The effects of tryptophan and manipulations of serotonergic receptors on tonic immobility in chickens

Serotoninergic activation of the basolateral amygdala and modulation of tonic immobility in guinea pig

Tonic immobility (TI), also known as death feigning or animal hypnosis, is a reversible state of motor inhibition that is not only triggered by postural inversion and/or movement restraining maneuvers but also by repetitive stimulation and pressure on body parts. Evidence has demonstrated that the basolateral nucleus of the amygdala (BLA) is particularly associated with defensive behavior that involves the emotional states of fear and anxiety. In addition, some reports have demonstrated that serotonin (5-HT) released in the amygdala is increased during states of stress and anxiety, principally in the BLA. In the present study, we investigated the effects of serotonergic activation of the BLA on the duration of TI. The results showed that the microinjection of 5-HT (3.0 microg) into the BLA decreased the duration of TI. Similarly, the administration of a 5-HT1A agonist (0.1 microg of 8-hydroxy-dipropylaminotretalin) or 5-HT2 agonist (0.1 microg of alpha-methyl-5-HT) into the BLA reduced the TI duration. The effect of 5-HT2 agonist was reversed by pretreatment with a dose that had no effect per se (0.01 microg) of ketanserin (5-HT2 receptor antagonists) into the BLA. Moreover, the activation of 5-HT1A and 5-HT2 receptors in the BLA did not alter the spontaneous motor activity in the open field test. The results of the present study indicate that the serotonergic system of the BLA possibly produces a reduction in fear and/or anxiety that reduces the TI duration in guinea pigs, but this is not due to increased spontaneous motor activity induced by serotonergic activation, which might affect TI duration non-specifically.

Serotonin 5-HT2 and 5-HT1A receptors in the periaqueductal gray matter differentially modulate tonic immobility in guinea pig

Tonic immobility (TI) is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. We have previously reported that cholinergic stimulation of the dorsal periaqueductal gray matter (PAG) decreases the duration of TI episodes, while stimulation of the ventrolateral region increases it. The ventrolateral PAG modulates this behavior via a similar neural circuit proposed to be involved in the antinociceptive system. Some studies have indicated that alterations in the levels of cerebral 5-hydroxytryptamine (5-HT) mediate or modulate the analgesic effect of PAG stimulation. Thus, in this study we investigated the possibility that the serotoninergic system is involved in the modulation of TI by this neural substrate. Our results showed that the effect of serotonin into the ventrolateral and dorsal PAG seems to be biphasic and dose dependent. The microinjection of low doses (0.1 microg) of 5-HT into the PAG increased the duration of TI, while high doses (1, 3 and 6 microg) decreased this behavior. Our results also showed that microinjection of a 5-HT(1A) agonist (0.003, 0.01 and 0.1 microg of 8-hydroxy-dipropylaminotretalin (8-OH-DPAT)) into the PAG increased the duration of TI episodes. However, the microinjection of 5-HT(2) agonist (0.01 and 0.1 microg of alpha-methyl-5-HT) into the PAG decreased the duration of TI and this effect could be reversed by pretreatment with an ineffective dose (0.01 microg) of ketanserin. In contrast, ketanserin (0.03 and 0.16 microg) increased this behavior in a dose-dependent manner. These results suggest that the PAG 5-HT(1A) and 5-HT(2) receptors have different roles in the modulation of TI in guinea pigs, since the 5-HT(1A) and 5-HT(2) agonists, respectively, increased and decreased the duration of TI.

Tonic immobility in guinea pigs: a behavioural response for detecting an anxiolytic-like effect?

Tonic immobility (TI) is considered to be an innate fear response characterized by a temporary state of profound and reversible motor inhibition. TI occurs in a wide range of species in a predator-prey confrontation and is hypothesized to be a terminal defence response occurring when there is physical contact between prey and predator. The objective of the present study was to investigate the validity of the TI model in guinea pigs for detection of anxiolytic and/or antidepressant drug activity. Compounds that reduced TI include the serotonin (5-HT) releaser fenfluramine, the 5-HT(1A) receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone, the 5-HT(2C/2B) receptor antagonist SB206553, the 5-HT(2A) receptor antagonist MDL 100.151 -- but only at doses thought also to inhibit 5-HT(2C) receptors--the noradrenaline (NA) reuptake inhibitor desipramine, the benzodiazepine inverse agonist FG-7142, the alpha(2)-adrenergic receptor antagonist yohimbine, the neurokinin (NK)(1) receptor antagonist L-733.060, and the NK(2) receptor antagonist SR-48968. Compounds that increased TI include the benzodiazepine agonists diazepam and alprazolam, and the alpha(2)-adrenergic receptor agonist clonidine. The selective 5-HT reuptake inhibitors citalopram, paroxetine and fluoxetine, the 5-HT(1A) receptor antagonist WAY100.635, the 5-HT(2C) receptor agonist MK-212, the 5-HT/NA reuptake inhibitor imipramine, the NA reuptake inhibitor talopram, the benzodiazepine antagonist flumazenil, the alpha(2)-adrenergic receptor antagonist idazoxan and the psychostimulant amphetamine did not have any effect. These findings indicate that the serotonergic, noradrenergic and neurokinin systems are involved in mediating or modulating TI behaviour in guinea pigs. The potential of TI as a behaviour for detecting anxiolytic-like effect may be questioned due to the contradictory effect of the benzodiazepine ligands, which may be attributed to the sedative and/or ataxic effects of the compounds. Nevertheless, there is preclinical evidence suggesting that 5-HT(1A) receptor agonists, 5-HT(2C) receptor antagonists and NK(1) and NK(2) receptor antagonists possess anxiolytic potential. Only when results of clinical investigations of the anxiolytic potential of non-benzodiazepine ligands (for example the NK receptor antagonists) are available, will it be possible to determine fully the predictive validity of the TI model.

