Elevation Of Tail Flick Latency Research Articles

A neuroanatomical correlate of sensorimotor recovery in response to repeated vaginocervical stimulation in rats

Gentle probing against the cervix via the vagina (vaginocervical stimulation, VCS) increases tail flick latency (TFL) to radiant heat; greater force abolishes the tail flick response and other withdrawal responses. This effect occurs in spinal cord-transected rats and in intact rats. On the basis of our earlier finding that VCS releases vasoactive intestinal peptide (VIP) into the spinal cord, and others’ reports of neurotrophic effects of VIP in vitro, we hypothesized that repeated VCS would stimulate sprouting and sensorimotor function of terminals of genital nerve primary afferents in the sacral spinal cord. To test this hypothesis, in the present study, we denervated the genital tract only unilaterally, which significantly reduced the TFL-elevating effect of VCS. Then we applied repeated daily VCS for 1 week and compared the subsequent effectiveness of acute VCS in elevating TFL. The rats that received the repeated daily VCS showed a significantly greater elevation in TFL in response to acute VCS than control rats that did not receive the repeated stimulation. Then, to test whether daily repeated VCS stimulates sprouting of genital primary afferents in such unilaterally genital tract-denervated rats, we transected the contralateral remaining intact pelvic nerve, applied horseradish peroxidase (HRP) to its proximal cut end for 1–2 h, and 2–3 days later counted HRP particles in its terminal zone (L6–S1) in the spinal cord. There were significantly more HRP particles in the rats that received the daily repeated VCS than in the control rats. In the context of these findings, we conclude that VCS in rats can produce a functional sensorimotor recovery via a neurotrophic effect on compromised primary afferents in the spinal cord.

The role of mu and kappa opioid receptors within the periaqueductal gray in the expression of conditional hypoalgesia

The periaqueductal gray (PAG) is a midbrain structure involved in the modulation of pain and expression of classically conditioned fear responses. Non-selective opioid antagonists applied to the PAG block the expression of hypoalgesia in rats exposed to a Pavlovian signal for shock. This study was conducted to determine the anatomical and pharmacological specificity of the PAG's role in conditional hypoalgesia. Rat subjects received injections of either the mu opioid antagonist CTAP (6.6 nMol), the kappa opioid antagonist Nor-binaltorphimine (Nor-BNI, 6.6 nMol) or saline. Injections were made into either the dorsolateral (dlPAG) or ventrolateral (vlPAG) PAG prior to the presentation of an auditory stimulus that had previously been paired with foot shock while measuring nociception with the radiant heat tail flick (TF) test. Elevation in TF latency in response to the auditory stimulus was blocked only by administration of CTAP into the vlPAG. These results suggest that conditional hypoalgesia (CHA) is subserved by mu but not kappa opioid receptors located in the vlPAG but not the dlPAG.

Neural systems for the expression of hypoalgesia during nonassociative fear.

A single brief exposure to moderately intense while noise is sufficient to produce opioid-mediated antinociception in rats. This form of stress-induced hypoalgesia represents a response to unconditional fear or anxiety. Three experiments compared the neural circuits responsible for learned versus unlearned fear responses. Male rats received lesions of the medial geniculate nucleus, lateral or central nuclei of the amygdala, or the ventral, dorsal lateral, or dorsal medial periaqueductal gray (PAG). Controls showed a pronounced elevation in tail-flick latency following presentation of 90-dB white noise. All lesions, with the exception of dorsolateral and dorsomedial PAG, significantly blocked this response. These results support the idea that hypoalgesia produced by aversive auditory stimuli uses a common neural circuit regardless of whether the response is a product of associative learning or unconditional fear/anxiety.

Effects of aging on spinal opioid-induced antinociception

Initial experiments were conducted to determine whether or not the aging process alters the ability of young, mature, or aged male Fischer 344 rats (5- to 6-, 15- to 16-, and 25- to 26-months-old, respectively) to respond to thermal nociceptive stimuli. Using the tail-flick analgesiometric assay, 25- to 26-month-old rats responded significantly faster to the heat source than 15- to 16-month-old animals, but no significant differences were noted between the 5- to 6-month-old and aged rats. Another series of investigations compared the effects of aging on the spinal antinociceptive properties of the μ opioid agonist [D-Ala 2,N-methyl-Phe 4,Gly 5-ol] enkephalin (DAMPGO) and the δ agonist [D-Pen 2,D-Pen 5] enkephalin (DPDPE). In these studies, young, mature, and aged rats were injected intrathecally (IT) with different doses of DAMPGO or DPDPE, and opioid-induced antinociception was tested on the tail-flick test. All three age groups responded to IT DAMPGO in a dose-dependent manner but, for the most part, higher spinal doses were required to produce significant elevations in tail-flick latency in the aged cohort of rats. The spinal analgesic effects of DPDPE also declined with advanced age. The aging process apparently alters the pain-inhibitory function of μ and δ opioid receptors in the rat spinal cord.

