Topographic Organization of Cholinergic Innervation From the Basal Forebrain to the Visual Cortex in the Rat.

Acetylcholine is an important neurotransmitter for the regulation of visual attention, plasticity, and perceptual learning. It is released in the visual cortex predominantly by cholinergic projections from the basal forebrain, where stimulation may produce potentiation of visual processes. However, little is known about the fine organization of these corticopetal projections, such as whether basal forebrain neurons projecting to the primary and secondary visual cortical areas (V1 and V2, respectively) are organized retinotopically. The aim of this study was to map these basal forebrain-V1/V2 projections. Microinjections of the fluorescent retrograde tracer cholera toxin b fragment in different sites within V1 and V2 in Long-Evans rats were performed. Retrogradely labeled cell bodies in the horizontal and vertical limbs of the diagonal band of Broca (HDB and VDB, respectively), nucleus basalis magnocellularis, and substantia innominata (SI), were mapped ex vivo with a computer-assisted microscope stage controlled by stereological software. Choline acetyltranferase immunohistochemistry was used to identify cholinergic cells. Our results showed a predominance of cholinergic projections coming from the HDB. These projections were not retinotopically organized but projections to V1 arised from neurons located in the anterior HDB/SI whereas projections to V2 arised from neurons located throughout the whole extent of HDB/SI. The absence of a clear topography of these projections suggests that BF activation can stimulate visual cortices broadly.

Expression, distribution and function of kinin B1 receptor in the rat diabetic retina.

BACKGROUND AND PURPOSE: The kinin B1 receptor contributes to vascular inflammation and blood-retinal barrier breakdown in diabetic retinopathy (DR). We investigated the changes in expression, cellular localization and vascular inflammatory effect of B1 receptors in retina of streptozotocin diabetic rats. EXPERIMENTAL APPROACH: The distribution of B1 receptors on retinal cell types was investigated by immunocytochemistry. Effects of B1 receptor agonist, R-838, and antagonist, R-954, on retinal leukocyte adhesion, gene expression of kinin and VEGF systems, B1 receptor immunoreactivity, microgliosis and capillary leakage were measured. Effect of B1 receptor siRNA on gene expression was also assessed. KEY RESULTS: mRNA levels of the kinin and VEGF systems were significantly enhanced at 2 weeks in streptozotocin (STZ)-retina compared to control-retina and were further increased at 6 weeks. B1 receptor mRNA levels remained increased at 6 months. B1 receptor immunolabelling was detected in vascular layers of the retina, on glial and ganglion cells. Intravitreal R-838 amplified B1 and B2 receptor gene expression, B1 receptor levels (immunodetection), leukostasis and vascular permeability at 2 weeks in STZ-retina. Topical application (eye drops) of R-954 reversed these increases in B1 receptors, leukostasis and vascular permeability. Intravitreal B1 receptor siRNA inhibited gene expression of kinin and VEGF systems in STZ-retina. Microgliosis was unaffected by R-838 or R-954 in STZ-retina. CONCLUSION AND IMPLICATIONS: Our results support the detrimental role of B1 receptors on endothelial and glial cells in acute and advanced phases of DR. Topical application of the B1 receptor antagonist R-954 seems a feasible therapeutic approach for the treatment of DR.

Cholinergic Potentiation of Restoration of Visual Function after Optic Nerve Damage in Rats.

Enhancing cortical plasticity and brain connectivity may improve residual vision following a visual impairment. Since acetylcholine plays an important role in attention and neuronal plasticity, we explored whether potentiation of the cholinergic transmission has an effect on the visual function restoration. To this end, we evaluated for 4 weeks the effect of the acetylcholinesterase inhibitor donepezil on brightness discrimination, visually evoked potentials, and visual cortex reactivity after a bilateral and partial optic nerve crush in adult rats. Donepezil administration enhanced brightness discrimination capacity after optic nerve crush compared to nontreated animals. The visually evoked activation of the primary visual cortex was not restored, as measured by evoked potentials, but the cortical neuronal activity measured by thallium autometallography was not significantly affected four weeks after the optic nerve crush. Altogether, the results suggest a role of the cholinergic system in postlesion cortical plasticity. This finding agrees with the view that restoration of visual function may involve mechanisms beyond the area of primary damage and opens a new perspective for improving visual rehabilitation in humans.

Cholinergic Potentiation Improves Perceptual-Cognitive Training of Healthy Young Adults in Three Dimensional Multiple Object Tracking.

