Independence of cocaine sensitization and amygdaloid kindling in the rat

The reported sensitization to repeated administration of cocaine has been described as a type of pharmacological kindling resembling the seizure development seen with repeated electrical stimulation of certain brain sites. If these two phenomena share a common neural mechanism, prior chronic administration of cocaine would be expected to facilitate the process of subsequent electrical kindling. Rats were pretreated with saline or cocaine (40 mg/kg IP) daily for 21 days, followed by electrical stimulation of the basolateral amygdala. Progressive sensitization to the behavioural effects of cocaine administration occurred. However, no significant differences in the subsequent development of electrically kindled seizures were observed between the cocaine and saline-treated groups. These results suggest that cocaine sensitization and electrical kindling of the amygdala are subserved by independent neural mechanisms.     

Social stress does not alter the expression of sensitization to cocaine

The effects of chronic social stress on behavioral sensitization to cocaine were investigated in the Syrian hamster. Adolescent animals received either 15 mg/kg i.p. of cocaine or saline twice per day for 7 consecutive days. Two weeks following the last injection (young adulthood), they were given a challenge dose of 5 mg/kg i.p. of cocaine and scored for locomotion. Motor activity was significantly greater in cocaine-treated animals, demonstrating sensitization to this psychostimulant. Following the results of the first study, another group of adolescent animals was exposed to either a novel clean cage (control) or an aggressive resident male hamster (social stress) for 15 min following an injection of cocaine (20 mg/kg i.p. once daily) or saline for 7 consecutive days. The groups were as follows: Social Stress/Cocaine (SSC), No Social Stress/Cocaine (NSSC), Social Stress/Saline (SSS) and No Social Stress/Saline (NSSS). Two weeks following the last injection (Day 21), all animals were given a challenge dose of cocaine (5 mg/kg i.p.) and were rescored for locomotion. At that time, the suppressive effect of stress on locomotion was no longer detectable, as the expression of sensitization was observed in the NSSC but not in the SSC group. These results suggest that chronic social stress administered during adolescence does not cross-sensitize with cocaine in young adult hamsters.     

Modulation of long-term potentiation in the rat hippocampus following cocaine self-administration

Long-lasting neuroadaptations that occur during drug use and remain after withdrawal are thought to contribute to the persisting and compulsive nature of drug addiction and relapse. At the molecular and cellular levels, mechanisms that have been implicated in the normal process of memory formation are increasingly being identified as potential contributors to the persistence of the addicted state. To investigate the effect of cocaine self-administration on synaptic plasticity, rats were allowed to self-administer 0.5 mg/kg/infusion cocaine or 0.9% NaCl during 90 min sessions for 15 consecutive days. These cocaine and saline self-administration subjects were then restricted to their home cages for 3, 30, or 100 days (3, 30, and 100 day cocaine/saline withdrawal groups) before the assessment of the induction and reversal of long-term potentiation (LTP) in the CA1 region of hippocampal slices. The magnitude of LTP was increased in the 3-day cocaine withdrawal group as compared with the 3-day saline withdrawal group, but this effect was short lived, as the 30-day cocaine and saline withdrawal groups exhibited similar LTP magnitudes. Interestingly, LTP was significantly decreased in the 100-day cocaine withdrawal group compared with the 100-day saline withdrawal group. These results support the hypothesis that the capacity for LTP is persistently altered after withdrawal from exposure to an addictive substance. In addition, this alteration can be differentially expressed such that depending upon the duration of the withdrawal period following the last drug exposure, LTP may be enhanced, unchanged, or suppressed.     

