Release, spread and persistence of immunoreactive neurokinin A in the dorsal horn of the cat following noxious cutaneous stimulation. Studies with antibody microprobes

In barbiturate anaesthetized spinal cats antibody microprobes were used to examine release of immunoreactive neurokinin A following cutaneous thermal and mechanical stimulation. In the absence of peripheral stimuli, microprobes detected a diffuse basal presence of immunoreactive neurokinin A. Noxious mechanical and to a lesser extent noxious thermal stimuli increased the levels of immunoreactive neurokinin A diffusely throughout the dorsal horn which, in many cases, spread into the adjacent white matter. These diffuse stimulus-evoked increases contrast with previous experiments where the same stimuli produced discrete focal increases in levels of immunoreactive substance P. Evidence was obtained that released immunoreactive neurokinin A persisted in the spinal cord for at least 30 min beyond the period of stimulation. Neurokinin A needs consideration as the agent responsible for the long-lasting increases in excitability of some spinal neurons found by several laboratories to follow a brief input from unmyelinated primary afferents.

Excitotoxin-induced lesions of the central but not basolateral nucleus of the amygdala modulate the baroreceptor heart rate reflex in conscious rats

The present study has examined the influence of the central nucleus of the amygdala (Ce) and the basolateral nucleus of the amygdala (BL) on the baroreceptor heart rate (HR) reflex in conscious, unrestrained rats. Baroreceptor HR reflex activity was examined in rats with bilateral excitotoxin (N-methyl-D-aspartate; 40 nmol/side)-induced lesions of the Ce or the BL and in control rats (artificial cerebrospinal fluid). After lesioning, the reflex HR responses were recorded following intravenous bolus doses of the pressor agent phenylephrine and the depressor agent sodium nitroprusside. Baroreceptor reflex parameters were determined by sigmoidal computerized curve-fitting. Lesions of either the Ce or the BL failed to affect resting mean arterial pressure (MAP) or HR. However, the Ce lesion reduced the maximum and average gain (sensitivity) of the baroreceptor reflex, and diminished the range of the reflex by altering the minimum, but not the maximum HR plateau. The upper and lower reflex thresholds and the MAP value corresponding to the midpoint of the HR range were not affected. Bilateral lesions of the BL failed to modify any baroreceptor reflex parameters. These results suggest that despite previous evidence for the involvement of the BL in cardiovascular regulation this nucleus does not exert a tonic influence on vasomotor neurons nor does it influence the baroreceptor HR reflex. In contrast, neurons projecting from the Ce appear to provide excitatory input to medullary neurons involved in baroreceptor reflex are regulation.

Strain differences in central nervous system concentrations of cholecystokinin between normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SH) rats

The regional brain and spinal cord concentrations of cholecystokinin-octapeptide (CCK-8) were measured in age-matched normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats. The relative order of distribution of CCK-8 in the WKY strain was hippocampus (20.5 +/- 1.3 pmol/g) greater than cortex greater than striatum = hypothalamus greater than midbrain = thalamus greater than spinal cord greater than medulla oblongata/pons (MO/P, 1.6 +/- 0.2 pmol/g) whereas in the SH strain this order was hippocampus (12.9 +/- 0.8 pmol/g) greater than cortex = striatum greater than hypothalamus greater than midbrain greater than thalamus = spinal cord greater than MO/P (1.4 +/- 0.2 pmol/g). The concentrations of CCK-8 in the cerebellum were at the level of assay sensitivity (0.5 pmol/g in both strains). In comparison to the WKY rats, the SH strain had significantly lower levels of CCK-8 in the hippocampus (-37%), cortex (-28%), spinal cord (-23%) and pituitary (-57%). The lowered levels of CCK-8 in the brain of the SH rat may be causally related to, or result from, the cardiovascular, behavioural or morphological abnormalities of this strain.

Serum glucose and insulin levels in normotensive (WKY) and spontaneously hypertensive (SH) rats during and after the cessation of continuous (10 day) clonidine infusion

The concentrations of glucose were elevated whereas those of insulin were decreased in the sera of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats on day 10 of a continuous subcutaneous infusion of clonidine (10 micrograms kg-1 h-1 for 10 days). Fifteen to 18 h after cessation of infusion, the glucose levels of both strains had fallen to below those of the respective controls whereas the insulin levels remained suppressed. As such the hypoglycaemia may be related to the general increase in metabolic requirements associated with the post-infusion 'withdrawal syndrome'.

