Serotonin-2C and -2a receptor co-expression on cells in the rat medial prefrontal cortex.

Neural function within the medial prefrontal cortex (mPFC) regulates normal cognition, attention and impulse control, implicating neuroregulatory abnormalities within this region in mental dysfunction related to schizophrenia, depression and drug abuse. Both serotonin-2A (5-HT2A) and -2C (5-HT2C) receptors are known to be important in neuropsychiatric drug action and are distributed throughout the mPFC. However, their interactive role in serotonergic cortical regulation is poorly understood. While the main signal transduction mechanism for both receptors is stimulation of phosphoinositide production, they can have opposite effects downstream. 5-HT2A versus 5-HT2C receptor activation oppositely regulates behavior and can oppositely affect neurochemical release within the mPFC. These distinct receptor effects could be caused by their differential cellular distribution within the cortex and/or other areas. It is known that both receptors are located on GABAergic and pyramidal cells within the mPFC, but it is not clear whether they are expressed on the same or different cells. The present work employed immunofluorescence with confocal microscopy to examine this in layers V-VI of the prelimbic mPFC. The majority of GABA cells in the deep prelimbic mPFC expressed 5-HT2C receptor immunoreactivity. Furthermore, most cells expressing 5-HT2C receptor immunoreactivity notably co-expressed 5-HT2A receptors. However, 27% of 5-HT2C receptor immunoreactive cells were not GABAergic, indicating that a population of prelimbic pyramidal projection cells could express the 5-HT2C receptor. Indeed, some cells with 5-HT2C and 5-HT2A receptor co-labeling had a pyramidal shape and were expressed in the typical layered fashion of pyramidal cells. This indirectly demonstrates that 5-HT2C and 5-HT2A receptors may be commonly co-expressed on GABAergic cells within the deep layers of the prelimbic mPFC and perhaps co-localized on a small population of local pyramidal projection cells. Thus a complex interplay of cortical 5-HT2A and 5-HT2C receptor mechanisms exists, which if altered, could modulate efferent brain systems implicated in mental illness.

The social defeat animal model of depression shows diminished levels of orexin in mesocortical regions of the dopamine system, and of dynorphin and orexin in the hypothalamus.

Anhedonia is a core symptom of clinical depression. Two brain neuropeptides that have been implicated in anhedonia symptomology in preclinical depression models are dynorphin and orexin; which are concentrated along lateral hypothalamic dopamine reward pathways. These affect regulating neuropeptides modulate each other's function, implicating an interactive dysfunction between them in anhedonia symptomology. But whether their influences are modified or imbalanced within the hypothalamus or dopamine system in anhedonic preclinical depression models is not yet clear. We used radioimmunoassay to determine this in the rat social defeat model of depression; at a time that anhedonic sexual disinterest was expressed. In tissue samples of the medial prefrontal cortex (mPFC), ventral tegmental area (VTA) and nucleus accumbens, basal dynorphin levels were similar to normal animals. But orexin was reduced in the VTA and mPFC. Also, dynorphin and orexin were both diminished in the hypothalamus which is noteworthy since nearly all hypothalamic orexin cells co-express dynorphin. These findings suggest that orexin and dynorphin function may be imbalanced between the hypothalamus and mesocortical dopaminergic brain regions in depression.

Chronic lithium treatment magnifies learning in rats.

Recent electrophysiological work shows that chronic lithium treatment increases long-term potentiation (LTP) in neurons of the hippocampus, and LTP is thought to be the major neurophysiological basis for the development of learning and memory. This suggests that lithium might enhance learning and memory. Available studies have mainly assessed memory using aversive conditioning paradigms, but very little is available on the effect of lithium on learning. Since lithium may diminish anxiety or negative affect in adult rats, which would hinder aversive learning, the present study used three different positive reinforcement spatial cognitive tasks to determine whether chronic lithium affects learning. Each task differed in complexity, in the type of learning required, and in the reward received. For 4 weeks prior to, and throughout all learning assessments, rats had continual access to lithium chow or to a control chow diet. After 4 weeks' access to their designated chow diet, rats began conditioning in the hole-board spatial discrimination or T-maze delayed alternation tasks in a counterbalanced fashion. They immediately began conditioning in the opposite task once completing the first. This was then followed with social place-preference conditioning, after 24-h isolation from their home-cage social partner. Chronic lithium increased learning in all three paradigms, regardless of the reward received. Indeed, both food and social interaction supported enhanced learning. Thus the learning effect was not merely due to an effect of lithium on food palatability. Importantly, clinically relevant serum lithium levels were evidenced at the time of testing. Lithium also marginally enhanced memory as well. Thus chronic lithium treatment may improve learning and memory in Alzheimer's disease, and do so not only by blocking the formation of beta-amyloid and neurofibrillary tangles as suggested by previous research, but also by enhancing mechanisms involved in basal learning and memory formation, such as hippocampal synaptic plasticity.

Localization of 5-HT(2A) receptors on dopamine cells in subnuclei of the midbrain A10 cell group.

