Widespread but regionally specific effects of experimenter- versus self-administered morphine on dendritic spines in the nucleus accumbens, hippocampus, and neocortex of adult rats

Increased AMPA GluR1 receptor subunit labeling on the plasma membrane of dendrites in the basolateral amygdala of rats self-administering morphine

Glutamate-dependent synaptic plasticity is emerging as an important neural substrate of addiction. These drug-dependent neural adaptations may occur within brain systems that mediate reward, emotion, and cognitive function such as the amygdala complex. Modification of glutamate receptor targeting may be a key mechanism mediating neural plasticity; however, evidence for alteration of amygdala AMPA receptor localization in response to drug self-administration is lacking. High-resolution immunogold electron microscopic immunocytochemistry was used to compare surface and intracellular labeling of the calcium sensitive AMPA GluR1 receptor subunit in the basolateral (BLA) and central (CeA) nuclei of the amygdala in rats self-administering escalating doses of morphine or saline. Morphine self-administration was associated with regionally diverse effects on dendritic GluR1 targeting in the BLA and CeA. In the BLA of morphine self-administering animals, there was a significant increase in the proportion of immunogold particles for GluR1 on the plasma membrane of dendrites, particularly in association with extrasynaptic sites, which was most prominent in large (2-4 microm) profiles. In contrast, there were no significant differences in surface or intracellular immunogold labeling in the CeA between morphine self-administering and control animals. In both amygdala regions, GluR1 and the micro-opioid receptor, the major cellular target of morphine, were only infrequently colocalized. These results indicate that GluR1 targeting is a dynamic process that can be differentially affected in distinct amygdala regions in response to chronic self-administration of morphine. Homeostatic adaptations in the subcellular localization of calcium sensitive AMPA receptors within the BLA may be an important neural substrate for alterations in reward, autonomic function, and behavioral processes associated with opiate addiction.

Differential involvement of the prelimbic cortex and striatum in conditioned heroin and sucrose seeking following long-term extinction

Relapse to drug taking is triggered by stimuli previously associated with consumption of drugs of misuse (cues) and involves brain systems controlling motivated behaviour towards natural reinforcers. In this study, we aimed to identify and compare neuronal pathways in corticostriatal systems that control conditioned heroin or natural reward (sucrose) seeking. To that end, rats were trained to self-administer heroin or sucrose in association with an identical compound cue. After more than 3 weeks of abstinence during extinction training, cue exposure robustly reinstated heroin and sucrose seeking, but induced distinct and even opposing changes in the expression of the neuronal activation marker zif268 in the prelimbic cortex and striatal complex, respectively. Because in the prelimbic area zif268 expression was enhanced during cue-induced heroin seeking but unaffected during sucrose seeking, a pharmacological intervention was aimed at this prefrontal region. Injection of a GABA agonist mixture within the prelimbic area enhanced conditioned heroin seeking, but had no effect on conditioned sucrose seeking. Our findings suggest a differential role of the prelimbic area and the striatum in the persistence of heroin vs. sucrose seeking following long-term extinction.

Morphine withdrawal-induced morphological changes in the nucleus accumbens

Morphine withdrawal produces a hypofunction of mesencephalic dopamine neurons that impinge upon medium spiny neurons (MSN) of the forebrain. After chronic treatment (from 20 to 140 mg/kg of morphine twice a day over 14 days at escalating doses) rats were withdrawn from chronic morphine spontaneously and pharmacologically. In these two distinct conditions we studied the effects of withdrawal on the morphology of MSN of the core and shell of the nucleus accumbens (Nacc). MSN were stained with the Golgi-Cox procedure and analysed by a confocal laser-scanning microscope (CLSM). Our analysis shows that, shell and core MSN differed significantly for perikarya size and spine density, and the various morphine treatments did not affect the perikarya morphometry. Both spontaneous and naloxone-induced withdrawal produced a similar reduction in spine density in MS shell neurons, as compared with MS core neurons. This effect is selectively localized at the level of second order dendritic trunks where afferents converge. By contrast, spine density counts of accumbens MSN from rats chronically treated with morphine, did not reveal any change. Collectively, the results of the present study are twofold: (i) spontaneous and pharmacologically precipitated withdrawal, but not chronic morphine per se, affects spine density of target structures of a reduced mesolimbic dopamine transmission, and (ii) the reduction of spine density in second order dendritic trunks is selectively segregated in the MSN of the shell of the Nacc. In conclusion, morphine withdrawal dramatically alters spine density, selectively in second order dendritic trunks of Nacc shell MSN, thereby further impoverishing the already abated dopamine (DA) transmission. This is in line with recent views suggesting the hypodopaminergic state as a cardinal feature of opioid dependence.