A large body of literature supports cognitive enhancement as an effect of cholinergic potentiation. However, it remains elusive whether pharmacological manipulations of cholinergic neurotransmission enhance complex visual processing in healthy individuals. To test this hypothesis, we randomly administered either the cholinergic transmission enhancer donepezil (DPZ; 5 mg P.O.) or placebo (lactose) to young adults (n = 17) 3 h before each session of the three-dimensional (3D) multiple object tracking (3D-MOT) task. This multi-focal attention task evaluates perceptual-cognitive learning over five sessions conducted 7 days apart. A significant amount of learning was observed in the DPZ group but not the placebo group in the fourth session. In the fifth session, this learning effect was observed in both groups. Furthermore, preliminary results for a subgroup of participants (n = 9) 4-14 months later suggested the cholinergic enhancement effect was long lasting. On the other hand, DPZ had no effect on basic visual processing as measured by a motion and orientation discrimination task performed as an independent one-time, pre-post drug study without placebo control (n = 10). The results support the construct that cholinergic enhancement facilitates the encoding of a highly demanding perceptual-cognitive task although there were no significant drug effects on the performance levels compared to placebo.

Pharmacological Mechanisms of Cortical Enhancement Induced by the Repetitive Pairing of Visual/Cholinergic Stimulation.

Repetitive visual training paired with electrical activation of cholinergic projections to the primary visual cortex (V1) induces long-term enhancement of cortical processing in response to the visual training stimulus. To better determine the receptor subtypes mediating this effect the selective pharmacological blockade of V1 nicotinic (nAChR), M1 and M2 muscarinic (mAChR) or GABAergic A (GABAAR) receptors was performed during the training session and visual evoked potentials (VEPs) were recorded before and after training. The training session consisted of the exposure of awake, adult rats to an orientation-specific 0.12 CPD grating paired with an electrical stimulation of the basal forebrain for a duration of 1 week for 10 minutes per day. Pharmacological agents were infused intracortically during this period. The post-training VEP amplitude was significantly increased compared to the pre-training values for the trained spatial frequency and to adjacent spatial frequencies up to 0.3 CPD, suggesting a long-term increase of V1 sensitivity. This increase was totally blocked by the nAChR antagonist as well as by an M2 mAChR subtype and GABAAR antagonist. Moreover, administration of the M2 mAChR antagonist also significantly decreased the amplitude of the control VEPs, suggesting a suppressive effect on cortical responsiveness. However, the M1 mAChR antagonist blocked the increase of the VEP amplitude only for the high spatial frequency (0.3 CPD), suggesting that M1 role was limited to the spread of the enhancement effect to a higher spatial frequency. More generally, all the drugs used did block the VEP increase at 0.3 CPD. Further, use of each of the aforementioned receptor antagonists blocked training-induced changes in gamma and beta band oscillations. These findings demonstrate that visual training coupled with cholinergic stimulation improved perceptual sensitivity by enhancing cortical responsiveness in V1. This enhancement is mainly mediated by nAChRs, M2 mAChRs and GABAARs. The M1 mAChR subtype appears to be involved in spreading the enhancement of V1 cortical responsiveness to adjacent neurons.

Distribution and effects of the muscarinic receptor subtypes in the primary visual cortex.

Muscarinic cholinergic receptors modulate the activity and plasticity of the visual cortex. Muscarinic receptors are divided into five subtypes that are not homogeneously distributed throughout the cortical layers and cells types. This distribution results in complex action of the muscarinic receptors in the integration of visual stimuli. Selective activation of the different subtypes can either strengthen or weaken cortical connectivity (e.g., thalamocortical vs. corticocortical), i.e., it can influence the processing of certain stimuli over others. Moreover, muscarinic receptors differentially modulate some functional properties of neurons during experience-dependent activity and cognitive processes and they contribute to the fine-tuning of visual processing. These functions are involved in the mechanisms of attention, maturation and learning in the visual cortex. This minireview describes the anatomo-functional aspects of muscarinic modulation of the primary visual cortex's (V1) microcircuitry.

Activation of the mouse primary visual cortex by medial prefrontal subregion stimulation is not mediated by cholinergic basalo-cortical projections.

