Cocaine- and alcohol-mediated expression of inducible transcription factors is blocked by pentobarbital anesthesia.

Identifying the neurocircuitry involved in behavioral responses to drugs of abuse is an important step towards understanding the mechanisms of drug addiction. The present study sought to distinguish brain regions involved in pharmacological effects of cocaine and ethanol from secondary effects by administering these drugs in the presence or absence of pentobarbital anesthesia. Changes in neuronal activity were assessed by immunohistochemical analysis of expression of an inducible transcription factor (ITF), c-Fos, in the brain of rats habituated to repeated pentobarbital anesthesia or saline administration. Cocaine administration (15 mg/kg, i.v.) in non-anesthetized animals produced a strong induction of c-Fos in the striatum and large number of other brain areas. Ethanol administration (2 g/kg, i.p.) induced c-Fos in a smaller number of characteristic brain areas, including the central nucleus of amygdala and paraventricular nucleus of hypothalamus. However, neither of these drugs was able to induce c-Fos in pentobarbital-anesthetized rats (50 mg/kg, i.v.). The suppressive effects of pentobarbital were not specific to c-Fos, such that pentobarbital also suppressed expression of ITFs FosB and Egr1 in the striatum of cocaine-treated rats. On the other hand, pentobarbital by itself strongly induced c-Fos expression in the lateral habenula of saline-, cocaine-, and ethanol-injected rats. It is not clear whether the suppressive effects of anesthesia on ITF expression in other areas are mediated by activation of lateral habenula, or are independent of this event. Our data suggest that in the absence of conscious awareness of drug-associated cues, cocaine and alcohol activate only a fraction of the neural elements engaged in the unanesthetized state.

Antagonist precipitated clonidine withdrawal in rat: effects on locus coeruleus neurons, sympathetic nerves and cardiovascular parameters.

The goal of the present study was to examine the effect of clonidine withdrawal on the neural control of blood pressure. Rats were treated for 7-13 days with clonidine via osmotic minipumps (200 microg kg(-1) day(-1), s.c.). Controls received saline or were sham operated. Withdrawal was precipitated by the alpha2-adrenergic receptor (alpha2-AR) antagonist atipamezole. Most experiments were done under halothane anesthesia. Chronic treatment with clonidine did not change mean arterial pressure (MAP) or heart rate (HR) but raised femoral artery resistance and the activity of locus coeruleus neurons slightly. Atipamezole given to rats treated chronically with clonidine produced the following effects: no change in MAP, severe tachycardia, sustained increase in splanchnic sympathetic nerve discharge (SND; +75 +/- 13%), transient increase in lumbar SND (+23 +/- 7%), ON-OFF activity pattern in the locus coeruleus (LC). The ON phase of LC activity was synchronized with upswings of SND and with small changes in MAP. A second alpha2-AR antagonist, methoxyidazoxan, produced effects identical to those of atipamezole. Atipamezole given to control rats produced no effect on MAP, HR, SND or LC activity. Atipamezole reversed the hypotension, sympathoinhibition and bradycardia produced by acute administration of clonidine. In awake rats treated chronically with clonidine, atipamezole did not change MAP but produced arterial pressure lability and tachycardia. In conclusion, under anesthesia, selective alpha2-AR antagonists elicit a clonidine withdrawal syndrome that displays autonomic characteristics reminiscent of the spontaneous withdrawal syndrome found in awake rats. The most prominent features of this syndrome are tachycardia, sympathoactivation, lack of hypertension and an oscillating activity pattern of brainstem neurons leading to abrupt changes in SND and in MAP.

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.