Tonic immobility and high-intensity calls in a precocial chick as a function of age, diet, and time of day

Tonic immobility (TI) and high-intensity vocalizations are two antipredator behaviors employed by domestic chicks (Gallus gallus) which vary in their function and the proximate mechanisms that govern them. In the present study, we sought to determine the influence of age (3 or 10 days old), diet (control or tryptophan-free), and time of testing (A.M. or P.M.) on the duration of TI and the number of calls produced in domestic chicks. Older chicks remained immobile significantly longer than younger chicks as did subjects on the control diet or those tested at night. However, 3-day-old chicks had significantly shorter TI durations only when tested in the A.M.: TI did not differ between age groups when subjects were tested in the P.M. Three-day-olds called significantly more than 10-day-olds when given the control diet and when tested in the P.M. but vocalization frequencies did not differ between age groups during the A.M. testing or when given the tryptophan-free diet (T-). Vocalization rate (calls/TI duration), however, was significantly greater during the A.M. testing. These results emphasize the importance of considering circadian rhythms and behavioral development. Furthermore, an integrative view of the proximate mechanisms and adaptive functions of TI and high-intensity vocalizations is presented.

Drug effects on active immobility responses: what they tell us about neurotransmitter systems and motor functions

The literature reviewed indicates that active immobility can be promoted by systemic injections of various neurotransmitter systems, as follows: (1) Dopaminergic blockade of both D1 and D2 receptor subtypes. (2) Cholinergic agonism of both muscarinic and nicotinic receptors. (3) Noradrenergic agonism of both alpha-1 and alpha-2 receptors (but these agonists may interfere with haloperidol- and reserpine-induced catalepsy). (4) GABA agonism. (5) Histamine agonism, particularly at the H1 receptor. (6) Opiate agonism, including action of many endogenous opiate peptides, particularly those affecting mu and delta receptors. (7) Agonism by certain other peptides (neurotensin, cholecystokinin). Among the major interactions of neurotransmitter systems that regulate immobility, are the following: (1) Cholinergic-dopaminergic (cholinolytics disrupt catalepsy of dopaminergic blockade and dopaminergic agonists tend to disrupt cholinomimetic catalepsy). (2) Opiate-induced catalepsy is antagonized by the dopamine agonist, apomorphine, but is enhanced by amphetamine. It is also antagonized by certain alpha-2 adrenergic agonists, while it does not seem to be antagonized by anticholinergics. (3) Numerous other interactions have been reported, involving opiates and MSH, serotonin and dopamine mimetics, serotonin and ketamine, GABA and neuroleptics, neurotensin and anticholinergics and histamine. The significance of the multiple neurotransmitter systems is unknown. One possible explanation is that the various neurotransmitter systems participate in mediating the sensory inputs that are involved in triggering immobility and regulate the higher-order limbic and basal ganglia processing reactions that engage a final motor output pathway from the brainstem. The brain is assumed to contain two sets of systems, each with its own, or possibly overlapping, set of neurotransmitter systems, that promote either active immobility or locomotion. The systems reciprocally inhibit each other. Another view, not mutually exclusive, is that output from the locomotor-promoting system provides a negative feedback, via the active immobility pathways, to act as a "brake" on movement, while at the same time maintaining the muscular tonus that is characteristic of active immobility.

Differential cholinergic influences on the immobility response in various strains of domestic fowl

A series of five experiments examined the effects of two anticholinergic drugs, atropine and scopolamine, on the duration of tonic immobility (TI) and susceptibility to the TI response in both Production Red and White Leghorn chickens (Gallus gallus), in an attempt to resolve previous contradictory findings about the effects of cholinergic manipulations on tonic immobility. These two anticholinergic drugs significantly reduced the duration of TI and, therefore, supported the conclusion that cholinergic systems are involved with the immobility response. However, the effects of these drugs on TI differed depending on the age, strain, local population, and handling experience of the individual birds.

