baroreceptor heart rate reflex gain was reduced (-51%) in kainic acid-treated rats, as was the maximal reflex bradycardia induced by the pressor agent, phenylephrine. Kainic acid treatment did not alter resting mean arterial pressure or heart rate. Vagal efferent neurons were spared by kainic acid treatment since bradycardic responses to electrical stimulation of the peripheral end of a cut vagus were not impaired. Histological studies showed marked destruction of perikarya within the nodose ganglia of kainic acid-treated rats: inflammatory and degenerative changes were evident at 2 days, and at 10 days there was considerable loss of neuronal cell bodies, but sparing of axons. Topical application to the nodose ganglion of alpha-methyl-DL-aspartic acid (6.8 nmol/microliters, 2 X 2 microliters), a non-excitotoxic dicarboxylic acid, failed to alter baroreflex sensitivity or produce perikaryal degeneration in nodose ganglia.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison between the in vivo and in vitro activity of five potent and competitive NMDA antagonists.

1. Phosphonate analogues of glutamate have been tested and compared as N-methyl-D-aspartate (NMDA) antagonists in electrophysiological and binding experiments. The compounds tested were three established NMDA antagonists: D-2-amino-5-phosphonopentanoate (D-AP5), DL-2-amino-7-phosphonoheptanoate (DL-AP7), 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), and two novel putative NMDA antagonists: 3-(2-carboxypiperidin-4-yl)propyl-1-phosphonate (CPPP) and 3-(2-carboxy-piperidin-4-yl)methyl-1-phosphonate (CPMP). 2. When administered electrophoretically to rat spinal neurones in vivo, these compounds were found to be selective NMDA antagonists with little effect on excitations evoked by quisqualate and kainate. CPMP and CPPP were approximately equipotent with CPP and about 5 times more potent than D-AP5. 3. Following systemic administration, 2-5 mg kg-1 i.v. of CPP, CPMP and CPPP reduced NMDA-evoked excitations by 70-100% whereas 50-100 mg kg-1 of D-AP5 and DL-AP7 produced a similar effect. The onset of the effects required 20-30 min and lasted more than six hours. 4. On bath application to cortical wedges, the IC50 values (microM) for antagonism of 40 microM NMDA were: CPP, 0.64 +/- 0.06 (mean +/- s.e.mean; n greater than 4); CPMP, 1.65 +/- 0.13; CPPP 0.89 +/- 0.09; D-AP5, 3.7 +/- 0.32; DL-AP7, 11.1 +/- 2.1; and DL-AP4 and DL-AP6 were inactive at 100 microM. 5. In binding studies with [3H]-CPP, the Ki values (nM) were: CPP, 446 + 150 (mean + s.e.mean; n > 3); CPMP, 183 + 74 and CPPP, 179 +/- 13 whereas against NMDA (lO microM)-stimulated [3H]- TCP (thienylcyclohexylpiperidine) binding the ICjo values (microM) for CPMP and CPPP respectively were 5.6 + 2.7 and 4.5 + 2.2. 6. Systemic administration of CPPP and CPMP, at doses sufficient to antagonize NMDA, also reduced cardiovascular responses to 5-hydroxytryptamine (Bezold-Jarisch reflex). This illustrates a role for NMDA receptors in central cardiovascular control. 7. The results indicate the systemic doses of piperidine and piperazine analogues of D-AP5 which may be used for assessing the role ofNMDA receptors in central synaptic function.

Withdrawal-like behaviour induced by inhibitors of biogenic amine reuptake in rats treated chronically with delta 9-tetrahydrocannabinol.

The effects of the biogenic amine reuptake inhibitors fluoxetine, clomipramine and imipramine on the behaviour of rats after chronic treatment with delta 9-tetrahydrocannabinol (delta 9-THC) for 5 and 10 days were examined. Rats with permanently in-dwelling IV cannulae were injected twice daily with doses of delta 9-THC (2-6 mg/kg). delta 9-THC treatment reduced the rate of body weight gain and induced the typical biphasic modifications of behaviour. Tolerance developed to both of these effects. On days 6 and 11 of the experiment, rats were injected IP with 15 mg/kg imipramine HCl, clomipramine HCl or fluoxetine HCl, and behaviour, consisting of writhes, backward kicks, jumps and wet shakes, was observed for the next 30 min. Each of the amine reuptake inhibitors induced changes in behaviour, the severity of which appeared to correlate with their ability to inhibit the reuptake of 5-hydroxytryptamine (5-HT). It is suggested that tryptaminergic mechanisms are involved in the production of a withdrawal-like behaviour after chronic delta 9-THC treatment.

Attenuation of delta 9-tetrahydrocannabinol-induced withdrawal-like behaviour by delta 9-tetrahydrocannabinol.

Rats chronically treated with increasing daily doses of delta 9-tetrahydrocannabinol (delta 9-THC) for up to 5 or 10 days exhibit a withdrawal-like behavioural response when challenged with clomipramine, a potent inhibitor of serotonin (5-hydroxytryptamine) reuptake, at the time when the next injection of delta 9-THC was due. To determine whether this response is indeed a withdrawal syndrome, different groups of rats were injected IV twice daily for up to 5 days with either delta 9-THC, in doses increasing from 2-6 mg/kg, or polyvinyl-pyrrolidone (PVP), the vehicle in which delta 9-THC was suspended. On day 6, an acute dose of 6 mg/kg delta 9-THC was given 30 min before IP clomipramine (15 mg/kg). The total behavioural response, which included writhing, backward kicking, jumping, head shaking, and paw tremor, was attenuated in rats chronically treated with delta 9-THC, but not in rats which received an acute dose of PVP. These results lend further evidence to our hypothesis that chronically administered delta 9-THC produces a state of physical dependence in the rat.

The effects of ?(9)-tetrahydrocannabinol ?(9)-THC) on the regional concentrations of spermidine in the rat brain.

The effects of intravenous administration of ?(9)-tetrahydrocannabinol (?(9)-THC, 2 mg/kg, i.v.) on the regional brain spermidine concentrations of the rat were examined. Thirty minutes after vehicle treatment, the spermidine concentrations were: for the medulla oblongata/pons, 68.2 +/- 7.7 ?g/g; the hypothalamus, 67.7 +/- 2.6 ?g/g; the midbrain, 59.1 +/- 4.4 ?g/g; the cerebellum, 47.3 +/- 5.9 ?g/g and for the cortex, 13.8 +/- 0.8 ?g/g. Thirty minutes after ?(9)-THC, these concentrations were reduced in the midbrain (47.0 +/- 8.0% of control, P < 0.0001) and cortex (69.4 +/- 7.4% of control, P < 0.009). The spermidine concentrations were not significantly altered in the medulla oblongata/pons (86.5 +/- 13.3%, P > 0.36), hypothalamus (107.2 +/- 11.8, P > 0.36) or cerebellum (89.0 +/- 14.4%, P < 0.48). These results suggest that spermidine within the midbrain and cortex may be involved in the expression of some of the actions of ?(9)-THC.