Considerable evidence suggests that a dysfunction of the dopamine and serotonin (5-hydroxytryptamine or 5-HT) neurotransmitter systems contributes to a diverse range of pathological conditions including schizophrenia, depression and drug abuse. Recent electrophysiological and behavioral studies suggest that 5-HT modulates dopaminergic neurons in the ventral tegmental area via activation of 5-HT(2A) receptors. It is currently unknown if 5-HT(2A) receptors mediate their actions on dopaminergic neurons in the ventral tegmental area via direct or indirect mechanisms. This study investigated whether 5-HT(2A) receptors were localized on dopamine cells within the A10 dopamine subnuclei of the rat, including the ventral tegmental area. We discovered that 5-HT(2A) receptor-like immunoreactivity colocalized with tyrosine hydroxylase, a marker for dopamine neurons, throughout the A10 dopamine cell population. Colocalization was most prominent in rostral and mid A10 regions, including the paranigral, parabrachial, and interfascicular subnuclei. Though more rare, non-dopaminergic neurons also expressed 5-HT(2A) receptor immunoreactivity in the ventral tegmental area. Additionally, although a dense population of 5-HT(2A) immunoreactive cells was observed in the rostral dorsal raphe nucleus, rarely were these cells immunoreactive for tyrosine hydroxylase. The linear raphe A10 dopamine subdivisions also displayed a low degree of 5-HT(2A) receptor and tyrosine hydroxylase colocalization. These findings provide an anatomical basis for the physiological modulation of dopamine neurons in the rostral ventral tegmental area either directly, by 5-HT(2A) receptors localized on dopamine cells, or indirectly, through a non-dopaminergic mechanism. Interestingly, 5-HT(2A) receptors were expressed on dopamine neurons in several A10 subnuclei that project to mesolimbic forebrain regions implicated in drug addiction, and recent evidence indicates that ventral tegmental area 5-HT(2A) receptor activation may modulate reward-related behavior in rodents. 5-HT(2A) receptors were also expressed on dopamine cells in A10 subnuclei that project to forebrain areas that have been implicated in schizophrenia, and atypical antipsychotic drugs have high affinities for 5-HT(2A) receptors. Thus, findings in this study could have important implications for understanding 5-HT and dopamine circuitry dysfunction in schizophrenia.

Ethanol consumption and place-preference conditioning in the alcohol-preferring C57BL/6 mouse: relationship with motor activity patterns.

Ethanol place-preference conditioning (PC) was conducted in drug-naive and ethanol pre-exposed female and male C57BL/6J (C57) mice to assess whether environmental cues can develop positive incentive value for ethanol-preferring animals when associated with administration of ethanol. After 12 days episodic access to free-choice ethanol and/or water self-administration, mice received eight ethanol injections (1.75 g/kg/i.p.) 5 min before placement in their nonpreferred PC chamber and eight saline injections paired with their preferred chamber. Control mice received eight saline injections (20 ml/kg) in both their preferred and nonpreferred chambers. Mice of both sexes developed strong ethanol PC. Correlational analysis indicated that the strength of ethanol PC for mice with a prior ethanol drinking experience was inversely related to the amount of ethanol consumed regardless of gender. Furthermore, depending on gender and previous ethanol drinking experience, ethanol PC was differentially related to initial baseline motor activity, the initial motor response to ethanol, or rapid change in the motor response to ethanol. Thus, a complicated relationship between neural systems that mediate ethanol reward and motor activity may exist as suggested by current addiction theory.

Connections of the piriform cortex in homing pigeons (Columba livia) studied with fast blue and WGA-HRP.

The piriform cortex in homing pigeons receives a projection from the olfactory bulb and is necessary for the operation of those aspects of the navigational map based on olfactory stimuli in these animals. The afferent and efferent projections of the piriform cortex were studied using retrograde migration of wheat-germ agglutinin horseradish peroxidase (WGA-HRP) and Fast Blue, and anterograde migration of WGA-HRP. The piriform cortex was found to receive projections from, and send projections to, numerous regions and nuclei in the telencephalon, diencephalon and lower brainstem. A reciprocal connection with the parahippocampal region suggests that the piriform cortex and hippocampal formation may be part of a neural system that regulates navigational map learning. The piriform cortex also connects reciprocally with a large portion of the anterior telencephalon, including the cortex prepiriformis and hyperstriatum dorsale. In general, the pathway connections of the piriform cortex in homing pigeons are similar to those of the piriform cortex in mammals.

Cocaine and vigilance task performance of rats: effects of delay of reinforcement.

In two experiments rats were food-reinforced for pressing one of two levers in an operant chamber, with the correct lever being indicated by the position of a briefly illuminated light. In Experiment 1 the levers were always in the chamber, whereas in Experiment 2 the levers were inserted into the chamber immediately after cue light termination and withdrawn immediately after a choice response. The rats were tested under four conditions: after an injection (SC) of saline or 2.5 mg/kg cocaine and with delay of reinforcement (DOR) of either 0 or 8 s. In both experiments, cocaine enhanced accuracy under the 0-s DOR condition. However, in neither experiment was there evidence of facilitation with cocaine under 8-s DOR, which by itself increased choice latencies and decreased accuracy when choice latencies exceeded 0.5 s. These results indicate that cocaine may only enhance performance in vigilance tasks under constrained conditions, e.g., those that require minimal levels of information processing.