Tonic immobility and the serotonergic system in chickens: differential blockade of receptors by cyproheptadine and cinanserin

Low doses of serotonin potentiated immobility durations in 2 1/2-week-old chickens (Gallus gallus) while high doses attenuated the response. Cyproheptadine, a serotonergic antagonist, eliminated increases generated by low doses of serotonin and reduced durations in control subjects, but had no effect on the attenuation produced by high doses of the drug. Cinanserin, another serotonergic blocker, also eliminated the potentiation generated by low doses of serotonin and partially reduced the attenuation caused by high doses, but had no effect on immobility durations in control animals. Larger doses of cinanserin eliminated the attenuation produced by high doses of serotonin and increased durations in control subjects. Results are discussed in terms of the differential blockade of various serotonin receptors.

Interaction of tonic immobility and dexamethasone in the modulation of hippocampal 5-HT activity in rabbits

The immobility reaction (animal hypnosis) in the rabbit is related to basal levels of corticosterone and to modifications in 5-HT activity in the brain. In order to investigate the relationship between adrenocortical function, brain 5-HT activity and immobility response in rabbits, the susceptibility to the reaction and 5-HT and 5-HIAA levels in the hippocampus were studied in animals whose pituitary-adrenal function was altered by chronic DEX treatment. It was still possible to induce immobility in animals treated with DEX. The reduction in duration observed under the present testing schedule (two sessions of immobility separated by a one week interval) did not occur in the treated animals. There was a reduction in 5-HT turnover as a consequence of the immobility response and this confirms previous findings in other brain areas of the same species. In animals pretreated with DEX, so as to drastically reduce ACTH and corticosterone, not only was a reduction in 5-HT turnover prevented, but there was also an increase in 5-HT and 5-HIAA levels when immobility was induced. These results are discussed in terms of the influence of the pituitary-adrenal system in the neuroendocrine and behavioural aspects of the immobility reaction.

Central and peripheral effects of serotonin on the immobility response in chickens

The effects of low and high doses of serotonin on tonic immobility (TI) duration and susceptibility in 10- and 45-day-old chickens were examined. High doses of serotonin reduced the number of inductions required to produce TI, regardless of the subject's age. In contrast, low and high doses of serotonin produced biphasic increases and decreases in TI duration in 10-day-old birds, but there were no apparent effects on immobility duration by either dose of this drug in older chickens. These results are discussed in terms of the formation of the blood-brain barrier in domestic fowl and the differential peripheral versus central actions by serotonin on TI susceptibility and response duration.

P-chlorophenylalanine attenuates tonic immobility duration in chickens

The results of two experiments demonstrated a dose-dependent reduction in tonic immobility (TI) duration by p-chlorophenylalanine (PCPA) in White Leghorn cockerels. In Experiment 1, five-day old chicks were given five intraperitoneal injections of either saline or 75 or 150 mg/kg of PCPA at a rate of one injection a day for five days. The effects of these injections were evaluated one day after the last injection. A total of 750 mg/kg of PCPA significantly reduced TI duration but 375 mg/kg did not. In Experiment 2, chicks were given either two saline injections, two 375 mg/kg PCPA injections, or one 375 mg/kg PCPA injection and one saline injection. The effects of these injections were evaluated either one or three days after injections. A total of 750 mg/kg of PCPA significantly reduced TI both one and three days after injections but 375 mg/kg of PCPA did not. Previous failures to obtain significant PCPA effects on TI may reflect insufficient drug amounts.

Duration of tonic immobility in chickens as a function of alpha-adrenergic receptor stimulation and blockade

Tonic immobility in chickens was influenced by a variety of drugs that act on the adrenergic neurochemical system of the body. Alpha 1 agonists such as methoxamine and phenylephrine produced decreases in the duration of immobility, although the former compound also caused a significant increase in the immobility response at high dosages. Alpha 2 agonists such as clonidine, naphazoline, and guanfacine enhanced the duration of immobility, but clonidine also produced an apparent reversal of this effect at high dosages. Subsequent experiments examined more fully the biphasic effects by methoxamine and clonidine on tonic immobility through interactions with alpha antagonists. Yohimbine, an alpha 2 blocker, attenuated the duration of immobility, either alone or in conjunction with various dosages of methoxamine. Prazosin, an alpha 1 blocker, had no direct effect on tonic immobility, but potentiated the duration of the response when given in conjunction with various dosages of methoxamine. When these antagonists were given in conjunction with clonidine, yohimbine reduced immobility durations, while prazosin had no apparent effect on this response. These results are discussed in terms of the relative contributions of the alpha 1 and alpha 2 adrenoceptors to the duration of the immobility response.