Cortical mechanisms of cocaine sensitization

Behavioral sensitization is the augmented motor-stimulant response that occurs with repeated, intermittent exposure to most drugs of abuse, including cocaine. Sensitization, which is a long-lasting phenomenon, is thought to underlie drug craving and relapse to drug use. Much research has been conducted to determine the neural mechanisms of sensitization. The bulk of this effort has focused on the nucleus accumbens and ventral tegmental area (VTA) that comprise a portion of the mesolimbic dopamine system. Recently, studies have begun to also explore the role of the medial prefrontal cortex (mPFC) in sensitization, in part because this region provides glutamatergic innervation to the VTA and nucleus accumbens. The present review will coalesce these studies into a working hypothesis that states that cocaine sensitization results from a decrease in inhibitory modulation of excitatory transmission from the mPFC to the VTA and nucleus accumbens. The discussion will revolve around how repeated cocaine exposure alters dopamine, gamma-aminobutyric acid (GABA), and glutamate regulation of pyramidal cell activity. It will be proposed that cocaine-induced alterations in cortical transmission occur in two phases. During early withdrawal from repeated cocaine exposure, changes in neurotransmitter release are thought to underlie the decreased inhibitory modulation of pyramidal projection neurons. Following more prolonged withdrawal, the attenuation in inhibitory transmission appears to occur at the receptor level. A model will be presented that may serve to direct future studies on the involvement of the mPFC in the development of cocaine sensitization, which ultimately could lead to development of pharmacotherapies for cocaine addiction.     

Minocycline affects cocaine sensitization in mice

Growing evidence has pointed to an interaction between the tetracycline antibiotic minocycline and drugs with abuse liability such as opioids and amphetamines. In this work, we tested the hypothesis that similar to its effects on methamphetamine-induced locomotor sensitization, minocycline may influence the behavioral effects of cocaine. Experiments were performed in male C57BL/6J mice using an automated system to measure locomotor activity. We found that 80 mg/kg minocycline significantly reduced locomotor activity when administered either alone or injected 30 min prior to cocaine, which increased locomotor activity. To investigate whether minocycline selectively affects the development of locomotor sensitization induced by four daily injections of 10 mg/kg cocaine, we sought a schedule of minocycline administration that does not per se affect locomotor activity. Thus, we selected 40 mg/kg minocycline administered 3 h prior to cocaine; minocycline did not affect cocaine-stimulated locomotor activity on the first day of administration but prevented the development of cocaine sensitization. We also tested whether minocycline would affect an already established cocaine sensitization. After establishing the sensitization effect by four daily injections, cocaine treatment was discontinued and mice were treated with minocycline daily (days 5–11) or on day 11 only. There was no effect of minocycline treatment on the response of cocaine-sensitized mice to the challenge dose of cocaine on day 11. The mechanisms by which minocycline interferes with the development of cocaine sensitization need to be characterized.     

The effect of cyclosporin A in delaying maturation of the small intestine during weaning in the rat.

As maturation of the small intestine has similar features to an immunologically mediated reaction, we studied the effect of the immunosuppressive agent, cyclosporin A (CyA), on the development of the small intestine during weaning in the DA x PVG rat. Intestinal development was measured by villus area, crypt length, crypt cell production rate (CCPR), and disaccharidase activity. Rat pups received either cyclosporin A (7.5 mg/kg daily subcutaneously) or polyethoxylated castor oil (Cremophor, drug vehicle) subcutaneously from 12 days of age. Cremophor- and CyA-treated litters were killed at 18, 20, 22, 24, and 26 days of age. CyA-treated animals had retarded weight gain, lower mesenteric lymph node and spleen weights, fewer intraepithelial lymphocytes, and reduced systemic secretion of rat mucosal mast cell protease II. CyA treatment retarded any increase in villus area, crypt length and CCPR until day 26 of age. Lactase activity was retained longer, and sucrase and maltase induction was delayed. We conclude that CyA retarded normal development of the small intestine, but some maturation still occurred at the end of weaning.     

