Desipramine attenuates working memory impairments induced by partial loss of catecholamines in the rat medial prefrontal cortex.

RATIONALE: The density of tyrosine hydroxylase-immunoreactive (TH-IR) axons in the prefrontal cortex of schizophrenic subjects may be reduced by as much as 50% in the deep cortical layers (Am J Psychiatry 156:1580-1589, 1999). Previously, we demonstrated that approximately 60% loss of TH-IR axons in the rat medial prefrontal cortex (mPFC) decreases local basal and stress-evoked extracellular dopamine (DA) concentrations, suggesting that moderate loss of DA axons in the mPFC is sufficient to alter the neurochemical activity of the remaining DA neurons (Neuroscience 93:497-505, 1999). OBJECTIVES: To further assess the functional consequences of partial mPFC DA depletion, we examined the effects of 6-hydroxydopamine lesions of the rat mPFC on behavior in a T-maze delayed-response task. We also assessed whether chronic administration of the norepinephrine (NE) uptake inhibitor, desipramine (DMI), attenuates lesion-induced deficits in T-maze performance. Previous research indicates that inhibition of NE transport in the mPFC results in a concomitant increase in extracellular DA and NE. RESULTS: Moderate loss of mPFC DA and NE (approximately 50 and 10% loss, respectively) was sufficient to impair delayed-response behavior, in part due to an increase in perseverative responding. Chronic DMI treatment (3 mg/kg delivered via osmotic pumps) impaired performance of control rats but attenuated the deficits in delayed-response behavior in rats previously sustaining loss of mPFC DA and NE (approximately 75 and 35% loss, respectively). CONCLUSION: These data suggest that moderate loss of DA and NE in the prefrontal cortex is sufficient to impair cognitive function, and these behavioral effects are attenuated by inhibition of the NE transporter.

Neonatal depletion of cortical dopamine: effects on dopamine turnover and motor behavior in juvenile and adult rats.

Abnormal development of mesoprefrontal dopamine (DA) neurons may contribute to the pathophysiology of schizophrenia. Consistent with this hypothesis, DA nerve terminal density is decreased in the cortex of schizophrenic subjects [M. Akil, J.N. Pierri, R.E. Whitehead, C.L. Edgar, C. Mohila, A.R. Sampson, and D.A. Lewis, Lamina-specific alterations in the dopamine innervation of the prefrontal cortex in schizophrenic subjects, Am. J. Psychiatry, 156 (1999) 1580-1589]. This abnormality may be present early in development, giving rise to dysfunction as an individual matures. The present studies examined the effects of early partial loss of medial prefrontal cortex (mPFC) DA on DA turnover and locomotor behavior in juvenile, pubertal, and adult rats (30, 45, and 60 days of age, respectively). Local infusions of 6-hydroxydopamine on postnatal day (PN) 12-14 produced persistent decreases in basal tissue DA concentrations and increases in 3,4-dihydroxyphenylacetic acid (DOPAC):DA ratios in the mPFC. In the nucleus accumbens of lesioned rats, basal DA concentrations were decreased and DOPAC:DA ratios were increased on PN30, but not PN45 or 60. Footshock (30 min at 0.6 mA) increased DOPAC and DOPAC:DA ratios in the mPFC of PN30 and 60 control rats. These effects were attenuated in age-matched rats previously sustaining approximately 50% loss of mPFC DA on PN12-14. Footshock did not affect DOPAC:DA ratios in the nucleus accumbens of control or lesioned rats. The lesion also failed to alter basal or stress-evoked motor activity. The present data suggest that a decreased density of mPFC DA nerve terminals occurring early in development results in persistent alterations in basal and stress-evoked activity of mesoprefrontal DA neurons, but not mesoaccumbens DA neurons.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats.

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

Mesoprefrontal dopamine neurons and schizophrenia: role of developmental abnormalities.

Schizophrenia is characterized by a complex array of symptoms that include hallucinations, delusions, abnormal affect, and cognitive deficits. It is becoming increasingly apparent that the symptoms of the disorder are paralleled by equally complex functional and structural abnormalities in brain regions such as the temporal lobe, prefrontal cortex, and thalamus. In addition, the delayed onset of symptoms in late adolescence/early adulthood suggests that abnormal development of the nervous system may ultimately contribute to the neurobiology of the disorder. The present review focuses on clinical and basic research that, together, suggests abnormal development of the dopamine (DA) innervation of prefrontal cortex plays a role in the cognitive deficits of schizophrenia.

