Unilateral Perforant Path Transection Does Not Alter Lateral Entorhinal Cortical or Hippocampal CA3 Arc Expression.

It is well established that degradation of perforant path fibers is associated with age-related cognitive dysfunction and CA3 hyperactivity. Whether this fiber loss triggers a cascade of other functional changes within the hippocampus circuit has not been causatively established, however. Thus, the current study evaluated the effect of perforant path fiber loss on neuronal activity in CA3 and layer II of the lateral entorhinal cortex (LEC) in relation to mnemonic similarity task performance. Expression of the immediate early gene Arc was quantified in rats that received a unilateral right hemisphere transection of the perforant path or sham surgery that cut the cortex but left the fibers intact. Behavior-related expression of Arc mRNA was measured to test the hypothesis that fiber loss leads to elevated activation of CA3 and LEC neurons, as previously observed in aged rats that were impaired on the mnemonic similarity task. Transection of perforant path fibers, which has previously been shown to lead to a decline in mnemonic similarity task performance, did not alter Arc expression. Arc expression in CA3, however, was correlated with task performance on the more difficult discrimination trials across both surgical groups. These observations further support a link between CA3 activity and mnemonic similarity task performance but suggest the reduced input from the entorhinal cortex to the hippocampus, as observed in old age, does not causatively elevate CA3 activity.

Regulation of cocaine-related behaviours by estrogen and progesterone.

Women are more sensitive to cocaine craving elicited by stimuli associated with relapse. Ovarian hormones modulate cocaine craving and may therefore function as risk factors or therapeutic agents for the development and treatment of cocaine use disorder, respectively. We review herein the neuropharmacological effects of the steroid hormones 17ß-estradiol, progesterone, and allopregnanolone, a progesterone metabolite, in relation to their effects on cocaine-induced locomotion, behavioural sensitization, conditioned place preference, and reinstatement of cocaine seeking. In general, the literature suggests that female rats are more sensitive to these cocaine-induced behaviours than males and that 17ß-estradiol facilitates the expression of these sex differences. Alternatively, in females, exogenous progesterone attenuates cocaine conditioned place preference, reinstatement, and possibly behavioural sensitization, either on its own or after conversion to allopregnanolone. These opposing effects of 17ß-estradiol and progesterone/allopregnanolone involve endocannabinoid, γ-aminobutyric acid, dopamine, and glutamate transmission in the medial prefrontal cortex and striatum. We conclude that 17ß-estradiol may be a risk factor for various components of cocaine use disorder in women, whereas progesterone and allopregnanolone may be potential treatment options.

Role of prefrontal cortex projections to the nucleus accumbens core in mediating the effects of ceftriaxone on cue-induced cocaine seeking.

Ceftriaxone is an antibiotic that reliably attenuates the reinstatement of cocaine seeking after extinction while preventing the nucleus accumbens (NA) core glutamate efflux that drives reinstatement. However, when rats undergo abstinence without extinction, ceftriaxone attenuates context-primed cocaine seeking but NA core glutamate efflux still increases. Here, we sought to determine if the same would occur when cocaine seeking is prompted by both context and discrete cues (cue-induced seeking) after cocaine abstinence. Male rats self-administered intravenous cocaine accompanied by drug-associated cues (light + tone) for 2 h/day for 14 days. Rats then experienced abstinence with daily handling but no extinction training for 2 weeks. Ceftriaxone (200 mg/kg IP) or vehicle was administered during the last 6 days of abstinence. During a cue-induced cocaine seeking test, microdialysis procedures were conducted. Rats were perfused at the end of the test for later Fos analysis. A separate cohort of rats was infused with the retrograde tracer cholera toxin B in the NA core and underwent the same self-administration and relapse procedures. Ceftriaxone increased baseline glutamate and attenuated both cue-induced cocaine seeking and NA core glutamate efflux during this test. Ceftriaxone reduced Fos expression in regions sending projections to the NA core (prefrontal cortex, basolateral amygdala, ventral tegmental area) and specifically reduced Fos in prelimbic cortex and not infralimbic cortex neurons projecting to the NA core. Thus, when cocaine seeking is induced by drug-associated cues, ceftriaxone is able to attenuate relapse by preventing NA core glutamate efflux, likely through reducing activity in prelimbic NA core-projecting neurons.

Ceftriaxone and mGlu2/3 interactions in the nucleus accumbens core affect the reinstatement of cocaine-seeking in male and female rats.

