Inhibitory effect of ondansetron on glycine response of dissociated rat hippocampal neurons.

We examined the effect of ondansetron, an antagonist of type 3 serotonin receptors, on the whole cell response of freshly isolated hippocampal CA1 pyramidal neurons of neonatal and "mature" rats to glycine using the gramicidin perforated patch technique. Ondansetron depressed the current induced by subsaturating concentrations of glycine (IGly) in a concentration-dependent manner. The ondansetron concentration needed to depress IGly induced by 30 microM glycine to half amplitude was 25 microM. Ondansetron (54 microM) shifted the glycine concentration-response curve to the right in a parallel manner, increasing the EC50 for glycine from 40 +/- 3 microM to 70 +/- 5 microM. Ondansetron increased the time constant of activation of IGly without affecting the time constant of deactivation. When examined under current clamp conditions, glycine induced depolarization and hyperpolarization in neonatal and mature neurons, respectively; ondansetron also suppressed these responses to glycine. The data suggest that ondansetron competitively inhibits the glycine receptor.

Effect of pre- and postnatal nicotine exposure on vasopressinergic system in rats.

Timed pregnant Sprague-Dawley rats were infused subcutaneously either with nicotine (NIC, 6 mg kg-1 day-1; n=17) or saline (control, n=15) on the 3rd day of gestation, via Alzet osmotic pumps, for 28 days. After the parturition, the pups of both, control and NIC infused dams, were each randomly divided into 2 groups and placed to be nursed as following: (1) control dams nursing pups born to control mother (control group); (2) control dams nursing pups born to NIC-infused mother (prenatal NIC group); (3) NIC-infused dams nursing pups born to control mother (postnatal NIC group); (4) NIC-infused dams nursing pups born to NIC-infused mother (pre- and postnatal NIC group). Vasopressin (VP) was measured by RIA in plasma, neurointermediate lobe (NIL) and hypothalamus (HT) in the pups of both sexes, at the following age: 0 (within 24 h after birth); 1, 2, 3, 4 and 6 weeks. At the age of 3, 4 and 6 weeks, the isolated NILs were individually superfused and VP was measured as a basal release and the response to a 10-min 56 mM potassium stimulation. A marked suppression in the activity of VP-ergic system was observed in both sexes of offspring exposed to NIC prenatally, being first detectable at the age of 3 weeks, when the HT-NIL system becomes fully developed. However, the significant changes were observed at the age of 6 weeks: decreased serum VP concentration, lower VP contents in the HT and NIL, and suppressed VP release, basal and stimulated, from the isolated NIL. Postnatal exposure to nicotine was ineffective.

Hypotension does not alter the antinociceptive effect of nifedipine.

We explored the relationship between antinociceptive and hypotensive effects of nifedipine (NIF) injected intraperitoneally ( ip, 15 mg/kg) and epidurally (epi, 20 microM), as compared to verapamil (VER, 10 mg/kg ip) and nitroglycerin (NTG, 0.1 and 0.15 mg/kg ip). The systolic blood pressure (BP) and tail-flick (TF) latencies were measured simultaneously every 10 min for 2 hours and individual values of both measurements were correlated. The highest antinociceptive as well as hypotensive effects were both measured in the group receiving NIF epi., with the correlation coefficient r2=0.2878. Injected ip., NIF revealed similar antinociceptive effect, whereas the other studied drugs were not effective. As to the degree of hypotensive activity, NIF epi was followed by VER, NTG 0.1, NIF ip. and NTG 0.15. No significant correlation was found between BP and TF latencies in any group receiving the drugs. We concluded that the antinociceptive response, measured by the tail-flick technique, is independent of the hypotensive activity of the studied drugs, including NIF.

The effects of serotonin biosynthesis inhibition on nicotine and nifedipine-induced analgesia in rats.