Hypoalgesia in response to sensitization during acute noise stress.

Three experiments examined the antinociceptive response shown by rats during exposure to loud noise. Noise exposure resulted in a time-dependent elevation of radiant heat tail flick latency that varied as a function of stimulus intensity. Noise stress hypoalgesia in response to a 90-dB stimulus was blocked by pretreatment with the opioid antagonist naltrexone (0.1-7.0 mg/kg). Systemic administration of midazolam (2 mg/kg) prior to exposure to the stressor attenuated the elevation in tail flick latency. Because topographically similar antinociceptive responses may be elicited with a low intensity noise stimulus that has served as a Pavlovian conditional stimulus for shock, the use of this paradigm may permit direct comparisons of associative and nonassociative fear responses using qualitatively similar auditory stimuli.

Lesions of the amygdala block conditional hypoalgesia on the tail flick test

Exposure to an innocuous stimulus that has been paired with footshock during Pavlovian conditioning results in the activation of descending antinociceptive systems in the rat. Several recent studies indicate that the hypoalgesia observed when contextual stimuli are paired with shock and the formalin test is used to measure antinociception depends on the integrity of a neural circuit which includes the amygdala and the periaqueductal gray. The present experiment was designed to determine if the amygdala is also critical for hypoalgesia in response to a discrete auditory signal for footshock when hypoalgesia is measured with the radiant heat tail flick test. Groups of rats were exposed to a series of paired presentations of a tone and footshock or associative control treatments. After training, one half of the animals received large electrolytic lesions of the amygdala. Lesions of the amygdala blocked the time dependent elevation in tail flick latency following tone presentation in animals given paired training, but did not alter baseline tail flick responding. These data indicate that the amygdala is also essential for fear-related modulation of spinally mediated nociceptive reflexes, and provide further support for our current model in which amygdalo-mesencephalic projections are critical for the expression of certain forms of stress-induced hypoalgesia.

The noradrenergic component contributing to spinal fentanyl-induced antinociception is supraspinally mediated

1. 1. Male Sprague-Dawley rats were fitted with intrathecal (i.t.) and intracerebroventricular (i.c.v.) catheters. Fentanyl was injected either i.t. or i.c.v., and the antinociceptive efficacy of fentanyl was evaluated using the tail-flick analgesiometric assay. 2. 2. Fentanyl dose-dependently elevated tail-flick latency (TFL) following i.c.v. or i.t. administration. The antinociceptive effects of fentanyl were reversed by naltrexone. 3. 3. Experiments were also designed to evaluate the effects of serotonin and α-adrenoceptor antagonists on i.t. or i.c.v. fentanyl-induced elevations in TFL. 4. 4. Phentolamine administered i.t. reversed both the spinal and supraspinal antinociceptive effects of fentanyl, whereas i.t. methysergide did not significantly alter the i.t. or i.c.v. effects of the μ agonist. 5. 5. These data suggest that fentanyl-induced antinociception does not rely on local serotonergic neuronal activation. Due to the highly lipophilic nature of fentanyl, it is possible that the noradrenergic component contributing to spinal fentanyl-induced analgesia is supraspinally-mediated.

Serotonin contributes to the spinal antinociceptive effects of morphine

This study was designed to determine if morphine administered intrathecally (IT) interacts with serotonergic or noradrenergic nerve terminals in the spinal cord to produce analgesia on the spinally mediated tail-flick test. Male Sprague-Dawley rats were fitted with IT catheters. One week later, animals were spinally pretreated with receptor antagonists selective for opioid, serotonin or α-adrenoceptors, and the ability of these agents to alter spinal morphine-induced antinociception was assessed. Morphine dose-dependently elevated tail-flick latency in a naltrexone-reversible manner. The serotonin receptor antagonists spiroxatrine (5-HT 1A), pindolol (5-HT 1B), ritanserin (5-HT 2) and ICS 205–930 (5-HT 3) attenuated the spinal analgesic effects of morphine. In contrast, the α 1 and α 2-adrenoceptor antagonists prazosin and yohimbine, respectively, did not alter morphine-induced elevations in tail-flick latency. These data substantiate earlier reports that spinal morphine-induced antinociception relies on an opioid receptor-mediated component in addition to a local serotonergic component. The finding that the α-adrenoceptor antagonists did not alter the antinociceptive effects of IT morphine suggests that spinal norepinephrine does not contribute to the analgesic effects of the opiate.

