A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

A single dose of methamphetamine leads to a long term reversal of the blunted dopamine D1 receptor-mediated neocortical c-fos responses in mice deficient for D2 and D3 receptors

Dopamine D(1) receptors play an essential role in the induction of expression of the immediate-early gene c-fos in response to pharmacological stimuli. In the forebrain of wild-type mice, administration of a D(1) receptor agonist leads to c-fos mRNA expression levels that are substantially higher than corresponding levels expressed after indirect stimulation of dopamine receptors with methamphetamine. In mice deficient for D(2) and D(3) receptors, c-fos mRNA levels expressed in response to D(1) agonist administration are significantly blunted. However, a single dose of methamphetamine (5 mg/kg) leads to a long lasting reversal of the blunted c-fos responses in these mutants. In the forebrain, this reversal is restricted to the neocortex. Moreover, methamphetamine also enhances c-fos expression levels in preadolescent wild-type mice that normally express low c-fos mRNA in response to D(1) agonist stimulation. Thus, a single dose of methamphetamine leads to a long term increase in D(1) receptor-dependent c-fos responses in brains with either low (preadolescent mice) or blunted (adult D(2) and D(3) mutant mice) c-fos expression levels. A similar long term reversal of the blunted c-fos responses is achieved with a single dose of a full D(1) agonist. These results indicate that the constitutive inactivation of D(2) and D(3) receptors leads to a decrease in agonist-promoted D(1) receptor activity that can be reversed by intermittent agonist stimulation.

ZIC2 and Sp3 repress Sp1-induced activation of the human D1A dopamine receptor gene

The human D(1A) dopamine receptor is transcribed from a tissue-specific regulated gene under the control of two promoters. An activator region (AR1) located between nucleotides -1154 and -1136 (relative to the first ATG) enhances transcription from the upstream promoter that is active in the brain. In this investigation, we sought to identify the nuclear factors that regulate the D(1A) gene through their binding to AR1 using yeast one-hybrid screening. Sp3 and Zic2 were among the positive clones isolated. Although Sp1 was not isolated from this screening and purified Sp1 alone does not bind to AR1 in gel shift experiments, this general transcription factor binds to AR1 in the presence of D(1A) expressing NS20Y nuclear extract and activates the D(1A) promoter. Thus, Sp1 appears to require an unknown factor(s) or post-translational modification to interact with AR1. On the other hand, Zic2 and Sp3 inhibit Sp1-induced activation of the D(1A) gene in an AR1-dependent manner. Zic2 and D(1A) genes have reciprocal brain regional distributions; Zic2 is expressed primarily in the cerebellum, and D(1A) is highly expressed in corpus striatum. These observations collectively suggest that one of the physiologic functions of Zic2 is repression of D(1A) gene transcription and that the intracellular balance among Sp1, Sp3 and Zic2 is important for regulating the tissue-specific expression of this dopamine receptor.

Cocaine, but not morphine, induces conditioned place preference and sensitization to locomotor responses in CB1 knockout mice

The involvement of cannabinoid CB1 receptors in morphine and cocaine motivational effects was investigated using CB1 knockout mice. For this purpose, we evaluated the rewarding effects in the place conditioning paradigm and the sensitization to the locomotor responses induced by these drugs. The hyperlocomotion induced by acute morphine administration (15 mg/kg, s.c.) was preserved, but the sensitization to this locomotor response induced by chronic morphine treatment was abolished in CB1 mutant mice. Morphine (5 mg/kg, s.c.) induced conditioned place preference in wild-type mice but failed to produce any response in knockout mice, indicating the inability of morphine to induce rewarding effects in the absence of CB1 cannabinoid receptors. When the aversive effects of morphine withdrawal were investigated using the place aversion paradigm, no differences between genotypes were observed. Acute cocaine (10 mg/kg, i.p.) induced hyperlocomotor responses in wild-type and knockout mice and a chronic cocaine treatment produced a similar sensitization to this response in both genotypes. In the conditioning place preference paradigm, cocaine (20 mg/kg, i.p.) produced rewarding responses in both wild-type and knockout mice. These results demonstrate that CB1 receptors are essential for adaptive responses produced by chronic morphine but not by chronic cocaine treatment.

Effects of administering cocaine at the same versus varying times of day on circadian activity patterns and sensitization in rats

The effects of different schedules of cocaine administration on circadian activity patterns and locomotor sensitization were studied. Rats received intraperitoneal injections of either saline or 20 mg/kg cocaine at either 24- or 33-hr intervals for 8 cycles (development). After a 2-day withdrawal, they were given a cocaine challenge in a novel environment. Rats given cocaine at 24-hr intervals were hypoactive 4 to 9 hr postinjection during development and, during cocaine challenge, showed sensitization of locomotor activity. Rats given cocaine at 33-hr intervals did not show these effects. On the 33-hr-period schedule, activity was enhanced beginning 24 hr after drug receipt. Different intermittent schedules of cocaine receipt may alter the vulnerability to cocaine, and altered vulnerability may be more likely when a subsequent cocaine injection interacts with a distal state of sensitivity produced by a prior injection.

Long-lasting stress sensitisation

Stressful experiences in humans can result in a spectrum of long-term changes in behavioural, autonomic and hormonal responsivity. An extreme form of such alterations is found in patients with post-traumatic stress disorder (PTSD). A number of animal models has been developed in which intense stressful experiences (shocks, social confrontations) result in longterm altered responsivity of behavioural, autonomic and hormonal responses to aversive challenges which mimic many of the changes seen in PTSD. These models of stress-induced sensitisation are beginning to generate a better understanding of the vulnerability factors, time-course and underlying neuronal substrates of the long-term disturbances experienced by humans as a result of stressful life events.

Long-term potentiation of excitatory inputs to brain reward areas by nicotine

Nicotine reinforces smoking behavior by activating nicotinic acetylcholine receptors (nAChRs) in the midbrain dopaminergic (DA) reward centers, including the ventral tegmental area (VTA). Although nicotine induces prolonged excitation of the VTA in vivo, the nAChRs on the DA neurons desensitize in seconds. Here, we show that activation of nAChRs on presynaptic terminals in the VTA enhances glutamatergic inputs to DA neurons. Under conditions where the released glutamate can activate NMDA receptors, long-term potentiation (LTP) of the excitatory inputs is induced. Both the short- and the long-term effects of nicotine required activation of presynaptic alpha7 subunit-containing nAChRs. These results can explain the long-term excitation of brain reward areas induced by a brief nicotine exposure. They also show that nicotine alters synaptic function through mechanisms that are linked to learning and memory.

Delayed effects of stress and immune activation

Stress responses play a crucial adaptive role but impose potentially subversive demands on the organism. The same holds for the symptoms of illness as seen after immune activation by pathogens or tissue damage. The responses to immune stimuli and stressors show remarkable similarities and rely on similar control mechanisms in the brain: i.e. they involve neuropeptides of the corticotropin releasing factor (CRF) family. Immune and non-immune challenges lead to responses that normally show a temporal relationship with the duration and intensity of the stimulus and the (re)activity of the stress-responsive systems return to their pre-challenged state within hours or days. However, exposure of animals or man to specific stimuli can induce delayed and long-lasting (weeks, months) alternation in stress responsive systems, resulting in a prolonged period of increased stress vulnerability. Immune stimuli are particularly powerful in eliciting such a stress vulnerable state. Various adaptive changes in the (neuro)biological substrate as seen during this stress vulnerable state also occur in depression, and may be causally related to the depressive symptoms that are often associated with infectious and inflammatory diseases.