UNLABELLED: The calcium channel blocker nifedipine has analgesic properties that are enhanced by nicotine. Although it is not known how this analgesic state might affect the awareness of anginal pain and impending myocardial infraction, recent studies have shown an increased mortality associated with the use of large doses of nifedipine. Because both nifedipine- and nicotine-induced analgesia involve serotonergic mechanisms, we studied the effects of the serotonin biosynthesis inhibitor parachlorophenylalanine (pCPA) on nifedipine- and nicotine-induced analgesia. Nociception was assessed by tail-flick method. Rats pretreated with pCPA (300 mg/kg intraperitoneally [IP]) followed by either nifedipine (15 mg/kg IP) or nicotine (1 mg/kg subcutaneously) had a increase in tail-flick latency of 41% (P = 0.09) and 50% (P = 0.05), respectively, compared with animals that did not receive pCPA. Additionally, rats pretreated with pCPA followed by a combination of nicotine and nifedipine doubled their tail-flick latency (P = 0.0001) compared with animals that were not treated with pCPA. These data further support the involvement of the serotonergic system in both nifedipine- and nicotine-induced analgesia and suggest that drugs that affect serotonin levels, including tricyclic antidepressants and serotonin-specific reuptake inhibitors, may also affect the analgesia induced by nifedipine and nicotine. IMPLICATIONS: This study examines the effect of serotonin depletion on nicotine- and nifedipine-induced analgesia. Nifidipine is a calcium channel blocker used to treat high blood pressure. It also has pain-relieving properties that are enhanced by nicotine. Because both nifedipine- and nicotine-induced analgesia involve the neurotransmitter serotonin, it is important to know how changes in serotonin concentration might affect both nicotine- and nifedipine-induced analgesia. This study not only supports the involvement of the serotonergic system in both nifidipine- and nicotine-induced analgesia, but also suggests that drugs that affect serotonin levels may also affect analgesia induced by nifidipine and nicotine.

Ondansetron exhibits the properties of a local anesthetic.

UNLABELLED: The purpose of this study was to determine whether ondansetron (OND) has local anesthetic effects. Using a patch-clamp technique, we showed that OND concentration dependently blocked Na channel currents in freshly isolated neurons of rat brains with a 50% inhibition concentration of 12 microM. The blockade started immediately when OND was applied to the cell body using a fast perfusion system, reached a plateau within 15 s, and recovered to the control level within 30 s after washout of the OND-containing solution. Because this is a known property of local anesthetics, we used the tail-flick technique to verify this effect in vivo in Sprague-Dawley rats (n = 46). OND was injected subcutaneously into the tail at the doses of 0.08, 0.16, and 0.2 mg. The tail-flick latency increased 2 min after OND injection, reaching the plateau within 5 min. This effect was dose-related, lasting from 10 to 25 min. These preliminary data indicate that OND, a selective 5-HT3 receptor antagonist, might serve as a prototype molecule for development of a novel series of local anesthetics. IMPLICATIONS: Ondansetron is a drug used to prevent vomiting, especially in cancer patients after chemotherapy. We found that it also causes numbness when injected under the skin. This new action may contribute to its role in "calming the stomach." We studied the effect of ondansetron on the isolated brain cells of live rats.

The effect of chronic nicotine and its withdrawal on ED50 inspired halothane requirement in rats.

