Prefrontal cortical GABA modulation of spatial reference and working memory.

BACKGROUND: Dysfunction in prefrontal cortex (PFC) GABA transmission has been proposed to contribute to cognitive dysfunction in schizophrenia, yet how this system regulates different cognitive and mnemonic functions remains unclear. METHODS: We assessed the effects of pharmacological reduction of GABAA signaling in the medial PFC of rats on spatial reference/working memory using different versions of the radial-arm maze task. We used a massed-trials procedure to probe how PFC GABA regulates susceptibility to proactive interference. Male rats were well-trained to retrieve food from the same 4 arms of an 8-arm maze, receiving 5 trials/day (1-2 min intervals). RESULTS: Infusions of the GABAA receptor antagonist bicuculline (12.5-50 ng) markedly increased working and reference memory errors and response latencies. Similar treatments also impaired short-term memory on an 8-baited arm task. These effects did not appear to be due to increased susceptibility to proactive interference. In contrast, PFC inactivation via infusion of GABA agonists baclofen/muscimol did not affect reference/working memory. In comparison to the pronounced effects on the 8-arm maze tasks, PFC GABAA antagonism only causes a slight and transient decrease in accuracy on a 2-arm spatial discrimination. CONCLUSIONS: These findings demonstrate that prefrontal GABA hypofunction severely disrupts spatial reference and short-term memory and that disinhibition of the PFC can, in some instances, perturb memory processes not normally dependent on the frontal lobes. Moreover, these impairments closely resemble those observed in schizophrenic patients, suggesting that perturbation in PFC GABA signaling may contribute to these types of cognitive deficits associated with the disorder.

unc5c haploinsufficient phenotype: striking similarities with the dcc haploinsufficiency model.

DCC and UNC5 homologs (UNC5H) are guidance cue receptors highly expressed by mesocorticolimbic dopamine neurons. We have shown that dcc heterozygous mice exhibit increased dopamine, but not norepinephrine, innervation and function in medial prefrontal cortex. Concomitantly, dcc heterozygotes show blunted mesolimbic dopamine release and behavioral responses to stimulant drugs. These changes appear only in adulthood. Recently, we found an adolescent emergence of UNC5H expression by dopamine neurons and co-expression of DCC and UNC5H by single dopamine cells. Here, we demonstrate selective expression of unc5 homolog c mRNA by dopamine neurons in adulthood. We show that unc5c haploinsufficiency results in diminished amphetamine-induced locomotion in male and female mice. This phenotype is identical to that produced by dcc haploinsufficiency and is observed after adolescence. Notably, and similar to dcc haploinsufficiency, unc5c haploinsufficiency leads to dramatic increases in tyrosine hydroxylase expression in the medial prefrontal cortex, but not in the nucleus accumbens. In contrast, medial prefrontal cortex dopamine-ß-hydroxylase expression is not altered. We confirmed that UNC5C protein is reduced in the ventral tegmental area of unc5c heterozygous mice, but that DCC expression in this region remains unchanged. UNC5C receptors may also play a role in dopamine function and influence sensitivity to behavioral effects of stimulant drugs of abuse, at least upon first exposure. The striking similarities between the dcc and the unc5c haploinsufficient phenotypes raise the possibility that functions mediated by DCC/UNC5C complexes may be at play.

Toll-like receptor-4 genotype influences the survival of cystic fibrosis mice.

