Table 2.

A summary table of neurotransmitters where sex differences have been reported.

Neurotransmitter	Sex difference	Putative disease association
Serotonergic (5-HT) system	Females have higher whole blood 5-HT levels (Gur and Gur, 1990), higher 5-HT transporter availability in the diencephalon and brainstem (Rodriguez et al., 1988), and higher 5-HT1A receptor numbers than males (Baxter et al., 1987; Staley et al., 2006). Males synthesize serotonin significantly faster than females (Andreason et al., 1994).	A wide range of psychiatric disorders, including, depression, anxiety and antisocial personality disorder (Hatazawa et al., 1987; Reiman et al., 1996).
Dopaminergic (DA) system	Females show a higher presynaptic dopaminergic synthesis in the striatum than age-matched males (Robinson et al., 1977). DA transporter, which regulates synaptic DA availability, is higher expressed in the caudate nucleus in females (Curtis et al., 2006).	Anxiety, depression, schizophrenia, addiction, and attention deficit hyperactivity disorder (Kuehner, 2003). DA transporter expression has been positively correlated with learning performance (Carter-Snell and Hegadoren, 2003).
Cholinergic (ACh) system	Females show higher frontal cortex cholinergic activity, while males have higher activity in the hippocampus (Proebstl et al., 2019).	Nicotine addiction (Weiss et al., 2005).
Gamma-aminobutyric acid (GABA)-ergic	Females have higher cortical GABA levels than males (Parsey et al., 2002).	Seizures and movement disorders (Löscher et al., 1987, in rats).
Opoid system	Sex differences exist in opioid affinity, receptor density, signal transduction, and tolerance development (Moen and Lee, 2021; Seeman and Lang, 1990).	Analgesia (Seeman and Lang, 1990). Blood pressure (Cruz and Rodríguez-Manzo, 2000, in rats).