A Proteomics Approach to Identify Long-Term Molecular Changes in Rat Medial Prefrontal Cortex Resulting from Sucrose Self-Administration

The medial prefrontal cortex (mPFC) is involved in the processing and retrieval of reward-related information. Here, we investigated long-lasting changes in protein composition of the mPFC in rats with a history of sucrose self-administration. Protein levels were analyzed using 2-D PAGE and MALDI-TOF sequencing. From approximately 1500 spots, 28 regulated proteins were unambiguously identified and were involved in cytoskeleton organization, energy metabolism, oxidative stress, neurotransmission, and neuronal outgrowth and differentiation. For several proteins, this change was also found as a long-lasting alteration in gene expression. We show that self-administration of sucrose produces long-lasting molecular neuroadaptations in the mPFC that may be involved in reward-related information processing.

Neural and behavioral plasticity associated with the transition from controlled to escalated cocaine use

BACKGROUND

Rats given extended access to cocaine develop several symptoms of addiction, including a gradual escalation of drug intake, whereas rats given limited access do not. We asked here whether extended access to cocaine also produces drug-induced sensitization, a form of neurobehavioral plasticity implicated in addiction.

METHODS

Rats were given limited (1 hour/session) or extended access (6 hours/session) to self-administered cocaine. Following a period of abstinence, rats were selected at random for assessment of their psychomotor response to cocaine or drug-seeking during extinction or for anatomic studies.

RESULTS

When re-exposed to cocaine, rats allowed extended drug access showed greater drug-seeking behavior and were hypersensitive (sensitized) to the psychomotor activating effects of cocaine compared with rats given limited access. Extended access to cocaine was also associated with a greater increase in the density of dendritic spines on neurons specifically in the core of the nucleus accumbens (and not in the shell or medial or orbital frontal cortex).

CONCLUSIONS

The transition from stable to escalated cocaine use, a hallmark of addiction, is associated with especially robust behavioral sensitization and synaptic reorganization in the core of the nucleus accumbens.

Mu-opioid receptors modulate the stability of dendritic spines

Opioids classically regulate the excitability of neurons by suppressing synaptic GABA release from inhibitory neurons. Here, we report a role for opioids in modulating excitatory synaptic transmission. By activating ubiquitously clustered mu-opioid receptor (MOR) in excitatory synapses, morphine caused collapse of preexisting dendritic spines and decreased synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Meanwhile, the opioid antagonist naloxone increased the density of spines. Chronic treatment with morphine decreased the density of dendritic spines even in the presence of Tetrodotoxin, a sodium channel blocker, indicating that the morphine's effect was not caused by altered activity in neural network through suppression of GABA release. The effect of morphine on dendritic spines was absent in transgenic mice lacking MORs and was blocked by CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2), a mu-receptor antagonist. These data together with others suggest that endogenous opioids and/or constitutive activity of MORs participate in maintaining normal morphology and function of spines, challenging the classical model of opioids. Abnormal alteration of spines may occur in drug addiction when opioid receptors are overactivated by exogenous opiates.

