The Cousa objective: a long-working distance air objective for multiphoton imaging in vivo.

Multiphoton microscopy can resolve fluorescent structures and dynamics deep in scattering tissue and has transformed neural imaging, but applying this technique in vivo can be limited by the mechanical and optical constraints of conventional objectives. Short working distance objectives can collide with compact surgical windows or other instrumentation and preclude imaging. Here we present an ultra-long working distance (20 mm) air objective called the Cousa objective. It is optimized for performance across multiphoton imaging wavelengths, offers a more than 4 mm2 field of view with submicrometer lateral resolution and is compatible with commonly used multiphoton imaging systems. A novel mechanical design, wider than typical microscope objectives, enabled this combination of specifications. We share the full optical prescription, and report performance including in vivo two-photon and three-photon imaging in an array of species and preparations, including nonhuman primates. The Cousa objective can enable a range of experiments in neuroscience and beyond.

Neurons in the most superficial lamina of the mouse superior colliculus are highly selective for stimulus direction.

The superior colliculus (SC) is a layered midbrain structure important for multimodal integration and sensorimotor transformation. Its superficial layers are purely visual and receive depth-specific projections from distinct subtypes of retinal ganglion cells. Here we use two-photon calcium imaging to characterize the response properties of neurons in the most superficial lamina of the mouse SC, an undersampled population with electrophysiology. We find that these neurons have compact receptive fields with primarily overlapping ON and OFF subregions and are highly direction selective. The high selectivity is observed in both excitatory and inhibitory neurons. These neurons do not cluster according to their direction preference and lack orientation selectivity. In addition, we perform single-unit recordings and show that direction selectivity declines with depth in the SC. Together, our experiments reveal for the first time a highly specialized lamina in the most superficial SC for movement direction, a finding that has important implications for understanding signal transformation in the early visual system.

Bidirectional encoding of motion contrast in the mouse superior colliculus.

Detection of salient objects in the visual scene is a vital aspect of an animal's interactions with its environment. Here, we show that neurons in the mouse superior colliculus (SC) encode visual saliency by detecting motion contrast between stimulus center and surround. Excitatory neurons in the most superficial lamina of the SC are contextually modulated, monotonically increasing their response from suppression by the same-direction surround to maximal potentiation by an oppositely-moving surround. The degree of this potentiation declines with depth in the SC. Inhibitory neurons are suppressed by any surround at all depths. These response modulations in both neuronal populations are much more prominent to direction contrast than to phase, temporal frequency, or static orientation contrast, suggesting feature-specific saliency encoding in the mouse SC. Together, our findings provide evidence supporting locally generated feature representations in the SC, and lay the foundations towards a mechanistic and evolutionary understanding of their emergence.

Visual Function, Organization, and Development of the Mouse Superior Colliculus.

The superior colliculus (SC) is the most prominent visual center in mice. Studies over the past decade have greatly advanced our understanding of the function, organization, and development of the mouse SC, which has rapidly become a popular model in vision research. These studies have described the diverse and cell-type-specific visual response properties in the mouse SC, revealed their laminar and topographic organizations, and linked the mouse SC and downstream pathways with visually guided behaviors. Here, we summarize these findings, compare them with the rich literature of SC studies in other species, and highlight important gaps and exciting future directions. Given its clear importance in mouse vision and the available modern neuroscience tools, the mouse SC holds great promise for understanding the cellular, circuit, and developmental mechanisms that underlie visual processing, sensorimotor transformation, and, ultimately, behavior.

Retinal origin of direction selectivity in the superior colliculus.

Detecting visual features in the environment, such as motion direction, is crucial for survival. The circuit mechanisms that give rise to direction selectivity in a major visual center, the superior colliculus (SC), are entirely unknown. We optogenetically isolate the retinal inputs that individual direction-selective SC neurons receive and find that they are already selective as a result of precisely converging inputs from similarly tuned retinal ganglion cells. The direction-selective retinal input is linearly amplified by intracollicular circuits without changing its preferred direction or level of selectivity. Finally, using two-photon calcium imaging, we show that SC direction selectivity is dramatically reduced in transgenic mice that have decreased retinal selectivity. Together, our studies demonstrate a retinal origin of direction selectivity in the SC and reveal a central visual deficit as a consequence of altered feature selectivity in the retina.

Brainstem injection of lidocaine releases the descending pain-inhibitory mechanisms in a rat model of mononeuropathy.

Lidocaine injections in the rostral ventromedial medulla (RVM) have been shown to produce significant reduction of neuropathic manifestations in rats. This effect has been attributed to selective block of a pain descending facilitatory system, responsible for chronic pain. However, recent observations from our laboratory did not provide confirmation to this hypothesis. We aimed, therefore, to investigate the spinal synaptic mechanisms activated by lidocaine injections in the RVM. Rats were subjected, under deep anesthesia, to the induction of mononeuropathy on one hindpaw, and to the stereotaxic implantation of chronic cannulae in the RVM for the injection of lidocaine or GABA antagonists. Implanted intrathecal catheter in the lumbosacral space was used for the injection of specific antagonists to GABA, 5HT, glycine, noreadrenaline and dopamine, prior to lidocaine. Tactile and cold hyperreactivity and heat hyperalgesia were assessed using von Frey hair filaments, acetone drop test and heat-induced paw withdrawal, respectively. Lidocaine injections produced significant inhibition of all neuropathic manifestations. Intrathecal injection of antagonists to GABA (bicucullin, picrotoxin and saclofen), serotonin 5HT(1-2) (ketanserin and methysergide) and α- (phentoalmine, yohimbine) and ß- (propranolol) adrenergic receptors, suppressed the lidocaine inhibitory effects; while partial or no attenuation were observed following pretreatment with glycine and dopamine D(2/3) antagonists. Comparable effects were observed with RVM injection of GABA antagonists. Lidocaine injection in the RVM results in a release of the descending pain-inhibitory systems from a tonic gabaergic inhibition. This descending system involves the activation of gabaergic, serotonergic and adrenergic mechanisms at the level of the spinal dorsal horn.