Chronic cold exposure potentiates CRH-evoked increases in electrophysiologic activity of locus coeruleus neurons.

BACKGROUND: Chronic stress exposure can produce sensitization of norepinephrine release in the forebrain in response to subsequent stressors. Furthermore, the increase in norepinephrine release in response to the stress-related peptide corticotropin-releasing hormone (CRH) is potentiated by prior chronic stress exposure. To explore possible mechanisms underlying these alterations in norepinephrine release, we examined the effect of chronic stress on the electrophysiologic activity of locus coeruleus (LC) neurons in response to centrally applied CRH. METHODS: Single-unit recordings of LC neurons in halothane-anesthetized rats were used to compare the effect of intraventricular administration of CRH (0.3-3.0 microg) in control and previously cold-exposed (2 weeks at 5 degrees C) rats. RESULTS: The CRH-evoked increase in LC neuron activity was enhanced following chronic cold exposure, without alteration in basal activity of LC neurons. The enhanced CRH-evoked activation was apparent at higher doses of CRH but not at lower ones, resulting in an increased slope of the dose-response curve for CRH in previously cold-exposed rats. CONCLUSIONS: These data, in combination with previous data, suggest that the sensitivity of LC neurons to excitatory inputs is increased following chronic cold exposure. The altered functional capacity of LC neurons in rats after continuous cold exposure may represent an experimental model to examine the role of central noradrenergic neurons in anxiety and mood disorders.

Effects of partial dopamine loss in the medial prefrontal cortex on local baseline and stress-evoked extracellular dopamine concentrations.

A reduction in the activity of mesoprefrontal dopamine neurons has been suggested to play a role in the pathophysiology of schizophrenia. Indeed, a recent study indicates that the density of tyrosine hydroxylase-immunoreactive axons is decreased in the deep layers of the prefrontal cortex of schizophrenic subjects [Akil et al., (1999) Am. J. Psychiatry, in press]. To determine the impact of partial loss of prefrontal dopamine axons on the activity of the remaining dopamine axons, we examined the effects of 6-hydroxydopamine lesions of the medial prefrontal cortex on local extracellular dopamine concentrations in the rat. In rats sustaining an average 63% loss of tyrosine hydroxylase-immunoreactive axons and no loss of dopamine-beta-hydroxylase-immunoreactive axons in the medial prefrontal cortex (smaller lesion), the baseline extracellular dopamine concentration was reduced by 63+/-9%. Thirty minutes of tail pressure increased extracellular dopamine in the medial prefrontal cortex by a maximum of 1.28+/-0.28 pg in control rats, but only 0.74+/-0.18 pg in rats with smaller lesions. In rats sustaining an average 80% loss of tyrosine hydroxylase-immunoreactive axons and 25% loss of dopamine-beta-hydroxylase-immunoreactive axons (larger lesion), the baseline extracellular dopamine concentration in the medial prefrontal cortex did not differ from control values. In addition, the maximum stress-evoked increase in dopamine concentration was also similar to that observed in control rats (+1.04+/-0.28 pg). The stress-induced increase in extracellular dopamine in the medial prefrontal cortex of rats sustaining smaller and larger lesions may occur in the absence of a corresponding increase in dopamine synthesis in mesoprefrontal dopamine neurons. This proposal is supported by our observation that stress did not alter tissue or extracellular 3,4-dihydroxyphenylacetic acid concentrations in the medial prefrontal cortex of lesioned rats. These data suggest that moderate loss of tyrosine hydroxylase-immunoreactive axons in the prefrontal cortex is sufficient to reduce extracellular dopamine concentrations in this brain region. In addition, a further reduction in tyrosine hydroxylase-immunoreactive axons in the medial prefrontal cortex, combined with the loss of dopamine-beta-hydroxylase-immunoreactive axons, results in normal extracellular dopamine concentrations in this area. We propose that the latter effect is due to increased neurochemical activity of remaining mesoprefrontal dopamine axons and/or decreased clearance of extracellular dopamine due to loss of both dopamine and norepinephrine transporters.