Olfactory learning-induced increase in spine density along the apical dendrites of CA1 hippocampal neurons

We have previously shown that rule learning of an olfactory discrimination task is accompanied by increased spine density along the apical dendrites of piriform cortex pyramidal neurons. The purpose of the present study was to examine whether such olfactory learning task, in which the hippocampus is actively involved, induces morphological modifications in CA1 pyramidal neurons as well. Rats were trained to discriminate positive cues in pairs of odors for a water reward. Morphological modifications were studied in Golgi-impregnated neurons with light microscopy, 1 and 3 days after training completion. Spine densities were measured on the proximal region of apical dendrites and on basal dendrites after rule learning. Three days after training completion, the mean spine density on apical dendrites in neurons from trained rats was significantly higher by 20.5% than in neurons from pseudo-trained and naive animals, which did not differ from each other. By contrast, there was no significant difference in spine density of basal dendrites among the three groups. As length and diameter of spiny dendritic segments did not change after learning, the learning-related increase in spine density in neurons from trained rats may reflect a net increase in the number of excitatory synapses in the hippocampus following olfactory rule learning.

Structural plasticity associated with exposure to drugs of abuse

Persistent changes in behavior and psychological function that occur as a function of experience, such those associated with learning and memory, are thought to be due to the reorganization of synaptic connections (structural plasticity) in relevant brain circuits. Some of the most compelling examples of experience-dependent changes in behavior and psychological function, changes that can last a lifetime, are those that accrue with the development of addictions. However, until recently, there has been almost no research on whether potentially addictive drugs produce forms of structural plasticity similar to those associated with other forms of experience-dependent plasticity. In this paper we summarize evidence that, indeed, exposure to amphetamine, cocaine, nicotine or morphine produces persistent changes in the structure of dendrites and dendritic spines on cells in brain regions involved in incentive motivation and reward (such as the nucleus accumbens), and judgment and the inhibitory control of behavior (such as the prefrontal cortex). It is suggested that structural plasticity associated with exposure to drugs of abuse reflects a reorganization of patterns of synaptic connectivity in these neural systems, a reorganization that alters their operation, thus contributing to some of the persistent sequela associated with drug use--including addiction.

Secrets of the opium poppy revealed

Studies concerning drugs of abuse have made major contributions in defining the circuitry, as well as cellular and molecular substrates that underlie certain behaviors. Opiate drugs for example, have revealed important insights concerning pain perception and reward. Up to the late 1960s, opiate drugs were suspected to work by mysteriously perturbing lipid membrane structure. We now know the following: the sequence and neuroanatomy of the G-protein coupled receptors that mediate opiate effects; that many proteins interact with opioid receptors such as G-protein sub-unit combinations, G-protein receptor kinases, arrestins and calmodulin; that many signaling molecules are modulated by opioid receptors, including ion channels, kinase cascades and adenyl cyclase. More than 20 different peptides, excised from three precursor proteins by specific proteases, have been shown to be endogenous ligands for opioid receptors. Revealing the molecules of the endogenous opioid system has inspired efforts for developing new opioid analgesics with the hope of minimizing abuse potential. This article will detail the current rationale for searching for less-addictive opiate analgesics and speculate on the future of drug abuse research in furthering our understanding of neural plasticity and the underpinnings of addictive behavior.

Cocaine makes actions insensitive to outcomes but not extinction: implications for altered orbitofrontal-amygdalar function

Addiction is characterized by persistent drug-seeking despite adverse consequences or outcomes. Such persistent behavior may result from drug-induced brain changes that increase the control of behavior by associations between antecedent cues and responses. However, it is equally plausible that brain changes cause a decrease in the control of behavior by the value of likely outcomes. To test whether drug exposure can cause persistent behavior, and to distinguish between these two accounts of such behavior, we tested cocaine-experienced rats in a Pavlovian 'reinforcer devaluation' task, which provides independent assessments of the control of behavior by antecedent cues and outcome representations. We found that cocaine exposure caused persistent responding in this setting a month after the last drug treatment, and that this deficit resulted from an inability to use representations of outcome value to guide behavior rather than from changes in stimulus-response learning or response inhibition.