The medial prefrontal cortex (mPFC) exerts top-down control of primary visual cortex (V1) activity. As there is no direct neuronal projection from mPFC to V1, this functional connection may use an indirect route, i.e., via basalo-cortical cholinergic projections. The cholinergic projections to V1 originate from neurons in the horizontal limb of the diagonal band of Broca (HDB), which receive neuronal projections from the ventral part of the mPFC, composed of prelimbic (PrL) and infralimbic cortices (IL). Therefore, the objective of this study was to determine whether electrical stimulation of mice mPFC subregions activate (1) V1 neurons; and (2) HDB cholinergic neurons, suggesting that the HDB serves as a relay point in the mPFC-V1 interaction. Neuronal activation was quantified using c-Fos immunocytochemistry or thallium autometallography for each V1 layer using automated particle analysis tools and optical density measurement. Stimulation of IL and PrL induced significantly higher c-Fos expression or thallium labeling in layers II/III and V of V1 in the stimulated hemisphere only. A HDB cholinergic neuron-specific lesion by saporin administration reduced IL-induced c-Fos expression in layers II/III of V1 but not in layer V. However, there was no c-Fos expression or thallium labeling in the HDB neurons, suggesting that this area was not activated by IL stimulation. Stimulation of another mPFC subarea, the anterior cingulate cortex (AC), which is involved in attention and receives input from V1, activated neither V1 nor HDB. The present results indicate that IL and PrL, but not AC, stimulation activates V1 with the minor involvement of the HDB cholinergic projections. These results suggest a functional link between the ventral mPFC and V1, but this function is only marginally supported by HDB cholinergic neurons and may involve other brain regions.

Boosting visual cortex function and plasticity with acetylcholine to enhance visual perception.

The cholinergic system is a potent neuromodulatory system that plays critical roles in cortical plasticity, attention and learning. In this review, we propose that the cellular effects of acetylcholine (ACh) in the primary visual cortex during the processing of visual inputs might induce perceptual learning; i.e., long-term changes in visual perception. Specifically, the pairing of cholinergic activation with visual stimulation increases the signal-to-noise ratio, cue detection ability and long-term facilitation in the primary visual cortex. This cholinergic enhancement would increase the strength of thalamocortical afferents to facilitate the treatment of a novel stimulus while decreasing the cortico-cortical signaling to reduce recurrent or top-down modulation. This balance would be mediated by different cholinergic receptor subtypes that are located on both glutamatergic and GABAergic neurons of the different cortical layers. The mechanisms of cholinergic enhancement are closely linked to attentional processes, long-term potentiation (LTP) and modulation of the excitatory/inhibitory balance. Recently, it was found that boosting the cholinergic system during visual training robustly enhances sensory perception in a long-term manner. Our hypothesis is that repetitive pairing of cholinergic and sensory stimulation over a long period of time induces long-term changes in the processing of trained stimuli that might improve perceptual ability. Various non-invasive approaches to the activation of the cholinergic neurons have strong potential to improve visual perception.

Impaired functional organization in the visual cortex of muscarinic receptor knock-out mice.

Acetylcholine modulates maturation and neuronal activity through muscarinic and nicotinic receptors in the primary visual cortex. However, the specific contribution of different muscarinic receptor subtypes in these neuromodulatory mechanisms is not fully understood. The present study evaluates in vivo the functional organization and the properties of the visual cortex of different groups of muscarinic receptor knock-out (KO) mice. Optical imaging of intrinsic signals coupled to continuous and episodic visual stimulation paradigms was used. Retinotopic maps along elevation and azimuth were preserved among the different groups of mice. However, compared to their wild-type counterparts, the apparent visual field along elevation was larger in M2/M4-KO mice but smaller in M1-KO. There was a reduction in the estimated relative receptive field size of V1 neurons in M1/M3-KO and M1-KO mice. Spatial frequency and contrast selectivity of V1 neuronal populations were affected only in M1/M3-KO and M1-KO mice. Finally, the neuronal connectivity was altered by the absence of M2/M4 muscarinic receptors. All these effects suggest the distinct roles of different subtypes of muscarinic receptors in the intrinsic organization of V1 and a strong involvement of the muscarinic transmission in the detectability of visual stimuli.

Roles of cannabinoid receptors type 1 and 2 on the retinal function of adult mice.

PURPOSE: Endocannabinoids are important modulators of synaptic transmission and plasticity throughout the central nervous system. The cannabinoid receptor type 1 (CB1R) is extensively expressed in the adult retina of rodents, while CB2R mRNA and protein expression have been only recently demonstrated in retinal tissue. The activation of cannabinoid receptors modulates neurotransmitter release from photoreceptors and could also affect bipolar cell synaptic release. However, the impact of CB1R and CB2R on the retinal function as a whole is currently unknown. METHODS: In the present study, we investigated the function of cannabinoid receptors in the retina by recording electroretinographic responses (ERGs) from mice lacking either CB1 or CB2 receptors (cnr1(-/-) and cnr2(-/-), respectively). We also documented the precise distribution of CB2R by immunohistochemistry. RESULTS: Our results showed that CB2R is localized in cone and rod photoreceptors, horizontal cells, some amacrine cells, and bipolar and ganglion cells. In scotopic conditions, the amplitudes of the a-wave of the ERG were increased in cnr2(-/-) mice, while they remained unchanged in cnr1(-/-) mice. The analysis of the velocity-time profile of the a-wave revealed that the increased amplitude was due to a slower deceleration rather than an increase in acceleration of the waveform. Under photopic conditions, b-wave amplitudes of cnr2(-/-) mice required more light adaptation time to reach stable values. No effects were observed in cnr1(-/-) mice. CONCLUSIONS: The data indicated that CB2R is likely to be involved in shaping retinal responses to light and suggest that CB1 and CB2 receptors could have different roles in visual processing.

