Sexually Monomorphic Maps and Dimorphic Responses in Rat Genital Cortex

Abstract

Mammalian external genitals show sexual dimorphism [1, 2] and can change size and shape upon sexual arousal. Genitals feature prominently in the oldest pieces of figural art [3] and phallic depictions of penises informed psychoanalytic thought about sexuality [4, 5]. Despite this longstanding interest, the neural representations of genitals are still poorly understood [6]. In somatosensory cortex specifically, many studies did not detect any cortical representation of genitals [7-9]. Studies in humans debate whether genitals are represented displaced below the foot of the cortical body map [10-12] or whether they are represented somatotopically [13-15]. We wondered what a high-resolution mapping of genital representations might tell us about the sexual differentiation of the mammalian brain. We identified genital responses in rat somatosensory cortex in a region previously assigned as arm/leg cortex. Genital responses were more common in males than in females. Despite such response dimorphism, we observed a stunning anatomical monomorphism of cortical penis and clitoris input maps revealed by cytochrome-oxidase-staining of cortical layer 4. Genital representations were somatotopic and bilaterally symmetric, and their relative size increased markedly during puberty. Size, shape, and erect posture give the cortical penis representation a phallic appearance pointing to a role in sexually aroused states. Cortical genital neurons showed unusual multi-body-part responses and sexually dimorphic receptive fields. Specifically, genital neurons were co-activated by distant body regions, which are touched during mounting in the respective sex. Genital maps indicate a deep homology of penis and clitoris representations in line with a fundamentally bi-sexual layout [16] of the vertebrate brain.

Sexually selected size differences and conserved sexual monomorphism of genital cortex

“…Growth of genital cortex during puberty has been reported in the rat (Lenschow et al, ). The area of penis and clitoris representation as defined by the layer 4 cytochrome ‐ oxidase activity showed morphological growth when comparing prepubertal to adult animals.…”

“…Lenschow et al () reported a cortical monomorphism of penis and clitoris representation in the rat. This result is surprising, given the marked sexual dimorphism of rat external genitals.…”

“…In accordance with the general somatotopy, genital cortex is anatomically attached to the trunk representation and separated from fore and hindpaw representations by non‐granular septa. With respect to the topographic body representation, it shows an erect posture (Lenschow et al, ). Taking these criteria into account, we were able to anatomically identify the genital representation in eight further mammals: mouse (Figure b), gerbil (Figure c), hamster (Figure d), guinea pig, chipmunk, degu (Figure e), chinchilla (Figure f), rabbit (top, cytochrome‐oxidase staining; bottom, reconstructed body map of the regions of interest).…”

“…Contrary to the external dimorphism of male and female genitals, the cortical representation was monomorphic. Furthermore, Lenschow et al () observed a high degree of plasticity of the genital cortex during puberty, showing greatly increased areal growth compared to the rest of somatosensory cortex.…”

Neuromodulation of maternal circuits by oxytocin

“…Indeed, in postpartum rodents, the frequency and duration of tactile stimulation from suckling pups is dramatically increased as infants spend around 50% of their awake time nursing (Champagne et al, 2007; Grota and Ader, 1974). Stronger afferent inputs from the nipples, resulting from the continuous nursing, may induce activity-dependent plasticity in S1 neurons similarly to the expansion of the female genital cortex during puberty (Lenschow et al, 2017,2016). …”