Heroin abusers' performance on the Tower of London Test relates to the baseline EEG alpha2 mean frequency shifts

Planning function deficit is the most consistent finding in neuropsychological studies of heroin addicts. The performance on the Tower of London Test (TLT) correlated with the duration of daily heroin abuse (DDHA) in our previous study. Alpha2 mean frequency in anterior/central derivations was also predicted by DDHA in the same patient population. This retrospective study was undertaken in order to understand better the relationships between observed neurological deviations in heroin abusers. Thirty-three heroin addicts and 12 healthy males were evaluated with 14 neuropsychological tests and resting eyes-closed electroencephalography (EEG). Multivariate tests showed that performance on the difficult (five-move) problems of TLT was strongly predicted by the EEG alpha2 mean frequency shifts, and these relationships were generally mediated by chronic heroin length. However, post-hoc analyses at separate leads demonstrated that the relationships between cognitive variables and alpha2 mean frequencies in the left hemisphere were independent of chronic heroin effects, whereas elevation of alpha2 frequency in the right hemisphere was strongly predicted by chronic heroin intake length. The patients with extremely high alpha2 mean frequency at the left central region were especially prone to failure in TLT due to the inability of the hypothesized alpha2-generating network, which normally projects to the central and temporal derivations bilaterally and to the right posterior temporal derivation to function appropriately. Hence, it was concluded that planning dysfunction in heroin abusers is related to alpha2 mean frequency shifts predominantly at the central regions.

Role of Cdk5 in Drug Abuse and Plasticity

Neuronal plasticity serves as the basis for learning and memory in the adult brain. Contextual, motor, and reward-based learning are important in reinforcing survival behavior in animals. Most psychostimulant drugs of abuse target the dopaminergic reward system of the brain. Drugs of abuse cause long-standing cellular and molecular neuroadaptations in the brain. The neuronal protein kinase Cdk5 is emerging as an important player in the cellular and physiological responses to drugs of abuse. Substantial evidence indicates that Cdk5 controls dopamine neurotransmission through regulation of the protein phosphatase-1 inhibitor, DARPP-32. Furthermore, the morphological changes associated with chronic cocaine exposure are dependent on Cdk5. Thus, Cdk5 mediates cellular responses to psychostimulant drug-induced changes in dopamine signal transduction and cytoskeletal reorganization. In this regard, Cdk5, through its targeting of various substrates, integrates a number of intracellular pathways that are targeted by psychostimulant drugs. These studies and the emerging role of Cdk5 in various forms of neuronal plasticity are reviewed.

Pleasure into pain: the consequences of long-term opioid use

One consequence of repeated drug administration is the development of adaptations in the nervous system, sometimes termed 'drug-opposite' responses. During administration, the effects of the drug are diminished by these adaptations (tolerance), while cessation of drug use results in the emergence of these drug-opposite responses as the withdrawal syndrome. Recent evidence on pain responses challenges this simple notion of withdrawal and suggests that aversive drug-opposite states may play a more important role in drug dependence than previously thought. While opioids such as heroin produce analgesia, people with a history of opioid self-administration are hypersensitive to certain kinds of pain during the time they are under the influence of the analgesic drug. This suggests that in pain systems, the drug-opposite response exceeds the pain inhibiting effect of the drug itself. This hyperalgesia is evident in people with a history of heroin use and is not modified by methadone or buprenorphine treatment but is reduced by long-term abstinence from opioids. This same pattern of the drug-opposite response exceeding the drug effect may also occur for mood. While opioids cause elevation of mood, commonly described as euphoria and reduction of emotional distress, methadone maintenance participants show significant negative mood disturbance relative to controls. Thus, for pain and mood, the chronic opioid user under the influence of the drug does not experience an opioid effect diminished by tolerance but a state opposite to the effect of the drug. Increases in drug concentration arising from administration serve only to reduce the degree of pain and mood disturbance. These aversive pain and mood states may contribute to the motivation for continued drug use and the dysfunction associated with drug dependence.

Cocaine-induced intracellular signaling and gene expression are oppositely regulated by the dopamine D1 and D3 receptors