Acute intracerebroventricular injections of the mast cell degranulator compound 48/80 and behavior in rats

The intracerebroventricular (ICV) injection of the mast cell degranulator Compound 48/80 (2.5-2.0 micrograms/kg) produced a marked behavioral syndrome in normotensive rats. The behaviors included head and body shakes, paw tremor, excessive grooming, unusual posture and gait, mild diarrhoea, piloerection, extreme agitation and irritability to touch, and a later phase of sedation. The highest doses (15 and 20 micrograms/kg) also produced catalepsy and episodes of "barrel rolling" (continuous rolling of 1-8 turns around the longitudinal axis). These behaviors were observed for approximately 15-30 min although the sedation and catalepsy were maintained for 90-120 min. A second ICV injection of the 10 micrograms/kg dose of Compound 48/80 given 2 hr after an initial injection of this dose, produced a much reduced response and the numbers of head and body shakes, and episodes of paw tremor and grooming were between 20-30% of those produced by the first injection. The reduced effect of the second injection indicates that the behavioral effects of Compound 48/80 may arise from the acute degranulation of mast cells rather than direct effects on neuronal populations or the cerebral vasculature.

The concentration of neuropeptide Y immunoreactivity falls with age selectively in microdissected regions of the ventrolateral medulla of spontaneously hypertensive and normotensive Wistar-Kyoto rats

The concentrations of neuropeptide Y immunoreactivity were measured in microdissected regions enriched in noradrenergic (A1, A2, A6) and adrenergic (C1, C2, C3) nuclei of the brainstem, and in the nucleus of the spinal tract of the trigeminal nerve (Sp5C) of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 8, 18 and 31 weeks of age. The aim of this study was to compare the manner in which changes in neuropeptide Y immunoreactivity levels related to increases in blood pressure with ageing in each rat strain. The concentration of neuropeptide Y immunoreactivity in the A1 nucleus progressively fell with increasing age in both SHR and WKY. In contrast, the levels of neuropeptide Y immunoreactivity in the C1 region fell at 18 weeks of age but did not fall further by 31 weeks. No significant age-related changes occurred in the concentrations of neuropeptide Y immunoreactivity in the A2, C2, A6 and Sp5C nucleus. The levels of neuropeptide Y immunoreactivity in the C3 region were below assay sensitivity. The neurochemical changes that occur in the A1 nucleus are consistent with the increase in blood pressure observed with ageing in both rat strains. However, this observation alone does not account for the elevated blood pressure measured in the SHR strain.

Pharmacokinetics and antihypertensive effects of low dose clonidine during chronic therapy

Using a sensitive and specific radioimmunoassay the pharmacokinetic disposition of clonidine was determined in hypertensive patients after a single dose and then after 5, 28 and 56 days of chronic dosing with 75 micrograms bd. Following a single dose of clonidine maximal plasma concentrations of 0.34 +/- 0.06 ng/ml were achieved after 3.6 +/- 1.2 hours. After 5 days of repetitive dosing the maximal concentration was significantly higher, 0.66 +/- 0.06 ng/ml and remained so throughout chronic therapy (P = 0.018). The AUC, Tmax and T1/2 did not differ significantly between the acute dose and the chronic dosing pharmacokinetic studies. Clonidine also produced a significant fall in blood pressure. Supine diastolic blood pressure fell from 106 +/- 5 mmHg predose to 99 +/- 6 mmHg 2 hours after the first dose (P less than 0.05). The corresponding values after cyclopenthiazide alone were 108 +/- 8 and 105 +/- 8 mmHg (P = 0.13). Similar falls in blood pressure were produced during chronic therapy.

Right ventricular neuropeptide Y levels in a rat model of left ventricular failure

Neuropeptide (NPY) and norepinephrine (NE) concentrations were measured in the right ventricle (RV) of rats with cardiac failure resulting from coronary artery ligation. RV NE concentrations fell significantly in rats with infarcts while RV NPY concentrations were unaltered.