BACKGROUND: Based on previously observed antinociceptive effect of chronic nicotine treatment in rats, the authors measured the ED50 inspired halothane, concentrations in rats having withdrawn from chronically infused nicotine, and in control rats. METHODS: Rats in nicotine group (n = 19) were chronically exposed to nicotine (6 mg/kg/day for 28 days) infusion via Alzet osmotic pumps. Those in the control group (n = 16) were subjected to a sham operation. In each group, half of the rats was used one month later to explore the effect of nicotine withdrawal. Halothane in oxygen was delivered at a rate of 2 L/min to a box measured 2,912 cm3. The ED50 halothane concentration, measured by mass spectrophotometry, was a midway gas tension at which 50% of rats responded to the tail pinching applied to the distal third of the tail for 30 s. RESULTS: In nicotine group, the results showed a lower concentration of ED50 inspired halothane on days 1 and 3 in the first week (p < 0.05). In the control group, repeated exposures to halothane gradually decreased the ED50, reaching lowest value on day 7 (p < 0.05 compared to day 1). Nicotine withdrawal did not alter the ED50 concentration of halothane in test rats compared with control rats. CONCLUSIONS: We conclude that chronic nicotine infusion could temporarily decrease the ED50 concentration of inspired halothane, and withdrawal of nicotine did not modify the ED50 concentration of halothane. In addition, the ED50 halothane concentration might be influenced by stress.

GABA(A) antagonist and nicotine-induced antinociception.

Rats, pretreated with saline or GABA(A) antagonist bicuculline (BIC) at the doses of 2, 4, 6, 10 and 20 mg/kg, were injected with nicotine (NIC, 1 mg/kg) 30 min later. Tail-flick (TF) latencies were measured before (baseline) and three times at 10 min interval after pretreated compounds, continued every 10 min up to 1 hr after NIC injection. In all groups, median TF latencies did not change from the baseline for the first 30 min after the pretreatment. Following NIC alone (control) and in the group pretreated with 2 mg/kg BIC, 60% rats reached ceiling TF latencies (20 sec) lasting for 10 min. In groups with higher BIC doses (4 to 10 mg/kg), median TF latencies were in the range of 5-7 sec with 30% rats reaching the ceiling TF latencies. Following 20 mg/kg BIC, one out of five rats reached 20 sec; the median was in the range of 4-5 sec. Significantly lower responses were observed following 4 mg BIC and higher doses with no difference among the groups. In conclusion, our novel data show that BIC alone, injected systemically, does not possess any effect on the thermal nociceptive transmission as measured by the tail flick test. However, pretreatment with BIC partially prevents NIC-induced antinociception, in a non dose related manner. This suggests that GABA(A) receptors may, at least in part, contribute to the complex mechanisms involved in NIC-induced antinociception.

Antinociceptive effect of nifedipine and verapamil tested on rats chronically exposed to nicotine and after its withdrawal.

Antinociceptive effect of nifedipine (15 mg/kg i.p.) and verapamil (10 mg/kg s.c.) was examined in rats chronically exposed to nicotine (6 mg/kg/day via Alzet osmotic pump for 28 days) and after nicotine withdrawal. Sham operated rats served as control for testing DMSO (dimethylsulfoxide, a solvent for nifedipine), nifedipine and verapamil alone. Nociception was measured by the tail-flick technique. Nifedipine, but not verapamil, injected to control rats produced a ceiling tail-flick latency (20 sec) 30 min after the injection, lasting for 10 min. In rats exposed to chronic nicotine for 3 days, nifedipine treatment exhibited ceiling tail-flick latency within 10 min lasting for 80 min. Tested in rats exposed to nicotine for 3 weeks, nifedipine treatment produced this effect 25 min after the injection lasting for 60 min. Nicotine withdrawal abolished this effect. Verapamil did not exhibit any significant changes in tail-flick latencies. These data support our hypothesis that smoking patients treated with nifedipine could be at a potential risk in developing a high pain threshold and missing the first sign of heart attack--a chest pain.

Ondansetron blocks nifedipine-induced analgesia in rats.