Repeated exposure to cocaine can induce neuroadaptations in the brain. One mechanism by which persistent changes occur involves alterations in gene expression mediated by the dopamine receptors. Both the dopamine D1 and D3 receptors have been shown to mediate gene expression changes. Moreover, the D1 and D3 receptors are also coexpressed in the same neurons, particularly in the nucleus accumbens and also caudoputamen (CPu). Little is known however, whether these two receptors coordinately regulate gene expression after cocaine administration and the underlying mechanisms. We have used various gene mutant mice to address this issue. We show that extracellular signal-regulated kinase (ERK) activation and c-fos induction in the CPu in response to acute cocaine administration is mediated by the D1 receptor and inhibited by the D3 receptor. Moreover, ERK activation mediates acute cocaine-induced expression of Fos family genes, including c-fos, fosB and fra2. Interestingly, dynorphin, neogenin, and synaptotagmin VII, genes that possess cAMP-response element binding protein and AP-1 transcription complex-binding consensus sequences in their promoters, are also oppositely regulated by the D1 and D3 receptors after repeated exposure to cocaine. Furthermore, such regulation depends on proper ERK activation and c-fos function. These results suggest that the D1 and D3 receptors elicit opposite regulation of target gene expression by regulating ERK activation and c-fos induction after acute and chronic cocaine treatment.

Neurotrophic mechanisms in drug addiction

The involvement of neurotrophic factors in neuronal survival and differentiation is well established. The more recent realization that these factors also play pivotal roles in the maintenance and activity-dependent remodeling of neuronal functioning in the adult brain has generated excitement in the neurosciences. Neurotrophic factors have been implicated in the modulation of synaptic transmission and in the mechanisms underlying learning and memory, mood disorders, and drug addiction. Here the evidence for the role of neurotrophins and other neurotrophic factors-and the signaling pathways they activate-in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction is reviewed.

Cocaine-experienced rats exhibit learning deficits in a task sensitive to orbitofrontal cortex lesions

Addictive drugs, such as cocaine, cause long-lasting neural changes in prefrontal cortex. It has been hypothesized that these changes affect the behavioural control mediated by orbitofrontal cortex. To test this hypothesis, rats were given injections of cocaine (30 mg/kg/d, i.p.) or vehicle for 14 days and then trained after a 2-week withdrawal period in an odor discrimination task sensitive to the effects of orbitofrontal cortex lesions. We found that cocaine-treated rats, who demonstrated long-lasting sensitization to the locomotor activating effects of cocaine, failed to show normal changes in response latency during discrimination learning and were also slower than controls to acquire serial reversals. These behavioural impairments are identical to the effects of orbitofrontal cortex lesions in this task and show that cocaine exposure in rats can cause long-lasting effects on orbitofrontal-dependent functions. Notably, these effects were not correlated with increases in locomotor activity linked to cocaine-induced psychomotor sensitization observed before or after training, suggesting that the brain changes underlying the behavioural effects in the discrimination task are different from those mediating psychomotor sensitization.

Genetic study of alcoholism and novel gene expression in the alcoholic brain

Alcohol dependence may result from neuroadaptation involving alteration of gene expression after long-term alcohol exposure. The systematic study of gene expression profiles of the human alcoholic brain was initiated using the method of polymerase chain reaction (PCR)-differential display and was followed by DNA microarray. To date, more than 100 alcohol-responsive genes have been identified from the frontal cortex, motor cortex and nucleus accumbens of the human brain. These genes have a wide range of functions in the brain and indicate diverse actions of alcohol on neuronal function. This review discusses the current information on the genetic basis of alcoholism and the induction and characterization of these alcohol-responsive genes.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

EEG spectral power and mean frequencies in early heroin abstinence

The purpose of the present study was to investigate cumulative heroin effects on brain functioning by studying relationships among electroencephalography (EEG) spectral power and mean frequencies and heroin abuse history. Eyes-closed resting EEG data were collected from the 19 monopolar electrode sites in 33 heroin abusers and 13 age-matched healthy volunteers. The mean age of the patients was 23.1+/-4.5 years, the duration of daily heroin abuse (DDHA) ranged from 4 to 44 months, the intravenous doses of heroin ranged from 0.04 to 1.00 g/day, the abstinence length ranged from 6 days to 4.5 months. General linear model (GLM) repeated measures procedure revealed a significant group effect on the distribution of the mean power spectrum between bands and mean frequencies in almost all analyzed derivations. Further analysis demonstrated that these intergroup differences were diversely related to at least three aspects of heroin-taking history. Frequency shifts in alpha2 range, most prominent in frontal and central derivations, were related to duration of daily heroin consumption. Slowing of alpha1 mean frequency, most prominent in central, temporal, and occipital derivations, was registered mainly in heroin addicts who abused high doses of the drug. Spectral power characteristics of brain electrical activities in our patient population were strongly predicted by abstinence length. The present results give grounds to suppose that chronic heroin-taking induces neuronal oscillation frequency changes, which may contribute to the development of antisocial trends and some semantic processes disturbances in these patients. Supplementary neurophysiological deficit is characteristic for heroin addicts who takes high doses of the drug, however, its relation to heroin abuse remains unclear. Pronounced desynchronization is observed in acute heroin withdrawal, and spectral power characteristics tend to normalize almost completely during several weeks of abstinence.