Development of cocaine sensitization before pregnancy affects subsequent maternal retrieval of pups and prefrontal cortical activity during nursing

Pups are a highly rewarding stimulus for early postpartum rats. Our previous work supports this notion by showing that suckling activates the mesocorticolimbic system in mothers. In the present study, we tested whether development of behavioral sensitization to cocaine before pregnancy affects the neural response to pups during the early postpartum days (PD). Virgin rats were repeatedly administered cocaine for 14 days (15 mg kg(-1)) and withdrawn from treatment during breeding and pregnancy. The neural response to suckling was measured at PD 4-8 using blood-oxygen-level-dependent (BOLD) MRI or microdialysis. Our results show that BOLD activation in the medial prefrontal cortex (PFC), septum and auditory cortex was curtailed in cocaine-sensitized dams. No differences between cocaine sensitized and saline control dams were observed in the nucleus accumbens, olfactory structures, or in 48 additional major brain regions that were analyzed. Baseline, but not pup-stimulated, dopamine (DA) levels in the medial PFC were lower in cocaine-sensitized dams than in controls. When tested for maternal behaviors, cocaine-sensitized dams showed significantly faster retrieval of pups without changes in other maternal behaviors such as grouping, crouching and defending the nest. Taken together, the present findings suggest that maternal motivation to retrieve pups was enhanced by repeated cocaine exposure and withdrawal, a result reminiscent of 'cross-sensitization' between the drug and a natural reward. Changes in retrieval behavior in cocaine-sensitized mothers might be associated with a hypo-responsive medial PFC.     

Testosterone is essential for cocaine sensitization in male rats

Most studies agree that males and females respond differently to drugs of abuse. In females, estradiol enhances the behavioral response to cocaine. However, studies on the role of testosterone and the locomotor response to psychostimulants in the male rat are inconclusive. Our study was designed to determine the behavioral effects of testosterone on the development and persistence of cocaine sensitization in male rats. We tested different doses of cocaine (10, 15 and 30 mg/kg) to determine which dose induced locomotor sensitization in intact (INT) and gonadectomized (GDX) animals. We also investigated if GDX males with testosterone replacement (GDX-T) showed a similar locomotor response to cocaine as INT males.Our data showed that gonadectomy enhanced the locomotor response to a single cocaine injection. This effect was not observed in gonadectomized rats that received testosterone replacement. However, GDX rats did not show a progressive increase in their locomotor response to repeated cocaine administration (15 and 30 mg/kg) (sensitization) as did INT and GDX-T animals. It is possible that in GDX males, the initial high locomotor response to cocaine creates a ceiling effect that limits further increase in cocaine-induced hyperactivity. These findings indicate that testosterone not only modulates the behavioral response to a single and to repeated cocaine injections, but is also essential for male rats to become sensitized to cocaine.     

Repeated cocaine exposure induces sensitization of ultrasonic vocalization in rats

Ultrasonic vocalization at 55 kHz (55 kHz-USVs) by rodents has been proposed to be a behavioral manifestation of affectively positive incentive motivation. To examine the extent to which 55 kHz-USV emissions correlate with cocaine-induced locomotor activity, we measured cocaine-induced 55 kHz-USVs and their relationship to cocaine-induced locomotor sensitization in rats. We demonstrate that similar to locomotor responses, 55 kHz-USVs are also sensitized by exposure to cocaine. Furthermore, we show that the magnitude of cocaine-induced 55 kHz-USV sensitization is positively correlated with that of locomotor sensitization. Moreover, we demonstrate that rats selectively bred for high rates of 55 kHz-USVs exhibit higher levels of cocaine-induced 55 kHz-USV sensitization than animals selectively bred for low levels of 55 kHz USVs. These results suggest that the neural circuits underlying 55 kHz-USV, which may directly reflect affective experience/motivation, can be sensitized by cocaine in a way that resembles locomotor sensitization.     

BDNF-dependent synaptic sensitization in midbrain dopamine neurons after cocaine withdrawal

The neural mechanism underlying the relapse to drug use after drug withdrawal is largely unknown. We found that after withdrawal from repeated cocaine exposure, excitatory synapses onto dopamine neurons in the ventral tegmental area (VTA) of the rat midbrain became highly susceptible to potentiation by weak presynaptic stimuli, an effect requiring endogenous brain-derived neurotrophic factor-tyrosine kinase B (BDNF-TrkB) signaling. The elevated BDNF expression in the VTA after cocaine withdrawal may prime these synapses for potentiation by cue-associated activity, triggering drug craving and relapse.     