The serotonergic system is involved in pain transmission and the 5-hydroxytryptamine (5-HT3) receptor subtype mediates some of these effects at the spinal level. Therefore, we explored the effects of the serotonergic system on nifedipine-induced analgesia by using the 5-HT3 receptor antagonist ondansetron. Male Sprague-Dawley rats were pretreated with ondansetron (1 mg/ kg intraperitoneally) or normal saline. After 15 min, rats received injections of nifedipine (15 mg/kg intraperitoneally) or dimethylsulfoxide (DMSO), solvent for nifedipine, as a control. Nociception was assessed by tail-flick method. Rats treated with nifedipine alone had an increase in tail-flick latency of 122%, as measured by the area under the curve, compared to rats treated with DMSO alone. Pretreatment with ondansetron, however completely blocked the analgesic effect of nifedipine, with tail-flick latency remaining at baseline throughout the measurement period. These results indicate that the 5-HT3 receptor plays an important role in the analgesic response to nifedipine and that medications that block this receptor may decrease the analgesic effectiveness of this type of therapy.

Tolerance-like phenomenon of nifedipine-induced antinociception.

We confirmed that repeated injection of nifedipine evokes an acute tolerance in rats. Nociceptive response was assessed by the tail-flick test. Rats, randomly divided into 3 groups, were injected once a week for 1 month with nifedipine intraperitoneally at the doses of: (A) 2 mg/kg; (B) 10 mg/kg; and (C) 10 mg/kg at the first injection and followed by dimethyl sulfoxide in the subsequent injections. After one month, group A and C were injected with 10 mg/kg and group B with 30 mg/kg nifedipine. In group A, 2 mg/kg of nifedipine did not change the TF latency but its repeated injections prevented the antinociceptive effect of 10 mg/kg nifedipine. In group B, only the first dose of 10 mg/kg and the last dose of 30 mg/kg produced a significant antinociception. In group C, the first but not the last dose of 10 mg/kg nifedipine, produced a significant antinociception. Our data suggest that a single dose of 10 mg/kg nifedipine or repeated doses of 2 mg/kg produced a tolerance-like phenomenon in nifedipine-induced antinociception.

Nifedipine-induced analgesia after epidural injection in rats.

We explored the analgesic effect of epidural nifedipine in male Sprague-Dawley rats. By using an implanted epidural catheter, the rats were given 35 microL of dimethylsulfoxide (DMSO) alone or DMSO containing 2.5, 5, 10, or 20 microM of nifedipine. Analgesia was measured by tailflick (TF) involving spinal reflexes, and by hotplate (HP) requiring an intact central nervous system. The latencies were recorded up to 120 min after the injection. The cutoff time of the noxious stimuli was 20 s in the TF and 60 s in the HP to prevent tissue damage. The TF technique revealed a significant difference from the control at doses of 5, 10, and 20 microM with no difference among the groups. Maximum latencies (cutoff time) lasted for 15, 30, and 40 min at doses of 5, 10, and 20 microM, respectively. The HP technique disclosed a dual effect: a significant decrease at the dose of 2.5 microM, no effect at 5 microM, and an increase at 10 and 20 microM. However, the median latency did not reach the cutoff time. We conclude that nifedipine, given epidurally, possesses antinociceptive properties at the dose of 5 microM and higher, detected better by the TF than HP. Our data suggest that the antinociceptive effect of nifedipine, at the studied doses, is more prominent at the spinal than the supraspinal level.

The effect of nifedipine and verapamil on nicotine-induced antinociception in rats.

We examined the modulatory effect of nifedipine (NIF, 0.04, 2, 5 and 15 mg/kg i.p.) and verapamil (VER, 2.5 and 10 mg/kg s.c.) on nicotine-induced antinociception in male rats, employing the tail-flick (TF) test. All rats were divided according to their responses to nicotine (1 mg/kg s.c.) alone, and classified as responders and nonresponders; both types were used. Both tested drugs were injected simultaneously with nicotine (NIC). In responders, NIC (1 mg/kg s.c.) alone produced a ceiling TF latency (15 sec) within 2.5 min lasting for 2.5 - 7.5 min. NIF, at the dose of 15 mg/kg i.p., prolonged this effect up to 40-50 min. Lower doses (5, 2 and 0.04 mg/kg) were not effective. VER in either dose did not prolong NIC-induced antinociception. In nonresponders, NIF also potentiated NIC-induced antinociception, and this effect appeared to be dose-related, lasting from 10 up to 40 min. VER was ineffective in either dose. These data support our hypothesis that calcium is involved, at least in part, in nicotine-induced antinociception. The potential risk for smoking patients treated with NIF is discussed.

