Lack of adaptive changes in 5-HT sites in perinatally undernourished rats after chronic stress: opioid influence

Differential response to gepirone but not to chlordiazepoxide in malnourished rats subjected to learned helplessness

The learned helplessness (LH) paradigm is characterized by learning deficits resulting from inescapable events. The aims of the present study were to determine if protein-calorie malnutrition (PCM) alters learning deficits induced by LH and if the neurochemical changes induced by malnutrition alter the reactivity to treatment with GABA-ergic and serotonergic drugs during LH. Well-nourished (W) and PCM Wistar rats (61 days old) were exposed or not to inescapable shocks (IS) and treated with gepirone (GEP, 0.0-7.5 mg/kg, intraperitoneally, N = 128) or chlordiazepoxide (0.0-7.5 mg/kg, intraperitoneally, N = 128) 72 h later, 30 min before the test session (30 trials of escape learning). The results showed that rats exposed to IS had higher escape latency than non-exposed rats (12.6 +/- 2.2 vs 4.4 +/- 0.8 s) and that malnutrition increased learning impairment produced by LH. GEP increased the escape latency of W animals exposed or non-exposed to IS, but did not affect the response of PCM animals, while chlordiazepoxide reduced the escape deficit of both W and PCM rats. The data suggest that PCM animals were more sensitive to the impairment produced by LH and that PCM led to neurochemical changes in the serotonergic system, resulting in hyporeactivity to the anxiogenic effects of GEP in the LH paradigm.

Altered serotonergic function of dorsal raphe nucleus in perinatally protein-deprived rats: effects of fluoxetine administration

We have previously described that perinatally undernourished rats showed increased locus coeruleus activity, a phenomenon reversed by repeated desipramine or fluoxetine administration. Since there is reciprocal modulation between the locus coeruleus and the dorsal raphe nucleus, and because these structures are associated with the pathophysiology of different states of anxiety, we evaluated the activity of serotonergic dorsal raphe neurons from early malnourished animals compared with controls, using in vivo extracellular single-unit recordings. The number of spontaneously active cells/track was significantly higher in protein-deprived animals, although the firing rate and the sensitivity of 5-HT(1A) receptors did not differ from those of controls. Five days of fluoxetine administration (5 mg/kg/day i.p.) was able to reverse the increased number of active serotonergic cells without affecting their firing rate. Furthermore, subsensitivity of 5-HT(1A) autoreceptors developed in the same way after repeated fluoxetine administration in both control and protein-deprived animals. These results suggest that the increased noradrenergic transmission observed in protein-deprived animals may induce an activation of serotonergic neurons in the dorsal raphe nucleus, and that this effect is normalized following fluoxetine treatment, which normalizes locus coeruleus activity.

Perinatal undernutrition: changes in brain opiate receptor density

The present work sought to study the binding properties of central mu-opiate receptors in whole brain and in different central areas in adult rats undernourished at perinatal age. Rats were undernourished with a hypoproteic diet containing 8% casein from day 14 of gestation until 50 days of age. The animals were thereafter fed a balanced commercial chow until 140 days of age. At this time point the experiments started. 3H-D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (3H-DAMGO) was used to selectively label the mu-receptors. The results obtained demonstrated that perinatal undernutrition induced, in the adult animal, a decreased mu-receptors density (Bmax) both in whole brain as well as in midbrain, without significant changes in affinity. In addition, no changes were found in mu-specific binding in the cortex of these undernourished animals. Taking into account that recent evidences from our laboratory have demonstrated a lower stress-induced analgesia following exposure to different stressful situations in rats undernourished in early life, the present findings seem to suggest that this lower analgesic response could be due, at least in part, to a lower density of mu-opiate receptors in the brain.

Behavioral effects Of 8-OH-DPAT in chronically stressed male and female rats

The present study tested the hypothesis that chronic stress desensitizes serotonergic 5-HT(1A) receptors and alters behavioral changes following 5-HT(1A) agonist administration. Eating, acoustic startle response (ASR), and locomotor activity were measured in stressed and nonstressed male and female rats after 8-OH-DPAT administration. Stressed rats were paired and stressed by around-the-clock intermittent foot shock. Controllable stress (CS) rats could avoid/terminate shock for themselves and their yoked partners by pulling a ceiling chain, whereas their partners, the uncontrollable stress (UCS) rats, could not. Rats earned their entire daily ration of food by pressing a lever. In previous experiments, this paradigm was stressful, but not debilitating and rats continued to eat, groom, sleep, and avoid/escape greater than 99% of shock trials. Locomotor activity and ASR were measured in the present study after saline and 8-OH-DPAT administration (0.25 mg/kg, IP) before, 24 h, and 72 h after shock onset. 8-OH-DPAT only decreased food intake significantly in male and female rats after the first administration. Stress decreased food intake in both the CS and UCS rats, with UCS rats eating the least. However, the effects of stress and 8-OH-DPAT were not additive. 8-OH-DPAT significantly increased peak startle amplitude at 100 and 120 dB, and decreased latency to peak startle amplitude at 100 dB in male and female rats. In contrast, 8-OH-DPAT did not alter percent prepulse inhibition (%PPI) at 100 dB, but significantly decreased %PPI in males but not females at 120 dB. Stress did not have a consistent effect on ASR, but reduced %PPI in males, but not females. Neither stress nor 8-OH-DPAT significantly altered locomotor activity. Although the results do not show an increased sensitivity to 8-OH-DPAT in stressed rats, the unexpectedly weak effects of 8-OH-DPAT alone on the behavioral measures chosen limits the conclusions that can be drawn.