The local monoaminergic dependency of spinal ketamine

The effects of s.c. doses of naloxone, methysergide and phentolamine on ketamine-induced spinal analgesia were assessed to determine the involvement of opiate, serotonergic and noradrenergic components mediating ketamine's antinociceptive action. Ketamine administered intrathecally (i.t.) produced a significant elevation in tail-flick latency in rats. The spinal antinociceptive effects of ketamine were dose dependently reversed by methysergide (ID 50 = 0.008 mg/kg s.c), phentolamine (ID 50 = 0.88 mg/kg s.c.) and naloxone (ID 50 = 3.0 mg/kg s.c). Unlike morphine, which remains analgesic and depedent on opiate interaction following bilateral lesions of the dorsolateral funiculus (DLF), ketamine analgesia was absent following DLF lesions. Thus, ketamine appears to produce an antinociceptive response which is dependent upon the neuronal activity of the descending pain-inhibitory pathways. The monoaminergic components comprising the descending pathways appear to be more prominent in the action of ketamine than they are in the spinal action of morphine. Furthermore, the spinal opioid receptors involved in ketamine's effect may be different from the μ subtype preferred by morphine.

Analgesic effects of serotonin and receptor-selective serotonin agonists in the rat spinal cord

1. 1. Serotonin (5-HT) and selective 5-HT receptor agonists were administered intrathecally (i.t.) in rats, and the antinociceptive efficacy of these agents was assessed on the tail-flick and hot plate tests. 2. 2. The 5-HT receptor agonists examined in this study included the 5-HT 1A agonist 8-hydroxy- N, N-dipropyl-2-aminotetralin (8-OH-DPAT), the 5-HT 1B agonist m-trifluoromethylphenylpiperazine (TFMPP), the 5-HT 2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT 3 agonist phenylbiguanide (PBG). 3. 3. None of these agents produced significant elevations in tail-flick latency (TFL) at doses which produced elevations in hot plate latency (HPL). 4. 4. In contrast, the i.t. dose of 5-HT which elevated TFL also produced analgesia on the hot plate test. 5. 5. Serotonin-induced elevations in TFL were reversed by pindolol, ritanserin and ICS 205–930, suggesting that 5-HT interacts with more than one 5-HT site in the spinal cord to produce analgesia on the tail-flick test. 6. 6. The finding that ritanserin reversed 5-HT-induced elevations in HPL suggests that the 5-HT 2 site is primarily responsible for mediating the spinal antinociceptive effects of 5-HT on the hot plate test.

Analgesic electrical stimulation of the hypothalamic arcuate nucleus: tolerance and its cross-tolerance to 2 Hz or 100 Hz electroacupuncture

Focal electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) for 5 min (1 session) produced a marked elevation of tail flick latency (TFL) to noxious heat in the pentobarbital-anesthetized rat. Repeated stimulation for a total of 11 sessions at 30 min intervals resulted in a gradual decline in the hypoalgesic action, and this tolerance may last for 7 days. Tolerance to the ARH analgesic stimulation reduced the analgesia produced by low (2 Hz) but not high (100 Hz) frequency electroacupuncture (EA); and tolerance to low frequency EA analgesia attenuated the ARH stimulation-produced analgesia without affecting high frequency EA analgesia. Alternatively, rats tolerant to high-frequency EA analgesia were still sensitive to either the ARH or low-frequency EA stimulation. These results suggest that the ARH stimulation and low-frequency EA administration produced analgesia via a common neural mechanism, supporting our hypothesis put forward previously that the ARH plays an important role in mediating low- but not high-frequency EA analgesia.

Spinal β-endorphin analgesia involves an interaction with local monoaminergic systems

β-Endorphin administered intrathecally (i.t.) in rats produced a dose-dependent elevation in tail-flick latency. Naltrexone administered i.t. as a pretreatment reversed the spinal antinociceptive action of β-endorphin, suggesting that the opioid interacts directly with spinal opiate receptors. Spinal administration of the α 1-adrenoceptor antagonist WB-4101 failed to alter the analgesic effects of the opioid, whereas the α 2-adrenoceptor antagonist yohimbine completely blocked β-endorphin-induced elevations intail-flick latency. Thus, there is an apparent specificity for the α 2-adrenoceptor to mediate the spinal action of β-endorphin. The 5-HT 1 and 5-HT 3 receptor antagonists (spiroxatrine and ICS 205–930, respectively) also reversed the analgesic effects of the opioid, while the 5-HT 2 receptor antagonist ritanserin only partially blocked β-endorphin-induced elevations in tail-flick latency. The present results suggest that β-endorphin produces analgesia at the spinal level via an opiate receptor-mediated interaction with spinal monoaminergic nerve terminals.