Relationship between the dose-response effects of diazepam and clobazam on electroencephalographic parameters and on kindled amygdaloid seizure activity in rats

1. The possibility that the anticonvulsant activity of the benzodiazepines, diazepam and clobazam, is related to changes in EEG parameters, particularly beta activity, was investigated in amygdaloid kindled rats. 2. The effects of diazepam (1, 2, 4, 8 and 16 mumols/kg), administered intraperitoneally (i.p.), clobazam (1, 2, 4, 8, 16 and 32 mumols/kg, i.p.) or vehicle (dimethyl sulfoxide) on the cortical EEG of amygdaloid kindled rats were quantitated for 15 min using computerized period amplitude analysis. Immediately afterwards, the amygdala was stimulated and the after-discharge duration (AD) and the seizures stage (SS) were determined. 3. The equivalent percentage time (EPT) of the diazepam-treated group was decreased in the theta band (4-8 Hz) and increased in the alpha (8-12 Hz) and first six beta (12-36 Hz) bands. The mean peak amplitude (MPA) was increased in the alpha (8-12 Hz) and all seven beta bands (12-40 Hz). Clobazam increased the EPT and MPA in the alpha (8-12 Hz) and all seven beta (12-40 Hz) bands. The MPA was also increased by clobazam in the theta (4-8 Hz) band. 4. Diazepam reduced both the AD and SS of the kindled seizures at doses of 4, 8 and 16 mumols/kg, whereas clobazam was anticonvulsant at doses of 16 and 32 mumols/kg. The reduction in both AD and SS correlated with increases in the EPT and MPA in the first beta (12-16 Hz) band in the diazepam-treated group and in the first four beta (12-28 Hz) bands in the clobazam-treated group.

Involvement of hypothalamic adrenaline in the clonidine withdrawal syndrome in normotensive and spontaneously hypertensive rats

1. The concentrations of adrenaline and other catecholamines (noradrenaline and its major metabolite DHPG, dopamine and its major metabolite DOPAC) were measured in the hypothalamus and medulla oblongata of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) which had received a continuous subcutaneous infusion of clonidine (10 micrograms/kg per h) for 10 days, and also in WKY and SHR rats which were killed 15-18 h after the cessation of the 10-day infusion. 2. The concentrations of adrenaline in the hypothalamus and medulla oblongata/pons of the clonidine treated WKY and SHR rats were not different from their respective controls. However, the adrenaline concentrations in the hypothalamus (but not the medulla oblongata) were significantly decreased in the post-infusion WKY and SHR. 3. These results suggest that hypothalamic adrenergic mechanisms may have a common involvement in the post-clonidine infusion syndromes displayed by the WKY and SHR strains.

Regional brain concentrations of cholecystokinin in the rat: the effects of kindled and non-kindled seizures

In an attempt to understand the neurochemical basis of kindling, this study investigated the effects on brain cholecystokinin (CCK) of amygdaloid kindled and non-kindled seizures. Thirteen brain regions were examined in rats sacrificed either 24 hr or 3 weeks after the last kindled seizure, or 24 hr after a suprathreshold stimulation-induced (non-kindled) seizure; and in sham kindled rats. There were no significant differences in CCK immunoreactivity between any of these groups. These results do not confirm a previous report of an increase in CCK in the hippocampus following amygdaloid kindling in the rat.

Lack of involvement of endogenous mu-receptor opioids in the hypothermic effects of clonidine in normotensive and spontaneously hypertensive rats

The effects of successive injections of the alpha-adrenoceptor agonist clonidine (25, 50 and 100 ug/kg given at hourly intervals) on the body temperature of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats, previously treated for 48 hr with slow release emulsions (subcutaneous) containing either morphine (morphine SR, 100 mg/kg), naloxone (naloxone SR, 80 mg/kg) or no drug (vehicle SR), were examined. The successive injections of clonidine produced dose-dependent falls in body temperature which were quantitatively similar in the vehicle-treated WKY and spontaneously hypertensive rats. The hypothermic effects of clonidine in the morphine-dependent WKY and spontaneously hypertensive rats, and in the naloxone-treated WKY and spontaneously hypertensive rats, were not different to those of the respective vehicle-treated controls. These results suggest that endogenous mu-receptor opioid peptides do not have a major involvement in the hypothermic actions of clonidine, in either normotensive or spontaneously hypertensive rats.

