Comment: The findings in male rats: Ejaculation initiated structural changes in the dopamine-producing nerve cells of the reward circuit – specifically the VTA. The nerve cell bodies in the VTA decreased in size, and stayed that way for at least 7 days (measurements were taken at 1, 7, and 30 days). The VTA supplies most of the dopamine for other reward circuit structures. In addition, ejaculation led to the rats being less responsive to injected morphine, indicating a (temporarily) numbed pleasure response. Keep in mind that dopamine is behind craving and wanting, whereas the actual pleasure of eating and sex arises from opioids released in the reward circuit.

J Neurosci.

2014 Jun 25;34(26):8825-36. doi: 10.1523/JNEUROSCI.0133-14.2014.


Natural reward and drugs of abuse converge on the mesolimbic pathway and activate common mechanism of neural plasticity in the nucleus accumbens. Chronic exposure to opiates induces plasticity in dopaminergic neurons of the ventral tegmental area (VTA), which regulates morphine reward tolerance.

Here, we test the hypotheses that mating-induced release of endogenous opioids in the VTA causes morphological changes of VTA dopamine cells in male rats, which in-turn regulate the long-term expression of experience-induced reinforcement of sexual behavior.

First, sexual experience decreased VTA dopamine soma size 1 and 7 days, but not 30 days after the last mating session. This effect was blocked with naloxone before each mating session; thus, VTA dopamine cell plasticity was dependent on action of endogenous opioids.

In turn, VTA plasticity was associated with altered opiate reward, as sexually experienced males did not form conditioned place preference for 0.5 mg/kg morphine.

Next, it was determined whether endogenous opioid action mediates sexual reward and memory in male rats treated with naloxone during mating experience, either systemically or intra-VTA. Naloxone did not prevent the initial experience-induced facilitation of sexual behavior over repeated mating sessions, or conditioned place preference for mating. However, naloxone treatment attenuated the longer-term expression of experience-induced facilitation of sexual behavior and neural activation in mesolimbic areas induced by mating-associated conditioned cues.

Together, these data demonstrate that endogenous opioids during mating induce neural plasticity in VTA dopamine neurons that appear critical for morphine reward and long-term memory for natural reward behavior.