Expression of cocaine sensitization: regulation by the medial prefrontal cortex.

Extracellular levels of dopamine are increased in response to systemic administration of cocaine in several brain areas including the nucleus accumbens and medial prefrontal cortex. While the cocaine-induced increase in extracellular dopamine levels in the nucleus accumbens is augmented after repeated daily cocaine, the response of extracellular dopamine levels in the medial prefrontal cortex is attenuated. Since dopamine in the medial prefrontal cortex has an inhibitory effect on nucleus accumbens dopamine levels and locomotor activity, the role of medial prefrontal cortex dopamine tolerance in the expression of sensitized locomotor behavior was further examined by injection of D-amphetamine sulfate into the prelimbic portion of the medial prefrontal cortex just prior to cocaine challenge in cocaine-sensitized rats. Male Sprague-Dawley rats were non-handled (naive) or injected with either saline (1 ml/kg, i.p.) or cocaine (15 mg/kg, i.p.) for five consecutive days. After a seven to 12 day withdrawal period, rats were microinjected with either saline or various doses of amphetamine into primarily the prelimbic region of the medial prefrontal cortex followed by systemic injection of saline or cocaine. In naive rats, intramedial prefrontal cortex amphetamine produced a trend toward decreased locomotor responding to cocaine challenge while no effect of amphetamine was evident in daily saline pretreated rats. Daily cocaine pretreated rats that received saline in the medial prefrontal cortex demonstrated a sensitized locomotor response compared to their daily saline pretreated counterparts. This sensitization was blocked by a low dose of amphetamine (0.175 microg/side) in the medial prefrontal cortex, an effect which disappeared in animals administered higher amphetamine doses. The results suggest that in rats sensitized to cocaine, decreased medial prefrontal cortex dopamine levels in response to cocaine challenge may contribute to behavioral sensitization. Furthermore, the data indicate the possibility that there is an optimal range at which medial prefrontal cortex amphetamine exerts maximal behavioral inhibition. These findings implicate a role for decreased cortical control in producing sensitized behavioral responding to cocaine.     

Shock sensitization of startle in the developing rat

Rats given a series of shocks exhibit a potentiated startle response to a loud acoustic stimulus compared to nonshocked animals. Experiment 1 showed that this shock sensitization of startle, like conditioned fear potentiation of startle to discrete cues, emerges relatively late in development (i.e., 23 days of age). Although different testing procedures were used in Experiment 2, preweanling rats still failed to exhibit the shock sensitization of startle effect. The failure to observe the shock sensitization of startle effect in preweanling rats was not due to age differences in contextual conditioning produced by the shock treatment (Experiment 3). The results of this study are discussed in terms of (a) the emergence of fear potentiation of startle during development, and (b) the relation between conditioned freezing and startle potentiation.     

Generalized and signal-specific long-term nociceptive sensitization in the common snail

It was found in experiments carried out in common snails that applications of a concentrated solution of quinine (10%) to the animal's head lead to long-term (more than 24 h) facilitation of defense reactions. Facilitation of synaptic components of the responses to test stimulation, an increase in the excitability of the membrane, and its depolarization correspond to the behavioral manifestations of long-term sensitization in the command neurons of defense behavior. The degree of manifestation of the sensitization effects depends on the duration of its development. After a one-day development of sensitization, manifestations of signal-specific sensitization, including site-specific sensitization (more pronounced synaptic facilitation in the responses to test stimulation of the segment of the body which had been subjected to the sensitizing influence, as compared with facilitation of responses to stimulation of other points) and modality-specific sensitization (more pronounced facilitation of responses to test stimulation which coincides in modality with the sensitizing stimulus, by comparison with the degree of manifestation of responses to a test stimulation of a different sensory modality), predominate. After a three-day development, signs of generalized sensitization predominated in the command neurons: marked facilitation of responses in all synaptic inputs of the command neurons, an increase in excitability, and depolarization of the membrane. P. K. Anokhin Research Institute of Normal Physiology, Russian Academy of Medical Sciences, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 45, No. 4, pp. 732–741, July–August, 1995.     