Differences in the development of tolerance to two anticonvulsant benzodiazepines in the amygdaloid kindled rat

The effects of chronic treatment with two benzodiazepines were studied on kindled amygdaloid seizures in rats. Clobazam (4 mg kg-1), clonazepam (0.3 mg kg-1) or vehicle (1 mL kg-1) was administered, by intraperitoneal injection, to fully kindled rats twice daily for nineteen days. Each rat was electrically stimulated 30 min after the morning dose on alternate days of treatment. Tolerance developed rapidly to the anticonvulsant effects of clobazam after only three days of treatment, following which only a small residual protection was maintained. Tolerance to clonazepam developed gradually over the course of the experiment, although this effect was relatively minor.

Sites of actions of alpha 2 agonists in the brain and periphery

Alpha 2 agonists including rilmenidine, clonidine and alpha methylnoradrenaline all seem to have a predominantly central action in lowering blood pressure. Studies using the peripheral decarboxylase inhibitor alpha-methyldopa hydrazine in both experimental animals and humans support the predominant central action of the active metabolite of methyldopa, alpha methylnoradrenaline. Comparative studies on the antihypertensive potency of selective alpha 2 agonists like rilmenidine, guanfacine, clonidine and lofexidine suggest that the relative ability to bind to alpha 1 and alpha 2 adrenoceptors is not critical but the ability to enter the central nervous system is essential for antihypertensive efficacy.

Effects of continuous infusions (10 days) and cessation of infusions of clonidine and rilmenidine (S 3341) on cardiovascular and behavioral parameters of spontaneously hypertensive rats

Clonidine is a centrally acting antihypertensive drug that acts in vivo at both alpha 1- and alpha 2-adrenoceptor sites, whereas rilmenidine (S 3341) is more selective for alpha 2 adrenoceptors. The present study compared the effects of continuous 10-day infusions of clonidine (5 micrograms/kg/hour) with those of rilmenidine (100 micrograms/kg/hour) on various cardiovascular and behavioral parameters in the spontaneously hypertensive rat. The changes in these parameters after cessation of the infusions were also compared. At these rates of infusion, clonidine and rilmenidine produced quantitatively similar reductions in mean arterial pressure (MAP), lability of MAP and cardiovascular responsiveness during normal behaviors such as eating and grooming. Neither drug infusion affected heart rate. The cessation of the clonidine infusion resulted in a "withdrawal" syndrome characterized by prominent rapid eye movement-sleep rebound, and cardiovascular and behavioral disturbances including an increased lability of MAP, exaggerated cardiovascular responses during normal behaviors, tachycardia, and an "opiate abstinence-like" syndrome including head and body shakes. Cessation of rilmenidine infusion resulted in somewhat similar cardiovascular and behavioral disturbances, but unlike clonidine there was a return to normal rapid eye movement sleep without rebound.

Altered levels of neuropeptides in the medulla and spinal cord of spontaneously hypertensive rats

1. Spontaneously hypertensive rats (SHR) are useful for investigating the possible pathophysiological and neurochemical basis of human essential hypertension. 2. The accepted pathogenic mechanism of hypertension in SHR is an increased central sympathetic drive which results in an increased peripheral resistance. 3. The neurochemical basis of the increased sympathetic drive is unknown. The observation that there are reduced levels of neuropeptides (vasoactive intestinal peptide, neuropeptide Y, cholecystokinin octapeptide, neurotensin and calcitonin gene related peptide) in the spinal cord in SHR rats compared with age and gender matched Wistar-Kyoto normotensive rats could provide a basis for understanding the mechanism of hypertension in SHR.

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.

Regional brain concentrations of neurotensin following amygdaloid kindled and cortical suprathreshold stimulation-induced seizures in the rat

There is now considerable evidence that putative neuropeptides are involved in epileptogenic processes. Neurotensin (NT), which affects neuronal excitability in many parts of the CNS, is potentially relevant to the kindling phenomenon, but has not previously been studied in the kindling model of epilepsy. This study compared the short term (24 hr) and long term (3 wk) effects of amygdaloid kindled seizures, and the short term effects of non-kindled seizures (cortical suprathreshold stimulation-induced seizures, STS), on the concentrations of NT in 13 brain regions in the rat. Whereas kindled seizures produced neither short term nor long term changes in NT, the STS resulted in altered NT concentrations, with a decrease in the cortex and increases in the hippocampus and cervical spinal cord. These results reveal seizure-type specific changes in NT in 3 brain regions; and indicate that although NT does not appear to be involved in kindling, it may be important in the neurochemical mechanisms underlying STS.

