Prolonged methamphetamine exposure during a critical period in neonatal Sprague Dawley rats does not exacerbate egocentric and allocentric learning deficits but increases reference memory impairments

Children exposed to methamphetamine (MA) in utero have cognitive deficits. MA administration in rats for 5-10 days between postnatal days (P)6 and 20 produces cognitive deficits. The purpose of this study was to determine if extending MA administration by 5 days within P6-20 would exacerbate allocentric (Morris water maze) and egocentric (Cincinnati water maze) learning deficits. Sprague Dawley female and male offspring (split-litter design) were administered saline (SAL) or MA (10 mg/kg) four times daily from P6 to 20 to create four groups: (a) SAL from P6 to 20, (b) MA from P6 to 20 (MA6-20), (c) MA from P6 to 15 (MA6-15), or (d) MA from P11 to 20 (MA11-20); the latter groups received saline on days they did not receive MA. Egocentric, allocentric, and conditioned freezing tests began on P60. The MA6-15 and MA6-20 groups showed egocentric deficits, all MA groups had allocentric deficits but no differences in conditioned freezing compared with SAL controls. The MA6-15 and MA6-20 groups had similar deficits in learning and memory that were larger than in the MA11-20 group. Learning in both mazes was sex dependent, but no interactions with MA were found. The data demonstrate that extending the exposure period of MA beyond the sensitive periods (P6-15 and P11-20) did not exacerbate the cognitive deficits.

High-frequency stimulation of the hippocampus blocks fear learning sensitization and return of extinguished fear

Patients with post-traumatic stress disorder (PTSD) present hippocampal (HPC) dysfunction, which may facilitate fear-related phenomena such as fear learning sensitization (i.e. potentiation of fear acquisition by initial fear conditioning (FC1)) and fear return (i.e. reactivation of extinguished fear). Fear return is sensitive to HPC high-frequency stimulation (HFS) in rats. The goal of the present study was to examine whether fear learning sensitization is also sensitive to HPC HFS in rats. We found in control conditions that, after FC1 (with 15 shock administrations) and extinction, conditioning in a different context with one shock administration was potentiated (proactive effect) and associated with fear return in the initial context (retroactive effect). Both phenomena were prevented by HPC HFS applied before the second conditioning. We also found that the effect of HPC HFS on fear learning sensitization required initial extinction. These findings suggest a pivotal role of the HPC in preventing proactive and retroactive effects of successive fear conditionings. These data also support the concept that HPC deactivation may be involved in fear learning sensitization and fear return in PTSD patients.

Dysregulation of brain adenosine is detrimental to the expression of conditioned freezing but not general Pavlovian learning

Glutamatergic and dopaminergic neurotransmission is modulated by adenosine, whose ambient level in the brain is in turn regulated by the metabolic enzyme, adenosine kinase (ADK). Brain adenosinergic tone can therefore be effectively reduced and increased by up- and down-regulation of ADK expression, respectively. Although changes in brain ADK levels can yield multiple behavioral effects, the precise functional significance of telencephalon (neocortical and limbic structures) adenosine remains ill-defined. Among the phenotypes identified in transgenic mice with brain-wide ADK overexpression (ADK(TG) mice) and reduced adenosinergic tone, working memory deficiency and potentiated response to systemic N-methyl-d-aspartate receptor blockade were exacerbated by the introduction of local ADK disruption (elevated adenosinergic tone) restricted to the telencephalon (ADK(TG):ADK(Tel-def) mice). These two phenotypes, which are central to schizophrenia cognitive/negative symptoms, appear to be regulated by adenosinergic activities within and outside the telencephalon in a complementary manner. Here, we extended this unique comparison between ADK(TG) mice ADK(TG):ADK(Tel-def) mice to another prominent phenotype previously documented in ADK(TG) mice - namely, impaired Pavlovian conditioned freezing. We found that ADK(TG):ADK(Tel-def) mice again were associated with a more severe phenotype while sharing a similar phenotype profile. Furthermore, we qualified that this Pavlovian phenotype did not translate into a general deficiency in associative learning, since no such deficit was evident in three other (aversive and appetitive) Pavlovian learning paradigms. The present study has thus identified a hitherto unknown function of brain adenosine: the execution of conditioned freezing behavior, which is dependent on the balance of adenosinergic changes between the telencephalon and the rest of the brain.