RATIONALE: The beta-lactam antibiotic ceftriaxone reliably attenuates the reinstatement of cocaine seeking. While the restoration of nucleus accumbens core (NA core) GLT-1 expression is necessary for ceftriaxone to attenuate reinstatement, AAV-mediated GLT-1 overexpression is not sufficient to attenuate reinstatement and does not prevent glutamate efflux during reinstatement. AIMS: Here, we test the hypothesis that ceftriaxone attenuates reinstatement through interactions with glutamate autoreceptors mGlu2 and mGlu3 in the NA core. METHODS: Male and female rats self-administered cocaine for 12 days followed by 2-3 weeks of extinction training. During the last 6-10 days of extinction, rats received ceftriaxone (200 mg/kg IP) or vehicle. In experiment 1, rats were killed, and NA core tissue was biotinylated for assessment of total and surface expression of mGlu2 and mGlu3 via western blotting. In experiment 2, we tested the hypothesis that mGlu2/3 signaling is necessary for ceftriaxone to attenuate cue- and cocaine-primed reinstatement by administering bilateral intra-NA core infusion of mGlu2/3 antagonist LY341495 or vehicle immediately prior to reinstatement testing. RESULTS: mGlu2 expression was reduced by cocaine and restored by ceftriaxone. There were no effects of cocaine or ceftriaxone on mGlu3 expression. We observed no effects of estrus on expression of either protein. The antagonism of mGlu2/3 in the NA core during both cue- and cocaine-primed reinstatement tests prevented ceftriaxone from attenuating reinstatement. CONCLUSIONS: These results indicate that ceftriaxone's effects depend on mGlu2/3 function and possibly mGlu2 receptor expression. Future work will test this hypothesis by manipulating mGlu2 expression in pathways that project to the NA core.

Regionally Specific Effects of Oxytocin on Reinstatement of Cocaine Seeking in Male and Female Rats.

Background: Oxytocin reduces cued reinstatement of cocaine seeking in male and female rats, but the underlying neurobiology has not been uncovered. The majority of effort on this task has focused on oxytocin and dopamine interactions in the nucleus accumbens core. The nucleus accumbens core is a key neural substrate in relapse, and oxytocin administration in the nucleus accumbens core reduces reinstatement to methamphetamine cues. Further, the nucleus accumbens core has strong glutamatergic innervation from numerous regions including the prefrontal cortex. Thus, we hypothesize that oxytocin regulates presynaptic glutamate terminals in the nucleus accumbens core, thereby affecting reinstatement. Methods: To begin to evaluate this hypothesis, we examined the effects of intra-nucleus accumbens core oxytocin on extracellular glutamate levels in this region. We next determined if direct infusion of oxytocin into the nucleus accumbens core could attenuate cued reinstatement of cocaine seeking in a manner dependent on metabotropic glutamate 2/3 receptors. Finally, we tested if site-specific application of oxytocin in the prefrontal cortex reduced cued reinstatement of cocaine seeking. Results: We found an increase in nucleus accumbens core extracellular glutamate for several minutes following reverse dialysis of oxytocin. In male and female rats with a history of cocaine self-administration, site-specific application of oxytocin in the nucleus accumbens core and prefrontal cortex had opposing effects, decreasing and increasing cued reinstatement, respectively. The mGlu2/3 antagonist LY-341495 reversed oxytocin's ability to attenuate cued reinstatement. Conclusions: While the precise mechanism by which oxytocin increases nucleus accumbens core glutamate is yet to be determined, the present results clearly support oxytocin mediation of glutamate neurotransmission in the nucleus accumbens core that impacts cued cocaine seeking.

Nucleus accumbens GLT-1a overexpression reduces glutamate efflux during reinstatement of cocaine-seeking but is not sufficient to attenuate reinstatement.

Cocaine use disorder is a chronically relapsing disease without FDA-approved treatments. Using a rodent model of cocaine relapse, we and others have previously demonstrated that the beta-lactam antibiotic ceftriaxone attenuates cue- and cocaine-primed reinstatement of cocaine-seeking. Ceftriaxone restores cocaine-induced deficits in both system xc- and GLT-1 expression and function in the nucleus accumbens core (NAc). We recently demonstrated that restoration of GLT-1 expression in the NAc is necessary for ceftriaxone to attenuate reinstatement of cocaine-seeking. Here we used an adeno-associated virus (AAV) to overexpress GLT-1a in the NAc to investigate whether such restoration is sufficient to attenuate cue- and cocaine-primed reinstatement. Rats self-administered cocaine for two weeks and received injections of either AAV-GFAP-GLT-1a or AAV-GFAP-eGFP in the NAc following the last day of self-administration. Rats then underwent three weeks of extinction training (during which time transduction and expression occurred) before undergoing a cue- or cocaine-primed reinstatement test. Microdialysis for the quantification of glutamate efflux in the NAc was conducted during the cocaine-primed test. Rats that received AAV-GFAP-GLT-1a reinstated cue-primed cocaine-seeking in a similar manner as rats that received the control AAV-GFAP-eGFP. Upregulation of GLT-1a attenuated glutamate efflux during a cocaine-primed reinstatement test, but was not sufficient to attenuate reinstatement. We confirmed that GLT-1a upregulation resulted in functional upregulation of glutamate transport and expression, without affecting sodium-independent glutamate uptake, indicating system xc-was not altered. These results indicate that upregulation of NAc GLT-1 transporters alone is not sufficient to prevent the reinstatement of cocaine-seeking and implicate additional mechanisms in regulating glutamate efflux.