Alterations in nociception following lesions of the A5 catecholamine nucleus

Neurons located in the nucleus raphe magnus (NRM), a region important in the control of nociception, appear to be tonically inhibited by noradrenergic (NA) neurons. Anatomical studies have suggested that the A5 catecholamine nucleus may be the primary source of noradrenergic neurons whose terminals are located in the NRM. The purpose of the present study was to examine the role of A5 neurons in the modulation of nociception. Bilateral electrolytic lesions of the A5 nuclei produced a marked and long-lasting antinociception as assessed by both the tail-flick and hot-plate tests. Unilateral A5 lesions also produced a long-lasting elevation in hot-plate latency, but the elevation of tail-flick latency was smaller in magnitude and was only observed one day following the lesion. This finding is consistent with previous studies which have shown that blockade of the NA input to the NRM by the microinjection of NA antagonists also produces antinociception. These data indicate that neurons located in the A5 nucleus may be the origin of this NA projection to the NRM. The elevation in tail-flick latency observed following A5 lesions was significantly attenuated by the intrathecal injection either the NA antagonist phentolamine or the serotonergic antagonist methylsergide. However, the elevation in hot-plate latency was not significantly altered by these monoaminergic antagonists. Similarly, previous studies have shown that the elevation in tail-flick, but not hot-plate latency, produced by the microinjection of NA antagonists in the NRM is attenuated by the intrathecal injection of either phentolamine or methysergide. These data suggest that the antinociception induced by lesions in the A5 region is mediated, at least in part, by the activation of spinally-projecting NA and serotonergic pathways. The results of these studies lead to the suggestion that the NA neurons located in the A5 modulate nociception by tonically inhibiting raphe-spinal neurons located in the NRM.

Characterization of dynorphin A-induced antinociception at spinal level

Dynorphin A (DYN A) injected intrathecally in the rat produced a significant elevation of the nociceptive threshold, measured by the tail flick test. The highest dose of DYN A (25 nmol) produced maximal elevation of tail flick latency to radiant heat together with hindlimb paralysis and tail flaccidity lasting several hours, thus confirming several previous reports. A lower dose of DYN A (12.5 nmol) produced only a smaller, not constant, short-lasting change in the nociceptive threshold. The vocalization test (electrical stimulation of the tail) gave a different result: the time course curve showed that the antinociceptive effect had worn off 60 min after DYN A 25 nmol. Thus it can be assumed that the prolonged depression of the tail flick reflex was related to motor dysfunction and did not completely reflect the animal's response to painful stimuli. Tolerance to the antinociceptive and motor effects developed after the chronic intrathecal infusion of DYN A with osmotic minipumps. Intrathecal MR 1452 (30 nmol.), a purported κ-receptor blocker, fully prevented the effects of DYN A but not morphine-induced antinociception. Naloxone antagonized DYN A only at a 4 fold higher dose. MR 1452 (90 nmol) administered after DYN A reversed the elevation of the vocalization threshold while tail flick latency remained unmodified. Analysis by high performance liquid chromatography of intrathecally injected radiolabelled DYN A revealed that DYN A was largely broken down about 10 min after its administration. Our results seem to indicate that DYN A in the spinal cord causes alterations in nociception and motor function, clearly distinguishable in time and both mediated by an opioid receptor, probably of the κ- type. However different mechanism(s), possibly non-opioid in nature, may contribute to the prolonged depression of the tail flick.

Ketamine analgesia is not related to an opiate action in the periaqueductal gray region of the rat brain

Ketamine is an injectable anesthetic agent that has been shown to interact as an agonist at opiate receptors. In addition, its antinociceptive action in rats is antagonized by the narcotic receptor antagonist naloxone. Thus it was assumed that the anesthetic may activate the pain inhibitory pathway, originating in the periaqueductal gray (PAG) and descending into the spinal cord, in a manner similar to that of narcotics like morphine. In the present study, it was verified that the systemic administration of naloxone (3 mg/kg i.p.) antagonized the elevation in tail-flick latency produced by an anesthetic dose of ketamine (160 mg/kg i.p.), but did not alter the duration of anesthesia (defined as duration of the loss of the righting reflex). However, when naloxone (3 μg/0.5 μl/30 sec) was given by microinjection into the PAG it was found to be ineffective against the ketamine-induced elevation of the tail-flick latency. In contrast, the microinjection of the antagonist significantly attenuated (halved) the elevated latency in response to systemically administered morphine (4 mg/kg s.c.). It was also shown that ketamine was unable to elicit an increase in the latency of the tail-flick reflex when administered directly into the PAG over a wide range (0.10–100 μg) of doses. On the other hand, a local anesthetic-like effect of ketamine, known to occur when the drug is used in high concentration, was observed when doses exceeding 0.1 μg were injected into the PAG. This action interfered with opiate actions in the PAG and made data from the microinjection studies difficult to interpret. The descending, pain inhibitory neuronal system originating in the PAG does not appear to participate in the antinociceptive action of ketamine measured by the tail-flick reflex. Perhaps the drug's effects are associated with alternative opiate mechanisms and/or opiate receptor subtypes not present on the cells of origin of the descending nerves within the PAG.