Sensitization of norepinephrine release in medial prefrontal cortex: effect of different chronic stress protocols.

Previously, we demonstrated that continuous exposure of rats to cold (5 degrees C) for 2-3 weeks potentiates the increase in extracellular norepinephrine in the medial prefrontal cortex produced by acute tail shock. In the present study, we used in vivo microdialysis to examine whether this sensitization of evoked norepinephrine release also occurs in the medial prefrontal cortex following exposure to other chronic stress protocols. Rats exposed to 30 min of intermittent foot shock (0.6 mA) each day for 14 days, did not exhibit a greater increase in extracellular norepinephrine in response to acute tail shock. To determine whether this discrepancy between cold exposure and foot shock might be related to differences in the nature or the pattern of exposure to the chronic stressor, we also examined the effect of intermittent exposure to cold or continuous exposure to a foot shock protocol on tail shock-evoked norepinephrine release. Sensitized norepinephrine release did not develop following either intermittent exposure to cold (5 degrees C; 4 h/day for 14 days) or continuous exposure to a foot shock protocol (0.6 mA trains at random intervals 24 h/day for 14 days), suggesting that both the nature of the stressor as well as the pattern of exposure to the chronic stressor play a role in the development of sensitized norepinephrine release.

Depletion of dopamine in the prefrontal cortex decreases the basal electrophysiological activity of mesolimbic dopamine neurons.

One hypothesis regarding the etiology of schizophrenia proposes that disruption of the dopaminergic innervation of the prefrontal cortex leads to an increase in dopamine (DA) transmission in subcortical regions. In the present study, we examined the effect of 6-hydroxydopamine lesions of the medial prefrontal cortex (mPFC) dopamine innervation on the spontaneous electrophysiological activity of ventral tegmental DA neurons recorded in vivo. DA cell activity was assessed along three dimensions: (1) the relative proportion of DA neurons exhibiting spontaneous activity, (2) their basal firing rate, and (3) the mean percentage of spikes fired in bursts. In lesioned rats, DA neurons in the ventral tegmental area (VTA) exhibited a significantly slower mean firing rate, as well as a significant reduction in the percentage of spikes fired in bursts relative to controls. In contrast, depletion of DA in the mPFC did not have a significant effect on the relative proportion of VTA DA neurons exhibiting spontaneous activity. We suggest that by reducing the basal electrophysiological activity of VTA DA neurons, mPFC DA depletion may lead to an increase in the level of responsivity of the system to excitatory stimuli. Thus, the magnitude of increase in action potential-dependent DA release that occurs in response to a challenge may be augmented in lesioned rats.

The effects of stress on central dopaminergic neurons: possible clinical implications.

The response of the central nervous system to stress is often critical to the adaptation of an organism to its environment. However, in humans the response to stress also can be maladaptive, resulting in the expression or exacerbation of many neurological and psychiatric disorders. In this review, we examine the impact of stress on the synthesis and release of dopamine within mesocortical, mesoaccumbens, and nigrostriatal dopamine projections. We note that whereas stress increases the neurochemical activity of each of these populations of dopamine neurons, heterogeneities do exist. Specifically, acute stress evokes a greater increase in dopamine metabolism and release within the prefrontal cortex than the subcortical sites. Furthermore, whereas prior exposure to chronic stress enhances the response of mesocortical dopamine neurons to an acute novel stressor, this does not occur in the subcortical sites. In addition to these regional heterogeneities, we also note that even within a single dopamine projection there can be heterogeneous regulation of dopamine synthesis and release. Specifically, whereas stress-induced dopamine release in the neostriatum is mediated by an action of glutamate on the dopamine cell body, stress-induced dopamine synthesis in the neostriatum is mediated by an action of glutamate on the dopamine nerve terminal. Finally, we propose that regional heterogeneities in the responsiveness of central dopamine neurons to stress may ultimately play a role in the expression and exacerbation of symptoms associated with schizophrenia.

Loss of dopamine terminals in the medial prefrontal cortex increased the ratio of DOPAC to DA in tissue of the nucleus accumbens shell: role of stress.