Toll-like receptor (Tlr) 4 is a lipopolysaccharide (LPS) receptor that contributes to the regulation of intestinal cell homeostasis, a condition that is altered in the intestines of cystic fibrosis mice. Herein, we assessed whether Tlr4 genotype influences cystic fibrosis intestinal disease by producing and phenotyping 12-wk (adult)- and 4-day (neonate)-old mice derived from BALB cystic fibrosis transmembrane conductance regulator, Cftr(+/tm1Unc) and C.C3-Tlr4(Lps-d)/J (Tlr4(-/-)), progenitors. Intestinal disease was assayed through mouse survival, crypt-villus axis (CVA) length, cell proliferation, bacterial load, bacterial classification, inflammatory cell infiltrate, and mucus content measures. Of the 77 Cftr(-/-) (CF) mice produced, only one Cftr/Tlr4 double-mutant mouse lived to the age of 12 wk while the majority of the remainder succumbed at approximately 4 days of age. The survival of CF Tlr4(+/-) mice exceeded that of both CF Tlr4(+/+) and Cftr/Tlr4 double-mutant mice. Adult CF mice presented increased Tlr4 expression, CVA length, crypt cell proliferation, and bacterial load relative to non-CF mice, but no differences were detected in Tlr4(+/-) compared with Tlr4(+/+) CF mice. The double-mutant neonates did not differ from Tlr4(+/+) or Tlr4(+/-) CF mice by intestinal CVA length or bacterial load, but fewer Tlr4(+/-) CF neonates presented with luminal mucus obstruction in the distal ileum, and the intestinal mast cell increase of CF mice was not evident in double-mutant neonates. We conclude that Tlr4 deficiency reduces the survival, but does not alter the intestinal phenotypes, of extended CVA or increased bacterial load in BALB CF mice.

Amelioration of cognitive impairments induced by GABA hypofunction in the male rat prefrontal cortex by direct and indirect dopamine D1 agonists SKF-81297 and d-Govadine.

Deficits in prefrontal cortex (PFC) GABAergic neurotransmission are linked to cognitive impairments seen in schizophrenia and other disorders, and pharmacological reduction of PFC GABAA transmission disrupts processes including working and spatial memory. This provides an opportunity to examine whether compounds capable of neutralizing GABAergic dysfunction may ameliorate these cognitive deficits. PFC dopamine (DA) D1 receptor activation enhances GABA transmission, raising the possibly that direct or indirect agonists of DA D1 receptors would be effective in reversing working memory and other forms of cognitive deficits. To test this, male rats were pre-treated with two drugs that augment PFC D1 signalling before PFC infusion of the GABAA antagonist, bicuculline (50 ng) and assessment of spatial working and reference memory function. A moderate dose of the full D1 agonist SKF-81297 (0.1 mg/kg) completely reversed PFC GABA hypofunction-induced working memory deficits assessed in an delayed-response task, whereas lower and higher doses (0.05 and 0.3 mg/kg respectively) were associated with mild improvements or deleterious effects. Treatment with the tetrahydroprotoberberine d-govadine (0.5 or 1.0 mg/kg), a synthetic compound known to enhance DA release selectively in the PFC, also significantly improved delayed-response working memory function induced by PFC GABAA antagonism. Furthermore, administration of the optimal dose of both drugs led to a partial rescue of PFC GABA hypofunction-induced reference and short-term spatial memory impairments assessed on a radial maze task. These findings suggest that modulation of PFC DA signalling via actions on the DA D1 receptor represents a promising therapeutic strategy for working memory and other cognitive impairments observed in psychiatric disorders, including those with causes that extend beyond DA dysfunction.

Disinhibition of the prefrontal cortex leads to brain-wide increases in neuronal activation that are modified by spatial learning.

Deficient prefrontal cortex (PFC) GABA function is hypothesized to play a role in schizophrenia and other psychiatric disorders. In rodents, PFC GABAA receptor antagonism produces cognitive and behavioral changes relevant to these disorders, including impaired spatial memory assessed with the traditional working/reference memory radial maze task. This aspect of spatial memory does not depend on PFC, suggesting that deficient PFC GABAergic transmission may interfere with non-PFC-dependent cognitive functions via aberrant increases in PFC output. To test this, we assessed whether PFC GABAA antagonism (50 ng bicuculline methbromide) alters neuronal activation in PFC terminal regions, including the striatum, thalamus, hippocampus, amygdala, and cortical regions, of adult male rats using the immediate early gene, c-Fos, as an activity marker. A subset of these animals were also trained and/or tested on the working/reference memory radial maze task. These treatments caused widespread increases in neuronal activation in animals under baseline conditions, with notable exception of the hippocampus. Furthermore, PFC GABAA antagonism impaired task performance. In most instances, training and/or testing on the radial maze had no additional effects on neuronal activation. However, in both the hippocampus and rhomboid thalamic nucleus, PFC GABAA antagonism caused a selective increase in neuronal activation in animals trained on the maze. These results indicate that deficiencies in PFC GABAergic transmission may have widespread impacts on neuronal activity that may interfere with certain PFC-independent cognitive functions. Furthermore, these alterations in activity are modulated by plasticity induced by spatial learning in the hippocampus and rhomboid thalamic nucleus.