Surgical and pharmacological reductions in sympathetic nerve activity increase the neuropeptide Y-immunoreactivity content of the rat iris but not the vas deferens

Decentralization of the superior cervical ganglion (S.C.G.) of the rat elevated the neuropeptide-Y immunoreactivity (NPY-ir) content of the ganglion on day 1 (+43%) but not on day 3 post-surgery. The content of NPY-ir in the iris was increased by decentralization (+40%) 3 days post-surgery, and treatment with clonidine (+43%), and pempidine (+82%). The levels of NPY-ir in the rat vas deferens were not affected by either surgical or pharmacological treatment. These results suggest NPY is released from sympathetic nerves in the iris but not vas deferens during normal sympathetic activity.

Differential effects of surgical sympathectomy on rat heart concentrations of neuropeptide Y-immunoreactivity and noradrenaline

The aim of this study was to estimate the proportion of cardiac neuropeptide Y-immunoreactivity (NPY-ir) which is not present in sympathetic neurones innervating the rat heart. The procedure employed was to surgically sympathectomize the heart and then measure the remaining cardiac concentrations of NPY-ir and noradrenaline (NA). Unilateral (left) sympathectomy significantly reduced the levels of NPY-ir and NA in all regions of the heart (by 40-70%) except for the NPY-ir in the right atrium which was unaltered. The effect of bilateral sympathectomy was significantly greater than that of unilateral sympathectomy. Unilateral and bilateral sympathectomy produced similar reductions in the concentrations of NPY-ir and NA in the ventricular tissue. In contrast dissimilar changes were produced in the atrium. Although bilateral sympathectomy almost totally depleted the NA from the right atrium (by 98%), the NPY-ir levels were only reduced by 50%. These results indicate that approximately half the content of NPY in the right atrium is not present in sympathetic noradrenergic neurones. This pool may occur in the previously described intrinsic neurones of the right atrium.

Medial prefrontal cortical lesions modulate baroreflex sensitivity in the rat

Previous neuroanatomical studies in rats have demonstrated that the medial prefrontal cortex sends projections to the nucleus of the solitary tract which also receives the bulk of baroreceptor information from primary afferents within the IXth and Xth cranial nerves. The present study examines the influence of the prefrontal cortex on baroreceptor heart rate reflex in conscious rats. Baroreceptor reflex activity was examined in rats with bilateral excitotoxin (N-methyl-D-aspartate)-induced lesions of the medial prefrontal cortex and in control rats (artificial cerebrospinal fluid). Seventeen to eighteen days after lesioning, reflex heart rate responses were recorded following intravenous bolus doses of the pressor agent phenylephrine and the depressor agent sodium nitroprusside. Baroreceptor reflex parameters i.e., maximum and average baroreceptor reflex gain (or sensitivity): minimum and maximum heart rate plateaus; heart rate range; upper and lower reflex thresholds, were determined by sigmoidal computerized curve-fitting. Lesioning the medial prefrontal cortex did not affect resting mean arterial pressure and heart rate. However, the lesion reduced maximum and average baroreceptor reflex gain and produced a small reduction in lower reflex threshold. The other parameters were unaffected by the lesion. These observations suggest that although the medial prefrontal cortex does not exert a tonic influence on brainstem vasomotor neurons, there may be a descending excitatory projection from this brain region to medullary neurones involved in the baroreceptor reflex arc.

The development of tolerance to the anticonvulsant effects of clonazepam, but not sodium valproate, in the amygdaloid kindled rat

The effects of chronic treatment with clonazepam and sodium valproate were studied on kindled amygdaloid seizures in rats. Fully kindled rats were given an intraperitoneal (i.p.) injection of either vehicle (1.0 ml/kg), clonazepam (0.3 mg/kg) or sodium valproate (200 mg/kg) twice daily for 12 days. Each rat was stimulated through an amygdaloid electrode 30 min after the morning dose. While both drugs initially blocked and kindled seizure (P less than 0.01), the results showed a significant trend (P less than 0.02) in the development of tolerance to the anticonvulsant action of clonazepam but no significant tolerance to the action of valproate.

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.