Electrical stimulation of the substantia nigra does not increase tail-flick latency in the rat

The effect of electrical stimulation at multiple sites in the midbrain was investigated in rats lightly anesthetized with barbiturate. The application of monopolar electrical stimulation to the substantia nigra (SN) or the adjacent cerebral peduncle failed to produce an elevation in tail-flick latency (TFL) in response to the application of noxious radiant heat. However, the same intensity of stimulation when applied to an area dorsal to the SN led to an elevation in TFL. This effect was not due to current spread to dopaminergic neurons in the zona compacta compartment of the SN because administration of cis-flupenthixol failed to block the effect. We conclude that, contrary to previous suggestions, discrete electrical stimulation of the SN itself does not give rise to a state of antinociception.

Antagonism of stimulation-produced antinociception by intrathecal administration of methysergide or phentolamine

This study examined whether the antinociception produced by electrical stimulation of medullary raphe nuclei is mediated by activation of monoaminergic neurons projecting to the spinal cord. Ninety-four sites distributed about the midline of the medulla were stimulated and their ability to produce antinociception was determined using two different analgesiometric tests. Stimulation of sites in the nuclei raphe pallidus and raphe obscurus did not produce antinociception, but rather, produced tremor and, on occasion, extensor rigidity. In contrast, stimulation at sites located in the nucleus raphe magnus and the adjacent nucleus reticularis paragigantocellularis produced antinociception, as indicated by increased tail flick latencies and decreased responsiveness to noxious pinch applied to the extremities. Intrathecal administration of saline prior to electrical stimulation of these sites did not attenuate either the elevation of tail flick latencies or the decreased responsiveness to pinch. However, intrathecal administration of 30 μg of either methysergide or phentolamine prior to stimulation at these same sites significantly attenuated the increase in tail flick latency and restored responsiveness to pinch. Naloxone (1 mg/kg, s.c.) did not attenuate the stimulation-produced antinociception evoked from any of these sites. These data support the postulate that the antinociceptive effect of electrical stimulation of the nucleus raphe magnus and the nucleus reticularis paragigantocellularis is mediated by activation of serotonergic and noradrenergic neurons projecting to the spinal cord. The inability of either methysergide or phentolamine alone to completely antagonize the elevation of tail flick latency further suggests that the serotonergic and noradrenergic bulbospinal systems are coactivated by electrical stimulation of these sites.

Naloxone partially antagonizes post-encounter analgesia and enhances defensive responding in male rats exposed to attack from lactating conspecifics

Recent evidence indicates that inter-male conflict in mice, but not rats, results in opiate-like analgesia in defeated animals. To test the hypothesis that differences in attack intensity/controllability may underlie this apparent species difference, male rats were exposed to non-ritualized attack from lactating conspecifics. In response to such encounters, male intruders exhibited significant elevations in tail-flick latency. Pretreatment with low (0.1–1.0 mg/kg), but not high (10 mg/kg), doses of naloxone partially antagonized the development of post-encounter analgesia and enhanced several components of male defensiveness during encounters. These results suggest that exposure to attack from lactating conspecifics activates both opioid and nonopioid analgesia mechanisms in male intruders. Correlational analyses revealed, however, that intruder analgesia was not related to the number of attacks received or the display of submissive postures. Data are discussed with reference to the possible importance of psychological factors in the activation of endogenous pain inhibitory mechanisms in rats.

Hypoalgesia induced by microinjection of a norepinephrine antagonist in the raphe magnus: reversal by intrathecal administration of a serotonin antagonist

Microinjection of phentolamine in the nucleus raphe magnus produces hypoalgesia as assessed by both the tail flick and hot plate tests. The elevation of tail flick latency appears to results from activation of serotonergic raphe-spinal projections, while the elevation of hot plate latency appears to be mediated by activation of non-serotonergic raphe-spinal neurons and/or by activation of raphe neurons which project to supraspinal nuclei.