Prefrontal cortical GABAergic and NMDA glutamatergic regulation of delayed responding.

NMDA glutamatergic and GABAergic transmission have both been implicated in regulating working memory functions mediated by the prefrontal cortex (PFC), and perturbations in these neurotransmitter systems have been proposed to underlie deficits in these functions observed in schizophrenia. Here, we examined the consequence of disrupting GABAergic or NMDA glutamatergic transmission within the medial PFC of rats on a delayed-response paradigm with translational relevance to working memory tasks used with humans. The operant delayed non-match to position task consisted of a sample phase (one lever extended) and a choice phase wherein rats were required to choose the opposite lever, separated by a variable delay (1-24 s). In well-trained rats, inactivation of the PFC via infusions of GABA agonists baclofen/muscimol (100 ng each) induced delay-independent deficits. Reducing PFC GABA transmission with the GABA-A receptor antagonist bicuculline (12.5-50 ng) also caused delay-independent impairments and increased trial omissions and response latencies during the sample and end-of-delay phases. On the other hand, non-selective blockade of PFC NMDA receptors with MK-801 (3-6 µg) disrupted performance, but these effects more closely resembled delay-dependent impairments. However, selective blockade of GluN2B-containing NMDA receptors with Ro-25-6981 (2.5 µg) did not affect any measures of performance. These results demonstrate that both intact PFC GABA and NMDA receptor signalling are integral for accurate delayed-responding, although they may differentially regulate encoding vs maintenance of information within working memory. Furthermore they suggest that perturbations of both of these neurochemical signals within the PFC may contribute differentially to impairments in working memory observed in schizophrenia.

Regulation of sustained attention, false alarm responding and implementation of conditional rules by prefrontal GABAA transmission: comparison with NMDA transmission.

RATIONALE: Both prefrontal cortex (PFC) GABAA and NMDA transmission regulate attentional processes, yet how they may differentially regulate signal detection or other aspects of attention is unclear. OBJECTIVES: We examined PFC GABAA and NMDA receptor regulation of attention using a sustained attention task (SAT) permitting identification of distinct forms of impairments. As this task requires implementation of conditional rules, we also investigated how reducing PFC GABA transmission affected performance of visual and auditory conditional discriminations. METHODS: Male rats were well-trained on the SAT that required identifying whether a brief visual stimulus (500-50 ms) was present/absent by pressing one of two levers. They then received intra-PFC infusions of the GABAA antagonist bicuculline (12.5-50 ng), the NMDA antagonist MK-801 (6 µg), and i.p. injections of MK-801 (0.1-0.3 mg/kg) prior to testing. Separate groups were trained either on a similar task where the visual stimulus was presented for 2.5 s, or a task where presentation of one of two auditory cues required responding on a left or right lever. RESULTS: Both doses of bicuculline impaired vigilance, selectively increasing errors during nonsignal trials. Intra-PFC MK-801 induced subtle impairments at short signal durations. Systemic MK-801 impaired performance and increased response latencies. Visual and auditory conditional discrimination was impaired by 50 ng, but not 12.5 ng of bicuculline. CONCLUSIONS: These findings highlight a key role for PFC GABA transmission in reducing sensitivity to distractors during attentional performance. Furthermore, they reveal that disruption of GABA signaling can interfere with the ability to implement conditional rules.