Anatomical and functional study of the cuneiform nucleus: A critical site to organize innate defensive behaviors

Abstract

The cuneiform nucleus (CUN) is a midbrain structure located lateral to the caudal part of the periaqueductal gray. In the present investigation, we first performed a systematic analysis of the afferent and efferent projections of the CUN using FluoroGold and Phaseolus vulgaris leucoagglutinin as retrograde and anterograde neuronal tracers, respectively. Next, we examined the behavioral responses to optogenetic activation of the CUN and evaluated the impact of pharmacological inactivation of the CUN in both innate and contextual fear responses to a predatory threat (i.e., a live cat). The present hodologic evidence indicates that the CUN might be viewed as a caudal component of the periaqueductal gray. The CUN has strong bidirectional links with the dorsolateral periaqueductal gray (PAGdl). Our hodological findings revealed that the CUN and PAGdl share a similar source of inputs involved in integrating information related to life-threatening events and that the CUN provides particularly strong projections to brain sites influencing antipredatory defensive behaviors. Our functional studies revealed that the CUN mediates innate freezing and flight antipredatory responses but does not seem to influence the acquisition and expression of learned fear responses.

Role of the rostral dorsomedial column of the periaqueductal gray during social defeat in rats

“…Our FG retrograde tract-tracing showed that the PAGdm receives inputs from prelimbic and anterior cingulate cortical areas, rostral zona incerta, a wealth of hypothalamic sites (such as the anterior part of the anterior hypothalamic nucleus, the juxtaparaventricular and juxtadorsomedial regions of the lateral hypothalamic area, ventromedial hypothalamic nucleus, and dorsal premammillary nucleus), the magnocellular part of the SPFm, PRC, and selective midbrain sites (such as SOM, caudal ventrolateral PAG, CUN, and latero-dorsal tegmental nucleus). Previous anterograde tract-tracing studies confirm most of the input sources to the PAGdm described here (prelimbic and anterior cortical areas, 13 anterior hypothalamic nucleus, 14 hypothalamic nucleus, 16 dorsal premammillary nucleus, 12 PRC, 17 caudal ventrolateral PAG, 18 CUN, 19 and latero-dorsal tegmental nucleus 20 ). Importantly, in animals exposed to an aggressive conspecific, we found that more than 30% of FOS-positive retrogradely labeled cells in several of the afferent sources of inputs, including the prelimbic and anterior cingulate cortical areas, rostral zona incerta, the anterior part of the anterior hypothalamic nucleus, the juxtaparaventricular and juxtadorsomedial regions of the lateral hypothalamic area, the ventrolateral part of the ventromedial hypothalamic nucleus, dorsal premammillary nucleus, magnocellular part of the SPFm, PRC, SOM, caudal ventrolateral PAG, CUN, and latero-dorsal tegmental nucleus.…”

“…Of particular relevance, however, is the dense projection of the PAGdm to several midbrain sites, including the caudal regions of dorsomedial, lateral, and ventrolateral parts of the PAG and the CUN. Previous retrograde findings also confirm the strong projection from the PAGdm to the CUN 19 . Conversely, only occasional anterograde labeling was found in the pontine and medular reticular core.…”

“…However, one of the main PAGdm targets is the CUN, which presented a decrease in activation in PAGdm‐lesioned animals exposed to social threat. As previously shown, the CUN provides extensive projections to hypothalamic circuits organizing defensive responses, such as AHNa, LHAjp, LHAjd, and LHA sf, which may ultimately influence the PMD 19 . In line with this view, optogenetic activation of the CUN yielded clear defensive responses, such as freezing and flight responses, 19 accompanied by a distinct FOS upregulation in the PMD.…”

“…Previous retrograde findings also confirm the strong projection from the PAGdm to the CUN. 19 Conversely, only occasional anterograde labeling was found in the pontine and medular reticular core. Therefore, PAGdm projection patterns are somewhat restricted, and most of the projections are limited to the PRC, other regions in the PAG, and the CUN.…”

“…21,22 In addition, the ventrolateral part of the ventromedial nucleus is part of the conspecific hypothalamic circuit responsive to aggressor exposure. 5,23 In the context of social defeat, it is noteworthy that the latero-dorsal tegmental nucleus, a cholinergic brain site, controls attentive states, and other midbrain sources of inputs to PAGdm are known to mediate either freezing (such as the PAGvl and CUN) 19,25 or escape (such as the SOM) 26 responses. Conversely, the magnocellular part of the SPFm is a likely candidate for mediating painful stimuli during the social agonistic interaction.…”