Participation of noradrenergic pathways in the expression of opiate withdrawal: biochemical and pharmacological evidence

Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

Mesolimbocortical dopamine (DA) neurotransmission is important in the mediation of the dependence-producing actions of nicotine and other drugs of abuse. Withdrawal from chronic treatment with various types of addictive drugs, including amphetamine, cocaine, ethanol and morphine is associated with a decrease in dopaminergic output in the nucleus accumbens (NAC), whereas the effects of withdrawal from these drugs on dopaminergic output in the medial prefrontal cortex (PFC), as yet, remain largely unknown. This study examined putative changes in the extracellular levels of dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC and in the PFC of rats displaying behavioral signs of nicotine withdrawal. Rats were infused for 7 days with nicotine via subcutaneously implanted minipumps, whereas control animals carried saline-containing pumps. On the fifth day of infusion a microdialysis probe was implanted in the NAC or the PFC of the rats. Forty-eight hours later the levels of DA and the monoamine metabolites were assessed in the dialysate. The behavioral and biochemical effects of a saline injection and a subsequent challenge with the nicotinic receptor antagonist mecamylamine (1 mg/kg s.c.) were determined. Following mecamylamine challenge in nicotine-treated animals, the levels of DA, DOPAC and HVA in the NAC, but not in the PFC, decreased below pre-injection levels and in relation to control animals. The score of abstinence signs increased in the nicotine-treated rats, as compared both to the score after saline and to that in control animals. The decreased DA output in the NAC in animals displaying nicotine withdrawal signs is similar to that seen after withdrawal of several other drugs of abuse, and may have bearing on motivational deficits associated with the abstinence reactions.

Molecular and cellular basis of addiction

Drug addiction results from adaptations in specific brain neurons caused by repeated exposure to a drug of abuse. These adaptations combine to produce the complex behaviors that define an addicted state. Progress is being made in identifying such time-dependent, drug-induced adaptations and relating them to specific behavioral features of addiction. Current research needs to understand the types of adaptations that underlie the particularly long-lived aspects of addiction, such as drug craving and relapse, and to identify specific genes that contribute to individual differences in vulnerability to addiction. Understanding the molecular and cellular basis of addictive states will lead to major changes in how addiction is viewed and ultimately treated.

Atipamezole-precipitated clonidine withdrawal induces c-Fos expression in rat central nervous system

We sought to identify a clonidine withdrawal syndrome in conscious rats by investigating the effects of a single injection of the specific alpha2-adrenergic antagonist atipamezole (1.5 mg/kg i.p.) after chronic treatment with the alpha2-adrenergic agonist clonidine (200 microg/kg per day via osmotic mini-pump for 7-10 days). Rats treated chronically with clonidine followed by atipamezole injection (clonidine-atipamezole) demonstrated dramatic behavioral effects including shaking, vigorous digging, and whole-body seizure-like movements. Control groups (saline-saline, clonidine-saline and saline-atipamezole) showed no overt unusual behavioral effects following injection. The brains of the clonidine-atipamezole group showed massive c-Fos expression (especially in di- and telencephalon) while the other groups showed either background levels of c-Fos-immunopositive cells (saline-saline and clonidine-saline groups) or a slight increase over background in selected areas (saline-atipamezole group). Maps of c-Fos-immunolabeled cells were generated at five representative coronal planes for each treatment group. C-Fos-immunopositive cells were counted in three representative brainstem structures (locus coeruleus, nucleus of the solitary tract, rostral ventrolateral medulla (RVL)) and in three regions of the thoracic spinal cord (dorsal horn, intermediate zone and ventral horn). In the three brainstem structures the number of c-Fos-positive cells was elevated 8-10-fold in the clonidine-atipamezole group compared to the other groups. No other treatment group was significantly different from the saline-saline group. An increased number of c-Fos-positive neurons was also noted in the dorsal horn and intermediate layers of the thoracic spinal cord in the clonidine-atipamezole group compared to a sham-operated atipamezole-injected group. In the RVL, 59% of c-Fos-positive cells contained alpha2A-adrenergic receptor-like immunoreactivity in clonidine-atipamezole treated (withdrawing) rats. In addition, one-third of the tyrosine hydroxylase (TH)-immunopositive cells in RVL were also c-Fos-positive in clonidine withdrawing rats where no TH-positive cells were also c-Fos-positive in RVL of control groups. Atipamezole injected 10 min after a single injection of clonidine (200 microg/kg, i.p.) produced no behavioral effect and did not increase c-Fos expression in brainstem. Injection of the opiate antagonist naltrexone (100 mg/kg, i.p.) in rats chronically treated with clonidine did not elicit behavioral effects or result in increased c-Fos expression in brainstem. In conclusion, administration of the selective alpha2-antagonist atipamezole to rats chronically treated with the alpha2-adrenergic agonist clonidine triggers a powerful withdrawal syndrome associated with massive CNS expression of c-Fos protein. The intensity of the withdrawal syndrome indicates that chronic exposure to alpha2-adrenergic receptor agonists produces strong dependence.