Changes in brain serotonin and 5-hydroxyindolacetic acid levels in the rabbit following tonic immobility

The effect of tonic immobility on serotonin (5-hydroxytryptamine, 5-HT) and on 5- hydroxyindolacetic acid (5-HIAA) levels in different brain areas has been investigated, following two different schedules of treatment. The massed treatment consisted of a series of consecutive trials up to 15 min, the spaced treatment in 4 series of trials within 24 hours. Massed treatment produced a decrease in 5-HT in the mesencephalon and of 5-HIAA in the pons-medullary area, but similar changes were also elicited in animals treated with the procedure of induction not followed by immobility. The spaced treatment, which resulted in a potentiation of the immobility duration, i.e., sensitization, produced a reduction of 5-HT levels in the mesencephalon and of 5-HT turnover in the striatum. The procedure of the induction per se acts preferentially on 5-HIAA by increasing its levels. In the striatum, which appears selectively responsive to the spaced treatment of immobility, a negative correlation has been found between serotonin levels and immobility duration. Results suggest that elicitation of tonic immobility is associated with reduced levels of 5-HT in the brain.

Tyrosine and tryptophan influence on tonic immobility in the chicken

The serotonergic and catecholaminergic systems have been shown to affect both tonic immobility and food intake in the domestic fowl. Tryptophan, the amino acid precursor of serotonin, has been demonstrated to influence tonic immobility and food intake when injected intraperitoneally. Tyrosine, amino acid precursor of the catecholamines, is also known to affect food intake when administered orally. In two experiments, tryptophan and tyrosine were intubated intragastrically into male chickens at levels known to affect food intake, and their effect on tonic immobility was measured. Tryptophan and tyrosine delivered intragastrically had no effect on either duration or susceptibility of tonic immobility. These data suggest that changes in appetite observed as a result of orally administered tyrosine and tryptophan were not due to a general inhibition or stimulation of the central nervous system.

Effects of neonatal treatment with 5,7-dihydroxytryptamine or 6-hydroxydopamine on the ontogenetic development of the audiogenic immobility reaction in the rat

The ontogenetic development of the audiogenic immobility reaction (freezing) was studied in rats given intracisternal injections of the neurotoxins 5,7-dihydroxytryptamine (5,7-DHT), 25 micrograms, or 6-hydroxydopamine (6-OHDA), 100 micrograms, neonatally (Day 1). The duration of the freezing response was strongly reduced in the 5,7-DHT-treated rats between 20-30 days of age, when normal animals show very prolonged responses. During the same period increased motor activity was observed in the 6-OHDA-treated rats while only a slight reduction of the freezing response was noted. Biochemical analyses performed on brains from animals 35 days of age showed a selective reduction (about 50%) of whole brain levels of serotonin in the 5,7-DHT-treated rats, while the noradrenaline levels were selectively reduced by about 60% in the 6-OHDA rats. A longitudinal investigation on the effects of neonatal treatment with 5,7-DHT showed a persistent selective reduction of the whole brain level of serotonin up to at least 90 days of age. Since 5,7-DHT mainly affects the serotonergic pathways, the results suggest that the disturbances noted in the ontogeny of the freezing response may be due to interference with the developing serotonergic system.

The effects of quipazine, fenfluramine and apomorphine on the morphine potentiation of tonic immobility

Acute administration of morphine (2 mg/kg, IM) enhanced tonic immobility (TI) durations in three-week old chickens. This effect could be reversed with the 5-HT receptor agonist quipazine. Similarly, promoting 5-HT release by fenfluramine antagonized the morphine potentiation of the response. Both 5-HT agonists reduced TI durations. Finally, the DA receptor agonist apomorphine produced decrements in TI duration and blocked the effect of morphine. The results suggest the involvement of serotonergic and dopaminergic mechanism in the morphine potentiation of the response. The findings are also discussed in terms of a revised serotonergic model of tonic immobility.

Tonic immobility in domestic fowl: possible interaction of serotonergic and dopaminergic mechanisms

Treatment with the dopamine (DA) receptor blocker, haloperidol, enhanced tonic immobility (TI) duration. Fenfluramine, a receptor agonist for serotonin (5-HT), reversed this effect. Tryptophan produced long TI reactions, and is believed to do so due to impaired synaptic transmission of 5-HT following its direct inhibitory effects on 5-HT neurons. DA receptor stimulation by apomorphine prevented the tryptophan potentiation of tonic immobility. The results suggest that serotonergic and dopaminergic systems may interact with respect to tonic immobility.