Prenatal nicotine exposure increased duration of nicotine-induced analgesia in adult rats.

The effect of prenatal exposure to nicotine on nicotine-induced analgesia was studied in rats. The analgesic effect of a single dose of nicotine (1 mg/kg SC) was measured by the tail-flick technique, and two subsequent studies were carried out. In the first study, 7-month-old male rats, born to dams chronically treated with nicotine during pregnancy (NIC), exhibited prolonged nicotine-induced analgesia compared to matched controls. The second study was designed to explore whether rats prenatally exposed to nicotine (NIC rats) are born with an increased sensitivity to nicotine and whether there is any sex difference. The analgesic effect of nicotine was tested on control and NIC rats of both sexes once a month from 2 to 7 months of age. At an early age, male but not female NIC rats, exhibited shorter analgesic responses to nicotine than did the matched controls. With increasing age, however, the duration of nicotine analgesia began to be prolonged in NIC rats of both sexes. Significant differences between control and NIC rats were found at the age of 6 and 7 months, in both sexes. Thus, rats prenatally exposed to nicotine are not born with an increased sensitivity to the analgesic effect of a single dose of nicotine. This phenomenon develops later, during the course of life, independently of gender.

An antinociceptive effect of the intraperitoneal injection of nifedipine in rats, measured by tail-flick test.

The tail-flick (TF) technique was used to assess the antinociceptive properties of nifedipine (NIF) given intraperitoneally (i.p.). First, the most suitable intensity of the noxious stimulus (temperature of the bulb) has been ascertained and used in the main study. Male Sprague-Dawley rats received NIF, dissolved in dimethyl sulfoxide (DMSO) at the doses of 0.0, 0.5, 2, 5, 10 and 15 mg/kg, or control with no injection. For the main study, the noxious stimulus was limited to 15 sec (cut-off time) and TF latencies were recorded up to 120 min. The antinociceptive response was expressed as the area under the curve for each rat and analyzed by one-way ANOVA. The antinociceptive response to the lower doses of NIF (0.5 and 2 mg/kg) did not differ from control (no injection) and DMSO alone. Significance was found at 5, 10 and 15 mg NIF with no difference among the doses. However, there was an increasing tendency of the mean values from 0.5 to 15 mg NIF resulting in a positive correlation. The correlation coefficient was 0.32483 (p = 0.015) and regression equation Y = (19.37) x dose + 1320. Our data suggest that spinal mechanisms are involved in NIF-induced antinociception.

Nicotine-induced analgesia in rats: the role of calcium and the diversity of responders and nonresponders.

Following a single dose of nicotine, (NIC, 1 mg/kg s.c.), 60% of tested rats revealed significant antinociception as measured by the tail-flick (TF) test, and were classified as responders, with those in which TF latencies did not change, nonresponders. The following experiments were carried out one week later. In nonresponders, pretreatment with ethylenediaminetetraacetic acid (EDTA, 250 microM/kg s.c. four times every 15 min) followed by 1 mg NIC, produced significant analgesia in 50% of rats, to the same magnitude as did nicotine alone (1 mg) in responders. The other 50% of rats which failed to respond to EDTA pretreatment, all revealed similar analgesia following the higher dose of NIC (1.5 mg/kg s.c.), with similar side effects, as generally observed in responders. In responders, pretreatment with CaCl2 (1.5 mM/kg s.c.) completely abolished NIC (1 mg/kg s.c.)--induced analgesia in all rats. Our data provide stronger evidence and a further verification that EDTA potentiates, whereas CaCl2 completely abolishes, nicotine-induced analgesia in rats; supporting our hypothesis of the involvement of calcium ions in this effect.