Opiate agonist-induced changes in behavioral sensitivity to clonidine are observed in perinatally malnourished rats exposed to chronic stress

Sensitivity of alpha2-adrenoceptors following repeated immobilization sessions plus morphine (MOR) or beta-endorphin (BETA) was assayed by examining clonidine (CLO)-induced hypoactivity in adult malnourished rats at perinatal age. As previously described, chronic restraint did not attenuate the hypoactivity elicited by CLO in malnourished rats, although chronic restraint did have such an effect on motor activity in control animals. MOR and BETA administration prior to each restraint session induced subsensitivity of alpha2-adrenoceptors in malnourished rats as determined by a blunted response to clonidine challenge. An injection of naloxone (NAL) prior to BETA before each stress session fully antagonized the subsensitivity to clonidine observed in malnourished animals. A possible deficiency in the functional role of the opiate system in the process of adaptation to chronic stress in perinatal malnourished rats is suggested.

Effect of perinatal malnutrition on the inactive behavior induced by long-term shock and conditioned analgesia

Inactive behavior in response to long-term inescapable foot shock (IS) and conditioned analgesia were evaluated in adult rats perinatally undernourished. During the IS session, control rats exhibited first a period of vigorous activity and then inactive behavior, during which they accepted the shock more passively. Moreover, when these same animals were subsequently placed in the environment associated with shock application, they showed an increase in the paw lick latency. Conversely, in malnourished rats, this IS schedule induced lower behavioral inactivity and rats failed to develop conditioned analgesia. When undernourished rats were injected with morphine (MOR) before IS exposure, it was observed that both phenomena--inactivity during shock and analgesia conditioned by submission to the shock context--were normalized. A possible alteration in the activation of an opiate process implicated in the stress response in early undernourished rats is suggested.

Effect of acute and chronic tramadol on [3H]-5-HT uptake in rat cortical synaptosomes

1. Tramadol hydrochloride is a centrally acting opioid analgesic, the efficacy and potency of which is only five to ten times lower than that of morphine. Opioid, as well as non-opioid mechanisms, may participate in the analgesic activity of tramadol. 2. [3H]-5-hydroxytryptamine (5-HT) uptake in rat isolated cortical synaptosomes was studied in the presence of tramadol, desipramine, fluoxetine, methadone and morphine. Methadone and tramadol inhibited synaptosomal [3H]-5-HT uptake with apparent Kis of 0.27 +/- 0.04 and 0.76 +/- 0.04 microM, respectively. Morphine essentially failed to inhibit [3H]-5-HT uptake (Ki 0.50 +/- 0.30 M). 3. Methadone, morphine and tramadol were active in the hot plate test with ED50s of 3.5, 4.3 and 31 mg kg-1, respectively. At the highest tested dose (80 mg kg-1) tramadol produced only 77 +/- 5.3% of the maximal possible effect. 4. When [3H]-5-HT uptake was examined in synaptosomes prepared from rats 30 min after a single dose of morphine, methadone or tramadol, only tramadol (31 mg kg-1, s.c., equal to the ED50 in the hot plate test) and methadone (35 mg kg-1, s.c., equal to the ED90 in the hot plate test) decreased uptake. 5. Animals were chronically treated for 15 days with increasing doses of tramadol or methadone (5 to 40 mg kg-1 and 15 to 120 mg kg-1, s.c., respectively). Twenty-four hours after the last drug injection, a challenge dose of methadone (35 mg kg-1, s.c.) or tramadol (31 mg kg-1, s.c.) was administered. [3H]-5-HT uptake was not affected in synaptosomes prepared from rats chronically-treated with methadone, whereas chronic tramadol was still able to reduce this parameter by 42%. 6. Rats chronically-treated with methadone showed a significant increase in [3H]-5-HT uptake (190%) 72 h after drug withdrawal. In contrast, [3H]-5-HT uptake in rats chronically-treated with tramadol (110%) did not differ significantly from control animals. 7. These results further support the hypothesis that [3H]-5-HT uptake inhibition may contribute to the antinociceptive effects of tramadol. The lack of tolerance development of [3H]-5-HT uptake, together with the absence of behavioural alterations after chronic tramadol treatment, suggest that tramadol has an advantage over classical opioids in the treatment of pain disorders.