Intermittent morphine treatment induces a long-lasting increase in cholinergic modulation of GABAergic synapses in nucleus accumbens of adult rats

Repeated exposure to drugs of abuse causes persistent behavioral sensitization and associated adaptations of striatal neurotransmission, which is thought to play an important role in certain aspects of drug addiction. Microdialysis and neurochemical studies suggest that intermittent morphine treatment may lead to a long-term increase in both ACh and dopaminergic neurotransmission in the nucleus accumbens (NAc). This implies that both cholinergic modulation of GABA synapses and their sensitivity to dopaminergic transmission might be changed, ultimately leading to a modified NAc output. Here we investigate to what extent cholinergic modulation and sensitivity to amphetamine, causing endogenous dopamine efflux, of GABAergic transmission in the nucleus accumbens are affected 3 weeks after a period of daily morphine injections in adult rats. To this end, we recorded medium spiny neurons using whole cell voltage clamp and monitored the frequency and amplitude of spontaneous GABAergic synaptic currents. We observed that the effect of nicotine on the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was suppressed in rats pretreated with morphine, whereas the effects of mecamylamine and tetrodotoxin (TTX) were increased. These results indicate that the probability of GABA release was increased and that this effect resulted from an upregulation of the endogenous activation of presynaptic nicotinic receptors. In addition, we observed an increased sensitivity to in vitro application of amphetamine. This suggests that the long-term increase in dopaminergic transmission caused by the morphine treatment affects GABA synapses in the NAc. Hence, there may be two parallel synaptic mechanisms by which drugs of abuse may affect processing and integration of NAc inputs.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.

Wheel access duration in rats: II. Day-night and within-session changes

Time of day and duration of wheel access were manipulated to see how running changed over days. Young male rats were given 2 hr of wheel access, during either the light or dark phase. Over 24 days, running increased 4-fold in the dark group but remained low and stable in the light group. With rats given either 1 or 4 hr of dark phase wheel access, running increased much more in the 4-hr group (even in Hour 1) than in the 1-hr group. The within-session running pattern for high runners changed from a habituation to a sensitization profile. Running and cocaine self-administration (S. H. Ahmed & G. F. Koob, 1998, 1999) are affected similarly by session length, suggesting running as an interesting addiction model.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Environmental complexity has different effects on the structure of neurons in the prefrontal cortex versus the parietal cortex or nucleus accumbens

Complex housing has been used widely as a model of experience-dependent change. Animals housed in complex environments typically show synaptogenesis throughout the sensory and motor cortex as well as the striatum and hippocampus, and thus it is generally assumed that such changes are likely to be found throughout the cerebrum. The purpose of the present study was to determine whether persistent alterations of dendritic morphology would be found in two regions that had previously not been examined, namely, the medial prefrontal region (Cg3) and nucleus accumbens (NAcc). The results show that housing female rats in complex environments for 3.5 months increased dendritic arborization on medium spiny neurons in the NAcc and on pyramidal cells in the somatosensory cortex (Par 1), but not in Cg3. Environmental complexity increased spine density in all three areas, however. The failure to find increased dendritic length or branching in Cg3 was unexpected. Thus, the data suggest that complex housing may not engage prefrontal neurons in the same manner as neurons in sensory or motor areas. It appears that complex housing may not produce generalized changes in cerebral morphology. The data further suggest that it is prudent to measure both dendritic length and spine density in studies of experience-dependent effects on synaptic plasticity.