We examined whether dopamine depletion in the medial prefrontal cortex of the rat differentially affects basal and evoked dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) content in the subareas of the neostriatum and nucleus accumbens. Loss of approximately 80% of tissue dopamine content in the medial prefrontal cortex did not significantly alter basal tissue concentrations of dopamine or DOPAC or the DOPAC:dopamine ratio in either the nucleus accumbens core or shell or the medial or lateral neostriatum. However, tail pressure stress significantly increased the DOPAC:dopamine ratio in the nucleus accumbens shell of lesioned rats. Because dorsal and ventral areas of the medial prefrontal cortex preferentially innervate the core and shell, respectively, we sought to determine whether the selective effect of lesions on dopamine terminals in the shell of the nucleus accumbens are paralleled by greater dopamine loss in the ventral medial prefrontal cortex. 6-Hydroxydopamine decreased tissue concentrations of dopamine in both the dorsal (-74%) and ventral medial prefrontal cortex (-68%). In lesioned rats, few tyrosine hydroxylase-immunoreactive fibers remained in the dorsal medial prefrontal cortex whereas a dense innervation remained in the ventralmost area. The present data suggest that the influence of mesocortical dopamine neurons on the dopamine projection to the nucleus accumbens shell is expressed only under conditions of stress. Furthermore, lesion-induced alterations in dopamine neurons projecting to the nucleus accumbens shell are not due to a more extensive loss of dopamine terminals in the ventral than in the dorsal medial prefrontal cortex.

Impact of corticotropin-releasing hormone on extracellular norepinephrine in prefrontal cortex after chronic cold stress.

We have previously demonstrated that exposing rats to cold (5 degrees C) for 3-4 weeks potentiates the increase in extracellular norepinephrine (NE) in the medial prefrontal cortex produced by acute tail shock. In the present study, we used microdialysis to determine the duration of cold exposure required to produce this sensitization and explored the mechanism of the phenomenon. Tail shock elicited a twofold greater increase in extracellular NE in the medial prefrontal cortex of rats exposed to cold for 2 weeks than in naive control rats or in rats exposed to cold for 1 week and tested either immediately or after a 2-week delay. Local infusion of 10 microM D-amphetamine or 30 mM K+ increased extracellular NE in the medial prefrontal cortex (approximately 350 and 190%, respectively) comparably in control rats and rats exposed to cold for 3 weeks. In contrast, intraventricular administration of 3.0 microg of corticotropin-releasing hormone increased extracellular NE in the medial prefrontal cortex by 65% in rats exposed to cold for 2 weeks, but only 35% in control rats. These results indicate that an enhanced responsiveness of noradrenergic neurons to acute tail shock (1) requires approximately 2 weeks of cold exposure to develop and (2) may be mediated by a change at the level of the noradrenergic cell bodies rather than the nerve terminals.

Effects of dopamine depletion in the medial prefrontal cortex on the stress-induced increase in extracellular dopamine in the nucleus accumbens core and shell.

In the present study we examined whether depletion of dopamine in the medial prefrontal cortex alters the neurochemical activity of mesoaccumbens dopamine neurons and/or their behavioral correlate, motor behavior. Infusion of 6-hydroxydopamine (1 microgram) into the medial prefrontal cortex of rats pretreated with a norepinephrine uptake blocker produced a 70% loss of tissue dopamine, with relative sparing of the norepinephrine content (-23%) in that region. Using in vivo microdialysis, we monitored basal and evoked extracellular dopamine in the nucleus accumbens core and shell of control and lesioned rats. The concentration of basal extracellular dopamine in the nucleus accumbens core was similar in control and lesioned rats; however, basal dopamine efflux in the nucleus accumbens shell was approximately 30% higher in lesioned rats than in controls. Lesions did not alter the ability of systemic D-amphetamine (1.5 mg/kg, i.p.) to increase extracellular dopamine in the nucleus accumbens shell, in contrast, the dopamine depletion in the medial prefrontal cortex attenuated the amphetamine-induced increase in extracellular dopamine in the nucleus accumbens core, as well as the amphetamine-induced increase in locomotor activity. Lesions did not significantly alter the effects of tail pressure (30 min) on extracellular dopamine in the nucleus accumbens core. However, the depletion of dopamine in the medial prefrontal cortex potentiated the stress-induced increase in extracellular dopamine in the nucleus accumbens shell. These data demonstrate that mesocortical dopamine neurons influence (i) amphetamine-induced dopamine efflux in the nucleus accumbens core and (ii) stress-evoked dopamine efflux in the nucleus accumbens shell. It has been proposed that a disruption in the interaction between cortical and subcortical dopamine neurons is involved in the pathophysiology of schizophrenia. The present data raise the possibility that a disruption in the interaction between mesocortical dopamine neurons and dopamine neurons projecting to the nucleus accumbens shell is involved in those symptoms of schizophrenia that are influenced by stress.