Cholinergic depletion in nucleus accumbens impairs mesocortical dopamine activation and cognitive function in rats.

In rats, selective depletion of the cholinergic interneurons in the ventral striatum (nucleus accumbens or N.Acc.) results in heightened behavioural sensitivity to amphetamine and impaired sensorimotor gating processes, suggesting a hyper-responsiveness to dopamine (DA) activity in the N.Acc. We hypothesized that local cholinergic depletion may also trigger distal functional alterations, particularly in prefrontal cortex (PFC). Adult male Sprague-Dawley rats were injected bilaterally in the N.Acc. with an immunotoxin targeting choline acetyltransferase. Two weeks later, cognitive function was assessed using the delayed alternation paradigm in the T-maze. The rats were then implanted with voltammetric recording electrodes in the ventromedial PFC to measure in vivo extracellular DA release in response to mild tail pinch stress. The PFC was also examined for density of tyrosine hydroxylase (TH)-labelled varicosities. In another cohort of control and lesioned rats, we measured post mortem tissue content of DA. Depletion of cholinergic neurons (restricted to N.Acc.) significantly impaired delayed alternation performance across delay intervals. While (basal) post mortem indices of PFC DA function were unaffected by N.Acc. lesions, in vivo mesocortical DA activation was markedly reduced; this deficit correlated significantly with cognitive impairments. TH-labelled varicosities however, were unaffected in cortical layer V relative to controls. These data suggest that selective depletion of cholinergic interneurons in N.Acc. triggers widespread functional impairments in mesocorticolimbic DA function and cognition. The possible relevance of these findings is also discussed in relation to schizophrenia, where reduced density of cholinergic neurons in ventral striatum has been reported.

D1-NMDA receptor interactions in the rat nucleus accumbens change during adolescence.

Many aspects of the dopamine (DA) system mature during adolescence. For example, the DA modulation of glutamate responses in the rat prefrontal cortex (PFC) acquires adult characteristics during late adolescence. In the striatum, D1 receptors modulate NMDA responses, but whether this behaviorally important interaction matures during adolescence is not known. Here, we tested whether the D1 agonist SKF38393 affects NMDA actions on nucleus accumbens medium spiny neuron (MSN) excitability in slices from juvenile and young adult rats. NMDA dose-dependently increased excitability in both age groups, and the D1 agonist produced a marginal increase of MSN excitability. In juvenile slices, the most common interaction was a downregulation of NMDA effects on excitability by the D1 agonist, whereas in most adult MSN, the D1 agonist increased NMDA effects on MSN excitability. These results suggest that D1-NMDA receptor interactions in the nucleus accumbens change during adolescence, a change that may result in different processing of reward functions during this critical developmental stage.

Periadolescent changes of D(2) -AMPA interactions in the rat nucleus accumbens.

Many aspects of dopamine (DA) systems mature during adolescence. In the nucleus accumbens, the modulation of prefrontal cortical synaptic responses by DA becomes refined during adolescence with the recruitment of a gamma-amino butyric acid (GABA) component. As this GABA component is depolarizing, it remains to be determined whether this change affects action potential firing in nucleus accumbens neurons. Here we tested whether a D(2) agonist affects AMPA-evoked cell firing in slices containing the nucleus accumbens from juvenile (postnatal day, PD 28-34) and adult (PD > 60) rats. 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) (0.1-0.4 µM) depolarized nucleus accumbens neurons and increased their firing in a dose-dependent manner. The D(2) agonist quinpirole (2 µM) had different effects in juvenile vs. adult slices. In the juvenile accumbens, quinpirole enhanced AMPA (0.2 µM) effects on evoked firing in a subset of neurons while it had no effect on the rest. In the adult accumbens, the D(2) agonist instead attenuated the effect of AMPA on evoked firing, an interaction that was blocked by the GABA-A antagonist picrotoxin (50 µM). Thus, D(2) receptors modulate AMPA responses in the nucleus accumbens differently in juvenile than adult rats, and the adult effect requires local GABA transmission. The incorporation of a GABA component in the modulation of information processing in the nucleus accumbens by DA during adolescence may allow for a better contrast in cortically activated ensembles.