Modulation of nicotine-induced analgesia by calcium agonist and antagonist in adult rats.

The involvement of calcium in nicotine-induced analgesia in male rats was explored using the tail-flick test. A single dose of nicotine (1 mg/kg SC) produced a maximal effect on tail-flick latency (15 s) within 8-10 min, which lasted for 4 min. Pretreatment with the calcium chelator, EDTA (250 microM/kg SC four injections at 15 min intervals), before the single dose of nicotine accelerated the onset and prolonged the duration of the nicotine-induced analgesia. The maximal effect on tail-flick latency occurred within 2 min and lasted for 10-20 min. Conversely, pretreatment with calcium chloride (1.5 mM/kg IP) attenuated nicotine-induced analgesia. It is suggested that nicotine may exert its antinociceptive effects via modulation of calcium fluxes across the neural membrane.

Antinociceptive effect of chronic nicotine and nociceptive effect of its withdrawal measured by hot-plate and tail-flick in rats.

Nicotine (6 mg/kg/day) was administered subcutaneously via Alzet osmotic pumps to rats for 28 days. Nociception was measured by the hot-plate and tail-flick methods. A significant antinociceptive effect was demonstrated during the first week of the treatment; after that tolerance developed, lasting for the remaining period of nicotine treatment. This effect was more pronounced in the habituated, rather than in the naive, rats. Rats subjected to nicotine withdrawal tended to exhibit the nociceptive effect for 10 days with an overall P value on the border of significance. This chronic nicotine-induced antinociception and nicotine withdrawal-induced nociception was detectable only by hot-plate but not the tail-flick technique.

Age-related differences in concomitant hormone release from the superfused rat adenopituitary.

We screened the concomitant release of five adenopituitary (AP) hormones, in vitro, at basal and stimulated conditions, in young and aged (2-3 and 25 months old, respectively) male Fischer 344 rats. The APs of aged rats exhibited significantly higher basal and total release of prolactin, but lower release of luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone and growth hormone than those of young rats. The APs of aged rats did not respond to high K+ depolarization by an increased release of any of the studied hormones, whereas the APs of young rats did release a significant amount in all instances. The results indicate marked age-related alterations affecting the release of the hormones, detectable even in the isolated AP alone.

Effect of chronic nicotine treatment and its withdrawal on the vasopressinergic system in rats.

Abstract We have studied the effect of chronic nicotine treatment and its withdrawal on the hypothalamo-neurohypophyseal Vasopressinergic system in male Sprague-Dawley rats. They were subcutaneously infused with low and high doses of nicotine, free base (0.6 and 6.0 mg/kg/day, respectively) for 28 days, via Alzet osmotic pumps. The studies were carried out immediately after the period of infusion and 1, 7, 14 and 28 (the latter in high dose only) days later. Basal, high K(+)-stimulated and total vasopressin release from the superfused neural lobes, the residual vasopressin content in the neural lobes, and hypothalamus and plasma vasopressin concentration were measured by radioimmunoassay. Treatment with the high dose of chronic nicotine alone decreased vasopressin content in and release from the neural lobe and its plasma concentration, but it did not change significantly the vasopressin content in the hypothalamus. A similar pattern of changes in vasopressin release and plasma concentration, though less pronounced, was observed in the rats infused with the low dose of nicotine. The withdrawal of the high dose of chronic nicotine gradually returned the decreased plasma vasopressin concentration, and its content in and release from the neural lobe to control values within 2 weeks. However, vasopressin content in the hypothalamus started to decline 1 week after nicotine withdrawal and persisted to decline for at least 3 subsequent weeks. The withdrawal of the high dose was associated with a marked suppression of high K(+)-stimulated vasopressin release from the neural lobe for a period of 4 weeks. The withdrawal of the low dose of nicotine exhibited a significant decline in plasma vasopressin concentration for up to 2 weeks only following chronic nicotine withdrawal, tending to return subsequently to control levels.