Reduced tolerance to certain pharmacological effects of ethanol after chronic administration in perinatally undernourished rats

We have previously reported that recovered adult rats undernourished at perinatal age failed to develop tolerance to the anticonflict effect of ethanol after chronic ethanol administration (1 g/kg/day during 30 days) (4). To further study the extent of this finding, we examined the effect of a similar chronic ethanol treatment on the hypothermic and anticonvulsant effects of ethanol in perinatally deprived rats. Hypoalgesic activity was assessed in ethanol treated rats during 15 days. After chronic ethanol treatment, a similar development of tolerance to the hypothermic effect of ethanol was observed in control and deprived rats. However, tolerance to the anticonvulsant and hypoalgesic effect of ethanol was significantly reduced in deprived as compared with control animals. Thus, early undernutrition differentially affects the development of tolerance elicited by chronic ethanol administration.

Analgesic response to stress is reduced in perinatally undernourished rats

Stress-induced analgesia was evaluated in adult rats submitted early in life to a protein deprivation schedule. Rats were undernourished with a hypoproteic diet containing 80 g casein/kg diet from d 14 of gestation until 50 days of age. Rats were thereafter fed a balanced nonpurified diet until 140 days of age, when they were exposed to two stressors: forced swimming and acute restraint, after which the analgesic response was evaluated. In addition, the analgesic response induced by different morphine doses was determined in another group of rats. Basal latency was not different in deprived and control rats. Undernourished rats presented a significantly lower analgesic response in both stress situations. However, when the analgesic response induced by different morphine doses (1, 2, 4 and 8 mg/kg, s.c.) was assessed, a significantly higher response occurred in undernourished rats compared to control rats. This lower stress-induced analgesia in undernourished rats may account for the behavioral alterations attributed to early undernutrition.

Modulation of 5-HT receptor subtype-mediated behaviours by corticosterone

Malfunction of the serotonergic system and dysregulation of the hypothalamo-pituitary-adrenocortical axis have been implicated in the pathophysiology of depression. Several studies provide evidence for reciprocal influences between glucocorticoids and 5-HT receptors. The effect of repeated treatment with a high dose of corticosterone (50 mg/kg s.c. twice daily for 4 days) on 5-HT receptor subtype-mediated behaviours was studied. It was found that in rats that were repeatedly treated with corticosterone the number of 2-chloro-6-(1-piperazinyl)pyrazine HCl (MK 212)-induced, 5-HT2C receptor-mediated penile erections were reduced, whereas both MK 212 and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced 5-HT2A receptor-mediated head shakes were increased. The (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-induced lower lip retraction mediated by presynaptic 5-HT1A receptors was unchanged, whereas the open field activity induced by 8-OH-DPAT was enhanced in corticosterone pretreated rats. These changes in 5-HT receptor subtype-mediated behaviours were not seen after a single injection with corticosterone given 24 h or 5 days before. The results suggest that 5-HT2A, 5-HT2C and postsynaptic 5-HT1A receptor-mediated behaviour can be modulated by repeated treatment with a high dose of corticosterone.

Malnutrition and reactivity to drugs acting in the central nervous system

There is a well-established body of data demonstrating that protein or protein-calorie malnutrition experienced early in life is associated with neuroanatomical, neurochemical, as well as behavioral alterations in both animals and humans. A number of studies has focused on the following question: are the neuroanatomical and/or neurochemical changes produced by early malnutrition responsible for the altered behaviors reported in malnourished animals? A tool that has been used to help answer this question is the administration of drugs with specific actions in the various neurotransmitter systems in the central nervous system (CNS). This neuropharmacological approach has produced a considerable amount of data demonstrating that malnourished animals react to drugs differently from controls, suggesting that the altered behavioral expression of these animals could be partly explained by the alterations in the brain function following malnutrition. The present review will provide an overview of the literature investigating the reactivity of malnourished animals to psychoactive drugs acting through GABAergic, catecholaminergic, serotonergic, opioid and cholinergic neurotransmitter systems. Altered responsiveness to psychoactive drugs in malnourished animals may be especially relevant to understanding the consequences of malnutrition in human populations.

Crude saponin extracted from Vietnamese ginseng and its major constituent majonoside-R2 attenuate the psychological stress- and foot-shock stress-induced antinociception in mice

Effects of Vietnamese ginseng (VG) crude saponin and majonoside-R2, a major saponin constituent, on the psychological stress- and foot shock stress-induced antinociception in the tail pinch test were examined in mice. VG crude saponin (6.2, 12.5, and 25 mg/kg, P.O.) attenuated psychological stress- but not foot shock stress-induced antinociceptive response, whereas majonoside-R2 (3, 6.2, and 12.5 mg/kg, P.O. and i.p.), as well as naloxone (2 mg/kg, i.p.), suppressed both psychological stress- and foot shock stress-induced antinociception. Pretreatment with the crude saponin (12.5 mg/kg, P.O.) or majonoside-R2 (6.2 mg/kg, P.O.) for 5 days followed by the treatment in combination with stress for next 5 days did not affect the development of adaptation to foot shock stress, but they significantly suppressed the antinociceptive action of the stress measured on the first, second, and third day during the stress exposure period. Majonoside-R2 (6.2 mg/kg, P.O.) but not the crude saponin (12.5 mg/kg, P.O.) significantly blocked the development of adaptation to psychological stress. These results suggest that VG crude saponin has the suppressing effect on psychological stress- and foot shock stress-induced antinociception and that majonoside-R2 is important for the action of the saponin.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.