Local influence of endogenous norepinephrine on extracellular dopamine in rat medial prefrontal cortex.

Noradrenergic and dopaminergic projections converge in the medial prefrontal cortex and there is evidence of an interaction between dopamine (DA) and norepinephrine (NE) terminals in this region. We have examined the influence of drugs known to alter extracellular NE on extracellular NE and DA in medial prefrontal cortex using in vivo microdialysis. Local application of the NE uptake inhibitor desipramine (1.0 microM) delivered through a microdialysis probe increased extracellular DA (+149%) as well as NE (+201%) in medial prefrontal cortex. Furthermore, desipramine potentiated the tail shock-induced increase in both extracellular DA (stress alone, +64%; stress + desipramine, +584%) and NE (stress alone, +55%; stress + desipramine, +443%). In contrast, local application of desipramine did not affect extracellular DA in striatum, indicating that this drug does not influence DA efflux directly. Local application of the alpha 2-adrenoceptor antagonist idazoxan (0.1 or 5.0 mM) increased extracellular NE and DA in medial prefrontal cortex. Conversely, the alpha 2-adrenoceptor agonist clonidine (0.2 mg/kg; i.p.) decreased extracellular NE and DA in medial prefrontal cortex. These results support the hypothesis that NE terminals in medial prefrontal cortex regulate extracellular DA in this region. This regulation may be achieved by mechanisms involving an action of NE on receptors that regulate DA release (heteroreceptor regulation) and/or transport of DA into noradrenergic terminals (heterotransporter regulation).

Effects of selective dopamine depletion in medial prefrontal cortex on basal and evoked extracellular dopamine in neostriatum.

In this study, we demonstrate that 6-hydroxydopamine (6-OHDA) can be used to produce a lesion of dopamine (DA) terminals in medial prefrontal cortex (mPFC) while sparing the noradrenergic innervation in this region. Furthermore, we determined the impact of these lesions on both extracellular DA in neostriatum, using in vivo microdialysis, and locomotor activity. Our results demonstrate that, whereas higher doses of 6-OHDA (> or = 4 micrograms) depleted both DA and norepinephrine (NE) in mPFC, 1 micrograms 6-OHDA produced a depletion of DA (-79%) without significantly affecting NE content (-13%). Selective depletion of DA content in mPFC did not alter basal levels of extracellular DA in neostriatum determined 14 days after the lesion. The lesion also did not alter the ability of acute tail pressure (30 min) to increase extracellular DA in neostriatum or to stimulate locomotor activity. Depletion of DA in mPFC did not alter the ability of d-amphetamine (1.5 mg/kg, i.p.) to increase intracellular DA in neostriatum. In contrast, the maximum amphetamine-induced increase in locomotor activity was attenuated in lesioned rats as compared with control rats (670 and 280 locomotor counts/15 min, respectively). These data suggest that in the intact system, DA terminals in mPFC do not regulate extracellular DA in neostriatum. In addition, these data confirm that DA terminals in mPFC can influence stimulant-induced locomotion.

Increased dopamine and norepinephrine release in medial prefrontal cortex induced by acute and chronic stress: effects of diazepam.