Covariant maturation of nocifensive oral behaviour and c-fos expression in rat superior colliculus

Injections of formalin into the rodent paw elicit a rapid orientation of the head and mouth to the source of discomfort, followed by licking and biting the injected area. Previous work has shown this response is dependent on the integrity of the midbrain superior colliculus. The present experiments were initiated to examine the ontogeny of this oral nocifensive reaction and to determine whether it is correlated with the functional maturation of collicular responses to noxious stimuli (as indicated by c-fos immunohistochemistry). Rat pups at various postnatal ages received formalin injections in either the hindpaw or perioral regions. Behaviour was videotaped, and after 120 min, animals were killed and the brain and spinal cord processed for Fos-like immunoreactivity. Uninjected controls were treated identically. Formalin-induced oral responses following injections into the hindpaw and the expression of Fos in the superior colliculus were virtually absent until 10 days postnatal, despite the presence of Fos-like immunoreactivity in many other structures (e.g. spinal cord, parabrachial area, periaqueductal grey). In contrast, animals from day 1 were able to use limbs to localise the perioral injection site. From day 10 onward, there was a progressive increase in oral nocifensive behaviours and Fos expression in the superior colliculus.Our observations are consistent with the hypothesis that the normal elaboration of pain-induced oral behaviour is initiated only after a functionally active superior colliculus has developed, and support previous observations that link the colliculus particularly with oral nocifensive behaviours.     

Changes in the expression of differentiated functions during long-term cultivation of rat hepatoma cells

The stability of the expression of six differentiated functions was examined during long-term cultivation of rat hepatoma cells. Faza 967 cell line—a clonal descendant of the Reuber H35 hepatoma—is characterized by the activity of tyrosine aminotransferase (TAT) and gluconeogenetic enzymes; secretion of serum albumin; and the presence of liver isozymes of alcohol dehydrogenase (ADH-L), aldolase (aldolase-B) and five isozymes of lactate dehydrogenase (LDH). During the 3-year-long cultivation of Faza 967 cells TAT specific activity, inducibility, and albumin production were reduced drastically whereas the expression of the three liver-specific isozymes examined was maintained. The majority of Faza 967 cells were able to perform gluconeogenesis after 3 years of continuous cultivation. Our results show that long-term cultivation of hepatoma cells may change the expression of certain liver-specific functions independently of the expression of other differentiated functions.     

Differential expression of respiratory long-term facilitation among inbred rat strains

We tested the hypotheses that: (1) long-term facilitation (LTF) following acute intermittent hypoxia (AIH) varies among three inbred rat strains: Fischer 344 (F344), Brown Norway (BN) and Lewis rats and (2) ventral cervical spinal levels of genes important for phrenic LTF (pLTF) vary in association with pLTF magnitude. Lewis and F344, but not BN rats exhibited significant increases in phrenic and hypoglossal burst amplitude 60 min post-AIH that were significantly greater than control experiments without AIH, indicating strain differences in phrenic (98%, 56% and 20%, respectively) and hypoglossal LTF (66%, 77% and 5%, respectively). Ventral spinal 5-HT_2A receptor mRNA and protein levels were higher in F344 and Lewis versus BN, suggesting that higher 5-HT_2A receptor levels are associated with greater pLTF. More complex relationships were found for 5-HT₇, BDNF and TrkB mRNA. BN had higher 5-HT₇ and TrkB mRNA versus F344; BN and Lewis had higher BDNF mRNA levels versus F344. Genetic variations in serotonergic function may underlie strain differences in AIH-induced pLTF.