Vasoactive intestinal polypeptide excites mammalian dorsal horn neurons both in vivo and in vitro.

Vasoactive intestinal polypeptide (VIP) applied by iontophoresis and/or pressure microinjection causes a strong excitation of more than 75% of all tested spinal neurons in laminae I-VII of both the cat intact spinal cord and the rat spinal cord slice preparation. In the cat intact spinal cord the excitation is not limited to a single population of neurons but is observed in all categories of units recognized in spinal preparations of cats in this area on the basis of their excitability by different kinds of cutaneous afferent input. In the rat spinal cord slice preparation, VIP depolarized dorsal horn neurons and increased their excitability. The depolarization was associated with a decrease in neuronal input resistance. These results are consistent with the possibility that VIP may have a physiological role in synaptic function, either as a transmitter or as a modulator.