We have examined the effects of diazepam on the stress-induced increase in extracellular dopamine and norepinephrine in the medial prefrontal cortex using in vivo microdialysis. In naive rats, acute tail pressure (30 min) elicited an increase in the concentrations of dopamine and norepinephrine in extracellular fluid of medial prefrontal cortex (+54 and +50%, respectively). Diazepam (2.5 mg/kg, i.p.) decreased the basal concentration of extracellular dopamine and norepinephrine. Diazepam also attenuated the stress-evoked increase in the absolute concentrations of extracellular dopamine (+17%), but did not alter the stress-induced increase in norepinephrine (+41%). However, when the drug-induced decrease in basal dopamine and norepinephrine concentration was taken into account, the stress-induced net increase in dopamine above the new baseline was equivalent to that obtained in vehicle pretreated rats, whereas the net increase in norepinephrine was almost twice that obtained in control subjects. In rats previously exposed to chronic cold (three to four weeks at 5 degrees C), tail pressure again produced an increase in the concentrations of dopamine and norepinephrine in the medial prefrontal cortex (+42% and +92%, respectively). However, in these chronically stressed rats, diazepam no longer decreased basal dopamine or norepinephrine in extracellular fluid, nor did it affect the stress-induced increase in the concentrations of these catecholamines. These data indicate that diazepam has complex effects on the extracellular concentrations of dopamine and norepinephrine which vary depending upon whether the rat is undisturbed or stressed during the period of drug exposure as well as the rat's prior history of exposure to stress. Moreover, these data raise questions regarding the role of catecholamines in the mechanism by which diazepam exerts its anxiolytic properties.

Stress-induced sensitization of dopamine and norepinephrine efflux in medial prefrontal cortex of the rat.

We examined whether prior exposure to chronic cold (17-28 days, 5 degrees C) alters basal or stress-evoked (30-min tail shock) catecholamine release in medial prefrontal cortex, nucleus accumbens, and striatum, using in vivo microdialysis. Basal norepinephrine (NE) concentrations in medial prefrontal cortex did not differ between chronically cold-exposed rats and naive control rats (2.7 +/- 0.3 vs. 2.5 +/- 0.2 pg/20 microliters respectively). Basal dopamine (DA) efflux in any of the brain regions was not significantly different between chronically cold-exposed rats and naive rats. However, a trend for lower basal DA efflux in the cold-exposed relative to naive rats was observed in medial prefrontal cortex (1.5 +/- 0.2 vs. 2.2 +/- 0.3 pg/20 microliters, respectively), nucleus accumbens (3.7 +/- 0.8 vs. 5.4 +/- 0.9 pg/20 microliters, respectively), and striatum (4.4 +/- 0.5 vs. 7.2 +/- 1.5 pg/20 microliters, respectively). In medial prefrontal cortex of rats previously exposed to cold, tail shock elicited a greater increase from baseline in both DA and NE efflux relative to that measured in naive rats (DA, 2.3 +/- 0.3 vs. 1.2 +/- 0.1 pg, respectively; NE, 3.8 +/- 0.4 vs. 1.4 +/- 0.2 pg, respectively). However, in nucleus accumbens or striatum of rats previously exposed to cold, the stress-induced increase in DA efflux was not significantly different from that of naive rats (nucleus accumbens, 1.8 +/- 0.7 vs. 1.5 +/- 0.3 pg, respectively; striatum, 1.9 +/- 0.4 vs. 2.6 +/- 0.7 pg, respectively). Thus, both cortical NE projections and cortically projecting DA neurons sensitize after chronic exposure to cold. In contrast, subcortical DA projections do not sensitize under these conditions.

Cocaine self-administration differentially alters mRNA expression of striatal peptides.

The influence of cocaine self-administration on the expression of messenger RNAs for dynorphin, enkephalin and substance P was analyzed in the rat striatum with in situ hybridization histochemistry. Cocaine, an indirect dopamine agonist, was found to differentially affect the levels of mRNA encoding these neuropeptides in different subregions of the striatum. Following a 7 day period of variable free access to cocaine, dynorphin and substance P mRNA levels were elevated throughout the striatum, but the increases were substantially greater in the dorsal striatum than in the nucleus accumbens. Enkephalin mRNA was not significantly altered in the dorsal striatum but was slightly elevated in the nucleus accumbens. These results suggest that cocaine self-administration has differential effects on striatonigral and striatopallidal projection neurons, and that these effects vary in subregions of the striatum.

Benzodiazepine-induced decreases in extracellular concentrations of dopamine in the nucleus accumbens after acute and repeated administration.

In vivo microdialysis was used to assess the effects of acute and repeated injections of the benzodiazepine midazolam on extracellular dopamine (DA) concentrations in the nucleus accumbens. Acute administration of midazolam (5 mg/kg, SC) elicited a 22% decrease in extracellular DA in the nucleus accumbens but failed to affect DA concentrations in the striatum. Similarly, six spaced intravenous infusions of midazolam, at a dose that has previously been found to support self-administration (0.05 mg per infusion), produced a 50% decrease in extracellular DA in the nucleus accumbens. In order to assess the effects of subchronic midazolam injections, two groups of rats were given injections of saline or midazolam (5 mg/kg, SC) for 14 days (two injections per day). A subsequent challenge injection of midazolam (5 mg/kg) decreased extracellular DA in the nucleus accumbens by 25% in both groups, indicating that neither tolerance nor sensitization occurred during the repeated drug administration. These experiments indicate (1) that midazolam differentially affects meso-accumbens and nigrostriatal DA neurons, and (2) that the midazolam-induced decrease in extracellular DA in the nucleus accumbens is not affected by repeated drug administration. The data further suggest that the rewarding effects of midazolam are not associated with increased release of DA in the nucleus accumbens.

Behavioral and neurochemical interactions between cocaine and buprenorphine: implications for the pharmacotherapy of cocaine abuse.

Intravenous self-administration studies in nonhuman primates suggest that the opioid receptor agonist-antagonist buprenorphine may be useful in the pharmacotherapy of cocaine abuse. In the present studies, behavioral and neurochemical interactions between cocaine and buprenorphine were examined using a conditioned place preference (CPP) procedure and in vivo microdialysis. Cocaine-induced CPP was linearly related to the dose administered (0-5.0 mg/kg). Buprenorphine (0-0.9 mg/kg) also elicited CPP in a dose-related manner; an inverted U-shaped function was obtained. Subthreshold doses of cocaine (1.5 mg/kg) and buprenorphine (0.01 mg/kg), themselves incapable of eliciting CPP, produced a significant CPP when given together. Moderate doses of cocaine (5.0 mg/kg) and buprenorphine (0.075 mg/kg), which were individually capable of eliciting CPP, produced a significantly larger CPP when given in combination. In the in vivo microdialysis studies, a low dose of buprenorphine (0.01 mg/kg) produced a progressive increase in extracellular dopamine in the nucleus accumbens, reaching approximately 200% of basal levels after 5 hr. Cocaine (5.0 mg/kg) rapidly increased extracellular dopamine concentrations (180% of basal values within 20 min), which returned to baseline in 2 to 3 hr. This effect of cocaine was significantly potentiated by coadministering buprenorphine (0.01 mg/kg); under this condition the peak increase in extracellular dopamine reached 260% of baseline values. These neurochemical findings are consistent with the CPP results and indicate that buprenorphine can interact with cocaine in a synergistic manner. In contrast to previous speculations, these results suggest that buprenorphine may enhance rather than attenuate the rewarding properties of cocaine.

Further characterization of intravenous self-administration of midazolam in the rat.

The present experiments further characterized intravenous self-administration of the short-acting benzodiazepine midazolam in rats under conditions of unlimited access to the drug. The results of the first experiment demonstrated that rats responded at higher rates on a lever that produced an infusion of 0.05mg midazolam than on a lever that did not result in reinforcement, and that they transferred responding to the other lever when the reinforcement contingencies were reversed. These results provide further evidence of the reinforcing effects of midazolam. In the second experiment, rats responding at stable rates for 0.05mg midazolam per infusion exhibited increased responding when transferred to a lower dose (0.0125mg/infusion) and decreased responding when transferred to a higher dose (0.20mg/infusion) of the drug. However, the inverse relationship between responding and drug dose was apparent only during the first transfer session, after which no consistent relationship was observed. In support of previous observations, all rats exhibited a temporal pattern of responding for midazolam over the 24h sessions, with maximal responding occurring during the dark phase of the 12h light/dark cycle. However, the present results also provide preliminary evidence that rats given prolonged access to midazolam (more than 49 days) develop a more constant within session pattern of responding for midazolam. This may be related to the development of physical dependence and reflect an attempt to avoid withdrawal effects by maintaining a stable intake of midazolam within a session.